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Showing papers in "Reports on Progress in Physics in 2011"


Journal ArticleDOI
TL;DR: In this paper, a spin fluctuation theory and the sign-changing s-wave symmetry of superconducting gap structures was proposed to account for the nonuniversality of the gap structures of FeNictide and chalcogenide superconductors.
Abstract: The recently discovered Fe-pnictide and chalcogenide superconductors display low-temperature properties suggesting superconducting gap structures which appear to vary substantially from family to family, and even within families as a function of doping or pressure. We propose that this apparent nonuniversality can actually be understood by considering the predictions of spin fluctuation theory and accounting for the peculiar electronic structure of these systems, coupled with the likely 'sign-changing s-wave' (s?) symmetry. We review theoretical aspects, materials properties and experimental evidence relevant to this suggestion, and discuss which further measurements would be useful to settle these issues.Satisfactoriness has to be measured by a multitude of standards, of which some, for aught we know, may fail in any given case; and what is more satisfactory than any alternative in sight, may to the end be a sum of pluses and minuses, concerning which we can only trust that by ulterior corrections and improvements a maximum of the one and a minimum of the other may some day be approached.??????????????????????William James, Meaning of Truth

840 citations


Journal ArticleDOI
TL;DR: The KKR-GF method as mentioned in this paper represents the electronic structure of a system directly and efficiently in terms of its single-particle Green's function (GF), which is in contrast to its original version and many other traditional wave-function-based all-electron band structure methods dealing with periodically ordered solids.
Abstract: The modern version of the KKR (Korringa–Kohn–Rostoker) method represents the electronic structure of a system directly and efficiently in terms of its single-particle Green's function (GF). This is in contrast to its original version and many other traditional wave-function-based all-electron band structure methods dealing with periodically ordered solids. Direct access to the GF results in several appealing features. In addition, a wide applicability of the method is achieved by employing multiple scattering theory. The basic ideas behind the resulting KKR-GF method are outlined and the different techniques to deal with the underlying multiple scattering problem are reviewed. Furthermore, various applications of the KKR-GF method are reviewed in some detail to demonstrate the remarkable flexibility of the approach. Special attention is devoted to the numerous developments of the KKR-GF method, that have been contributed in recent years by a number of work groups, in particular in the following fields: embedding schemes for atoms, clusters and surfaces, magnetic response functions and anisotropy, electronic and spin-dependent transport, dynamical mean field theory, various kinds of spectroscopies, as well as first-principles determination of model parameters.

758 citations


Journal ArticleDOI
TL;DR: The current status of theoretical studies on the quantum chromodynamics (QCD) phase diagram at finite temperature and baryon chemical potential is reviewed with special emphasis on the origin of various phases and their symmetry breaking patterns as mentioned in this paper.
Abstract: The current status of theoretical studies on the quantum chromodynamics (QCD) phase diagram at finite temperature and baryon chemical potential is reviewed with special emphasis on the origin of various phases and their symmetry breaking patterns. Topics include quark deconfinement, chiral symmetry restoration, order of the phase transitions, QCD critical point(s), colour superconductivity, various inhomogeneous states and implications from QCD-like theories.

727 citations


Journal ArticleDOI
TL;DR: A brief overview of the current status of different types of qubits can be found in this paper, where a hybrid device made of natural/artificial atoms and photons may provide the next generation design for quantum computers.
Abstract: Remarkable progress towards realizing quantum computation has been achieved using natural and artificial atoms as qubits. This paper presents a brief overview of the current status of different types of qubits. On the one hand, natural atoms (such as neutral atoms and ions) have long coherence times, and could be stored in large arrays, providing ideal 'quantum memories'. On the other hand, artificial atoms (such as superconducting circuits or semiconductor quantum dots) have the advantage of custom-designed features and could be used as 'quantum processing units'. Natural and artificial atoms can be coupled with each other and can also be interfaced with photons for long-distance communications. Hybrid devices made of natural/artificial atoms and photons may provide the next-generation design for quantum computers.

