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: \ensuremath{\alpha}-(ET${)}_{2}$${\mathrm{I}}_{3}$, \ensuremath{\beta}-(ET${)}_{2}$${\mathrm{I}}_{3}$, \ensuremath{\kappa}-(ET${)}_{2}$Cu(NCS${)}_{2}$, \ensuremath{\kappa}-(ET-${\mathit{d}}_{8}$${)}_{2}$Cu(NCS${)}_{2}$, and \ensuremath{\kappa}-(ET${)}_{2}$Cu[N(CN${)}_{2}$]Br. All compounds show clear evidence for superconductivity, although there is a marked difference in the quality of the crystals as reflected in the microwave signals. The latter three materials show sharp, well-defined transitions that are comparable to those observed in the high-${\mathit{T}}_{\mathit{c}}$ superconductors, and they exhibit highly anisotropic vortex motion. We confirm the ${\mathit{T}}_{\mathit{c}}$ of 11.6 K for the organic superconductor \ensuremath{\kappa}-(ET${)}_{2}$Cu[N(CN${)}_{2}$]Br, which is 2\char21{}3 K higher than that of \ensuremath{\kappa}-(ET${)}_{2}$Cu(NCS${)}_{2}$. We also confirm the approximately 0.5-K isotopic enhancement in the ${\mathit{T}}_{\mathit{c}}$ of \ensuremath{\kappa}-(ET-${\mathit{d}}_{8}$${)}_{2}$Cu(NCS${)}_{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\char22{}the measurement is nondestructive, contactless, and may be carried out on microgram samples.