First-Principles Study on Dirac Cones in a Single-Component Molecular Crystal Under High Pressure

Most single-component molecular crystals show insulating or semiconducting properties at ambient pressure. Recently, metal dithiolene complexes have attracted much attention ever since a metallic state was realized in Ni(tmdt)$_2$ at ambient pressure. Even if the system is insulating at ambient pressure, it possibly turns into a metallic or superconducting state by application of pressure. In this study, we have found anisotropic linear (tilted Dirac cone) dispersions near the Fermi level in a single-component molecular crystal, Pd(dddt)$_2$ at 8 GPa by first-principles density functional theory calculations. Recent electrical resistivity at 12.6 GPa shows temperature independent behavior as is observed in the massless Dirac fermion system, $\alpha$-(BEDT-TTF)$_2$I$_3$. Our analysis of the electronic structure indicates that the band structure at ambient pressure has quasi-one-dimensional character, which corresponds to the stacking of Pd(dddt)$_2$ molecules along the $b$-axis, and the dimensionality of the band structure near the Fermi level is changed under the pressure of 8~GPa, where intermolecular hybridization increases due to the reduced intermolecular distances. We also discuss anisotropy of the Dirac cones and their possible origin in the multi-orbital system.