Interlayer electrodynamics and unconventional vortex state inYBa2Cu3Oy

Abstract

We report on the $c$-axis magneto-optical response of $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{y}$ ($y=6.65$ and 6.75) single crystals, with magnetic fields oriented both parallel and perpendicular to the $\mathrm{Cu}{\mathrm{O}}_{2}$ 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 $8\phantom{\rule{0.3em}{0ex}}\mathrm{T}$. Fields oriented perpendicular to the $\mathrm{Cu}{\mathrm{O}}_{2}$ planes are shown to shift the edge of the JPR and also reduce the weight of the so-called ``$400\text{\ensuremath{-}}{\mathrm{cm}}^{\ensuremath{-}1}$ mode,'' shedding light on this enigmatic feature. In the $H\ensuremath{\Vert}\mathrm{Cu}{\mathrm{O}}_{2}$ 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: ${\mathrm{La}}_{2\ensuremath{-}x}\mathrm{Sr}\mathrm{Cu}{\mathrm{O}}_{4}$ compounds that we have investigated earlier, and ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}\mathrm{Ca}{\mathrm{Cu}}_{2}{\mathrm{O}}_{8\ensuremath{-}\ensuremath{\delta}}$. 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.