Josephson plasma resonance in the mixed state of the organic superconductor κ-(BEDT-TTF${)}_2$Cu(NCS${)}_2$

Magnetic-field dependence of microwave surface resistance measured using a cavity perturbation technique shows a peak structure below ${\mathrm{T}}_{\mathrm{c}}$ in the organic layered superconductor κ-(BEDT-TTF${)}_2$Cu(NCS${)}_2$. It is observed only when ${\mathrm{E}}_{\mathrm{\omega }}$ is perpendicular to the superconducting layer and the peak field shows an anticyclotronic frequency dependence. All these observations suggest the excitation of Josephson plasma across the superconducting layers as the origin of the resonance. The interlayer phase correlation increases as the temperature is lowered even below the irreversibility line in contrast to the case of the ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{8+\mathrm{y}}$ high-${\mathrm{T}}_{\mathrm{c}}$ superconductor.