Annual Review of Condensed Matter Physics - Volume 2, 2011
Volume 2, 2011
- Preface
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Reflections on My Career in Condensed Matter Physics
Vol. 2 (2011), pp. 1–9More LessI present some personal views on the field of condensed matter physics (CMP), primarily as a long-time practitioner, but also as a physicist who has had the opportunity to play broader roles in scientific affairs. I comment on how CMP has evolved in the past half century, and express my optimism for its future.
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The Ubiquity of Superconductivity
Vol. 2 (2011), pp. 11–30More LessAfter a brief review of the phenomenology of superconductivity and of its generic explanation in terms of the concept of off-diagonal long-range order, I first survey the original Bardeen-Cooper-Schrieffer (BCS) weak-coupling model and some extensions of it. I then turn to systems such as the heavy fermions and the cuprates where the superconductivity is generally believed to be due to an all-electronic mechanism, and ask how much information we get about this mechanism from quite general energetic and other considerations, without committing ourselves to any particular microscopic model.
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The Quantum Spin Hall Effect
Vol. 2 (2011), pp. 31–53More LessMost quantum states of condensed matter are classified by the symmetries they break. For example, crystalline solids break translational symmetry, and ferromagnets break rotational symmetry. By contrast, topological states of matter evade traditional symmetry-breaking classification schemes, and they signal the existence of a fundamentally different organizational principle of quantum matter. The integer and fractional quantum Hall effects were the first topological states to be discovered in the 1980s, but they exist only in the presence of large magnetic fields. The search for topological states of matter that do not require magnetic fields for their observation led to the theoretical prediction in 2006 and experimental observation in 2007 of the so-called quantum spin Hall effect in HgTe quantum wells, a new topological state of quantum matter. In this article, we review the theoretical foundations and experimental discovery of the quantum spin Hall effect.
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Three-Dimensional Topological Insulators
Vol. 2 (2011), pp. 55–78More LessTopological insulators in three dimensions are nonmagnetic insulators that possess metallic surface states (SSs) as a consequence of the nontrivial topology of electronic wavefunctions in the bulk of the material. They are the first known examples of topological order in bulk solids. We review the basic phenomena and experimental history, starting with the observation of topological insulator behavior in BixSb1−x by angle and spin-resolved photoemission spectroscopy (spin-ARPES) and continuing through measurements on other materials and by other probes. A self-contained introduction to the single-particle theory is then given, followed by the many-particle definition of a topological insulator as a material with quantized magnetoelectric polarizability. The last section reviews recent work on strongly correlated topological insulators and new effects that arise from the proximity effect between a topological insulator and a superconductor. Although this article is not intended to be a comprehensive review of what is already a rather large field, we hope that it serves as a useful introduction, summary of recent progress, and guideline to future directions.
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Unconventional Quantum Criticality in Heavy-Fermion Compounds
O. Stockert, and F. SteglichVol. 2 (2011), pp. 79–99More LessWe review magnetic quantum-critical points (QPCs) in heavy-fermion compounds separating at zero temperature: an antiferromagnetically ordered state and a nonordered ground state. At the magnetic instability, the Fermi-liquid (FL) description valid for normal metals breaks down, giving rise to unusual, non-Fermi-liquid (NFL) low-temperature behavior. After a short introduction to phase transitions and to T = 0 phase transitions in general as well as to the physics of heavy-fermion systems, the two main theoretical scenarios describing the physics at QPCs in these systems are presented, the conventional spin-density-wave (SDW) scenario and the unconventional Kondo-breakdown scenario. Whereas for the conventional scenario experimental data for CeCu2Si2 and Ce1−xLaxRu2Si2 are discussed only briefly, we focus in more detail on the unusual behavior of CeCu6−xAux and YbRh2Si2 at their respective QPCs and show that these systems are best described within the unconventional scenario.
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Electronic Transport in Graphene Heterostructures
Vol. 2 (2011), pp. 101–120More LessThe elementary excitations of monolayer graphene, which behave as massless Dirac particles, make it a fascinating venue in which to study relativistic quantum phenomena. One notable example is Klein tunneling, a phenomena in which electrons convert to holes to tunnel through a potential barrier. However, the omnipresence of charged impurities in substrate-supported samples keep the overall charge distribution nonuniform, obscuring much of this “Dirac” point physics in large samples. Using local gates, one can create tunable heterojunctions in graphene, isolating the contribution of small regions of the samples to transport. In this review, we give an overview of quantum transport theory and experiment on locally gated graphene heterostructures, with an emphasis on bipolar junctions.
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Materials and Novel Superconductivity in Iron Pnictide Superconductors
Hai-Hu Wen, and Shiliang LiVol. 2 (2011), pp. 121–140More LessSince the first report on superconductivity at 26 K in F-doped LaOFeAs at the end of February 2008, the iron pnictide superconductor family has been quickly expanded to six different structures, and the superconducting transition temperature has been rapidly raised to approximately 57 K. Meanwhile, the pairing mechanism has been clarified as nonconventional, and the antiferromagnetic (AF) spin fluctuations (SFs) seem to play important roles in making these pairs. This article covers a brief overview of the materials and novel superconductivity in this fascinating field, including issues concerning the materials and structures, phase diagram, pairing symmetry, and spin excitations. We give perspectives on these topics and raise questions instead of simply summarizing the theoretical and experimental achievements about the superconducting mechanism.
