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

TL;DR

This study reveals that the quasi-two-dimensional molecular conductor ${\alpha}$-(BETS)$_2$I$_3$ hosts ambient pressure Dirac electrons, exhibits a metal-insulator transition at 50 K, and is characterized as a weak topological insulator with potential for exploring 2D Dirac physics.

Contribution

The paper provides the first detailed structural and electronic characterization of ${\alpha}$-(BETS)$_2$I$_3$ as a weak topological insulator with Dirac cones at ambient pressure.

Findings

Presence of anisotropic Dirac cones at the Fermi level.

Metal-insulator crossover at 50 K.

Insulating gap of approximately 2 meV due to spin-orbit interaction.

Abstract

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 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 $T_{MI}$. 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 ${\alpha}$-(BETS)$_2$I$_3$ is a promising material for studying the bulk Dirac electron system in two dimensions.