Interlayer electrodynamics and unconventional vortex state in Y Ba2 Cu3 Oy

We report on the c -axis magneto-optical response of Y Ba2 Cu3 Oy (y=6.65 and 6.75) single crystals, with magnetic fields oriented both parallel and perpendicular to the Cu O2 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 Cu O2 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 Cu O2 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-x SrCu O4 compounds that we have investigated earlier, and Bi2 Sr2 Ca Cu2 O8-δ. 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. © 2007 The American Physical Society.