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Overview

Introduction

Condensed-matter and materials physics is the branch of physics that studies the properties of the large collections of atoms that compose both natural and synthetic materials. The roots of condensed-matter and materials physics lie in the discoveries of quantum mechanics in the early part of the twentieth century. Because it deals with properties of matter at ordinary chemical and thermal energy scales, condensed-matter and materials physics is the subfield of physics that has the largest number of direct practical applications. It is also an intellectually vital field that is currently producing many advances in fundamental physics.

Fifty years ago the transistor emerged from this area of physics. High-temperature superconductivity was discovered by condensed-matter physicists, as were the fascinating low-temperature states of superfluid helium. Scientists in this field have long-standing interests in electronic and optical properties of solids and all aspects of magnetism and magnetic materials. They investigate the properties of glasses, polymeric materials, and granular materials as well as composites, in which diverse constituents are combined to produce entirely new substances with novel properties.

Condensed-matter and materials physics has played a key role in the technological advances that have changed our lives so dramatically in the last 50 years. Driven by discoveries in condensed-matter and materials physics, these advances have brought us the integrated circuit, magnetic resonance imaging (MRI), low-loss optical fibers, solid-state lasers, light-emitting diodes, magnetic recording disks, and high-performance composite materials. These in turn have led to the



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Page 5 Overview Introduction Condensed-matter and materials physics is the branch of physics that studies the properties of the large collections of atoms that compose both natural and synthetic materials. The roots of condensed-matter and materials physics lie in the discoveries of quantum mechanics in the early part of the twentieth century. Because it deals with properties of matter at ordinary chemical and thermal energy scales, condensed-matter and materials physics is the subfield of physics that has the largest number of direct practical applications. It is also an intellectually vital field that is currently producing many advances in fundamental physics. Fifty years ago the transistor emerged from this area of physics. High-temperature superconductivity was discovered by condensed-matter physicists, as were the fascinating low-temperature states of superfluid helium. Scientists in this field have long-standing interests in electronic and optical properties of solids and all aspects of magnetism and magnetic materials. They investigate the properties of glasses, polymeric materials, and granular materials as well as composites, in which diverse constituents are combined to produce entirely new substances with novel properties. Condensed-matter and materials physics has played a key role in the technological advances that have changed our lives so dramatically in the last 50 years. Driven by discoveries in condensed-matter and materials physics, these advances have brought us the integrated circuit, magnetic resonance imaging (MRI), low-loss optical fibers, solid-state lasers, light-emitting diodes, magnetic recording disks, and high-performance composite materials. These in turn have led to the

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Page 6 spectacular growth of modern computer and telecommunications industries and, consequently, to the information revolution. For many years after the invention of the transistor, the major intellectual challenge facing researchers in condensed-matter and materials physics was to understand the physical properties of nearly perfect single crystals of elements, simple compounds, and alloys. Most of these materials occur in some form in nature. On a basis of increased knowledge and powerful new synthesis techniques, today's condensed-matter and materials physics is directed toward creating entirely new classes of materials—so-called "artificially structured" materials—that do not exist in nature and whose sizes reach all the way down to the atomic domain. At the same time, a growing number of researchers are using new theoretical and experimental tools to extend our understanding to much more complex forms of matter—high-temperature superconductors, multicomponent magnetic materials, semicrystalline polymers, and glasses. These tools are, in turn, giving greater insight into more complex phenomena like the fracture of solids and the continuous transition from liquid to glass in the process of cooling. Ever in view in current condensed-matter and materials physics are research opportunities presented by dramatic progress in the biological sciences. Condensed-matter and materials physicists are working with biological scientists to develop a new field of "physical biology" in which physics-based techniques and approaches are applied to the study of biological materials and processes. Indeed, condensed-matter and materials physics is distinguished by its extraordinary interdependence with other science and engineering fields. It is a multifaceted and diverse interdisciplinary field, strongly linked to other science and engineering disciplines that both benefit from and contribute to its successes. Important examples of this collaboration include fullerenes (physics and chemistry), macromolecules (physics, chemistry, and biology), structural alloys (physics and materials engineering), and silicon technology (physics and electrical engineering). Condensed-matter and materials physics also has strong interrelationships to other branches of physics. Prominent examples include Bose-Einstein condensation (with atomic physics) and the fractional quantum Hall effect (elementary-particle physics). Its practitioners include those who discover and develop new materials, those who seek to understand such materials at a fundamental level through experiments and theoretical analysis, and those who apply the materials and understanding to create new devices and technologies. This work is done in universities, in industry, and in government laboratories. Advances in basic research inspire new ideas for applications, and applications-driven technological advances provide tools that enable new fundamental investigations. Technological advances provide new tools such as synchrotrons, neutron sources, electron microscopes, computers, and scanning-probe microscopes. These new tools are leading to new advances in the fundamental understanding of materials and to a wide-ranging impact on other fields—biology, chemistry, environmental sciences, and engineering.

