Showing papers on "High-temperature superconductivity published in 2007"

Scanning tunneling spectroscopy of high-temperature superconductors

Abstract: Tunneling spectroscopy has played a central role in the experimental verification of the microscopic theory of superconductivity in classical superconductors. Initial attempts to apply the same approach to high-temperature superconductors were hampered by various problems related to the complexity of these materials. The use of scanning tunneling microscopy and spectroscopy (STM and STS) on these compounds allowed the main difficulties to be overcome. This success motivated a rapidly growing scientific community to apply this technique to high-temperature superconductors. This paper reviews the experimental highlights obtained over the last decade. The crucial efforts to gain control over the technique and to obtain reproducible results are first recalled. Then a discussion on how the STM and STS techniques have contributed to the study of some of the most unusual and remarkable properties of high-temperature superconductors is presented: the unusually large gap values and the absence of scaling with the critical temperature, the pseudogap and its relation to superconductivity, the unprecedented small size of the vortex cores and its influence on vortex matter, the unexpected electronic properties of the vortex cores, and the combination of atomic resolution and spectroscopy leading to the observation of periodic local density of states modulations in the superconducting and pseudogap states and in the vortex cores.

Handbook of high-temperature superconductivity : theory and experiment

Abstract: From Single- to Bipolarons with Jahn-eller Character and Metallic Cluster-Stripes in Hole-Doped Cuprates.- Tunneling Measurements of the Cuprate Superconductors.- Angle-Resolved Photoemission Spectroscopy on Electronic Structure and lectron-Phonon Coupling in Cuprate Superconductors.- Microwave Electrodynamics of High Temperature Superconductors.- Magnetic Resonance Studies of High Temperature Superconductors.- Neutron Scattering Studies of Antiferromagnetic Correlations in Cuprates.- Optical Conductivity and Spatial Inhomogeneity in Cuprate Superconductors.- What Tc can Teach About Superconductivity.- High-Tc Superconductors: Thermodynamic Properties.- Normal State Transport Properties.- High-Pressure Effects.- Superconductivity in Organic Conductors.- Numerical Studies of the 2D Hubbard Model.- t-J Model and the Gauge Theory Description of Underdoped Cuprates.- How Optimal Inhomogeneity Produces High Temperature Superconductivity.- Superconducting States on the Border of Itinerant Electron Magnetism.

Large intrinsic effect of axial strain on the critical current of high-temperature superconductors for electric power applications

Abstract: A remarkably large reversible reduction in the critical current of “second generation” high-temperature superconductors for electric power applications has been measured with a new technique over a wide range of mechanical strain. The effect amounts to a 40% reduction in critical current at 1% compressive strain in self-magnetic field, and is symmetric for compressive and tensile strains. The intrinsic effect is measured in highly aligned multigranular YBa2Cu3O7−d coated conductors made by different processes, including superconductors with nanoscale pinning centers. This effect and its magnitude are expected to have a significant impact on power applications and provide a useful new parameter for probing the fundamental nature of current transport in high-temperature superconductors.

Hierarchy of Multiple Many-Body Interaction Scales in High-Temperature Superconductors

Abstract: To date, angle-resolved photoemission spectroscopy has been successful in identifying energy scales of the many-body interactions in correlated materials, focused on binding energies of up to a few hundred meV below the Fermi energy. Here, at higher energy scale, we present improved experimental data from four families of high-T{sub c} superconductors over a wide doping range that reveal a hierarchy of many-body interaction scales focused on: the low energy anomaly ('kink') of 0.03-0.09eV, a high energy anomaly of 0.3-0.5eV, and an anomalous enhancement of the width of the LDA-based CuO{sub 2} band extending to energies of {approx} 2 eV. Besides their universal behavior over the families, we find that all of these three dispersion anomalies also show clear doping dependence over the doping range presented.

