1 Ambient pressure Dirac electron system in quasi-two-dimensional molecular conductor α-(BETS)2I3 Shunsuke Kitou 1,*,† , Takao Tsumuraya 2,‡,† , Hikaru Sawahata 3 , Fumiyuki Ishii 4 , Ko-ichi Hiraki 5 , Toshikazu Nakamura 6 , Naoyuki Katayama 1 , and Hiroshi Sawa 1,# 1 Department of Applied Physics, Nagoya University, Nagoya 464-8603, Japan 2 Priority Organization for Innovation and Excellence, Kumamoto University, 2-39-1 Kurokami, Kumamoto 860-8555, Japan 3 Graduate School of Natural Science and Technology, Kanazawa University, Kanazawa 920-1192, Japan 4 Nanomaterials Reserach Institute, Kanazawa University, 920-1192 Kanazawa, Japan 5 Department of Natural Sciences, Fukushima Medical University, Fukushima 960-1295, Japan 6 Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan (Dated: June 16, 2020) 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  MI = 50 K. Our x-ray diffraction and 13 C nu- clear magnetic resonance experiments revealed that α-(BETS)2I3 maintains the inversion symmetry below  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. Introduction. A massless Dirac electron system, in which two linear band dispersions intersect at the Fermi level (EF), is one of the central themes of modern physics [1-6]. When a system has such emergent band structure, the electron behavior, such as the electronic transport, follows a relativistic Dirac equation, and the charge carri- ers move, as if they had no mass, at the speed of light in a material. However, there are very few material systems, in which the Dirac point is located at the EF, and the band gap is zero, that can be named massless Dirac electron system. Such an electronic state is realized in two-dimensional (2D) layer of graphene [1], bismuth [7-9], and the surface of topological insulators [10,11]. In these systems, unique physical properties such as quantum Hall effect [1], quantum spin Hall effect [2], and unscreened long-range Coulomb interaction [6] attributed to the Dirac cone band structure have been proposed. In addition, po- tential applications to a high mobility electronic device have been reported [12,13]. Recently, the existence of massless Dirac electron sys- tems has been suggested in some organic molecular solids [14-30], in which Dirac cones are formed by the bands of the same character of wavefunctions as frontier orbitals of consistent molecules at different sites. The massless Dirac electron system in bulk was first realized in a quasi-2D molecular conductor, α-(ET)2I3 [ET = BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene] [Fig. 2(a)] [16]. Un- like graphene [1], α-(ET)2I3 has a pair of anisotropic Dirac cones [16]. However, the massless Dirac state in α-(ET)2I3 is realized only under high-pressure (  >1 .2 GPa) [30]. At ambient pressure and  MI = 135 K, α-(ET)2I3 shows a metal–insulator (MI) transition, which causes a charge ordering (CO) associated with the lack of inversion center, and the system turns to a nonmagnetic ferroelectric phase [31-39]. Further, the CO transition can be suppressed by applying pressure, and an anomalous electronic conduct- ing phase including the massless Dirac electron system can be realized under high-pressure [15-17]. Although the quantum Hall effect [25], discrete Landau levels [26], and unscreened long-range Coulomb interactions [28,29] are observed under high-pressure in α-(ET)2I3, experimental determination of the detailed crystal structure and physical property measurements in the Dirac state are still limited. To address the above-mentioned limitations, we searched for a bulk Dirac electron system at ambient pressure and found a promising candidate in the seleni- um-substituted analog of α-(ET)2I3, α-(BETS)2I3 [BETS = BETS-TSF = bis(ethylenedithio)tetraselenafulvalene] [Fig. 2(b)], where the central four S atoms in the ET molecule were replaced with Se atoms. α-(BETS)2I3 shows a resis- tivity behavior similar to that of α-(ET)2I3, and the MI crossover temperature of α-(BETS)2I3 (  MI = 50 K) [40] is less than the CO transition temperature of α-(ET)2I3 [31]. As the temperature decreases from room temperature to low-temperature (LT), the magnetic susceptibility of α-(BETS)2I3 gradually decreases, and no anomaly is ob- served at  MI [41]. These electronic properties are dif- ferent from α-(ET)2I3, and the origin of the insulating state