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Condensed-Matter and Materials Physics: The Science of the World Around Us 8 The Impact of Condensed-Matter and Materials Physics Research IMPACT ON SOCIETY Condensed-matter and materials physics (CMMP) is remarkable for the breadth and depth of its impact on society as well as on other scientific disciplines. While it is not possible to review all of these connections exhaustively, in this chapter a few examples in the areas of education, the economy, energy, and medicine and health care are used to illustrate the major impact that CMMP has had, and continues to have, on U.S. society. Education Condensed-matter and materials physics describes and shapes the world we see. It is critical to educate a new generation with a deeper understanding of the role of CMMP (and of science in general) in society. Yet few people ponder the quantum mysteries of the magnet on their refrigerator or realize that they are working against entropy when they stretch a rubber band. Many people benefit from the torrent of new and improved electronic devices, but few are aware that these products are the fruits of a rich and coherent scientific discipline characterized by an inseparable mix of fundamental and applied research. Limited public awareness and understanding of science constitute an increasing danger to U.S. economic security and are most dramatically reflected in the current crisis in primary and secondary school science education. The CMMP community must now extend educational efforts not only to improve general scientific literacy but also to
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Condensed-Matter and Materials Physics: The Science of the World Around Us increase the pool of students interested in science and engineering. It is critical to infuse a new generation of scientists with the knowledge, skills, creativity, versatility, and sense of wonder needed to meet the challenges ahead. Along with the six scientific challenges for CMMP that are identified in this report, the challenge to educate the next generation of scientists and citizens is equally important. In further refinement of this challenge, the Committee on CMMP 2010 identified three key issues: how to educate the next generation of CMMP researchers, how to attract talented people to the field, and how to increase the scientific literacy of the general public and of school-age children. These three issues are addressed below. Regarding the education of the next generation of CMMP researchers, the research community perceives that significant changes have occurred in the field during the past decade and that these changes have had and will continue to have implications for the education of this next generation. Growing interdisciplinarity at the frontiers of CMMP is evident in all the scientific challenges discussed in this report, including the nanoworld, emergent and far-from-equilibrium phenomena, energy for the future, the physics of life, and the evolution of the information age. This interdisciplinarity is manifested as a broadening of the interface between CMMP and other areas of physics and also other disciplines, such as chemistry, biology, mathematics, and computer science. More exposure to these fields is therefore needed in the undergraduate and graduate educational programs of students who might be seeking careers in CMMP. Such interdisciplinary education should include both formal course work and hands-on exposure to how research is done in these fields, as well as an introduction to the vocabulary and culture of these diverse fields as they are now practiced in their interface with CMMP. This new emphasis is believed to be essential for working at the cutting edge of CMMP in the coming decade and beyond. The great challenge that faces the academic community thus is to create an undergraduate curriculum that balances the need for breadth against the depth of the traditional physics culture, from which the community draws its strength. Innovative, experimental approaches to this problem should be encouraged, as should more flexible curricula that can transmit at least part of the physicist’s intellectual style to students considering careers in areas such as business, finance, and law. There is a strong sentiment that the achievements and experience level of students receiving undergraduate and graduate degrees in U.S. universities must be very thorough and fully competitive with their counterparts in the best universities worldwide. The implementation of these new educational directions for students and faculty in an exciting way, without increasing the length of undergraduate, graduate, and postdoctoral programs, is a significant challenge. Attracting top-level talent to CMMP is identified as the second significant challenge to the field. It is expected that transforming the undergraduate educational
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Condensed-Matter and Materials Physics: The Science of the World Around Us experience to be more interdisciplinary and more flexible, while emphasizing educational achievement and research leadership at the highest levels, will help attract high-level talent to the U.S. CMMP research enterprise. Emphasis on rewarding hands-on research experiences at the undergraduate level is regarded as a critical component in exposing young people to the excitement and intellectual rewards of CMMP research. The committee recommends that the Research Experience for Undergraduates (REU) program that is now working well be further expanded. Thoughtful mentoring and attention to young people in the classroom and research laboratory are also regarded as important ingredients in effective career development in the changing environment that CMMP faces, both in supporting the intellectual development of the field and in providing more opportunities to young people in developing their independent but CMMP-related careers in the United States. Finding more pathways for rewarding careers for young people in the changing landscape of CMMP-related careers in industry, in national laboratories, and in other sectors remains a challenge for the community. To make academic careers more attractive, efforts are needed to increase the probability of funding new research proposals so that more time is devoted to doing research rather than to writing proposals seeking funding for it. Enhancing the degree to which CMMP researchers can work on the topics in which they are genuinely most interested, as opposed to those most in fashion with the funding agency, was identified as another way that academic CMMP careers could be made more attractive and more scientifically productive. Increasing the scientific literacy of the general public remains a high-priority challenge, especially for the CMMP community, because CMMP involves so many captivating and often everyday phenomena that are of great general interest. At the same time, the general public is largely unaware that CMMP is the science behind many of the technological marvels that they take for granted. Introducing interesting CMMP phenomena at all levels of science teaching, from “gee-whiz” talks in the local public library to science classes in elementary school and the undergraduate lecture hall, is an opportunity that the CMMP community should not miss. Through common examples of CMMP occurring in daily life, from the growth of soap bubbles to the beautiful symmetry of ice crystals, the imagination of young people can be captured to increase their interest in science and to improve their retention of scientific concepts. Such examples could also be used by science teachers seeking to focus more on presenting fundamental concepts than on preparing students to pass standardized tests. For more advanced students in high school and college and for the general public, more attention to current interesting examples of CMMP should be introduced in order to achieve similar educational objectives. Examples such as the physics behind the operation of a cellular telephone, or the dynamics of biological motion in a zebra fish, or why nanoscience offers promise of better utilization of renewable energy sources will resonate and inspire. Indeed,
