Showing papers in "Annual Review of Condensed Matter Physics in 2012"

Superconducting Microresonators: Physics and Applications

Abstract: Interest in superconducting microresonators has grown dramatically over the past decade. Resonator performance has improved substantially through the use of improved geometries and materials as well as a better understanding of the underlying physics. These advances have led to the adoption of superconducting microresonators in a large number of low-temperature experiments and applications. This review outlines these developments, with particular attention given to the use of superconducting microresonators as detectors.

Pairing mechanism in Fe-based superconductors

Abstract: I review recent works on the symmetry and the structure of the superconducting gap in Fe-based superconductors (FeSCs) and on the underlying pairing mechanism in these systems. The experimental data on superconductivity show very rich behavior, with potentially different symmetry of a superconducting state for different compositions of the same material. The variety of different pairing states raises the issue of whether the physics of FeSCs is model dependent or is universal, governed by a single underlying pairing mechanism. I argue that the physics is universal and that all pairing states obtained so far can be understood within the same universal pairing scenario and are well described by the effective low-energy model with small numbers of input parameters.

What Can Gauge-Gravity Duality Teach Us about Condensed Matter Physics?

Abstract: I discuss the impact of gauge-gravity duality on our understanding of two classes of systems: conformal quantum matter and compressible quantum matter. The first conformal class includes systems, such as the boson Hubbard model in two spatial dimensions, which display quantum critical points described by conformal field theories. Questions associated with non-zero temperature dynamics and transport are difficult to answer using conventional field theoretic methods. I argue that many of these can be addressed systematically using gauge-gravity duality, and discuss the prospects for reliable computation of low frequency correlations. Compressible quantum matter is characterized by the smooth dependence of the charge density, associated with a global U(1) symmetry, upon a chemical potential. Familiar examples are solids, superfluids, and Fermi liquids, but there are more exotic possibilities involving deconfined phases of gauge fields in the presence of Fermi surfaces. I survey the compressible systems studied using gauge-gravity duality, and discuss their relationship to the condensed matter classification of such states. The gravity methods offer hope of a deeper understanding of exotic and strongly-coupled compressible quantum states.

Spin Ice, Fractionalization, and Topological Order

Abstract: The spin ice compounds Dy2Ti2O7 and Ho2Ti2O7 are highly unusual magnets that epitomize a set of concepts of great interest in modern condensed matter physics: Their low-energy physics exhibits an emergent gauge field and their excitations are magnetic monopoles that arise from the fractionalization of the microscopic magnetic spin degrees of freedom. In this review, we provide an elementary introduction to these concepts and we survey the thermodynamics, statics, and dynamics—in and out of equilibrium—of spin ice from these vantage points. Along the way, we touch on topics such as emergent Coulomb plasmas, observable Dirac strings, and irrational charges. We close with the outlook for these unique materials.

Quantum Coherence in Photosynthetic Light Harvesting

Abstract: Recent two-dimensional (2D) electronic spectroscopic experiments revealed that electronic energy transfer in photosynthetic light harvesting involves long-lived quantum coherence among electronic excitations of pigments. These findings have led to the suggestion that quantum coherence might play a role in achieving the remarkable quantum efficiency of photosynthetic light harvesting. Further, this speculation has led to much effort being devoted to elucidation of the quantum mechanisms of the photosynthetic excitation energy transfer (EET). In this review, we provide an overview of recent experimental and theoretical investigations of photosynthetic electronic energy transfer, specifically addressing underlying mechanisms of the observed long-lived coherence and its potential roles in photosynthetic light harvesting. We close with some thoughts on directions for future developments in this area.

Studying Two Dimensional Systems With the Density Matrix Renormalization Group

Abstract: The density matrix renormalization group (DMRG) is one of the most powerful numerical methods for studying two-dimensional quantum lattice systems, despite a perception that it is only suitable for one dimension. Reviewing past applications of DMRG in 2D demonstrates its success in treating a wide variety of problems, although it remains underutilized in this context. We present techniques for performing cutting-edge 2D DMRG studies including methods for ensuring convergence, extrapolating finite-size data, and extracting gaps and excited states. Finally, we consider a selection of recently developed 2D tensor network methods and compare the performance of one of these to 2D DMRG.

