Evidence from resistivity measurements along the c axis for a transition within the vortex state for H∥ab in single-crystal YBa2Cu3O7

Abstract

The broadening of the resistive transition along the c axis of a ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ single crystal in the presence of strong magnetic fields (H>${\mathit{H}}_{\mathit{c}1}$)∥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 ${\mathit{T}}_{\mathit{m}}$(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.