V. M. Svistunov

The nature of transport critical current hysteresis in HTSC: magnetic fields and high pressures

Abstract It was found that pressure has a strong influence on the critical current hysteresis loop of ceramics at H ∼ 10 kOe. The phenomenon is attributed to the critical current hysteresis of separate Josephson contacts and is due to the Abrikosov vortex density gradient within granules. The gradient defines both the sign and the value of the pinning current, whereas the sign of Meissner reversible surface current component is determined by the external field H direction. As a result the critical current of Josephson contacts defined by the total surface value depends on the magnetic prehistory of a sample.

Zero bias conductance peak enhancement in Bi2Sr2CaCu2O8+δ-SiO-Ag planar tunnel junctions

We report the experimental results of quasiparticle tunneling using Bi 2 Sr 2 CaCu 2 O 8+δ (BSCCO)-SiO-Ag planar junctions. For the {001}-oriented junctions, we have measured a well-known V-shaped gap structure expected for d -wave superconductors. For the {100}- and {110}-oriented junctions, a reproducible zero bias conductance peak (ZBCP) enhancement has been observed. The temperature dependence of the ZBCP is also presented. The ZBCP was observed below T c . Our results are consistent with those of recent studies on the Andreev reflection between a d -wave superconductor and a normal metal.

Current transfer in HTSC and at high pressures

The authors have measured the pressure effects on the superconducting Tc and the current density Ic of the YBaCuO (123) phase and the BiSrCaCuO (2223) phase up to 10 kbar. They have found that for Bi ceramics the increase in Jc with pressure is related to the Tc change, while for YBCO it is due to the Josephson medium weak-link effects. This result provides the evidence that in Bi compounds strong-current Josephson contacts are possible. The influence of high magnetic fields on the Josephson critical current is briefly discussed.

Phonon spectra in the conductance of Andreev junctions on Bi2223 at high pressure

Abstract A softening of the high-frequency phonon modes in Bi2223 ( T c =110 K) at high pressure is found. These modes correspond to the oxygen breathing mode and also the Cu–O bending modes. Qualitatively, the result obtained explains the observed rise in R =2 Δ / kT c ( P ) ratio in the framework of the strong electron–phonon interaction model. However, the analysis of the R -ratio in the framework of the s+d model has revealed a possibility of explanation of the observed rise in R ( P ) also for the d-wave pairing.

CRITICAL CURRENT AND TRANSITION TEMPERATURE WIDTH IN OXIDE SUPERCONDUCTORS UNDER PRESSURE

The critical current density of Y-based and Bi-based high-Tc superconductors has been measured under pressure. The substantial increase of jc with pressure is found. The results of our measurements can be explained if the sample is regarded as a network of S-N-S junctions with arbitrary parameters. These junctions can be formed by integrain layers and/or twin boundaries.

PRESSURE EFFECTS ON HIGH-Tc SUPERCONDUCTORS CURRENT TRANSFER

The efficiency of high pressure technique for the evidence of the Josephson nature of the intergranular contacts in metal ceramics has been demonstrated. With this technique it has been found that the abnormal hysteresis in fields of the order of tens kOe was not connected with the short-circuited contacts between the granules. The reason for the hysteresis lies in the ambiguity of the magnetization of the granules versus the change in the magnetic field direction. The critical current stability of the Josephson conteacts to high magnetic fields is due to both the inhomogeneity of the contacts and the existence of Abrikosov vortices within the superconducting electrode links.

Spectroscopy of bosonic excitations in manganite-based heterostructures

The tunneling current transport is investigated in layered structures based on lanthanum manganite. The spectral function of the electron–boson interaction is reconstructed over a wide energy range. High-energy excitations localized in the barrier and near-barrier regions are revealed. The inelastic scattering of charge carriers on these states can lead to suppression of the magnetoresistance of tunnel junctions with increasing applied voltage.

Dephasing effects in superconducting heterojunctions: a crossover from coherent to sequential transmission

Abstract We present first experimental and theoretical results demonstrating a crossover from mesoscopic (phase-coherent transmission) to macroscopic (two resistors in series) behavior of conductance spectra for a normal-superconducting heterostructure. Following the lines of the Buttikers approach to normal double-barrier devices, we show that phase-randomizing events in a mesoscopic interbarrier layer together with Andreev-reflection processes drastically influence corresponding conductance vs. voltage characteristics. An anomalous temperature shift of a gap-like feature in an Ag/LaBa 2 Cu 3 O 7− x point contact is interpreted as an effect of the partial phase decoherence in the intermediate magnetically active region between the normal-metal tip and the bulk 123-type superconducting cuprate.

ANOMALOUS TEMPERATURE EFFECT ON POINT-CONTACT Ag–LaBa2Cu3O7-x CONDUCTANCE SPECTRA

The observation of an anomalous temperature behavior of the differential conductance versus voltage curves in contacts formed by an Ag tip and a bulk ceramic LaBa2Cu3O7-x with Tc around 92 K is reported. For a wide range of temperatures from Tc to helium-liquid ones, we have found a crossover from curves typical for a pure conducting normal metal–superconductor interface up to Giaever tunneling characteristics with gap features shifted to high biases. We take into account the existence of a degraded interlayer with suppressed superconducting parameters between a normal injector and a superconducting bulk and interpret qualitatively the data in terms of mesoscopic proximity effects. We argue that as the temperature is decreased, (i) the electron localization in a disordered region is enhanced, and (ii) in the interlayer, inelastic scattering processes become more effective. The latter was considered as a result of the inelastic scattering rate changes for charge carriers interacting with magnetic excitations in the near-interface region of high-Tc compound junctions.

Structural ordering effects on conductances of metaloxide compounds and tunnel structures on their base

Abstract Lattice change effects on the conductance of Y-123, Dy-123 and Bi- 2223 compounds have been studied. Their manifestations lead to the local increase in dI/dU(T). An anomaly in R(T) curve of normal state Dy-123 conducting below 2.5 K is attributed to the possible structural transition preceding magnetic ordering of the rare-earth ions.