Eur. Phys. J. B (2021) 94: 17
https://doi.org/10.1140/epjb/s10051-020-00038-y

Regular Article - Solid State and Materials

First-principles study of the effective Hamiltonian for Dirac fermions with spin-orbit coupling in two-dimensional molecular conductor $\alpha$-(BETS)${}_2{\text{I}}_3$

¹ Priority Organization for Innovation and Excellence, Kumamoto University, 860-8555, Kumamoto, Japan
² Department of Physics, Nagoya University, 464-8603, Nagoya, Japan

^a tsumu@kumamoto-u.ac.jp

Received: 18 June 2020
Accepted: 16 December 2020
Published online: 12 January 2021

Abstract

We employed first-principles density-functional theory (DFT) calculations to characterize Dirac electrons in quasi-two-dimensional molecular conductor $\alpha$-(BETS)${}_2{\text{I}}_3$ [= $\alpha$-(BEDT–TSeF)${}_2{\text{I}}_3$] at a low temperature of 30 K. We provide a tight-binding model with intermolecular transfer energies evaluated from maximally localized Wannier functions, where the number of relevant transfer integrals is relatively large due to the delocalized character of Se p orbitals. The spin–orbit coupling gives rise to an exotic insulating state with an indirect band gap of about 2 meV. We analyzed the energy spectrum with a Dirac cone close to the Fermi level to develop an effective Hamiltonian with site potentials, which reproduces the spectrum obtained by the DFT band structure.