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Yoshiaki Ōno

Publications and source records attributed to Yoshiaki Ōno.

At least 19 recordsLinked to original sources
arXiv2026

GPU Acceleration of Collinear and Noncollinear DFT Using a Numerical Atomic Orbital-Based DFT Code

We implement GPU acceleration of collinear and noncollinear density functional theory (DFT) calculations in the numerical atomic orbitals (NAOs) code OpenMX by offloading matrix multiplications and eigenvalue solves (plus selected auxiliary steps) to cuBLAS/cuSOLVER and OpenACC. Benchmarks on the Pegasus supercomputer (per node: a 48-core Intel Xeon Platinum 8468 CPU and one NVIDIA H100 GPU) compare GPU-accelerated and CPU-only runs under identical settings. For a 512-atom collinear case on two nodes (two GPUs total), the GPU-accelerated calculation achieves a 2.02 times speedup over a CPU-only run on two nodes (96 CPU cores total); for a 384-atom noncollinear case on two nodes (two GPUs total), the speedup is 2.60 times over two CPU-only nodes (96 cores). These results demonstrate practical GPU-accelerated DFT in an NAO-based code for both collinear and noncollinear calculations.

physics.comp-ph↗
arXiv2025

Carrier Density Dependence of Superconducting Transition Temperature in Electron-doped $\rm{SrTiO_3}$ Based on the First-principles Calculations

Electron-doped strontium titanate $\rm{SrTiO_3}$, known to be one of the most dilute superconductors, is investigated on the basis of the first-principles calculations. When the carrier density n decreases, the frequencies of the ferroelectric optical phonons near the $Γ$-point monotonically decreases in the overdoped regime with $n<10^{20}/\rm{cm}^{3}$, while unphysical imaginary phonon frequencies due to ferroelectric instabilities appear in the underdoped regime with $n>10^{20}/\rm{cm}^{3}$. We estimate the superconducting transition temperature $T_{\rm{c}}$ by using the McMillan equation in the overdoped regime and find that $T_{\rm{c}}$ increases with decreasing n as consistent with experiments in the overdoped regime. Detailed analysis of the Eliashberg function reveals that the increases in $T_{\rm{c}}$ with decreasing n in the overdoped regime is mainly due to the contributions from the ferroelectric soft-mode optical phonons.

cond-mat.supr-con↗
arXiv2020

FFLO Superconductivity Mediated by Excitonic Fluctuation in Semimetallic Ta$_2$NiSe$_5$

We investigate the excitonic fluctuation and its mediated superconductivity in the quasi one-dimensional three-chain Hubbard model for Ta$_2$NiSe$_5$ known as a candidate material for the excitonic insulator. In the semiconducting case and the semimetallic case with a small band-overlapping where one conduction ($c$) band and one valence ($f$) band cross the Fermi level, the excitonic fluctuation with $\bm{q}=\bm{0}$ is enhanced due to the $c$-$f$ Coulomb interaction and diverges towards the uniform excitonic order corresponding to the excitonic insulator. On the other hands, in the semimetallic case with a large band-overlapping where two $c$ bands and one $f$ band cross the Fermi level, the non-uniform excitonic fluctuation with $\bm{q}\neq \bm{0}$ corresponding to the nesting vector between the $c$ and $f$ Fermi-surfaces (FSs) becomes dominant and results in the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) excitonic order characterized by the condensation of excitons with finite center-of-mass momentum $\bm{q}$. Near the instability, the largely enhanced excitonic fluctuations mediate the $c$-$f$ interband Cooper pairs with finite center-of-mass momentum resulting in the FFLO superconductivity, which is expected to be realized in the semimetallic Ta$_2$NiSe$_5$ under high pressure.

cond-mat.supr-con↗
arXiv2020

Transition Temperature of Superconductivity in Sodium Tungsten Bronze -Theoretical Study Based on First-principles Calculations-

Using first-principles calculations, we examine the transition temperature $T_{\rm c}$ of superconductivity in sodium tungsten bronze ( Na$_x$WO$_3$, where $x$ is equal to or less than unity ). Although $T_{\rm c}$ is relatively low $T_{\rm c}( <\sim 3 {\rm K})$, it is interesting that its characteristic exponential dependence on $x$ has been experimentally observed at $\sim 0.2 < x < \sim 0.4$. On the basis of the McMillan equation for $T_{\rm c}$ including the effect of plasmons, we succeed in reproducing the absolute values of $T_{\rm c}$ and its $x$ dependence. We also find that the plasmon effect is crucial for the estimation of $T_{\rm c}$ as well as phonons. Since the calculated $T_{\rm c}$ may not exceed $\sim 20$ K even for $x <\sim 0.1$, the superconductivity at a low $T_{\rm c}$ can be interpreted by the usual phonon mechanism, including the plasmon effect. On the other hand, a high $T_{\rm c}$ up to about 90 K, which is found on the surface of a Na$_x$WO$_3$ system at $x\sim 0.05$ by recent experiments, cannot be explained by our results. This discrepancy suggests that another mechanism is required to clarify the nature of the high-$T_{\rm c}$ superconductivity of Na$_x$WO$_3$.

