Importance of Spin-Orbit Coupling in Organic BEDT-TTF and BEDT-TSF Salts

Abstract

We investigate the spin-orbit coupling (SOC) effects in $\alpha$- and $\kappa$-phase BEDT-TTF and BEDT-TSF organic salts. Contrary to the assumption that SOC in organics is negligible due to light C, S, H atoms, we show the relevance of such an interaction in a few representative cases. In the weakly correlated regime, SOC manifests primarily in the opening of energy gaps at degenerate band touching points. This effect becomes especially important for Dirac semimetals such as $\alpha$-(ET)$_2$I$_3$. Furthermore, in the magnetic insulating phase, SOC results in additional anisotropic exchange interactions, which provide a compelling explanation for the controversial field-induced behaviour of the quantum spin-liquid candidate $\kappa$-(ET)$_2$Cu$_2$(CN)$_3$. We conclude by discussing the importance of SOC for the description of low-energy properties in organics.