Behavior of bulk high-temperature superconductors of finite thickness subjected to crossed magnetic fields: Experiment and model

Abstract: Crossed-magnetic-field effects on bulk high-temperature superconductors have been studied both experimentally and numerically. The sample geometry investigated involves finite-size effects along both (crossed-)magnetic-field directions. The experiments were carried out on bulk melt-processed Y-Ba-Cu-O single domains that had been premagnetized with the applied field parallel to their shortest direction (i.e., the $c$ axis) and then subjected to several cycles of the application of a transverse magnetic field parallel to the sample $ab$ plane. The magnetic properties were measured using orthogonal pickup coils, a Hall probe placed against the sample surface, and magneto-optical imaging. We show that all principal features of the experimental data can be reproduced qualitatively using a two-dimensional finite-element numerical model based on an $E\text{\ensuremath{-}}J$ power law and in which the current density flows perpendicularly to the plane within which the two components of magnetic field are varied. The results of this study suggest that the suppression of the magnetic moment under the action of a transverse field can be predicted successfully by ignoring the existence of flux-free configurations or flux-cutting effects. These investigations show that the observed decay in magnetization results from the intricate modification of current distribution within the sample cross section. The current amplitude is altered significantly only if a field-dependent critical current density ${J}_{\mathrm{c}}(B)$ is assumed. Our model is shown to be quite appropriate to describe the cross-flow effects in bulk superconductors. It is also shown that this model does not predict any saturation of the magnetic induction, even after a large number $(\ensuremath{\sim}100)$ of transverse field cycles. These features are shown to be consistent with the experimental data.

Fluctuating Cu–O–Cu bond model of high-temperature superconductivity

Abstract: Twenty years of research have yet to produce a consensus on the origin of high-temperature superconductivity (HTS). However, several generic characteristics of the copper oxide superconductors have emerged as the essential ingredients of and/or constraints on any viable microscopic model of HTS. Besides a critical temperature Tc of the order of 100 K, they include a d-wave superconducting gap with Fermi liquid nodal excitations, a pseudogap with d-symmetry and the characteristic temperature scale T*, an anomalous doping-dependent oxygen isotope shift, nanometre-scale gap inhomogeneity and so on. The isotope shift implies a key role for oxygen vibrations, but conventional Bardeen–Cooper–Schrieffer single-phonon coupling is essentially forbidden by symmetry and by the on-site Coulomb interaction U. Here we invoke the nonlinear modulation of the Cu–Cu bond by planar oxygen vibrations. The Fermi liquid nature of the d-wave superconducting ground state supports a weak-coupling treatment of this modulation. The dominant fluctuations are manifested in a pattern of oxygen vibrational square amplitudes with quadrupolar symmetry around a given Cu site. On the basis of such bond fluctuations, both dynamic and static, we can understand the salient features of HTS.

Angle-Resolved Photoemission Spectroscopy on Electronic Structure and Electron-Phonon Coupling in Cuprate Superconductors

