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Suggested Citation:"6 New Tools for Research." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
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6
New Tools For Research

Scientific progress is predicated on the observation of new phenomena, and there are two basic paradigms for making scientific observations. The first is the Galilean paradigm, which calls for building a better tool, such as a telescope, to investigate a familiar object, which in Galileo's case was Jupiter. The second could be called the Columbian paradigm, which calls for using existing technology, such as a small fleet of ships with the best available equipment, to investigate previously uncharted waters. Most of this report is dedicated to the Columbian paradigm and its technological consequences. This chapter summarizes where the Galilean paradigm has led us in the last 10 years and where building better tools might lead in the coming decade.

What we are really dealing with are new ways of seeing what has been there all along. The suite of small-to-large scale facilities that have enabled condensed-matter physicists to image atoms and electrons is as essential to the condensed-matter enterprise as the network of telescopes and detectors probing optical and cosmic ray spectra are fundamental to astronomy and cosmology. For more than a century, the condensed-matter suite has included small apparatus such as magnetometers and calorimeters. During the last few decades the suite has expanded to include synchrotrons and free-electron lasers, which produce highly coherent light of wavelengths from the far infrared to hard x-rays; nuclear reactors optimized for neutron yields; proton accelerators with targets for neutron and meson production; electron microscopes; and scanning-probe microscopes sensitive to everything from electron densities to magnetization at surfaces. Other exploratory tools include machines for subjecting matter to extreme conditions such as high magnetic and electric fields and pressures or ultralow temperatures.

Suggested Citation:"6 New Tools for Research." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
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Finally, in the past decade, direct computation or simulation has become an increasingly routine and reliable method for seeing and understanding condensed matter.

This chapter consists of sections devoted to each of the tools noted. Each section describes specific accomplishments these tools made possible in the last decade as well as opportunities and challenges for the future. Even though the sections deal with quite distinct facilities and techniques, there are certain overarching themes. An excellent example of an important scientific contribution over the last 10 years has been the effort to unravel the astonishing properties of the high-Tc cuprates and their siblings. It would be very difficult to imagine where our knowledge of the cuprates would be without the atomic coordinates given by neutron diffraction carried out at proton accelerators, the electronic bands given by photoemission at synchrotron sources, the defects found by electron microscopy, the magnetic order and fluctuations discovered using both reactor- and accelerator-based neutron sources, the charge transport measured in extreme pressures or magnetic fields, and the computer calculations of electronic energy levels. The experience with the cuprates shows that each of the facilities used is both unique and indispensable, and that their power is vastly amplified by combining data from the entire suite.

In addition to addressing specific scientific problems, another overarching theme includes the invigorating effects of new facilities, be they large national resources such as the Advanced Photon Source, the new hard x-ray synchrotron at Argonne National Laboratory; medium-scale installations such as the newly formed National High Field Magnet Laboratory operated in Florida and New Mexico; or electron microscopes and surface characterization equipment in central materials research facilities. The commercial availability of increasingly powerful workstations, electron microscopes, piezoelectric scanning-probe tools, and superconducting magnets have played an equally important but different role—namely, that of democratizing access to atomic resolution and high magnetic fields by giving individual investigators with small laboratories extraordinary capabilities formerly limited to those with access to large facilities.

A final thread linking the tools is a direct product of the information revolution seeded by condensed-matter physics and discussed at length elsewhere in the report—specifically, the proliferation of information the tools provide and the increasingly quantitative nature of the information. The most obvious manifestation is the trend away from simple black-and-white x-y plots and toward digital color images as experimental outcomes. Such images were exotic and laboriously produced 10 years ago. (The original scanning-tunneling microscopy images of silicon surfaces by Binnig and Rorer were actually photographs of cardboard models constructed from chart-recorder traces.) Today, color images are a routine feature of output from all of the techniques and facilities described below.

The future holds many opportunities and challenges including raising probe particle brilliance, improving instrumental resolution, extending spectral ranges,

Suggested Citation:"6 New Tools for Research." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
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and diagnosing increasingly complex phenomena in areas from ceramic processing to biology. Less obvious but equally important is the need to continue to collect and take full advantage of the large and quantitative data sets that the tools of today and tomorrow promise. This implies a broad program including elements such as quoting results that had hitherto been considered qualitative in absolute units, modeling strong probe-sample interactions, and taking advantage of the most advanced data collection and display technologies available.

Atomic Visualization Through Microscopy

A quick glance at the illustrations in this report confirms that atomic visualization underpins much of condensed-matter and materials physics. Knowledge of the arrangements of atoms is a prerequisite for understanding and controlling the physical properties of solids. The techniques needed to visualize atoms in solids themselves challenge our scientific and engineering capabilities. Research in atomic visualization techniques has often lead to improved manufacturing technologies, for example, in semiconductor fabrication and quality control. Tools used for atomic visualization are small enough to fit into an average-sized laboratory and are inexpensive enough to fit into the budget of a small-instrumentation grant, but cooperative usage (as facilities) and especially cooperative instrumental development can be invaluable.

Our ability to see atomic arrangements and identify local electronic structure has progressed dramatically in the last decades. The Nobel Prize in Physics of 1986 recognized the development of the two most important techniques for this purpose—scanning-tunneling microscopy and transmission-electron microscopy (TEM) (see Table O.1). Since then there has been astounding progress. The tunneling microscope has given birth to a burgeoning industry of versatile "scanning-probe" microscopes that, while sharing many characteristics with the scanning-tunneling microscope, do not rely on vacuum tunneling for image formation. Whereas the tunneling microscope is sensitive to local electronic states, probe microscopies can examine chemical reactivity, magnetism, optical absorption, mechanical response, and a host of other properties of surfaces on a near-atomic scale. The United States is a leader in research with probe microscopes, and this is the only microscopy area in which we dominate commercially.

Probe microscopy is undoubtedly powerful, but it is to a large extent limited to surface imaging. There are interesting exceptions, such as ballistic emission electron microscopy (BEEM) in which fast electrons are injected into a layer and their propagation is influenced by interfacial structure. Other complementary surface microscopy techniques that have grown in the last decade include low-energy electron microscopy (LEEM) and near-field scanning optical microscopy (NSOM). TEM, however, remains the dominant tool used for the microstructural characterization of thin films and bulk materials because its images are not confined to the surface. In the transmission-electron microscope, a high-energy

Suggested Citation:"6 New Tools for Research." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
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electron beam, guided by magnetic lenses, is scattered by a thin specimen. Diffraction makes it possible to study atomic structures inside solids and examine microstructure on scales from 0.1 nm to 100 µm. One example of the innovations achieved in the last decade with TEM is the discovery and structural solution of carbon nanotubes and nanoparticles. There has been significant progress in the last decade in the TEM field as well, for example, improved resolution (now at about 1 Å). Resolution is likely to be improved even further, using innovative aberration-correction techniques. Concomitant with improved spatial resolution in microscopy has been an improvement in efficiency and resolution in spectroscopy with electrons, which has enabled atomic-scale characterization of electronic structure. These techniques are complementary to, and synergistic with, improved neutron and x-ray tools described elsewhere.

Despite the undoubted value of improved resolution, a more important frontier in electron microscopy involves the ability to extract reliable quantitative information from images. An example is the use of fluctuation microscopy to go beyond the limits of diffraction in studying disordered materials. We anticipate much progress in the quantitative arena in the next decade. Although the proverb holds that a picture is worth a thousand words (no doubt true aesthetically), in science a few well-chosen words are sometimes worth a thousand pictures. This is because scientific questions involve precise answers, and pictures are by their nature imprecise. However, the theory of high-energy electron scattering is well developed, and continuing improvements in electron image detection and image analysis permit quantitative interpretation of images at the atomic level. We can expect that this capability will eventually reach a level at which nonexperts can use TEM as a quantitative structure analysis tool.

Similar progress can be expected in electron spectroscopy. Local spectroscopy allows not only atomic visualization, but also characterization of the electronic and chemical states of individual atoms or groups of atoms. Spectroscopy of surface atoms is the natural result of scanning-tunneling microscopy and can also be obtained (on groups of atoms) using TEM and surface-electron microscopy by electron energy-loss spectroscopy. Near-edge structure observed at characteristic x-ray energies can be used to determine band structure at buried interfaces, for example. Recent work has directly revealed the importance of metal-induced gap states in metal-ceramic bonding. One expects improvements both in the sensitivity of these techniques and in the quantitative modeling and data analysis needed to interpret their results. Ultimately, we need to obtain both atomic positional and chemical information for full structural characterization.

Although probe microscopes and some electron microscopes can flourish in the individual-investigator or small-facility setting, some instruments required for the future growth of atomic visualization will be of a scale such that they will need to be located in regional, if not national, centers. With computer network

Suggested Citation:"6 New Tools for Research." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
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access, remote control of the instruments is likely to become widespread. So even though instruments may be located in only a few institutions, accessibility will be universal. It remains desirable to maintain centers of excellence where experts in the appropriate techniques can be available for consultation and collaboration. Also, instrument and technique development could be facilitated on the regional-center scale and should be encouraged because although it has historically been underemphasized, it is critical to scientific and technological success. In addition, centers facilitate education in instrumentation, so critical for industrial competitiveness.

Atomic Structure

Scanning-probe microscopes have made atomic resolution imaging of surfaces almost routine, with tremendous impact on surface science. We are finally beginning to understand the important subject of thin-film growth, one atom at a time, and can observe how atomic steps can prevent atom migration in one direction compared with another, leading to undesirable roughness in deposited films. Here, there is close interaction between experimental visualization and computer modeling. A particularly exciting development in scanning-probe microscopy has been the imaging of chemical and biochemical molecules and the possibility of monitoring chemical reactions. By choosing one molecule as the tip of the atomic-force microscope (AFM), the forces between molecules can be directly measured and chemical reactions sensed with unprecedented molecular sensitivity. This has already led to new insights into the rheology of macromolecules (see Chapter 5), and we can expect great advances in the near future, especially in the biological sciences. For example, the use of "smart" tips would allow recognition of molecules using specific receptors adhered to the tip.

The scanning-tunneling microscope (STM) views the local electronic structure, so careful image simulations must be made to deduce atomic structure. In general, for structural studies on surfaces, the best results have been obtained by a combination of direct STM imaging with diffraction—for example, by x-rays or electrons. The highest directly interpretable spatial resolution for atomic structure has been obtained with TEM (see Box 6.1); instruments capable of resolving 1 Å have recently been demonstrated. The committee notes that, partly because of the ~$50 million price tag for these instruments and partly because of the damage accompanying the high accelerating voltages required, no such instrument can be found in the United States. Researchers' hopes are pinned on lower accelerating voltage approaches to improved TEM resolution, such as holographic reconstruction, focus variation, incoherent Z-contrast, and aberration correction. However, it is troubling that work in these areas is predominantly located in Europe and Japan; a notable exception is work on incoherent Z-contrast imaging (see Box 6.1). A relatively recent study of trends in atomic resolution

Suggested Citation:"6 New Tools for Research." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
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BOX 6.1 Being Certain About Atom Positions at Interfaces

Identification of atomic structure at interfaces has been one of the important applications of high-resolution transmission electron microscopy. Interfaces control mechanical strength in ceramics, electrical transport in transistors, corrosion problems in aircraft, tunneling currents in superconductor junctions, and a myriad of other practical materials behavior. Yet, with rare exceptions, interfaces are not amenable to diffraction analysis because they are very thin and not usually uniform. Figure 6.1.1 shows an example of a high-resolution transmission-electron microscope image, using ''z-contrast" of a grain boundary in MgO (courtesy of Oak Ridge National Laboratory), in which atomic columns at the boundary are revealed. Images like this are beginning to be analyzed in a quantitative manner, using accurate measurements of intensity, simulations of electron propagation, and computational modeling of atomic structure, to achieve unprecedented reliability in analysis of interfaces.

image

 

Figure 6.1.1
High-resolution transmission electron
micrograph, using z-contrast, in MgO.

microscopy was published by the National Science Foundation.1 Advances in electron microscopy enable advances in related industrial technologies, especially semiconductors; so the value of U.S. investment in this area extends far beyond atomic visualization.

