Annual Review of Condensed Matter Physics - Volume 6, 2015
Volume 6, 2015
-
-
Innovations in Statistical Physics
Vol. 6 (2015), pp. 1–14More LessIn 1963–71, a group of people, myself included, formulated and perfected a new approach to physics problems, which eventually came to be known under the names of scaling, universality, and renormalization. This work formed the basis of a wide variety of theories ranging from a starting point in critical phenomena, moving out to particle physics and relativity, and then into economics and biology. This work was of transcendental beauty and of considerable intellectual importance.
This left me with a personal problem. What next? Constructing the answer to that question would dominate the next 45 years of my professional life. I would
- ■ Try to help in finding and constructing new fields of science.
- ■ Do research and give talks on the science-society borderline.
- ■ Provide constructive criticism of scientific and technical work.
- ■ Help students and younger scientists.
- ■ Demonstrate scientific leadership.
-
-
-
Many-Body Localization and Thermalization in Quantum Statistical Mechanics
Vol. 6 (2015), pp. 15–38More LessWe review some recent developments in the statistical mechanics of isolated quantum systems. We provide a brief introduction to quantum thermalization, paying particular attention to the eigenstate thermalization hypothesis (ETH) and the resulting single-eigenstate statistical mechanics. We then focus on a class of systems that fail to quantum thermalize and whose eigenstates violate the ETH: These are the many-body Anderson-localized systems; their long-time properties are not captured by the conventional ensembles of quantum statistical mechanics. These systems can forever locally remember information about their local initial conditions and are thus of interest for possibilities of storing quantum information. We discuss key features of many-body localization (MBL) and review a phenomenology of the MBL phase. Single-eigenstate statistical mechanics within the MBL phase reveal dynamically stable ordered phases, and phase transitions among them, that are invisible to equilibrium statistical mechanics and can occur at high energy and low spatial dimensionality, where equilibrium ordering is forbidden.
-
-
-
Composite Fermion Theory of Exotic Fractional Quantum Hall Effect
Vol. 6 (2015), pp. 39–62More LessThe fractional quantum Hall effect (FQHE) arises from strong correlations between electrons when they are confined to two dimensions and exposed to a strong magnetic field. The underlying physics is the formation of topological particles called composite fermions (CFs), electron-vortex bound states whose integer quantum Hall effect explains a large majority of the observed FQHE states. In recent years, the focus has shifted to the more exotic states that originate from a weak residual interaction between composite fermions. These include chiral p-wave paired states of composite fermions at certain even denominator fractions, unconventional FQHE of composite fermions, and a series of CF crystals at low fillings. Aside from these states, we also review the FQHE in multicomponent systems, which has attracted renewed attention because of the observation of well-developed FQHE in several multivalley systems, such as graphene and AlAs quantum wells.
-
-
-
The Statistical Physics of Athermal Materials
Vol. 6 (2015), pp. 63–83More LessAt the core of equilibrium statistical mechanics lies the notion of statistical ensembles: a collection of microstates, each occurring with a given a priori probability that depends on only a few macroscopic parameters, such as temperature, pressure, volume, and energy. In this review, we discuss recent advances in establishing statistical ensembles for athermal materials. The broad class of granular and particulate materials is immune to the effects of thermal fluctuations because the constituents are macroscopic. In addition, interactions between grains are frictional and dissipative, which invalidates the fundamental postulates of equilibrium statistical mechanics. However, granular materials exhibit distributions of microscopic quantities that are reproducible and often depend on only a few macroscopic parameters. We explore the history of statistical ensemble ideas in the context of granular materials, clarify the nature of such ensembles and their foundational principles, highlight advances in testing key ideas, and discuss applications of ensembles to analyze the collective behavior of granular materials.
-
-
-
Napoleon Is in Equilibrium
Vol. 6 (2015), pp. 85–111More LessIt has been said that the cell is the test tube of the twenty-first century. If so, the theoretical tools needed to quantitatively and predictively describe what goes on in such test tubes lag sorely behind the stunning experimental advances in biology seen in the decades since the molecular biology revolution began. Perhaps surprisingly, one of the theoretical tools that has been used with great success on problems ranging from how cells communicate with their environment and each other to the nature of the organization of proteins and lipids within the cell membrane is statistical mechanics. A knee-jerk reaction to the use of statistical mechanics in the description of cellular processes is that living organisms are so far from equilibrium that one has no business even thinking about it. But such reactions are probably too hasty given that there are many regimes in which, because of a separation of timescales, for example, such an approach can be a useful first step. In this article, we explore the power of statistical mechanical thinking in the biological setting, with special emphasis on cell signaling and regulation. We show how such models are used to make predictions and describe some recent experiments designed to test them. We also consider the limits of such models based on the relative timescales of the processes of interest.
