Experimental evidence for vortex-glass superconductivity in Y-Ba-Cu-O

We demonstrate experimentally the existence of a continuous phase transition between a normal and a true superconducting phase (with zero linear resistivity) in epitaxial films of Y-Ba-Cu-O in strong magnetic fields fields, H>>Hc1. The nonlinear I-V curves show scaling behavior near the transition and the relevant critical exponents are extracted. These exponents are consistent with values expected for freezing into a superconducting vortex-glass phase.