A clear example of the value of improved resolution in TEM is tomography. Tomography has been widely used in biology to reconstruct objects at about 1-nm resolution. Only with a resolution of about 0.5 Å will it be possible to

1National Science Foundation Panel Report on Atomic Resolution Microscopy: Atomic Imaging and Manipulation (AIM) for Advanced Materials, U.S. Government Printing Office, Washington, D.C. (1993).

Suggested Citation:"6 New Tools for Research." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
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reconstruct objects in three dimensions at the atomic scale. This would be particularly exciting for amorphous and disordered materials; knowledge of their atomic structure is limited to statistical averages from diffraction. Instruments to enable this will require ˜0.5 Å resolution combined with high specimen-tilt capability (>45º). Such will be possible either with very high voltages or with aberration correction.

Electronic Structure

For many research problems in condensed-matter and materials physics, it is important to visualize the electronic structure on a near-atomic scale. STM provides direct information about electronic states at surfaces but is often used for purely structural analysis and has had tremendous impact on surface science. Examples in the report include the germanium "huts" in Figure 2.13. In general, probe microscopy combined with electron microscopy has revolutionized our understanding of thin-film growth and epitaxy (see Chapter 2).

STM has been profitably used to examine surface electronic states and chemical reactions on the atomic level. Although detailed electronic structure calculations are needed to interpret STM images in terms of atomic positions, often the electronic structure information is directly useful. For example, Box 6.2 gives an example of direct STM imaging of the electronic states associated with individual dopant atoms in semiconductors.

Electron energy-loss spectroscopy in TEM provides an important method to obtain electronic structure from the interior of samples on a near-atomic level. Improvements in the sensitivity of detection, using more monochromatic field-emission electron sources and parallel detection, have led to important advances in the last decade. For example, dopant segregation at semiconductor grain boundaries has been identified.

Nanoproperties Of Materials

One of the most significant developments of the last decade is the proliferation of scanning-probe techniques for measuring the nanoproperties of materials. Figure 6.1 shows a large variety of signals that are now detectable. Nanomechanical (force) measurements can be used to watch the behavior of individual dislocations; optical measurements can visualize single luminescent states; piezoelectric measurements can identify the effect of defects on ferroelectrics, which have potential for high-density nonvolatile memory; magnetic measurements can show the effect of single atoms on spin alignment in atomic layers; ballistic electron transport can identify the electronic states associated with isolated defects inside a film. We can expect these capabilities to revolutionize our ability to characterize the physical properties of nanoscale materials.

Suggested Citation:"6 New Tools for Research." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
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BOX 6.2 Single Impurity Atoms Imaged in Semiconductor Layers

One critical issue, as semiconductor devices are scaled down in size for higher density and speed, is the stochastic nature of the location of dopant atoms. These atoms, which lend electronic carriers to the active semiconductor layers, are typically present in densities of only about 1 in a million. Until recent years, it was an impossible dream to identify the exact location of these dopant atoms, but this has recently proved possible with scanning-tunneling microscopy. Figure 6.2.1 shows detection of the local electronic state generated by the impurity. When a semiconductor structure is cleaved in vacuum, the individual impurity atoms near the surface are clearly visible. The image (courtesy of Lawrence Berkeley Laboratory) shows the position of Si dopants in GaAs as bright spots. Also present in the image are Ga vacancies, which appear as dark spots.

image

 

Figure 6.2.1 Local electronic states in GaAs generated by Si impurities.

image

Figure 6.1
Schematic drawing of the signals detected in scanning-probe 
microscopy. (Courtesy of the University of Illinois at 
Urbana-Champaign.)

Suggested Citation:"6 New Tools for Research." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
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Similar developments have occurred in other imaging systems. A beautiful example of a technique known as ''scanning electron microscopy with polarization analysis," which allows imaging of magnetic monolayers at surfaces in a modified scanning-electron microscope, is discussed in Chapter 1.

Atomic Manipulation

Whether intended or not, our atomic-scale characterization tools can change the structures they are examining. This can be used to our advantage in manipulating atoms on the atomic scale for making nanostructures. Figure 6.2 shows the classic example of a ring of iron atoms assembled by the tip of a scanning-tunneling microscope. The circular atomic corral shows the resonant quantum states expected from simple theory. The imagination boggles at the possibilities with related techniques. In principle, we can assemble arbitrary structures to test our understanding of the physics of nanostructures and perhaps make useful devices at unprecedented density. Two major issues will need to be addressed before these methods can reach their full potential. First, even when we place atoms where we choose, with few exceptions (such as the Fe atoms in Figure 6.2 at ultralow temperatures), they will not stay there. So, to assemble structures that

image

Figure 6.2
Atomic manipulation. The image shows the atomic scale capability for 
patterning that is possible with the scanning-probe microscope. Atoms 
of Fe (high peaks) were arranged in a circle on the surface of Cu and 
caused resonant electron states (the ripples) to appear in the Cu surface. 
The structure is dubbed the "quantum corral." Related structures might 
one day be useful for electronic devices, where as many devices as 
there are humans in the world could be assembled on an area 
the size of a pinhead (1 mm2). (Courtesy IBM Research.)

Suggested Citation:"6 New Tools for Research." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
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retain their integrity, we need to understand the stability of materials on this scale. Second, the speed with which we can pattern structures with a single scanning probe is far too slow to allow practical device fabrication on the scale of modem semiconductor technology. Alternate methods involving massive arrays of tips, projection electron lithography, or other short-exposure techniques must be developed.

Conclusions

Atomic visualization is a crucial part of condensed-matter and materials physics. It is a thriving area in which advances usually driven by physics and engineering have wide impact on science and technology. Many manufacturing technologies depend on innovations enabled by atomic visualization equipment, so research in the field has important economic value. We expect continued developments, but attention must be paid to nurturing the development of appropriate instrumentation in close connection with scientific experiments. Depending on the nature of the visualization tool, the funding scope ranges from individual investigator to small groups, to national centers of excellence in instrumentation. From our success in probe microscopy, it appears we are stronger at the individual-investgator level but weaker at the medium- and larger-scale instrumentation development levels. A concern is that many new students are attracted by computer visualization rather than experimental visualization. The two methods are obviously complementary, and we are not yet near the point where we can rely only on computer experiments. Thus funding must be maintained at a level sufficient to create opportunities that will attract high-quality students into this field.

Neutron Scattering

The neutron is a particle with the mass of the proton, a magnetic moment because of its spin-1/2, and no electrical charge. It probes solids through the magnetic dipolar interaction with the electron spins and via the strong interaction with the atomic nuclei. These interactions are weak compared to those associated with light or electrons. They are also extremely well known, which makes it possible to use neutrons to identify spin and mass densities in solids with an accuracy that in many cases is greater than with any other particle or electromagnetic probe. The wavelengths of neutrons produced at their traditional source, nuclear research reactors with moderator blankets of light or heavy water held near room temperature, are on the order of inter-atomic spacings in ordinary solids. In addition, their energies are on the order of the energies of many of the most common collective excitations—such as lattice vibrations—in solids. To image spin and mass densities, condensed-matter physicists usually aim neutrons moving at a single velocity and in a single direction, that is, with well-specified momentum

Suggested Citation:"6 New Tools for Research." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
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and energy, at a sample and then measure the energy and momentum distribution of the neutrons emerging from the sample. Such neutron-scattering experiments have been important for the development of condensed-matter physics over the last half century. Indeed, the impact of the technique has been such that C. Shull (Massachusetts Institute of Technology) and B. Brockhouse (McMaster University) were awarded the 1994 Nobel Prize in Physics for its development (see Table O.1). In previous decades, neutron scattering provided key evidence for many important phenomena ranging from antiferromagnetism, as originally posited by Neel, to unique quantum oscillations (called rotons) in superfluid helium. But what has happened in the last decade in the area of neutron scattering from solids and liquids, and what is its potential for the coming decade?

The Past Decade
Overview

Three major developments of the last decade are (1) the emergence of neutron scattering as an important probe for "soft" as well as "hard" condensed matter, (2) the coming of age of accelerator-based pulsed neutron sources, and (3) the revival of neutron reflectometry. The first development has expanded the user base for neutron scattering far beyond solid-state physicists and chemists, who had been essentially the only users of neutrons. The second development is associated with a method for producing neutrons not from a self-sustaining fission reaction, but from the spallation—or evaporation—that occurs when energetic protons strike a fixed target. As depicted in Figure 6.3, a spallation source consists of a proton accelerator that produces short bursts of protons with energies generally higher than 0.5 GeV, a target station containing a heavy metal target that emits neutrons in response to proton bombardment, and surrounding moderators that slow the neutrons to the velocities appropriate for experiments. Until the mid-1980s, the leading facility of this type was the Intense Pulsed Neutron Source (IPNS) at the Argonne National Laboratory. In the last decade, the clear leader by a very wide margin has been the ISIS facility in the United Kingdom. Successful developments, especially at ISIS, have given the neutron-scattering field growth prospects that it has not had since the original high-flux nuclear reactor core designs of the 1960s. This follows because pulsed sources are more naturally capable of taking advantage of the information and electronics revolutions and because the unit of cooling power required per unit of neutron flux is almost one order of magnitude less than for nuclear reactors.

The revival of neutron reflectometry seems at first glance less momentous than the emergence of neutron scattering as a soft condensed-matter probe or the emergence of accelerator-based pulsed neutron sources. However, as so much of modern condensed-matter physics and materials science revolves about surfaces and interfaces, neutron scattering could hardly be considered a vital technique

Suggested Citation:"6 New Tools for Research." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
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image

Figure 6.3
Drawing of the planned Spallation Neutron Source at Oak Ridge National 
Laboratory. The basic design features are similar to those for the Los 
Alamos Neutron Scattering Center (LANSCE) and the planned 
European Spallation Source (ESS). The linear accelerator takes 
protons to 1.33 GeV, while the accumulator ring groups them into 
1 µsec bursts, occurring at a repetition rate of 60 Hz, which then 
impinge onto a liquid mercury target. The neutrons emanate in 
corresponding bursts from the target and feed scattering instruments 
with flight paths with lengths from 2 to 100 m. 
(Courtesy of Oak Ridge National Laboratory.)

without some clearly defined contribution in these areas. The revival of reflectometry has enabled neutrons, in spite of their weak coupling nature, to become a legitimate probe of surfaces and interfaces. Here we use long wavelengths and incident and reflected beams that nearly graze the sample, so that we are in the surface-sensitive regime near the condition of total external reflection.