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Page 7 A New Era The world of condensed-matter and materials physics is entering a new era. Extraordinary advances in instrumentation are providing access to the world of atoms and molecules on an unprecedented scale. Powerful new experimental tools, from national synchrotron and neutron facilities to bench-scale atomic-probe microscopes, are opening up new windows for visualizing and manipulating materials on the atomic scale. Applications range from nanofabrication of electronic devices to probing the secrets of superconductivity and protein folding. These changes are far-reaching. Many research areas, previously inaccessible, are yielding to new and unanticipated advances in atomic-scale synthesis, characterization, and visualization. Advances in computational power have made it possible, for the first time, to simulate the behavior of complex materials systems and large assemblies of atoms. As a result, numerical simulation is approaching parity with laboratory experiments and analytic theory in many areas of condensed-matter and materials physics research. Based on benchmarks provided by experimentation, and enlightened by a proper consideration of theory, the new computational tools provide synergy to accelerate the understanding of ever more-complex systems. Again, this is a qualitative change—with each new generation of computational power, opportunities are emerging that could only be imagined a few years earlier. The combined power of the new experimental tools and computational advances are having an enormous impact on condensed-matter and materials physics, particularly in those areas where the ability to span length scales from the atomic to macroscopic is of fundamental importance, that is, where the properties of atoms and molecules—especially quantum phenomena—become relevant to large-scale phenomena. This new capability to span length scales is bringing the world of atoms and molecules closer to the world of our experience, from the mysteries of quantum mechanics, to the mechanical properties of materials, to the self-assembly of biological systems. Many of these problems, which underlie technological innovation and revolution, could not have been addressed on a fundamental basis even a few years ago. The developments described in this report present a condensed-matter and materials physics profoundly different than it has been at any other time in history. The ability to control and manipulate atoms, to observe and simulate collective phenomena, to treat complex materials systems, and to span length scales from atoms to our everyday experience, provides opportunities that were not even imagined a decade ago. These developments underlie current progress in condensed-matter and materials physics and provide tremendous optimism for the future vitality of the field. They also underlie a new unity in science. Advances in condensed-matter and materials physics increasingly interface with and relate to nearly all areas of science and engineering, including atomic and molecular physics, particle physics, materials science, chemistry, biology, and com-

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Page 8 putational sciences. The next decade will bring extraordinary benefits from this unity, especially as the new capabilities in condensed-matter and materials physics bridge the gap between physics and biology, revealing the molecular-physics basis of biological phenomena. The Science of Modern Technology The information age owes its rapid growth to technological advances that depend on progress in science. Scientific understanding of fundamental phenomena has been closely tied to the development of materials with special properties as carriers and controllers of electrical current, light waves, and magnetic fields. Equally important is understanding, on both fundamental and quantitative levels, the processes that enable cost-competitive manufacturing of devices and systems based on these materials for the rapidly changing electronics and telecommunications industries. Silicon is the foundation of today's integrated circuit technology. The science and technology of this material have built on each other, leading to progressively smaller, faster, and more-complex devices. In turn, semiconductor fabrication technology has enabled the construction of exotic quantum devices that are essentially man-made two-dimensional atoms. A range of new insights into the behavior of large collections of electrons have come from studying electron behavior in these devices. As these circuits continue to develop, doubling their speed and power every 18 months in accordance with the empirical Moore's Law, they march farther into the quantum domain where new physical phenomena arise that must be understood and controlled to ensure continuing progress. Compound semiconductors such as gallium arsenide, gallium nitride, and others are essential to the field of telecommunications. They have characteristics that enable the production of electronic and optoelectronic devices with exceptional performance characteristics. They also underlie the solid-state laser technology that converts digital electronic signals into optical signals that travel great distances on fiber-optic cable. The development of lasers at the right wavelengths is essential to minimize losses and to enable the amplification of the signal by purely optical (nonelectronic) means. A particularly exciting recent development is the gallium nitride laser, which produces blue light suitable for next-generation optical storage systems. Another important development is vertical-cavity surface-emitting lasers, which will bring the mass-production advantages of integrated circuits to diode lasers. Their geometry will enable chip-to-chip optical interconnects in future generations of integrated circuits. Fiber-optic cable, an extraordinary product of optical materials research, enables the transmission of information by modulation of light waves traveling in a glass fiber. Because light, the carrier of the information, has a frequency of at least 1014 Hz (much higher than ordinary radio waves), the rate of information transmission achievable with fiber optics is extremely high. The mechanisms that

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Page 9 scatter light in these fibers had to be understood in great detail before it was possible to develop today's low-loss fibers that enable a signal to travel 800 km (500 miles) before it is attenuated. Modern optical fibers are so perfect that the dominant loss mechanism is the scattering of light by density fluctuations frozen in when the fiber is cooled to become a glass. Fiber-optic amplifiers have been developed that greatly extend the reach of these systems. A great challenge is to develop an inexpensive interface that will bring fiber information links to the home user. Optics enables data storage in compact disks, in the new high-capacity digital video disks (DVDs), and high-density storage systems now in development. To continue this progress, materials scientists must develop commercially manufacturable solid-state lasers that emit blue light. Optics is also essential to display, printing, and copying technologies. Optics is playing an increasingly important role in the telecommunications revolution. Further research into the properties of materials for fiber-optic amplifiers, fast optical switches, and many other optical technologies will be necessary for future advances in information technologies. Magnetism has presented physics with some of its most challenging theoretical problems and also with some of its most important applications. Driven by the need for progressively more data-storage capacity, the science of magnetism has yielded new technology for the devices that read and write data on computer disk drives. Using the recently discovered phenomenon of giant magnetoresistance, technologists have found ways to fabricate read/write heads that have allowed development of these devices to keep pace with rapid improvements in integrated circuitry. Progress in magnetic materials has also yielded a new class of small motors and new transformer core materials for power distribution. And magneto-electronic devices have moved from the laboratory to applications with amazing speed. Among the sensors based on these new technologies are superconducting quantum interference devices (SQUIDs) that enable the detection of minuscule magnetic fields emitted by the human brain and heart, and magnetic force microscopes that can image magnetic properties with nearly atomic-scale resolution. As integrated circuitry becomes ever smaller and closer to the realm of quantum physics, new ways of constructing the logic functions that are the building blocks of these circuits may be discovered. It may even become possible to develop a new form of logic circuitry that exploits the properties of the strange world of quantum mechanics. Until then, a number of challenges face the manufacturers of information systems. The tiny aluminum (and recently, copper) layers that connect the logic devices and their insulating glassy sheaths may have to be replaced with something better in order to continue increasing speed. Optical interconnections may play a role. Will some new way of producing digital switches, the building blocks of computer logic, emerge? Can optical technology be developed so that the essential functions of the communications network (such as switching, now carried out electronically) can all be carried out with optical devices? Will the key information technologies be reducible to the atomic-size

