Scaling of voltage-current characteristics of thin-film Y-Ba-Cu-O at low magnetic fields

Abstract

We have measured voltage-current characteristics for ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ thin films in magnetic fields from 5 T to ambient, including some in the millitesla range. In all cases, the resistivity--current density isotherms can be separated into two classes: those which exhibit upward curvature, and have constant resistivity at low currents, and those which exhibit downward curvature at all currents. These two classes of isotherms are separated by a field-dependent temperature ${\mathit{T}}_{\mathit{g}}$. For each field, the isotherms scale in a manner consistent with a three-dimensional vortex-liquid to vortex-glass phase transition. The region that can be scaled is several kelvin at 5 T and narrows with decreasing field. The low-current resistivity above ${\mathit{T}}_{\mathit{g}}$ varies as \ensuremath{\Vert}1-T/${\mathit{T}}_{\mathit{g}}$${\mathrm{\ensuremath{\Vert}}}^{\mathit{s}}$ with s\ensuremath{\approxeq}7.4 at all fields, but the critical scaling exponents z and \ensuremath{\nu} become field dependent below about 10 mT.