arXiv subjects

Daigo Ohki

Publications and source records attributed to Daigo Ohki.

7 recordsLinked to original sources

arXiv2023

Gap opening mechanism for correlated Dirac electrons in organic compounds $α$-(BEDT-TTF)$_2$I$_3$ and $α$-(BEDT-TSeF)$_2$I$_3$

To determine how electron correlations open a gap in two-dimensional massless Dirac electrons in the organic compounds $α$-(BEDT-TTF)$_2$I$_3$ [$α$-(ET)$_2$I$_3$] and $α$-(BEDT-TSeF)$_2$I$_3$ [$α$-(BETS)$_2$I$_3$], we derive and analyze $ab$ $initio$ low-energy effective Hamiltonians for these two compounds. We find that the horizontal stripe charge ordering opens a gap in the massless Dirac electrons in $α$-(ET)$_2$I$_3$, while an insulating phase without explicit symmetry breaking appears in $α$-(BETS)$_2$I$_3$. We clarify that the combination of the anisotropic transfer integrals and the electron correlations induces a dimensional reduction in the spin correlations, i.e., one-dimensional spin correlations develop in $α$-(BETS)$_2$I$_3$. We show that the one-dimensional spin correlations open a gap in the massless Dirac electrons. Our finding paves the way for opening gaps for massless Dirac electrons using strong electronic correlations.

cond-mat.str-el↗

arXiv2022

Interaction-induced quantum spin Hall insulator in the organic Dirac electron system $α$-(BEDT-TSeF)$_2$I$_3$

Focusing on the recently-discovered candidate topological insulator $α$-(BEDT-TSeF)$_2$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 non-local 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 $\sim50$ K as well as the Berry phase change from 0 to $π$ under hydrostatic pressure. We argue that such a QSH insulating phase does not necessitate a sizeable 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.

cond-mat.mes-hall↗

arXiv2020

Tight-Binding Model and Electronic Property of Dirac Nodal Line in Single-Component Molecular Conductor [Pt(dmdt)$_{2}$]

Motivated by the recent discovery of Dirac nodal line in the single-component molecular conductor [Pt(dmdt)$_{2}$], we propose a three-orbital tight-binding model based on the Wannier fitting of the first-principles calculation, and address the problems of edge states, topological properties and magnetic susceptibility. We find that logarithmic peaks of the local density of states emerge near the Fermi energy, owing to pseudo-one-dimensional edge states that appear between the Dirac nodal lines. Magnetic susceptibility calculated in our model can explain the experimental result at a high temperature. In the presence of a realistic spin-orbit coupling, we show that [Pt(dmdt)$_{2}$] is a topological nodal line semimetal with isolated electron and hole pockets.

cond-mat.mes-hall↗

arXiv2020

Transport properties of organic Dirac electron system α-(BEDT-TSeF)$_2$I$_3$

Motivated by the insulating behavior of $α$-(BEDT-TSeF)$_2$I$_3$ at low temperatures ($T$'s), we first performed first-principles calculations based on the crystal structural data at 30 K under ambient pressure, and we constructed a two-dimensional effective model using maximally localized Wannier functions. As possible causes of the insulating behavior, we studied the effects of the on-site Coulomb interaction $U$ and spin-orbit interaction (SOI) by investigating the electronic state and the transport coefficient using the Hartree approximation and the $T$-matrix approximation. The calculations at a finite $T$ demonstrated that a spin-ordered massive Dirac electron (SMD) appeared due to the on-site Coulomb interaction. We had an interest in the anomalous competitive effect with $U$ and SOI when the SMD phase is present in $α$-(BETS)$_2$I$_3$, and we investigated these contributions to the electronic state and conductivity. The SMD is not a conventional spin order, but it exhibits the spin-valley Hall effect. Direct current resistivity in the presence of a spin order gap increased divergently and exhibited negative magnetoresistance in the low $T$ region with decreasing $T$. The charge density hardly changed below and above the $T$ at which this insulating behavior appeared. However, when considering the SOI alone, the state changed to a topological insulator phase, and the electrical resistivity is saturated by edge conduction at quite low $T$. When considering both the SMD and the SOI, the spin order gap was suppressed by the SOI, and gaps with different sizes opened in the left and right Dirac cones. This phase transition leads to distinct changes in microwave conductivity, such as a discontinuous jump and a peak structure.

