Theory of superconductor-insulator transition in organic superconductor λ -(BETS)₂Fe_xGa_1-xCl₄

We investigate superconductivity and magnetism in the organic conductor λ-(BETS)₂Fe_xGa_1-xCl₄. In this material, it is known that the d-electrons on Fe-ions are ordered antiferromagnetically (T_N<8.5-9 K) for 0.4<x ≤ 1, while superconductivity (T_SC<6 K) of π-electrons appears when 0 ≤ x<0.4. In this paper, including the detailed band structure of this material, we discuss the exchange interaction between the d- and π-electrons, focusing on its role in the antiferromagnetism and superconductivity. We show that the inclusion of only this π-d exchange interaction is difficult to explain both the observed x-dependence of T_N and T_SC. To understand the phase diagram of λ-(BETS)₂Fe_xGa _1-xCl₄, we point out that the Coulomb interaction between π-electrons is also important.