Two Phase Collective Modes in Josephson Vortex Lattice in Intrinsic Josephson Junction Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$

TL;DR

This study investigates Josephson plasma excitations in Bi$_2$Sr$_2$CaCu$_2$O$_{8+eta}$ under in-plane magnetic fields, revealing two distinct collective modes of the Josephson vortex lattice with different magnetic field dependences.

Contribution

It reports the first observation of a novel phase collective mode of the Josephson vortex lattice in high-$T_c$ superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+eta}$, distinguished from the plasma resonance mode.

Findings

Identified two resonance modes with different magnetic field dependences.

The higher energy mode aligns with the Josephson plasma resonance along the vortex lattice.

The lower energy mode is a new phase collective mode of the vortex lattice.

Abstract

Josephson plasma excitations in the high $T_c$ superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ have been investigated in a wide microwave frequency region (9.8 -- 75 GHz), in particular, in magnetic field applied parallel to the $ab$ plane of the single crystal. In sharp contrast to the case for magnetic fields parallel to the c axis or tilted from the $ab$ plane, it was found that there are two kinds of resonance modes, which are split in energy and possess two distinctly different magnetic field dependences. One always lies higher in energy than the other and has a shallow minimum at about 0.8 kOe, then increases linearly with magnetic field. On the other hand, another mode begins to appear only in a magnetic field (from a few kOe and higher) and has a weakly decreasing tendency with increasing magnetic field. By comparing with a recent theoretical model the higher energy mode can naturally be attributed to the Josephson plasma resonance mode propagating along the primitive reciprocal lattice vector of the Josephson vortex lattice, whereas the lower frequency mode is assigned to the novel phase collective mode of the Josephson vortex lattice, which has never been observed before.