cond-mat.mes-hall↗

arXiv2020

Effect of Coulomb Interaction on Seebeck Coefficient of Organic Dirac Electron System $α$-(BEDT-TTF)$_2$I$_3$

Motivated by the results of recent thermoelectric effect studies, we show the effects of Coulomb interactions on the Seebeck coefficient based on an extended Hubbard model that describes the electronic states of a slightly doped organic Dirac electron system, $α$-(BEDT-TTF)$_2$I$_3$. Our results indicate that the Hartree terms of the Coulomb interactions enhance the electron-hole asymmetry of the energy band structure and change the energy dependence of the relaxation time from impurity scattering, which reflects the shape of the density of states. Thus, the Seebeck coefficient exhibits a non-monotonic $T$ dependence which qualitatively agrees with the experimental results. Furthermore, we also show that the signs of the Seebeck coefficient and the Hall coefficient calculated by linear response theory do not necessarily correspond to the sign of the chemical potential using a modified Weyl model with electron-hole asymmetry. These results point out that changing the electron-hole asymmetry by strong Coulomb interaction has the potential to controllable the sign and value of the Seebeck coefficient in the Dirac electron systems.

cond-mat.mes-hall↗

arXiv2019

Excitonic instability of two-dimensional tilted Dirac cones

The electron-electron Coulomb interaction in Dirac-Weyl semimetals harbours a novel paradigm of correlation effects that hybridizes diverse realms of solid-state physics with their relativistic counterpart. Driving spontaneous mass acquisition, the excitonic condensate of strongly-interacting massless Dirac fermions is one such example whose exact nature remains debated. Here, by focussing on the two-dimensional tilted Dirac cones in the organic salt $α$-(BEDT-TTF)$_2$I$_3$, we show that the excitonic instability is controlled by a small chemicalpotential shift and an in-plane magnetic field. In combined analyses based on renormalization-group approaches and ladder approximation, we demonstrate that the nuclear relaxation rate is an excellent probe of excitonic-spin fluctuations in an extended parameter region. Comparative nuclear magnetic resonance (NMR) experiments show good agreements with this result, jointly revealing the importance of intervalley nesting between field-induced, spin-split Fermi pockets of opposite charge polarities. Our work provides an accurate framework to search for excitonic instability of strongly-interacting massless fermions.

cond-mat.str-el↗

arXiv2019

Domain Wall Conductivity with strong Coulomb interaction of two-dimensional massive Dirac Electrons in the Organic Conductor $α$-(BEDT-TTF)$_2$I$_3$

Motivated by the results of recent transport and optical conductivity studies, we propose a semi-infinite two-dimensional lattice model for interacting massive Dirac electrons in the pressurized organic conductor $α$-(BEDT-TTF)$_2$I$_3$, and address the problem of domain wall conductivity in a charge-ordered insulating phase under realistic experimental conditions. Using the extended Hubbard model at a mean field level, we present results of extensive numerical studies around the critical region of the model, reporting on the resistivity and optical conductivity calculated by means of the Nakano-Kubo formula. We find that the activation gap extracted from the resistivity data can be much smaller than the optical gap in the critical region, which is induced by metallic conduction along an one-dimensional domain wall emerging at the border of two charge-ordered ferroelectric regions with opposite polarizations. The data are consistent with the observed transport gap in real $α$-(BEDT-TTF)$_2$I$_3$ samples that is reduced remarkably faster than the optical gap upon suppressing charge order with pressure. Our optical conductivity also reveals an additional shoulder-like structure at low energy inside the gap, which is argued to be directly relevant to the metallic bound states residing on the domain wall.

cond-mat.mes-hall↗