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Page 10 scale? Can the complex physics and chemistry of current and future materials be mastered to enable us to meet this grand challenge? New Materials and Structures The effort to develop new materials and new configurations of matter, driven largely by the potential for innovation that such novelties make possible, has led to the creation of structures that have turned out to be microphysics laboratories on the atomic scale. Some of these new materials and structures have provided the environment for completely unexpected phenomena to emerge. A few examples include high-temperature superconductors, organic superconductors, buckyballs and related structures, and giant magnetoresistance materials. High-temperature superconductors are superconducting above the boiling point of liquid nitrogen. Before the discovery of superconductivity in this class of materials, the effort to increase the superconducting transition temperature (Tc) of materials focused on painstaking efforts to combine various metals into superconducting alloys. This effort seemed to hit a brick wall at about 23 degrees above absolute zero—too low for widespread applications outside of research. Unlike conventional superconductors, the new high-temperature materials are not metals but ceramics, which one would expect to be insulators. The fact that they conduct electricity at all is quite surprising. These materials have a complicated perovskite crystal structure (like many naturally occurring minerals) with planes of copper and oxygen atoms. At the right temperatures, these planes can function as superconductors. These ceramics are much more complex than most of the materials previously studied in condensed-matter physics. There are many possible permutations and combinations of constituents, making these materials difficult to prepare and characterize. Because of the short range of superconducting correlations, the materials are extremely sensitive to defects, which adds to the difficulty. Notwithstanding the complexity, high-Tc superconducting films have already found application in the SQUID devices described above. The discovery of high-Tc superconducting materials has had a profound influence on researchers in condensed-matter physics. The classic paradigm in the field was to seek systems that exhibit some special simplification as an aid to understanding the physics of solids, even the simplest of which are fairly complicated. But the emergence of startling properties in materials with more complex composition and structure has convinced researchers that admitting higher complexity to the field of study can also offer the possibility of new and unexpected phenomena and insights in all areas of condensed-matter and materials physics. One effect of the study of high-Tc materials was a renewed interest in complex perovskites. That, in turn, led to the discovery that the electrical resistance of lanthanum manganate can be extremely sensitive to the presence of magnetic fields. This effect, known as "colossal magnetoresistance," is of great interest

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Page 11 because of its potential application to read data from a new generation of ultra-high-density magnetic storage disks. Until recently, carbon was thought to exist in only two crystal structures—diamond and graphite. The discovery of new crystalline structures, generically called ''buckminsterfullerenes,'' was a great surprise. Variations of these structures can have amazing properties, including forms with a tensile strength 100 times that of steel. The name "buckminsterfullerene" was chosen because the structure of the first one discovered resembles that of a geodesic dome. Their properties depend primarily on this special shape. The structure can be imagined by starting with a two-dimensional hexagonal lattice of carbon atoms found in graphite. If pentagons are substituted for some of the hexagons, the surface develops positive curvature and can be made to close on itself, forming a soccer-ball structure (a "buckyball") or various other possible shapes, including tubes. These tubes can vary from metallic to semiconducting, depending on their geometry. By exposing fullerene molecules (C60) to alkali or alkaline-earth metal vapors, organic super-conductors can be prepared. These examples have all been startling breakthroughs. But amazing out-comes can also result from the steady, evolutionary development of the properties of materials as some property is refined past previous technological barriers. For example, steady improvements in the purity of semiconductor materials used in high-frequency applications such as cellular phones eventually led to the fabrication of quantum-dot structures. These structures, fabricated on the quantum-size domain, have energy states similar to those of atoms but with optical and electronic properties that can be tailored for a wide variety of applications. Several themes and challenges are apparent—the role of molecular geometry and reduced dimensionality, the synthesis and processing and understanding of more complex materials, tailoring the composition and structure of materials on very small scales, and incorporation of new materials and structures in existing technologies. Progress in these areas holds the promise of further startling break-throughs, yielding materials with unexpected and useful properties and extending the understanding of condensed-matter and materials physics. Novel Quantum Phenomena Perhaps the most important lesson learned from studying the physics of systems that contain many particles is that when the number of particles in the system is large enough, entirely new phenomena can appear. These new behaviors of the whole system may not have any obvious relationship to the properties of the individual particles, but rather may arise from collective or cooperative behavior of all the particles. Such phenomena are often referred to as "emergent phenomena" because they emerge as the complexity of a system grows with the addition of more particles.

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Page 12 In some materials, at sufficiently low temperatures, small motions of the crystal lattice create interactions among the electrons that cause them to pair in such a way that they form an electron liquid that no longer experiences friction when it flows. Thus, because the flow of electrical current experiences no resistance in such materials, they are known as "superconductors." Superconductivity is a good example of a quantum emergent phenomenon. Its existence was unanticipated when only the properties of individual atoms were considered. A rich variety of other new effects emerge when large ensembles of atoms are brought together. The length scale of an atom is about 1 Å, and typical quantum energy levels of the electrons in atoms are in the range of 1 to 10 electron volts (eV). As atoms are assembled in a solid, and more collective effects emerge, length scales become larger and energy scales become smaller. It is often convenient to view such systems not in terms of their building blocks, but rather in terms of "elementary excitations" that may have properties very different from those of the electrons and atoms that compose the system. As it becomes possible to make materials and have more and more control over their structure, impurities, and imperfections, collective excitations at smaller and smaller energy scales have been observed. These excitations sometimes have bizarre properties, such as carrying a charge that is a fraction of the electron charge. Because these emergent phenomena present new and unexpected properties, it can be said that one of the new frontiers of condensed-matter physics is at low energies and at length scales large compared to atoms. By contrast, in elementary-particle physics, the frontier is at increasingly higher energies and shorter length scales. A particularly fascinating emergent phenomenon occurs in helium at very low temperatures. Helium has an integer spin, so it is not bound by the Pauli exclusion principle. As a result, many helium atoms can all occupy the same quantum state, and this effect actually occurs at very low temperatures. In a manner somewhat analogous to the loss of electrical resistivity in superconductivity, liquid helium at very low temperatures loses all viscosity. It can flow without resistance through very small orifices and climb up the walls of its containers. This phenomenon is called "superfluidity." The general phenomenon whereby many integer-spin particles occupy the lowest energy state is called "Bose-Einstein condensation," after Bose and Einstein, who first described the statistical behavior of ensembles of quantum-mechanical particles with integer spin. Physicists were surprised to learn that helium-3 atoms can also exhibit super-fluidity. Helium-3 is a half-integral spin atom, so it must obey the Pauli exclusion principle, and it should not be able to undergo Bose-Einstein condensation. But two helium-3 atoms can pair up to make a particle with integer spin, and that composite particle can then undergo condensation. This phenomenon is similar to the way electrons in a superconducting solid form pairs. Helium-3 forms a super-fluid only at very low temperatures because the particles of which it is formed

