Annual Review of Condensed Matter Physics - Volume 5, 2014

Subfields of physics are born, expand, and develop in intellectual scope, then can spawn new offspring by subdividing, can disappear by being absorbed in new definitions of the fields of physics, or may merely decline in vigor and membership. Textbooks, seminar programs, graduate courses, and the chosen structure of industrial laboratories all contributed to making solid state physics a vibrant subfield for 30 years, to ultimately disappear into regroupings with names such as condensed matter, materials science, biological physics, complexity, and quantum optics. This review traces the trajectory of the subfield solid state physics through the experiences of the author in relationship to major university departments and Bell Labs, with digressions into how he became a physicist, physics education, and choosing research problems.

Physics of noncentrosymmetric superconductors is reviewed. We explain the mixing between singlet and triplet superconducting order parameters when parity symmetry is absent. Some exotic properties are summarized, including magnetoelectric effects, the helical phase, topological properties, and unusual surface states.

Since the discovery of high-Tc cuprates, the quest for new superconductors has shifted toward more anisotropic, strongly correlated materials with lower carrier densities and competing magnetic and charge-density wave orders. Although these materials’ features enhance superconducting correlations, they also result in serious problems for applications at liquid nitrogen (and higher) temperatures and strong magnetic fields so that such conventional characteristics as the critical temperature Tc and the upper critical field Hc2 are no longer the main parameters of merit. This happens because of strong fluctuations of the order parameter, thermally activated hopping of pinned vortices, and electromagnetic granularity, as has been established after extensive investigations of cuprates and Fe-based superconductors (FBSs). In this paper, I give an overview of those mechanisms crucial for power and magnet applications and discuss the materials’ restrictions that must be satisfied to make superconductors useful at high temperatures and magnetic fields. These restrictions become more and more essential at higher temperatures and magnetic fields, particularly for the yet-to-be-discovered superconductors operating at room temperatures. In this case, the performance of superconductors is limited by destructive fluctuations of the order parameter so that higher superfluid density and weaker electronic anisotropy, which reduce these fluctuations, can become far more important than higher Tc.

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.

Understanding Dirac-like fermions has become an imperative in modern condensed matter sciences: All across the research frontier, from graphene to high Tc superconductors to the topological insulators and beyond, various electronic systems exhibit properties that can be well described by the Dirac equation. Such physics is no longer the exclusive domain of quantum field theories and other esoteric mathematical musings; instead, physics of real condensed matter systems is governed by such equations, and important materials science and practical implications hinge on our understanding of Dirac particles in two and three dimensions. Although the physics that gives rise to the massless Dirac fermions in each of the above-mentioned materials is different, the low-energy properties are governed by the same Dirac kinematics. The aim of this article is to review a selected cross-section of this vast field by highlighting the generalities and contrasting the specifics of several physical systems.

Whether a quantum critical point (QCP) lies beneath the superconducting dome has been a long-standing issue that remains unresolved in many classes of unconventional superconductors, notably cuprates, heavy fermions, and, most recently, iron pnictides. The existence of a QCP may offer a route to understanding the origin of unconventional superconductors’ anomalous non-Fermi liquid properties, the microscopic coexistence between unconventional superconductivity and magnetic or some other exotic order, and, ultimately, the mechanism of superconductivity itself. The isovalent substituted iron pnictide BaFe2(As1−xPx)2 offers a new platform for the study of quantum criticality, providing a unique opportunity to study the evolution of the electronic properties in a wide range of the phase diagram. Recent experiments in BaFe2(As1−xPx)2 have provided the first clear and unambiguous evidence of a second-order quantum phase transition lying beneath the superconducting dome.