Abstract: In addition to the record high superconducting transition temperature (T{sub c}), high temperature cuprate superconductors are characterized by their unusual superconducting properties below T{sub c}, and anomalous normal state properties above T{sub c}. In the superconducting state, although it has long been realized that superconductivity still involves Cooper pairs, as in the traditional BCS theory, the experimentally determined d-wave pairing is different from the usual s-wave pairing found in conventional superconductors. The identification of the pairing mechanism in cuprate superconductors remains an outstanding issue. The normal state properties, particularly in the underdoped region, have been found to be at odd with conventional metals which is usually described by Fermi liquid theory; instead, the normal state at optimal doping fits better with the marginal Fermi liquid phenomenology. Most notable is the observation of the pseudogap state in the underdoped region above T{sub c}. As in other strongly correlated electrons systems, these unusual properties stem from the interplay between electronic, magnetic, lattice and orbital degrees of freedom. Understanding the microscopic process involved in these materials and the interaction of electrons with other entities is essential to understand the mechanism of high temperature superconductivity. Since the discovery of high-T{sub c} superconductivity in cuprates, angle-resolved photoemission spectroscopy (ARPES) has provided key experimental insights in revealing the electronic structure of high temperature superconductors. These include, among others, the earliest identification of dispersion and a large Fermi surface, an anisotropic superconducting gap suggestive of a d-wave order parameter, and an observation of the pseudogap in underdoped samples. In the mean time, this technique itself has experienced a dramatic improvement in its energy and momentum resolutions, leading to a series of new discoveries not thought possible only a decade ago. This revolution of the ARPES technique and its scientific impact result from dramatic advances in four essential components: instrumental resolution and efficiency, sample manipulation, high quality samples and well-matched scientific issues. The purpose of this treatise is to go through the prominent results obtained from ARPES on cuprate superconductors. Because there have been a number of recent reviews on the electronic structures of high-T{sub c} materials, we will mainly present the latest results not covered previously, with a special attention given on the electron-phonon interaction in cuprate superconductors. What has emerged is rich information about the anomalous electron-phonon interaction well beyond the traditional views of the subject. It exhibits strong doping, momentum and phonon symmetry dependence, and shows complex interplay with the strong electron-electron interaction in these materials. ARPES experiments have been instrumental in identifying the electronic structure, observing and detailing the electron-phonon mode coupling behavior, and mapping the doping evolution of the high-T{sub c} cuprates. The spectra evolve from the strongly coupled, polaronic spectra seen in underdoped cuprates to the Migdal-Eliashberg like spectra seen in the optimally and overdoped cuprates. In addition to the marked doping dependence, the cuprates exhibit pronounced anisotropy with direction in the Brillouin zone: sharp quasiparticles along the nodal direction that broaden significantly in the anti-nodal region of the underdoped cuprates, an anisotropic electron-phonon coupling vertex for particular modes identified in the optimal and overdoped compounds, and preferential scattering across the two parallel pieces of Fermi surface in the antinodal region for all doping levels. This also contributes to the pseudogap effect. To the extent that the Migdal-Eliashberg picture applies, the spectra of the cuprates bear resemblance to that seen in established strongly coupled electron-phonon superconductors such as Pb. On the other hand, the cuprates deviate from this conventional picture. In the underdoped regime, the carriers are best understood as small polarons in an antiferromagnetic, highly electron correlated background, while the doped compounds require an anisotropic electron-phonon vertex to detail the prominent mode coupling signatures in the superconducting state. Electronic vertex corrections to the electron-phonon coupling furthermore may enhance, and for certain phonons, determine, the anisotropy of the electron-phonon coupling. A consistent picture emerges of the cuprates, combining strong, anisotropic electron-phonon coupling, particular phonon modes that could give rise to such a coupling, and an electron-electron interaction modifying the el-ph vertex.

Transport evidence of a magnetic quantum phase transition in electron-doped high-temperature superconductors

Abstract: We present magnetotransport evidence for antiferromagnetism in films of the electron-doped cuprates ${\mathrm{Pr}}_{2\ensuremath{-}x}{\mathrm{Ce}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$. Our results show clear signature of static or quasistatic antiferromagnetism up to optimal doping $x=0.15$, with a quantum phase transition close to $x=0.16$, and a coexistence of antiferromagnetism and superconductivity for $0.12\ensuremath{\leqslant}x\ensuremath{\leqslant}0.15$.

Interaction of magnetic field and magnetic history in high-temperature superconductors

Abstract: Yttrium barium copper oxide (YBCO) coated conductors are now the most promising high-temperature superconducting tapes in terms of current capacity and price. One form of these conductors utilizes YBCO films on Ni–W metallic tapes and is being considered for a number of power engineering applications. In these applications, the conductor will carry an ac current, leading to energy losses, which are the focus of significant technical and experimental efforts. Our measurements of the ac losses of YBCO/Ni–W conductors carrying ac currents in applied dc magnetic fields have revealed a complex interaction between the magnetic materials present, the geometry of the conductor, the ac and dc magnetic fields, and the electromagnetic “history” of the sample. The investigation of this interaction is the main subject of this paper.

Microstructure and Properties of High-Temperature Superconductors

Abstract: Superconductors and Superconductivity: General Issues.- High-Temperature Superconductors: Overview.- Composition Features and HTSC Preparation Techniques.- Experimental Investigations of HTSC.- Carbon Problem.- General Aspects of HTSC Modeling.- Modeling of BSCCO Systems and Composites.- Modeling of YBCO Oxide Superconductors.- Modeling Conductive and Elastic Properties of Superconductive Composites.- Computer Simulation of HTSC Microstructure and Toughening Mechanisms.- Mechanical Destructions of HTSC Josephson Junctions and Composites.- Modeling of Electromagnetic and Superconducting Properties of HTSC.- Classification of Superconductors.- Finite Element Implementation of Carbon-Induced Embrittlement Model.- Macrostructure Modeling of Heat Conduction.- Computational Algorithms for Definition of Structure-Sensitive Properties of some HTSC Ceramics and Composites.- Eden Model.

