Chemical-Strain Induced Tilted Dirac Nodes in (BEDT-TTF)$_2$X$_3$ (X = I, Cl, Br, F) Based Charge-Transfer Salts

TL;DR

This study explores how chemical strain from halogen substitution in (BEDT-TTF)$_2$X$_3$ salts induces tilted Dirac nodes, revealing new electronic properties in these quasi-2D materials.

Contribution

It demonstrates that chemical strain from halogen substitution can induce tilted Dirac nodes in (BEDT-TTF)$_2$X$_3$ salts, a novel finding in this class of materials.

Findings

Over-tilted Dirac-type-II nodes found in $ ext{(BEDT-TTF)}_2$F$_3$

No topological transition in $ ext{(BEDT-TTF)}_2$F$_3$ for $ ext{kappa}$-phase

Chemical strain significantly alters electronic structure

Abstract

The identification of novel multifunctional Dirac materials has been an ongoing effort. In this connection quasi 2-dimensional (BEDT-TTF)-based charge transfer salts are widely discussed. Here, we report about the electronic structure of $\alpha$-(BEDT-TTF)$_2$I$_3$ and $\kappa$-(BEDT-TTF)$_2$I$_3$ under a hypothetical substitution of iodine with the halogens bromine, chlorine and fluorine. The decreasing size of the anion layer corresponds to applying chemical strain which increases tremendously in the case of (BEDT-TTF)$_2$F$_3$. We performed structural optimization and electronic structure calculations in the framework of density functional theory, incorporating, first, the recently developed strongly constrained and appropriately normed semilocal density functional SCAN, and, second, van der Waals corrections to the PBE exchange correlation functional by means of the dDsC dispersion correction method. In the case of $\alpha$-(BEDT-TTF)$_2$F$_3$ the formation of over-tilted Dirac-type-II nodes within the quasi 2-dimensional Brillouin zone can be found. For $\kappa$-(BEDT-TTF)$_2$F$_3$, the recently reported topological transition within the electronic band structure cannot be revealed.