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Page 13 are, themselves, only weakly bound together. Creating the super-low-temperature environment necessary to achieve superfluidity in helium-3 was an amazing experimental tour de force. The discovery of superfluidity enabled by these experimental techniques was awarded the 1996 Nobel Prize for physics. (Table O.1 lists Nobel Prizes awarded for research related to condensed-matter and materials physics.) The mechanism involved may have analogies to those responsible for high-Tc superconductivity. An even more extreme form of Bose-Einstein condensation was recently achieved by the atomic physics community when alkali metal (such as sodium) vapor was held in an atomic trap and cooled to temperatures within a few microdegrees of absolute zero. The gas is very dilute and the atoms are far apart, but the atoms behave coherently and can be directed in a laser-like beam. This incredibly difficult experiment is at the outermost reaches of the low-energy, large-distance quantum frontier. These achievements have built bridges among the condensed-matter, atomic-physics, and quantum-optics communities because condensation phenomena are important in all of these areas. For example, optical lasers depend on the ability to place a large ensemble of atoms in the same excited energy state. The atoms then decay in a collective, coherent transition to a lower energy state, releasing coherent light in the process. An issue at the forefront of condensed-matter theory concerns quantum spin chains and ladders. Quantum spins are typically associated with magnetic dipoles. Magnets are simply solids in which the atomic spins in the crystal lattice and their associated magnetic dipoles are all pointing in the same direction. An important consequence of the discovery of high-Tc superconductors has been progress in learning how to synthesize compounds that have spins arranged in unusual configurations. Among these configurations are two-dimensional planes, one-dimensional chains, and other more complex structures such as ladders. The synthesis of these and other new families of organic and inorganic compounds has reinvigorated the study of quantum magnetism. The role of magnetism in high-Tc superconductors is among the outstanding questions in this area. Probably the most remarkable collective phenomenon discovered in the latter half of the twentieth century is the quantum Hall effect. The ordinary Hall effect arises when electric current passes through a semiconductor film in the presence of a magnetic field perpendicular to the plane of the film. The current-bearing electrons moving in a magnetic field experience a force perpendicular to both the magnetic field and the direction of motion. As a result, electrons are pushed to one side of the film, which creates a transverse electric field and a voltage across the film. The more current is passed through the film, the greater the voltage. The ratio of the applied current to this voltage has the units of a conductance (or inverse electrical resistance) and is called the "Hall conductance." If this experiment is carried out at high magnetic fields and low temperatures, quantum mechanics comes into play and the Hall conductance becomes

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Page 14 TABLE O.1 Nobel Prizes Awarded for Research Related to Condensed-Matter and Materials Physics Since 1986 Year Field Citation Laureates 1986 Physics For design of the first electron microscope (Ruska) and the scanning-tunneling microscope (Binnig and Rohrer) Ernst Ruska, Gerd Binnig, and Heinrich Rohrer 1987 Physics For discovery of superconductivity in ceramic materials Johannes Georg Bednorz and Karl A. Müller 1991 Physics For discovery of methods for studying order phenomena in complex forms of matter, particularly liquid crystals and polymers Pierre-Gilles de Gennes 1994 Physics For development of neutron-scattering techniques for studies of condensed matter Clifford G. Shull and Bertram N. Brockhouse 1996 Chemistry For the discovery of fullerenes Harold Kroto, Robert Curl Jr., and Richard E. Smalley 1996 Physics For the discovery of superfluidity in helium-3 David M. Lee, Douglas D. Osheroff, and Robert C. Richardson 1997 Physics For development of methods to cool and trap atoms with laser light Steven Chu, Claude Cohen-Tannoudji, and William D. Phillips 1998 Chemistry For development of the density-functional theory (Kohn) and computational methods in quantum chemistry (Pople) Walter Kohn and John A. Pople 1998 Physics For discovery of a new form of quantum fluid with fractionally charged excitations Robert B. Laughlin, Horst L. Störmer, and Daniel C. Tsui