cond-mat.supr-con↗
arXiv2018

Excitonic Phase Diagram of the Three-Chain Hubbard Model for Semiconducting and Semimetallic Ta$_2$NiSe$_5$

Transition metal chalcogenide Ta$_2$NiSe$_5$, a promising material for the excitonic insulator, is investigated on the basis of the quasi-one-dimensional three-chain Hubbard model with two conduction ($c$) bands and one valence ($f$) band. In the semimetallic case where only one of two $c$ bands and the $f$ band cross the Fermi level, the transition from the $c$-$f$ compensated semimetal to the uniform excitonic insulator takes place at low temperature as the same as in the semiconducting case. On the other hand, when another $c$ band also crosses the Fermi level, the system shows three types of Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) excitonic orders characterized by the condensation of excitons with finite center-of-mass momentum $q$ corresponding to the three types of nesting vectors between the imbalanced two $c$ and one $f$ Fermi surfaces. The obtained FFLO states are metallic in contrast to the excitonic insulator and are expected to be observed in semimetallic Ta$_2$NiSe$_5$ under high pressure.

cond-mat.str-el↗
arXiv2017

Fermi surface, pressure-induced antiferromagnetic order, and superconductivity in FeSe

The pressure dependence of the structural ($T_s$), antiferromagnetic ($T_m$), and superconducting ($T_c$) transition temperatures in FeSe is investigated on the basis of the 16-band $d$-$p$ model. At ambient pressure, a shallow hole pocket disappears due to the correlation effect, as observed in the angular-resolved photoemission spectroscopy (ARPES) and quantum oscillation (QO) experiments, resulting in the suppression of the antiferromagnetic order, in contrast to the other iron pnictides. The orbital-polarization interaction between the Fe $d$ orbital and Se $p$ orbital is found to drive the ferro-orbital order responsible for the structural transition without accompanying the antiferromagnetic order. The pressure dependence of the Fermi surfaces is derived from the first-principles calculation and is found to well account for the opposite pressure dependences of $T_s$ and $T_m$, around which the enhanced orbital and magnetic fluctuations cause the double-dome structure of the eigenvalue $λ$ in the Eliashberg equation, as consistent with that of $T_c$ in FeSe.

cond-mat.supr-con↗
arXiv2016

FFLO Excitonic State in the Three-Chain Hubbard Model for Ta$_2$NiSe$_5$

The three-chain Hubbard model for Ta$_2$NiSe$_5$ known as a candidate material for the excitonic insulator is investigated over the wide range of energy gap $D$ between the two-fold degenerate conduction bands and the nondegenerate valence band including both semiconducting ($D>0$) and semimetallic ($D<0$) cases. In the semimetallic case, the difference of the band degeneracy inevitably causes the imbalance of each Fermi wavenumber, resulting in a remarkable excitonic state characterized by the condensation of excitons with finite center-of-mass momentum $q$, the so-called Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) excitonic state. With decreasing $D$ corresponding to increasing pressure, the obtained excitonic phase diagram shows a crossover from BEC ($D\simg 0$) to BCS ($D\siml 0$) regime, and then shows a distinct phase transition at a certain critical value $D_c(<0)$ from the uniform ($q=0$) to the FFLO ($q\ne 0$) excitonic state, as expected to be observed in Ta$_2$NiSe$_5$ under high pressure.

cond-mat.str-el↗
arXiv2015

Hole-$s_{\pm}$ State Induced by Coexisting Ferro- and Aniferromagnetic and Aniferro-orbital Fluctuations in the Iron Pnictides

The multi-orbital Hubbard model is investigated in order to clarify the electron correlation effects and the superconductivity in the iron-based superconductor. The renormalization effects on the self-energy and the two-particle irreducible vertex function are calculated on the basis of the dynamical mean field theory. We find that the vertex function exhibits a strong renormalization with the significant orbital dependence as compared with the renormalization factor, when the antiferromagnetic and the antiferro-orbital fluctuations are comparably enhanced due to the electron and the hole Fermi surfaces nesting effects. The orbital-dependent vertex function together with the $q\sim0$ nesting between the two-hole Fermi surfaces results in the inter-orbital ferromagnetic fluctuation gradually enhanced which is expected to be observed in LiFeAs. We show that the hole-$s_{\pm}$-wave superconductivity with the sign change of the two-hole Fermi surfaces is realized by the enhanced ferromagnetic fluctuation accompanied by the antiferromagnetic fluctuation and the antiferro-orbital fluctuation.

