ULTRASONIC DETERMINATION OF THE SUPERCONDUCTING ENERGY-GAP IN DOMAIN BOUNDARIES OF MELT-TEXTURED YBA2CU3O7

Ultrasonic attenuation and velocity measurements as a function of magn etic field in the superconducting state of a melt-textured sample of Y Ba2Cu3O7 with T(c) = 91 K are reported. Sharp rises in both attenuatio n alpha and relative velocity dv/v are observed at the penetration fie lds H(c1)' of its domain boundaries. The change in dv/v at H(c1)' is p roportional to the repulsive force between vortices. The change in alp ha at H(c1)' is produced by a relaxation process involving the excitat ion and decay of superconducting quasiparticles in the vicinity of the normal core of the vortices. The relaxation time associated with this process yields the Burdeen-Cooper-Schrieffer temperature-dependent en ergy gap.