Emergence of the Dirac Electron System in a Single-Component Molecular Conductor under High Pressure

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)2] (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)2] 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.