Vortex dynamics and correlated disorder in high-Tc superconductors

We develop a theory for the vortex motion in the presence of correlated disorder in the form of twin boundaries and columnar defects. Mapping vortex trajectories onto boson world lines enables us to establish the duality of the vortex transport in systems with correlated disorder and the hopping conductivity of charged particles in 2D systems. A glassy-like dynamics of the vortex lines with zero linear resistivity and strongly nonlinear current-voltage behavior as V ∝ exp(-const/J^μ) in a Bose glass state is predicted.     

Current-influenced voltage noise in bulk polycrystalline high-Tc superconductors

The 1/f voltage noise in bulk polycrystalline high-temperature superconductors (HTSC) under bias current and magnetic field has its origin in the noise current-dependence of the grain boundary junctions (GBJs), due in turn to the correlated effects of junction critical current and normal resistance fluctuations. The analogy between the results obtained by varying the bias current through the specimen and those performed with temperature as variable is evidenced. The noise maxima obtained in both sets of measurements turn out to be caused by the junction critical current fluctuations, which dominate when the currents flowing through the GBJs are close to the Josephson critical current. The anti-phase correlation between the normal resistance and the critical current fluctuations is responsible for the monotonical decrease of the noise at constant bias current, with the temperature exceeding the value corresponding to the noise maximum. In contrast, varying the bias current at fixed temperature, the voltage noise exhibits a local minimum followed by an increasing tendency after passing through the maximum.     

高温超导体中约瑟夫森涡旋流阻的振荡效应

通过数值计算耦合sine-Gordon方程组研究高温超导体中约瑟夫森涡旋的运动,得到约瑟夫森涡旋电压和流阻随平面磁场和驱动电流的变化规律.固定驱动电流,约瑟夫森涡旋电压和流阻随着磁场的增大出现周期性的振荡行为,振荡周期与每层约瑟夫森结中进入一个磁通量子相对应.分析和阐明了产生这种周期性振荡的原因. 关键词： 约瑟夫森涡旋 涡旋格子 高温超导     

Hole distribution and madelung energy in the high-Tc oxide superconductors

Evidence is presented that in high-T_c superconductors the electrostatic interaction as expressed by the Madelung energy is the key factor in determining the charge distribution and in particular the distribution of the holes between the CuO₂-planes and other structural subunits. Specifically, this explains the phase diagram (T_c as a function of oxygen content and oxidizing power) of cation-substituted YBa₂Cu₃O_6+y and the oxygen ordering in YBa₂Cu₃O_6.5. Moreover, we argue that the dependence of T_c on strontium content in La_2−xSr_xCuO₄ is similarly due to the fact that holes go into the (La,Sr)O-sheets as well as into the CuO₂-planes.     

Lattice instabilities and c-axis resistivity in high-Tc superconductors

We propose a general model to describe the coupling of polaronic charge carriers with double well potentials due to local lattice instabilities. A path integral formalism accounts for the retardation effects in the attractive polaron-polaron interaction mediated by the double well structure. The model is applied to the normal state of YBa₂Cu₃O_7−δ superconductor and it is shown that the attractive forces are relevant in a strong coupling regime for realistic values of the input parameters. The calculated c-axis electrical resistivity shows a metallic like behaviour with deviations from a T linear dependence due to the role of the attractive interaction. The absolute values of resistivity are consistent with the experimental data in the case of strong electron-lattice coupling.     

High-Tc superconductors and AC loss in electrotechnical devices

To replace conventional normal conducting solutions in electrotechnical devices, high-T_c superconductors must offer distinct economical and technical benefits in terms of lower overall loss, volume and weight. Based on AC loss theory we design appropriate 50 Hz reference conductors for cables, transformers and other applications, calculate admissible limits for the conductor variables filament diameter, twist and matrix resistance and compare this to the present state of Bi-2223-tape conductors and AC loss measurements. Further the influence of perpendicular AC field components on losses is addressed. High current devices will require multistrand conductors, where nonuniform current distribution due to unbalanced magnetic coupling may result in partial saturation and enhanced losses. As an example we discuss the multilayer HTSC-cable and present a solution based on a ‘zero flux condition' for azimuthal and axial magnetic fields.     

Direct observation of flux-creep in high-Tc superconductors using the high-resolution Faraday effect

Using the capability of the high-resolution Faraday effect (HRF) to allow dynamic observations of the Shubnikov phase, we directly observed effects of flux-creep in high -T_c superconductors. In this paper we show measurements on (RE) Ba₂Cu₃O_7-δ with RE=Y, Dy, Gd and on Bi₂Sr₂CaCu₂O_8−δ single crystals and also on YBaCuO thin films and sintered materials. Furthermore, this technique enables us also to measure the flux creep locally. From the obtained photographs we determine the time dependence of the total flux in the samples. Using these data and the model of Hagen et al. the effective activation energies have been determined for these materials.     

