LOW-TEMPERATURE MICROWAVE SURFACE IMPEDANCE OF THE CONVENTIONAL ORGANIC SUPERCONDUCTOR KAPPA-(BEDT-TTF)2CU(NCS)2

We have performed measurements of the microwave surface impedance Z(S) = R(S) + iX(S) of the organic superconductor kappa-(BEDT-TTF)2Cu(NCS) 2 down to T = 0.8 K. The reactance X(S), and therefore the penetration depth lambda, exhibits no temperature dependence for T << T(c). This result is in full agreement with calculations of the BCS ground state and gives no indication of unconventional pairing. Furthermore, we hav e evaluated the complex conductivity sigma = sigma1 + isigma2 and find a pronounced peak in sigma1 below T(c) which can be identified as a c oherence peak.