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

TL;DR

This study uses first-principles DFT calculations to analyze Dirac electrons and spin-orbit effects in a quasi-two-dimensional molecular conductor, deriving an effective Hamiltonian that captures the low-energy electronic spectrum.

Contribution

It presents a detailed tight-binding model and effective Hamiltonian for Dirac fermions in $ ext{α}$(BETS)$_2$I$_3$, incorporating spin-orbit coupling effects and complex transfer integrals.

Findings

Spin-orbit coupling induces a 2 meV indirect band gap.

The effective Hamiltonian accurately reproduces the DFT band structure.

Multiple transfer integrals reflect the delocalized Se p orbitals.

Abstract

We employed first-principles density-functional theory (DFT) calculations to characterize Dirac electrons in quasi-two-dimensional molecular conductor $\alpha$-(BETS)$_2$I$_3$ [= $\alpha$-(BEDT-TSeF)$_2$I$_3$] at a low temperature of 30K. 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.