672 citations


Journal ArticleDOI
TL;DR: In this article, the state of the art of diamond-based single-photon emitters and their fabrication methodologies are discussed, and the main challenges and perspectives for employing diamond emitters in quantum information processing are discussed.
Abstract: The exploitation of emerging quantum technologies requires efficient fabrication of key building blocks. Sources of single photons are extremely important across many applications as they can serve as vectors for quantum information—thereby allowing long-range (perhaps even global-scale) quantum states to be made and manipulated for tasks such as quantum communication or distributed quantum computation. At the single-emitter level, quantum sources also afford new possibilities in terms of nanoscopy and bio-marking. Color centers in diamond are prominent candidates to generate and manipulate quantum states of light, as they are a photostable solid-state source of single photons at room temperature. In this review, we discuss the state of the art of diamond-based single-photon emitters and highlight their fabrication methodologies. We present the experimental techniques used to characterize the quantum emitters and discuss their photophysical properties. We outline a number of applications including quantum key distribution, bio-marking and sub-diffraction imaging, where diamond-based single emitters are playing a crucial role. We conclude with a discussion of the main challenges and perspectives for employing diamond emitters in quantum information processing.

564 citations


Journal ArticleDOI
TL;DR: In this article, the basic modes of operation in cantilever-like micromechanical sensors and optical and electrical means for signal transduction are discussed with focus on silicon-and polymer-based technologies.
Abstract: The field of cantilever-based sensing emerged in the mid-1990s and is today a well-known technology for label-free sensing which holds promise as a technique for cheap, portable, sensitive and highly parallel analysis systems. The research in sensor realization as well as sensor applications has increased significantly over the past 10 years. In this review we will present the basic modes of operation in cantilever-like micromechanical sensors and discuss optical and electrical means for signal transduction. The fundamental processes for realizing miniaturized cantilevers are described with focus on silicon- and polymer-based technologies. Examples of recent sensor applications are given covering such diverse fields as drug discovery, food diagnostics, material characterizations and explosives detection.

525 citations


Journal ArticleDOI
TL;DR: A detailed review of the technical progress that made this new level of acuity possible and a survey of the new insights gained from an atomic level perspective of structural dynamics can be found in this article.
Abstract: One of the great dream experiments in Science is to directly observe atomic motions as they occur. Femtosecond electron diffraction provided the first 'light' of sufficient intensity to achieve this goal by attaining atomic resolution to structural changes on the relevant timescales. This review covers the technical progress that made this new level of acuity possible and gives a survey of the new insights gained from an atomic level perspective of structural dynamics. Atomic level views of the simplest possible structural transition, melting, are discussed for a number of systems in which both thermal and purely electronically driven atomic displacements can be correlated with the degree of directional bonding. Optical manipulation of charge distributions and effects on interatomic forces/bonding can be directly observed through the ensuing atomic motions. New phenomena involving strongly correlated electron?lattice systems are also discussed in which optically induced changes in the potential energy landscape lead to ballistic structural changes. Concepts such as the structural order parameters are now directly observable at the atomic level of inspection to give a remarkable view of the extraordinary degree of cooperativity involved in strongly correlated electron?lattice systems. These recent examples, in combination with time-resolved real space imaging now possible with electron probes, are truly defining an emerging field that holds great promise to make a significant impact in how we understand structural dynamics.This article is dedicated to the memory of Professor David John Hugh Cockayne, a world leader in electron microscopy, who sadly passed away in December.

479 citations


Journal ArticleDOI
TL;DR: The totally asymmetric exclusion process as discussed by the authors is regarded as a paradigmatic model for non-equilibrium statistical mechanics, much like the role the Ising model played for equilibrium statistical mechanics.
Abstract: Unlike equilibrium statistical mechanics, with its well-established foundations, a similar widely accepted framework for non-equilibrium statistical mechanics (NESM) remains elusive. Here, we review some of the many recent activities on NESM, focusing on some of the fundamental issues and general aspects. Using the language of stochastic Markov processes, we emphasize general properties of the evolution of configurational probabilities, as described by master equations. Of particular interest are systems in which the dynamics violates detailed balance, since such systems serve to model a wide variety of phenomena in nature. We next review two distinct approaches for investigating such problems. One approach focuses on models sufficiently simple to allow us to find exact, analytic, non-trivial results. We provide detailed mathematical analyses of a one-dimensional continuous-time lattice gas, the totally asymmetric exclusion process. It is regarded as a paradigmatic model for NESM, much like the role the Ising model played for equilibrium statistical mechanics. It is also the starting point for the second approach, which attempts to include more realistic ingredients in order to be more applicable to systems in nature. Restricting ourselves to the area of biophysics and cellular biology, we review a number of models that are relevant for transport phenomena. Successes and limitations of these simple models are also highlighted.

