SUBPICOVOLT RESOLUTION MEASUREMENTS OF THE CURRENT-VOLTAGE CHARACTERISTICS OF TWINNED CRYSTALLINE YBA2CU3O7-X - NEW EVIDENCE FOR A VORTEX-GLASS PHASE

We present an improved contactless technique that allows measurement o f the current-voltage (I-V) curves of YBa2Cu3O7-x single crystals in l arge magnetic fields with 5 orders of magnitude improved voltage resol ution compared to conventional transport measurements. The crystals ar e shaped in the form of rings. The I-V curves are obtained by applying a small triangular oscillation of magnetic field and measuring the in duced current in the ring using a local Hall probe. The magnetic field resolution achieved is 1 ppm. The I-V curves are obtained very close to the glass transition temperature T-G, down to voltage levels of 0.1 pV, and show the characteristic change of slope on a log-log plot typ ical of a glassy transition.