研究者詳細 - 杉本　高大

論文 - 杉本　高大

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Symmetry, topology, duality, chirality, and criticality in a spin- 12 XXZ ladder with a four-spin interaction

Fontaine M., Sugimoto K., Furukawa S.

Physical Review B （Physical Review B） 109 （ 13 ） 2024年04月

査読有り, ISSN 24699950

We study the ground-state phase diagram of a spin-12 XXZ model with a chirality-chirality interaction (CCI) on a two-leg ladder. This model offers a minimal setup to study an interplay between spin and chirality degrees of freedom. The spin-chirality duality transformation allows us to relate the regimes of weak and strong CCIs. By applying the Abelian bosonization and the duality, we obtain a rich phase diagram that contains distinct gapped featureless and ordered phases. In particular, Néel and vector chiral orders appear for easy-axis anisotropy, while two distinct symmetry-protected topological (SPT) phases appear for easy-plane anisotropy. The two SPT phases can be viewed as twisted variants of the Haldane phase. We also present an effective description in terms of (spinor) hard-core bosons, which reveals critical behavior on the self-dual line in the easy-axis and easy-plane regimes. We perform numerical simulations to confirm the predicted phase structure and critical properties. We further demonstrate that the two SPT phases and a trivial phase are distinguished by topological indices in the presence of certain symmetries. A similar phase structure is expected in a spin-12 XXZ ladder with four-spin ring exchange.

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Pump-probe spectroscopy of the one-dimensional extended Hubbard model at half filling

Sugimoto K., Ejima S.

Physical Review B （Physical Review B） 108 （ 19 ） 2023年11月

筆頭著者, 責任著者, 査読有り, ISSN 24699950

By utilizing time-dependent tensor-network algorithms in the infinite matrix-product-state representation, we theoretically investigate the pump-probe spectroscopy of the one-dimensional extended Hubbard model at half filling. Our focus lies on nonequilibrium optical conductivity and single-particle excitation spectra. In the spin-density-wave (SDW) phase, we identify an in-gap state in the nonequilibrium optical conductivity due to the formation of excitons (or doublon-holon pairs), generated by the pulse through nonlocal interactions. In the strong-coupling regime, we discern additional multiple in-gap and out-of-gap states. In the charge-density-wave (CDW) phase, we detect not only an in-gap state but also a finite Drude weight, which results from the dissolution of charge order by photoexcitation. Analyzing time-dependent single-particle excitation spectra directly in the thermodynamic limit confirms the origin of these new states in the SDW and CDW phases as the excitation of newly emerged dispersions. We illustrate that the pump-probe spectroscopy simulations in the thermodynamic limit furnish unambiguous spectral structures that allow for direct comparison with experimental results, and the integration of nonequilibrium optical conductivity and time- and angle-resolved photoemission spectroscopy provides comprehensive insights into nonequilibrium states.

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Zigzag chain order of LiVSe2 developing away from the vanadium trimer phase transition boundary

Kojima K., Katayama N., Sugimoto K., Hirao N., Ohta Y., Sawa H.

Physical Review B （Physical Review B） 108 （ 9 ） 2023年09月

査読有り, ISSN 24699950

The phenomenon of self-assembly of constituent elements to form molecules at low temperatures appears ubiquitously in transition metal compounds with orbital degrees of freedom. Recent progress in local structure studies using synchrotron radiation x rays is shifting the interest in structural studies in such molecule-forming systems from the low-temperature ordered phase to the short-range order that appears like a precursor at high temperatures. In this study, we discuss both experimentally and theoretically the relationship between the trimer structure that appears in the layered LiVX2(X = O, S, Se) system with a two-dimensional triangular lattice of vanadium and the zigzag chainlike local structure that appears near the phase transition boundary where molecular formation occurs. The vanadium trimerization that persistently appears in both low-temperature phases of LiVO2 and LiVS2 disappears in LiVSe2, and a regular triangular lattice is thought to be realized in LiVSe2, but this study reveals that the zigzag chain local distortion appears with a finite correlation length. These zigzag chain state local distortions are similar to the motif of local distortions in the high-temperature phase of LiVS2, indicating that the local distortions are persistent away from the trimer phase transition boundary. On the other hand, it is concluded that the zigzag chain order appearing in LiVSe2 is more stable than that in LiVS2 in terms of the temperature variation of atomic displacement and correlation length. The zigzag chain order is considered to be competitive with the trimer order appearing in the LiVX2 system. In this paper, we discuss the similarities and differences between the parameters that stabilize these electronic phases and the local distortions that appear in other molecular formation systems.

