Chemical Control of Electrical Properties and Phase Diagram of a Series of λ-Type BETS Superconductors, λ-(BETS)<sub>2</sub>GaBr<i><sub>x</sub></i>Cl<sub>4-</sub><i><sub>x</sub></i>

λ-(BETS)<sub>2</sub>GaBr<i><sub>x</sub></i>Cl<sub>4</sub><sub>-</sub><i><sub>x</sub></i> [BETS = bis(ethylenedithio)tetraselenafulvalene; 0 ≤ <i>x</i> ≤ 2] is a molecular superconductor with strongly correlated conduction electrons. The electrical transport properties of λ-(BETS)<sub>2</sub>GaBr<i><sub>x</sub></i>Cl<sub>4</sub><sub>-</sub><i><sub>x</sub></i> are drastically changed by varying the bromine content <i>x</i> or by applying pressure. At ambient pressure, the superconducting transition could be observed for <i>x</i> < 0.75. The pressure and <i>x</i> dependencies of <i>T</i><sub>c</sub> were examined. The <i>M</i>−<i>H</i> curve (<i>M</i> = magnetization; <i>H</i> = magnetic field) at 2 K indicated the almost perfect Meissner state of the superconducting phase of λ-(BETS)<sub>2</sub>GaCl<sub>4</sub>. The <i>H</i><sub>c1</sub> is ∼8 Oe for <i>H</i><sub>⊥</sub> and 12 Oe for <i>H</i><sub>∥</sub>, where <i>H</i><sub>⊥</sub> and <i>H</i><sub>∥</sub> are the magnetic fields perpendicular and parallel to the <i>c</i> axis, respectively. The magnetic susceptibility of λ-(BETS)<sub>2</sub>GaBr<i><sub>x</sub></i>Cl<sub>4</sub><sub>-</sub><i><sub>x</sub></i> increases with decreasing temperature to ∼60 K, below which the susceptibility becomes <i>x</i>-dependent and tends to be suppressed with increasing <i>x</i>. The isotropic decrease of the static susceptibility at lower temperature observed in the insulating system with <i>x</i> > 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)<sub>2</sub>GaBr<i><sub>x</sub></i>Cl<sub>4</sub><sub>-</sub><i><sub>x</sub></i> 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 <i>x</i>-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 <i>x</i>. There exists one intermolecular overlap integral exhibiting a large temperature and <i>x</i>-dependence, which seems to play a crucial role in determining the nature of the ground state.