SCALING OF VOLTAGE-CURRENT CHARACTERISTICS OF THIN-FILM Y-BA-CU-O AT LOW MAGNETIC-FIELDS

We have measured voltage-current characteristics for YBa2Cu3O7 thin fi lms in magnetic fields from 5 T to ambient, including some in the mill itesla range. In all cases, the resistivity-current density isotherms can be separated into two classes: those which exhibit upward curvatur e, and have constant resistivity at low currents, and those which exhi bit downward curvature at all currents. These two class of isotherms a re separated by a field-dependent temperature T(g). For each field, th e isotherms scale in a manner consistent with a three-dimensional vort ex-liquid to vortex-glass phase transition. The region that can be sca led is several kelvin at 5 T and narrows with decreasing field. The lo w-current resistivity above T(g) varies as \1-T/T(g)\s with s almost-e qual-to 7.4 at all fields, but the critical scaling exponents z and v become field dependent below about 10 mT.