Anisotropy of the flux pinning and elastic anomalies under high magnetic fields in La_2-xSr_xCuO₄

Temperature dependence of the sound velocity of La_1.85Sr_0.15CuO₄ single crystals has been measured in high magnetic fields up to 23 T. The anisotropy of the activation energy of the flux pinning is evaluated from the analysis of the excess elastic modulus due to the flux line lattice. The large activation energy of U(0 K, 14 T)=972 K and the small field dependence ∝ H^-0.3 are obtained for H//c-plane where the intrinsic pinning mechanism due to the layered structure is effective. The value of U rapidly decreases with increasing the tilt angle θ between H and the c-plane. The angular dependence of U is explained on the basis of the three-dimensional stepwise-flux-line structure. Moreover, elastic anomalies have been observed at T_c and near irreversibility line. The upper critical field is evaluated from the anomaly at T_c and origin of the elastic anomaly near irreversibility line is discussed.