Transition to the normal state induced by high current densities in YBa_2Cu_3O_(7-δ) thin films: A thermal runaway account

摘要

By using a finite-element method we analyze at a quantitative level the abrupt jump to the normal state in high-T_c films observed when measuring their current-voltage characteristics at current densities, J~*, which are between two to three times their critical current density, J_c. The experimental data that this analysis focuses on are from YBa_2Cu_3O_(7-δ) films, measured between 75 K and T_c ≈ 90 K and under zero applied magnetic field. Our main starting point is the assumption that the constant-temperature curves, i.e., that would be measured at arbitrarily short measuring time, are smooth and so jumpless. When taking into account the finite measuring times, the highly nonlinear nature of the film's electrical conductivity, and the thermal properties of the substrate, simulation by the finite-element method shows that a thermal runaway takes place that explains, without free parameters, the experimental jumps to a 5% accuracy. The voltage values prior to the jump are also coherently accounted for with similar accuracy. No critical mechanism such as the vortex instability model from Larkin-Ovchinikov or any others are needed for this quantitative agreement to our measurements, though they can become dominant under different refrigeration conditions, temperature range, or magnetic-field application.