Chemical Control of Electrical Properties and Phase Diagram of a Series of λ-Type BETS Superconductors, λ-(BETS)2GaBrxCl4-x

λ-(BETS) 2 GaBr x Cl 4 - x [BETS = bis(ethylenedithio)tetraselenafulvalene; 0 ≤ x ≤ 2] is a molecular superconductor with strongly correlated conduction electrons. The electrical transport properties of λ-(BETS) 2 GaBr x Cl 4 - x are drastically changed by varying the bromine content x or by applying pressure. At ambient pressure, the superconducting transition could be observed for x < 0.75. The pressure and x dependencies of T c were examined. The M − H curve ( M = magnetization; H = magnetic field) at 2 K indicated the almost perfect Meissner state of the superconducting phase of λ-(BETS) 2 GaCl 4 . The H c1 is ∼8 Oe for H ⊥ and 12 Oe for H ∥, where H ⊥ and H ∥ are the magnetic fields perpendicular and parallel to the c axis, respectively. The magnetic susceptibility of λ-(BETS) 2 GaBr x Cl 4 - x increases with decreasing temperature to ∼60 K, below which the susceptibility becomes x -dependent and tends to be suppressed with increasing x . The isotropic decrease of the static susceptibility at lower temperature observed in the insulating system with x > 1.0 indicates the insulating ground state seems not to be antiferromagnetic but probably nonmagnetic. The crystal structure determinations of a series of λ-(BETS) 2 GaBr x Cl 4 - x and the calculations of the intermolecular overlap integrals of the highest occupied molecular orbital of BETS were made to elucidate a key factor of the superconducting transition mechanism. The x -dependence of intermolecular overlap integrals seems to suggest that the magnitude of the “spin gap” of the nonmagnetic insulating state tends to be diminished with decreasing x . There exists one intermolecular overlap integral exhibiting a large temperature and x -dependence, which seems to play a crucial role in determining the nature of the ground state.