Pressure-induced Dirac nodal-loop semimetal and topological phase transition in a single-component molecular crystal, [Pd(dddt)₂]

A single-component molecular crystal, [Pd(dddt)₂] is insulating at ambient pressure, but it becomes conducting under a pressure of 12.6 GPa [1]. By performing structural optimization with first-principles calculations, we have ascribed an unusual conducting state in [Pd(dddt)₂], to the formation of Dirac cones under the pressure of 8 GPa. Based on our tight-binding analysis for the optimized structure at 8 GPa, multi-orbital nature (HOMO-LUMO mixing) is essential for the formation of the Dirac cones. The contact points of Dirac cones form a single nodal-loop in Brillouin zone. By including spin-orbit coupling, we find that the electronic state changes from the nodal-loop semimetal to an insulator with a small band gap. To understand whether it is a topological or normal insulator, we calculate Z₂ topological invariants by two methods, Fu-Kane’s TRIM and Fukui-Hatsugai's methods. The calculated Z₂ indices for the band structure at 8 GPa suggest the system is 3D “strong topological insulator”. On the other hand, Z₂ indices calculated for the ambient-pressure band structure show that the system is a normal insulator. Therefore, our results strongly suggest that pressure can induce topological phase transition in this system. [1] R. Kato et al, J. Am. Chem. Soc. 139, 1770 (2017).