Phenomenological description of flux pinning in non-uniform high-temperature superconductors in magnetic fields lower than the self-field

Abstract: Critical currents measured on a commercial Bi-2223/Ag tape in applied magnetic fields ranging from zero to 100 mT are analysed in order to demonstrate the validity of the phenomenological description proposed by the authors for flux pinning in polycrystalline high-temperature superconductors in low magnetic fields. The four-parameter model is a generalization of the Kim formula describing the magnetic field dependence of the critical current density. When combined with the finite-element calculation, it is able to predict the critical currents measured in different field orientations with an average accuracy better than 0.2%. The volume density of the flux pinning force is easily evaluated from the parameters of the model. Particular attention is paid to the problem of a possible sample non-uniformity. In fact, the experimental data can be better explained assuming the hypothesis of better filament texturing in outer filaments. This also means that the accurate determination of critical currents in low magnetic fields can be used to reveal a non-uniformity in the current-carrying capability of the superconducting material.

Temperature Dependency of Levitation Force and Its Relaxation in HTS

Abstract: High-temperature superconductors (HTS) have high potential for various engineering applications such as a superconducting flywheel energy storage system. The attractive feature of the superconducting magnetic bearings in the energy storage system is a reduction in the rotational loss. In this study, we studied the feasibility of a superconducting magnet to support bulk superconductors in magnetic bearings with the aim of increasing the stored energy. The superconducting magnet has a room temperature bore of 100 mm with the maximum magnetic field of 3.5 T. In addition, we used a GM freezer (10 K in the lowest attainment temperature), which can control the temperature of the bulk superconductors. The used bulk samples were Y-Ba-Cu-O and Sm-Ba-Cu-O 46 mm in diameter and 15 mm in thickness fabricated with a top-seeded melt-growth process. We have studied the effects of the temperature and magnetic field on the levitation force and its time relaxation. The levitation forces were measured by applying the magnetic fields of 0-2 T at 10-77.3 K. It was found that the levitation force increased with lowering temperature in both samples. The time relaxation of the levitation force was also reduced with lowering temperature. Sm-Ba-Cu-O exhibited better performance in the levitation force and its time relaxation in a high temperature and high-field region.

Study of microstructure and magnetic properties in copper oxide superconducting systems through AC magnetic susceptibility

Abstract: Two samples of high temperature superconductors with the general compositions YBa 2 Cu 3 O 7 - δ and GdBa 2 Cu 3 O 7 - δ were prepared by solid state reaction. The X-ray diffraction patterns and the Scherrer's equation were used to obtain the lattice parameters and the average grain size, D. We explored the microstructure of both samples using the temperature dependence of AC magnetic susceptibility at different magnetic field amplitudes. We discussed the flux flow regime and pinning and used the Cole–Cole plots to calculate the grain fraction in the samples. Finally, real parts was used to calculate the lower critical magnetic field H c 1 ( T ) as a function temperature.

Magnetic ion doped Cu1- xTlxBa2Ca2- yMgyCu1.5Ni1.5O10- δ (y = 0, 0.5, 1.0, 1.5) superconductors and their improved inter-plane coupling

Abstract: We have synthesized Cu 0.5 Tl 0.5 Ba 2 Ca 2 Cu 3− y Ni y O 10− δ ( y = 0, 0.5, 1.0, 1.5) superconductors at normal pressure and studied the enhanced correlation among CuO 2 /NiO 2 planes of Cu 0.5 Tl 0.5 Ba 2 Ca 2− y Mg y Cu 1.5 Ni 1.5 O 10− δ ( y = 0.5, 1.0, 1.5) superconductors by doping Mg at the Ca sites. Surprisingly, we have not observed any substantial depression of critical temperature with Ni doping in Cu 0.5 Tl 0.5 Ba 2 Ca 2 Cu 3− y Ni y O 10− δ ( y = 0, 0.5, 1.0, 1.5) superconductors. The main objective of Mg substitution is to enhance the inter-plane coupling which may increase the interactions of the spins of Ni atoms in different planes. The increased inter-plane coupling may enhance interactions of the atomic spins with the free carriers and might be a key to understanding the effects of spin scattering and their role in the mechanism of high temperature superconductivity. We have observed a decrease of c -axis length with increased Mg concentration in Cu 0.5 Tl 0.5 Ba 2 Ca 2− y Mg y Cu 1.5 Ni 1.5 O 10− δ ( y = 0.5, 1.0, 1.5) superconductors, showing an enhanced inter-plane coupling. The room temperature resistivity of the samples is decreased and the zero resistivity critical temperature [ T c ( R = 0)] and the magnitude of diamagnetism are increased with higher Mg doping in Cu 0.5 Tl 0.5 Ba 2 Ca 2− y Mg y Cu 1.5 Ni 1.5 O 10− δ superconductors. A maximum diamagnetism is observed in the samples when Mg atoms replace 75% of the Ca atoms, which shows that a higher Mg doping concentration enhances the inter-plane coupling which possibly promotes the de-localization of the carriers to the Ni 2+ sites and results in enhanced superconducting properties.

