MICROWAVE RESPONSE OF JOSEPHSON PLASMA IN THE MIXED-STATE OF KAPPA-(BEDT-TTF)(2)CU(NCS)(2)

A cavity perturbation technique is used to measure the magnetic-field dependence of surface impedance at 24 and 41 GHz in superconducting ka ppa-(BEDT-TTF)(2)Cu(NCS)(2) single crystals. A peak structure in the s urface resistance R(s)(H) is observed (1) below T-c, (2) when microwav e electric field is perpendicular to the superconducting layer, and (3 ) at higher magnetic field for the lower measurement frequency (anticy clotronic behavior). These observations are identified as the Josephso n plasma mode. The peak field increases as the temperature is lowered even below the irreversibility line in contrast to the case of Bi2Sr2C aCu2O8+y high-T-c superconductor.