Layered Oxide Superconductors Research Articles

Intrinsic pinning of FeSe1−xSx single crystals probed by torque magnetometry

Intrinsic pinning is caused by natural pinning centers that occur because of the modulation of the order parameter or weak superconducting layers. Early work has shown that intrinsic pinning generates a high pinning force and critical current density in some layered oxide superconductors. Studying the intrinsic pinning of superconductors is crucial for both fundamental studies and potential applications. Herein, we use torque magnetometry to study angle-resolved in-plane and out-of-plane magnetic torque for a series of high-quality FeSe1−xSx single crystals. A fourfold torque signal was observed when the magnetic field was within the ab plane. We interpret that this fourfold in-plane irreversible torque is from the intrinsic pinning due to combined effects of gap nodes/minimum and twin domains. Additionally, we attributed the observed out-of-plane torque peaks to intrinsic pinning due to the layered structure.

From Mott insulator to overdoped superconductor: evolution of the electronic structure of cuprates studied by ARPES

We review our angle-resolved photoemission spectroscopy (ARPES) results on different layered oxide superconductors, and on their insulating parent compounds. The low energy excitations are discussed with emphasis on some of the most recent issues, such as the Fermi surface and remnant Fermi surface, the pseudogap and d-wave-like dispersion, and lastly the signatures in the ARPES spectra of multiple electronic components, many body effects, and the superfluid density. We will focus on systematic changes in the electronic structure which may be relevant to the development of a comprehensive picture for the evolution from Mott insulators to overdoped superconductors.

Electron-transport properties of a two-dimensional electron gas interacting with phonons of a defect-containing lattice: A paradigm for layered oxide superconductors.

We study the electron-transport properties of a two-dimensional electron gas interacting with phonons of a lattice containing isotope defects. This serves as a heuristic model for the layered oxide superconductors to help understand the anomalous transport properties of these high-T c superconductors. Electron-phonon interaction is described by the deformation-potential approximation, and the lattice dynamics is simplified by the continuum approximation. The integral equation describing the vertex corrections to the electron-transport relaxation time is solved numerically

The upper critical field of an anhanharmonic superconductor

We investigate the orbital upper critical field of a dirty superconductor with strong anharmonic lattice vibrations. We show that anharmonicity gives rise to an anomalously large upper critical field with an upward curvature in the temperature dependence of H c2 ( T). It is found that the magnitude of the corresponding electron-phonon coupling constant λ is quite reasonable. Possible implications for the layered oxide superconductors as short coherence length and large gap are briefly discussed.

Flux Pinning in Artificial Layered Oxide Superconductors

Multilayered YBa2Cu3O6.8/YBa2(Cu1-xNix)3O7-y and YBa2Cu3O6.8/Bi2Sr2Can-1CunOx (n=1, 2, 3) superconducting films have been grown by laser deposition technique. Critical currents were measured in the external magnetic field up to 5 tesla with different thicknesses of the interlayers. Results show that control of the number of layers as well as the thickness in multilayer structures could provide an effective means for improving the critical current in magnetic fields.

Thin film processing for high-Tc superconductors of the Bi-system

Basic thin film deposition processes for controlled deposition of the high-Tc superconductors of the Bi-systems are described. The layered structures of Bi-oxide superconductors are fabricated by a multitarget sputtering process. The multitarget sputtering process realizes the controlled deposition of single phase Bi-oxide superconductors, Bi2O2 · 2SrO · (n −1)CaO · nCuO2 for n = 1 to 5. The minimum thickness controlled by the multitarget sputtering is a half crystal unit-cell of around 15 Å, and the superlattices comprising (AkBk) · m, where A and B denote the Bi-oxide superconductors with different numbers of Cu–O layers, could be fabricated for k > 1, although ion mixing takes place during the sputtering deposition due to the bombardment of the highly energetic sputtered adatoms. Multitarget sputtering will be available for the fabrication of the artificially-made layered oxide superconductors (ALOS).

Effect of interlayer coupling in YBa2Cu3O7-y

The layered oxide superconductors are essentially two-dimensional in character with a weak coupling between copper oxide planes. Using perturbation arguments of scaling theory, we compare the scaling function in a quasi-two-dimensional layered material with that in a (2+e) dimensional system and relate the interlayer coupling to an effective dimensionality. The effect of interlayer coupling on metallic conduction and superconductivity in YBa2Cu3O7−y is discussed.

Superconducting properties in layered oxide superconductors

The copper oxide superconductors have layer structures with strongly and weakly superconducting layers. Using the layer structure of YBa 2 Cu 3 O 7 , the tunneling conductance and the specific heat are calculated. The spectrum of the tunneling conductance shows a fine structure inside the superconducting gap at low temperatures in agreement with the results of tunneling experiments. The specific heat has a small electronic contribution even at low temperatures.

Anomalous temperature dependence of Hc1 in layered oxide superconductors

To study theoretically the structure of a flux line in copper oxide superconductors, we use a model in which their crystals are composed of arrays of superconducting and normal layers, and superconductivity in the normal layers is induced by a proximity effect. The superconducting order parameter in the normal layers is much enhanced at low temperatures. As a result of this enhancement, the lower critical field H c1 is drastically enhanced at low temperatures. The anomalous enhancement of H c1 at low temperatures has been observed in several oxide superconductors. The lower critical field perpendicular to the layers is expressed as H c 1 =(Г(T)/2К s ) log К s [Ø 0 /{2πξ s (T)λ s (T)}] , К s , ξ s ( T ) and λ s ( T ) being the parameters defined on the superconducting layers. The quantity Г( T ) regulates the enhancement.

Tunneling conductance in layered oxide superconductors

The copper oxide superconductors are considered to have stacking structures of strongly and weakly super-conducting layers. Using this model, the tunneling conductance in a junction of the superconductor and a normal metal is calculated as a function of bias voltage. The calculation predicts that a characteristic fine structure appears inside the superconducting gap in the tuneling spectrum at low temperatures.

Anisotropy of critical current in layered oxide superconductors

Employing the layer structures of the superconducting copper oxides as intrinsic flux pinning centers, we calculate the critical current density as a function of the direction of an external magnetic field. The critical current J c has a maximum when the magnetic field is parallel to the CuO 2 layer. When the magnetic field is tilted from this layer, J c drastically decreases. The predicted anisotropy of J c is in good agreement with that observed by experiments.

Spin singlet d wave hole pairing in the new high-Tcsuperconductors

The authors examined the mechanism for, and the nature of pairing in the newly discovered layered oxide superconductors modelled by a two-dimensional Hubbard Hamiltonian having a low concentration nh holes in the lower Hubbard band. They find spin-singlet d wave pairing. The calculated binding energies typically approximately=100 K show a maximum at nh approximately=0.15 per atom, in accord with experiment. The pairing potential originates from a wake of higher energy spin configurations generated by a moving hole.