High temperature superconductivity

Abstract: The critical temperature, Tc, for all presently known superconductors does not exceed 21'K. This fact obviously limits the range of applications of superconductivity in technology in a very fundamental way. On the whole, the reason why the value of Tc for “ordinary” superconductors would not exceed 20-40oK is fairly well understood. At the same time, there apparently could exist high temperature superconductors for which Tc would reach liquid air temperature or even room temperatures. Possible means of producing high temperature superconductors are considered in this article which is based on the review published in Contemporary Physics, 9, 355 (1968), but includes also some new material. Special attention is paid to what can be called the exciton mechanism of superconductivity and its application to systems of the “sandwich” type and metallic conductors with layer structures.

Search for the existence of circulating currents in high-Tc superconductors using the polarized neutron scattering technique

Abstract: We review experimental attempts using polarized neutron scattering technique to reveal the existence in high temperature superconductors of a long-range ordered state characterized by the spontaneous appearance of current loops. We draw particular attention to our recent results (B. Fauque et al., Phys. Rev. Lett. 96 (2006) 197001) that, up to now, can be explained only by the theory of circulating currents proposed by Varma.

High‐Tc cuprate superconductivity as a new paradigm

Abstract: High-Tc cuprate superconductivity was discovered two decades ago by Bednorz and Muller. Even now the origin and the nature of this remarkable group of superconductors appear to be hotly debated in the literature. However, we believe that a few things are clearly established: (i) the high-Tc cuprate superconductors in the overdoped region are the BCS d-wave superconductors (dSC), (ii) the pseudogap phase is a d-wave spin density wave (dSDW) and (iii) the superconductivity in the underdoped region is gossamer (i.e. dSC in coexistence with dSDW). Here we review these developments and consider a class of nodal superconductors, which share similar phase diagrams to those for high-Tc cuprate superconductors. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Intrinsic electron doping in nominal “nondoped” superconducting (La,Y)2CuO4 thin films grown by magnetron sputtering

Abstract: The superconducting nominal “nondoped” La1.85Y0.15CuO4 (LYCO) thin films are prepared by magnetron sputtering. The best TC0 of more than 13K is achieved in the optimal films with highly pure c-axis oriented T′-type structure. The quasiquadratic temperature dependence of resistivity and the negative Hall coefficient in the normal state, effect of oxygen content are all quite similar to the known Ce-doped n-type cuprate superconductors, suggesting the electron-doping nature in T′-LYCO. The charge carriers are considered to arise from oxygen deficiency.

Cu 0.5 Tl 0.5 Ba 2 Ca 3 Cu 4- y Zn y O 12- δ ( y=0, 1.0, 2.0, 3.0, 3.5): Superconductor with Four ZnO 2 Planes

Abstract: A new Cu0.5Tl0.5Ba2Ca3Cu4−yZnyO12−δ (y=0, 1.0, 2.0, 3.0, 3.5) superconductor with four ZnO2 planes is reported. The structure of the material remains tetragonal for all Zn doping concentration. The substitution of Zn at CuO2 planar site was carried out following Cu0.5Tl0.5Ba2Ca3Cu4−yZnyO12−δ (y=0, 1.0, 2.0, 3.0, 3.5) formula. Contrary to all previous studies of Zn doping in all copper oxide high temperature superconductors, the zero resistivity critical temperature Tc(R=0), critical current density and quantity of diamagnetism increase with increased Zn concentration. The onset temperature of superconductivity in these samples was observed at 128 K and Tc(R=0) at 122 K for y=3.5. The volume of the unit cell observed through X-ray diffraction scan is found to decrease with increase Zn doping; promoting an increase in Fermi vector KF and effective density of states which results in enhanced superconductivity parameters. The synthesis of Cu0.5Tl0.5Ba2Ca3Cu4−yZnyO12−δ material by this method is highly reproducible.

