Interaction-induced quantum spin Hall insulator in the organic Dirac electron system <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>α</mml:mi><mml:mtext>−</mml:mtext><mml:msub><mml:mrow><mml:mtext>(BEDT-TSeF)</mml:mtext></mml:mrow><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">I</mml:mi><mml:mn>3</mml:mn></mml:msub></mml:math>
Daigo Ohki, Kazuyoshi Yoshimi, Akito Kobayashi
Abstract
Focusing on the recently-discovered candidate topological insulator $\ensuremath{\alpha}\text{\ensuremath{-}}{\text{(BEDT-TSeF)}}_{2}{\mathrm{I}}_{3}$---having two-dimensional charge-neutral Dirac cones in a low symmetry lattice---we combine ab initio and extended-Hubbard model calculations to deal with spin-orbit and nonlocal repulsive interactions, and find a realization of an interaction-induced quantum spin Hall (QSH) insulator, similar to the one proposed in the honeycomb lattice under next-nearest-neighbor repulsions. In the absence of repulsive interactions, a topological insulator appears by the spin-orbit coupling and is characterized by a nonzero spin Chern number. By considering up to next-nearest-neighbor repulsions at Hartree-Fock level, the intrinsic spin-orbit gap is found to grow by orders of magnitude and a QSH insulating phase appears that has both a finite spin Chern number and order parameter. Transport coefficients and spin susceptibility are calculated and found to consistently account for most of the experimental findings, including the metal-to-insulator crossover occurring at $\ensuremath{\sim}50\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ as well as the Berry phase change from 0 to $\ensuremath{\pi}$ under hydrostatic pressure. We argue that such a QSH insulating phase does not necessitate a sizable spin-orbit interaction to generate a large insulating gap, which is highly advantageous for the search of novel topological phases in generic materials having low symmetry lattice and/or small spin-orbit coupling.