-
-
-
Assembly of Biological Nanostructures: Isotropic and Liquid Crystalline Phases of Neurofilament Hydrogels
Vol. 6 (2015), pp. 113–136More LessNeurofilaments are the building blocks of the major cytoskeletal network found in the axons of vertebrate neurons. The filaments consist of three distinct molecular-weight subunits—neurofilament-low, neurofilament-medium, and neurofilament-high—which coassemble into 10-nm flexible rods with protruding intrinsically disordered C-terminal sidearms that mediate interfilament interactions and hydrogel formation. Molecular neuroscience research includes areas focused on elucidating the functions of each subunit in network formation, during which disruptions are a hallmark of motor-neuron diseases. Here, modern concepts and methods from soft condensed matter physics are combined to address the role of subunits as it relates to interfilament forces and phase behavior in neurofilament networks. Significantly, the phase behavior studies reveal that although neurofilament-medium subunits promote nematic liquid crystal hydrogel phase stability with parallel filament orientation, neurofilament-high subunits stabilize the hydrogel in the nematic phase close to the isotropic gel phase with random, crossed-filament orientation. This indicates a regulatory role for neurofilament-high subunits in filament orientational plasticity required for organelle (e.g., membrane-bound vesicle or mitochondrion) transport along microtubules embedded in neurofilament hydrogels. Future studies—for example, on neurofilament subunits mixed with tubulin and microtubule-associated proteins—should lead to a deeper understanding of forces and heterogeneous structures in neuronal cytoskeletons.
-
-
-
Plutonium-Based Heavy-Fermion Systems
E.D. Bauer, and J.D. ThompsonVol. 6 (2015), pp. 137–153More LessAn effective mass of charge carriers that is significantly larger than the mass of a free electron develops at low temperatures in certain lanthanide- and actinide-based metals, including those formed with plutonium, owing to strong electron-electron interactions. This heavy-fermion mass is reflected in a substantially enhanced electronic coefficient of specific heat γ, which for elemental Pu is much larger than that of normal metals. By our definition, there are twelve Pu-based heavy-fermion compounds, most discovered recently, whose basic properties are known and discussed. Relative to other examples, these Pu-based heavy-fermion systems are particularly complex owing in part to the possible simultaneous presence of multiple, nearly degenerate 5fn configurations. This complexity poses significant opportunities as well as challenges, including understanding the origin of unconventional superconductivity in some of these materials.
-
-
-
Exciton-Polariton Bose-Einstein Condensates
Vol. 6 (2015), pp. 155–175More LessExciton-polaritons, mixed light-matter quasiparticles in semiconductors, have recently shown evidence for Bose-Einstein condensation. Some of the properties of condensates of exciton-polaritons are reviewed in this article. We first discuss the spontaneous appearance of long-range order and the way this can be easily accessed in the case of polariton fluids. We show that the Penrose-Onsager criterion is valid even for such a very special case of condensate. We then describe the experiments that allow observation of topological defects in the fluid: quantized vortices, half vortices, and hyperbolic spin vortices. We demonstrate through the comparison with the gross Pitaevskii equation that the appearance and stability of such vortices are linked with the dissipative nature of the condensate together with the presence of disorder. We then briefly summarize the experiments on superfluid behavior of the polaritons at large-enough densities and expand somewhat more on the dynamical behavior of turbulence in the wake of an obstacle, with the appearance of vortex streets. We finally show that the Bogoliubov transformation has been revealed through four-wave mixing experiments.
-
-
-
Marginal Stability in Structural, Spin, and Electron Glasses
Vol. 6 (2015), pp. 177–200More LessWe revisit the concept of marginal stability in glasses and determine its range of applicability in the context of an avalanche-type response to slow external driving. We argue that there is an intimate connection between a pseudogap in the distribution of local fields and crackling in systems with long-range interactions. We classify glassy systems according to the presence or absence of marginal stability, providing a unifying perspective on the phenomenology of systems as diverse as spin and electron glasses, hard spheres, pinned elastic interfaces, and soft amorphous solids undergoing plastic deformation.
-
-
-
Ultracold Atoms Out of Equilibrium
Vol. 6 (2015), pp. 201–217More LessThe relaxation of isolated quantum many-body systems is a major unsolved problem connecting statistical and quantum physics. Studying such relaxation processes remains a challenge despite considerable efforts. Experimentally, it requires the creation and manipulation of well-controlled and truly isolated quantum systems. In this context, ultracold neutral atoms provide unique opportunities to understand nonequilibrium phenomena because of the large set of available methods to isolate, manipulate, and probe these systems. Here, we give an overview of the rapid experimental progress that has been made in the field over the past few years and highlight some of the questions that may be explored in the future.
-
-
-
Motility-Induced Phase Separation
Vol. 6 (2015), pp. 219–244More LessSelf-propelled particles include both self-phoretic synthetic colloids and various microorganisms. By continually consuming energy, they bypass the laws of equilibrium thermodynamics. These laws enforce the Boltzmann distribution in thermal equilibrium: The steady state is then independent of kinetic parameters. In contrast, self-propelled particles tend to accumulate where they move more slowly. They may also slow down at high density for either biochemical or steric reasons. This creates positive feedback, which can lead to motility-induced phase separation (MIPS) between dense and dilute fluid phases. At leading order in gradients, a mapping relates variable-speed, self-propelled particles to passive particles with attractions. This deep link to equilibrium phase separation is confirmed by simulations but generally breaks down at higher order in gradients: New effects, with no equilibrium counterpart, then emerge. We give a selective overview of the fast-developing field of MIPS, focusing on theory and simulation but including a brief speculative survey of its experimental implications.
