Effect of microwave-enhanced superconductivity in YBa2Cu3O7 Bi-crystalline grain bounda ry weak-links

We have studied systematically the effect of microwave irradiation on the temperature dependent resistivity (R(I) and the current-voltage (I-V) characteristics of YBa2Gu3O(7 - x) (YBCO) bicrystalline grain boundary weak-links (GBWL's), with grain boundary of three different tilt angles. The superconducting transition temperature, T(sub c), has significant enhancement upon microwave irradiation. The microwave enhanced T(sub c) is increased as a function of incident microwave power, but limited to an optimum power level. The GBWL's of 45 deg tilt boundary has shown to be most sensitive to the microwave irradiation power, and the GBWL's of 36.8 deg tilt boundary has displayed a moderate response. In contrast, no enhancement of T(sub c) was observed in the GBWL's of 24 deg tilt boundary, as well as in the uniform films. Under the microwave irradiation, the R(T) dependent is hystertic as the transition taken from superconducting state to normal state and vice versa. Mechanisms associated with the redistribution of nonequilibrium quasiparticles under microwave irradiation are discussed.