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 flatbands, 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 [bis(3,4-diiodo-3′,4′-ethyleneditio- tetrathiafulvalene), 2,3-dichloro-5,6-dicyanobenzoquinone, acetenitrile] $\left[{\left(\mathrm{EDT}\text{−}\mathrm{TTF}\text{−}{\mathrm{I}}_2\right)}_2\right]\left(\mathrm{DDQ}\right)·\left({\mathrm{CH}}_3\mathrm{CN}\right)$ and $2,2^{\prime },5,5^{\prime }\text{−}\mathrm{tetraselenafulvalene}–7,7,8,8\text{−}\mathrm{tetracyano}\text{−}p\text{−}\mathrm{quinodimethane}$ (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 ${\left(\mathrm{EDT}\text{−}\mathrm{TTF}\text{−}{\mathrm{I}}_2\right)}_2\left(\mathrm{DDQ}\right)·\left({\mathrm{CH}}_3\mathrm{CN}\right)$ and TSeF-TCNQ opens an excitonic gap on the order of 60 and 100 meV, which is in good agreement with previous experiments on these materials.

• Received 8 June 2020
• Accepted 10 November 2020

DOI:https://doi.org/10.1103/PhysRevB.102.205134

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. Funded by Bibsam.

Published by the American Physical Society