The anisotropy of the upper critical field Hc2 has been measured at ambient pressure on a layered organic superconductor β-(BEDT-TTF)2I3 both in the states before and after annealing at 109 K for 120 hours. By annealing, the critical temperature Tc increases from 1.4 to 2.08 K, while the Hc2±plane increases and the anisotropy, Hc2//plane/Hc2±Plane, decreases. The angular dependences of Hc2 for both states are explained in terms of the effective mass model. It is clarified that the enhanced Tc which is ascribed to the annealing-induced configurational orderings of ethylene groups is closely related to the enhanced effective mass in the basal plane.
- Critical field anisotropy