EVIDENCE FROM RESISTIVITY MEASUREMENTS ALONG THE C-AXIS FOR A TRANSITION WITHIN THE VORTEX STATE FOR H-PARALLEL-TO-AB IN SINGLE-CRYSTAL YBA2CU3O7

The broadening of the resistive transition along the c axis of a YBa2Cu3O7 single crystal in the presence of strong magnetic fields (H > H(c1)) parallel to ab disappears below a well-defined line in the H-T phase diagram. Above this line, dissipation is Ohmic and field dependent. Below this line, a discontinuity appears in the resistivity curves, and the I-V behavior is strongly nonlinear. The shape of the resistive transition below the T(m)(H) line has a universal character, which remains unaffected by changes in the amplitude of the field. This unusual behavior contradicts any collective-flux-pinning model that predicts a field-dependent activation energy, and suggests a phase transition related to the depinning line.