Dressed Josephson junction network models and low-field magnetic response of high-Tc granular superconductors

A bare three-dimensional model, in which grains are reduced to points, cannot fully account for the magnetic properties of granular superconductors. A dressed version of these network models is proposed to discuss the quantitative link between the low-field magnetic response of high-T_c superconducting granular samples and the characteristic properties of Josephson junction network models. By means of dressed models, the temperature dependence of the d.c. field-cooled susceptibility of a simple three-dimensional granular system, consisting of eight grains in a cubic arrangement, is studied.     

Hole dispersion and enhancement of antiferromagnetic interaction of localized spins in high-Tc superconductors

In the framework of the extended Hubbard model (copper-oxygen and oxygen-oxygen hopping) the energy and effective mass of a mobile hole is expressed in terms of hopping parameters and the correlators between spins of the background S_nS_n+1. Due to interaction of the mobile hole with localized holes, its effective mass increases about four times.     

Four contact electrical measurements on the anisotropic high-Tc superconductors: voltage reversals

We describe three mechanisms for the anomalous voltage reversals which are occasionally observed during four contact current-voltage measurements on the high temperature superconductors. Voltage reversals have been frequently observed for whiskers of the bismuth strontium calcium copper oxide (BSSCO) compounds, and we describe measurements made on high quality whiskers of the 2:2:1:2 and 2:2:2:3 phases. A simple model based on extreme anisotropy plus offset contacts can explain the gross features of four contact measurements in the ohmic flux-flow regime, but not the wealth of reversal phenomena found for the IVVI configuration in the lower temperature irreversibility region.     

Microwave absorption properties of Sr2FeMoO6 nanoparticles

The microwave absorption properties of nanosized double perovskite Sr₂FeMoO₆ and epoxy resin composites were investigated in the frequency range of 2-18 GHz using the coaxial method. The Sr₂FeMoO₆ composites with an optimal 20 wt% epoxy resin showed a strong electromagnetic attenuation of −49.3 dB at 8.58 GHz with a matching thickness of 2.15 mm. Moreover the optimum absorption frequency at which the reflection loss is less than −20 dB, which corresponds to 99% reflection loss of the incident microwave, is from 5.7 to 13.2 GHz with the matching thickness ranging from 3.0 to 1.5 mm. The excellent microwave-absorption properties are a consequence of a proper electromagnetic match due to the existence of the insulating matrix of anti-site defects and anti-phase domains, which not only contribute to the dielectric loss but also to the reduced eddy current loss.     

Properties of grain boundaries in high-Tc superconductors – Notes on a recent presentation

The purpose of this article is to discuss a view concerning key datasets of the properties of grain boundaries in high-T_c superconductors that was recently expressed in Ref. [R. Gross, Physica C 432 (2005) 105]. The reference also criticizes our previous research. Using examples, I disprove this criticism.     

液氮温区镍氧化物高温超导体的发现

非常规超导材料探索和机理研究是物理学中重要的研究内容和科学问题。目前主要的非常规超导体材料包括重费米子超导体、铜氧化物高温超导体和铁基超导体。其中只有铜氧化物超导临界温度高于液氮沸点77 K,处于液氮温区。镍氧化物与铜氧化物具有相似的材料结构体系,其中+1价镍离子与+2价铜离子具有相同的电子填充数,是此前研究人员探索超导电性的重点。具有钙钛矿结构单元、双镍氧层的La₃Ni₂O₇中镍的价态为+2.5,常压下呈现顺磁性的金属态。近期作者在14 GPa压力下的La₃Ni₂O₇单晶样品中发现了高达80 K的超导临界温度,超出此前理论预期。文章将简要介绍非常规超导材料体系的研究历史、La₃Ni₂O₇单晶样品的生长方法、常压下的物理性质和高压下的结构表征、以及通过电学和磁性测量对超导电性的确认。     

Electromagnetic and microwave absorption properties of carbonyl iron/La0.6Sr0.4MnO3 composites

In this work carbonyl iron/La_0.6Sr_0.4MnO₃ composites were prepared to develop super-thin microwave absorbing materials. The complex permittivity, permeability and microwave absorption properties are investigated in the frequency range of 8-12 GHz. An optimal reflection loss of −12.4 dB is reached at 10.5 GHz with a matching thickness of 0.8 mm. The thickness of carbonyl iron/La_0.6Sr_0.4MnO₃ absorber is thinner, compared with conventional carbonyl iron powders with the same absorption properties. The bandwidth with a reflection loss exceeding −7.4 dB is obtained in the whole measured frequency range with the thickness of 0.8 mm. The excellent microwave absorption properties are attributed to a better electromagnetic matching established by the combination of the enhanced dielectric loss and nearly invariable magnetic loss with the addition of La_0.6Sr_0.4MnO₃ nanoparticles in the composites. Our work indicates that carbonyl iron/La_0.6Sr_0.4MnO₃ composites may have an important application in wide-band and super-thin electromagnetic absorbers in the frequency range of 8−12 GHz.     

