ELASTIC PROPERTIES AND ANISOTROPIC PINNING OF THE FLUX-LINE-LATTICE IN SINGLE-CRYSTALLINE LA1.85SR0.15CUO4

The temperature dependence of the sound velocity and the sound-attenua tion coefficient in single-crystalline La1.85Sr0.15CuO4 have been meas ured in the mixed state with an ultrasonic technique. An increase in t he sound velocity and an attenuation peak, due to the elasticity of th e flux-line lattice and the flux-pinning effect, are observed at a tem perature lower than the superconducting transition temperature T(c). T he compression and tilt moduli of the flux-line lattice are found to b e softened in the case of H perpendicular-to c. The anisotropic activa tion energies that are necessary to depin the flux-line lattice are se parately evaluated by measurements under various settings of the direc tions of the wave vector k, the polarization vector u, and the magneti c field H using an analysis based on the thermally assisted flux-flow model. The estimated activation energies are 1140 K (at 0 K, 6 T) for H perpendicular-to c, u parallel-to c, 201 K (at 0 K, 14 T) for H perp endicular-to c, u parallel-to (c X H), and 93 K (at 0 K, 6 T) for H pa rallel-to c, u perpendicular-to c. These results are consistent with t he intrinsic pinning mechanism. The activation energies determined by ultrasonic measurements are found to be smaller than those determined by resistivity measurements.