Single-component molecular conductors can provide a variety of electronic states. We demonstrate here that the Dirac electron system emerges in a single-component molecular conductor under high pressure. First-principles density functional theory calculations revealed that Dirac cones are formed in the single-component molecular conductor [Pd(dddt)] (dddt = 5,6-dihydro-1,4-dithiin-2,3-dithiolate), which shows temperature-independent resistivity (zero-gap behavior) at 12.6 GPa. The Dirac cone formation in [Pd(dddt)] can be understood by a tight-binding model. The Dirac points originate from the HOMO and LUMO bands, each of which is associated with different molecular layers. Overlap of these two bands provides a closed intersection at the Fermi level (Fermi line) if there is no HOMO-LUMO coupling. Two-step HOMO-LUMO couplings remove the degeneracy on the Fermi line, resulting in gap formation. The Dirac cones emerge at the points where the Fermi line intersects with a line on which the HOMO-LUMO coupling is zero.

The electronic structure of an isostructural series of molecular conductors, β ′ -X[Pd(dmit) 2 ] 2 , is systematically studied by a first-principles method based on the density-functional theory. The calculated band structures are fitted to the tightbinding model based on Pd(dmit) 2 dimers on the triangular lattice. We find a systematic variation in the anisotropy of the transfer integrals along the three directions of the triangular lattice taking different values. The transfer integral along the face-to-face stacking direction of Pd(dmit) 2 dimers is always the largest. Around the quantum spin liquid, X = EtMe 3 Sb, the other two transfer integrals become comparable. We also report sensible differences in the distribution of wavefunctions near the Fermi level between the two dmit ligands of the Pd(dmit) 2 molecule.

We investigated the precise crystal structures and electronic states in a quasi-two-dimensional molecular conductor α-(BETS)2I3 at ambient pressure. The electronic resistivity of this molecular solid shows a metal-to-insulator (MI) crossover at 𝑇 MI = 50 K. Our x-ray diffraction and 13 C nuclear magnetic resonance experiments revealed that α-(BETS)2I3 maintains the inversion symmetry below 𝑇 MI . The first-principles calculations found a pair of anisotropic Dirac cones at a general k-point, where the degenerated contact points are located at the Fermi level. Furthermore, the origin of the insulating state in this system is explained by a small energy gap of ~2 meV opened by a spin-orbit interaction, in which the Z2 topological invariants indicate a weak topological insulator. Our results suggest that α-(BETS)2I3 is a promising material for studying the bulk Dirac electron system in two-dimension.

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