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Page 15 quantized in precise integer multiples of the fundamental constant e2/h where e is the charge of the electron and h is Planck's constant. It is remarkable that this universal result is completely independent of all microscopic details of the sample such as the density and types of impurities, the precise value of the magnetic field, etc. Among the consequences of this extraordinary phenomenon is that it is possible to make a high-precision measurement of the fine-structure constant (which expresses the strength of electromagnetic forces) and also to realize a highly reproducible standard of electrical resistance. This phenomenon is used in standards laboratories throughout the world to maintain the unit of electrical resistance (the ohm). An even more surprising phenomenon occurs with samples of very high purity at very low temperatures and in very high magnetic fields. In these conditions it is possible to observe a Hall effect in which the conductivity is a fraction of e2/h—for example, (1/3)e2/h. Physicists were quite startled by this observation. It turns out to result from the formation of quasiparticles whose effective charge is one-third (or various other rational fractions) of the electron's charge. These quasiparticles are a collective mode of a quantum fluid. The low-energy excitations of this weird fluid consist of vortices bound to a fraction of an electron charge. These objects have been recently observed by direct measurement of their charge and by tunneling experiments in which an electron added to the system breaks up into three excitations, each with one-third of the electron's charge. This discovery, which earned the 1998 Nobel Prize in Physics (see Table O.1), offers a whole universe of intriguing possibilities for experimental and theoretical exploration of new collective modes. Nonequilibrium Physics Nonequilibrium physics is the study of systems that are out of balance with their surroundings. They may be changing their states as they are heated or cooled, deforming as a result of external stresses, or generating complex or even chaotic patterns in response to forces imposed on them. Examples include water flowing under pressure through a pipe, a solid breaking under stress, or a snowflake forming in the atmosphere. Understanding nonequilibrium phenomena is of great practical importance in such diverse areas as optimizing manufacturing technologies, designing energy-efficient transportation, processing structural materials, or mitigating the damage caused by earthquakes. At the same time, the theory of nonequilibrium phenomena contains some of the most challenging and fundamental problems in physics. A central theme in this field is that the physics of ordinary materials and processes is a rich source of inspiration for basic research. Because nonequilibrium physics touches on such a wide range of different areas of science and technology, it is an important channel through which physics makes contact with other disciplines. For example, its concepts help explain

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Page 20 creation of extreme environments in which to explore the behavior of matter and the synthesis of materials with extraordinary properties. It also provides new "eyes" to observe and new "hands" to manipulate at the atomic scale. Examples of such equipment range from benchtop-scale atomic-force microscopes used by individual investigators, to storage rings the size of a small town that generate the x-rays used by collaborative research groups with many members. These experimental tools enable new insights into systems of recognized importance and the exploration of completely new regimes. One example of scientific progress that depended on modem instruments of research involved unraveling the properties of the high-Tc superconductors. Developing today's understanding of these materials depended on the use of experimental tools: • Neutron diffraction was used to determine atomic coordinates. • Synchrotron radiation was used to determine the electronic structure. • Electron microscopy made it possible to determine the microstructure. • Neutron scattering was used to determine magnetic order. • High magnetic fields and high pressures were used to gain understanding of charge transport. Many of these studies were performed at large- or medium-scale facilities. Although large facilities are critical to condensed-matter and materials physics, another theme that pervades this report is the importance of atomic-scale observation and manipulation. Two of the most important tools for this purpose are the scanning-tunneling microscope and the transmission electron microscope. The equipment is on the small-to-medium scale. Scanning-tunneling microscopes work by placing a probe that is sharp, on an atomic scale, so close to the sample that the quantum wave function of the electron allows it to jump the gap. By scanning this probe over the sample, using sophisticated positioning technology, the surface can be mapped atom by atom. The resolution is far better than anything that can be achieved with light waves, because the wavelength of light is thousands of times too large to visualize atoms. Various kinds of scanning-probe microscopes are now commonly available, including instruments that can examine chemical reactivity, magnetism, optical absorption, mechanical response, and other properties. A particularly promising development is the imaging of molecules, including rather large ones that play a role in biological processes. Scanning-tunneling microscopes can go beyond measuring structures to actually creating them by positioning individual atoms. In principle, it is possible to create any structure to test our understanding of the physics of devices at this scale. One of the challenges in this area is to learn to control the stability of atomic-scale structures. In general, individual atoms placed on a surface will not stay put unless the temperature is extremely low. Another challenge is that, if this technique is ever to lead to practical devices, it would have to be much faster than it is now.

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Page 21 Electron microscopes, which use beams of electrons to probe the sample, are able to penetrate below the surface. They have much higher resolution than optical microscopes because of the shortness of the electron wavelength. Instruments with 1-Å resolution have been demonstrated. Development of electron microscopes has occurred primarily in Europe and Japan. These instruments show great promise for reconstructing three-dimensional structures of biological interest as well as for studying the properties of amorphous and disordered materials. We can be sure that improved brightness, spectroscopy, and resolution in electron microscopes will allow more precise determination of structure and composition in ultra-small volumes of materials, even when these are embedded below the surface. This will have a continuing major impact on the microstructural study of all materials, for example, identifying interfacial structures, solving important problems of support effects on small clusters, and understanding the structural basis of adhesion and fracture in materials. These new capabilities will require a significant reinvestment in infrastructure and increased investment in instrumental research and development. An equally important theme is that, to a growing degree, significant advances in a number of sciences depend on large national facilities. The United States has been particularly strong in the development of synchrotron radiation sources. These sources depend on the fact that when an electron is accelerated, it gives off light. The most sophisticated (third-generation) synchrotron radiation source in the United States is the Advanced Photon Source (APS) at Argonne National Laboratory. The APS uses devices called ''undulators'' that wiggle an electron beam by passing it through an array of powerful magnets to generate tunable beams of very high intensity x-rays. The power and controllability of the x-rays from the APS have made possible a new generation of experiments that have resolved structures with unprecedented precision. The technology now being developed for a fourth generation of light sources may be eight orders of magnitude more powerful than even the APS and will have pulse lengths less than a picosecond. These devices, called "free electron lasers," use undulators configured so that the radiation given off bathes the electron beam and stimulates further emission of radiation in a process closely analogous to the operation of a laser. If past experience is a guide, the greater intensity and coherence that these devices will one day offer will lead to new classes of experiments and new insights into the structure of materials. Synchrotron radiation is being used to conduct research in a number of areas. Inelastic x-ray scattering has provided unique information about the dynamics of fluids and glasses. Photoemission experiments have provided much information about the electronic structure of solids, which is essential to understanding the physical details of the operation of semiconductor devices and integrated circuits. X-ray studies of disordered systems have given insight into inorganic glasses and biological structures. Information can be obtained about chemical states and