413 citations


Journal ArticleDOI
TL;DR: In this paper, a review of far-field optical techniques that have sufficient sensitivity to detect single metal particles is presented, and the importance of single-particle detection techniques in their development is emphasized.
Abstract: Since the first report on the far-field optical detection of single metal nanoparticles in the late 1990s, the field has rapidly developed and new methods and concepts have been introduced Eliminating averaging over the broad size, shape and crystallinity distributions produced by even the best of current synthesis methods, these techniques have proven extremely useful for gaining a deeper insight into many of the properties of metal nanoparticles These techniques have already led to the first applications specifically directed at using single particles In this review we describe far-field optical techniques (both linear and nonlinear) that have sufficient sensitivity to detect single metal particles We further discuss emerging applications, and emphasize the importance of single-particle detection techniques in their development

263 citations


Journal ArticleDOI
TL;DR: Graphene is possibly one of the largest and fastest growing fields in condensed matter research as mentioned in this paper, however, graphene is only one example in a large class of two-dimensional crystals with unusual properties.
Abstract: Graphene is possibly one of the largest and fastest growing fields in condensed matter research. However, graphene is only one example in a large class of two-dimensional crystals with unusual properties. In this paper we briefly review the properties of graphene and look at the exciting possibilities that lie ahead.

261 citations


Journal ArticleDOI
TL;DR: In this article, the interplay of frustration and strong electronic correlations in quasi-two-dimensional organic charge transfer salts, such as (BEDT-TTF)2X and EtnMe4−nPn[Pd(dmit)2]2] has been discussed.
Abstract: We review the interplay of frustration and strong electronic correlations in quasi-two-dimensional organic charge transfer salts, such as (BEDT-TTF)2X and EtnMe4−nPn[Pd(dmit)2]2. These two forces drive a range of exotic phases including spin liquids, valence bond crystals, pseudogapped metals and unconventional superconductivity. Of particular interest is that in several materials pressure drives a first-order transition from a spin liquid Mott insulating state to a superconducting state. Experiments on these materials raise a number of profound questions about the quantum behaviour of frustrated systems, particularly the intimate connection between spin liquids and superconductivity. Insights into these questions have come from a wide range of theoretical techniques including first principles electronic structure, quantum many-body theory and quantum field theory. In this review we introduce some of the basic ideas of the field by discussing a simple frustrated Heisenberg model with four spins. We then describe the key experimental results, emphasizing that for two materials, κ-(BEDT-TTF)2Cu2(CN)3 and EtMe3Sb[Pd(dmit)2]2, there is strong evidence for a spin liquid ground state, and for another, EtMe3P[Pd(dmit)2]2, there is evidence of a valence bond crystal ground state. We review theoretical attempts to explain these phenomena, arguing that they can be captured by a Hubbard model on the anisotropic triangular lattice at half filling, and that resonating valence bond wavefunctions capture most of the essential physics. We review evidence that this Hubbard model can have a spin liquid ground state for a range of parameters that are realistic for the relevant materials. In particular, spatial anisotropy and ring exchange are key to destabilizing magnetic order. We conclude by summarizing the progress made thus far and identifying some of the key questions still to be answered.