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Condensed-Matter and Materials Physics: The Science of the World Around Us CMMP affects daily life and influences important societal and governmental decisions concerning issues such as energy policy, industrial competitiveness, and the preservation of the global environment. Increased public appreciation of this interplay between CMMP and society is sure to pay dividends to the nation. The CMMP community believes that the level of science education in high schools needs to be elevated in order to decrease the gap between high school science and introductory college-level courses. Not only does this gap present a hurdle to enthusiastic future scientists, but it also scares away undergraduates who are less certain that science and engineering will figure largely in their careers. Strong input from the focus groups that were convened for the CMMP 2010 study indicated a great desire for CMMP participation in outreach programs, kindergarten through grade 12 (K-12) education, and increased public understanding of science. Nonetheless, the present system, in which the quality of outreach programs is a criterion for the evaluation of scientific research grants, confuses two conceptually distinct goals to the point that neither is optimally served. The Committee on CMMP 2010 therefore concluded that outreach, K-12 education, and undergraduate science education initiatives should be supported by supplemental or stand-alone grants administered by separate National Science Foundation (NSF) and Department of Education programs. Further discussion between the funding agencies and the CMMP community is needed on how best to implement this common goal. The Economy The impact of science and technology on economic growth and the quality of life has been tremendous over the past hundred years. Technology has now replaced capital and the workforce as the major growth factor in developed countries and in developing nations as well. While there is a variation in the quantitative value assigned to economic growth owing to science and technology, all agree that science and technology are the principal factors in the past and continued growth of national economies. Usually, the impact is determined by subtracting the effect of capital and labor and attributing the remaining economic growth to research and development (R&D). A summary by Boskin and Lau gives a range of 25 to 50 percent, depending on how the education of workers is included.1 Whatever the impact, most economic regions have large and growing efforts in research in all scientific fields in the belief that it will impact economic growth. The growth in research in CMMP has been especially large in Southeast Asia and Western Europe. The U.S. CMMP research effort is conducted in government laboratories, industrial laboratories, and universities. The industrial laboratories grew from 1 Nathan Rosenberg, Ralph Landau, and David Mowery, Technology and the Wealth of Nations, Palo Alto, Calif.: Stanford University Press, 1992, p. 32.
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Condensed-Matter and Materials Physics: The Science of the World Around Us a start in the late-19th century, with major investments following World War II. The national policy on government laboratories was set by Vannevar Bush in the report Science, The Endless Frontier,2 initiated by President Roosevelt and presented to President Truman in 1945. The U.S. research universities benefited from wartime investments and the destruction of research institutions in many other countries in the 1940s. Today, the United States leads the world in investment in R&D and as a result, many believe, has a major market position in key industries. However, this situation is changing, as other economic regions are making large government investments with the conviction that these investments will lead to additional economic growth and perhaps market leadership in the key industries of the 21st century. The period from 1940 to 1980 in the United States saw the federal government as the leading investor in R&D. After 1980, U.S. industry’s investment exceeded that of the government and is now more than two-thirds of the national R&D effort.3 However, most of the industrial R&D effort is focused on product development, with less than 10 percent going to fundamental research. The U.S. economic leadership, measured in gross domestic product (GDP) per person or in total dollars, is unquestioned. The portion of that leadership owing to R&D is not widely acknowledged. However, there is no question that physics research, and especially CMMP research, has had a major impact on almost every aspect of life today. Examples range from the invention of the transistor/integrated circuit and the laser to their application in personal computers and supercomputers and the instrumentation that is used to diagnose and treat disease—for example, computer-aided tomography, positron emission tomography, and magnetic resonance imaging. Tools developed by physicists are also essential to the advancement of science in other fields, such as astronomy, biology, chemistry, and many of the life sciences. CMMP has been one of the leading contributors to economic advances and better health care and life style. The electronics industry alone is about a $1.5 trillion business worldwide, based principally on technology arising from CMMP research, some done more than 50 years ago.4 A NASA report estimated that the direct contributions of granular materials and fluids processing alone account for roughly 5 percent of the GDP and almost one-third of the manufacturing output of the 2 Vannevar Bush, Science, The Endless Frontier, Washington, D.C.: U.S. Government Printing Office, 1945. 3 National Science Foundation, Division of Science Resources Statistics, U.S. R&D Increased 6.0% in 2006 According to NSF Projections, NSF 07-317, Arlington, Va., 2006. Available at http://www.nsf.gov/statistics/infbrief/nsf07317/; last accessed September 17, 2007. 4 David C. Mowery and Nathan Rosenberg, Paths of Innovation, New York: Cambridge University Press, 1998, p. 175.
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Condensed-Matter and Materials Physics: The Science of the World Around Us United States alone (about $850 billion per year).5 Thus, better understanding of far-from-equilibrium behavior (Chapter 5) can lead to better packing of granular materials with large economic benefit. Other applications such as financial systems, health care, and education are major economic areas in which CMMP research has had an effect. The CMMP research underlying these areas has been recognized by a significant number of Nobel Prizes (see Chapter 9) and many other awards. It is clear that there has been a significant impact of CMMP research on society and the economy. For continued growth in the world’s economy and the improvement of health care, research in the fundamental aspects of CMMP, both experimental and theoretical, is essential. Energy The introduction to the energy challenge in Chapter 3 of this report outlines the energy and environmental issues that will face society during the 21st century. Recent reports from the United Nations climate panel6-8 estimate with 90 percent certainty that average global temperature increases during the second half of the 20th century were due to anthropogenic greenhouse gas emissions, and they predict further increases in temperatures during the 21st century accompanied by substantial rises in sea level. Even if greenhouse gas emission can be stabilized, atmospheric carbon dioxide (CO2) levels will take hundreds of years to return to their pre-industrial level. Against this backdrop, the United States will face some difficult decisions concerning its energy policy. With U.S. oil and gas resources dwindling, it will be tempting to place more emphasis on power generation from coal, especially because the nation has massive coal assets, which represent more than 25 percent of the world’s recoverable reserves. At the present time, approximately 50 percent of U.S. electricity is produced from coal, and this could certainly be increased.9 Coal, therefore, could present a partial solution to the nation’s burgeoning energy 5 Paul Chaikin and Sidney Nagel, Report on the NASA Soft and Complex Condensed Matter Workshop, 2003. Available at http://gltrs.grc.nasa.gov/reports/2003/CR-2003-212618.pdf; last accessed September 17, 2007. 6 Intergovernmental Panel on Climate Change, Working Group I Report, The Physical Science Basis, 2007. Available at http://ipcc-wg1.ucar.edu/wg1/wg1-report.html; last accessed September 17, 2007. 7 Intergovernmental Panel on Climate Change, Working Group II Report, Impacts, Adaptation and Vulnerability, 2007. “Summary for Policymakers” available at http://www.ipcc.ch/SPM13apr07.pdf; last accessed September 17, 2007. 8 Intergovernmental Panel on Climate Change, Working Group III Report, Mitigation of Climate Change, 2007. “Summary for Policymakers” available at http://www.ipcc.ch/SPM040507.pdf; last accessed September 17, 2007. 9 For further U.S. energy statistics, see http://www.eia.doe.gov; last accessed September 17, 2007.