Hypercomplex fluids are amalgamations of polymers, colloids, or amphiphilic molecules that exhibit emergent properties not observed in elemental systems alone. Especially promising building-blocks for assembly of hypercomplex materials are molecules with anisotropic shape. Alone, these molecules form numerous liquid crystalline phases with symmetries and properties that are fundamentally different from those of conventional liquids or solids. When combined with other complex fluids, liquid crystals form materials with diverse emergent properties. In equilibrium, the interactions, dimensions, and shapes of these hypercomplex materials can be precisely controlled. When driven far from equilibrium, these materials can deform and even spontaneously flow in the absence of external forces. Here we describe recent experimental accomplishments in this rapidly developing research area. We emphasize how the common theme underlying these diverse efforts is their reliance on the basic physics of molecular liquid crystals developed in the 1970s.

The condensation of excitons, bound electron-hole pairs in a solid, into a coherent collective electronic state was predicted more than 50 years ago. Perhaps surprisingly, the phenomenon was first observed in a system consisting of two closely spaced parallel two-dimensional electron gases in a semiconductor double quantum well. At an appropriate high magnetic field and low temperature, the bilayer electron system condenses into a state resembling a superconductor, only with the Cooper pairs replaced by excitons consisting of electrons in one layer bound to holes in the other. In spite of being charge neutral, the transport of excitons within the condensate gives rise to several spectacular electrical effects. This article describes these phenomena and examines how they inform our understanding of this unique phase of quantum electronic matter.

Flocking is a paradigmatic case of self-organized collective behavior in biology and a living example of active matter. Several models and theories have been developed in recent years to address these kinds of systems. However, unlike granular materials and biological systems at the microscale, experiments have been scarce until recently, preventing the necessary comparison between theory and data. In this review, we discuss a novel approach to flocking, in which experimental data are used as a starting point to empirically characterize flocking as a collective phenomenon—as the term is understood in statistical and condensed matter physics—and build models directly from the data.

The crossover from weak-coupling Bardeen-Cooper-Schrieffer (BCS) pairing to a Bose-Einstein condensate (BEC) of tightly bound pairs, as a function of the attractive interaction in Fermi systems, has long been of interest to theoretical physicists. The past decade has seen a series of remarkable experimental developments in ultracold Fermi gases that have realized the BCS-BEC crossover in the laboratory, bringing with it fresh new insights into the very strongly interacting unitary regime in the middle of this crossover. In this review, we start with a pedagogical introduction to the crossover and then focus on recent progress in the strongly interacting regime. Although our focus is on new theoretical developments, we also describe three key experiments that probe the thermodynamics, transport, and spectroscopy of the unitary Fermi gas. We discuss connections between the unitary regime and other areas of physics—quark-gluon plasmas, gauge-gravity duality, and high-temperature superconductivity—and conclude with open questions about strongly interacting Fermi gases.

Recent experimental and theoretical progress on the study of crackling noise in the plastic deformation of crystals, ferroelastics, and porous materials is reviewed. We specifically point out opportunities and potential pitfalls in this approach to the study of the nonequilibrium dynamics of disordered materials. Direct optical observation of domain boundary movement under stress and experimental results from acoustic emission and heat-flux measurements lead to power-law scaling of the jerk distribution with energy exponents between 1.3 and 2.3. The collapse of porous materials under stress leads to exceptionally large intervals of power-law scaling (seven decades). Applications in geology and materials sciences are discussed.

The question of whether the dramatic slowing down of the dynamics of glass-forming liquids near the structural glass transition is caused by the growth of one or more correlation lengths has received much attention in recent years. Several proposals have been made for both static and dynamic length scales that may be responsible for the growth of timescales as the glass transition is approached. These proposals are critically examined with emphasis on the dynamic length scale associated with spatial heterogeneity of local dynamics and the static point-to-set or mosaic length scale of the random first-order transition theory of equilibrium glass transition. Available results for these length scales, obtained mostly from simulations, are summarized, and the relation of the growth of timescales near the glass transition with the growth of these length scales is examined. Some of the outstanding questions about length scales in glass-forming liquids are discussed, and studies in which these questions may be addressed are suggested.

