Novel Dirac Electron in Single-Component Molecular Conductor [Pd(dddt)₂] (dddt = 5,6-dihydro-1,4-dithiin-2,3-dithiolate)

Abstract

Dirac electrons in a single-component molecular conductor [Pd(dddt) 2 ] (dddt=5,6dihydro-1,4-dithiin-2,3-dithiolate) under pressure have been examined using a tight-binding model which consists of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) functions in four molecules per unit cell. The Dirac cone between the conduction and valence bands originates from the property that the HOMO has ungerade symmetry and the 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 time reversal invariant momentum (TRIM) is calculated using the inversion symmetry around the lattice point of the crystal. It is shown that such an exotic Dirac electron can be understood from the parity of the wave function at the TRIM and also from an effective Hamiltonian. Recently, a Dirac electron was found in the single-component molecular conductor [Pd(dddt) 2 ] (dddt=5,6-dihydro-1,4-dithiin-2,3-dithiolate), which shows a constant resistivity with decreasing temperature under pressure. 4, 5) Based on first-principles calculation, which shows the existence of a Dirac cone, 6) a tight-binding model of [Pd(dddt) 2 ] consisting of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)functions in four molecules per unit cell was proposed. 7) In the crystal, there are two crystallographically independent layers given by layers 1 and 2 (Fig. 1), and the Dirac cone originates from the HOMO-based band in layer 1 and the LUMO-based band in layer 2. 4) The interplay *

Electric Transport of Nodal Line Semimetals in Single-Component Molecular Conductors

“…A TB model corresponding to Figure 1 has been recently obtained using the crystal structure observed under pressure [36]. There are several kinds of transfer energies between two molecular orbitals, which are listed in Table 1.…”

“…i and j are the lattice sites of the unit cell with N being the total number of the unit cells, α and β denote the 8 molecular orbitals given by HOMO (H1, H2, H3, H4) and LUMO (L1, L2, L3, L4). These energies in the unit of eV are listed in Table 1 where the gap between the energy of HOMO and that of LUMO is taken as ∆E = 0.696 eV to reproduce the energy band of the first principle calculation [28]. Using a Fourier transform…”

“…The matrix HamiltonianĤ(k) is given in Appendix A. The nodal line has been found using H(k) [28] where the Dirac point is supported by the existence of the inversion center [2]. The energy band E j (k) and the wave function…”

“…[Pd(dddt) 2 ] under high pressure exhibits almost temperature independent resistivity [26]. First-principles calculations indicate that this material belongs to the three-dimensional Dirac electron system [27] consisting of HOMO (Highest Occupied Molecular Orbital) and LUMO (Lowest Unoccupied Molecular Orbital) bands, and that a TB model exhibits a loop of Dirac points called a nodal line semimetal [28]. There are several studies on the effective Hamiltonian, where a general two-band model is introduced [29] and the explicit calculation is performed for the nodal line semimetal [30].…”

“…In this case, the TB model suggests the almost constant resistivity in [Pd(dddt) 2 ] due to the decrease of the DOS, but not by the property relevant to the Dirac cone. Recently, a TB model was re-examined using the crystal structure, which was obtained under high pressure [36]. Surprisingly, this band calculation shows the DOS, which depends linearly on a wide region of the energy being compatible with temperature of almost constant conductivity.…”