Ambient-Pressure Organic Dirac Electron State in $α$-(BETS)$_2$AuCl$_2$

Abstract

We report an ambient-pressure Dirac electron (DE) state in a new organic conductor, $α$-(BETS)$_2$AuCl$_2$ (BETS = bis(ethylenedithio)tetraselenafulvalene). This salt exhibits characteristic transport properties, including large positive in-plane and anomalous negative interlayer magnetoresistance. These signatures closely resemble the high-pressure DE states of $α$-(ET)$_2$I$_3$ (ET = bis(ethylenedithio)tetrathiafulvalene). First-principles calculations including spin-orbit coupling identify the electronic state as a quasi-three-dimensional massive Dirac semimetal with residual Fermi pockets. This discovery provides a valuable platform for exploring bulk Dirac fermions without the complexity of high-pressure measurements.

Figures (4)

• Figure 1: Crystal structures of (a) $\alpha$-(BETS)$_2$AuCl$_2$ and (b) $\alpha$-(BETS)$_2$I$_3$ viewed along the $a$ axis, showing alternating stacks of donor molecules and anions. Arrangement of donor molecules and anions in (c) $\alpha$-(BETS)$_2$AuCl$_2$ and (d) $\alpha$-(BETS)$_2$I$_3$, viewed along the molecular long axis. Black lines indicate the unit cell. The structure of $\alpha$-(BETS)$_2$I$_3$ was drawn using the crystallographic data deposited as CCDC 2008983 Kitou2021a.
• Figure 2: Temperature dependence of (a) in-plane resistivity $\rho_{//}$ and (b) interlayer resistivity $\rho_{\perp}$ at various magnetic fields applied perpendicular to the conducting plane.
• Figure 3: (a) Band dispersion of $\alpha$-(BETS)$_2$AuCl$_2$. The symbols represent the band structure calculated by the first-principles calculations, whereas the solid curves denote the Wannier-interpolated band structure derived from MLWFs. The Fermi energy is indicated at $E_{\rm F}=4.30$ eV. The band dispersion is plotted along the high-symmetry $k$-path defined in fractional crystal coordinates as $\Gamma(0,0,0)$, Z$(0,0,1/2)$, Y$(0,1/2,0)$, Y$^\prime(0,-1/2,0)$, X$(1/2,0,0)$, V$^\prime(1/2,-1/2,0)$, U$^\prime(-1/2,0,1/2)$, T$^\prime(0,-1/2,1/2)$, and R$^\prime(-1/2,-1/2,1/2)$. (b) Real-space representation of the MLWFs, visualized using the VESTAMommaVESTA2011. The black lines are guides for the eyes. (c) Schematic model of the conducting layer. Ellipses represent BETS molecules, and colored bonds indicate the dominant intermolecular transfer integrals ($t_{\rm a1}$--$t_{\rm a3}$ and $t_{\rm b1}$--$t_{\rm b4}$). (d) Transfer integrals of $\alpha$-(BETS)$_2$AuCl$_2$ obtained from the Wannier-based spinor tight-binding model including SOC. The arrows $\uparrow$ and $\downarrow$ denote the spin indices of the corresponding matrix elements.
• Figure 4: Band dispersion in the presence of SOC, projected onto the $k_a$ axis for fixed values of $k_c$. Each panel shows the energy dispersion along $k_a$ at a given $k_c$, where the energy is measured relative to the Fermi energy ($E_{\rm F}=0$).