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Annual Review of Condensed Matter Physics - Volume 13, 2022
Volume 13, 2022
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Reflections on 65 Years of Helium Research
Vol. 13 (2022), pp. 1–12More LessIn this autobiographical article, I discuss a number of topics that have absorbed my interest over the years and illustrate how advances in experimental technique, such as the superfluid gyroscope and torsional oscillators, were entwined with expanding knowledge of the properties of helium.
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My Life and Science
Vol. 13 (2022), pp. 15–31More LessIn this article, I tried to compress the events of my long life and scientific career into a readable manuscript. The choice of scientific problems in development of which I was involved and people with whom I contacted naturally is not complete. I hope, however, that my selection more or less correctly reflects my teaching activity and my participation in the enormous progress of quantum mechanics, statistical physics, and condensed matter physics in the second part of the previous and in the beginning of the current century.
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Russell Donnelly and His Leaks
Vol. 13 (2022), pp. 33–47More LessRussell James Donnelly (b. 1930) was an exceptionally creative physicist with many other interests: art, music, history, and scientific societies and their scholarly journals. He reinvigorated the maturing field of low temperature physics by linking it strongly to fluid turbulence through bold and optimistic leadership at the intersection of the two fields. Immediately after achieving his Ph.D. at Yale University with C.T. Lane and L. Onsager, Russ joined the University of Chicago in 1956, where he became a professor at the first possible opportunity. After some ten years at University of Chicago, where he worked for a time with S. Chandrasekhar, he moved to the University of Oregon and led a vigorous life until his death in 2015. Russ was an excellent organizer of scientific meetings and an enthusiastic expositor of his science. He had a profound sense of service to the community, both civic and scientific, and showed exceptional scientific openness and generosity to his colleagues. He was greatly admired by his community.
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Director Deformations, Geometric Frustration, and Modulated Phases in Liquid Crystals
Vol. 13 (2022), pp. 49–71More LessThis article analyzes modulated phases in liquid crystals, from the long-established cholesteric and blue phases to the recently discovered twist-bend, splay-bend, and splay nematic phases, as well as the twist-grain-boundary (TGB) and helical nanofilament variations on smectic phases. The analysis uses the concept of four fundamental modes of director deformation: twist, bend, splay, and a fourth mode related to saddle-splay. Each mode is coupled to a specific type of molecular order: chirality, polarization perpendicular and parallel to the director, and octupolar order. When the liquid crystal develops one type of spontaneous order, the ideal local structure becomes nonuniform, with the corresponding director deformation. In general, the ideal local structure is frustrated; it cannot fill space. As a result, the liquid crystal must form a complex global phase, which may have a combination of deformation modes, and may have a periodic array of defects. Thus, the concept of an ideal local structure under geometric frustration provides a unified framework to understand the wide variety of modulated phases.
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Thin Film Skyrmionics
Vol. 13 (2022), pp. 73–95More LessIn condensed matter physics, magnetic skyrmions, topologically stabilized magnetic solitons, have been discovered in various materials systems, which has intrigued the community in terms of not only fundamental physics but also with respect to engineering applications. In particular, skyrmions in thin films are easily manipulable by electrical means even at room temperature. Concomitantly, a variety of possible applications have been proposed and proof-of-concept devices have been demonstrated. Recently, the field of skyrmion-based electronics has been referred to as skyrmionics and this field has been rapidly growing and extended in multiple directions. This review provides recent progress for skyrmion research in thin film systems and we discuss promising new directions, which will further invigorate the field.
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The Physics of Dense Suspensions
Vol. 13 (2022), pp. 97–117More LessDense suspensions of particles are relevant to many applications and are a key platform for developing a fundamental physics of out-of-equilibrium systems. They present challenging flow properties, apparently turning from liquid to solid upon small changes in composition or, intriguingly, in the driving forces applied to them. The emergent physics close to the ubiquitous jamming transition (and to some extent the glass and gelation transitions) provides common principles with which to achieve a consistent interpretation of a vast set of phenomena reported in the literature. In light of this, we review the current state of understanding regarding the relation between the physics at the particle scale and the rheology at the macroscopic scale. We further show how this perspective opens new avenues for the development of continuum models for dense suspensions.
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Topological Magnets: Functions Based on Berry Phase and Multipoles
Vol. 13 (2022), pp. 119–142More LessMacroscopic responses of magnets are often governed by magnetization and, thus, have been restricted to ferromagnets. However, such responses are strikingly large in the newly developed topological magnets, breaking the conventional scaling with magnetization. Taking the recently discovered antiferromagnetic (AF) Weyl semimetals as a prime example, we highlight the two central ingredients driving the significant macroscopic responses: the Berry curvature enhanced because of nontrivial band topology in momentum space, and the cluster magnetic multipoles in real space. The combination of large Berry curvature and multipoles enables large macroscopic responses such as the anomalous Hall and Nernst effects, the magneto-optical effect, and the novel magnetic spin Hall effect in antiferromagnets with negligible net magnetization, but also allows us to manipulate these effects by electrical means. Furthermore, nodal-point and nodal-line semimetallic states in ferromagnets may provide the strongly enhanced Berry curvature near the Fermi energy, leading to large responses beyond the conventional magnetization scaling. These significant properties and functions of the topological magnets lay the foundation for future technological development such as spintronics and thermoelectric technology.
