Band Structure and Physical Properties of α-STF₂I₃: Dirac Electrons in Disordered Conduction Sheets

• Main Authors: Toshio Naito, Ryusei Doi
• Format: Article
• Language: English
• Published: MDPI AG 2020-04-01
• Series: Crystals
• Subjects: organic conductors; charge-transfer complexes; Dirac cones; zero-gap semiconductors; electrons with relativistic behavior; disorder
• Online Access: https://www.mdpi.com/2073-4352/10/4/270

The compound being investigated is an organic charge-transfer complex of the unsymmetrical donor STF with I₃ [STF = bis(ethylenedithio)diselenadithiafulvalene], which is isostructural with α-ET₂I₃ and α-BETS₂I₃ [ET = bis(ethylenedithio)tetrathiafulvalene, BETS = bis(ethylenedithio)tetraselenafulvalene]. According to recent studies, the calculated band structure should represent a zero-gap semiconductor at 1 bar that is similar to α-ET₂I₃ under high pressure (>15 kbar). Such materials have attracted extensive interest because the electrons at the Fermi level can be massless Dirac fermions (MDFs), with relativistic behaviors like those seen in graphene. In fact, α-STF₂I₃ exhibited nearly temperature-independent resistivity, <i>ρ</i>, (~100–300 K), a phenomenon that is widely observed in zero-gap semiconductors. The non-Arrhenius-type increase in <i>ρ</i> (<~100 K) was consistent with the characteristics of interacting MDFs. The paramagnetic susceptibility, <i>χ</i>, (2–300 K)—as well as the reflectivity, <i>R</i> and optical conductivity, <i>σ</i>, (25–300 K; 400–25,000 cm⁻¹)—were also almost temperature independent. Furthermore, <i>σ</i> was practically independent of wavenumber at ~6000–15,000 cm⁻¹. There was no structural transition based on X-ray studies (90–300 K). Considering all the electrical, magnetic, optical and structural properties of α-STF₂I₃ at 1 bar, it was concluded that the salt possesses a band structure characterized with Dirac cones, which was consistent with the calculation.