Locating The Atoms

The major contributions of neutrons to condensed-matter and materials physics in the last decades come from using neutrons to answer the most fundamental question that always arises when new materials are discovered—"Where are the atoms?" Although this question is generally answered using x-ray diffraction, the unique properties of neutrons offer significant advantages in many important cases. However, since neutrons couple via the nuclear interaction to the atomic

Suggested Citation:"6 New Tools for Research." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
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cores rather than via the electromagnetic interaction to the atomic electrons, neutrons can be equally sensitive to light (low-Z) and heavy (high-Z) atoms, whereas x-rays always couple much more strongly to the heavy elements. Neutrons are especially sensitive to the lightest and arguably most important element of all, hydrogen, and quite sensitive to its rival in importance, oxygen. In addition, it is possible to change atoms' visibility to neutrons, without appreciably changing the bonding or chemistry of a particular atom, by changing the isotope. Thus, particular sites in a material can be labeled for investigation of their microscopic coordinates and motion. Finally, the combination of various neutron sources, as well the ability to tailor wavelength distributions even at a single source, permits the examination of structures with characteristic length scales from angstroms to microns. The weak coupling nature of the probe means that even as the wavelengths of the neutrons used experimentally change over three orders of magnitude, the scattering cross sections do not and absorption and resolution corrections remain simply calculable.

One of the most lively areas in condensed-matter science over the last decade has been that of transition metal oxides, a field dramatically revived by the discovery of high-temperature superconductivity in oxides of copper. The materials are generally combinations of relatively heavy lanthanides, medium-weight transition metals, and light oxygen atoms. With this set of constituents, neutron scattering was ideally positioned to make an important contribution to the structure determination. The technique did not disappoint. First it has been demonstrated that the key structural elements common to all of the cuprate superconductors are nearly square planar arrays of copper and oxygen. The significance of this simple finding is impossible to overstate. That copper oxygen planes are the key feature of the high-temperature superconductors has been the starting point for essentially all of our thinking about high-temperature superconductivity as well as searches for materials with better superconducting properties. Beyond revealing the ubiquity of the copper oxygen planes, neutron diffraction has revealed how the planes appear singly, in pairs, or even as triplets, sometimes with and sometimes without copper oxide chains in intervening layers. The picture of the intervening layers as reservoirs that provide charges for the copper oxygen planes is largely the result of a combination of neutron diffraction and classical measurements of bulk electrical properties such as resistivity.

Extensive work has shown close correlations between structural details and superconducting properties. For example, in a mercury-based compound exhibiting an extraordinarily high Tc, which itself is very sensitive to pressure, neutron diffraction showed dramatic changes in the atomic coordinates with applied pressure (see Figure 6.4).

Even after 10 years of indispensable contributions to the understanding of high-Tc superconductivity, neutron diffraction retains its unique, driving role in this field. A recent illustration of this is the excitement generated by the discovery that certain materials very closely related to the high-temperature supercon-

Suggested Citation:"6 New Tools for Research." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
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imageimage

Figure 6.4
Mercury-based cuprates exhibit not only the highest transition temperatures Tc
for superconductivity, but also extraordinarily pressure-dependent Tc's High-
resolution neutron diffraction at pulsed spallation sources has revealed the 
complex structures (at right) of these compounds. In addition, the penetrating 
power of the technique has been exploited to examine the pressure dependence 
of the structure. There is an astonishing 0.25 Å, contraction of the marked 
copper-to-oxygen distance as pressure is applied to raise Tc from 138 to 160 K.

ductors undergo phase transitions to states with large-scale superstructures. Neutron scattering has provided atomic-scale information not only about the high-temperature superconductors, but also about many other transition metal oxides. Notable examples are the perovskite manganites whose "colossal" magnetoresistance—large changes in the electrical resistivity when external magnetic fields are imposed—has recently been rediscovered. Neutron diffraction has been used to identify the structural parameters most strongly correlated with the magnetoresistance.

Seeing The Spins

Once they know the locations of atoms in a particular solid or fluid, condensed-matter physicists generally would like to know what the electrons are

Suggested Citation:"6 New Tools for Research." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
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doing. The electrons of particular interest are the outer electrons because they account for the chemical bonding and electrical and magnetic properties of a solid. The neutron couples to these electrons through the magnetic dipole interaction; because its energy is typically much too small to excite the electrons from the core where they form a closed shell with zero net orbital and spin-angular momenta, the cores are invisible. The outer electrons are easiest to see with neutrons when they live on a regular lattice and their spin orientations repeat periodically. In this case, they produce diffraction spots entirely analogous to those associated with atomic nuclei. As Shull showed in the 1950s, it is then possible to do magnetic crystallography to image the spin arrangements in virtually any magnet. In the last decade, magnetic crystallography with neutrons has continued to be among the most essential tools in condensed-matter physics. Again, high-temperature superconductivity has been an area of accomplishment. The important experiment was that which showed, shortly after the superconductivity's discovery, that the insulating and undoped parent compounds of the superconductors are actually very simple antiferromagnets. In the decade since this experiment, the superconductivity and magnetism of the cuprates have been inextricably intertwined. As for the neutron diffraction experiments that revealed the microscopic structures of the high-Tc compounds, the last decade's progress in high-temperature superconductivity would be unimaginable without the early magnetic diffraction data on the parent compounds.

Magnetic diffraction has played a similar role in other subfields that have been active in the last decade. For example, it established a definite link between the magnetism and exotic superconductivity of certain actinide and rare-earth intermetallics, also known as heavy fermion compounds. Also, a particularly important and elegant set of experiments explored the coupling between magnetic layers through intervening nonmagnetic layers in thin-film multilayer structures grown by molecular-beam epitaxy. The structures show great promise as "spin valves" for application to computer disk drive read heads. The optimization of their performance requires complete knowledge of the atomic and spin densities responsible for the desirable giant magnetoresisant behavior. Using polarized-neutron reflectivity, one can obtain a depth profile of the direction and magnitude of the magnetic moment in these materials with 2- to 3-Å resolution. Early polarized-neutron reflectivity studies confirmed that maximum giant magnetoresistance is correlated with an antiparallel alignment of the magnetic layers across the nonmagnetic interlayers. More recent experiments revealed the complex interplay between the magnetic structure and the physical characteristics.

Imaging Vortices In Superconductors

A seemingly different type of magnetic structure is that of mesoscopic field inhomogeneities. Mesoscopic inhomogeneities are seen by the neutrons in the same way in which they see the microscopic field inhomogeneities associated

Suggested Citation:"6 New Tools for Research." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
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with the electron spins—namely, through the magnetic dipole coupling between the neutron spin and the magnetic fields. The relevant wave numbers and corresponding apparatus are different, but the concepts remain the same. The most famous mesoscopic field inhomogeneities in condensed-matter physics are those associated with type-II superconductors. Here, the superconductor accommodates an external field by admitting quantized vortices containing normal (metallic) state cores embedded in a superconducting matrix. The vortices typically arrange themselves to form a lattice with inter-vortex separations of order 100 to 1000 Å. One of the triumphs of neutron scattering in the 1960s was the verification of the vortex lattice picture for conventional, low-temperature type-II superconductors. Given this early success, it should come as no surprise that as unconventional superconductors such as actinide intermetallics and cuprates were discovered in the 1980s and 1990s, neutrons were used to image their vortex lattices. They provided key evidence for two of the most important new ideas about superconductivity. The first idea is that real solids could actually display superconductivity more akin to the superfluidity of helium-3 than to the superconductivity of ordinary solids like aluminum; the second is that collections of vortices can have intricate phase diagrams much like those of complicated organic molecules in solution.

Pictures Of Soft Matter

The committee has focused so far on neutron scattering from intermetallic compounds and their oxides, which, although they are complex, are solids of long-standing interest to condensed-matter physicists. Indeed, manganites were among the first materials to be investigated by neutron diffraction shortly after the invention of the technique in the 1950s. The last decade has witnessed a huge growth in the use of neutrons to image structures formed at surfaces and interfaces, as well as the large-scale structures that emerge in materials with genuinely large molecular units, such as polymers and water-based biomolecules. The universe of such structures is actually much larger than that of traditional condensed-matter and materials physics and contains most of the matter essential for our lives. One particularly successful application of the neutron technique has been to diblock coploymers, which show a huge variety of disordered and mesoscopically ordered states. As in classic condensed-matter physics, the goal of the investigations is to relate the structures to properties, such as elasticity, which determine functionality. Neutron scattering has also measured the sizes and shapes of micelles that appear in microemulsions. Model systems mainly of interest to statistical physicists, as well as biologically interesting micelles such as ribosomes, have been examined. Another development of the last decade has been the application of neutron scattering to surfaces, interfaces, and membranes involving polymers and other large organic molecules. Knowledge of local structural features as well as interface profiles feeds into a vast array of scientific

Suggested Citation:"6 New Tools for Research." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
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and technical fields from biology to integrated circuit packaging. Figure 6.5 shows the shape change undergone by diblock copolymers adsorbed on a glass substrate. As the conditions are changed, the copolymers undergo a transition from a mushroom- to a brush-like shape, which correlates with a change in the adhesive properties of the coated surface.

Dynamics

Nuclear and magnetic structure determinations represent the most common and widely understood application of neutron scattering. However, since the work of Brockhouse in the 1950s, the study of lattice vibrations and magnetic fluctuations has also had an impact on condensed-matter physics and materials science. As have neutron determinations of magnetic and nuclear structure,

image

Figure 6.5
One of the most important developments of the 1990s has been the revival of 
neutron reflectometry. Formerly used as a tool for establishing absolute 
neutron-scattering cross sections, it has become a major technique for surface 
and interface science, with particularly significant accomplishments in the 
fields of soft matter and magnetoresistive films. The figure shows data for 
the ''mushroom" to "brush" transition for polymers attached to a substrate. 
Raw data are at right, the directly deduced density profiles are in the middle, 
and the inferred morphology is shown at left. The radically different reflectivity 
profiles at right attest to the ability of the technique to discriminate between 
the different arrangements of the polymers at the surface. 
(Courtesy of Los Alamos National Laboratory.)

Suggested Citation:"6 New Tools for Research." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
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neutron scattering from excitations in solids has strongly influenced our thinking about transition metal oxides. For example, work on the cuprate superconductors includes measurements of the phonon density of states, which can be used as inputs into traditional calculations of the superconducting transition temperature. The apparent failure of such calculations remains an important motivation to search for a new theory for the superconductivity of the cuprates. Much more recent experiments provide similarly complete magnetic excitation spectra, which can now be used in analogous tests of "conventional" magnetic theories of high-temperature superconductivity.

An interesting development has been the use of neutrons to probe the electronic gap function and pair-breaking excitations in the superconducting state. Neutrons are unique for this application because they allow the only superconducting spectroscopy that is a true bulk probe capable of examining short-wavelength phenomena with high energy resolution.

The continuing work on the dynamics of ordered solids has coexisted with a rapidly growing enterprise concerned with the dynamics of fluids and soft matter. Important experiments include those that have verified one of the key concepts in polymer science—that polymers in a melt move in snake-like fashion within tubular structures formed by their neighbors. The experiments are noteworthy not only for their scientific impact, but also because they required the use of an instrument—the neutron spin echo spectrometer—that operates on a principle unknown to the founders of inelastic neutron scattering, Fermi and Brockhouse.