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Page 22 environments of individual atoms in the structure of disordered systems. Structural changes associated with the functioning of metalloproteins in processes like nitrogen fixation, photosynthesis, and respiration can be observed. The most rapidly growing and arguably the most important use of synchrotron radiation is in protein crystallography, in which the structure of biologically important proteins is determined. Crystallography of big molecules using synchrotrons is the primary source of insight into three-dimensional biological structures. Biologists are just scratching the surface of the possibilities. Approximately two-thirds of all new structures published in Science and Nature in 1995 used synchrotron radiation, and the number is growing. Progress in this area in the next 10 years should be breathtaking, both in its intellectual scope and its real-world implications. Pharmaceutical companies are pursuing a "drugs-by-design" approach to relating the structure of complex molecules to their function and activity in biological systems; this approach has the potential to revolutionize the industry. Another area in which the use of synchrotron radiation has been of benefit involves providing the means for researchers in the life sciences to understand the workings of a variety of biological structures: • Scientists now understand how muscle contraction works at the molecular level. • Scientists now know how living cells mobilize energy. • Scientists understand how nitrogen-fixing bacteria work. • Scientists have learned the structure of ribozyme, a catalytic form of RNA. • Scientists have determined plant and animal viral structures. One of the key factors that enabled these results was that both the physics community and the sponsoring agencies (primarily the Department of Energy) adopted a philosophy that synchrotron facilities are national resources that should be designed and implemented to serve all the branches of science. That has meant investing resources in making the machines and experimental facilities reliable, predictable, and easily used by researchers unfamiliar with accelerator facilities. It has meant providing the human infrastructure necessary to support such users. It has meant husbanding the special institutional arrangements necessary to make such users successful. Nonphysicist users are now in the majority at most synchrotron facilities. Neutron scattering is a technique particularly sensitive to spin states and low-atomic-weight atoms. It is therefore particularly well suited for the study of magnetism, high-Tc materials, polymers, and biological materials. The major research facilities for neutron scattering in the United States have been the Department of Energy's high-flux reactors at Brookhaven National Laboratory and at Oak Ridge National Laboratory, the Department of Commerce's reactor at the

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Page 23 National Institute of Standards and Technology, and accelerator-based pulsed sources at Argonne National Laboratory and Los Alamos National Laboratory. These facilities have not kept pace with developments in Europe, and in the last decade, leadership decisively passed to the Institute Laue-Langeven reactor facility in France and the ISIS pulsed facility in the United Kingdom. Among the recent successes of neutron scattering has been the demonstration that the high-Tc superconductors have a common feature—nearly square planar arrays of copper and oxygen atoms. Neutron scattering also was used to image vortex lattices in high-Tc superconductors. These studies gave evidence for two of the most important new ideas about superconductivity. The first is that solids can become super-conducting by mechanisms analogous to those responsible for superfluidity of helium-3. The second is that vortices in superconductors can have intricate phase diagrams. Structural information about the perovskite manganites that exhibit "colossal magnetoresistance" has also come from neutron scattering. So far we have discussed strategies for research that involves using powerful tools to probe the structure of matter, to visualize what is happening at the atomic and molecular levels. Another strategy that has proved effective in flushing out new insights is placing matter in extreme conditions of temperature, pressure, or magnetic field. A recurring theme of this study has been exploration of what might be called the "low-energy frontier." One aspect of this frontier is extremely low temperatures. In this regime, very gentle, low-energy effects such as superconductivity and superfluidity begin to emerge; but these effects are washed out by the chaotic thermal motion of the atoms and electrons at higher temperatures. An experimental tour de force of the last decade was the discovery of collective nuclear spin order at nanokelvin temperatures in elemental copper. The rapidly growing capability of computers for experimental control, visualization, and numerical simulation is having a significant impact on condensed-matter and materials physics. Computers used to be viewed not as part of the instrumentation armamentarium but as something outside it. But that may be changing. Modem computers and their associated visualization capabilities are now so powerful, they are beginning to provide "virtual laboratories" that offer new ways to acquire physical insight by exploring the effects of varying physical parameters. Real physical experiments will always be necessary, but computation may provide a virtual experimental space that will greatly speed up decisions about what kinds of experiments to do and where to look for new phenomena. A major event of the decade for individual-researcher laboratories has been the development of scanning-probe microscopes as routine, off-the-shelf analytical tools. The technologies involved hold the promise of control and manipulation of surface materials at the atomic scale as well as study of large molecules in fine detail. On the intermediate scale of infrastructure, there have been three important developments: greater access to electron microscopes and related equipment, the exploitation of university-based microfabrication centers, and reinvigoration of

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Page 24 the U.S. high-field magnet research through the establishment of the National High-Field Magnet Laboratory. The establishment of third-generation synchrotron light sources at Lawrence Berkeley National Laboratory and at Argonne National Laboratory and the decision to construct the new Spallation Neutron Source (SNS) have dominated the decade as far as large national facilities are concerned. It is particularly crucial to move forward with construction of the SNS to make the United States competitive in neutron-scattering studies. Once the SNS is commissioned at Oak Ridge National Laboratory, U.S. researchers can begin the process of recapturing the lead in the use of neutrons to study structure and spins in superconductors, polymers, and other materials. We can confidently predict a rapid growth in knowledge and understanding of biological materials and living organisms resulting from the exploitation of the Advanced Photon Source by the biology community. That progress will be accelerated once the SNS becomes operational. The extensive deployment of various microscopies in many laboratories can be expected to implement great strides in understanding surface physics. The rapid pace of development of research at various scales has depended on accelerator science and research on the physics of smaller instrumentation. This work has had to be parasitic on various enterprises, despite the fact that advances in scientific equipment propel science forward in great leaps. How much better could we exploit the leverage that instrumentation research has if we were to recognize it as an important enterprise worthy of planned investment! The institutional frameworks for such investments clearly depend on scale, but there are natural environments at each level. Expertise in tomorrow's beam physics, for example, partly resides at the major centers of high-energy physics research, but development for low-energy applications will likely occur elsewhere. The materials research science and engineering centers are one set of obvious potential homes for intermediate-scale instrumentation development. Findings And Recommendations Condensed-matter and materials physics is entering an era of great excitement and anticipation as powerful new experimental and computational capabilities are brought to bear on some of the most fundamental scientific and technical challenges of our time. Underlying these challenges is the knowledge that drives the information revolution, modern materials technology, and biotechnology enabled by understanding of the molecular basis of life. We have seen astounding developments over the past decade such as buckyballs and carbon nanotubes, high-temperature superconductivity, giant and colossal magnetoresistance, and large-scale quantum phenomena. The next decade, enriched by powerful new research tools, promises to be even more extraordinary.

