Theoretical Model for a Novel Electronic State in a Dirac Electron System Close to Merging: An Imaginary Element between Sulphur and Selenium

Abstract

Topological materials with Dirac electron systems have been extensively studied. Organic crystalline materials form a unique group of such compounds with well-defined crystal structures. While most organic compounds require high pressures to exhibit Dirac-cone-type band structures, the title compound, α-STF2I3, has garnered increasing interest due to its Dirac-cone-type band structure under ambient pressure. Various experiments have been conducted under ambient pressure; their results can be compared with those of theoretical calculations to obtain insights into Dirac electron systems. However, structural disorder peculiar to the STF molecules in the solid-state has prevented any type of theoretical calculation of the states. In this study, we report a new method for calculating intermolecular interactions in disordered systems based on the extended Hückel approximation. This method enables band calculations, suggesting that this material is a rare example of a system close to merging. The obtained band structure indicates that the characteristic disorder in the STF solids distributed electrons equally on the sulphur and selenium atoms as if they belong to an imaginary element between sulphur and selenium and are arranged without disorder.

Conductivity of Two-dimensional Dirac Electrons Close to Merging in Organic Conductor $α$-STF$_2$I$_3$ at Ambient Pressure

“…10,[27][28][29] In this paper, we examine the conductivity of α-STF 2 I 3 using a TB model with transfer energies, which are improved from a previous work. 15) The model 30) provides an electronic state with Dirac points close to merging, which has been theoretically shown in terms of the effective Hamiltonian. [31][32][33] This paper is organized as follows.…”

“…Figure 2(a) shows the conduction and valence bands, E 1 (k) and E 2 (k), respectively, measured from the chemical potential (µ) on the plane of the 1st Brillouin zone. Note that the k x axis of the present choice 30) corresponds to −k x in Ref. 15 due to a different choice of a unit cell.…”

“…The present model was obtained by averaging the transfer energies of two extreme cases, which were deduced from the following consideration. 30) Note that single-crystal X-ray structural analyses revealed that all the inner four chalcogen atoms in Fig. 1(a) possess equal electron densities; 28,35) thus, all the electron densities are obtained by averaging all the inner chalcogen atoms, suggesting a delocalized wave function.…”

“…2, we briefly mention the difference in the estimation of transfer energies between our previous model 15) and the present one. 30) Our Hamiltonian consists of a TB model, impurity scattering, and e-p interaction with a reasonable coupling constant (λ) taken for an organic conductor. In Sect.…”

“…Expressing the wave function in terms of atomic orbitals and discarding the off-diagonal elements between S and Se, we obtain the following transfer energies. By rewriting the linear combination in terms of orbitals consisting of only S or Se, they are given by 30) g 1 = 0.0535, g 2 = 0.132, g 3 = 0.0475, h 1 =-0.0295, h 2 = 0.295, h 3 =0.1415, and h 4 =0.009.…”