Penetration field studies on melt-textured YBa/sub 2/Cu/sub 3/O/sub 7- delta / from low field ultrasonic measurements

Both attenuation and velocity measurements have been performed in low magnetic fields on a melt-textured sample of YBa/sub 2/Cu/sub 3/O/sub 7- delta /. Transverse waves 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 perpendicular, at 45 degrees , or parallel to the polarization direction. The fourth orientation was parallel to the c-axis. The results suggest that the penetration field observed by the ultrasonic measurements is associated with flux line penetration into the matrix surrounding the oriented crystallites. The velocity jump changes as the inverse of the separation between the vortex lines, and the maximum in the attenuation jump appears to be the result of the fact that the attenuation is proportional to the density of the flux lines but the interaction is described by a thermally activated process.<>