PENETRATION FIELD STUDIES ON MELT-TEXTURED YBa2Cu3O7-delta FROM LOW FIELD ULTRASONIC MEASUREMENTS

Both attenuation and velocity measurements have been performed in low magnetic fields on a melt-textured sample of YBa2Cu3O7-delta. Transverse waves were propagated along the c-axis at 76 MHz while the temperature was kept constant. Four orientations of the magnetic field H were used. In the a-b plane the field was (1) perpendicular, (2) at 45, or (3) parallel to the polarization direction. The fourth orientation (4) was H parallel to the c-axis. At 77 K, an abrupt change in velocity and attenuation was observed at a field of 10 Gauss for all four orientations. However, the magnitude of the effect decreased as the field orientation was varied from (1) to (4). The position of the abrupt change depends quadratically on the temperature. Because of the lack of anisotropy of the value of the penetration field with magnetic field orientation, we identify it as the penetration field of the matrix surrounding the individual oriented crystallites. There is a maximum in the attenuation change at the penetration field at about 56 K. The effects appear to be produced by the interaction of sound waves with the flux lines which percolate through the Josephson junctions connecting the individual superconducting crystallites.