Journal ArticleDOI
TL;DR: In this article, it was shown that the process of smoothing over structure can contribute to an acceleration term and so alter the apparent value of the cosmological constant, and that concordance cosmology may be affected by back-reaction effects.
Abstract: Structure occurs over a vast range of scales in the Universe. Our large-scale cosmological models are coarse-grained representations of what exists, which have much less structure than there really is. An important problem for cosmology is determining the influence the small-scale structure in the Universe has on its large-scale dynamics and observations. Is there a significant, general relativistic, backreaction effect from averaging over structure? One issue is whether the process of smoothing over structure can contribute to an acceleration term and so alter the apparent value of the cosmological constant. If this is not the case, are there other aspects of concordance cosmology that are affected by backreaction effects? Despite much progress, this 'averaging problem' is still unanswered, but it cannot be ignored in an era of precision cosmology, for instance it may affect aspects of baryon acoustic oscillation observations.

Journal ArticleDOI
TL;DR: An overview of the behavior of Fe-based superconductors in magnetic fields is given with the emphasis on the materials features important for pinning of vortices, critical current densities, melting of the vortex structures and the upper critical fields.
Abstract: An overview of the behavior of Fe-based superconductors (FBSs) in magnetic fields is given with the emphasis on the materials features important for pinning of vortices, critical current densities, melting of the vortex structures and the upper critical fields. We also discuss how the multiband electronic structure of FBSs can result in extremely high upper critical fields tunable by doping or in the Fulde–Ferrel–Larkin–Ovchinnikov inhomogeneous state at high fields and low temperatures.

Journal ArticleDOI
TL;DR: In this paper, a review of both the quasi-probability representations of infinite-dimensional quantum theory (including the Wigner function) and the more recently defined quasi probability representations of finite dimensional quantum theory is presented, with an emphasis toward quantum information theory.
Abstract: This paper comprises a review of both the quasi-probability representations of infinite-dimensional quantum theory (including the Wigner function) and the more recently defined quasi-probability representations of finite-dimensional quantum theory. We focus on both the characteristics and applications of these representations with an emphasis toward quantum information theory. We discuss the recently proposed unification of the set of possible quasi-probability representations via frame theory and then discuss the practical relevance of negativity in such representations as a criteria for quantumness.

Journal ArticleDOI
TL;DR: In this paper, the fixed-node/fixed-phase diffusion Monte Carlo (FPMC) method is applied to the electronic structure of solids and other extended many-particle systems.
Abstract: Quantum Monte Carlo methods represent a powerful and broadly applicable computational tool for finding very accurate solutions of the stationary Schrequation for atoms, molecules, solids and a variety of model systems. The algorithms are intrinsically parallel and are able to take full advantage of present-day high-performance computing systems. This review paper concentrates on the fixed-node/fixed-phase diffusion Monte Carlo method with emphasis on its applications to the electronic structure of solids and other extended many-particle systems. (Some figures in this article are in colour only in the electronic version) This article was invited by M-Y Chou.

Journal ArticleDOI
TL;DR: In the first three years since the discovery of Fe-based high Tc superconductors, scanning tunneling microscopy (STM) and spectroscopy have shed light on three important questions as discussed by the authors.
Abstract: In the first three years since the discovery of Fe-based high Tc superconductors, scanning tunneling microscopy (STM) and spectroscopy have shed light on three important questions. First, STM has demonstrated the complexity of the pairing symmetry in Fe-based materials. Phase-sensitive quasiparticle interference (QPI) imaging and low temperature spectroscopy have shown that the pairing order parameter varies from nodal to nodeless s± within a single family, FeTe1−xSex. Second, STM has imaged C4 → C2 symmetry breaking in the electronic states of both parent and superconducting materials. As a local probe, STM is in a strong position to understand the interactions between these broken symmetry states and superconductivity. Finally, STM has been used to image the vortex state, giving insights into the technical problem of vortex pinning, and the fundamental problem of the competing states introduced when superconductivity is locally quenched by a magnetic field. Here we give a pedagogical introduction to STM and QPI imaging, discuss the specific challenges associated with extracting bulk properties from the study of surfaces, and report on progress made in understanding Fe-based superconductors using STM techniques.