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Condensed-Matter and Materials Physics: The Science of the World Around Us needs, but the inevitable production of greenhouse gases from the use of coal as a fuel makes this strategy very unattractive from an environmental perspective unless the CO2 emissions can be ameliorated. Another viable option would be to generate more electricity from nuclear power, but here there are societal concerns about safety, nuclear waste disposal, and security. So what are the alternatives to oil, gas, coal, and nuclear power? Renewable energy sources, such as solar, wind, and hydroelectric power, have low carbon emissions but currently contribute only about 6 percent to the energy supply in the United States. Clearly there will have to be major changes in U.S. energy policy if the nation is to make a significant effort to reduce greenhouse gas emissions. The CMMP community cannot solve the political and economic issues that surround the energy challenge, but it can aspire to creating technological breakthroughs that will provide lawmakers with more options. For example, progress in the area of CO2 sequestration could substantially reduce the environmental impact of using coal for power generation, and the development of closed-loop nuclear fuel cycles could mitigate concerns about nuclear waste disposal.10 Improvements in the efficiency of solar cells may make the price of solar energy competitive with that of energy from fossil fuels, and success in the areas of hydrogen generation and storage could make the hydrogen economy a reality. The possible use of hydrogen as a fuel is complex from an emissions standpoint, because it depends on how the hydrogen is produced. About 95 percent of the current hydrogen production derives from natural gas, so the carbon content of the natural gas ultimately ends up as CO2. The greater efficiency of fuel cells compared with internal combustion engines mitigates the level of emissions, but hydrogen must be efficiently generated from solar energy or by electrolysis based on hydroelectric or nuclear power to be a viable fuel. The use of ethanol or diesel fuel from natural sources is similar in that, while these sources are renewable, the required processing and infrastructure will nevertheless contribute greenhouse gas emissions. An exciting long-term goal in the fuels area would be to make renewable liquid fuels from CO2 and sunlight through a water-gas shift reaction with carbon monoxide as an intermediate, but many difficult obstacles remain in this pathway. With respect to the energy challenge, an area of great societal concern, the CMMP community has an exciting opportunity to make major contributions based on new materials, new systems, and advanced computation and modeling. For it to be able to do so, it is necessary that there be a broad research investment strategy, based on multiple energy technologies: oil, gas, coal, solar, wind, nuclear, biofuels, hydroelectric, thermoelectric, and so forth. This is, of course, an interdis- 10 For a description of research and development needs, see the Technology Roadmap for Generation IV Nuclear Energy Systems, available at http://gif.inel.gov/roadmap/pdfs/gen_iv_roadmap.pdf; last accessed September 17, 2007.
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Condensed-Matter and Materials Physics: The Science of the World Around Us ciplinary challenge that will require close cooperation between CMMP and other disciplines. In addition, given that the energy challenge is clearly a global issue, the area also presents an opportunity for international collaboration on these different technologies. Medicine and Health Care Condensed-matter and materials physics has a long history of seeding not only developments in fundamental biology—the progression from the early work on x-ray diffraction from crystals to the invention of molecular biology comes to mind here—but also in the practice of medicine. This trend has been accelerating, involving more people and entailing larger consequences over the past decade. While this trend is well known, it is still worth noting a few examples of recent progress. In particular, there has been widespread adoption of reliable home pregnancy tests based on gold nanoparticles, routine magnetic resonance imaging—employing ever-improving superconducting magnets and other hardware and software, but also nanoparticle-based contrast enhancement—of everything from tumors to brain function, and the development of new materials to increase the lifespan of surgical implants. The committee expects similarly rapid progress in the coming decade, especially given the growing emphasis on interdisciplinary and translational (laboratory bench to bedside) research at universities and hospitals. Important areas will be nanotechnology, photonics, novel x-ray sources and optics, superconducting devices, and tissue engineering. Nanotechnology is of particular interest, as it has captured the public imagination with its tremendous potential for diagnostics, public health, and therapeutics. Indeed, two of the examples listed in the preceding paragraph as recent advances demonstrate that nanotechnology has already arrived in clinical medicine. Today, there are journals with “Nanomedicine” in their titles, and a look at such journals as well as more established publications shows the huge scope of activity, on themes ranging from the targeting of tumors by functionalized nanoparticles to single (or few)-molecule detection using nanomechanical and nanophotonic devices instead of chemical amplification. The reverse side of therapeutics is toxicology, which will be essential for the further take-up of nanotechnology, and especially nanoparticles, in the clinic and elsewhere. The underlying science here will be cell physiology at the nanoscale, and many of the tools to be used, including electron and scanning probe microscopes, will be provided by condensed-matter physics. At a more general level, health care is facing several major simultaneous challenges. The most important is to bring affordable personalized health care to all, thus finally reaping the fruits of the genomics revolution. Many secondary but nonetheless large challenges also exist, ranging from the drug pipeline crisis in
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Condensed-Matter and Materials Physics: The Science of the World Around Us the pharmaceutical industry to the evolution of bacteria to circumvent antibiotics. While it would clearly be an exaggeration to claim that CMMP will play the leading role in meeting these challenges, its contributions will nevertheless be very important. Sensors embodying novel transduction mechanisms, such as field-effect transistors with chemical modulation of gate voltages, micro- and nanofluidic reactors made according to recipes developed for the computer and telecommunications industries, and sensitive but inexpensive magnetometers are among the many technologies that condensed-matter physicists and materials scientists will be called on to develop for clinical and pharmaceutical use. Indeed, what the committee foresees is a merger of the health care and information technology sectors, with the mobile phone network eventually being the platform for diagnostics and the real-time management of therapies. The hope is that there will finally be a Moore’s law for health care provision, with productivity increases finally exceeding spending increases as insatiable demands