The Next Decade

Because of unique properties associated with their mass, charge, and spin, neutrons have a scientific future as bright as their past. Most likely, key accomplishments with neutrons will be as unexpected as were those of the last decade, when they were linked to a largely unexplored class of materials—the cuprates—that happened to display an extraordinary and unexpected property, high-Tc superconductivity. Thus, the cuprates offer for neutron scattering, as they do for condensed-matter physics as a whole, a lesson in humility to all who wish to plan future accomplishments. At the same time, the success of neutrons in meeting the challenges of high-temperature superconductivity was not entirely serendipitous. Indeed, the discovery and subsequent intensive study of the cuprates coincided with other developments:

1. The rapid development of accelerator-based pulsed neutron sources and instrumentation, whose operating paradigms are entirely different from those invented by Shull and Brockhouse for nuclear reactors;

2. Progress in electronics, data visualization, and computation driven by the microelectronics revolution;

Suggested Citation:"6 New Tools for Research." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
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3. Extension of the routinely examinable spectral range from its traditional 0.5- to 50-meV domain to its current 1-µeV to 1-eV band;

4. Development of increasingly efficient beam optics and scattered-neutron analysis and detection schemes; and

5. Extension of sample environment capabilities to lower temperatures, higher magnetic fields, and higher pressures.

These other developments not only coexisted with the great materials discoveries of the last decade, but were actually a prerequisite for the significant contributions ultimately made by neutron scattering to the elucidation of these discoveries. It is our judgment that further improvements in all five of the listed categories are inevitable in the next decade. The inevitability follows from the continued effects of accelerator-based pulsed neutron sources and instrumentation, and of advances driven by the microelectronics revolution on the entire field, that has been hampered by the limits imposed by the modest incident fluxes that even modern research reactors can provide. Advances in both accelerator-based pulsed neutron research and microelectronics have made it possible to multiplex many experiments on an enormous scale, for example, simultaneously collecting 106 usable pixels of information where the old reactor-based methods would yield a single pixel. Thus, the field of neutron scattering has changed qualitatively over the last decade, even though only one major new source (ISIS in the United Kingdom) has been completed. The figure of merit for many important experiments has been transformed from the reactor power to the information rate. In the coming decade, we expect the useable information rates as measured by the product of incident flux delivered by the beam optics and the number of independent pixels to grow in tandem with the microelectronics revolution (Figure 6.6). Beam optics are also on a growth curve driven by improvements in thin film-technology and x-ray and light optics, and so are also likely to improve. The continued growth in capabilities will make many new experiments possible, as well as allow old measurements to be performed with greater precision. The new experiments might include measurements of vortex lattice dynamics in type-II superconductors, investigations of the magnetic aspects of the quantum Hall effects, characterization of fluid flow in small capillaries, and studies of electromigration at silicon-metal interfaces. Of course, the most exciting experiments will be those dealing with phenomena we are unaware of today.

Neutron experiments have continued to be popular even in the absence of a new neutron source because of the neutron's uniqueness as a probe of condensed matter and because neutron experiments are so readily improved by ongoing advances in microelectronics and thin-film technology. However, merely transferring technology developed for other uses to its antiquated neutron-scattering centers will not allow the United States to recapture its lead in neutron science. There is no substitute for constructing a new high-power spallation source with many high-flux beam lines. In recognition of this, the government is supporting

Suggested Citation:"6 New Tools for Research." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
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image

Figure 6.6
The information acquisition rate (left) for single-crystal inelastic experiments is the product of the flux at the sample (expressed in nuclear reactor equivalent MW units) and the number of useable pixels within which the scattered and incident neutrons fall. Brockhouse was co-recipient of the 1994 Nobel Prize for developing the single-pixel triple-axis spectometer (see Table O.1), which dominated inelastic neutron scattering until around a decade ago. The development of pulsed spallation sources and fast rotor chopper spectrometers has moved inelastic neutron scattering onto a growth curve (Moore's Law) driven largely by the electronic data-processing industry. The neutron sources identified are the HFBR (High Flux Beam Reactor, Brookhaven National Laboratory), HFIR (High Flux Isotope Reactor, Oak Ridge National Laboratory), and ILL (Institut Laue-Langevin, France) reactors and the ISIS (Rutherford-Appleton Laboratory, United Kingdom), SNS (proposed Spallation Neutron Source, Oak Ridge National Laboratory), and ESS (European Spallation Source, currently unsited) accelerator-based facilities. MAPS, HET, and MARI correspond to ISIS instruments at different stages of development. [Physics World, 33 (December 1997).]

the construction of precisely such a source, the Oak Ridge Spallation Neutron Source, whose completion will be the big event of the next decade for neutron science.

Synchrotron Radiation

In the past 30 years, the use of infrared, ultraviolet, and x-ray synchrotron radiation (SR) for condensed-matter and materials physics research, as well as research in the other natural sciences, engineering, and technology, has blossomed. The pace and scientific range of SR utilization has increased even more rapidly during the past decade because of source improvements, advanced instru-

Suggested Citation:"6 New Tools for Research." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
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mentation, and more beam time than was possible a decade ago. Further impetus was provided by the construction of new facilities with extreme performance. As a consequence of these developments, approximately 4,000 scientists from academia, industry, and government laboratories now use U.S. SR facilities.

In the 1960s and 1970s, research was initiated using SR produced by the bending magnets at storage rings designed for high-energy physics. As shown in Figure 6.7, such rings provided about four orders of magnitude greater brightness than the best in-laboratory sources. In addition, the radiation covered a very broad spectrum, in contrast to the line source x-ray tubes then available. These features made a number of previously unfeasible experiments possible.

image

Figure 6.7
History of (8 keV) x-ray sources. Brilliance (or brightness) is 
defined as source intensity per illuminated solid angle. 
(Courtesy of Argonne National Laboratory.)

Suggested Citation:"6 New Tools for Research." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
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Because of the science and the large user communities resulting from the first-generation sources, they were joined by second-generation, high-brilliance rings designed specifically for SR research in the mid-1980s. The increased brightness and the greater availability of these sources, as well as the increased flux achieved by insertion devices at all sources, further expanded both the science and the user community. (Insertion devices, known as wigglers and undulators, are magnetic arrays that cause the charged particles to undergo quasi-sinusoidal paths, producing far brighter radiation than can be achieved with bending magnets at the same storage ring.)

During the past decade, third-generation rings [the Advanced Light Source (ALS) and Advanced Photon Source (APS; shown in Figure 6.8) in the United States, SPRING-8 in Japan, and the European Synchrotron Radiation Facility in France], with still higher brightness (by 4 to 5 orders) and many straight sections for insertion devices, have been constructed. At the same time, the first- and second-generation rings have been modified so that their performances have increased markedly. Such increases form the basis of revolutions in the research that utilizes SR—a process that is likely to continue well into the next century with new sources.

image

Figure 6.8
Overview of the recently completed Advanced Photon Source 
(APS) at Argonne National Laboratory. Electrons circulate in the 
storage ring and emit brilliant x-ray beams that are used to probe 
the structure of condensed matter at scales ranging from the 
atomic to the macroscopic. (Courtesy of Argonne National 
Laboratory.)

Suggested Citation:"6 New Tools for Research." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
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The Past Decade
Protein Crystallography

The goal of understanding life has evolved into a large interdisciplinary effort that integrates information extending from experimental results at the atomic and molecular levels to studies of organelle, cellular, and tissue organization and function. Atomic-level information will increasingly provide the means through which biological function, and malfunction that leads to disease, will be understood. Macromolecular crystallography has provided the vast majority of information about three-dimensional biological structure and will play an even greater role in the future. Information relating structure to function has also led to the development and successes of new approaches to drug discovery (often called structure-based drug design).

The unique properties of SR—namely, its tunability and high brilliance—have allowed it to play a seminal role in these advances. So important is SR to protein crystallography that 73 percent of new structures published in Nature and 60 percent of those published in Science in 1995 used synchrotron-based data, and this percentage continues to grow. Some of the most important results include the structure of the myosin head, which has led to a molecular-level interpretation of muscle contraction; the structure of cytochrome oxidase, which is the enzyme that carries out the final step in mammalian respiration; the structure of the enzyme nitrogenase responsible for production of most of the assimilable (fixed) nitrogen in our biosphere; the structure of the ribozyme, which is a catalytic form of RNA; numerous plant and animal virus structures (for an important example, see Figure 6.9), as well as studies of their interaction with potential antiviral drugs; and structures of a variety of enzymes, like topoisomerases, involved in DNA transformations and regulation.

Kinetic Studies Of Structure

The five order of magnitude increase in photon flux provided by the first x-ray SR sources immediately enabled time-resolved diffraction studies. Structural biologists addressed the changes in muscle tissue as it contracts and expands and have come to a detailed understanding of the mechanism of force generation. Subsequently, time-resolved scattering and x-ray absorption spectroscopy (XAS) studies, as well as the related studies of systems in excited states, have blossomed, providing kinetic understanding of reactions and processes that cannot be obtained in other ways. Following are examples of other types of studies that have benefited from time-resolved scattering and XAS:

1. In situ studies of thin film growth by sputtering, organometallic vapor phase epitaxy, and molecular-beam epitaxy;

Suggested Citation:"6 New Tools for Research." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
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2. Both protein crystallographic and depolarization studies of atomic reorganization in photosensitive biological molecules subjected to light pulses (see Figure 5.3);

3. In situ small-angle x-ray scattering studies of the refinement of heavy crude oils; and

4. Structural studies of phases formed during various stages of welding.

The examples illustrate a portion of the range of time-resolved studies that have been performed thus far. With third-generation sources completed and fourth-generation sources being planned, applications to faster processes, shorter-lived states and more weakly scattering systems can be anticipated as we seek to understand aspects of reactions at the atomic and molecular levels.

Surfaces And Interfaces

Both fundamental and applied x-ray scattering studies of surfaces and interfaces have flourished over the past decade at all of the x-ray facilities. Among the

image

Figure 6.9
Structural information is central to the development of models 
and cures for disease, and today is largely established using 
methods and large facilities originally developed for the 
condensed-matter and materials physics community. The 
figure shows the exterior envelope protein (upper right) of 
the AIDS (acquired immunodeficiency syndrome) virus together 
with a neutralizing antibody (left) and the human CD4 receptor 
(lower right). The structures are deduced from x-ray diffraction 
data collected at the National Synchrotron Light Source (NSLS) 
at Brookhaven National Laboratory.

Suggested Citation:"6 New Tools for Research." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
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most exciting of the fundamental studies have been those of continuous phase transformations. SR studies of transformations between the different phases of monolayers absorbed on flat, well-ordered substrates, as well as reconstructed surfaces, have enabled significant tests of exact results from the theory of two-dimensional physics. Similarly, the step-bunching transition on single crystal surfaces, originally predicted theoretically, has been studied experimentally with SR. The surface-scattering methods have also been used for studies of liquid-surface and amorphous thin-film structures. A particularly interesting result is that near the surface of liquid metals, there is metal atom layering.

Surface sensitive SR techniques have been applied increasingly to significant technological problems. Several major projects have been aimed at understanding thin-film formation via vapor deposition and sputtering. Because the normal surface-sensitive techniques cannot be used in these ''high-pressure" situations, synchrotron-based surface-scattering studies now account for a large portion of the existing in situ characterization of these processes. An important extension of the surface-scattering technique is grazing incidence fluorescence now being applied by several semiconductor manufacturing companies to micro-contamination analysis of Si wafers.

Also of technological importance are the surface-sensitive electron-yield techniques in the vacuum ultraviolet (VUV)/soft x-ray region, which measure bond lengths of adsorbate/surface bonds and orientations of molecules adsorbed on surfaces. These are now being used for practical applications such as determining the mechanisms governing the orientation of molecules of importance to liquid-crystal displays. In addition, x-ray magnetic circular dichroism is having a significant impact on the science of magnetic recording.

