We have measured voltage-current characteristics for YBa2Cu3O7 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 curbature, 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 temperture Tg. 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 kelving at 5 T and narrows with decreasing field. The low-current resistivity above Tg varies as |1-T/Tg|^s with s≈7.4 at all fields, but the critical scaling exponentsz z and v become fileld dependent below about 10 mT.
Brown, Brandon R., "Scaling of Voltage-current Characteristics of Thin-film Y-Ba-Cu-0 at Low Magnetic Fields" (1994). Physics and Astronomy. Paper 26.