Observation of separate vortex-melting and vortex-glass transitions in defect-enhanced YBa2Cu3O7 single crystals

We have observed two distinct phase transitions in a magnetic field, separating the normal and superconducting phases in single-crystal YBa2Cu3O7 samples with enhanced disorder, either naturally occurring or deliberately introduced by radiation damage using 3-MeV protons or 1-GeV Au ions. At low currents, the I-V data are linear on both sides of the upper transition, whereas the lower transition separates Ohmic behavior from nonlinear (i.e., superconducting) behavior. We interpret the upper transition as the remnant of the clean system's first-order vortex-lattice melting and the lower transition as the transition into a true superconducting phase, a vortex glass. © 1992 The American Physical Society.