The number of applied problems involving surfaces, surface layers, and interfaces is enormous. We anticipate enormous growth in experiments related to corrosion, electrochemistry, tribology, environmental interfaces, and the like as more beam lines are commissioned around the world. Electrochemistry deserves special mention because already SR together with probe microscopies has transformed this field from one primarily dependent on electrochemical measurements and related modeling to the study of electrode processes at the molecular level.

Microspectroscopy

The availability of the third-generation sources has made possible higher resolution microspectroscopies. The higher brightness at ALS has enabled construction of an improved scanning transmission x-ray microscope (STXM) as well as a scanning photoelectron microscope (SPEM). The STXM is especially useful for micro-composition and orientation measurements in multicomponent polymers and organic systems. Spatially resolved x-ray photoelectron spectroscopy is now being applied to a range of materials issues, such as examining

Suggested Citation:"6 New Tools for Research." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
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chemical structure of Ti-A1 alloys reacted with graphite and chemical speciation on bond pads of integrated circuits in order to correlate chemical state with phenomena like adhesion and chemical residues in vias.

At APS, an x-ray microprobe with a FWHM focal spot size of 0.33 µm with a flux density exceeding 5 x 1010 photons/m2 s (0.01 percent bandwidth) has been developed. Using a root specimen in its natural hydrated state, elemental sensitivity significantly better than 10 ppb and minimum elemental detection limit of 0.3 fg have been demonstrated. The x-ray microprobe is being used in variety of environmental and biological research projects. Figure 6.10 shows that it is also

image

Figure 6.10
X-ray microbeam diffraction study at the NSLS on an electro-
absorption modulator/laser device (lower figure). The upper 
figure shows the change in the strain distribution on going 
from the modulator region to the laser region of 
the device. (Courtesy of Bell Laboratories, Lucent
 Technologies.)

Suggested Citation:"6 New Tools for Research." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
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extremely useful for the characterization of technologically important man-made microstructures.

Insertion devices provide brilliance adequate for performing high-resolution inelastic scattering measurements of excitations in solids and liquids. X-rays have the special advantage that they are not subject to the kinematic restrictions that prevent neutrons from probing medium-energy excitations at long wavelengths. In addition, their coherence as they emerge from modern undulators is such that photon correlation spectroscopies, hitherto limited to visible wavelengths emitted by ordinary lasers, can be extended to examine slow dynamic processes at shorter distances than previously possible. Given these twin advantages, it should come as no surprise that x-rays have yielded some of the most exciting results in the physics of fluids and glasses, where they have been able to examine portions of phase space inaccessible to both neutron- and light-scattering techniques. Other progress has occurred in lower-resolution measurements to examine excitations in various simple metals as well as more complex oxides.

Photoemission Spectroscopy

Angle-resolved photoemission spectroscopy (ARPES) at SR sources has proven to be a unique tool when addressing the question of the electronic structure of solids, most notably the variation of the band energies with respect to momentum.

Over the last decade, such experiments have played an important role in advancing our understanding of high-temperature superconductors. The significant improvements in beam intensity and energy resolution obtained from undulators and new spectrometers have facilitated the discovery of a number of fascinating features in the electronic structure of the high-Tc superconductors. The most notable consequence is the beginning of a detailed view of how conventional band theory breaks down for these materials. In addition, ARPES has provided images of the unconventional superconducting gap functions of the cuprates.

Magnetic Scattering

Although the coupling of x-rays to magnetic moments is considerably smaller than that of neutrons or electrons, the extremely high-SR intensities have enabled qualitatively new kinds of experiments complementary to those performed with neutrons. In particular, the availability of radiation of tunable energy and polarization has led to spectroscopies that promise the separation of the orbital and spin magnetization densities in solids. SR has made very precise characterizations of the magnetic behavior of a variety of rare-earth, transition-metal, and actinide systems possible. The naturally high resolution not only allows the

Suggested Citation:"6 New Tools for Research." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
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determination of the magnetic periodicities with unprecedented accuracy, but also allows magnetic correlations to be explored on micron length scales.

Infrared Investigations

One development that was not foreseen a decade ago was the rise of infrared techniques using SR. For example, the vacuum ultraviolet ring at the National Synchrotron Light Source (NSLS) at Brookhaven provides infrared light that is 103 times brighter than typical thermal sources and highly stable. Similarly, it produces more power than thermal sources in the far infrared and is also a pulse source suitable for time-resolved spectroscopy with subnanosecond resolution. This source has enabled infrared spectroscopy to be applied to problems such as the dynamics of adsorbates on metals and semiconductors, and photoconductivity.

X-Ray Absorption Spectroscopy

This is a very simple and powerful technique with applications too numerous to list. To give an idea of what is possible, the committee considers briefly its use in environmental science, a field driven by the tremendous need for new remediation and prevention technologies. Environmental science is therefore a growth area for the application of methods from condensed-matter and materials physics. The application of SR techniques. particularly XAS, to problems in environmental science has grown rapidly during the past decade. XAS is particularly useful because easily interpretable data on chemical states can be collected in environmentally relevant conditions (e.g., in the presence of water, at ambient pressures and temperatures, at dilute metal ion concentrations greater than 10 ppm). The resulting information is critically important for determining the toxicities, bioavailabilities, transport properties, and environmental fate of metal ions in soils and aquifers and for subsequently designing cost-effective and reliable remediation.

The Next Decade

The main focus of U.S. efforts will be to fully develop the third-generation sources that have only just come online. This means building functioning beam lines capable of what is currently routine at the European Synchrotron Radiation Facility (ESRF) in Grenoble (e.g., high-resolution inelastic scattering and photon correlation spectroscopies, high-pressure diffraction). However, past experience indicates that many of the new experimental techniques that will be developed at the third-generation SR sources have not yet emerged. It seems likely that the history of first- and second-generation sources, where some of the most fruitful techniques were advanced only after experience with the source and instrumentation had been obtained, will be repeated.

Suggested Citation:"6 New Tools for Research." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
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Nevertheless, SR scientists are already planning fourth-generation sources that will provide still greater brightness (about two orders of magnitude) and subpicosecond pulses at successively shorter wavelengths using single-pass free-electron lasers. The plans include a deep-ultraviolet source at Brookhaven National Laboratory, a soft x-ray source at Argonne National Laboratory, and an x-ray source providing radiation at wavelengths down to approximately 1.5 Å at the Stanford Linear Accelerator Center. The ultimate facility will be based on a superconducting linac of some 20-30 GeV feeding a farm of 50 or more 100-m-long undulators with radiation from the ultraviolet to the hard x-ray.

As indicated in Figure 6.7, the FELs promise about eight-orders-of-magnitude increases in peak brightness. They will provide diffraction-limited radiation at their operating wavelengths with sufficient numbers of photons so that data will generally be acquired with a single pulse. Thus, they are likely to usher in a new era of short wavelength coherent imaging and subpicosecond studies of electronic and atomic structure. Moreover the development of these sources will enhance the impact of SR on biology, soil science, agriculture, archeology, and other fields.

The Reinvention Of Traditional Condensed-Matter Experiments

At the same time that the capabilities of large-scale facilities have been dramatically expanding, there has been a quiet revolution in small-scale instrumentation. This revolution has had an enormous impact on both the efficiency and capabilities of single investigators and groups working on small-scale materials experiments in traditional laboratory settings. From spectrum analyzers, to top-loading dilution refrigerators, to personal computer (PC)-controlled parameter analyzers, the tools of the trade have evolved to the point where measurements that once took from weeks to entire graduate student careers, can now be routinely done in days, hours, or less. Laboratory instruments, computers, and software to enable quick and easy automation of most laboratory measurements are now available. Almost all commercial instruments come with IEEE GPIB interfaces. Inexpensive PCs are pervasive, and commercial software packages have been designed specifically for laboratory automation. The ongoing advances in microelectronics technology have spawned new generations of inexpensive yet extremely high-performance digital oscilloscopes, voltmeters, and all sorts of parameter analyzers. In parallel there have been continuous improvements in performance and reductions in the cost of systems such as dilution refrigerators, pulsed lasers, and superconducting magnets. There are also new types of commercial instruments that act as platforms for performing a number of complex experiments on a sample, all under computer control. For example, such probes had a huge impact on the development of the field of high-Tc super-

Suggested Citation:"6 New Tools for Research." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
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conductivity and are playing a very important role in the development and exploration of new magnetic materials.

Another very exciting area of ''small-scale" instrumentation involves leveraging microfabrication technology, as driven by the microelectronics industry, to do or enable physics experiments. There are several different aspects of this. The first relates to the custom design of electronic circuitry specifically configured for some special laboratory instrumentation function. What used to require the effort of numerous people hand-wiring large numbers of components together to eventually produce a rack full of instrumentation can now frequently be reduced to an application-specific integrated circuit (ASIC) designed to the need, along with a few other high-function, but standard, integrated circuits.

A second aspect of leveraging microfabrication involves special-purpose technology developed to fulfill some engineering need, but using it for physics applications as well. A good example of this is low-Tc superconducting electronics. For the past several years high-quality foundry service has been available for producing prototype superconducting digital circuitry. This same foundry service has been used to fabricate on-chip experiments to study the physics of Josephson junctions, the behavior of arrays of superconducting devices, the performance of high-frequency mixers and antennae for radio astronomy, and so on. In addition, this technology can be used to fabricate all manner of integrated SQUIDs including, for example, magnetometers with small pickup-loop structures to be used in scanning SQUID microscopy applications. This is a fabrication service available to everyone at a very modest cost.

A third aspect of leveraging is related to microelectromechanical systems (MEMS). MEMS is a rapidly growing engineering field, closely linked to the microelectronics industry, that has developed a wide variety of devices such as micromotors, microactuators, and microflow-controllers. MEMS in the form of microcantilever structures are at the heart of many scanning-probe implementations. MEMS technology is beginning to provide some exciting opportunities to do physics in unconventional ways on very small quantities of matter. At present, cantilever structures in one form or another are the basis for many such experiments, but it is clear that MEMS can provide an ideal platform for a wide variety of physics experimentation. We can expect to see a rapid expansion of "Iaboratory-on-a-chip" concepts and implementations in the near future.

In their pursuit to gain control of entities of the very smallest dimensions, scientists are developing extremely sensitive sensors to detect and analyze very weak physical and chemical effects involving minute amounts of material. The basic sensing element used in one such study is a silicon microcantilever like that used in an atomic-force microscope. This microcantilever bends in reaction to the forces imposed on it by various phenomena under investigation. Several methods can be applied to detect the motion and deformation of the cantilever including optical and electrical techniques, the latter using piezoresistors.

Suggested Citation:"6 New Tools for Research." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
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Because of their very small size, such microcantilevers—if properly designed— feature high sensitivity and short response times.

There are two obvious prerequisites for experiments in condensed-matter and materials physics. The first is measuring equipment, the second is interesting specimens. As regards small laboratories, the key development is that interesting specimens are becoming more and more indistinguishable from the measuring equipment, in the shape of "experiments on chips." Certainly examples exist of outstanding physics being done with 20-year-old equipment on samples made in a great variety of traditional ways. However, the overall efficiency and productivity of the research will be significantly enhanced through ongoing upgrades of instrumentation and automation, both for measurement and fabrication. Incorporating upgrades in all the physics laboratories in the nation represents a substantial ongoing investment required to keep small-scale laboratory operations competitive.

