High pressure investigation of an organic three-dimensional Dirac semimetal candidate having a diamond lattice (original) (raw)
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Molecular diamond lattice antiferromagnet as a Dirac semimetal candidate
Physical Review B, 2019
The ground state of a molecular diamond-lattice compound (ET)Ag 4 (CN) 5 is investigated by the magnetization and nuclear magnetic resonance spectroscopy. We found that the system exhibits antiferromagnetic long-range ordering with weak ferromagnetism at a high temperature of 102 K owing to the strong electron correlation. The spin susceptibility is well fitted into the diamond-lattice Heisenberg model with a nearest neighbor exchange coupling of 230 K, indicating the less frustrated interactions. The transition temperature elevates up to ∼195 K by applying pressure of 2 GPa, which records the highest temperature among organic molecular magnets. The first-principles band calculation suggests that the system is accessible to a three-dimensional topological semimetal with nodal Dirac lines, which has been extensively searched for a half-filling diamond lattice.
$\beta$-As$_2$Te$_3$: Pressure-Induced 3D Dirac Semi-Metal
arXiv: Materials Science, 2020
We report a theoretical \textit{ab-initio} study of beta\betabeta-As$_2$Te$_3$ ($R\bar{3}m$ symmetry) at hydrostatic pressures up to 12 GPa. We have systematically characterized the vibrational and electronic changes of the system induced by the pressure variation. The electronic band dispersions calculated at different pressures using \textit{QS}GW show an insulator-metal transition. At room pressure the system is a semiconductor with small band-gap, and the valence and conduction bands present a parabolic conventional dispersion. However around 2 GPa the parabolic shape of the bands become linear and touch at the Fermi level. This means that this compound undergoes a pressure-induced topological phase transition to a 3D analog of graphene, known as a 3D Dirac semi-metal, with gapless electronic excitations. At increasing pressures the gap reopens and variation of the character of the electronic band-gap from direct to indirect is evidenced. At 7 GPa we observe the formation of a negativ...
Physical review B, 2021
An ab-initio study of beta-As2Te3 (R3m symmetry) at hydrostatic pressures shows that this compound is a trivial small band-gap semiconductor at room pressure that undergoes a quantum topological phase transition to a 3D topological Dirac semi-metal around 2 GPa. At higher pressures, the band-gap reopens and again decreases above 4 GPa. Our calculations predict an insulator-metal transition above 6 GPa due to the closing of the band-gap, with strong topological features persisting between 2 and 10 GPa with Z4=3 topological index. By investigating the lattice thermal-conductivity (κL), we observe that close to room conditions κL is very low, either for the in-plane and the out-of-plane axis, with 0.098 and 0.023 Wm −1 K −1 , respectively. This effect occurs due to the presence of two low-frequency optical modes, namely Eu and Eg, which increase the phononphonon scattering rate. Therefore, our work suggests that ultra-low lattice thermal-conductivities, which enable highly efficient thermoelectric materials, can be engineered in systems that are close to a structural instability derived from phonon Kohn anomalies. At higher pressures, the values of the in-and out-of-plane thermal-conductivities not only increase in magnitude, but also approximate in value as the layered character of the compound decreases.
Scientific Reports, 2015
The three dimensional (3D) Dirac semimetal is a new quantum state of matter that has attracted much attention recently in physics and material science. Here, we report on the growth of large plate-like single crystals of Cd 3 As 2 in two major orientations by a self-selecting vapor growth (SSVG) method, and the optimum growth conditions have been experimentally determined. The crystalline imperfections and electrical properties of the crystals were examined with transmission electron microscopy (TEM), scanning tunneling microscopy (STM), and transport property measurements. This SSVG method makes it possible to control the as-grown crystal compositions with excess Cd or As leading to mobilities near 5-10 5 cm 2 V −1 s −1. Zn-doping can effectively reduce the carrier density to reach the maximum residual resistivity ratio (RRR≡ρ 300K /ρ 5K) of 7.6. A vacuum-cleaved single crystal has been investigated using angle-resolved photoemission spectroscopy (ARPES) to reveal a single Dirac cone near the center of the surface Brillouin zone with a binding energy of approximately 200 meV. Cadmium arsenide (Cd 3 As 2) is a degenerate n-type semiconductor of the II-V family with high mobility, low effective mass, and a highly non-parabolic conduction band 1. It exhibits an inverted band structure (optical energy gap E g < 0) comparable to the strained topological insulator HgTe; however, the conduction and valence bands touch at the Dirac nodes in the bulk band structure, giving rise to bulk Dirac fermions featuring robust topologically protected linear dispersion in all three dimensions. These
Ambient-pressure Dirac electron system in the quasi-two-dimensional molecular conductor α−(BETS)2I3
Physical Review B
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 nuclear 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.
Three-dimensional Dirac semimetal and quantum transport in Cd_{3}As_{2}
Physical Review B, 2013
Based on the first-principles calculations, we recover the silent topological nature of Cd3As2, a well known semiconductor with high carrier mobility. We find that it is a symmetry-protected topological semimetal with a single pair of three-dimensional (3D) Dirac points in the bulk and non-trivial Fermi arcs on the surfaces. It can be driven into a topological insulator and a Weyl semi-metal state by symmetry breaking, or into a quantum spin Hall insulator with gap more than 100meV by reducing dimensionality. We propose that the 3D Dirac cones in the bulk of Cd3As2 can support sizable linear quantum magnetoresistance even up to room temperature.
Coexistence of Dirac and massive carriers in α-(BEDT-TTF)_{2}I_{3} under hydrostatic pressure
Physical Review B, 2013
Transport measurements were performed on the organic layered compound α − (BEDT − TTF)2I3 under hydrostatic pressure. The carrier types, densities and mobilities are determined from the magneto-conductance of α − (BEDT − TTF)2I3 . While evidence of high-mobility massless Dirac carriers has already been given, we report here, their coexistence with low-mobility massive holes. This coexistence seems robust as it has been found up to the highest studied pressure. Our results are in agreement with recent DFT calculations of the band structure of this system under hydrostatic pressure. A comparison with graphene Dirac carriers has also been done.
2020
Department of Applied Physics, Nagoya University, Nagoya 464-8603, Japan Priority Organization for Innovation and Excellence, Kumamoto University, 2-39-1 Kurokami, Kumamoto 860-8555, Japan Graduate School of Natural Science and Technology, Kanazawa University, Kanazawa 920-1192, Japan Nanomaterials Reserach Institute, Kanazawa University, 920-1192 Kanazawa, Japan Department of Natural Sciences, Fukushima Medical University, Fukushima 960-1295, Japan Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan