Ambient-pressure Dirac electron system in the quasi-two-dimensional molecular conductor \nα−(BETS)2I3

Abstract

We investigated the precise crystal structures and electronic states of a quasi-two-dimensional molecular conductor $\\ensuremath{\\alpha}\\text{\\ensuremath{-}}{(\\mathrm{BETS})}_{2}{\\mathrm{I}}_{3}$ at ambient pressure. The electronic resistivity of this molecular solid shows metal-to-insulator (MI) crossover behavior at ${T}_{\\mathrm{MI}}=50\\phantom{\\rule{4pt}{0ex}}\\mathrm{K}$. Our x-ray diffraction and $^{13}\\mathrm{C}$ nuclear magnetic resonance experiments revealed that $\\ensuremath{\\alpha}\\text{\\ensuremath{-}}{(\\mathrm{BETS})}_{2}{\\mathrm{I}}_{3}$ maintains the inversion symmetry below ${T}_{\\mathrm{MI}}$. 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 $\\ensuremath{\\sim}2\\phantom{\\rule{0.16em}{0ex}}\\mathrm{meV}$ opened by the spin-orbit interaction. The ${Z}_{2}$ topological invariants indicate that this system is a weak topological insulator. Our results suggest that $\\ensuremath{\\alpha}\\text{\\ensuremath{-}}{(\\mathrm{BETS})}_{2}{\\mathrm{I}}_{3}$ is a promising material for studying the bulk Dirac electron system in two dimensions.