Effect of an in-plane magnetic field on the interlayer phase coherence in the extreme-2D organic superconductor k-(BEDT-TTF)2Cu(NCS)2

Using a high-sensitivity cavity perturbation technique (40 to 180 GHz), we have probed the angle dependent interlayer magneto-electrodynamic response within the vortex state of the extreme two-dimensional organic superconductor k-(BEDT-TTF)2Cu(NCS)2. A previously reported Josephson plasma resonance [M. Mola et al., Phys. Rev. B 62, 5965 (2000)] exhibits a dramatic re-entrant behavior for fields very close (<1 degree) to alignment with the layers. In this same narrow angle range, a new resonant mode develops which appears to be associated with the non-equilibrium critical state. Fits to the angle dependence of the Josephson plasma resonance provide microscopic information concerning the superconducting phase correlation function within the vortex state. We also show that the effect of an in-plane magnetic field on the temperature dependence of the interlayer phase coherence is quite different from what we have previously observed for perpendicular magnetic fields.