2K-superconducting state with less disorder in the organic superconductor ß-(BEDT-TTF)₂I₃

Annealing of the title compound at about 110 K for 20-40 hr causes a change in the superstructure, and gives rise to two superconducting states with the critical temperatues of 2 K and ∼7.5 K. The former state is found to occupy most of the sample volume. This state having T_c ∼ 2K, found in the present study for the first time,has a lower resistance and a narrower EPR linewidth in its normal state than this compound without annealing has. A conjecture is given that the state with T_{c ∼} 2 K has only the A-type conformation of BEDT-TTFmolecules leading to the reduction of disorder related to the conformational degree of freedom and to the resultant increase of T_c from 1-1.5 K of the so-called "low-T_c" state of this material. The other state with T_c ∼ 7.5K is interpreted to be nothing but the "high-T_c" state having no superstructure.