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Register a new userFirst-principles study of the effective Hamiltonian for Dirac fermions with spin-orbit coupling in two-dimensional molecular conductor $alpha$-(BETS)$_2$I$_3$
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We employed first-principles density-functional theory (DFT) calculations to characterize Dirac electrons in quasi-two-dimensional molecular conductor $alpha$-(BETS)$_2$I$_3$ [= $alpha$-(BEDT-TSeF)$_2$I$_3$] at a low temperature of 30K. We provide a tight-binding model with intermolecular transfer energies evaluated from maximally localized Wannier functions, where the number of relevant transfer integrals is relatively large due to the delocalized character of Se $p$ orbitals. The spin-orbit coupling gives rise to an exotic insulating state with an indirect band gap of about 2 meV. We analyzed the energy spectrum with a Dirac cone close to the Fermi level to develop an effective Hamiltonian with site-potentials, which reproduces the spectrum obtained by the DFT band structure.
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We investigated the precise crystal structures and electronic states in a quasi-two-dimensional molecular conductor ${alpha}$-(BETS)$_2$I$_3$ at ambient pressure. The electronic resistivity of this molecular solid shows metal-to-insulator (MI) crossover at $T_{MI}$=50 K. Our x-ray diffraction and $^{13}$C nuclear magnetic resonance experiments revealed that ${alpha}$-(BETS)$_2$I$_3$ maintains the inversion symmetry below $T_{MI}$. First-principles calculations found a pair of anisotropic Dirac cones at a general k-point, with the degenerate contact points at the Fermi level. The origin of the insulating state in this system is a small energy gap of ~2 meV opened by the spin-orbit interaction. The Z$_2$ topological invariants indicate that this system is a weak topological insulator. Our results suggest that ${alpha}$-(BETS)$_2$I$_3$ is a promising material for studying the bulk Dirac electron system in two dimensions.
Effect of spin-orbit coupling (SOC) on Dirac electrons in the organic conductor $alpha$-(BETS)$_2$I$_3$ [BETS = bis(ethylenedithio)tetraselenafulvalene] has been examined by calculating electric conductivity and spin magnetic susceptibility. A tight-binding (TB) model with transfer energies consisting of real and imaginary parts is evaluated using first-principles density-functional theory calculation. The conductivity without SOC depends on both anisotropies of the velocity of the Dirac cone and the tiling of the cone. Such conductivity is suppressed by the SOC, which gives rise to the imaginary part of the transfer energy. It is shown at low temperatures that the conductivity decreases due to the SOC and the Dirac cone with linear dispersion. A nearly constant conductivity at high temperatures is obtained by an electron-phonon (e--p) scattering. Further, the property of the Dirac cone is examined for spin susceptibility, which is mainly determined by the density of states (DOS). The result is compared with the case of the organic conductor $alpha$-(BEDT-TTF)$_2$I$_3$ [BEDT-TTF=bis(ethylenedithio)tetrathiafulvalene], which provides the Dirac cone without SOC. The relevance to experiments is discussed.
Two-dimensional Dirac fermions are subjected to two types of interactions, namely the long-range Coulomb interaction and the short-range on-site interaction. The former induces excitonic pairing if its strength $alpha$ is larger than some critical value $alpha_c$, whereas the latter drives an antiferromagnetic Mott transition when its strength $U$ exceeds a threshold $U_c$. Here, we study the impacts of the interplay of these two interactions on excitonic pairing with the Dyson-Schwinger equation approach. We find that the critical value $alpha_c$ is increased by weak short-range interaction. As $U$ increases to approach $U_c$, the quantum fluctuation of antiferromagnetic order parameter becomes important and interacts with the Dirac fermions via the Yukawa coupling. After treating the Coulomb interaction and Yukawa coupling interaction on an equal footing, we show that $alpha_c$ is substantially increased as $U rightarrow U_c$. Thus, the excitonic pairing is strongly suppressed near the antiferromagnetic quantum critical point. We obtain a global phase diagram on the $U$-$alpha$ plane, and illustrate that the excitonic insulating and antiferromagnetic phases are separated by an intermediate semimetal phase. These results provide a possible explanation of the discrepancy between recent theoretical progress on excitonic gap generation and existing experiments in suspended graphene.
We theoretically study hydrogen-bonded molecular conductors synthesized recently, $kappa$-H$_3$(Cat-EDT-TTF)$_2$ and its diselena analog, $kappa$-H$_3$(Cat-EDT-ST)$_2$, by first-principles density-functional theory calculations. In these crystals, two H(Cat-EDT-TTF/ST) units share a hydrogen atom with a short O--H--O hydrogen bond. The calculated band structure near the Fermi level shows a quasi-two-dimensional character, with a rather large interlayer dispersion due to the absence of insulating layers in contrast with conventional molecular conductors. We discuss effective low-energy models based on H(Cat-EDT-TTF/ST) units and its dimers, respectively, where the microscopic character of the orbitals composing them are analyzed. Furthermore, we find a stable structure which is different from the experimentally determined structure, where the shared hydrogen atom becomes localized to one of the oxygen atoms, in which charge disproportionation between the two types of H(Cat-EDT-TTF) units is associated. The calculated potential energy surface for the H atom is very shallow near the minimum points, therefore the probability of the H atom can be delocalized between the two O atoms.
We demonstrate the formation of a two-dimensional electron gas (2DEG) at the $(100)$ surface of the $5d$ transition-metal oxide KTaO$_3$. From angle-resolved photoemission, we find that quantum confinement lifts the orbital degeneracy of the bulk band structure and leads to a 2DEG composed of ladders of subband states of both light and heavy carriers. Despite the strong spin-orbit coupling, our measurements provide a direct upper bound for potential Rashba spin splitting of only $Delta{k}_parallelsim0.02$ AA$^{-1}$ at the Fermi level. The polar nature of the KTaO$_3(100)$ surface appears to help mediate formation of the 2DEG as compared to non-polar SrTiO$_3(100)$.
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