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Wannier-Stark ladders and Stark shifts of excitons in Mott insulators

Udono M., Kaneko T., Sugimoto K.

Physical Review B （Physical Review B） 108 （ 8 ） 2023年08月

最終著者, 責任著者, 査読有り, ISSN 24699950

External-field driven energy-level discretization, such as Landau quantization or Stark localization, is one of the most intriguing phenomena in quantum systems. We investigate the emergence of the Wannier-Stark ladder coming from the particle-hole continuum and the Stark shifts of the exciton levels in one-dimensional Mott insulators under the dc electric field. The discretized peak structure in the optical-conductivity spectra newly appears by applying the dc electric field, and the positions of these peaks can be reproduced from the energy levels of a simple effective model in the strong-coupling regime. Our results not only suggest that Mott insulators can serve as a viable platform for Stark discretization, but also pave the way for investigations of dynamical properties in correlated many-body systems under a dc electric field.

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Collective mode excitations and simulated L -edge resonant inelastic x-ray scattering spectra in antiferromagnetic Ca2 RuO4

Yamamoto S., Ohta Y., Sugimoto K.

Physical Review B （Physical Review B） 106 （ 4 ） 2022年07月

最終著者, 査読有り, ISSN 24699950

Using the three-orbital Hubbard model, we investigate the low-energy excitation spectra in the antiferromagnetic phase of Ca2RuO4. We calculate the dynamical susceptibilities in the low-energy region by the random phase approximation and find that the anisotropic dispersion of the transverse mode is in good agreement with the spectra recently reported by inelastic neutron scattering experiments. By the fast-collision approximation, we simulate the resonant inelastic x-ray scattering (RIXS) spectra of the Ru L3 edge from the dynamical susceptibilities. We show that the dispersion of the transverse mode is clearly observed in the calculated RIXS spectra and that the polarization dependence of the incident x rays enables one to distinguish between the excitations of the in-plane transverse mode and out-of-plane transverse mode.

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Excitonic effects on high-harmonic generation in Mott insulators

Udono M., Sugimoto K., Kaneko T., Ohta Y.

Physical Review B （Physical Review B） 105 （ 24 ） 2022年06月

査読有り, ISSN 24699950

To study excitonic effects on high-harmonic generation (HHG) in Mott insulators, we investigate pumped nonequilibrium dynamics in the one-dimensional extended Hubbard model. By employing time-dependent calculations based on the exact diagonalization and infinite time-evolving block decimation methods, we find the strong enhancement of the HHG intensity around the exciton energy. The subcycle analysis in the sub-Mott-gap regime shows that the intensity region of the time-resolved spectrum around the exciton energy splits into two levels and oscillates following the driving electric field. This excitonic dynamics is qualitatively different from the dynamics of free doublon and holon but favorably contributes to HHG in the Mott insulator.

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Determination of the magnetic structures in the heavy fermion superconductor Ce<inf>3</inf>PtIn<inf>11</inf>

Shioda N., Kazuki , Kumeda , Fukazawa H., Ohama T., Kohori Y., Das D., Blawat J., Kaczorowski D., Sugimoto K.

Journal of Physics: Conference Series （Journal of Physics: Conference Series） 2164 （ 1 ） 012032 - 012032 2022年03月

査読有り, ISSN 17426588

Comprehensive analyses of the 115In-NQR spectra employing a magnetic dipolar model and a transferred hyperfine model are performed to determine the magnetic structures of Ce3PtIn11. Based on this analysis, we confirmed that the magnetic moment at the Ce(1) site is very small (∼4% of that at the Ce(2) site) but finite. Further, for T N2 < T < T N1 and T < T N2, the magnetic moments at the Ce(1) and Ce(2) sites are parallel to the c-axis, and the in-plane propagation vectors in the two Ce atoms sublattices are (qa, qb ) = (1/2, 1/2). In the temperature interval T N2 <T < T N1, for example, both of them have a form of q = (1/2, 1/2, 0). For T < T N2, the magnetic structure in the Ce(2) sublattice is defined by the propagation vector q = (1/2, 1/2, 1/2 or 0), whereas the Ce(1) sublattice forms a six-fold period structure given by q = (1/2, 1/2, 1/6).