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Active Turbulence
Vol. 13 (2022), pp. 143–170More LessActive fluids exhibit spontaneous flows with complex spatiotemporal structure, which have been observed in bacterial suspensions, sperm cells, cytoskeletal suspensions, self-propelled colloids, and cell tissues. Despite occurring in the absence of inertia, chaotic active flows are reminiscent of inertial turbulence, and hence they are known as active turbulence. Here, we survey the field, providing a unified perspective over different classes of active turbulence. To this end, we divide our review into sections for systems with either polar or nematic order, and with or without momentum conservation (wet or dry). Comparing to inertial turbulence, we highlight the emergence of power-law scaling with either universal or nonuniversal exponents. We also contrast scenarios for the transition from steady to chaotic flows, and we discuss the absence of energy cascades. We link this feature to both the existence of intrinsic length scales and the self-organized nature of energy injection in active turbulence, which are fundamental differences from inertial turbulence. We close by outlining the emerging picture, remaining challenges, and future directions.
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Topological Magnons: A Review
Vol. 13 (2022), pp. 171–190More LessAt sufficiently low temperatures, magnetic materials often enter correlated phases hosting collective, coherent magnetic excitations such as magnons or triplons. Drawing on the enormous progress on topological materials of the past few years, recent research has led to new insights into the geometry and topology of these magnetic excitations. Berry phases associated with magnetic dynamics can lead to observable consequences in heat and spin transport, whereas analogs of topological insulators and semimetals can arise within magnon band structures from natural magnetic couplings. Magnetic excitations offer a platform to explore the interplay of magnetic symmetries and topology, drive topological transitions using magnetic fields, examine the effects of interactions on topological bands, and generate topologically protected spin currents at interfaces. In this review, we survey progress on all these topics, highlighting aspects of topological matter that are unique to magnon systems and the avenues yet to be fully investigated.
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Olfactory Sensing and Navigation in Turbulent Environments
Vol. 13 (2022), pp. 191–213More LessFluid turbulence is a double-edged sword for the navigation of macroscopic animals, such as birds, insects, and rodents. On the one hand, turbulence enables pheromone communication among mates and the possibility of locating food by their odors from long distances. Molecular diffusion would indeed be unable to spread odors over relevant distances in natural conditions. On the other hand, turbulent flows are hard to predict, and learning effective maneuvers to navigate them is challenging, as we discuss in this review. We first provide a summary of the olfactory organs that sense airborne or surface-bound odors, as well as the computational tasks that animals face when extracting information useful for navigation from an olfactory signal. A compendium of the dynamics of turbulent transport emphasizes those aspects that directly impact animals’ behavior. The state of the art on navigational strategies is discussed, followed by a concluding section dedicated to future challenges in the field.
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Irreversibility and Biased Ensembles in Active Matter: Insights from Stochastic Thermodynamics
Vol. 13 (2022), pp. 215–238More LessActive systems evade the rules of equilibrium thermodynamics by constantly dissipating energy at the level of their microscopic components. This energy flux stems from the conversion of a fuel, present in the environment, into sustained individual motion. It can lead to collective effects without any equilibrium equivalent, some of which can be rationalized by using equilibrium tools to recapitulate nonequilibrium transitions. An important challenge is then to delineate systematically to what extent the character of these active transitions is genuinely distinct from equilibrium analogs. We review recent works that use stochastic thermodynamics tools to identify, for active systems, a measure of irreversibility comprising a coarse-grained or informatic entropy production. We describe how this relates to the underlying energy dissipation or thermodynamic entropy production, and how it is influenced by collective behavior. Then, we review the possibility of constructing thermodynamic ensembles out of equilibrium, where trajectories are biased toward atypical values of nonequilibrium observables. We show that this is a generic route to discovering unexpected phase transitions in active matter systems, which can also inform their design.
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The Hubbard Model
Vol. 13 (2022), pp. 239–274More LessThe repulsive Hubbard model has been immensely useful in understanding strongly correlated electron systems and serves as the paradigmatic model of the field. Despite its simplicity, it exhibits a strikingly rich phenomenology reminiscent of that observed in quantum materials. Nevertheless, much of its phase diagram remains controversial. Here, we review a subset of what is known about the Hubbard model based on exact results or controlled approximate solutions in various limits, for which there is a suitable small parameter. Our primary focus is on the ground state properties of the system on various lattices in two spatial dimensions, although both lower and higher dimensions are discussed as well. Finally, we highlight some of the important outstanding open questions.
