Novel Dirac Electron in Single-Component Molecular Conductor [Pd(dddt)$_2$]

Dirac electrons in a single-component molecular conductor [Pd(dddt)$_2$] under pressure have been examined using a tight-binding model which consists of HOMO (Highest Occupied Molecular Orbital) and LUMO (Lowest Unoccupied Molecular Orbital) functions in four molecules per unit cell. The Dirac cone between the conduction and valence bands comes from the property that HOMO has ungerade symmetry and LUMO has gerade symmetry. The Dirac point forms a loop in the three-dimensional Brillouin zone, which is symmetric with respect to the plane of $k_y$ = 0 where $k_y$ is the intralayer momentum along the molecular stacking direction, i.e., with the largest (HOMO-HOMO, LUMO-LUMO) transfer energy. The parity at TRIM (time reversal invariant momentum) is calculated using the inversion symmetry around the lattice point of the crystal. It is shown that such an exotic Dirac electron is understood from the parity of the wave function at the TRIM and also from an effective Hamiltonian.