Vortex state in the highly anisotropic superconductor, α-(BEDT-TTF)2NH4Hg(SCN)4

The highly anisotropic superconductor, α-(BEDT-TTF)2NH4Hg(SCN)4 , was studied by specific heat and ac susceptibility measurements in magnetic fields perpendicular to the conducting plane. The specific heat measurement gave a mean field Hc2 line. The diamagnetic signal in both in-plane (hac ⊥ conducting plane) and out-of-plane (hac // conducting plane) ac susceptibility drastically increased far below the Hc2 line. The plot of the onset temperature vs field forms an additional boundary where the vortex mobility changes drastically.