Tilt angle effect of intrinsic flux pinning in a single crystal of La_1.85Sr_0.15CuO₄

The anisotropy of the activation energy of the flux pinning in a La_1.85Sr_0.15CuO₄ single crystal was investigated by ultrasonic measurements in various directions of wave vector k, polarization vector u and magnetic fields H. The large activation energy of U(0 K, 14 T) = 972 K and small field dependence ∝H^-0.3 were obtained for H ∥ c-plane, when the flux motion is parallel to the c-axis where the intrinsic pinning mechanism due to the layered structure is effective. The elastic modulus of the flux-line lattice which moves parallel to the c-axis is proportional to H² cos ²θ, and the value of U rapidly decreases with increasing tilt angle θ between H and the c-plane (U(θ = 10°) ∼ 300 K, U(θ = 30°) ∼ 75 K). These angular dependences of the elastic modulus and the activation energy can be explained on the basis of the intrinsic pinning mechanism and the three-dimensional stepwise flux-line structure.