Ambient-pressure Dirac electron system in the quasi-two-dimensional molecular conductor <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>α</mml:mi><mml:mtext>−</mml:mtext><mml:msub><mml:mrow><mml:mo>(</mml:mo><mml:mi>BETS</mml:mi><mml:mo>)</mml:mo></mml:mrow><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">I</mml:mi><mml:mn>3</mml:mn></mml:msub></mml:mrow></mml:math>
Shunsuke Kitou, Takao Tsumuraya, Hikaru Sawahata, Fumiyuki Ishii, K. Hiraki, Toshikazu Nakamura, Naoyuki Katayama, Hiroshi Sawa
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.