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Interface Physics in Complex Oxide Heterostructures
Vol. 2 (2011), pp. 141–165More LessComplex transition metal oxides span a wide range of crystalline structures and play host to an incredible variety of physical phenomena. High dielectric permittivities, piezo-, pyro-, and ferroelectricity are just a few of the functionalities offered by this class of materials, while the potential for applications of the more exotic properties like high temperature superconductivity and colossal magnetoresistance is still waiting to be fully exploited. With recent advances in deposition techniques, the structural quality of oxide heterostructures now rivals that of the best conventional semiconductors, taking oxide electronics to a new level. Such heterostructures have enabled the fabrication of artificial multifunctional materials. At the same time they have exposed a wealth of phenomena at the boundaries where compounds with different structural instabilities and electronic properties meet, giving unprecedented access to new physics emerging at oxide interfaces. Here we highlight some of these exciting new interface phenomena.
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Mott Physics in Organic Conductors with Triangular Lattices
Kazushi Kanoda, and Reizo KatoVol. 2 (2011), pp. 167–188More LessElectron correlation and spin frustration are among the central issues in condensed matter physics, and their interplay is expected to bring about exotic phases with both charge and spin fluctuations. Molecular materials are playgrounds suitable for this study. Fundamentals in physics of Mott transition and spin frustration on triangular lattices are seen in the organic materials ET and Pd(dmit)2 compounds. We review the experimental studies on the criticality of Mott transition with a continuously controllable pressure technique and on the ground state of the quasi-triangular-lattice Mott insulator. Mott criticality is well characterized in both charge and spin channels with unconventional critical exponents of possibly quantum nature. The ground state of the triangular-lattice Mott insulator is changed from antiferromagnet to spin liquid as the triangular lattice becomes more isotropic. The various experiments probing the nature of spin liquid are described in the light of proposed mechanisms.
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Hybrid Solid-State Qubits: The Powerful Role of Electron Spins
Vol. 2 (2011), pp. 189–212More LessWe review progress on the use of electron spins to store and process quantum information, with particular focus on the ability of the electron spin to interact with multiple quantum degrees of freedom. We examine the benefits of hybrid quantum bits (qubits) in the solid state that are based on coupling electron spins to nuclear spins, electron charge, optical photons, and superconducting qubits. These benefits include the coherent storage of qubits for times exceeding seconds; fast qubit manipulation; single qubit measurement; and scalable methods for entangling spatially separated, matter-based qubits. In this way, the key strengths of different physical qubit implementations are brought together, laying the foundation for practical solid-state quantum technologies.
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Quantum Turbulence
Vol. 2 (2011), pp. 213–234More LessWe examine developments in the study of quantum turbulence with a special focus on clearly defining many of the terms used in the field. We critically review the diverse theoretical, computational, and experimental approaches from the point of view of experimental observers. Similarities and differences between the general properties of classical and quantum turbulence are elucidated. The dynamics and interactions of quantized vortices and their role in quantum turbulence are discussed with particular emphasis on reconnection and vortex ring collapse. A stark distinction between the velocity statistics of quantum and classical turbulence is exhibited and used to highlight a potential analogy between quantum turbulence and magnetohydrodynamic (MHD) turbulence in astrophysical plasmas. Although much of this review pertains to superfluid 4He (He II), the underlying science is broadly applicable to other quantum fluids such as 3He-B, type-II superconductors, Bose-Einstein condensates, Weinberg-Salam fields, and grand-unified-theory (GUT) Higgs fields.
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Electron Glass Dynamics
Ariel Amir, Yuval Oreg, and Yoseph ImryVol. 2 (2011), pp. 235–262More LessExamples of glasses are abundant, yet it remains one of the phases of matter whose understanding is very elusive. In recent years, remarkable experiments have been performed on the dynamical aspects of glasses. Electron glasses offer a particularly good example of the trademarks of glassy behavior, such as aging and slow relaxations. In this work we review the experimental literature on electron glasses, as well as the local mean-field theoretical framework put forward in recent years to understand some of these results. We also present novel theoretical results explaining the periodic aging experiment.
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Characterizing Structure Through Shape Matching and Applications to Self-Assembly
Vol. 2 (2011), pp. 263–285More LessStructural quantities such as order parameters and correlation functions are often employed to gain insight into the physical behavior and properties of condensed matter systems. Although standard quantities for characterizing structure exist, often they are insufficient for treating problems in the emerging field of nano- and microscale self-assembly, wherein the structures encountered may be complex and unusual. The computer science field of shape matching offers a robust solution to this problem by defining diverse methods for quantifying the similarity between arbitrarily complex shapes. Most order parameters and correlation functions used in condensed matter apply a specific measure of structural similarity within the context of a broader scheme. By substituting shape matching quantities for traditional quantities, we retain the essence of the broader scheme, but extend its applicability to more complex structures. Here we review some standard shape-matching techniques and discuss how they might be used to create highly flexible structural metrics for diverse systems such as self-assembled matter. We provide three proof-of-concept example problems applying shape-matching methods to identifying local and global structures and tracking structural transitions in complex assembled systems. The shape-matching methods reviewed here are applicable to a wide range of condensed matter systems, both simulated and experimental, provided particle positions are known or can be accurately imaged.