for ever-better prevention and treatment are met. Because of the associated demands for miniaturization and quantitative rigor as well as the fact that medicine ultimately deals with the materials systems of life, there are tremendous opportunities for contributions from CMMP, and especially from subfields such as soft matter and nanometrology. There are also less immediately obvious subfields that will contribute to meeting the needs of health care; these include even the area of devices for quantum encryption, which might find use in ensuring the confidentiality of the exchange of patient records. IMPACT ON OTHER SCIENTIFIC DISCIPLINES CMMP plays a vital role in other disciplines of science in two ways. First, CMMP technologies, such as materials and devices ranging from nonlinear organic materials to charge-coupled-device detectors, are ubiquitous in laboratories throughout the scientific enterprise. Second, as science expands and the disciplinary boundaries blur, concepts originating in CMMP—for example, fermion pairing or the statistical mechanics of biological molecules (Chapter 4)—find increasing relevance. Examples of the impact of CMMP on other subfields of physics and on other disciplines are illustrated by atomic, molecular, and optical (AMO) physics; nuclear and high-energy physics; astronomy; chemistry; biology; and information technology and computer science. Atomic, Molecular, and Optical Physics Scientific and technological connections between CMMP and atomic, molecular, and optical physics are historically strong, principally because of similarities between the energy and length scales of the two fields. Laser and optical technol-
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Condensed-Matter and Materials Physics: The Science of the World Around Us ogy, developed primarily within the AMO community, has wide application in CMMP research for materials characterization and processing. While lasers are the most prominent and wide-reaching connection so far, the recent development of methods to trap and cool atoms into the nano-kelvin regime also has the potential to affect CMMP profoundly, by enabling the realization of some of the most fundamental models of condensed-matter physics.11 Lasers have long been used to characterize materials by a variety of techniques. Photoluminescence spectroscopy, for example, is widely employed to measure band gaps and to identify defects and impurities. Sensitivities down to the single-molecule level have been achieved. Ultrafast laser technology, with pulse durations in the femtosecond regime, can reveal critical information about charge carrier dynamics, such as relaxation processes, needed for the characterization of semiconductor materials for the potential application of these materials in devices. Femtosecond pulses are also used in time-resolved photoemission spectroscopy to determine surface states and relaxation processes in solids, and they can be used to study structural phase transitions in real time. Bound electron-hole pairs, known as excitons, may be created by the laser excitation of semiconductors and their dispersion properties measured using laser spectroscopy. Laser-induced light scattering is also commonly used to characterize the distribution of particles in random and diffuse media. Laser technology is routinely employed in CMMP research to process and anneal materials. While CMMP benefits from laser technology developed by the AMO community, optical materials developed by CMMP scientists are essential to many laser applications. Nonlinear optical materials, for example, have profoundly affected almost all applications of lasers. Newly developed organic and polymer nonlinear materials, a forefront area of investigation in CMMP, are being integrated with semiconductors and used for optical information processing and data storage. Fundamental connections between the AMO and CMMP communities have grown recently owing to developments in ultracold atomic research. In the past 10 years, laser cooling and evaporation techniques have been employed by AMO physicists to cool composite atomic bosons, such as 87Rb and 7Li, as well as the fermions 40K and 6Li. Temperatures in the nano-kelvin regime are routinely achieved. At these temperatures, quantum-state occupancy is of order unity, and the gases may undergo Bose-Einstein condensation or fermion pairing into a superfluid state, phenomena long ago discovered and explained by CMMP scientists in the context of superconductivity and superfluidity of liquid helium 4He and 3He. An emerging theme is that systems of ultracold atoms may be used as idealized and highly tunable model systems for fundamental problems in condensed- 11 For additional detail, see National Research Council, Controlling the Quantum World: The Science of Atoms, Molecules, and Photons, Washington, D.C.: The National Academies Press, 2007.
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Condensed-Matter and Materials Physics: The Science of the World Around Us matter physics. Their utility as model systems derives from the fact that they are inherently “clean,” with no uncontrolled defects, impurities, or disorder, and that their physical properties, such as density, temperature, geometry, and perhaps most importantly, interaction strength, are widely tunable. By imposing an optical lattice formed by the interference of laser beams, periodic potentials in various dimensions and geometries may be created, thus simulating an underlying crystal lattice (Figure 8.1). Spurred by prior discoveries in CMMP, recent experiments with ultracold atoms have been remarkably successful in modeling important problems in condensed-matter physics, including the observation of the superfluid to Mott insulator transition, the Bose-Einstein condensation to Bardeen-Cooper-Schrieffer (BEC-BCS) crossover, vortex lattices, and the Josephson effect. Several of these are examples of emergent phenomena and are discussed in more detail in Chapter 2. The Mott insulator, which arises in periodic structures with strong on-site repulsion, is an example of strong particle correlations resulting in a dramatic change in the macroproperties of the gas. It was realized with cold atoms by imposing an optical lattice onto a Bose-Einstein condensate. The BEC-BCS crossover in paired fermions is a concept going back more than 25 years in the CMMP community, where it was first realized that there is an intimate connection between BCS pairing, the underlying mechanism of superconductivity and superfluidity in 3He, and Bose-Einstein condensation of composite bosons that are created by binding two fermions. Although apparently quite different, the BEC and BCS regimes are just the extreme limits of a continuum. In the BEC limit, the superfluid corresponds to a condensation of tightly bound bosonic molecules, while in the BCS limit, pairing is a many-body phenomenon involving correlated but spatially diffuse pairs of fermions of opposite spin. Probing the crossover requires that the strength of interaction between particles be varied, which can be accomplished with ultracold atoms using a Feshbach resonance. The effect of pairing when the two spin states have unequal Fermi energies has long been of interest to CMMP. Exotic new phases with broken-space symmetries FIGURE 8.1 Optical lattices in (a) one dimension, (b) two dimensions, and (c) three dimensions may be used to create arrays of confined atoms in lower dimensions. SOURCE: E. Mueller, Cornell University.
