JOSEPHSON PLASMA EDGE IN LA1.85SR0.15CUO4

The far-infrared dielectric c-axis response for La1.85Sr0.15CuO4, show s a pronounced plasma edge below the BCS superconductive gap frequency . The low frequency of this plasmon was previously interpreted as an i ndication for anomalous mass enhancement for the c-axis transport. How ever, a natural explanation for this low-frequency plasmon can be give n in terms of Josephson plasma oscillations of the order parameter pha se difference between adjacent planes in the layered high-T-c material s. In this contribution we concentrate on the effect of a large magnet ic held parallel to the planes of the superconducting layers. We will demonstrate that the spectroscopic measurement of the plasma frequency and damping as a function of magnetic field yields direct information about the c-axis critical current density and the vortex dynamics for this configuration. In particular, we find for La1.85Sr0.15CuO4 that the vortices for B parallel to the planes are not pure Josephson vorti ces but rather in between the Abrikosov and Josephson limits.