cond-mat.str-el↗
arXiv2014

Metal-insulator transition and superconductivity in the two-orbital Hubbard-Holstein model for iron-based superconductors

We investigate a two-orbital model for iron-based superconductors to elucidate the effect of interplay between electron correlation and Jahn-Teller electron-phonon coupling by using the dynamical mean-field theory combined with the exact diagonalization method. When the intra- and inter-orbital Coulomb interactions, $U$ and $U'$, increase with $U=U'$, both the local spin and orbital susceptibilities, $χ_{s}$ and $χ_{o}$, increase with $χ_{s}=χ_{o}$ in the absence of the Hund's rule coupling $J$ and the electron-phonon coupling $g$. In the presence of $J$ and $g$, there are distinct two regimes: for $J \stackrel{>}{_\sim} 2g^2/ω_0$ with the phonon frequency $ω_0$, $χ_{s}$ is enhanced relative to $χ_{o}$ and shows a divergence at $J=J_c$ above which the system becomes Mott insulator, while for $J \stackrel{<}{_\sim} 2g^2/ω_0$, $χ_{o}$ is enhanced relative to $χ_{s}$ and shows a divergence at $g=g_c$ above which the system becomes bipolaronic insulator. In the former regime, the superconductivity is mediated by antiferromagnetic fluctuations enhanced due to Fermi-surface nesting and is found to be largely dependent on carrier doping. On the other hand, in the latter regime, the superconductivity is mediated by ferro-orbital fluctuations and is observed for wide doping region including heavily doped case without the Fermi-surface nesting.

cond-mat.str-el↗
arXiv2014

A High-Tc Mechanism of Iron Pnictide Superconductivity due to Cooperation of Ferro-orbital and Antiferromagnetic Fluctuations

The electronic states and superconductivity in iron pnictides are studied on the basis of the 16 band $d$-$p$ model which includes both the onsite Coulomb interaction between Fe $d$ electrons and the intersite one between Fe $d$ and pnictogen $p$ electrons. The model well accounts for experimentally observed two fluctuations: the $d$-$d$ interaction-enhanced antiferromagnetic (AFM) fluctuation and the $d$-$p$ interaction-enhanced ferro-orbital (FO) fluctuation responsible for the $C_{66}$ elastic softening. The AFM fluctuation induces the repulsive pairing interaction for $\bm{q}\sim \bm{Q}_{\rm AF}$ while the FO does the attractive one for $\bm{q}\sim \bm{0}$ resulting in the $s_{\pm}$-wave superconductivity where the two fluctuations cooperatively enhance the superconducting transition temperature $T_{c}$ without any competition by virtue of the $\bm{q}$-space segregation.

cond-mat.supr-con↗
arXiv2012

Dynamical Mean-Field Study of Local Pairing Interaction Mediated by Spin and Orbital Fluctuations in Iron Pnictide Superconductors

We investigate the two-orbital Hubbard model, which reproduces the electron and hole Fermi surfaces in the iron pnictide superconductors, in the presence of the Jahn-Teller electron-phonon coupling by using the dynamical mean-field theory. When the intra- and inter-orbital Coulomb interactions, $U$ and $U'$, increase with $U=U'$, both the local spin and orbital susceptibilities, $χ_{s}$ and $χ_{o}$, increase with $χ_{s}=χ_{o}$ because of the spin-orbital symmetry. Due to the Hund's rule coupling $J$, $χ_{s}$ is enhanced and dominates over $χ_{o}$ resulting in the repulsive local pairing interaction $V_{\rm loc}>0$, while due to the electron-phonon coupling $g$, $χ_{o}$ is enhanced and dominates over $χ_{s}$ resulting in the attractive one $V_{\rm loc}<0$ which induces the intra-orbital s-wave pairing. Remarkably, $V_{\rm loc}$ is weakly dependent on doping and can be attractive for heavily electron-doped regime where the superconductivity is observed without Fermi surface nesting.

cond-mat.supr-con↗
arXiv2012

Dynamical mean-field theory for the anisotropic Kondo semiconductor: Temperature and magnetic field dependence

