Ambient-pressure Dirac electron system in the quasi-two-dimensional molecular conductor α−(BETS)2I3

Abstract

We investigated the precise crystal structures and electronic states in a quasi-two-dimensional molecular conductor α-(BETS)2I3 at ambient pressure. The electronic resistivity of this molecular solid shows a metal-to-insulator (MI) crossover at 𝑇 MI = 50 K. Our x-ray diffraction and 13 C nuclear magnetic resonance experiments revealed that α-(BETS)2I3 maintains the inversion symmetry below 𝑇 MI . The first-principles calculations found a pair of anisotropic Dirac cones at a general k-point, where the degenerated contact points are located at the Fermi level. Furthermore, the origin of the insulating state in this system is explained by a small energy gap of ~2 meV opened by a spin-orbit interaction, in which the Z2 topological invariants indicate a weak topological insulator. Our results suggest that α-(BETS)2I3 is a promising material for studying the bulk Dirac electron system in two-dimension.

Electric Double Layer Doping of Charge-Ordered Insulators α-(BEDT-TTF)2I3 and α-(BETS)2I3

“…The Arrhenius plots of ∆σ (Figure 6b) show that unlike in α-(BEDT-TTF) 2 I 3 , the activation energy near the transition temperature significantly decreases with increasing positive V G . The mechanism is unclear because the insulating state of α-(BETS) 2 I 3 is not a simple charge-ordered state, and the insulating mechanism is under debate [14][15][16]. However, the present results may provide clues to understanding the insulating phase of this compound.…”

“…Therefore, it is considered that the electronic state of α-(BETS) 2 I 3 is similar to that of moderately pressurized α-(BEDT-TTF) 2 I 3 . However, recent X-ray diffraction and 13 C nuclear magnetic resonance experiments revealed that α-(BETS) 2 I 3 maintains inversion symmetry below the transition temperature [14]. These results imply a different insulating mechanism from simple charge ordering (the spin-orbit interaction may play an important role).…”

“…These results imply a different insulating mechanism from simple charge ordering (the spin-orbit interaction may play an important role). The mechanism is still under debate [14][15][16].…”

“…Under electron doping, the gate-induced conductivity exceeds the Mott-Ioffe-Regel conductivity limit in two dimensions ( around which the metallic conduction appears in Si-MOSFET [20] and de κ-type BEDT-TTF salts [21,22]. However, in α-(BETS)2I3 EDLT does not conduction below the transition temperature of ~50 K, as shown in Figure nius plots of (Figure 6b) show that unlike in α-(BEDT-TTF)2I3, the act near the transition temperature significantly decreases with increasing po mechanism is unclear because the insulating state of α-(BETS)2I3 is not a s ordered state, and the insulating mechanism is under debate [14][15][16]. How sent results may provide clues to understanding the insulating phase of th…”

Unusual stoichiometry, band structure and band filling in conducting enantiopure radical cation salts of TM-BEDT-TTF showing helical packing of the donors

“…Consequently, if the coupling through donors B could be reinforced, for instance through applied pressure, and the metallic state could be stabilized, the Fermi level would occur at the K point and thus would coincide with a Dirac cone which could lead to the exotic transport properties predicted for this situation. 35 Among molecular conductors, only (EDT-TTF-CONH 2 ) 6 [Re 6 Se 8 (CN) 6 ], 36,37 a-(BEDT-TTF) 2 I 3 under pressure, 38 [Pd(dddt) 2 ] (dddt = 5,6-dihydro-1,4-dithiin-2,3-dithiolate) under pressure, 39 [Pt(dmdt) 2 ] (dmdt = dimethyl-tetrathiafulvalenedithiolate) 40 and, very recently, a-(BETS) 2 I 3 (BETS = bis (ethylenedithio)-tetraselenafulvalene) 41 have been previously shown to exhibit such an uncommon electronic feature. Unfortunately, our measurements suggest that in the present salts the electrons remain localized at the pressures we could reach.…”