PARALLEL FIELD PENETRATION IN A LAYERED SUPERCONDUCTOR

The Bean-Livingston entrance field H(s) for vortex penetration at the surface is calculated for layered superconductors with Josephson interlayer coupling and field parallel to the layers. Two regimes must be distinguished: close to T(c), one can use the anisotropic London theory, so H(s) is of the order of H(c), the thermodynamic critical field, and is the same as for a field normal to the layers. On the opposite, when thc transverse coherence length-xi(c) is smaller than the interlayer distance d, H(s) becomes smaller than H(c) and is of the order of (xi(c)/d)H(c). Contrary to the entrance field for pure Josephson vortices (fluxons) in junctions, this field is still much larger than the first critical field H(c1 parallel-to). This behaviour is a consequence of the specific structure of the vortex core in a layered superconductor and results in a crossover from a linear to a square root temperature dependence as the temperature is lowered.