We investigate the periodic Anderson model with $\bm{k}$-dependent $c$-$f$ mixing reproducing the point nodes of the hybridization gap by using the dynamical mean-field theory combined with the exact diagonalization method. At low temperature below a coherence temperature $T_0$, the imaginary part of the self-energy is found to be proportional to $T^2$ and the pseudogap with two characteristic energies $\tilde{\it Δ}_1$ and $\tilde{\it Δ}_2$ is clearly observed for $T\ll T_0$, while the pseudogap is smeared with increasing $T$ and then disappears at high temperature $T \simg T_0$ due to the evolution of the imaginary self-energy. When the Coulomb interaction between $f$ electrons $U$ increases, $\tilde{\it Δ}_1$, $\tilde{\it Δ}_2$, and $T_0$ together with $T_{\rm max}$ at which the magnetic susceptibility is maximum decrease in proportion to the renormalization factor $Z$ resulting in a heavy-fermion semiconductor with a large mass enhancement $m^*/m=Z^{-1}$ for large $U$. We also examine the effect of the external magnetic field $H$ and find that the magnetization $M$ shows two metamagnetic anomalies $H_1$ and $H_2$ corresponding to $\tilde{\it Δ}_1$ and $\tilde{\it Δ}_2$ which are reduced due to the effect of $H$ together with $Z$. Remarkably, $Z^{-1}$ is found to be largely enhanced due to $H$ especially for $H_1 \siml H \siml H_2$, where the field induced heavy-fermion state is realized. The obtained results seem to be consistent with the experimental results observed in the anisotropic Kondo semiconductors such as CeNiSn.

cond-mat.str-el↗
arXiv2010

Orbital Order, Structural Transition and Superconductivity in Iron Pnictides

We investigate the 16-band d-p model for iron pnictide superconductors in the presence of the electron-phonon coupling g with the orthorhombic mode which is crucial for reproducing the recently observed ultrasonic softening. Within the RPA, we obtain the ferro-orbital order below TQ which induces the tetragonal-orthorhombic structural transition at Ts = TQ, together with the stripe-type antiferromagnetic order below TN. Near the phase transitions, the system shows the s++ wave superconductivity due to the orbital fluctuation for a large g case with TQ > TN, while the s+- wave due to the magnetic fluctuation for a small g case with TQ < TN. The former case is consistent with the phase diagram of doped iron pnictides with Ts > TN.

cond-mat.supr-con↗
arXiv2010

Effect of the spin-orbit interaction and the electron phonon coupling on the electronic state in a silicon vacancy

The electronic state around a single vacancy in silicon crystal is investigated by using the Green's function approach. The triply degenerate charge states are found to be widely extended and account for extremely large elastic softening at low temperature as observed in recent ultrasonic experiments. When we include the LS coupling $λ_{\rm Si}$ on each Si atom, the 6-fold spin-orbital degeneracy for the $V^{+}$ state with the valence +1 and spin 1/2 splits into $Γ_{7}$ doublet groundstates and $Γ_{8}$ quartet excited states with a reduced excited energy of $O(λ_{\rm Si}/10)$. We also consider the effect of couplings between electrons and Jahn-Teller phonons in the dangling bonds within the second order perturbation and find that the groundstate becomes $Γ_{8}$ quartet which is responsible for the magnetic-field suppression of the softening in B-doped silicon.

cond-mat.str-el↗
arXiv2010

Dynamical Mean-Field Study of Metamagnetism in Heavy Fermion Systems

We investigate the metamagnetism in the periodic Anderson model with the $\bm{k}$-dependent $c-f$ mixing by using the dynamical mean-field theory combined with the exact diagonalization method. It is found that both effects of the $\bm{k}$-dependent $c-f$ mixing and strong correlation due to the Coulomb interaction between $f$ electrons are significant for determining both the magnetization and the mass enhancement factor. For the case away from the half-filling, the results is consistent with the metamagnetic behavior observed in CeRu$_2$Si$_2$.

cond-mat.str-el↗
arXiv2010

Cooperative effects of Coulomb and electron-phonon interactions in the two-dimensional 16-band d-p model for iron-based superconductors

We study the electronic states and the superconductivity in the two-dimensional 16-band d-p model coupled with A1g, B1g and Eg local phonons and obtain the rich phase diagram including the magnetic, charge and orbital ordered phases on the parameter plane of the Coulomb and electron-phonon interactions. When the electron-phonon interaction is dominant, the charge fluctuations induce the s++ wave superconductivity, while when the Coulomb interaction is dominant, the magnetic fluctuations induce the s+- wave superconductivity. Remarkably, the orbital fluctuations are enhanced due to the cooperative effects of the Coulomb and electron-phonon interactions and induce the s++ wave and the nodal s+- wave superconductivities.

cond-mat.supr-con↗
arXiv2010

Two types of s-wave pairing due to magnetic and orbital fluctuations in the two-dimensional 16-band d-p model for iron-based superconductors

We study superconductivity in the two-dimensional 16-band d-p model extracted from a tight-binding fit to the band structure of LaFeAsO, using the random phase approximation. When the intraorbital repulsion U is larger than the interorbital one U', an extended s-wave (s+--wave) pairing with sign reversal of order parameter is mediated by antiferromagnetic spin fluctuations, while when U U'.

cond-mat.supr-con↗