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Condensed-Matter and Materials Physics: The Science of the World Around Us were predicted more than 40 years ago but have been difficult to confirm owing to the fundamental incompatibility of magnetism with the usual forms of superconductivity. In contrast, making unbalanced mixtures of atomic Fermi gases is straightforwardly accomplished by driving radio-frequency transitions between the states. New experiments have begun to explore the phase diagram. The relevance of these experiments goes beyond CMMP, as unbalanced pairing is expected to be an important ingredient in the quark-gluon plasmas formed in high-energy collisions of heavy nuclei and could also play a central role in understanding neutron stars (see the following subsection on connections between CMMP and nuclear and high-energy physics). As discussed in Chapter 2, strong correlation effects often manifest themselves in lower dimensions, where particle interactions can more easily dominate the kinetic energy. Atoms confined to lower dimensions can be achieved using various optical lattice configurations. For example, two counter-propagating laser beams can divide a cloud into a number of quasi-two-dimensional sheets (Figure 8.1b). The two-dimensional Kosterlitz-Thouless transition, a transition to a superfluid state without Bose-Einstein condensation previously studied in CMMP in connection with interacting spin systems, was realized with cold atoms using this configuration. Four lasers in a plane can be used to make a square array of ultracold atoms confined to quasi-one-dimensional tubes (Figure 8.1a), which has promise to realize the exactly solvable model of an interacting one-dimensional gas of fermions. One of the most significant outcomes of strong correlations is the emergence of quasi-particles that are no longer the single-particle-like excitations of the conventional Landau-Fermi liquid theory, as discussed in Chapter 2. A one-dimensional Fermi gas, which should exhibit a separation of the single-particle spin and charge degrees of freedom, may provide a highly idealized system for studying this remarkable effect. One of the most significant opportunities for CMMP would be the realization of the Hubbard model, in which particles can hop from lattice site to lattice site, interacting repulsively on doubly occupied sites. While the Hubbard model is the most prominent model of high-temperature superconductors, it is still not known for sure whether it reproduces the essential properties of these materials. One can envision that systems of ultracold atoms, acting as highly tunable quantum simulators, can resolve this and other similarly vexing issues in CMMP in the near future. These growing connections between CMMP and AMO physics have blurred the boundaries where these two fields intersect. Increasingly, young CMMP physicists, particularly theorists, are engaging in ultracold atom research. Similarly, AMO physicists are applying their expertise to CMMP-related investigations. An indication of the growing interconnection is that an increasing fraction of the program of the American Physical Society’s annual March Meeting, the traditional venue for presenting CMMP research, is being devoted to ultracold atom physics. Federal
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Condensed-Matter and Materials Physics: The Science of the World Around Us funding agencies, in particular the Department of Defense, have recognized the intellectual fervor developing at this interdisciplinary boundary by establishing new funding initiatives. Some university physics departments have already introduced new graduate-level courses to prepare students for interdisciplinary CMMP/AMO research, and more are in development. As in other areas of scientific research, interdisciplinary research can be very productive scientifically, but it also challenges the existing infrastructure to adapt to changes. Nuclear and High-Energy Physics CMMP has had a significant impact on nuclear and particle physics over the years, ranging from the development of advanced detector materials to fundamental studies of nuclei, neutron stars, and elementary particles. Examples of detector technology made possible by CMMP research include ultrapure silicon wafers used for charged-particle detection, avalanche photodiodes, and large, very pure crystal scintillation materials. Such connections are not unexpected, since nuclei, as well as the vacuum, as complex many-body systems, present intellectual challenges with many similarities to those of condensed-matter physics. For example, the shell structure of nuclei owes its existence to the Pauli exclusion principle lengthening mean free paths of nucleons near the Fermi surface, in precisely the same way that electrons in normal metals often behave as nearly free particles. Indeed, corrections to free-particle behavior, seen in experiments on nuclei in which particles are removed or added, are understood in the same way as in condensed-matter systems such as metals. The discovery of the BCS theory of superconductivity had an immediate impact on nuclear physics: Cooper pairing of neutrons and of protons explained many features of the single-particle excitation levels, as well as the larger-than-expected spacing of the energies of states of rotating nuclei in terms of a reduced moment of inertia—the analog of the Meissner effect in superconductors. In neutron stars—in essence, giant nuclei with masses somewhat greater than that of the Sun, and the driving engines of pulsars and related high-energy astrophysical phenomena—the neutron liquid is expected to be superfluid, and the proton liquid, superconducting. The superfluidity in rotating stars gives rise to vortices, as in superfluid liquid helium, whose sporadic motions explain the observed sudden speedups (glitches) of pulsars. Indeed, elucidation of the properties of neutron stars, as well as of other astrophysical systems including the early universe, has depended considerably on techniques—for example, transport theory—previously developed for condensed-matter systems. The ideas of BCS theory have also been extended to quark-gluon plasmas, the liquid of quarks and gluons that constituted matter in the early universe prior to a few microseconds after the big bang, the matter that is produced in ultrarelativ-
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Condensed-Matter and Materials Physics: The Science of the World Around Us istic heavy-ion collisions, and that may possibly be present in neutron stars. Cold quark-gluon plasmas are predicted to have a rich structure of Cooper-paired states, which could influence observed properties of neutron stars. More generally, the concept of the spontaneous breaking of symmetries, which arose in describing condensed-matter systems such as antiferromagnets and superconductors, has come to play a dominant role in elementary particle physics. For example, the internal symmetry of the standard model of strong and electroweak interactions is spontaneously broken as the temperature in the early universe falls. This breaking leads to a finite expectation value of the Higgs field in the vacuum, which is responsible for the masses of the fundamental quarks, vector bosons, and leptons—analogous to the way the Anderson modes of a superconductor develop a finite frequency gap in a superconductor. The intellectual flow between work on condensed-matter and nuclear systems has in fact been in both directions. Research in one area frequently inspires new insights and approaches in other areas. For example, work on vortices in neutron stars has led to new insights into Bose-Einstein condensates, superfluids, and superconductors. Similarly, studies of neutron matter have led to new insights