研究者詳細 - 杉本　高大

論文 - 杉本　高大

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Typicality-based variational cluster approach to thermodynamic properties of the Hubbard model

Nishida H., Fujiuchi R., Sugimoto K., Ohta Y.

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan） 89 （ 2 ） 023702 - 023702 2020年

研究論文（学術雑誌）, 共著, 査読有り, ISSN 00319015

© 2020 The Physical Society of Japan We develop the variational-cluster-approximation method based on the thermal-pure-quantum-state approach and apply the method to the calculations of the thermodynamic properties of the Hubbard model. We thereby obtain the temperature dependence of the single-particle excitation spectra, entropy and specific heat, and order parameter of the antiferromagnetic phase efficiently. We thus demonstrate that the method developed here has a potential power for elucidating finite-temperature properties of strongly correlated electron systems.

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Quantum phase transitions in the dimerized extended Bose-Hubbard model

Sugimoto K., Ejima S., Lange F., Fehske H.

Physical Review A （Physical Review A） 99 （ 1 ） 012122 2019年01月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 24699926

© 2019 American Physical Society. We present an unbiased numerical density-matrix renormalization group study of the one-dimensional Bose-Hubbard model supplemented by nearest-neighbor Coulomb interaction and bond dimerization. It places the emphasis on the determination of the ground-state phase diagram and shows that, besides dimerized Mott and density-wave insulating phases, an intermediate symmetry-protected topological Haldane insulator emerges at weak Coulomb interactions for filling factor one, which disappears, however, when the dimerization becomes too large. Analyzing the critical behavior of the model, we prove that the phase boundaries of the Haldane phase to Mott insulator and density-wave states belong to the Gaussian and Ising universality classes with central charges c=1 and c=1/2, respectively, and merge in a tricritical point. Interestingly we can demonstrate a direct Ising quantum phase transition between the dimerized Mott and density-wave phases above the tricritical point. The corresponding transition line terminates at a critical end point that belongs to the universality class of the dilute Ising model with c=7/10. At even stronger Coulomb interactions the transition becomes first order.

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Spin texture and spin current in excitonic phases of the two-band Hubbard model

Nishida H., Miyakoshi S., Kaneko T., Sugimoto K., Ohta Y.

Physical Review B （Physical Review B） 99 （ 3 ） 035119 2019年01月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 24699950

© 2019 American Physical Society. Using the mean-field approximation, we study the k-space spin textures and local spin currents emerged in the spin-triplet excitonic insulator states of the two-band Hubbard model defined on the square and triangular lattices. We assume a noninteracting band structure with a direct band gap and introduce s-, p-, d-, and f-type cross-hopping integrals, i.e., the hopping of electrons between different orbitals on adjacent sites with four different symmetries. First, we calculate the ground-state phase diagrams in the parameter space of the band filling and interaction strengths, whereby we present the filling dependence of the amplitude and phase of the excitonic order parameters. Then, we demonstrate that the spin textures (or asymmetric band structures) are emerged in the Fermi surfaces by the excitonic symmetry breaking when particular phases of the order parameter are stabilized. Moreover, in the case of the p-type cross-hopping integrals, we.

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Nonequilibrium dynamics in the pump-probe spectroscopy of excitonic insulators

Tanabe T., Sugimoto K., Ohta Y.

Physical Review B （Physical Review B） 98 （ 23 ） 235127 2018年12月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 24699950

© 2018 American Physical Society. We study the nonequilibrium dynamics in the pump-probe spectroscopy of excitonic insulators using the spinless two-orbital model with phonon degrees of freedom in the time-dependent mean-field approximation. We introduce the pulse light as a time-dependent vector potential via the Peierls phase in the Hamiltonian. We find that, in the Bose-Einstein condensation regime where the normal state is semiconducting, the excitonic order is suppressed when the frequency of the pulse light is slightly larger than the band gap, while the order is enhanced when the frequency of the pulse is much larger than the band gap. We moreover find that the excitonic order is completely destroyed in the former situation if the intensity of the pulse is sufficiently strong. In the BCS regime where the normal state is semimetallic, we find that the excitonic order is always suppressed, irrespective of the frequency of the pulse light. The quasiparticle band structure and optical conductivity spectrum after the pumping are also calculated for the instantaneous states.

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Large entropy change derived from orbitally assisted three-centered two-electron σ bond formation in metallic Li0.33VS2

Katayama N., Tamura S., Yamaguchi T., Sugimoto K., Iida K., Matsukawa T., Hoshikawa A., Ishigaki T., Kobayashi S., Ohta Y., Sawa H.

Physical Review B （Physical Review B） 98 （ 8 ） 081104(R) 2018年08月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 24699950

© 2018 American Physical Society. We discuss herein the emergence of a large entropy change in metallic Li0.33VS2 derived from orbitally assisted loose σ bond formation. Comprehensive structural studies based on synchrotron x-ray and neutron diffraction analyses clarify the fabrication of ribbon chains at 375 K, consisting of multiple three-centered two-electron σ bonds based on the viewpoint of local chemical bonding. Although the metallic conductivity persists down to the lowest temperature measured, an exceptionally large entropy change as a metal, as much as ΔS=6.6Jmol-1K-1, appears at the transition. The emergence of a large entropy change in a metallic state may possibly lead to different functional materials, such as a heat-storage material with a rapid thermal response.

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Strong Coupling Nature of the Excitonic Insulator State in Ta2NiSe5

Sugimoto K., Nishimoto S., Kaneko T., Ohta Y.