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Page 25 New capabilities for condensed-matter and materials physics research include spectacular advances in the atomic-scale characterization and manipulation of materials, computer simulations of large interacting systems, and the ability to relate properties and phenomena from molecular- to macroscopic-length scales. These new capabilities are uniting the worlds of atomic-scale behavior and macroscopic phenomena in ways that provide avenues for understanding and designing materials and processes from the atoms up. In turn, this new understanding holds the promise of breakthroughs at a time when the limits of incremental progress are being tested in materials-based technologies ranging from integrated circuits and magnetic storage devices to the synthesis of advanced polymers to the performance of materials under extreme conditions. Perhaps the greatest impact will be felt at the interface between biology and physics, where the convergence of condensed-matter and materials physics and molecular biology is expected to drive important advances in the fundamental understanding of biological processes. Condensed-matter and materials physics faces critical challenges in realizing this future. Investments in facilities and research infrastructure are essential to provide a world-class research environment and to enable breakthrough opportunities. Partnerships across disciplines and among universities, government laboratories, and industry are essential to leverage resources and strengthen interdisciplinary research and connections to technology. Finally, special attention must be given to condensed-matter and materials physics education to ensure the availability of intellectual capital to sustain the vitality of the field and its contributions to society. Research Infrastructure The United States has a strong foundation of research groups and small-scale centers located in universities and government laboratories. Centers play an essential role in a number of areas including microcharacterization, processing, synthesis, and state-of-the-art instrumentation development. Research groups and centers are a crucial reservoir of expertise. They also play an important institutional role by providing a meeting ground for research and development personnel in industry and students and researchers in universities and government laboratories. Centers bring together the problem-definition capabilities of industry with the educational role of the universities and the research missions of government laboratories. As a result, leading-edge research capabilities are applied to important areas of microcharacterization, processing, synthesis, and instrumentation. Centers also bring a long-term commitment to applying intellectual excellence to research problems and to developing expertise in the next generation of researchers in these essential areas of study. The role that small-scale centers now play has been fostered also in major industrial research laboratories as well as by the research strategy of the Department of Defense. But the burden now falls much more heavily on research groups

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Page 26 and small centers in universities and government laboratories. Therefore, it is appropriate to strengthen this part of the nation's research infrastructure. Additional support, long-term commitments, and oversight structures that involve all the interested parties (universities, government laboratories, and industry) are necessary to accomplish this fortification. Recommendations for Upgrading the Infrastructure • The National Science Foundation, the U.S. Department of Energy, the U.S. Department of Defense, and other agencies that support condensed-matter and materials physics research should continue to nurture the core research at the heart of the field. The research areas described in the Overview provide a guide to the scientific arenas at the forefront of this work. • The agencies that support and direct research should plan for increased investment in modernizing the condensed-matter and materials physics research infrastructure at universities and government laboratories. • The National Science Foundation should increase its investment in state-of-the-art instrumentation and fabrication capabilities, including centers for instrumentation R&D, nanofabrication, and materials synthesis and processing at universities. The Department of Energy should increase its support for such programs at national laboratories and universities. Major Facilities The emergence of national synchrotron and neutron facilities has revolutionized our understanding of the atomic-scale structure and dynamics of materials. The nation is fortunate to have world-class facilities for synchrotron research. However, the situation is strikingly different for neutrons, where we find ourselves with fewer facilities than those judged inadequate by national review committees more than a decade ago. Many of the advances in structural biology, polymers, magnetic materials, and superconductivity depend on access to state-of-the-art neutron-scattering facilities. Without a new neutron source, the nation cannot be competitive in these and other areas of enormous scientific and technological significance. This is an urgent and immediate need, and the committee strongly recommends construction of the Spallation Neutron Source (SNS). Upgrades at existing neutron-scattering facilities are also essential to sustaining neutron-scattering research in the United States during SNS construction as well as to strengthen the field and provide broad access to the user community. Over the past decade there has been an explosion in the use of synchrotron facilities. A great success of these facilities has been the rapid growth in their use across the broad spectrum of science. At national synchrotron facilities biologists are attacking the structure of biological molecules, chemists are improving drug designs, and environmental scientists are following the migration of envi-

