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: α-(ET${)}_2$ ${\mathrm{I}}_3$, β-(ET${)}_2$ ${\mathrm{I}}_3$, κ-(ET${)}_2$Cu(NCS${)}_2$, κ-(ET-${\mathit{d}}_8$ ${)}_2$Cu(NCS${)}_2$, and κ-(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 κ-(ET${)}_2$Cu[N(CN${)}_2$]Br, which is 2–3 K higher than that of κ-(ET${)}_2$Cu(NCS${)}_2$. We also confirm the approximately 0.5-K isotopic enhancement in the ${\mathit{T}}_{\mathit{c}}$ of κ-(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—the measurement is nondestructive, contactless, and may be carried out on microgram samples.