MICROWAVE-LOSS STUDIES OF ORGANIC SUPERCONDUCTORS

We have conducted microwave-loss studies on organic superconductors. We report measurements on a series of crystals prepared from the bis(ethylenedithio)tetrathiafulvalene (ET) donor: alpha-(ET)2I3, beta-(ET)2I3, kappa-(ET)2Cu(NCS)2, kappa-(ET-d8)2Cu(NCS)2, and kappa-(ET)2Cu[N(CN)2]Br. All compounds show clear evidence for superconductivity, although there is a marked difference in the quality of the crystals as reflected in the microwave signals. The latter three materials show sharp, well-defined transitions that are comparable to those observed in the high-T(c) superconductors, and they exhibit highly anisotropic vortex motion. We confirm the T(c) of 11.6 K for the organic superconductor kappa-(ET)2Cu[N(CN)2]Br, which is 2-3 K higher than that of kappa-(ET)2Cu(NCS)2. We also confirm the approximately 0.5-K isotopic enhancement in the T(c) of kappa-(ET-d8)2Cu(NCS)2. The microwave-loss experiment is ideally suited for the search for superconductivity in organics and the experiment may be performed with the ease and quantity of material required for a melting-point determination-the measurement is nondestructive, contactless, and may be carried out on microgram samples.