The scientific and technological future of nanofabrication and nanoscale processing, which enables modern "experiments on chips," is bright and exciting. If, however, the momentum of this nanoscale revolution is to continue to grow and its full promise to be realized, steps must be taken to ensure that the broad research community has appropriate and effective access to the full arsenal of capabilities in this area, capabilities that must be at the very forefront of nanofabrication and nanoprocessing technology. Because of the complex and multifaceted nature of this technology, an essential component of this access must be via national user facilities, such as the current National Nanofabrication Users Network (NNUN) supported by the National Science Foundation. It is essential that each facility of this sort be adequately supported so that it can broadly provide world-class research capabilities to academic, industrial, and government researchers and thus continue to push the state of the art. It is also essential that each encompass a broad range of nanofabrication and processing capabilities so that users can take a nanoscale science or technology research project from concept through to a working device or functional structure. Further, it is essential that each facility be adequately staffed with highly competent professionals oriented toward introducing new users to the technology, educating the research community about nanofabrication, and facilitating users in successfully exploiting exciting new research opportunities.

Man-Made Extreme Conditions

The urge to discover new states of matter has been one of the deepest motivations in condensed-matter and materials physics. An important route to discovery has been the fabrication of new materials—new compounds, alloys, and combinations of metals, ceramics, and organic matter. Frequently it has been possible to design new materials for specifically desired properties. It seems likely that modern fabrication techniques will permit us to make a host of new

Suggested Citation:"6 New Tools for Research." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
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objects and devices that are beyond our current imagination. Numerous advances in the fabrication of new materials are discussed in this report.

Another path to the discovery of new phenomena has been through subjecting matter to unusual or extreme conditions of low temperature, pressure, and magnetic field. Measurements under such unusual conditions have sometimes led to dramatic surprises, with important results that could not have been anticipated in advance. Here, the committee looks at a few of the results and describes the present state of the technology and future prospects. There are common themes for research under extreme conditions: (1) The limit in the equilibrium or static value of minimum temperature, maximum pressure, or maximum magnetic field is 10 or more times less than transient values that can be achieved. (2) The instrumentation required for preparing specimens and performing measurements has become increasingly sophisticated and frequently requires facilities available only at large laboratories. (3) The miniaturization of specimens and apparatus is becoming increasingly beneficial to each technology.

Matter at Very Low Temperatures

The classic unexpected result in condensed-matter physics was the discovery of superconductivity in 1911, just a few years after helium was first liquefied. The electric resistance of superconductors becomes zero. The low temperatures were critical for producing the phenomenon because thermal energy at higher temperatures disrupts the delicate interactions between electrons. At low-enough temperatures, the electrons form the paired state responsible for superconductivity. It took another 50 years before the underlying phenomenon was really understood. The discovery was made primarily because the experimenter wanted to see how the nature of conductors changed as they were cooled. Similarly, the superfluid states of the helium isotopes,4He and3He, were discovered because of developments in low-temperature technology and curiosity about how matter behaved at lower temperatures. When the thermal motion is reduced, delicate new phenomena appear.

The minimum temperature achieved under static conditions with the thermometer, coolant, and specimen under investigation is 1 mK. Temperatures down to 10 mK can be produced routinely with commercially available apparatus in which pumped3He is circulated through heat exchangers. Lower temperatures require a different principle: magnetic cooling. Nuclei of a metal (usually copper) are polarized at millikelvin temperatures in a magnetic field of 10 T. The copper and specimens attached to it are then thermally isolated and the magnetic field is removed. The polarized nuclear magnetic moments become very cold. Electrons in the copper transfer heat between the specimen and the cold nuclei.

An important challenge that has attracted quite a number of experimentalists in this field has been the attempt to cool dilute mixtures of 3He in liquid 4He to

Suggested Citation:"6 New Tools for Research." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
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very low temperatures. The goal has been to discover a superfluid pairing transition similar to that which occurs in pure liquid3He. The paired state might be quite different from that in the pure liquid. No one has succeeded in cooling the dilute mixtures to temperatures less than 200 µK. Heat transfer between the metal coolant and the dilute mixture is quite difficult.

Significantly lower temperatures have been achieved for isolated systems not in thermal equilibrium with surrounding matter. One class of experiments has been the study of spontaneous nuclear magnetism in metals such as copper, silver, and platinum. At the end of the magnetic cooling process, after the large magnetic field used to polarize the nuclei is removed, the magnetic moments are quite cold. The thermal equilibrium times are quite long, sometimes more than 108 seconds. Through clever determinations of the spin-entropy, temperatures as low as a tens of picokelvin have been deduced. The method has been used to examine a variety of unusual states of magnetic order. As Figure 6.11 illustrates, some of the experiments have even been conducted in connection with neutron diffraction at reactor facilities. The neutrons were used to image the nuclear magnetic moments in the ordered state.

A spectacular example of the cooling of a metastable isolated system of matter has been the studies of Bose-Einstein condensation in gases of sodium, rubidium, and lithium. Modern optical techniques in conjunction with magnetic traps and radio frequency fields have been used to cool dilute gases of these atoms to sub-microkelvin temperatures. The hot atoms are kicked out of the magnetic trap by the radio frequency electromagnetic fields. At the very low temperatures, the atoms obeying Bose statistics can simultaneously occupy the same state—the condition for Bose-Einstein condensation. Quantum interference between clusters of atoms has been demonstrated. The effect is analogous to interference between two sources of coherent light.

Matter at Very High Pressures

For most of this century, progress in achieving high pressure in matter was achieved by building ever larger series of cascaded metal pistons to compress material. By 1980 a dead end had apparently been reached. Even with the strongest steel alloys, the limiting static pressure that could be produced on samples of milliliter volumes was in the range of several hundred kilobars. The invention of the diamond anvil pressure cell led to a major advance in highpressure science. Small diamond crystals are formed into narrow tips to be pressed against each other. Typical contact regions have a diameter less than 10 µm. Forces of order of only a few Newtons can produce megabar pressures over such small areas. To achieve high pressures, the region around the narrow tips must be defect free. The smaller the region, the higher the probability that there are no defects.

The maximum static pressure achieved by the diamond anvil method is a

Suggested Citation:"6 New Tools for Research." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
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image

Figure 6.11
Photograph of nuclear demagnetization apparatus installed 
on a coldneutron guide emanating from a nuclear reactor. 
The apparatus was used to discover the ordering of the 
nuclear spins in elemental copper at less than 58 × 10-9 K. 
(Courtesy RISØ National Laboratory, Roskilde, Denmark.)

Suggested Citation:"6 New Tools for Research." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
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little more than 1 Mbar. Quite a number of new dense phases of matter have been discovered. Of particular interest has been the transition of normally completely insulating materials such as solid xenon and sulfur in conducting metallic states. As the atoms are squeezed closer together, the outer electrons become free.

The small size of the specimens presents a special challenge in instrumentation. The entire experiment has to be built in micron-sized volumes. Nevertheless, nanotechnology has been used to apply electrical leads and even magnetic resonance coils to diamond anvil devices. The crystal structure of new highpressure phases of matter has been determined with x-rays from synchrotron sources. The measurement of the pressure is also a difficult matter. Calculation of the force per unit area is frequently insufficient because the stresses are not uniformly distributed. Instead, a combination of calculated pressures and extrapolation of material properties such as fluorescence frequencies must be carefully compared in many experiments to establish a reliable pressure scale.

A special goal in high-pressure research in recent years has been the search to find the elusive metallic state of solid hydrogen. It would be an especially interesting discovery because hydrogen should be one of the easiest materials for which to calculate an equation of state with fundamental theory. The pressure predicted for the metallic transition in hydrogen is very close to the values currently being produced.

Beyond providing information about systems of fundamental interest to condensed-matter physicists, high-pressure research is essential for understanding the composition and properties of Earth's interior. Recent experiments have led to significant new findings on phase transformations associated with deep earthquakes, for example.

Further progress in achieving higher pressures will probably be achieved through use of stronger materials. For example, studies of tungsten and iron suggest that they become even stronger at megabar pressures.

Transient pressures greater than 2 Mbars have been obtained in shock waves. The maximum pressure lasts only a few nanoseconds. Nevertheless, most of the existing high-pressure and high-temperature data have been obtained with the use of gas guns, high explosives, and even nuclear detonations. The development of high-intensity lasers provides a potentially attractive complement to these methods, particularly for equation of state studies at high energy densities. By focusing a short-pulse, intense laser beam on a sample, a rapidly expanding plasma is created, which, in turn, drives a shock wave into the sample; laser-induced shockwave experiments to obtain high-pressure data (in excess of a megabar) have been carried out for more than a decade. However, concerns have existed regarding the accuracy of the data owing to the lack of planarity of the shock front, preheating of the material ahead of the shock front, difficulty in determining the steadiness of the wave front because of the small sample size, and the absence of absolute pressure and volume data. Recent improvements in beam smoothing

Suggested Citation:"6 New Tools for Research." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
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and other experimental developments have improved the quality of the propagating shock wave.

Matter in Large Magnetic Fields

Large magnetic fields represent yet another extreme condition to impose on matter. Frequently the large fields are imposed in conjunction with low temperatures. A large magnetic field can orient material, confine electrons in conductors to particular energy states and locations, and produce specially selected spin states of nuclei and electrons. Figure 6.12 illustrates how the range of accessible phenomena grows with the magnetic field strength, while Figure 6.13 shows the steady growth in man-made fields over the last century. Examples of dramatic discoveries in recent years are the integer and fractional quantum Hall effects. Both were discovered by accident—again—because the technology was available to produce the required extreme condition and experimentalists were interested in how matter changed under the new conditions. Both are discussed in other sections of this report.

image

Figure 6.12
Higher fields are associated with higher energies, smaller length 
scales, and more extreme technologies and environments. 
(Courtesy of Bell Laboratories, Lucent Technologies.)

Suggested Citation:"6 New Tools for Research." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
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image

Figure 6.13
Magnet technology, as expressed in the maximum field reached 
in nondestructive experiments, has grown exponentially over the 
last century. We anticipate that the incorporation of high- Tc 
superconductors will assure continued growth. 
(Courtesy of Bell Laboratories, Lucent Technologies.)

The highest static magnetic field achieved is 37 T (80,000 times Earth's magnetic field strength). The large field is achieved with a combination of two concentric magnets—a water-cooled copper solenoid carrying a large current on the outside and a superconducting solenoid on the inside. Today's superconductors alone cannot carry enough current to produce a magnetic field greater than about 15 T. The outer copper (resistive) magnet produces the additional 22 T.

The ultimate design limit on the maximum strength of steady magnetic field is determined by the strength of the material that carries the current. The field produces a radial outward force proportional to the current passing through the solenoid wire. Using the strongest materials that exist, the maximum field is approximately 50 T.

Much larger fields can be achieved for shorter times. Large current pulses through wire coils have been used to produce fields greater than 75 T for tens of microseconds. These magnets can be repeatedly cycled with current pulses to obtain masses of data. The maximum fields, those in excess of 100 T, are produced by explosive technology. Typically, current-carrying cylinders are rapidly imploded and a very large magnetic field is produced as the tube collapses. The transient-field experiments share a challenge with the transientpressure experiments—the data must be obtained in nanoseconds or less. The

Suggested Citation:"6 New Tools for Research." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
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transient experiments have been important in gaining information about the highfield behavior of high-temperature superconductors, the optical properties of matter, and the conducting properties of unusual metallic compounds.

