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Ambient pressure Dirac electron system in quasi-two-dimensional molecular conductor $α$-(BETS)$_2$I$_3$

We investigated the precise crystal structures and electronic states in a quasi-two-dimensional molecular conductor $α$-(BETS)$_2$I$_3$ at ambient pressure. The electronic resistivity of this molecular solid shows metal-to-insulator (MI) crossover at $T_{MI}$=50 K. Our x-ray diffraction and $^{13}$C nuclear magnetic resonance experiments revealed that $α$-(BETS)$_2$I$_3$ maintains the inversion symmetry below $T_{MI}$. First-principles calculations found a pair of anisotropic Dirac cones at a general k-point, with the degenerate contact points at the Fermi level. The origin of the insulating state in this system is a small energy gap of ~2 meV opened by the spin-orbit interaction. The Z$_2$ topological invariants indicate that this system is a weak topological insulator. Our results suggest that $α$-(BETS)$_2$I$_3$ is a promising material for studying the bulk Dirac electron system in two dimensions.