Journal ArticleDOI
TL;DR: In this paper, the intrinsic in-plane electronic anisotropy of Fe-arsenide superconductors is determined by resistivity, reflectivity and angle-resolved photoemission spectroscopy measurements.
Abstract: The parent phases of the Fe-arsenide superconductors harbor an antiferromagnetic ground state. Significantly, the Neel transition is either preceded or accompanied by a structural transition that breaks the four-fold symmetry of the high-temperature lattice. Borrowing language from the field of soft condensed matter physics, this broken discrete rotational symmetry is widely referred to as an Ising nematic phase transition. Understanding the origin of this effect is a key component of a complete theoretical description of the occurrence of superconductivity in this family of compounds, motivating both theoretical and experimental investigation of the nematic transition and the associated in-plane anisotropy. Here we review recent experimental progress in determining the intrinsic in-plane electronic anisotropy as revealed by resistivity, reflectivity and angle-resolved photoemission spectroscopy measurements of detwinned single crystals of underdoped Fe-arsenide superconductors in the '122' family of compounds.

Journal ArticleDOI
TL;DR: In the past 15 years, several thousands of papers related to spin polarized tunneling and transport have been published, making this topic one of the hottest areas in condensed matter physics from both fundamental science and applications viewpoints.
Abstract: The phenomenon of quantum tunneling, which was discovered almost a century ago, has led to many subsequent discoveries. One such discovery, spin polarized tunneling, was made 40 years ago by Robert Meservey and Paul Tedrow (Tedrow and Meservey 1971 Phys. Rev. Lett. 26 192), and it has resulted in many fundamental observations and opened up an entirely new field of study. Until the mid-1990s, this field developed at a steady, low rate, after which a huge increase in activity suddenly occurred as a result of the unraveling of successful spin tunneling between two ferromagnets. In the past 15 years, several thousands of papers related to spin polarized tunneling and transport have been published, making this topic one of the hottest areas in condensed matter physics from both fundamental science and applications viewpoints. Many review papers and book chapters have been written in the past decade on this subject. This paper is not exhaustive by any means; rather, the emphases are on recent progress, technological developments and informing the reader about the current direction in which this topic is moving.

Journal ArticleDOI
TL;DR: In this article, the authors provide sufficient experimental evidence to support the reliability and the consistency of the angle-resolved photoemission spectroscopy measurements over a wide range of material compositions.
Abstract: Angle-resolved photoemission spectroscopy allows direct visualization and experimental determination of the electronic structure of crystals in the momentum space, including the precise characterization of the Fermi surface and the superconducting order parameter. It is thus particularly suited for investigating multi-band systems such as the Fe-based superconductors. In this review, we cover several aspects of these recently discovered materials that have been addressed by this technique, with a special emphasis on their superconducting gap and their Fermi surface topology. We provide sufficient experimental evidence to support the reliability and the consistency of the angle-resolved photoemission spectroscopy measurements over a wide range of material compositions.

Journal ArticleDOI
TL;DR: In this paper, the authors consider the positive role that grain boundaries (GBs) play in the metallic, low-temperature superconductors and then review the theoretical background and current experimental data relating to the properties of GBs.
Abstract: The Fe-based superconductors (FBSs) are an important new class of superconducting materials. As with any new superconductor with a high transition temperature and upper critical field, there is a need to establish what their applications potential might be. Applications require high critical current densities, so the usefulness of any new superconductor is determined both by the capability to develop strong vortex pinning and by the absence or ability to overcome any strong current-limiting mechanisms of which grain boundaries (GBs) in the cuprates are a cautionary example. In this review we first consider the positive role that GB properties play in the metallic, low-temperature superconductors and then review the theoretical background and current experimental data relating to the properties of GBs in FBS polycrystals, bicrystal thin films and wires. Based on this evidence, we conclude that GBs in FBS are weak linked in a qualitatively similar way to GBs in the cuprate superconductors, but also that the effects are a little less marked. Initial experiments with the textured substrates used for cuprate coated conductors show similar benefit for the critical current density of FBS thin films too. We also note that the particular richness of the pairing symmetry and the multiband parent state in FBS may provide opportunities for GB modification as a better understanding of their pairing state and GB properties are developed.

