Interlayer electrodynamics and unconventional vortex state in YBa2Cu3Oy

We report on the c-axis magneto-optical response of YBa2Cu3Oy (y=6.65 and 6.75) single crystals, with magnetic fields oriented both parallel and perpendicular to the CuO2 planes. The dominant characteristic of the c-axis electrodynamics in the superconducting state, the Josephson plasma resonance (JPR), is remarkably sensitive to fairly modest magnetic fields below 8 T. Fields oriented perpendicular to the CuO2 planes are shown to shift the edge of the JPR and also reduce the weight of the so-called 400-cm(-1) mode, shedding light on this enigmatic feature. In the H parallel to CuO2 geometry, where the magnetic field initiates Josephson vortices, we observed a strong mode in the far-infrared which hardens with increasing field. The field dependence of the low-frequency resonance behavior is contrasted to that of two other cuprate materials: La2-xSrCuO4 compounds that we have investigated earlier, and Bi2Sr2CaCu2O8-delta. Specifically, there exist disparities in the number and field dependence of longitudinal modes measured for each system. Many of these differences can be explained through a new numerical solution of the interlayer phase equations which includes effects of both in-plane and c-axis dissipation parameters. Support for this approach is given by calculations of the Josephson vortex lattice ground state configuration, and further insight is gained through the phenomenological framework of the transverse JPR model, as well as a classical model of vortex dynamics.