Critical currents in vicinal YBa$_2$Cu$_3$O$_{7-\delta}$ films

TL;DR

This paper investigates the angular dependence of critical current density in vicinal YBa$_2$Cu$_3$O$_{7-\delta}$ films, revealing flux line lattice transitions and proposing a kinked vortex model to explain anisotropic behavior.

Contribution

It introduces measurements on vicinal films to better understand flux lattice structures and their impact on critical current anisotropy in YBCO.

Findings

Evidence for flux line lattice transitions based on field, orientation, and temperature.

Failure of simple scaling laws to describe critical current anisotropy.

Successful modeling of $j_c( heta)$ with a kinked vortex structure.

Abstract

Most measurements of critical current densities in YBa$_2$Cu$_3$O$_{7-\delta}$ thin films to date have been performed on films where the \textit{c}-axis is grown normal to the film surface. With such films, the analysis of the dependence of $j_c$ on the magnetic field angle is complex. The effects of extrinsic contributions to the angular field dependence of $j_c$, such as the measurement geometry and disposition of pinning centres, are convoluted with those intrinsically due to the anisotropy of the material. As a consequence of this, it is difficult to distinguish between proposed FLL structure models on the basis of angular critical current density measurements on \textit{c}-axis films. Films grown on mis-cut (vicinal) substrates have a reduced measurement symmetry and thus provide a greater insight into the critical current anisotropy. In this paper previous descriptions of the magnetic field angle dependence of $j_c$ in YBa$_2$Cu$_3$O$_{7-\delta}$ are reviewed. Measurements on YBa$_2$Cu$_3$O$_{7-\delta}$ thin films grown on a range of vicinal substrates are presented and the results interpreted in terms of the structure and dimensionality of the FLL in YBa$_2$Cu$_3$O$_{7-\delta}$. There is strong evidence for a transition in the structure of the flux line lattice depending on magnetic field magnitude, orientation and temperature. As a consequence, a simple scaling law can not, by itself, describe the observed critical current anisotropy in YBa$_2$Cu$_3$O$_{7-\delta}$. The experimentally obtained $j_c(\theta)$ behaviour of YBCO is successfully described in terms of a kinked vortex structure for fields applied near parallel to the \textit{a-b} planes.