Physical Review Letters （Physical Review Letters） 120 （ 24 ） 247602 2018年06月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 00319007

© 2018 American Physical Society. We analyze the measured optical conductivity spectra using the density-functional-theory-based electronic structure calculation and density-matrix renormalization group calculation of an effective model. We show that, in contrast to a conventional description, the Bose-Einstein condensation of preformed excitons occurs in Ta2NiSe5, despite the fact that a noninteracting band structure is a band-overlap semimetal rather than a small band-gap semiconductor. The system above the transition temperature is therefore not a semimetal but rather a state of preformed excitons with a finite band gap. A novel insulator state caused by the strong electron-hole attraction is thus established in a real material.

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Multipole ordering and collective excitations in the excitonic phase of Pr<inf>0.5</inf>Ca<inf>0.5</inf>CoO<inf>3</inf>

Yamaguchi T., Sugimoto K., Ohta Y.

Physica B: Condensed Matter （Physica B: Condensed Matter） 536 37 - 39 2018年05月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 09214526

© 2017 Elsevier B.V. As an extension of our previous paper (Yamaguchi et al., 2017) [24], we study the carrier doping dependence of the excitonic condensation in Pr0.5Ca0.5CoO3 using the random-phase and mean-field approximations for the realistic five-orbital Hubbard model. We show that the spin-triplet excitonic phase with a magnetic multipole ordering is stable against the doping of carriers in a considerable range around Co3+ (or 3d6). We discuss experimental relevance of our results.

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Ferromagnetic Peierls insulator state in AMg4Mn6 O15 (A= K,Rb,Cs)

Yamaguchi T., Sugimoto K., Ohta Y., Tanaka Y., Sato H.

Physical Review B （Physical Review B） 97 （ 16 ） 161103(R) 2018年04月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 24699950

© 2018 American Physical Society. Using the density-functional-theory-based electronic structure calculations, we study the electronic state of recently discovered mixed-valent manganese oxides AMg4Mn6O15(A=K,Rb,Cs), which are fully spin-polarized ferromagnetic insulators with a cubic crystal structure. We show that the system may be described as a three-dimensional arrangement of the one-dimensional chains of a 2p orbital of O and a 3d orbital of Mn running along the three axes of the cubic lattice. We thereby argue that in the ground state the chains are fully spin polarized due to the double-exchange mechanism and are distorted by the Peierls mechanism to make the system insulating.

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Excitonic order and superconductivity in the two-orbital hubbard model: Variational cluster approach

Fujiuchi R., Sugimoto K., Ohta Y.

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan） 87 （ 6 ） 063705 - 063705 2018年

研究論文（学術雑誌）, 共著, 査読有り, ISSN 00319015

© 2018 The Physical Society of Japan. Using the variational cluster approach based on the self-energy functional theory, we study the possible occurrence of excitonic order and superconductivity in the two-orbital Hubbard model with intra- and inter-orbital Coulomb interactions. It is known that an antiferromagnetic Mott insulator state appears in the regime of strong intra-orbital interaction, a band insulator state appears in the regime of strong inter-orbital interaction, and an excitonic insulator state appears between them. In addition to these states, we find that the s±-wave superconducting state appears in the small-correlation regime, and the dx2y2-wave superconducting state appears on the boundary of the antiferromagnetic Mott insulator state. We calculate the single-particle spectral function of the model and compare the band gap formation due to the superconducting and excitonic orders.

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Coherence Factors and Quantum Interferences in Excitonic Condensation of Ta2NiSe5

Koudai Sugimoto, Tatsuya Kaneko, Yukinori Ohta

Journal of Physics: Conference Series （Institute of Physics Publishing） 807 （ 11 ） 112001 2017年04月

筆頭著者, 責任著者, 査読有り, ISSN 1742-6596

In order to elucidate whether Ta2NiSe5 is in an excitonic condensation state or not, we study macroscopic quantum interferences in ultrasonic attenuation rate and nuclear magnetic resonance relaxation rate. Using the three-chain model describing the excitonic condensation of Ta2NiSe5, we demonstrate analytically that the ultrasonic attenuation rate shows a characteristic peak just below the transition temperature of the excitonic condensation, while the nuclear magnetic resonance relaxation rate shows a rapid drop. In particular, we find that the constructive interference originates from the hybridization between the conduction and valence bands induced by an external field.

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Low-Energy Excitation Spectra in the Excitonic Phase of Cobalt Oxides

Tomoki Yamaguchi, Koudai Sugimoto, Yukinori Ohta

Journal of the Physical Society of Japan （Physical Society of Japan） 86 （ 4 ） 043701 - 043701 2017年03月

ISSN 0031-9015

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Orbital diamagnetic susceptibility in excitonic condensation phase

Koudai Sugimoto, Yukinori Ohta

Physical Review B （American Physical Society (APS)） 94 （ 8 ） 2016年08月

筆頭著者, 責任著者, 査読有り, ISSN 2469-9950

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Microscopic quantum interference in excitonic condensation of Ta<sub>2</sub>NiSe<sub>5</sub>

Koudai Sugimoto, Tatsuya Kaneko, Yukinori Ohta

Physical Review B （American Physical Society (APS)） 93 （ 4 ） 041105(R) 2016年01月

筆頭著者, 責任著者, 査読有り, ISSN 2469-9950

The microscopic quantum interference associated with excitonic condensation in Ta2NiSe5 is studied in a BCS-type mean-field approximation. We show that in ultrasonic attenuation the coherence peak appears just below the transition temperature Tc, whereas in NMR spin-lattice relaxation the rate rapidly decreases below Tc; these observations can offer a crucial experimental test for the validity of the excitonic condensation scenario in Ta2NiSe5. We also show that excitonic condensation manifests itself in a jump of the heat capacity at Tc as well as in a softening of the elastic shear constant, in accordance with the second-order phase transition observed in Ta2NiSe5.