Journal ArticleDOI
TL;DR: In this paper, a self-consistent Eilenberger two-band model that relates the measurable superfluid density and temperature dependences of the superconducting gaps is presented.
Abstract: Measurements of London penetration depth, a sensitive tool to study multiband superconductivity, have provided several important insights into the behavior of Fe-based superconductors. We first briefly review the 'experimentalist-friendly' self-consistent Eilenberger two-band model that relates the measurable superfluid density and temperature dependences of the superconducting gaps. Then we focus on BaFe2As2-derived materials, for which the results are consistent with (1) two distinct superconducting gaps; (2) development of strong in-plane gap anisotropy with departure from optimal doping; (3) development of gap nodes along the c direction in a highly overdoped regime; (4) significant pair-breaking, presumably due to charge doping; (5) fully gapped intrinsic behavior (exponential at low temperatures) at optimal doping if the scattering is removed (probed in the 'self-doped' stoichiometric LiFeAs); (6) competition between the magnetically ordered state and superconductivity, which do coexist in underdoped compounds. Overall, it appears that while there are common trends in the behavior of Fe-based superconductors, the gap structure is non-universal and is quite sensitive to the doping level. It is plausible that the rich variety of possible gap structures within the general s± framework is responsible for the observed behavior.

Journal ArticleDOI
TL;DR: How atomic force microscopy (AFM) can be applied for structural and functional investigations of native membrane proteins is reviewed and Dynamic SMFS discloses fascinating insights into the free energy landscape of membrane proteins.
Abstract: Membranes confining cells and cellular compartments are essential for life. Membrane proteins are molecular machines that equip cell membranes with highly sophisticated functionality. Examples of such functions are signaling, ion pumping, energy conversion, molecular transport, specific ligand binding, cell adhesion and protein trafficking. However, it is not well understood how most membrane proteins work and how the living cell regulates their function. We review how atomic force microscopy (AFM) can be applied for structural and functional investigations of native membrane proteins. High-resolution time-lapse AFM imaging records membrane proteins at work, their oligomeric state and their dynamic assembly. The AFM stylus resembles a multifunctional toolbox that allows the measurement of several chemical and physical parameters at the nanoscale. In the single-molecule force spectroscopy (SMFS) mode, AFM quantifies and localizes interactions in membrane proteins that stabilize their folding and modulate their functional state. Dynamic SMFS discloses fascinating insights into the free energy landscape of membrane proteins. Single-cell force spectroscopy quantifies the interactions of live cells with their environment to single-receptor resolution. In the future, technological progress in AFM-based approaches will enable us to study the physical nature of biological interactions in more detail and decipher how cells control basic processes.

Journal ArticleDOI
TL;DR: What is known about spectral and intensity variations of terrestrial solar radiation as well as its penetration through the atmosphere and into human skin and tissue and action spectra relevant for health are described.
Abstract: The Sun has played a major role in the development of life on Earth. In Western culture, people are warned against Sun exposure because of its adverse effects: erythema, photoimmunosuppression, photoageing, photocarcinogenesis, cataracts and photokeratitis. However, Sun exposure is also beneficial, since moderate doses give beneficial physiological effects: vitamin D synthesis, reduction of blood pressure and mental health. Shortage of Sun exposure may be even more dangerous to human health than excessive exposure. Avoiding Sun exposure leads to vitamin D deficiency which is associated not only with rickets and osteomalacia, but also with increased risk of cardiovascular disease, multiple sclerosis, rheumatoid arthritis, diabetes, influenza, many types of cancer and adverse pregnancy outcomes. Solar radiation induces nitric oxide release in tissue and immediate pigment darkening which certainly play important roles, although these are still unknown. Action spectra relevant for health are described. We will also review what is known about spectral and intensity variations of terrestrial solar radiation as well as its penetration through the atmosphere and into human skin and tissue.