into the behavior of strongly coupled atomic clouds, and into the general properties of superfluid Fermi gases. Current areas of study include questions about the similarity of the transition between Bose-Einstein condensation and BCS pairing in condensed matter and the deconfinement transition of dense hadronic matter; the observed low viscosity of strongly interacting quark-gluon plasmas and condensed atomic clouds; and BCS pairing of imbalanced Fermi seas in superconductors, trapped fermionic atomic clouds, and quark-gluon plasmas. In addition, expertise developed for condensed-matter systems, such as quantum Monte Carlo computational techniques and techniques of many-body theory at finite temperatures, have played important roles in modern nuclear physics, leading, for example, to exact Green’s function Monte Carlo calculations of the levels of all nuclei up to mass number eight. Astronomy The contribution of CMMP to astronomy and experimental astrophysics is hard to overstate. Virtually every modern telescope, whether observing gamma rays, visible photons, or radio waves, has a sophisticated solid-state detector at its focus. At optical wavelengths, silicon charge-coupled-device (CCD) arrays long ago supplanted photographic plates, in much the same way that they have recently done in ordinary photography. In the millimeter band (which includes the cosmic microwave remnants of the big bang and accounts for most of the electromagnetic energy in the universe), semiconductor diode detectors and, more recently, superconducting transition-edge sensors are used for photometric observations.
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Condensed-Matter and Materials Physics: The Science of the World Around Us Heterodyne mixers based on superconducting tunnel junctions are now in wide use for high-resolution studies of molecular absorption lines. At very high energies, NASA’s Chandra X-ray Observatory, launched aboard the space shuttle in 1999, uses silicon CCD detectors to study gamma-ray bursts and other spectacular astronomical events. The catalog of the various solid-state detectors in use in modern astronomy is a very long one, and a thorough review would be inappropriate here. Instead, a single new detector concept that promises to have a large impact on astronomy in the coming decade is discussed: the so-called kinetic inductance detector (KID). The KID is a particularly appropriate example, since it relies on aspects of non-equilibrium superconductivity that themselves are not far from the research cutting edge in CMMP. The heart of a KID consists of a small strip of superconducting metal forming the inductor of a resonant “tank” circuit embedded in a conventional transmission line. On resonance (at typically ~10 GHz), the tank circuit loads down the transmission line and reduces its transparency, an easily detected effect. A remarkable property of a superconductor is that the inertia of moving Cooper pairs within it contributes significantly to its inductance. Incoming photons, with energies larger than the superconducting energy gap, tear apart a number of Cooper pairs and thus modify this inductance. This in turn shifts the resonant frequency of the tank circuit and changes the transmission through the line. The KID concept has a number of key advantages. Perhaps most important is its straightforward extension into an array geometry. Pixelated array detectors are essential in modern astronomy and astrophysics, in part because in many cases the noise in individual detector elements is actually due to photon-counting statistics, not to external amplifier noise. Detector arrays gather more photons and thus increase the net signal-to-noise ratio. The resonant tank circuit and transmission line of the kinetic inductance detector are readily fabricated by conventional lithographic means. It is therefore easy to embed many resonant circuits in the same transmission line, each designed with a slightly different resonant frequency. In this way a detector array can be made, and each “pixel” can be rapidly addressed via a simple frequency multiplexing scheme. In addition, kinetic inductance detectors are sensitive over an extremely broad energy band, from the very low energies characteristic of the cosmic microwave background to the very high energies encountered in x-ray astronomy. The connections between CMMP and astronomy and astrophysics are certainly not limited to detector technology. There are many fundamental scientific connections as well. (Connections between CMMP and astronomy, astrophysics, and cosmology are discussed in the previous subsections.) For example, the classic CMMP concepts of superfluidity and superconductivity are quite relevant to the internal structure and dynamics of neutron stars. Similarly, the notion of
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Condensed-Matter and Materials Physics: The Science of the World Around Us spontaneous symmetry breaking discussed in Chapter 2 in the context of phase transitions in condensed matter is also a cornerstone of our understanding of the very early universe, just moments after the big bang. Finally, neutron stars and other astrophysical objects are, in effect, laboratories for the study of materials under conditions (ultrahigh density, magnetic field, and so forth) that no terrestrial laboratory will ever replicate. Chemistry Advances in a number of areas of CMMP have had an important impact on developments in chemistry, especially in the areas of materials chemistry and physical chemistry. In the area of computation, the importance of density functional theory—which is now used extensively by chemists to calculate the electronic structures of materials and by polymer physicists to calculate the structure of polymer molecules in solutions and melts—was recognized by the sharing of the Nobel Prize in chemistry by physicist Walter Kohn in 1998. In the field of soft condensed matter, theoretical concepts and methods from statistical physics such as percolation theory, renormalization group, scaling, (self-consistent) field theory, disordered systems (quenched disorder, spin glasses), and to some extent liquid-state physics, have had an enormous impact on macromolecular science. Ideas taken from the understanding of systems such as Ising models and magnetism, for example, are now applied to the interpretation of phase transitions in polymer mixtures and block copolymers. CMMP has also had a very great impact on the chemistry of materials through the development of advanced characterization tools, such as light sources for synchrotron x-ray diffraction and extended x-ray absorption fine structure studies, neutron sources for diffraction, spectroscopic and small-angle scattering studies, and advanced transmission electron microscopes for high-resolution imaging. Instruments based on the scanning tunneling microscope (STM), for which Rohrer and Binnig won the Nobel Prize in physics in 1986, have made key contributions to the chemistry of nanomaterials in recent years. In particular, the atomic force microscope (AFM), which uses broadly the same principle as that used by the STM but does not require the sample to be conducting, is now used ubiquitously by chemists and others to obtain images of surfaces with atomic resolution. Sometimes, of course, the fundamental developments in physics take many years to make their mark. The discovery of nuclear magnetic resonance (Nobel Prize in physics, 1952) only gradually developed into one of the most powerful characterization tools in organic chemistry, polymer science, and molecular biology, but finally led to several Nobel Prizes in chemistry (e.g., 1991, 2002), as well as the 2003 Nobel Prize in physiology or medicine for the development of magnetic resonance imaging (MRI).
