Anomalous vortex dynamics in κ-[bis(ethylenedithio)tetrathiafulvalene]2Cu[N(CN)2]Br: Evidence for field-induced magnetic order and unconventional superconductivity

Abstract

Anisotropic superconducting flux-line behavior of single-crystal κ-[bis(ethylenedithio)tetrathiaful- valene${]}_2$Cu[N(CN${)}_2$]Br has been studied with dc magnetization and vibrating-reed (VR) techniques. A peak at ${\mathit{T}}^{\mathrm{*}}$(H) in the temperature dependence of the VR inverse quality factor 1/Q exhibits a shoulder for magnetic fields 0<${\mathrm{\mu }}_0$H≤0.5 T parallel to the conducting ac plane. The shoulder becomes unresolved near a kink in T*(H) at ${\mathrm{\mu }}_0$H≊0.7 T. Discontinuities in 1/Q and the VR frequency shift occur for 0.1≤${\mathrm{\mu }}_0$H≤1.3 T applied perpendicular to the ac plane and temperatures T≤20 K, well into the normal state. These anomalies are preliminary evidence for a field-induced magnetic phase transition and unconventional superconductivity.