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The Hubbard Model: A Computational Perspective
Vol. 13 (2022), pp. 275–302More LessThe Hubbard model is the simplest model of interacting fermions on a lattice and is of similar importance to correlated electron physics as the Ising model is to statistical mechanics or the fruit fly to biomedical science. Despite its simplicity, the model exhibits an incredible wealth of phases, phase transitions, and exotic correlation phenomena. Although analytical methods have provided a qualitative description of the model in certain limits, numerical tools have shown impressive progress in achieving quantitative accurate results over the past several years. This article gives an introduction to the model, motivates common questions, and illustrates the progress that has been achieved over recent years in revealing various aspects of the correlation physics of the model.
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Understanding Hydrophobic Effects: Insights from Water Density Fluctuations
Vol. 13 (2022), pp. 303–324More LessThe aversion of hydrophobic solutes for water drives diverse interactions and assemblies across materials science, biology, and beyond. Here, we review the theoretical, computational, and experimental developments that underpin a contemporary understanding of hydrophobic effects. We discuss how an understanding of density fluctuations in bulk water can shed light on the fundamental differences in the hydration of molecular and macroscopic solutes; these differences, in turn, explain why hydrophobic interactions become stronger upon increasing temperature. We also illustrate the sensitive dependence of surface hydrophobicity on the chemical and topographical patterns the surface displays, which makes the use of approximate approaches for estimating hydrophobicity particularly challenging. Importantly, the hydrophobicity of complex surfaces, such as those of proteins, which display nanoscale heterogeneity, can nevertheless be characterized using interfacial water density fluctuations; such a characterization also informs protein regions that mediate their interactions. Finally, we build upon an understanding of hydrophobic hydration and the ability to characterize hydrophobicity to inform the context-dependent thermodynamic forces that drive hydrophobic interactions and the desolvation barriers that impede them.
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Modeling of Ferroelectric Oxide Perovskites: From First to Second Principles
Vol. 13 (2022), pp. 325–364More LessTaking a historical perspective, we provide a brief overview of the first-principles modeling of ferroelectric perovskite oxides over the past 30 years. We emphasize how the work done by a relatively small community on the fundamental understanding of ferroelectricity and related phenomena has been at the origin of consecutive theoretical breakthroughs, with an impact going often well beyond the limit of the ferroelectric community. In this context, we first review key theoretical advances such as the modern theory of polarization, the computation of functional properties as energy derivatives, the explicit treatment of finite fields, or the advent of second-principles methods to extend the length and timescale of the simulations. We then discuss how these have revolutionized our understanding of ferroelectricity and related phenomena in this technologically important class of compounds.
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How Cross-Link Numbers Shape the Large-Scale Physics of Cytoskeletal Materials
Vol. 13 (2022), pp. 365–384More LessCytoskeletal networks are the main actuators of cellular mechanics, and a foundational example for active matter physics. In cytoskeletal networks, motion is generated on small scales by filaments that push and pull on each other via molecular-scale motors. These local actuations give rise to large-scale stresses and motion. To understand how microscopic processes can give rise to self-organized behavior on larger scales it is important to consider what mechanisms mediate long-ranged mechanical interactions in the systems. Two scenarios have been considered in the recent literature. The first scenario is systems that are relatively sparse, in which most of the large-scale momentum transfer is mediated by the solvent in which cytoskeletal filaments are suspended. The second scenario is systems in which filaments are coupled via cross-link molecules throughout. Here, we review the differences and commonalities between the physics of these two regimes. We also survey the literature for the numbers that allow us to place a material within either of these two classes.
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Studying Quantum Materials with Scanning SQUID Microscopy
Vol. 13 (2022), pp. 385–405More LessElectronic correlations give rise to fascinating macroscopic phenomena such as superconductivity, magnetism, and topological phases of matter. Although these phenomena manifest themselves macroscopically, fully understanding the underlying microscopic mechanisms often requires probing on multiple length scales. Spatial modulations on the mesoscopic scale are especially challenging to probe, owing to the limited range of suitable experimental techniques. Here, we review recent progress in scanning superconducting quantum interference device (SQUID) microscopy. We demonstrate how scanning SQUID combines unmatched magnetic field sensitivity and highly versatile designs, by surveying discoveries in unconventional superconductivity, exotic magnetism, topological states, and more. Finally, we discuss how SQUID microscopy can be further developed to answer the increasing demand for imaging new quantum materials.
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Coherently Coupled Mixtures of Ultracold Atomic Gases
Vol. 13 (2022), pp. 407–432More LessThis article summarizes some of the relevant features exhibited by binary mixtures of Bose–Einstein condensates in the presence of coherent coupling at zero temperature. The coupling, which is experimentally produced by proper photon transitions, can involve either negligible momentum transfer from the electromagnetic radiation (Rabi coupling) or large momentum transfer (Raman coupling) associated with spin–orbit effects. The nature of the quantum phases exhibited by coherently coupled mixtures is discussed in detail, including their paramagnetic, ferromagnetic, and, in the case of spin–orbit coupling, supersolid phases. The behavior of the corresponding elementary excitations is discussed, with explicit emphasis on the novel features caused by the spin-like degree of freedom. Focus is further given to the topological excitations (solitons, vortices) as well as to the superfluid properties. This review also points out relevant open questions that deserve more systematic theoretical and experimental investigations.
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