The Josephson plasma resonance has been investigated in Bi2Sr2CaCu2O8+δ single crystals in parallel magnetic fields to the ab-plane. We found two resonance modes; one appears at higher frequency in high fields above the plasma frequency ωp at zero field and absolute zero, and the other lies below ωp and is observed only in magnetic fields without considerable field dependence. Two resonance lines were also found in numerical simulations in a single junction model with randomness of the critical current. The higher frequency mode is attributed to the Josephson plasma mode modified by the periodic structure of Josephson vortices, while the lower frequency mode is interpreted as oscillations of Josephson vortices.
@article{arxiv.cond-mat/0111094,
title = {Josephson Plasma Excitation and Vortex Oscillation Mode in Josephson Vortes State},
author = {I. Kakeya and T. Wada and K. Kadowaki and M. Machida},
journal= {arXiv preprint arXiv:cond-mat/0111094},
year = {2007}
}