-
-
-
Physics of Viral Shells
Vol. 6 (2015), pp. 245–268More LessWe review the application of statistical mechanics, elasticity theory, and condensed matter physics to the assembly and maturation of viral capsids.
-
-
-
Amplitude/Higgs Modes in Condensed Matter Physics
David Pekker, and C.M. VarmaVol. 6 (2015), pp. 269–297More LessThe order parameter and its variations in space and time in many different states in condensed matter physics at low temperatures are described by the complex function Ψ(r, t). These states include superfluids, superconductors, and a subclass of antiferromagnets and charge density waves. The collective fluctuations in the ordered state may then be categorized as oscillations of phase and amplitude of Ψ(r, t). The phase oscillations are the Goldstone modes of the broken continuous symmetry. The amplitude modes, even at long wavelengths, are well defined and are decoupled from the phase oscillations only near particle-hole symmetry, where the equations of motion have an effective Lorentz symmetry, as in particle physics and if there are no significant avenues for decay into other excitations. They bear close correspondence with the so-called Higgs modes in particle physics, whose prediction and discovery are very important for the standard model of particle physics. In this review, we discuss the theory and the possible observation of the amplitude or Higgs modes in condensed matter physics—in superconductors, cold atoms in periodic lattices, and uniaxial antiferromagnets. We discuss the necessity for at least approximate particle-hole symmetry as well as the special conditions required to couple to such modes because, being scalars, they do not couple linearly to the usual condensed matter probes.
-
-
-
Symmetry-Protected Topological Phases of Quantum Matter
Vol. 6 (2015), pp. 299–324More LessWe describe recent progress in our understanding of the interplay between interactions, symmetry, and topology in states of quantum matter. We focus on a minimal generalization of the celebrated topological band insulators (TBIs) to interacting many-particle systems known as symmetry-protected topological (SPT) phases. As with the TBIs, these states have a bulk gap and no exotic excitations but have nontrivial surface states that are protected by symmetry. We describe the various possible phases and their properties in three-dimensional systems with realistic symmetries. We develop many key ideas for the theory of these states using simple examples. The emphasis is on physical rather than mathematical properties. We survey insights obtained from the study of SPT phases for a number of other theoretical problems.
-
-
-
Spatial Localization in Dissipative Systems
Vol. 6 (2015), pp. 325–359More LessSpatial localization is a common feature of physical systems, occurring in both conservative and dissipative systems. This article reviews the theoretical foundations of our understanding of spatial localization in forced dissipative systems, from both a mathematical point of view and a physics perspective. It explains the origin of the large multiplicity of simultaneously stable spatially localized states present in a parameter region called the pinning region and its relation to the notion of homoclinic snaking. The localized states are described as bound states of fronts, and the notions of front pinning, self-pinning, and depinning are emphasized. Both one-dimensional and two-dimensional systems are discussed, and the reasons behind the differences in behavior between dissipative systems with conserved and nonconserved dynamics are explained. The insights gained are specific to forced dissipative systems and are illustrated here using examples drawn from fluid mechanics (convection and shear flows) and a simple model of crystallization.
-
-
-
Topological Crystalline Insulators and Topological Superconductors: From Concepts to Materials
Yoichi Ando, and Liang FuVol. 6 (2015), pp. 361–381More LessIn this review, we discuss recent progress in the explorations of topological materials beyond topological insulators; specifically, we focus on topological crystalline insulators and bulk topological superconductors. The basic concepts, model Hamiltonians, and novel electronic properties of these new topological materials are explained. The key role of the symmetries that underlie their topological properties is elucidated. Key issues in their materials realizations are also discussed.
-
-
-
Universal Dynamics and Renormalization in Many-Body-Localized Systems
Ehud Altman, and Ronen VoskVol. 6 (2015), pp. 383–409More LessWe survey the recent progress made in understanding nonequilibrium dynamics in closed random systems. The emphasis is on the important role played by concepts from quantum information theory and on the application of systematic renormalization group methods to capture universal aspects of the dynamics. Finally, we outline some outstanding open questions, which include the description of the many-body-localization phase transition and the identification of physical systems that allow systematic experimental study of these phenomena.
-
-
-
Quantum Oscillations in Hole-Doped Cuprates
Vol. 6 (2015), pp. 411–430More LessOne of the leading challenges of condensed matter physics in the past few decades is an understanding of the high-temperature copper-oxide superconductors. Although the d-wave character of the superconducting state is well understood, the normal state in the underdoped regime has eluded understanding. Here, we review the past few years of quantum oscillation measurements performed in the underdoped cuprates that have culminated in an understanding of the normal ground state of these materials. A nodal electron pocket created by charge order is found to characterize the normal ground state in YBa2Cu3O6+δ and is likely universal to a majority of the cuprate superconductors. An open question remains regarding the origin of the suppression of the antinodal density of states at the Fermi energy in the underdoped normal state, either from mainly charge correlations or, more likely, from mainly pairing and/or magnetic correlations that precede charge order.
-