Kink in Stoner Factor as a Signature of Changing Magnetic Fluctuations in Organic Conductor $λ$-(BETS)$_2$GaCl$_4$
Abstract
We have theoretically investigated the magnetic properties of the quasi-two-dimensional organic conductor $\lambda$-(BETS)$_2$GaCl$_4$ using a multi-band Hubbard model and the two-particle self-consistent method. We have employed a four-band model, where each BETS molecule is considered as a site, and a two-band model, treating each BETS dimer as a site. Our results for the temperature dependence of the Stoner factor reveal a kink around $T_\mathrm{kink} \approx 5 \mathrm{meV}$, indicating a change in the dominant magnetic fluctuations. Above $T_\mathrm{kink}$, it shows a broad structure indicating smeared antiferromagnetic (AFM) fluctuations, while below $T_\mathrm{kink}$, the spin susceptibility peaks at a wavevector corresponding to spin-density-wave (SDW)-like fluctuations. As the intra-dimer transfer integral increases, the kink disappears, and the AFM fluctuations are enhanced. Our findings are consistent with experimental observations, which also report a change in magnetic properties from AFM to SDW-like fluctuations upon cooling.