Process for the production of highly-textured, band-shaped, high-temperature superconductors

Abstract: A wet-chemical method for producing strip-shaped high-temperature superconductors with a substrate, optionally with a buffer layer and with a high-temperature superconductive layer is improved by increasing the texturing and the layer thickness of the high-temperature superconductive layer. To this end, precursor solutions are applied in layers to the substrate, of which the first is low in fluorine or does not contain fluorine, and the following have a fluorine concentration that increases with each layer.

Superfluid Suppression in d-Wave Superconductors due to Disordered Magnetism

Abstract: High temperature superconductors (HTS) are an idealclass of materials with which to study electronic corre-lations in superconductivity because the correlations canbe tuned from weak to strong via chemical doping. Aconsequence of strong-correlationphysics is that the BCStheory of conventional superconductors fails to describeHTS. Uemura demonstrated that, unlike in BCS theorywhere the superfluid density ρ

I‐V characteristics of La1.84Sr0.16CuO4∕Nb-doped SrTiO3 heterojunction

Abstract: Current-voltage (I‐V) characteristics of the La1.84Sr0.16CuO4∕1.0wt% Nb-doped SrTiO3 heterojunction were measured in the temperature range from 5to290K under magnetic fields up to 14T. All I‐V curves show a fine rectifying property. A visible reduction of the diffusion potential (Vd) can be observed at the superconducting transition temperature due to the opening of the superconducting gap in La1.84Sr0.16CuO4. The reduction of Vd from its original track, ΔVd, is found to be of similar temperature and field dependencies as those of the superconducting gap. This suggests an alternative technique for detecting the superconductivity related features in superconducting films.

Gap-function symmetry and spin dynamics in electron-doped cuprate superconductors

Abstract: An antiferromagnetic (AF) spin fluctuation induced pairing model is proposed for the electron-doped cuprate superconductors. It suggests that, similar to the hole-doped side, the superconducting gap function is monotonic d(x)(2)-y(2) wave and explains why the observed gap function has a nonmonotonic d(x)(2)-y(2)-wave behavior when an AF order is taken into account. Dynamical spin susceptibility is calculated and shown to be in good agreement with the experiment. This gives a strong support to the proposed model.

Bulk Superconducting Nano-Composites With High Critical Currents

Abstract: Flux pinning sites are most effective if their size is comparable to the superconducting coherence length, which is on the nano-meter scale for RE-Ba-Cu-O superconductors [RE = rare earth element]. Introducing nano-phase inclusions directly into the bulk superconducting material has only been partially successful to date, however, due primarily to the absence of chemically stable phases that can co-exist with RE-Ba-Cu-O without suppressing its key superconducting properties. We have identified novel isostructural phases based on (RE)2Ba4CuMOy (where M = W, Zr, Nb, Ag and Bi) and have fabricated successfully superconducting bulk nano-composites with a high current carrying capability. The average size of the nano-inclusions is observed to vary from 20 nm to 300 nm depending on element M. An observed improvement in Jc under low and high external magnetic fields at 77 K correlates directly with an increased density of nano-inclusions in the superconducting matrix.

Gapless Fermi surfaces of anisotropic multiband superconductors in a magnetic field.

Abstract: We propose that a new state with a fully gapless Fermi surface appears in quasi-2D multiband superconductors in magnetic field applied parallel to the plane. It is characterized by a paramagnetic moment caused by a finite density of states on the open Fermi surface. We calculate thermodynamic and magnetic properties of the gapless state for both s-wave and d-wave cases, and discuss the details of the first order metamagnetic phase transition that accompanies the appearance of the new phase in s-wave superconductors. We suggest possible experiments to detect this state both in the s-wave (2-H NbSe2) and d-wave (CeCoIn5) superconductors.