H[sub [ital C]1] and BCS energy gap from low field ultrasonic measurements on melt-textured YBa[sub 2]Cu[sub 3]O[sub 7]

Ultrasonic attenuation and velocity measurements have been performed on melt-textured YBa[sub 2]Cu[sub 3]O[sub 7]. In low magnetic field sweeps at constant temperatures, one sees sharp changes in both the velocity and the attenuation at the lower critical field. The attenuation jump per vortex can be fit to a Debye relaxation formula and the temperature dependence of the relaxation time in the superconducting state can be determined. The final results appear to be consistent with BCS energy gap [Delta](0)=1.76 k[sub [ital B]]T[sub [ital C]].