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.
Highlights
Semimetals have attracted huge attention due to their striking transport properties, analogies to high-energy physics phenomena, and potential for functionalization [1–3]. Their realization relies on a delicate combination of symmetry, electron filling, and band ordering enforcing the existence of the nodes in the band structure at the chemical potential while having a vanishing density of states (DOS) at the crossing point [4–6]
With the goal of identifying experimentally feasible materials to investigate interaction effects in nodal semimetals we focus on organic crystals
To explain the discrepancy between density functional theory (DFT) semimetallic phases and experimentally observed semiconductivity in (EDT-TTF-I2)2(DDQ) · (CH3CN) and (TSeF-TCNQ) we argue that both systems are likely to undergo an excitonic instability
Summary
Semimetals have attracted huge attention due to their striking transport properties, analogies to high-energy physics phenomena, and potential for functionalization [1–3] Their realization relies on a delicate combination of symmetry, electron filling, and band ordering enforcing the existence of the nodes in the band structure at the chemical potential while having a vanishing density of states (DOS) at the crossing point [4–6]. It has been shown extensively for the case of Dirac semimetals that under a sufficiently high interaction strength a dynamical mass term can be generated leading to a quantum phase transition into a gapped phase [7–10].
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