E. A. Pashitskii

Mechanisms of limitation and nature of field dependence of critical current in HTS epitaxial YBaCuO films

Magnetic field and temperature dependencies of the critical current density, J/sub c/(H/spl par/c, T) were measured by SQUID-magnetometry, ac magnetic susceptibility, and dc transport current techniques in the single-crystalline epitaxially-grown by off-axis dc magnetron sputtering YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) films with J/sub c/(H/spl par/c, 77 K) /spl ges/ 2 /spl middot/ 10/sup 6/ A/cm/sup 2/. The mechanism of vortex depinning from growth-induced linear defects, i.e., out-of-plane edge dislocations in low-angle tilt domain boundaries, is shown to describe quantitatively measured J/sub c/(H/spl par/c, T). The developed model takes into account a statistical distribution of the dislocation domain boundaries ordered in a network as well as the interdislocation spacing within boundaries. Actual structural features of YBCO film known from HREM data turn out to be extracted from J/sub c/(H/spl par/c, T)-curves by a fitting procedure within the proposed model.

Linear defects in epitaxial Y-Ba-Cu-O films: their role in anisotropic vortex pinning and microwave surface resistance

YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// films exhibit two orders of magnitude higher critical current density, J/sub c/, and a few times higher microwave surface resistance, R/sub s/, than single crystals. This evidences a different nature of crystal defects, which are responsible for the pinning and HF losses. Several types of dislocation arrays are identified in YBCO films by TEM/HREM and XRD. An analysis of J/sub c/(H,T,/spl theta/) and R/sub s/(T)-dependencies along with X-ray and HREM data for films grown via various modes allows to differentiate contributions to J/sub c/ and R/sub s/ from out-of-plane and in-plane edge dislocations. The higher the density of edge dislocations within YBCO film the higher are J/sub c/ and R/sub s/. Stress-strain field calculations allow one to comprehend the difference between out-of-plane and in-plane dislocations effects.

Magnetic field dependence of the critical current density in thin epitaxial HTS YBa2Cu3O7−δ films with dislocation low-angle domain boundaries

Highly biaxially oriented YBa 2 Cu 3 O 7-δ (YBCO) films exhibit critical current densities, J c , two orders of magnitude higher than any sort of bulk samples. Out-of-plane edge dislocations (mean 2D density is 10 11 lines/cm 2 in the film area) in low-angle tilt domain boundaries are shown to play a crucial role in J c (H||c, T)-behavior. Measurements of the magnetic field dependence of J c for the pulse-laser deposited YBCO films are carried out by AC magnetic susceptibility technique. Dislocation cores are shown to provide an extremely strong pinning force leading to J c (77 K) ≅ 2 × 10 6 A/ cm 2 . Peculiarities of J c (H||c) are found in the low-field range. A simple model of pinning by dislocation domain boundaries is developed. The model consistently describes the J c (H||c, T)/J c (0, T)-dependencies, accounting for the fraction of vortices pinned by dislocations.

The role of pair correlations in the formation of the ground state and the elementary excitation spectrum in a superfluid Bose liquid (A Review)

The paradoxes and disparities in the contemporary microscopic theory of superfluid helium (He–II) are discussed along with possible ways of resolving them by taking pair correlations of 4He atoms into consideration. It is shown that most paradoxes are associated with the commonly accepted initial assumption concerning the dominating role of single-particle Bose condensate (SPBC) in the quantum microstructure of the superfluid component ρs. The existence of intensive SPBC leads to a strong hybridization of the elementary excitation branches and to a common dispersion law for all boson branches, which is identified with the quasiparticle spectrum E(p) observed experimentally from slow neutron scattering in liquid helium. However, the stability of this spectrum during a transition through the λ-point and the large value of the gap in the vicinity of the “rotonic” minimum contradict both the Landau theoretical criterion of superfluidity and the small value of experimentally measured critical velocity. At the ...

Nature of magnetic field and angular dependencies of the critical current density in epitaxial HTS YBa2Cu3O7−δ films

Abstract Out-of-plane edge dislocations (mean 2D density is 10 11 lines/cm 2 ) in low angle tilt domain boundaries of single-crystalline YBCO films are shown to play a crucial role in J c ( H ∥ c , T ) behavior and J c angular dependence. Dislocation cores and their vicinity, where T c is locally suppressed, are shown to provide an extremely strong pinning force resulting in J c (77 K )≈2×10 6 A/cm 6 . Peculiarities of J c ( H ∥ c ) are found in the low-field range. A simple model of pinning by dislocation domain boundaries is developed. The model consistently describes the J c ( H ∥ c , T )/ J c (0, T )-dependencies, accounting for the fraction of vortices pinned by dislocations. Angular dependencies of J c in rotating magnetic field were measured by four-probe transport technique. A satisfactory agreement with theoretically predicted behavior has been found.

Defects-induced thermal instability in YBCO films in microwave field

Abstract The heat instability induced by linear defects is assumed to enhance the remarkable difference between microwave properties of YBCO single crystals and thin films due to extended strain fields near out-of-plane edge dislocations. We have shown theoretically and confirmed experimentally that a single dislocation cannot have a strong effect on the surface resistance R s , but dislocation arrays, which were observed experimentally, can induce the thermal instability, if edge dislocations in the arrays are spaced closer than the heat relaxation length. Ordered dislocation structures provide much higher local temperature perturbation than randomly distributed dislocations.