3-DIMENSIONAL TO 2-DIMENSIONAL CROSSOVER IN LAYERED HIGH-T-C SUPERCONDUCTORS

We calculate vortex lattice properties in a highly anisotropic layered high-T-c superconductor using Monte Carlo simulations. The supercondu cting order parameter is expanded in products of lowest Landau level s tates in the ab plane and tight-binding Bloch states in the c directio n. The phase diagram is then a universal function of a dimensionless e ffective temperature T and effective interlayer coupling eta, both of which depend on temperature T and magnetic field B. We characterize th e vortex lattice by the helicity modulus (superfluid density) Upsilon in the c direction, and shear modulus mu in the ab plane. There appear s to be no phase transition separating two-dimensional (2D) and 3D sol ids. Instead, the 3D to 2D transition is manifested by a smooth crosso ver of Upsilon from nearly mean-field 3D behavior Upsilon similar to e ta at large eta, to a 2D, fluctuation-dominated regime Upsilon similar to eta(2) at small eta. In the limit eta --> 0 the shear modulus smoo thly approaches a finite value characteristic of a purely 2D vortex la ttice. We discuss the possibility that this dimensional crossover may account for the disappearance of the neutron scattering peaks in BiSr2 Ca2CuO8+delta at high fields.