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Condensed-Matter and Materials Physics: The Science of the World Around Us The symbiotic nature of the relationship between CMMP and chemistry is reflected in the fact that synthetic chemists have enabled some of the most exciting advances in condensed-matter physics in recent years. Advances in conducting polymers and molecular electronics over the past 25 years were initially launched by the synthesis of polyacetylene films in the laboratory of Shirakawa in Japan. Subsequent collaborations between physicists and chemists led to the electrical characterization of this and related materials, and the resulting explosion of work in the area is now impacting technologies as disparate as biosensors and photovoltaic cells. Similarly, the discovery of buckminsterfullerene, C60, in the Chemistry Department at Rice University in 1985 (Nobel Prize in chemistry, 1996) provided the impetus for the identification of carbon nanotubes in 1991 and arguably for the dramatic advances in nanoscience witnessed during the past decade. In the hard condensed-matter area, the lanthanum cuprate high-temperature superconductors, for which Bednorz and Muller received the 1987 Nobel Prize in physics, were first synthesized by chemists in France and Russia in the 1970s and 1980s, although the remarkable electronic properties of these materials were not appreciated at the time. The same can be said for magnesium diboride (MgB2), which was synthesized by chemists as early as 1954, but it was not until 2001 that the exciting superconducting properties of MgB2 were revealed; 6 years later it is already being developed for use in a new generation of superconducting magnets for MRI. Physics and chemistry will continue to be inextricably linked, and many exciting future discoveries in the CMMP area will be found at the interface between these two important fields. It is essential that this interface be nurtured by bringing chemists and physicists together at interdisciplinary research centers such as the NSF Materials Research Science and Engineering Centers and the Department of Energy Nanoscale Science Research Centers. It is also important to recognize the role of national facilities, such as the neutron and light sources, in providing opportunities for scientists from different disciplines to discuss their findings and different perspectives on CMMP research. Finally, the committee would like to stress the importance of such interfaces in education, as discussed elsewhere in this report. Biology The experimental methods of CMMP have had an enormous impact on biology and medicine, and the committee expects that this will continue and grow in the coming decade. While the discussion of the challenges to CMMP researchers in biology (Chapter 4) emphasized new questions that physicists ask about living systems, this subsection focuses on the application of CMMP techniques to questions posed by biologists; for the impact of CMMP on medicine and health care, see the earlier discussion in this chapter.
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Condensed-Matter and Materials Physics: The Science of the World Around Us One of the lasting legacies of the revolution that created molecular biology is the idea that biological function is linked tightly to molecular structure; nowhere is this more obvious than in the case of deoxyribonucleic acid (DNA). This link makes the determination of molecular structures a central problem in biology. Roughly 50 years have passed since the first protein structures were solved using x-ray crystallography. Today, many biologists look forward to the day when they will be able to determine the structure of all proteins at atomic resolution. This grand effort, sometimes referred to as structural genomics, is conceivable only because of dramatic improvements in the performance of synchrotron light sources (see Chapter 11). The increased brilliance of these sources means that data collection is faster, that radiation damage is reduced, and that smaller crystals are sufficient. At the same time, a number of groups are investigating the physics of protein crystallization itself, in an attempt to increase the efficiency of this most problematic step in sample preparation. Finally, as with all fields of science, mastery over the physics of semiconductor devices has driven the dramatic improvements in computational power that make solving a complex molecular structure a routine calculation. Thus, the biologists’ dream of a complete catalog of biomolecular structures is being enabled by the technology generated in the CMMP community. For the CMMP community, some of the most striking aspects of magnetic resonance involve relaxation processes: how spins exchange energy with each other and with their surroundings, and how these interactions build a bridge between the coherent quantum dynamics of isolated spins and the dissipative behavior of macroscopic samples as they come to thermal equilibrium. It is the understanding of these relaxation processes that has made magnetic resonance such a useful tool in investigating biological systems. In proteins, the fact that energy transfer or cross-relaxation between two spins depends on their spatial separation means that relaxation experiments can measure, more or less directly, the distance between atoms even as the protein tumbles freely in solution. These distance measurements can be combined to generate accurate three-dimensional structures with nearly the same accuracy as that of x-ray diffraction but without the need to form crystals. Improvements in high magnetic field techniques will extend the range of applicability of these methods to yet larger molecules. The bulk of any living organism is water, and the easiest magnetic resonance signal to detect is thus from the water protons. But the relaxation dynamics of these protons is sensitive to their environment, responding even to relatively subtle changes such as the oxygen content of blood. The beautiful brain images seen even in the popular press are derived from this subtle effect: when cells in a particular region of the brain are more active, they use more oxygen, and the change in oxygenation level of the blood flowing to these regions changes the relaxation time of the proton spins. Images of spin-relaxation time thus provide an image of neural activity, literally showing which regions of the brain are involved in particular
