Measurements of the magnetic field and temperature dependences of the critical current in YBCO films and procedures for an appropriate theoretical model selection

Abstract

The magnetic field and temperature dependences of the critical current density jc(H,T) were studied in thin epitaxial YBCO films (laser ablated on single-crystal SrTiO3 substrates). Two complementary magnetic methods, namely, measurements of virgin magnetization m(H = const,T) and magnetic susceptibility loss-maximum on alternating magnetic field h, were used. The slope of the obtained magnetic field dependences of jc(H,T)/jc(0,T) in the inflection point is analysed in semi-logarithmic (jc–ln(H)) coordinates. It is shown that this parameter is rather informative for the governing pinning mechanism in the specific film, and together with the low-field ‘plateau’ temperature dependence it can be made the basis of the respective identification criterion. Of the five critical current limitation mechanisms considered, only one meets this criterion—the model of collective pinning of the distorted vortex lattice on a system of low-angle grain boundaries formed by edge dislocation walls threading through the whole film thickness. Scaling of the magnetic field and temperature curves in this model allows the emergence of all experimental data to a single general experimental curve specific for an individual film. Fitting of such curves with the theoretical formulae provides important information about the film parameters such as average grain size, dispersion of grain size distribution, and the average misorientation angle of crystallites. The results obtained lead to the conclusion that the laser ablated films studied are characterized by rather large sizes and low misorientation angles of single-crystal blocks as compared with films made using other procedures.

Two components of critical current in YBa$_2$Cu$_3$O$_{7-δ}$ films

“…In EDP model 14 the critical current depends on an average size of the crystallites, i. e. CDB size in the diffraction experiments, as well as on a misorientation angle ω between them. 42 As seen in Table I, in our samples the CDB size changes by one order and the angle ω varies almost seven times, but these parameters do not correlate with J 2 . Despite absence of the correlation, we compare below J 2 (T ) with J c (T ) calculated for pinning on both non-superconducting inclusions and the edge dislocations for analysis to be comprehensive.…”

“…14,28,41 For some samples this model approximates well the field dependence of the current in a wide range of fields. 14,41,42 At the same time the EDP model is not universal for all YBa 2 Cu 3 O 7−δ films, some conditions are necessary for its correct application. 42 Restriction of the EDP model may be caused by neglecting of weak pinning which may influence J(H) behavior at low temperatures.…”

“…14,41,42 At the same time the EDP model is not universal for all YBa 2 Cu 3 O 7−δ films, some conditions are necessary for its correct application. 42 Restriction of the EDP model may be caused by neglecting of weak pinning which may influence J(H) behavior at low temperatures. J(T ) behavior that follows from the EDP model haven't been tested yet.…”

Nonlinear microwave response of a superconducting YBa2Cu3O7−δ microstrip transmission line with a constriction

“…However, it can be supposed on the basis of the results obtained that for the YBCO films studied in the present work Josephson-type weak links arising at grain boundaries with large angles of disorientation or some other planar defects and resulting in a local nonlinear high-frequency response, recorded in HTSC films using a scanning laser or microwave microscope, 19,31,32 do not play a substantial role in the integral characteristics of the film ͑IVCs at dc current, surface impedance of the film at microwave frequencies, or the dependence studied in the present work of the high-frequency power transmission coefficient of a film microwave transmission line͒. This indicates that small-angle intergrain boundaries, which for angles of disorientation of the grains ഛ1-2°i n the ab plane are transparent to the flow of the superfluid condensate current and, moreover, contain c-oriented edge dislocations which are strong pinning centers for vortices, [33][34][35][36] play the dominant role in films of this type. This conjecture concerning the structure of the films used in the present work agrees with high values of the critical current density and the form of the IVCs.…”

Features of the temperature dependence and magnetic-field dependence of the critical current density close to the critical temperature in YBa2Cu3O7−δ thin films

“…The presence or absence of H / scaling can be regarded as an indicator of a definite temperature dependence of the vortex-pinning energy. 26 A paper by Gurevich and Pashitskii ͑G&P͒ 27 proposed another mechanism for limiting the critical current in YBCO thin films, associated with their block structure-not in connection with vortex pinning on it, but because the capability of the interblock boundaries to transmit SC current ͑their "transparency" for an SC current͒ is reduced by comparison with the internal sections of the blocks ͑the G&P model͒. The nonsuperconducting cores of the EDs at the interblock boundaries construct the channels by which the SC current flows through the boundaries.…”