Angular-dependent upper critical field studies of (formula presented)

It has been suggested that the strong upward curvature and large enhancement of the b axis critical fields in (formula presented) are the result of a magnetic field-induced dimensional crossover (FIDC) effect. In this paper we present a critical test of the FIDC for this material at a pressure of 5.7 kbar. Decoupled two-dimensional layers should exhibit a cusp in (formula presented) vs angle near H parallel to the layers. Rather we see no cusp in (formula presented) and the anisotropy decreases as temperature is reduced. Our data, in this pressure regime, are more consistent with a recently proposed insulator-superconductor slab model.