M. T. Gonzalez

Vortex dynamics at the transition to the normal state in YBa2Cu3O7- δ films

Abstract We propose a description of the vortex dynamics in YBa 2 Cu 3 O 7− δ films from the critical to the normal states. This description supposes that the vortex motion is thermally activated along the twin boundaries of the films. The discontinuity observed in the current–voltage curves at the transition to the normal state is explained by the sudden increase in the dissipated power rate due to vortex depinning. However, near the critical temperature, this phenomenon does not occur because the vortex activation energy is near zero. We also show how the current at the transition to the normal state can be computed from the current–voltage curves measured at low currents. The predictions of this description are compared to the data published by [M.T. Gonzalez, J. Vina, S.R. Curras, J.A. Veira, J. Maza, F. Vidal, Phys. Rev. B 68 (2003) 054514].

Normal-state resistivity versus critical current in YBa2Cu3O7−δ thin films at high current densities

Abstract We report measurements at zero applied magnetic field of the normal state resistivity against temperature, ρ ( T ), and of the characteristic curves ( E – J ) in YBa 2 Cu 3 O 7− δ thin films. The E – J curves are measured at different temperatures, between 77 K and the superconducting transition temperature, and up to current densities one order of magnitude larger than the critical current density, J c , with the samples submerged in a liquid nitrogen bath, controlled in temperature by its vapor pressure. These E – J curves allow us to observe the abrupt electric-field jump from the superconducting state to an ohmic highly dissipative state appearing at a current density J * ≫ J c . Our results confirm the absence of any scaling between ρ and J c for thin films having different structural quality. In contrast, we have found that J * scales with the slope of the E – J curves, ρ n (=E/J) , for J > J * , which is very close to the extrapolation of the normal-state resistivity to the range of temperatures where the E – J curves are measured.

Electric field versus current density curves in melt-textured samples of YBa2Cu3O7−δ under currents well above the critical current

Abstract In this work, we present measurements of the E – J curves of top seeded melt-textured YBa 2 Cu 3 O 7− δ . The samples, with a typical 0.5 mm 2 cross section, were cut from monoliths grown up to 2 cm in diameter ( ab -planes direction) and 0.7 cm in height ( c -axis direction). The E – J characteristics were measured up to current densities several times larger than the critical current density J C . To avoid spurious heating, current pulses up to 200 A and 1–3 ms long were applied through the samples immersed in liquid nitrogen. Measurements were carried out in the temperature range 80–93 K by pressuring the nitrogen bath. An abrupt electric-field jump is observed, in the E – J curves, for a current density J * several times higher than J C . This jump at J * is to some extent similar to the one observed in thin films.

The Transition to the Normal State of YBCO Films

We propose a description of the transition of YBa2Cu3O7-? films from the superconducting to the normal state. The discontinuity observed in the current-voltage curves at the transition is explained by the sudden increase in the dissipated power rate due to vortex depinning. However, near Tc, this phenomenon does not occur because the vortex activation energy in this range of temperature is near zero. We also show how the current at the transition to the normal state is related to the critical current of the films. The predictions of this description are compared to the data published by Gonz?lez et al. [Phys.Rev. B 68, 054514 (2003)].

Dependence of critical-state models parameters on the intergrain resistivity in granular Y1Ba2Cu3O7−δ

Abstract By comparing critical-state models to current density measurements we show that the characteristic magnetic field H 0 that signals the beginning of the field-sensitive regime of critical currents in granular high-T c superconductors is a material parameter that may obtained from the integrain resistivity, i.e., from normal-state resistivity measurements.