Phase Change Materials: Challenges on the Path to a Universal Storage Device

Abstract: Phase change materials, in which a material is reversibly switched between an amorphous and crystalline state with corresponding contrast in optical and electronic transport properties, are excellent nonvolatile storage media. Rewritable digital versatile disks (DVDs) and Blu-ray discs are based on such materials in which the optical contrast between the amorphous and crystalline phases enables data storage. Additionally, the large change in electronic transport properties with resistivity contrast of up to six orders of magnitude on crystallization and fast switching speed is at the heart of a new class of nonvolatile data storage devices with electronic read/write operation and potential for future miniaturization. The amorphous state is characterized by saturated covalent bonds, whereas the crystalline phase forms resonant bonds. This bonding mechanism can account for the high electronic polarizabilities that characterize crystalline phase change materials. Interestingly, the relevant electronic states...

Mechanical Instabilities of Gels

Abstract: Although the study of gels undoubtedly takes its roots within the field of physicochemistry, the interest in gels has flourished and they have progressively become an important object in the study of the mechanics of polymeric materials and volumetric growth, raising some fascinating problems, some of them remaining unsolved. Because gels are multiphase objects, their study represents an important step in the understanding of the mechanics of complex soft matter as well as for the process of shape generation in biological bodies. The scope of this article is to review the understanding we have of the mechanical behavior of gels, with a strong focus on the development of instabilities in swelling gels.

Quantum Computation by Local Measurement

Abstract: Quantum computation is a novel way of information processing that allows, for certain classes of problems, exponential speedups over classical computation. Various models of quantum computation exist, such as the adiabatic, circuit, and measurement-based models. They have been proven equivalent in their computational power, but operate very differently. As such, they may be suitable for realization in different physical systems, and also offer different perspectives on open questions such as the precise origin of the quantum speedup. Here, we give an introduction to the one-way quantum computer, a scheme of measurement-based quantum computation (MBQC). In this model, the computation is driven by local measurements on a carefully chosen, highly entangled state. We discuss various aspects of this computational scheme, such as the role of entanglement and quantum correlations. We also give examples for ground states of simple Hamiltonians that enable universal quantum computation by local measurements.

Planetary Atmospheres as Nonequilibrium Condensed Matter

Abstract: Planetary atmospheres, and models of them, are discussed from the viewpoint of condensed matter physics. Atmospheres are a form of condensed matter, and many of the most interesting concepts of condensed matter systems are realized by them. The essential physics of the general circulation is illustrated with idealized two-layer and one-layer models of the atmosphere. Equilibrium and nonequilibrium statistical mechanics are used to directly ascertain the statistics of these models.

Physics of Cancer: The Impact of Heterogeneity

Abstract: It is a common mistake to view cancer as a single disease with a single possible cure which we have just not found yet. In reality cancer takes on many forms that share a common symptom: uncontrolled cell growth and successful invasion of cancer colonies to remote regions of the body. The key reason why we may never be able to defeat cancer may lie in the extreme heterogeneity of the population of the cells in a tumor: there is no one magic bullet. We will try in this review to show how the developing field of the physics of biological heterogeneity can help us understand and quantify the emergent heterogeneity that makes cancer such a fundamental puzzle.

Bose Gases with Nonzero Spin

Abstract: Gaseous Bose-Einstein condensates with nonzero spin feature multicomponent order parameters that reflect the internal degrees of freedom. They exhibit a rich variety of superfluid and magnetic phenomena similar or complementary to those of superfluid helium-3. The unprecedented degree of manipulability of atomic gases makes spinor condensates a unique playground for exploring symmetry breaking, topological excitations, and the interplay between superfluidity and magnetism. An overview of these subjects is provided.

Sixty Years of Condensed Matter Physics: An Everlasting Adventure

Abstract: Condensed matter physics has changed since the fifties: I attempt to retrace its evolution in the light of my own trajectory. It was and it remains a living field, in constant renewal. New ideas, new concepts keep appearing along with new experimental and theoretical tools. The danger lies in the bureaucratic evolution of scientific research, which might sterilize imagination and innovation. The future lies in the hands of young physicists who should defend their independence and creativity against fashions and competition.