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Condensed-Matter and Materials Physics: The Science of the World Around Us tasks or even particular thoughts. Functional magnetic resonance imaging, as it is called, has evolved in roughly 15 years from a physics experiment into a standard technique for psychology laboratories. While knowing which areas of the brain are involved in a process does not describe how things work, the images that emerged from this work have completely changed the language for discussion of cognition, and in this sense are beginning to transform our view of ourselves as humans. Optical microscopy is a venerable technique that has undergone a renaissance in the past decade, especially in its application to biological systems. Lasers have made confocal microscopy commonplace, and scanning multiphoton fluorescence microscopies have made it possible to reach deep into tissues such as the brain and observe dynamics on the micron and submicron scales, revealing, for example, the continual making and breaking of connections between neurons as animals learn about their environment. Microscopy using evanescent waves makes it possible to focus on events within 100 nanometers of the cell surface, and near-field scanning probes provide even higher resolution. Recent developments combine scanning microscopy with photo-switchable probes to literally count every molecule of a given class. Related ideas combine surface microscopies (either optical or electron microscopy) with laser ablation to provide detailed, three-dimensional structures of fixed tissue, holding out the potential to revolutionize the study of anatomy in general and the “wiring diagram” of the brain in particular. Information Technology and Computer Science In this subsection, the intellectual connections between CMMP and information technology and computer science are discussed. The economic connections are discussed in Chapter 7 and earlier in this chapter. CMMP has contributed enormously to the development of devices for information technology; it has made modern computers possible. These contributions are outlined further in Chapter 7. Less obviously, statistical physics concepts developed in CMMP in order to understand complex and emergent phenomena in materials have also contributed to the development of computer science. The Metropolis algorithm and Monte Carlo methods, which compute properties by the judicious sampling of possibly favorable configurations, were first used in nuclear physics and further developed in condensed-matter physics. They were the first of a broad class of methods than can be used to find approximate but very accurate solutions to difficult search problems. More sophisticated search algorithms followed, motivated by the need to analyze systems which have many distinct and unrelated configurations that are comparably favorable. The simulated annealing algorithm of Kirkpatrick, Gelatt, and Vecchi is a generalization of a Monte Carlo method, inspired by slow cooling techniques in crystal growth, for examining the possible states of many-body
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Condensed-Matter and Materials Physics: The Science of the World Around Us systems. In this method, once the system has found one favorable configuration, it can draw energy from a thermal reservoir to jump away and search further for more favorable configurations of distinct character. Such Monte Carlo methods have since been applied to classic optimization problems in computer science such as the traveling salesman problem. The Swendsen-Wang algorithm, another method for making large jumps to explore distinct classes of configurations to identify favorable ones, has been applied to problems such as graph partitioning and computer vision. Studies of the spin glass problem—the problem of calculating the lowest energy state of a disordered system—have contributed to understanding in the theory of computational complexity. For a spin glass, like many other so-called nondeterministic polynomial problems, the running time of all known algorithms increases exponentially with the size of the problem, and the challenge is to determine if there is no algorithm for which the running time is polynomial rather than exponential. Correlation and scaling laws from statistical mechanics are used to describe and understand the structure and emergent behaviors of large computer networks. Concepts of self-organized collective behavior from CMMP are now at the frontier of computer science and robotics, as researchers strive to find general rules for the emergence of sophisticated collective behavior from networks of computing agents interacting by simple rules. Finally, it has become clear in recent years that the theory of the transmission and processing of intact quantum states represents a profound extension of the classical theories of information and computation, significantly altering the assessment of the kind and quantity of physical resources needed to solve various computational problems. The developing theory already has some applications to cryptography, and, if general-purpose quantum computers can be built, a large class of optimization problems will be solvable in a time proportional to the square root of the time presently required. A few problems, such as factoring of large integers, would be sped up even more. Thus, the theoretical study of quantum computation has become a new and vital branch of both computer science and theoretical physics, and the CMMP research community is making enormous contributions to the development of the new devices that will be required in the future for practical quantum computing. INTERDISCIPLINARY RESEARCH IN CMMP Increasingly, the nature of CMMP research is becoming more interdisciplinary, and its scope is broadening. CMMP researchers are jointly working with other physicists in areas such as atomic, molecular, and optical science and particle physics, and with researchers in other disciplines such as chemistry, biology, and astronomy. CMMP approaches are being applied to problems ranging from
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Condensed-Matter and Materials Physics: The Science of the World Around Us energy conversion to information technology to biological systems and other far-from-equilibrium systems such as the climate. If the United States is to continue to be a leader in these scientifically and economically important areas, funding agencies should support the emerging interdisciplinary research that underpins them. However, the current organizational structure at funding agencies is based on subfield boundaries established decades ago. This structure hinders individual researchers from venturing into nontraditional, rapidly evolving areas. The first of the committee’s two recommendations below is intended to promote more efficient approaches toward advancing emerging interdisciplinary research areas. This recommendation is further supported by and discussed in a general context in the National Research Council report Facilitating Interdisciplinary Research.12 RECOMMENDATIONS Recommendation: Funding agencies should work to develop more-effective approaches to nurturing emerging interdisciplinary areas for which no established reviewer base now exists. The CMMP community should organize sessions at national meetings to engage funding agencies and the community in a dialogue on best practices for proposal review and for the support of nontraditional, rapidly evolving areas. Recommendation: Outreach, K-12, and undergraduate science education initiatives should be supported through supplemental or stand-alone grants administered by separate National Science Foundation and Department of Education programs, instead of through individual research grant awards. In the present system, the quality of outreach programs is a criterion in the evaluation of NSF/Division of Materials Research grants. The present approach confuses two conceptually distinct goals to the point that neither is optimally served. The funding agencies and the research community both want outreach programs to succeed, and they should confer to determine how best to implement an effort to achieve that goal. 12 National Academy of Sciences, National Academy of Engineering, and Institute of Medicine, Facilitating Interdisciplinary Research, Washington, D.C.: The National Academies Press, 2005.