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Page 27 ronmental pollutants, all alongside materials researchers. In fact, life scientists are the fastest growing user community at national synchrotron facilities and currently occupy more than 25 percent of available beam time. This large influx of users from other scientific disciplines is remarkably productive and is creating a healthy scientific melting pot, but it is also straining the ability of the facilities to respond. Modest investments at existing synchrotron facilities can greatly expand current capabilities and help alleviate this problem. On the horizon are fourth-generation light sources that offer enormous gains in intensity and coherence over existing sources. The committee recommends upgrades at existing synchrotron facilities and research and development to explore possible options for fourth-generation sources. Recommendations for Major Materials Research Facilities • The insufficiency of neutron sources in the United States should be addressed in the short term by upgrading existing neutron-scattering facilities and in the long term by the construction of the Spallation Neutron Source. • Support for operations and upgrades at synchrotron facilities, including research and development on fourth-generation light sources, should be strengthened. • The broad use of synchrotron and neutron facilities across scientific disciplines and sectors should be considered when establishing agency budgets. Partnerships Condensed-matter and materials physics is becoming increasingly interrelated with other fields of science and technology, with important links to many disciplines including other branches of physics, chemistry, materials science, biology, and engineering. At the same time, the field has advanced to the point where it is often impractical and sometimes impossible to assemble in one place all of the intellectual resources and specialized equipment for a given research project. Continued progress in the field depends on establishing effective partnerships across disciplines and among universities, government laboratories, and industry. These partnerships enable cross-disciplinary research, leverage resources, and provide awareness of technological drivers and potential applications. The extraordinary scientific and technological success of the major industrial laboratories over the past half-century resulted from their ability to integrate long-term fundamental research, cross-disciplinary teams involving experimentalists and theorists, materials synthesis and processing, and a strategic intent. Virtual elements of this fertile ground exist in potential partnerships among universities, government laboratories, and industry. Federal R&D agencies should encourage partnerships that recreate this environment in appropriate subfields of condensed-matter and materials physics.

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Page 28 Recommendations for Research and Development Partnerships • Federal agencies should provide incentives for formation of partnerships among universities and government and industrial laboratories that carry out research in condensed-matter and materials physics. These research and development partnerships should be encouraged in order to: — Optimize the use of infrastructure and facilities, — Enable cross-disciplinary research, — Improve university and government laboratory appreciation of industry priorities and needs, — Share the risks and returns of long-term research, and — Assemble teams that can recreate the fertile research environment of the large industrial research laboratories of the past half-century. These partnerships should be fostered by: — Making resources available through special programs that stimulate part-nerships, — Developing effective protocols for resolving intellectual property issues in cooperative research, — Encouraging university and government laboratory internships and sabbaticals in industry, and — Requiring partners to have a stake in the partnership (e.g., for universities and government laboratories, the partnership should add value to core missions). Education Intellectual capital is probably the single most important investment for science and technology. Intellectual capital in condensed-matter and materials physics occupies a special place in the national economy, underpinning many of the technological advances that drive economic growth. The U.S. system of graduate education, research universities, government and industrial laboratories, and national facilities for condensed-matter and materials physics is a major reason for rapid progress in research and technological applications. Maintaining this progress requires continued commitment to strengthening these institutions. In addition, condensed-matter and materials physics must play a crucial role in engaging undergraduates in research and improving their understanding of science and technology. Making investments to develop the human capital essential for leadership in condensed-matter and materials physics and related technologies will pay rich dividends to the nation. Successful accomplishment of these

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Page 29 objectives will also help the larger field of physics to adjust to a new role in which economic security becomes the dominant justification for national investments in research. Recommendations for Education • Universities should endeavor to enhance their students' understanding of the role of knowledge integration and transfer as well as knowledge creation. • Universities should develop ways to bring the excitement and creativity of research and discovery into education at an earlier stage. • University departments should consider new professional degree programs that link undergraduate physics education with, for example, engineering-oriented disciplines. • Universities should foster joint academic appointments across departments to break down disciplinary barriers. Campuses should experiment with the creation of "virtual departments" to aid intellectual restructuring to better achieve their research and education missions in changing times. • Universities need to expand existing programs that enable undergraduates to have research experiences in faculty laboratories or summer internships in industry or at national laboratories. • Universities should recognize the importance of knowledge integration and transfer in addition to knowledge creation. • Applied physics departments and programs should link to industrial liaison programs, which generally are strong in colleges of engineering. • Agencies, particularly the National Science Foundation, should provide incentives for action in these areas. Research Themes Throughout this study the themes of new experimental and computational capabilities, the ability to address problems of increasing complexity, and the importance of relationships with other fields pervade the subdisciplines of condensed-matter and materials physics (see Box O.1). These themes provide a sense of vitality and optimism for the future of condensed-matter and materials physics. Maintaining scientific excellence, a long-term perspective, and a world-class environment for research are essential. Investing in facilities, encouraging partnerships, integrating research and education, and encouraging discovery are critical elements. But where is the field headed? Although it is often dangerous to predict the future in science, the committee identified 10 areas that span and underpin the subdiscipline-specific scientific priorities of condensed-matter and materials physics as described in the body of this report. These areas, listed in Box O.1, encompass the committee's view of the high-level strategic priorities that have emerged from the internal dynamics of the field and that are likely to

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Page 30 BOX O.1 Strategic Scientific Themes in Condensed-Matter and Materials Physics • The quantum mechanics of large, interacting systems • The structure and properties of materials at reduced dimensionality • Materials with increasing complexity in composition, structure, and function • Nonequilibrium processes and the relationship between molecular and mesoscopic properties • Soft condensed matter and the physics of large molecules, including biological structures • Controlling electrons and photons in solids on the atomic scale • Understanding magnetism and superconductivity • Properties of materials under extreme conditions • Materials synthesis, processing, and nanofabrication • Moving from empiricism toward predictability in the simulation of materials properties and processes characterize condensed-matter and materials physics research over the next decade. Condensed-matter and materials physics lies at the heart of revolutionary advances in broad areas of science and technology. The next decade promises exciting new discoveries and technology impacts as powerful new capabilities in synchrotron and neutron research, atomic-scale visualization, nanofabrication, computing, and many other areas probe the secrets of materials and materials-related phenomena. This is a new era. Vast new arenas, ranging from subtle quantum phenomena, to macromolecular science, to the realm of complex materials, are increasingly accessible to fundamental study. It is a time of exceptional opportunity to perform pioneering research at the technological frontier—a frontier enabled by advances in condensed-matter and materials physics.