Dissipation in high tc thin films

Current methods of fabricating Y₁Ba₂Cu₃O_7-δ thin films by pulsed laser deposition result in films that are multicrystalline, with grain sizes of typically 1 μm, or are quasi-monocrystalline, with no discernable grain boundaries, according to whether the deposition is carried out at high or low temperatures, respectively. The authors studied the electrical characteristics of both these types of material. It was found that the I-V characteristics of the granular material follow the rule Vα (I-I_c)^x(T) over at least six orders of magnitude in voltage, at all temperatures and magnetic fields. In addition, x(T) changes sharply from x = 1 to x = 2 at the temperature at which I_c becomes finite. This behavior can be explained quantitatively by a phase-ordering transition. The nongranular material has a much sharper resistive transition and larger critical current. The authors examined the variation of critical current with magnetic field and temperature in this material and observed an exponential suppression of J_c with magnetic field.