Novel Microwave Absorption Due to Strong Coupling between Josephson Plasma and the Josephson Vortex Array in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$

TL;DR

This study explores microwave excitations in Bi$_2$Sr$_2$CaCu$_2$O$_{8+eta}$, revealing two distinct phase-collective modes linked to Josephson plasma and vortex oscillations, influenced by magnetic fields.

Contribution

It identifies and characterizes two different Josephson plasma modes in high-temperature superconductor crystals under magnetic fields, advancing understanding of vortex dynamics.

Findings

Two phase-collective modes observed: one increases with magnetic field, approaching linear dependence.

Higher mode attributed to Josephson plasma propagating along vortex lattice.

Lower mode linked to oscillation of Josephson vortices.

Abstract

We have investigated the Josephson plasma excitations in magnetic fields parallel to the $ab$-plane in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ single crystals in a wide microwave frequency region (9.8 -- 75 GHz). It was found that there are two kind of phase-collective modes: one increases with magnetic fields in higher fields, approaching linear asymptotic dependence with $ck$, which lies well above the inherent plasma frequency $\omega_p$, while the other does not show considerable field dependence. The higher linear mode is attributed to the Josephson plasma mode propagating along the reciprocal lattice vector of the Josephson vortex, whereas the lower one can be ascribed to the oscillation mode of Josephson vortices.