Lock-in transition in layered superconductors

Abstract

We have studied the equilibrium vortex state of various layered superconductors by torque magnetometry. From the angle dependence of the torque the anisotropy of the magnetic penetration depth γ= λ⊥/ λ| is determined. In Tl 2Ca 2Ba 2Cu 3O 10 and Bi 2Sr 2CaCu 2O 8 we find extreme values up to γ=900. At field orientations close to parallel to the layers a double peak of the torque appears as a fingerprint of the vortex lock-in transition. In the locked state we observe ideal screening of the field component perpendicular to the planes.