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Controlling the Functionality of Materials for Sustainable Energy
Vol. 2 (2011), pp. 287–301More LessOur understanding and control of sustainable energy technologies is in its infancy. Many sustainable energy phenomena depend on the exchange of photons and electrons among quantized energy levels of semiconductors, molecules, and metals at nanoscale spatial scales and at fast or ultrafast time scales. Improving the performance of sustainable energy technologies to make them competitive with fossil technologies requires probing and understanding these quantum phenomena with advanced scientific techniques. This understanding must then be translated into control of the functionality and performance of the materials and chemistry that govern sustainable energy technologies.
The review begins by contrasting the foundations of fossil fuel technology based on combustion, heat, and classical thermodynamics with the foundations of sustainable energy technology based on quantum exchange of energy among photons, chemical bonds, and electrons without conversion to heat. Two sets of tools that are essential to observe, understand, and control the quantum phenomena of sustainable energy are described: in situ and time-resolved experiments and theory, and numerical modeling of the functionality of large assemblies of atoms. Finally, the challenges and opportunities for understanding and ultimately controlling sustainable energy phenomena are presented for catalysis, solar water splitting, and superconductivity.
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Energy Conversion in Photosynthesis: A Paradigm for Solar Fuel Production
Vol. 2 (2011), pp. 303–327More LessSolar energy has the capacity to fulfill global human energy demands in an environmentally and socially responsible manner, provided efficient, low-cost systems can be developed for its capture, conversion, and storage. Toward these ends, a molecular-based understanding of the fundamental principles and mechanistic details of energy conversion in photosynthesis is indispensable. This review addresses aspects of photosynthesis that may prove auspicious to emerging technologies. Conversely, areas in which human ingenuity may offer innovative solutions, resulting in enhanced energy storage efficiencies in artificial photosynthetic constructs, are considered. Emphasis is placed on photoelectrochemical systems that utilize water as a source of electrons for the production of solar fuels.
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Equalities and Inequalities: Irreversibility and the Second Law of Thermodynamics at the Nanoscale
Vol. 2 (2011), pp. 329–351More LessThe reason we never observe violations of the second law of thermodynamics is in part a matter of statistics: When ∼1023 degrees of freedom are involved, the odds are overwhelmingly stacked against the possibility of seeing significant deviations away from the mean behavior. As we turn our attention to smaller systems, however, statistical fluctuations become more prominent. In recent years it has become apparent that the fluctuations of systems far from thermal equilibrium are not mere background noise, but satisfy strong, useful, and unexpected properties. In particular, a proper accounting of fluctuations allows us to rewrite familiar inequalities of macroscopic thermodynamics as equalities. This review describes some of this progress, and argues that it has refined our understanding of irreversibility and the second law.
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Deformation and Failure of Amorphous, Solidlike Materials
Vol. 2 (2011), pp. 353–373More LessSince the 1970s, theories of deformation and failure of amorphous, solidlike materials have started with models in which stress-driven, molecular rearrangements occur at localized flow defects via shear transformations. This picture is the basis for the modern theory of shear transformation zones (STZs), which is the focus of this review. We begin by describing the structure of the theory in general terms and by showing several applications, specifically, interpretation of stress-strain measurements for a bulk metallic glass, analysis of numerical simulations of shear banding, and the use of the STZ equations of motion in free-boundary calculations. In the second half of this review, we focus for simplicity on what we call an athermal model of amorphous plasticity, and use that model to illustrate how the STZ theory emerges within a systematic formulation of nonequilibrium thermodynamics.
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Life is Physics: Evolution as a Collective Phenomenon Far From Equilibrium
Vol. 2 (2011), pp. 375–399More LessEvolution is the fundamental physical process that gives rise to biological phenomena. Yet it is widely treated as a subset of population genetics, and thus its scope is artificially limited. As a result, the key issues of how rapidly evolution occurs and its coupling to ecology have not been satisfactorily addressed and formulated. The lack of widespread appreciation for, and understanding of, the evolutionary process has arguably retarded the development of biology as a science, with disastrous consequences for its applications to medicine, ecology, and the global environment. This review focuses on evolution as a problem in nonequilibrium statistical mechanics, where the key dynamical modes are collective, as evidenced by the plethora of mobile genetic elements whose role in shaping evolution has been revealed by modern genomic surveys. We discuss how condensed matter physics concepts might provide a useful perspective in evolutionary biology, the conceptual failings of the modern evolutionary synthesis, the open-ended growth of complexity, and the quintessentially self-referential nature of evolutionary dynamics.
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