On the spin gap phase in λ-(BETS)2GaXzY4−z

The nature of the insulating state in the quasi-two-dimensional organic conductor λ-(BETS)2GaXzY4 − z, where the existence of a spin gap is suggested by susceptibility measurement, has been studied theoretically. The Hartree-Fock calculations at absolute zero temperature show that, ifthe on-site Coulomb interaction exceeds some critical value, antiferromagnetic spin ordering emerges and eventually leads to an insulating state which can be considered as a two-dimensional localized spin system. Based on the quantumMonte Carlo simulations by Katoh and Imada, we will show that this system is located near the boundary between the antiferromagnetic phase and the spin gap phase.