The Next Decade

Matter under extreme conditions is as obvious a frontier of science as highenergy physics or astronomy. It is also equally easy to state simply the future program for this field—to subject matter to ever higher pressures, lower temperatures, and higher magnetic fields and to use every conceivable visualization tool to see what happens. Spectacular opportunities will arise because of new infrastructure, such as the National Ignition Facility, actually designed for fusion research at the Lawrence Livermore Laboratory, and the 100 T pulsed magnet foreseen at Los Alamos. Other significant advances will arise from improved visualization capabilities, which will follow from installation of high-pressure cells and high-field magnets at advanced light and neutron sources.

The scientific problems addressable by experiments with samples in extreme environments will span the range of condensed-matter physics. Past performance suggests that such experiments will make important contributions to resolving the problems posed by the high-temperature superconductors as well as many other fascinating materials both known and unknown. We also look forward to breakthroughs in areas much further from the traditional core of condensedmatter physics. Recent examples include optical tweezers, whose development was closely interwined with the quest for ultra-cold laser-cooled matter, and magnetic resonance imaging.

Computational Materials Physics

The modem high-speed computer is a remarkable device, made possible in part by fundamental discoveries and continuing advances in condensed-matter and materials physics. With clock speeds now routinely reaching as high as 500 MHz, small, mass-produced workstations have a computational power that would have been possible only with giant supercomputers (now viewed as dinosaurs) just a few years ago. Computers and their components are now so sophisticated that each succeeding generation cannot be built without making full use of the computational power of the existing generation. In addition to improving themselves, computers have become powerful tools in the study of a wide variety of condensed-matter phenomena and materials. Figure 6.14 shows the remarkable progress in high-end computing, together with its implications for one particular problem, the computation of turbulence in fluids.

The history of computing has been one of frantic attempts to find architectures and software that can deal with ever-changing hardware limitations. Less than 20 years ago, memory was very expensive (witness today's Year 2000

Suggested Citation:"6 New Tools for Research." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
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image

Figure 6.14.
The plot shows the growth of the number of operations per 
second from 1940 to 2010 for the fastest available 
''supercomputers." Objects of different shapes are used 
to distinguish serial, vector, and parallel architectures. 
All processors until Cray-1 were single-processor machines. 
The line marked "three-dimensional Navier-Stokes turbulence" 
shows, in rough terms, the extent to which the increased 
computing power has been harnessed to obtain turbulent 
solutions by solving three-dimensional Navier-Stokes 
equations. Turbulence is used here as an example of one 
of the grand and difficult problems needing large 
computing power. The computing power limits the size 
of the spatial domain over which computations can be 
performed. The Reynolds number (marked on the right 
as Rl) is an indicator of this size. (Courtesy of Los Alamos 
National Laboratory.)

Suggested Citation:"6 New Tools for Research." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
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problems) and microprocessors were much slower than discrete component designs. Today memory density has risen enormously and prices have fallen dramatically. Microprocessors have risen several orders of magnitude in speed and gone from 8- to 64-bit word lengths. After some tumultuous history involving exploration of different parallel architectures, shared-memory parallel systems combining many processors communicating via high-speed digital switches are now rapidly developing and have largely replaced pure vector processors. Clock speeds for microprocessors are now so high that memory access time is often far and away the greatest limitation on overall speed. One of the great software challenges now is to find algorithms that can take maximum advantage of parallel architectures consisting of many fast processors coupled together.

In addition to hardware advances, the last decade has seen some revolutionary advances in algorithms for the study of materials and quantum many-body systems. Improved algorithms are crucial to scientific computation because the combinatorial explosion of computational cost with increasing number of degrees of freedom can never be tamed by raw speed alone. (Consider the daunting fact that in a brute force diagonalization of the lowly Hubbard model, each site added multiplies the computational cost by a factor of approximately 64.)

In the last two decades computational condensed-matter and materials science has moved from the initial exploratory stages (in which numerical studies were often little more than curiosities) into the main stream of activity. In some areas today, such as the study of strongly correlated low-dimensional systems, numerical methods are among the most prominent and successful methods of attack. As new generations of students trained in this field have begun to populate the community, numerical approaches have become much more common. Nevertheless it is fair to say that computational physics is still in its infancy.

Pushing the frontiers of computational physics and materials science is important in its own right but also important because training students in this area provides industry and business with personnel who not only have expertise on the latest hardware architectures but also bring with them physicists' methods and points of view in analyzing and solving complex problems.

Progress in Algorithms

In spite of its great enthusiasm, the committee offers a warning before proceeding. Specifically, numerical methods have become more and more powerful over time, but they are not panaceas. Vast lists of numbers, no matter how accurate, do not necessarily lead to better or deeper understanding of the underlying physics. It is impossible to do computational physics without first being a good physicist. One needs a sense of the various scales relevant to the problem at hand, an understanding of the best available analytical and perturbative approaches to the problem, and a thorough understanding of how to formulate the interesting questions.

Suggested Citation:"6 New Tools for Research." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
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Electronic Structure Algorithms

The goal of electronic structure calculations is to compute from first principles or with approximate methods the quantum states of electrons in solids and large molecules. This information is then used to predict the mechanical, structural, thermal, and optical properties of the materials. The outstanding problems are ones of computational efficiency for large-scale calculations and convergence to the thermodynamic limit. Indeed, the calculations are so complex and time consuming that real-time dynamics can be followed only for pico- or nanoseconds.

Perhaps the single most dramatic development in the last decade has been the advent of the Car-Parrinello method, which has enormously enhanced the efficiency of electronic structure calculations. This method calls for adjusting the atomic positions and the electronic wave functions at the same time to optimize the Hohenburg-Kohn-Sham density functional. Additional efficiencies come from use of fast Fourier transform techniques to compute the action of the Hamiltonian on the wave functions without the necessity of computing the full Hamiltonian matrix.

Another area of intensive investigation has been the search for so-called ''Order N" methods. The idea is to find approximation schemes in which the computational cost rises only as the first power of the number atoms or electrons, as opposed to some higher power (˜3) as is typically the case. So far, this has been attempted only for tight-binding models involving spatially localized orbitals for the electrons. It is not yet clear that the problem will be solvable, but research in this direction is important if we are going to be able to do larger and more complex structures. Other techniques under investigation include adaptive coordinate, wavelet, and direct grid/finite element methods that are useful in situations in which the number of plane waves needed to represent atomic orbitals is very large.

The Kohn-Sham local-density functional approximates the many-body exchange-correlation corrections to the energy by a functional of the local density. It has been very successful and is finally winning support within the computational chemistry community. An important area of current research involves generalized gradient expansion corrections to the local-density approximation. In several examples, simple local-density approximations fail to give correct structures but appropriate gradient expansion functionals work. In general, however, it is often still difficult to obtain the chemical accuracy required.

Monte Carlo Methods

Fermion Monte Carlo techniques continue to be plagued by the "sign problem.'' Because of the sign reversals that occur in quantum wave-functions when two particles exchange places, not all time histories have positive weights in the

Suggested Citation:"6 New Tools for Research." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
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Feynman path integral. This means that the weights cannot be interpreted as probabilities that can be sampled by Monte Carlo methods. The fixed-node approximation attempts to get around this problem by specifying a particular nodal structure of the wave function. This has yielded very useful results in some cases in which the nodal structure is understood a priori. Some workers are now moving beyond small atoms and molecules to simple solids and have obtained good results for lattice constants, cohesive energies, and bulk moduli.

Fermion Monte Carlo path integral methods continue to be applied successfully to lattice models such as the Hubbard model, but again the sign problem is a serious limitation. For example, it is still difficult to go to low-enough temperatures to search for superconductivity, even in highly simplified models of high-Tc materials.

Bosons, which are much easier to treat numerically, also pose interesting problems. "Dirty boson" models have been used to describe helium films adsorbed on substrates and to treat the superconductor-insulator transition. With this model one makes the approximation that Cooper pairs are bosons and assumes (not necessarily justifiably) that there are no fermionic degrees of freedom at zero temperature.

Cluster Algorithms in Statistical Mechanics

One serious problem in the Monte Carlo simulation of statistical systems near critical points is the divergence of the characteristic timescales. The computer time needed to evolve the system to a new statistically independent state diverges as some power of the correlation length or system size, MCd+z where d is the dimensionality. Cluster algorithms have been extremely successful on certain classes of problems (such as the Ising and XY models and certain vertex models) and are able to reduce the dynamical exponent zMC to nearly zero. This is accomplished by constructing clusters of spins, and for each cluster, choosing a random value of spin that is assigned to the individual spins it contains. Such a move cannot be implemented in an ordinary Metropolis algorithm because the Boltzmann factor would make the acceptance rate of the move essentially zero.

The trick is to have the probability that a cluster grows to a particular size and shape be precisely the Boltzmann factor for the energy cost of flipping the cluster. This is a very tiny probability, but it is canceled by the fact that there are a huge number of different possible clusters that could have resulted from the random-growth process.

This has been a very important advance. Unfortunately there are still many cases (such as frustrated spin systems) for which cluster methods cannot (as yet) be applied because of technical problems similar to the fermion minus-sign problem.

Suggested Citation:"6 New Tools for Research." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
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Density-Matrix Renormalization Group

A revolutionary development in computational techniques for quantum systems is the "density matrix renormalization group." The essential idea is to very efficiently determine which basis states are the most important to keep to be able to describe the quantum ground state. The procedure is the first one ever found that gives exponentially rapid convergence as the number of basis states is increased. It applies to essentially any one-dimensional model with short-range forces, even random systems without translation symmetry and fermion systems. Using this technique it is now easy to compute ground state energies and correlation functions to 10-digit accuracy on a desktop workstation. Ongoing work is extending the technique to excited states and to higher dimensions.

Computational Physics in A Teraflop World

In this section we contemplate questions of the future of computation and what can be (optimistically) done with the next factor of 1000 in computing power.

Glassy Systems, Disorder, and Slow Dynamics

At first sight, these problems do not seem well suited for more computer time. They are too hard. Experimentally, the phenomena are spread over fifteen decades in frequency, and even that dynamical range in the experiments is often not enough to reach firm conclusions. Current simulations span perhaps three decades: one might think that three more won't make an overwhelming improvement.

There are two reasons to be optimistic. First, in numerical simulations, it is straightforward to watch individual atoms/spins/automata relax. The last decade has seen tremendous progress in the visualization and study of spin glasses, charge density waves, glassy behavior in martensites, and "real" glasses. We are, however, barely into the scaling region for many of these simulations: even if the scaling region grows only logarithmically with the timescale, three more decades might make the patterns clear.

Second, there is every reason to believe that we can get around these slow timescales. There is no reason for our methods for relaxing glassy systems to be as inefficient as nature. Until now we have mainly developed techniques to mimic nature with as little wasted effort as possible. This was sensible for studying systems in which nature relaxes efficiently; when you are barely able to follow the system for a nanosecond, you study systems that relax rapidly. Now that we are turning to problems for which nature is slow (for example, glasses and phase transitions) we are making rapid strides in developing acceleration algorithms. In particular, because we are more likely to gain the next factor of 1000 in computing power by increasing the number of processors rather than through

Suggested Citation:"6 New Tools for Research." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
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raw speed increases, we will naturally learn new algorithms for relaxation to exploit the extra processors.

