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Kink in Stoner Factor as a Signature of Changing Magnetic Fluctuations in Organic Conductor $λ$-(BETS)$_2$GaCl$_4$

Hirohito Aizawa

Abstract

We have theoretically investigated the magnetic properties of the quasi-two-dimensional organic conductor $λ$-(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.

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 -(BETS)GaCl 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 , indicating a change in the dominant magnetic fluctuations. Above , it shows a broad structure indicating smeared antiferromagnetic (AFM) fluctuations, while below , 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.

Paper Structure

This paper contains 1 section, 2 equations, 4 figures.

Table of Contents

  1. Acknowledgments

Figures (4)

  • Figure 1: Crystal and electronic structure of $\lambda$-(BETS)$_2$GaCl$_4$. (a) Four-band model, where each BETS molecule is treated as a site, and (b) two-band model, where a BETS dimer is treated as a site. (c) Band structure, where the red dotted curves represent the DFT band structure, and the blue solid (green dashed) curves correspond to the four-band (two-band) model Aizawa2018. (d) Fermi surface obtained from the four-band model.
  • Figure 2: Temperature dependence of the Stoner factor $U_\mathrm{sp}\chi_0\left( Q_\mathrm{max} \right)$ for various transfer integrals in the four-band and two-band models, with symbols and curves as indicated in the legend.
  • Figure 3: The spin susceptibility $\chi_\mathrm{sp}\left( \bm{q} \right)$ of the four-band model with $t_\mathrm{A}=t^\mathrm{DFT}_\mathrm{A}$ at (a) a temperature below $T_\mathrm{kink}$ ($T=2~\mathrm{meV}$) and (b) a temperature above $T_\mathrm{kink}$ ($T=10~\mathrm{meV}$), where $T_\mathrm{kink} \approx 5~\mathrm{meV}$.
  • Figure 4: The spin susceptibility $\chi_\mathrm{sp}\left( \bm{q} \right)$ at $T=6~\mathrm{meV}$ for (a) the four-band model with $t_\mathrm{A}=4 t_\mathrm{A}^\mathrm{DFT}$, and (b) the two-band model.