Annual Review of Condensed Matter Physics: Most Cited Articles
http://www.annualreviews.org/content/journals/conmatphys?TRACK=RSS
Please follow the links to view the content.Many-Body Localization and Thermalization in Quantum Statistical Mechanics
http://www.annualreviews.org/content/journals/10.1146/annurev-conmatphys-031214-014726?TRACK=RSS
We 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.Rahul Nandkishore and David A. Huse2023-12-08T10:22:22ZSearch for Majorana Fermions in Superconductors
http://www.annualreviews.org/content/journals/10.1146/annurev-conmatphys-030212-184337?TRACK=RSS
Majorana fermions (particles that are their own antiparticle) may or may not exist in nature as elementary building blocks, but in condensed matter they can be constructed out of electron and hole excitations. What is needed is a superconductor to hide the charge difference and a topological (Berry) phase to eliminate the energy difference from zero-point motion. A pair of widely separated Majorana fermions, bound to magnetic or electrostatic defects, has non-Abelian exchange statistics. A qubit encoded in this Majorana pair is expected to have an unusually long coherence time. I discuss strategies to detect Majorana fermions in a topological superconductor, as well as possible applications in a quantum computer. The status of the experimental search is reviewed.C.W.J. Beenakker2024-01-16T22:55:06ZThe Mechanics and Statistics of Active Matter
http://www.annualreviews.org/content/journals/10.1146/annurev-conmatphys-070909-104101?TRACK=RSS
Active particles contain internal degrees of freedom with the ability to take in and dissipate energy and, in the process, execute systematic movement. Examples include all living organisms and their motile constituents such as molecular motors. This article reviews recent progress in applying the principles of nonequilibrium statistical mechanics and hydrodynamics to form a systematic theory of the behavior of collections of active particles–active matter–with only minimal regard to microscopic details. A unified view of the many kinds of active matter is presented, encompassing not only living systems but inanimate analogs. Theory and experiment are discussed side by side.Sriram Ramaswamy2023-12-08T10:21:12ZTopological Materials: Weyl Semimetals
http://www.annualreviews.org/content/journals/10.1146/annurev-conmatphys-031016-025458?TRACK=RSS
Topological insulators and topological semimetals are both new classes of quantum materials, which are characterized by surface states induced by the topology of the bulk band structure. Topological Dirac or Weyl semimetals show linear dispersion around nodes, termed the Dirac or Weyl points, as the three-dimensional analog of graphene. We review the basic concepts and compare these topological states of matter from the materials perspective with a special focus on Weyl semimetals. The TaAs family is the ideal materials class to introduce the signatures of Weyl points in a pedagogical way, from Fermi arcs to the chiral magnetotransport properties, followed by hunting for the type-II Weyl semimetals in WTe2, MoTe2, and related compounds. Many materials are members of big families, and topological properties can be tuned. As one example, we introduce the multifunctional topological materials, Heusler compounds, in which both topological insulators and magnetic Weyl semimetals can be found. Instead of a comprehensive review, this article is expected to serve as a helpful introduction and summary by taking a snapshot of the quickly expanding field.Binghai Yan and Claudia Felser2023-12-08T10:21:35ZCorrelated Quantum Phenomena in the Strong Spin-Orbit Regime
http://www.annualreviews.org/content/journals/10.1146/annurev-conmatphys-020911-125138?TRACK=RSS
We discuss phenomena arising from the combined influence of electron correlation and spin-orbit coupling (SOC), with an emphasis on emergent quantum phases and transitions in heavy transition metal compounds with 4d and 5d elements. A common theme is the influence of spin-orbital entanglement produced by SOC, which influences the electronic and magnetic structure. In the weak-to-intermediate correlation regime, we show how nontrivial band-like topology leads to a plethora of phases related to topological insulators (TIs). We expound these ideas using the example of pyrochlore iridates, showing how many novel phases, such as the Weyl semimetal, axion insulator, topological Mott insulator, and TIs, may arise in this context. In the strong correlation regime, we argue that spin-orbital entanglement fully or partially removes orbital degeneracy, reducing or avoiding the normally ubiquitous Jahn-Teller effect. As we illustrate for the honeycomb-lattice iridates and double perovskites, this leads to enhanced quantum fluctuations of the spin-orbital entangled states and the chance to promote exotic spin liquid and multipolar ordered ground states. Connections to experiments, materials, and future directions are discussed.William Witczak-Krempa, Gang Chen, Yong Baek Kim and Leon Balents2023-12-08T10:23:23ZMotility-Induced Phase Separation
http://www.annualreviews.org/content/journals/10.1146/annurev-conmatphys-031214-014710?TRACK=RSS
Self-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.Michael E. Cates and Julien Tailleur2023-12-08T10:20:50ZInterface Physics in Complex Oxide Heterostructures
http://www.annualreviews.org/content/journals/10.1146/annurev-conmatphys-062910-140445?TRACK=RSS
Complex 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.Pavlo Zubko, Stefano Gariglio, Marc Gabay, Philippe Ghosez and Jean-Marc Triscone2023-12-08T08:20:43ZEqualities and Inequalities: Irreversibility and the Second Law of Thermodynamics at the Nanoscale
http://www.annualreviews.org/content/journals/10.1146/annurev-conmatphys-062910-140506?TRACK=RSS
The 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.Christopher Jarzynski2023-12-08T08:19:13ZSuperconducting Qubits: Current State of Play
http://www.annualreviews.org/content/journals/10.1146/annurev-conmatphys-031119-050605?TRACK=RSS
Superconducting qubits are leading candidates in the race to build a quantum computer capable of realizing computations beyond the reach of modern supercomputers. The superconducting qubit modality has been used to demonstrate prototype algorithms in the noisy intermediate-scale quantum (NISQ) technology era, in which non-error-corrected qubits are used to implement quantum simulations and quantum algorithms. With the recent demonstrations of multiple high-fidelity, two-qubit gates as well as operations on logical qubits in extensible superconducting qubit systems, this modality also holds promise for the longer-term goal of building larger-scale error-corrected quantum computers. In this brief review, we discuss several of the recent experimental advances in qubit hardware, gate implementations, readout capabilities, early NISQ algorithm implementations, and quantum error correction using superconducting qubits. Although continued work on many aspects of this technology is certainly necessary, the pace of both conceptual and technical progress in recent years has been impressive, and here we hope to convey the excitement stemming from this progress.Morten Kjaergaard, Mollie E. Schwartz, Jochen Braumüller, Philip Krantz, Joel I.-J. Wang, Simon Gustavsson and William D. Oliver2023-09-12T20:08:54ZStrong Correlations from Hund’s Coupling
http://www.annualreviews.org/content/journals/10.1146/annurev-conmatphys-020911-125045?TRACK=RSS
Strong electronic correlations are often associated with the proximity of a Mott-insulating state. In recent years however, it has become increasingly clear that the Hund’s rule coupling (intra-atomic exchange) is responsible for strong correlations in multiorbital metallic materials that are not close to a Mott insulator. Hund’s coupling has two effects: It influences the energetics of the Mott gap and strongly suppresses the coherence scale for the formation of a Fermi liquid. A global picture has emerged recently, which emphasizes the importance of the average occupancy of the shell as a control parameter. The most dramatic effects occur away from half-filling or single occupancy. We review the theoretical understanding and physical properties of these Hund’s metals, together with the relevance of this concept to transition-metal oxides (TMOs) of the 3d, and especially 4d, series (such as ruthenates), as well as to the iron-based superconductors (iron pnictides and chalcogenides).Antoine Georges, Luca de' Medici and Jernej Mravlje2024-01-16T22:56:02Z