Ambient-Pressure Organic Dirac Electron State in $\alpha$-(BETS)${}_2$AuCl${}_2$

We report an ambient-pressure Dirac electron (DE) state in a new organic conductor, $\alpha$-(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 $\alpha$-(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.