Chemically synthesized semiconductor nanocrystals (NCs) have been extensively studied as a test bed for exploring the physics of strong quantum confinement and as a highly flexible materials platform for the realization of a new generation of solution-processed optical, electronic, and optoelectronic devices. Because of readily tunable, size-dependent emission and absorption spectra, colloidal NCs are especially attractive for applications in light-emitting diodes, solid-state lighting, lasing, and solar cells. It is universally recognized that the realization of these and other prospective applications of NCs requires a detailed understanding of carrier-carrier interactions in these structures, as they have a strong effect on both recombination and photogeneration dynamics of charge carriers. For example, nonradiative Auger recombination is one of the key factors limiting the performance of NC-based lasers and light-emitting diodes. The inverse of this process, carrier multiplication, plays a beneficial role in light harvesting and can be used to boost the efficiency of photovoltaics through increased photocurrent. This article reviews recent progress in the understanding of multicarrier processes in NCs of various complexities, including zero-dimensional spherical quantum dots, quasi-one-dimensional nanorods, and various types of core-shell heterostructures. This review’s specific focus is on recent efforts toward controlling multicarrier interactions using traditional approaches, such as size and shape control, as well as newly developed methods involving interface engineering for suppression of Auger decay and engineering of intraband cooling rates for enhancement of carrier multiplication.

The response of polycrystals to plastic deformation is studied at the level of variations within individual grains, and comparisons are made to theoretical calculations using crystal plasticity (CP). We provide a brief overview of CP and a review of the literature, which is dominated by surface observations. The motivating question asks how well does CP represent the mesoscale behavior of large populations of dislocations (as carriers of plastic strain). The literature shows consistently that only moderate agreement is found between experiment and calculation. We supplement this with a current example of microstructure evolution in the interior of a copper sample subjected to tensile deformation. Nondestructive measurements of orientation fields were performed using the near-field high-energy X-ray diffraction microscopy (nf-HEDM) technique at the Advanced Photon Source (APS). Starting at highly ordered grains, a single two-dimensional slice of microstructure containing ∼150 grains was followed through multiple strain states, where it tracked lattice rotations and defect accumulation of up to 14% elongation. In accord with the literature, at the scale of individual grains, comparison of observations with CP models indicates reasonable qualitative agreement but significant variations between simulation and experiment are apparent. The conclusion is that in order to be able to quantify the effects of microstructure on the distributions of slip, orientation change, and damage accumulation, the empirically derived constitutive relations used in continuum-scale simulations need to be improved. Equally important will be the development of large-scale simulations of polycrystals that directly model dislocations.

Among very-low-disorder systems of condensed matter, the high-mobility two-dimensional electron gas (2DEG) confined in aluminum gallium arsenide (AlGaAs)–gallium arsenide (GaAs) heterostructures holds a privileged position as a platform for the discovery of new electronic states driven by strong Coulomb interactions. Molecular beam epitaxy (MBE), an ultra-high vacuum (UHV), thin-film deposition technique, produces the highest quality 2DEGs and has played a central role in a number of discoveries that have at their root the interplay of reduced dimensionality, strong electron-electron interactions, and disorder. This review attempts to describe the latest developments in heterostructure design, MBE technology, and the evolution of our understanding of disorder that result in improved material quality and facilitate discovery of new phenomena at ever finer energy scales.

Despite 5,000 years of metals in technology and 80 years since dislocations were postulated as the carriers of deformation, we still lack fundamental theories of dislocation substructure development and its relation to the stress-strain response in real materials. In this review, we focus on one type of tool being used to explore dislocation-based plasticity: the modeling and simulation of dislocation structures and properties. Before a discussion of the methods, we present a brief summary of aspects of dislocation theory that are critical to understanding plasticity. We then review three approaches to modeling and simulating dislocation properties at the mesoscale: discrete dislocation dynamics, phase-field dislocation methods, and continuous dislocation theory. We discuss the power and limitations of the methods when applied to real-world problems.