Journal ArticleDOI
TL;DR: In this article, the authors present and discuss two-body beta decays, namely bound-state beta decay and orbital electron capture, and provide general requirements common to any other experiment in this context.
Abstract: Beta decay of highly charged ions has attracted much attention in recent years. An obvious motivation for this research is that stellar nucleosynthesis proceeds at high temperatures where the involved atoms are highly ionized. Another important reason is addressing decays of well-defined quantum-mechanical systems, such as one-electron ions where all interactions with other electrons are excluded. The largest modifications of nuclear half-lives with respect to neutral atoms have been observed in beta decay of highly charged ions. These studies can be performed solely at ion storage rings and ion traps, because there high atomic charge states can be preserved for extended periods of time (up to several hours). Currently, all experimental results available in this field originate from experiments at the heavy-ion complex GSI in Darmstadt. There, the fragment separator facility FRS allows the production and separation of exotic, highly charged nuclides, which can then be stored and investigated in the storage ring facility ESR. In this review, we present and discuss in particular two-body beta decays, namely bound-state beta decay and orbital electron capture. Although we focus on experiments conducted at GSI, we will also attempt to provide general requirements common to any other experiment in this context. Finally, we address challenging but not yet performed experiments and we give prospects for the new radioactive beam facilities, such as FAIR in Darmstadt, IMP in Lanzhou and RIKEN in Wako.

Journal ArticleDOI
TL;DR: Optical lattices have emerged as ideal simulators for Hubbard models of strongly correlated materials, such as the high-temperature superconducting cuprates as mentioned in this paper, but this benefit of using optical lattices to study Hubbard models comes with one clear disadvantage: the energy scales in atomic systems are typically nanokelvin compared with kelvin in solids, with correspondingly miniscule temperature scale required to observe exotic phases such as d-wave superconductivity.
Abstract: Optical lattices have emerged as ideal simulators for Hubbard models of strongly correlated materials, such as the high-temperature superconducting cuprates. In optical lattice experiments, microscopic parameters such as the interaction strength between particles are well known and easily tunable. Unfortunately, this benefit of using optical lattices to study Hubbard models comes with one clear disadvantage: the energy scales in atomic systems are typically nanokelvin compared with kelvin in solids, with a correspondingly miniscule temperature scale required to observe exotic phases such as d-wave superconductivity. The ultra-low temperatures necessary to reach the regime in which optical lattice simulation can have an impact—the domain in which our theoretical understanding fails—have been a barrier to progress in this field. To move forward, a concerted effort is required to develop new techniques for cooling and, by extension, techniques to measure even lower temperatures. This paper will be devoted to discussing the concepts of cooling and thermometry, fundamental sources of heat in optical lattice experiments, and a review of proposed and implemented thermometry and cooling techniques.

Journal ArticleDOI
TL;DR: In this paper, the authors present a review of recent progress in the field of superconducting thin films of iron chalcogenides, with primary focus on FeSe1?xTex (0? x? 1).
Abstract: Iron chalcogenides are of great interest for both basic physics and high-field applications. Although their superconducting transition temperatures are typically lower than those of iron pnictides, iron chalcogenides exhibit lower anisotropies with very high upper critical field slopes near the superconducting transition temperatures. They also have the simplest structures among the iron-based superconductors. This review covers recent progress in the field of superconducting thin films of iron chalcogenides, with primary focus on FeSe1?xTex (0 ? x ? 1). High quality superconducting thin films with x = 0 and 0.5 have been fabricated by several groups. Of particular interest is that some of them exhibit significantly higher superconducting transition temperatures than those of bulk polycrystalline samples and single crystals over the entire doping regime. Upon the incorporation of small amounts of oxygen, superconductivity is seen to emerge in a thin film of FeTe (parent compound of the iron chalcogenides), but not in the bulk. Advances in superconducting-tape fabrication of iron chalcogenides are also described since the very high upper critical fields and critical current densities of these films suggest that they are prospective candidates for high-field applications. In addition, we present a brief comparison between iron chalcogenide films and iron pnictide films.

Journal ArticleDOI
TL;DR: In this article, the role of pressure may be complicated by the level of hydrostaticity, and high-pressure studies on two iron-based families of RFeAsO and AFe2As2 (A = alkaline-earth metals) are reviewed.
Abstract: Insight into the mechanism of high-temperature superconductivity can be gained by pressure-dependent studies of structural, thermodynamics and transport data. The role of pressure may be complicated by the level of hydrostaticity. High-pressure studies on two iron-based families of RFeAsO (R = rare-earth metals) and AFe2As2 (A = alkaline-earth metals) are reviewed here.

