Ambient pressure Dirac electron system in quasi-two-dimensional molecular conductor ${\alpha}$-(BETS)$_2$I$_3$.

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