LOCK-IN TRANSITION IN LAYERED SUPERCONDUCTORS

We have studied the equilibrium vortex state of various layered superc onductors by torque magnetometry. From the angle dependence of the tor que the anisotropy of the magnetic penetration depth gamma = lambda(pe rpendicular-to)/lambda(parallel-to) is determined. In Tl2Ca2Ba2Cu3O10 and Bi2Sr2CaCu2O8 we find extreme values up to gamma = 900. At field o rientations close to parallel to the layers a double peak of the torqu e appears as a fingerprint of the vortex lock-in transition. In the lo cked state we observe ideal screening of the field component perpendic ular to the planes.