Quantum Chemistry and Electronic Structure of Materials

With these problems we are not confused about the physical behavior. For these problems, the answers to the interesting questions inherently demand immense precision. Quantum chemistry is difficult not because the systems are complex and subtle, but because the standards are high. All of chemistry is controlled by reaction and binding energies that are tiny compared to total energies. Electronic-structure calculations for materials face exactly the same problem. We can now study only relatively simple molecules and crystal structures; with the next generations of machines and algorithms, this will change qualitatively.

Structured Systems: From Inorganic Industrial Materials to Proteins

These are systems for which there are huge ranges of length scales and timescales, which interact in nontrivial ways. We have to understand the physics and materials science on each scale and connect together the properties at different scales. The algorithms appropriate to the models at different scales can be quite different from each other.

The category of "industrial materials" includes ceramics, concrete, polycrystalline metals and alloys, and composites. Their important properties are normally almost completely removed from the world of perfect crystals and equilibrium systems often studied by mainstream physics. The wearing properties of steel, the resistance of concrete to cracks, the thermal and electrical properties of polycrystalline metals—all are dominated by the mesostructure, the detailed arrangement of domain walls, pebbles, and grains.

Three issues must be confronted to make progress. First, the materials are disordered. Second, they display history dependence; for example, the polycrystalline domains in metals are dependent in detail on how the metal was cast, rolled, and stamped during its manufacture. Third, the systems have a large range of scales. The dynamics of grain boundaries under external strain is determined by the dynamics of the individual line dislocations that make them up. The line dislocations interact logarithmically (in inscrutable ways), and one can only simulate them at the current level of knowledge. Their dynamics, in turn, is determined by atomic-scale motion; the diffusion of vacancies and the pinning to inhomogeneities (and to other line dislocations) are crucial to understanding their motions. It is this enormous range of scales that we can only hope to disentangle with large-scale simulations (see Figure 6.15).

Proteins and biomolecules provide similar problems. The molecular biologists separate their structures into primary, secondary, and tertiary precisely as a set of length scales on which the structure is organized. The functional behavior

Suggested Citation:"6 New Tools for Research." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
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image

Figure 6.15
Million-atom molecular dynamics simulation of ductile behavior 
in nanophase silicon nitride, which is being explored for its 
extraordinary resistance to fracturing under strain: a 30 percent 
strain is required to completely fracture the nanophase system, 
while only 3 percent is required for single-crystal silicon nitride. 
Shown is the system before it fractures under an applied strain 
of 30 percent and a zoom-in to atomic scale visualizing that the 
crack front advances along disordered interfacial regions in the 
system. It is along the amorphous intercluster regions where the 
crack propagates by coalescence of the primary crack with 
voids and secondary cracks. (Courtesy of Louisiana 
State University.)

on the largest scales depends in detail on the dynamics and energetics not only down to the protein level, but even down to the way in which each protein is hydrated by its aqueous environment.

Quantum Computers

Theoretical analysis of the quantum computer, in which computation is performed by the coherent manipulation of a pure quantum state, has advanced extremely rapidly in recent years and indicates that such a device, if it could ever be constructed, could solve some classes of computational problems now considered intractable. A quantum computer is a quantum mechanical system able to evolve coherently in isolation from irreversible dephasing effects of the environment. The "program" is the Hamiltonian. The "input data" is the initial quantum state into which the system is prepared. The ''output result" is the final, timeevolved state of the system. Because quantum mechanics allows a system to be in a linear superposition of a large number of different states at the same time, a quantum computer would be the ultimate "parallel'' processor.

The basic requirement for quantum computation is the ability to isolate, control, and measure the time evolution of an individual quantum system, such as an atom. To achieve the goal of single-quantum sensitivity, condensed-matter experimentalists are pursuing studies of systems ranging from few-electron quantum dots to coherent squeezed photon states of lasers. When any of these reach the desired single-quantum limit, experiments to probe the action of a quantum

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gate could be immediately designed. Recent theory shows in principle how to form different types of gates and provides error-correcting codes to enhance robustness. At this point it is quite unclear if a practical system can be developed, but many clever ideas are being explored. Interesting physics is sure to result and there is at least a remote possibility of a tremendous and revolutionary technological payoff.

Several groups have reported an experimental realization of quantum computation by nuclear magnetic resonance (NMR) techniques. The race is now on to demonstrate more complex quantum algorithms, to compute with more quantum bits than the two bits of the first demonstration, and to verify error-correction techniques.

Future Directions and Research Priorities

Tools for visualizing atoms and electrons have been at the center of condensed-matter and materials physics since Bragg and von Lane first observed x-ray diffraction from crystals nearly 100 years ago. These tools will remain at the center of the field and many others, from catalysis to biochemistry. The last decade has seen great progress in research performed using apparatus of all scales.

In the area of medium-scale infrastructure, the three important developments have been widespread access to sophisticated electron microscopes and related equipment, the exploitation of the Cornell nanofabrication center, and the reinvigoration of U.S. high-field magnet research by the founding of the National High Field Magnet Laboratory. Access to equipment has fueled and will doubtless continue to fuel improved understanding and applications of bulk materials, surfaces, and interfaces. Beyond enabling U.S. academe to participate in and thereby greatly accelerate the development of mesoscale (between atomic and macrosopic scales) physics, the Cornell nanofabrication center has been an extraordinarily fertile training ground for the U.S. microelectronics industry. The National High Field Magnet Laboratory will provide access to a scientific frontier—a key site for discoveries and technological developments ranging from magnetic resonance imaging to the quantum Hall effect.

Turning finally to large-scale facilities of a type that can only exist at national laboratories, the major events have been the commissioning of third-generation synchrotrons at the Argonne and Lawrence Berkeley laboratories and the decision to recapitalize U.S. neutron science via construction of a pulsed spallation source at Oak Ridge. The synchrotrons will produce the x-rays and light necessary for the United States to compete in emerging areas such as timeresolved protein crystallography. Even though a U.S. scientist (Shull) shared the 1994 Nobel Prize for inventing neutron scattering in the 1950s (see Table O.1), the Europeans have since then established a clear lead. The Oak Ridge source will reestablish U.S. competitiveness in this area, which over the last decade has

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proven so vital for imaging atoms and spins in materials ranging from hightemperature superconductors to polymers.

In previous decades, key events in condensed-matter and materials physics have been the exploitation of inventions and investments in large facilities. The inventions and the facilities are devices with the special purpose of being tools for condensed-matter and materials physics. The last decade is unique in that the major event relating to such tools is actually not directly connected with inventions and facilities. Instead, it is the same phenomenon that has profoundly transformed nearly all other aspects of our society—namely, the information revolution. An obvious consequence of the information revolution for condensedmatter and materials physics is the recent progress in computational materials science. Less obvious but equally important is the ability to collect and manipulate progressively larger quantitative data sets and reliably execute increasingly complex experimental protocols. For example, in neutron scattering, data gathering rates and, more crucially, the meaningful information content, have risen in tandem with the exponential growth of information technology

What will happen in the next decade? Although we cannot predict inspired invention, we anticipate progress with ever-shrinking and more-brilliant probe beams and increasingly complete, sensitive, and quantitative data collection. One result will be the imaging and manipulation of steadily smaller atomic landscapes. Another will be the analysis and successful modeling of complex materials with interesting properties in fields from biology to superconductivity.

The promised performance improvements with applications throughout materials science will come about only if balanced development of both large-scale facilities and technology for small laboratories takes place. For example, determination of the crystal structures of complex ceramics and biological molecules is likely to remain the province of neutron and synchrotron x-ray diffraction, performed at large facilities, while defects at semiconductor surfaces will most likely remain a topic for electron and scanning-probe microscopy, carried out in individual investigators' laboratories and small facilities. Thus, the cases for large facilities and small-scale instruments are equally strong. Although the larger items such as the neutron and photon sources appear much more expensive than those that benefit a single investigator, recent European experience suggests that the costs per unit of output do not depend very strongly on the scale of the investment, provided of course that it is properly chosen, planned, and managed. Information technology is also blurring the difference between large and small facilities, as they all become nodes on the Internet. One important upshot will be that the siting of large facilities as well as the large-versus-small facility debates will largely cease to be of importance to scientists.

In addition to the construction of large facilities such as the SNS and APS, healthy research in instrumentation science is crucial to the development of improved tools for atomic visualization and manipulation. Although we have impressive success stories to point at, as in the dominance of the probe micros-

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copy business, we strive for similar success in other areas of instrumentation that are important for both research and manufacturing. In the United States, scientific research and instrumentation have traditionally had an uncomfortable relationship. Although it is very important that instrumentation programs be science-driven and not isolated, sometimes long lead times and the need for expert research in the instrumentation itself (for example, in advanced lithography and electron, x-ray and neutron optics) require that special investment be allocated for instrumentation. The absence of such middle-scale investment as well as a perceived lack of intellectual respectability are key reasons why the nation is lagging in beam technology and science. A solution would be the development of centers of excellence in instrumentation research and education, the latter being an equally important role for this research. A model might be the National High Field Magnet Laboratory, which has recently revived magnet research in the United States. It is also clear that viable centers can exist in already strong centers of materials research.

The committee's list of priorities is designed to enable the United States to recapture its leadership in scientific tools for condensed-matter and materials physics and their exploitation. The goals to be achieved by the large neutron and synchrotron facilities are obvious—namely, to duplicate and then to exceed what the Europeans can do today. The recapitalization of the university laboratories will serve the similarly obvious purpose of maintaining the efficiency and quality of university research. The nanolithography investment will maintain user facilities in an area of extraordinary importance in materials research as well the U.S. economy. The medium-scale centers devoted to topics such as electron optics and high magnetic fields will serve not only to develop new technologies in the areas they are specifically devoted to, but also to establish a flourishing culture of scientific instrumentation within condensed-matter and materials physics. Finally, condensed-matter and materials physics needs to take advantage of all available information technology to continue to move toward its central goal of seeing all the atoms and electrons all of the time.

Outstanding Scientific Questions

• Can we manipulate single atoms fast enough to make devices?

• Can we use computation to predict superconductivity in complex materials?

• Can we make inelastic scattering using x-rays, neutrons, and electrons as important to materials science and biology as elastic scattering is today?

• Can we image and manipulate spins on the atomic scale?

• Can we develop a nondestructive subsurface probe with nanometer resolution in three dimensions?

Suggested Citation:"6 New Tools for Research." National Research Council. 1999. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. Washington, DC: The National Academies Press. doi: 10.17226/6407.
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Priorities

• Build the Spallation Neutron Source and upgrade existing neutron sources.

• Fully instrument and exploit the existing synchrotron light sources and do R&D on the x-ray laser.

• Build state-of-the-art nanofabrication facilities staffed to run user programs for the benefit of not only the host institutions but also universities, government laboratories, and businesses that do not have such facilities.

• Recapitalize university laboratories with state-of-the-art materials fabrication and characterization equipment.

• Build medium-scale centers devoted to single issues such as high magnetic fields or electron microscopy,

• Exploit the continuing explosion in information technology to visualize and simulate materials.

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This book identifies opportunities, priorities, and challenges for the field of condensed-matter and materials physics. It highlights exciting recent scientific and technological developments and their societal impact and identifies outstanding questions for future research. Topics range from the science of modern technology to new materials and structures, novel quantum phenomena, nonequilibrium physics, soft condensed matter, and new experimental and computational tools.

The book also addresses structural challenges for the field, including nurturing its intellectual vitality, maintaining a healthy mixture of large and small research facilities, improving the field's integration with other disciplines, and developing new ways for scientists in academia, government laboratories, and industry to work together. It will be of interest to scientists, educators, students, and policymakers.

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