Collapse of the vortex pinning force in LaSrCuO for H//a-b planes

The irreversible magnetization and the associated critical current density of the high-temperature superconductor La_1.85Sr_0.15CuO₄ are investigated as a function of the orientation ( theta , phi ) of the applied field (H) with respect to the crystallographic axes. For small angles (v= pi /2- theta ) between vortex lines and a-b planes, the authors show for the first time that the decrease of the pinning force component parallel to these planes occurs in a narrow range v approximately=1 degrees where it is of first order in mod v mod . This behaviour seems to be related to the existence of a 'staircase' (kinked) type of vortex lattice. Neither the usual anisotropic lattice (valid for a homogeneous superconductor), nor the more recently proposed combined lattice scheme, is compatible with the authors' results in this range of field orientation.