Identification of strongly interacting organic semimetals

TL;DR

This paper combines machine learning, density functional theory, and effective models to identify organic semimetals that can undergo excitonic instabilities, revealing potential new materials with interaction-driven phases.

Contribution

It introduces a novel approach integrating computational methods to identify organic semimetals prone to excitonic phase transitions, highlighting specific candidate materials.

Findings

Identified organic charge transfer salts as semimetals in ab initio calculations.

Predicted excitonic gaps of 60-100 meV in selected materials.

Results align with experimental observations of these materials.

Abstract

Dirac- and Weyl point- and line-node semimetals are characterized by a zero band gap with simultaneously vanishing density of states. Given a sufficient interaction strength, such materials can undergo an interaction instability, e.g., into an excitonic insulator phase. Due to generically flat bands, organic crystals represent a promising materials class in this regard. We combine machine learning, density functional theory, and effective models to identify specific example materials. Without taking into account the effect of many-body interactions, we found the organic charge transfer salts (EDT-TTF-I$_2$)$_2$(DDQ)$\cdot($CH$_3$CN) and TSeF-TCNQ and a bis-1,2,3-dithiazolyl radical conductor to exhibit a semimetallic phase in our ab initio calculations. Adding the effect of strong particle-hole interactions for (EDT-TTF-I$_2$)$_2$(DDQ)$\cdot($CH$_3$CN) and TSeF-TCNQ opens an excitonic gap in the order of 60 meV and 100 meV, which is in good agreement with previous experiments on these materials.