Condensed-Matter and Materials Physics Basic Research for Tomorrow's Technology (1999) / Chapter Skim
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6 New Tools for Research
6 New Tools for Research
Pages 225-273
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From page 225... ...
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.
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From page 226... ...
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.
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From page 227... ...
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.
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From page 228... ...
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
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From page 229... ...
The scanning-tunneling microscope (STM) views the local electronic structure, so careful image simulations must be made to deduce atomic structure.
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From page 230... ...
FIGURE 6.1.1 High-resolution transmission electron micrograph, using z-contrast, in MgO. microscopy was published by the National Science Foundation.i Advances in electron microscopy enable advances in related industrial technologies, especially semiconductors; so the value of U.S.
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From page 231... ...
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.
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From page 232... ...
FIGURE 6.2.1 Local electronic states in GaAs generated by Si impurities. Tunneling \ Current \l/ 1\ ~ Light, Sound (electrostatic, (magnetic)
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From page 233... ...
Figure 6.2 shows the classic example of a ring of iron atoms assembled by the tip of a scanningtunneling microscope. The circular atomic corral shows the resonant quantum states expected from simple theory.
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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.
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From page 235... ...
the coming of age of accelerator-based pulsed neutron sources, and (3) the revival of neutron reflectometry.
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From page 236... ...
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?
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From page 237... ...
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.
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From page 238... ...
CONDENSED-MATTER AND MATERIALS PHYSICS 100 ~ ~Cu2-03 FIGURE 6.4 Mercury-based cuprates exhibit not only the highest transition temperatures TC for superconductivity, but also extraordinarily pressure-dependent TC'S. Highresolution neutron diffraction at pulsed spallation sources has revealed the complex structures (at right)
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From page 239... ...
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.
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From page 240... ...
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.
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From page 241... ...
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.
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From page 242... ...
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;
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From page 243... ...
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.
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From page 244... ...
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.
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From page 245... ...
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.
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From page 246... ...
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 Francei, with still higher brightness (by 4 to 5 orders)
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From page 247... ...
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)
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From page 248... ...
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 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.
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From page 249... ...
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.
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From page 250... ...
250 CONDENSED-MATTER AND MATERIALS PHYSICS 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 vies. At APS, an x-ray microprobe with a FWHM focal spot size of 0.33 ,um with a flux density exceeding 5 x 10~° photons/m2 s (0.01 percent bandwidth)
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From page 251... ...
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.
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From page 252... ...
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.
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From page 253... ...
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.
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From page 254... ...
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.
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From page 255... ...
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.
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From page 256... ...
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.
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From page 257... ...
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.
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From page 258... ...
258 CONDENSED-MATTER AND MATERIALS PHYSICS 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 x lO-9 K
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From page 259... ...
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.
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From page 260... ...
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.
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From page 261... ...
(Courtesy of Bell Laboratories, Lucent Technologies.) The highest static magnetic field achieved is 37 T (80,000 times Earth's magnetic field strength)
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From page 262... ...
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.
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From page 263... ...
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.
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From page 264... ...
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.)
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From page 265... ...
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.
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From page 266... ...
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.
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From page 267... ...
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)
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From page 268... ...
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.
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From page 269... ...
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.
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From page 270... ...
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.
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From page 271... ...
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.
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From page 272... ...
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.
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From page 273... ...
· 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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