Emergence of Dirac Electron Pair in Charge Ordered State of Organic Conductor $\alpha$-(BEDT-TTF)$_2$I$_3$

TL;DR

This paper investigates how pressure induces a topological transition in the band structure of an organic conductor, leading to the emergence of Dirac electron pairs in its charge-ordered state.

Contribution

It reveals a pressure-driven topological transition from a conventional insulator to a phase with Dirac electrons, modeled by an effective low-energy Hamiltonian and characterized by Berry curvature analysis.

Findings

Transition from single-minimum to double-minimum dispersion

Emergence of Dirac electrons with finite mass

Topological transition confirmed by Berry curvature analysis

Abstract

We re-examine the band structure of the stripe charge ordered state of $\alpha$-(BEDT-TTF)$_2$I$_3$ under pressure by using an extended Hubbard model within the Hartree mean-field theory. By increasing pressure, we find a topological transition from a conventional insulator with a single-minimum in the dispersion relation at the M-point in the Brillouin zone, towards a new phase which exhibits a double-minimum. This transition is characterized by the appearance of a pair of Dirac electrons with a finite mass. Using the Luttinger-Kohn representation at the M-point, it is shown that such a variation of the band structure can be described by an effective $2 \times 2$ low energy Hamiltonian with a single driving parameter. The topological nature of this transition is confirmed by the calculation of the Berry curvature which vanishes in the conventional phase and has a double peak structure with opposite signs in the new phase. We compare the structure of this transition with a simpler situation which occurs in two-component systems, like boron-nitride.