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

Anisotropic superconducting flux-line behavior of single-crystal -[bis(ethylenedithio)tetrathiaful- valene]2Cu[N(CN)2]Br has been studied with dc magnetization and vibrating-reed (VR) techniques. A peak at T*(H) in the temperature dependence of the VR inverse quality factor 1/Q exhibits a shoulder for magnetic fields 0<0H0.5 T parallel to the conducting ac plane. The shoulder becomes unresolved near a kink in T*(H) at 0H0.7 T. Discontinuities in 1/Q and the VR frequency shift occur for 0.10H1.3 T applied perpendicular to the ac plane and temperatures T20 K, well into the normal state. These anomalies are preliminary evidence for a field-induced magnetic phase transition and unconventional superconductivity.