Temperature dependence of thermal and electrical conductivity of Bi-based high-Tc (2 2 2 3) superconductor

Superconducting samples with nominal composition Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O_δ were prepared by the conventional solid-state reaction technique. The samples have been characterized by X-ray diffraction, dc electrical resistivity, ac magnetic susceptibility and thermal conductivity. The X-ray diffraction studies were done at room temperature and the lattice constants of the material were determined by indexing all the peaks. All the above measurements show that, there exists two phases i.e. high-T_c (2 2 2 3) and low-T_c (2 2 1 2). The information obtained from dc electrical resistivity data agrees with ac magnetic susceptibility measurements. The onset temperature T_c (onset) and zero resistivity temperature T_c (R = 0) of the samples remains within the temperature 120 ± 1 K and 103 ± 1 K. Thermal conductivity has been measured with a transient plane source (TPS) technique in the temperature range 77–300 K. The estimation of the electrical resistivity change due to scattering by phonons and impurities has been discussed. An increase in thermal conductivity is observed above and below T_c (R = 0). The electron–phonon scattering time, phonon-limited mobility and the size of the electron–phonon constant are also calculated. Wiedemann–Franz law is applied to gain prediction about the magnitude of electronic and phonon contribution to the total thermal conductivity of the samples. It is observed that heat is mainly conducted by the phonons in this system.     

A possible mechanism of superconductivity in high-Tc cuprates

This paper derives a generic T_c formula by using the long-range phase coherence condition in quantum phase fluctuation of the order parameter. Taking the two-local-spin-mediated interaction (TLSMI) proposed by Liu and Chen [Phys. Rev. B 58 (1998) 8812] as a Cooper pair potential, and the T_c formula, this paper explains five basic experimental facts in high-T_c cuprates. The aim of this paper is to show that TLSMI is a possible pairing mechanism of superconductivity in high-T_c cuprates.     

Statistical modeling of inductances in high-Tc superconductor electronic structures

In the design of electronic applications, e.g. rapid single flux quantum (RSFQ) circuits, with high-T_c superconductors there is an obvious necessity of having precise knowledge of loop inductances. In general, these inductances have to be computed numerically due to the small sizes of the structures and the large contribution of the kinetic inductance. The present work is based on the analysis of a large number of inductances which have been calculated numerically for several high-T_c superconducting loop configurations. By means of statistical modeling, analytical formulae, which are customized for the considered type of structure, are derived. They can efficiently be used to simplify and accelerate the process of layout design in high-T_c RSFQ electronics.     

O KVV Auger emission versus resonant photoemission at the O K edge of high-Tc superconductors

Photoelectron spectroscopy results on single crystals of the superconductors Bi₂Sr₂CaCu₂O₈,Bi₂Sr₂CuO₆, Ba_0.6K_0.4BiO₃ and the semiconductor Ba_0.9K_0.1BiO₃ are reported for the photon energy region around the O K absorption threshold. The development of the O-KVV Auger structure has been carefully monitored as a function of photon energy. A non-monotonic behavior displaying a feature at a constant binding energy of about 14 eV was found for Bi₂Sr₂CaCu₂O₈ and Bi₂Sr₂CuO₆ in a narrow photon energy region of 1 eV at the main edge of the O K absorption spectrum around 530 eV. The corresponding enhancement, connected with the autoionization of O 2p states, is absent in Ba_1−xK_xBiO ₃ in contrast to Bi₂Sr₂CaCu₂O₈ and Bi₂Sr₂CuO₆. The resonant enhancement is more pronounced for Bi₂Sr₂CuO₆ as compared to Bi₂Sr₂CaCu₂O₈, which can be explained by a lower charge carrier concentration in the former case, leading to a more localized nature of intermediate O 2p states. The model parameters Cu d–d and O p–p Coulomb interactions and the charge transfer energy Δ are estimated from the experiments.     

Size dependence of the thermo-electrodynamics states of composite high-Tc superconductors and its effect on the current instability conditions

The effect of transverse geometries of the slab of composite high-T_c superconductors on their stable and unstable thermal and electrodynamics transient states in the incomplete and complete penetration modes during the current charging are discussed. The transient period when the electric field that is induced by the charged current becomes more homogeneous during the initial stage of the complete penetration mode in the sub-critical voltage range is studied. In the over-critical voltage range, the cross-section shape of the slab affects its stable and unstable temperature variation. As a result, the current instability condition is not identical for high-T_c superconducting composite tapes that have the same cross-sectional area with various shapes of the cross-section. The condition depends on their thickness: the less thickness, the more stable the current distribution in the composite superconductors with the same cross-sectional area. This feature is a result of the unavoidable reduction of the current-carrying capacity of a high-T_c superconducting composite by the temperature increase. This reduction is caused by the relevant temperature dependence of electrodynamics states of the composite. This temperature dependence happens even during a stable stage of the current charging. These mechanisms must be considered during experiments at which the critical or quenching currents are defined.