Journal ArticleDOI
TL;DR: In this paper, the authors review a number of experimental techniques that are beginning to reveal fine details of the bosonic spectrum α2F(Ω) that dominates the interaction between the quasiparticles in high-temperature superconductors.
Abstract: We review a number of experimental techniques that are beginning to reveal fine details of the bosonic spectrum α2F(Ω) that dominates the interaction between the quasiparticles in high-temperature superconductors. Angle-resolved photoemission spectroscopy (ARPES) shows kinks in electronic dispersion curves at characteristic energies that agree with similar structures in the optical conductivity and tunnelling spectra. Each technique has its advantages. ARPES is momentum resolved and offers independent measurements of the real and imaginary part of the contribution of the bosons to the self-energy of the quasiparticles. The optical conductivity can be used on a larger variety of materials and with the use of maximum entropy techniques reveals rich details of the spectra including their evolution with temperature and doping. Scanning tunnelling spectroscopy offers spatial resolution on the unit cell level. We find that together the various spectroscopies, including recent Raman results, are pointing to a unified picture of a broad spectrum of bosonic excitations at high temperatures which evolves, as the temperature is lowered, into a peak in the 30–60 meV region and a featureless high-frequency background in most of the materials studied. This behaviour is consistent with the spectrum of spin fluctuations as measured by magnetic neutron scattering. However, there is evidence for a phonon contribution to the bosonic spectrum as well.

Journal ArticleDOI
TL;DR: Point-contact Andreev-reflection spectroscopy has proven to be one of the most powerful tools in the investigation of superconductors, where it provides information on the order parameter (OP), a fundamental property of the superconducting state as mentioned in this paper.
Abstract: Point-contact Andreev-reflection spectroscopy (PCAR) has proven to be one of the most powerful tools in the investigation of superconductors, where it provides information on the order parameter (OP), a fundamental property of the superconducting state. In the past 20 years, successive improvements of the models used to analyze the spectra have continuously extended its capabilities, making it suited to study new superconductors with 'exotic' properties such as anisotropic, nodal and multiple OPs. In Fe-based superconductors, the complex compound- and doping-dependent Fermi surface (FS) and the predicted sensitivity of the OP to fine structural details present unprecedent challenges for this technique. Nevertheless, we show here that PCAR measurements in Fe-based superconductors carried out so far have already greatly contributed to our understanding of these materials, despite some apparent inconsistencies that can be overcome if a homogeneous treatment of the data is used. We also demonstrate that, if properly extended theoretical models for Andreev reflection are used, directional PCAR spectroscopy can provide detailed information not only on the amplitude and symmetry of the OPs, but also on the nature of the pairing boson, and even give some hints about the shape of the FS.

Journal ArticleDOI
TL;DR: In this article, the authors present a summary of results on single crystal growth of two types of iron-chalcogenide superconductors, Fe1+yTe1?xSex (11), and AxFe2?ySe2 (A = K, Rb, Cs, Tl, Tls/K, TL/K and Tl/Rb), using Bridgman, zone-melting, vapor self-transport and flux techniques.
Abstract: In this review, we present a summary of results on single crystal growth of two types of iron-chalcogenide superconductors, Fe1+yTe1?xSex (11), and AxFe2?ySe2 (A = K, Rb, Cs, Tl, Tl/K, Tl/Rb), using Bridgman, zone-melting, vapor self-transport and flux techniques. The superconducting and magnetic properties (the latter gained mainly from neutron scattering measurements) of these materials are reviewed to demonstrate the connection between magnetism and superconductivity. It will be shown that for the 11 system, while static magnetic order around the reciprocal lattice position (0.5,?0) competes with superconductivity, spin excitations centered around (0.5,?0.5) are closely coupled to the materials' superconductivity; this is made evident by the strong correlation between the spectral weight around (0.5,?0.5) and the superconducting volume fraction. The observation of a spin resonance below the superconducting temperature, Tc, and the magnetic-field dependence of the resonance emphasize the close interplay between spin excitations and superconductivity, similar to cuprate superconductors. In AxFe2?ySe2, superconductivity with Tc ~ 30?K borders an antiferromagnetic insulating phase; this is closer to the behavior observed in the cuprates but differs from that in other iron-based superconductors.