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Charge-ordered state satisfying the Anderson condition in LiRh2 O4 arising from local dimer order

Shiomi M., Kojima K., Katayama N., Maeda S., Schneeloch J.A., Yamamoto S., Sugimoto K., Ohta Y., Louca D., Okamoto Y., Sawa H.

Physical Review B （Physical Review B） 105 （ 4 ） L041103 2022年01月

査読有り, ISSN 24699950

We report on the charge-ordered structure of LiRh2O4 arising below the metal-insulator transition at 170 K. Structural studies using synchrotron x rays have revealed that the charge-ordered states of Rh3+ and Rh4+ with dimerization are realized in the low-temperature phase below 170 K. Although the low-temperature ground state resembles that of CuIr2S4, a charge-ordering pattern satisfying the Anderson condition is realized in LiRh2O4. Based on structural information such as the short-range order of dimers appearing above the transition temperature and the weakening of the correlation between rhodium one-dimensional chains appearing in the crystal structure, we argue that the Coulomb interaction plays an important role in determining the charge-ordering patterns.

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Fate of Measurement-Induced Phase Transition in Long-Range Interactions

Minato T., Sugimoto K., Kuwahara T., Saito K.

Physical Review Letters （Physical Review Letters） 128 （ 1 ） 010603 2022年01月

査読有り, ISSN 00319007

We consider quantum many-body dynamics under quantum measurements, where the measurement-induced phase transitions (MIPs) occur when changing the frequency of the measurement. In this work, we consider the robustness of the MIP for long-range interaction that decays as r-a with distance r. The effects of long-range interactions are classified into two regimes: (i) the MIP is observed (a>ac), and (ii) the MIP is absent even for arbitrarily strong measurements (a<ac). Using fermion models, we demonstrate both regimes in integrable and nonintegrable cases. We identify the underlying mechanism and propose sufficient conditions to observe the MIP, that is, a>d/2+1 for general bilinear systems and a>d+1 for general nonintegrable systems (d: spatial dimension). Numerical calculation indicates that these conditions are optimal.

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Determination of the magnetic q vectors in the heavy fermion superconductor Ce3PtIn11

Shioda N., Kumeda K., Fukazawa H., Ohama T., Kohori Y., Das D., Bławat J., Kaczorowski D., Sugimoto K.

Physical Review B （Physical Review B） 104 （ 24 ） 245119 2021年12月

査読有り, ISSN 24699950

An analysis with a transferred hyperfine field has been performed on the spectra of the In115 nuclear quadrupole resonance experiments of heavy fermion superconductor Ce3PtIn11 exhibiting co-occurrence of two successive antiferromagnetic orderings (TN1 = 2.2 K and TN2 = 2.0 K) followed by a superconducting transition (Tc = 0.32 K). The spectral changes at magnetic transition temperatures TN1 and TN2 indicate that the Ce(2) site has the dominant magnetic contribution, with a small magnetic moment, but not negligible, at the Ce(1) site. Our analysis using a transferred hyperfine field which overcomes the previous simple dipolar model evinces the magnetic structures of Ce3PtIn11. ForTN2<T<TN1, the propagation vectors at the Ce(1) and Ce(2) sublattices are q1=q2=(12,12,0or12). On the other hand, for T<TN2, the propagation vectors are q1=(12,12,16or13) and q2=(12,12,12or0), respectively.

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Slow dynamics of disordered zigzag chain molecules in layered LiVS<inf>2</inf> under electron irradiation

Katayama N., Kojima K., Yamaguchi T., Hattori S., Tamura S., Ohara K., Kobayashi S., Sugimoto K., Ohta Y., Saitoh K., Sawa H.

npj Quantum Materials （npj Quantum Materials） 6 （ 1 ） 2021年12月

査読有り

Electronic instabilities in transition metal compounds often spontaneously form orbital molecules, which consist of orbital-coupled metal ions at low temperature. Recent local structural studies utilizing the pair distribution function revealed that preformed orbital molecules appear disordered even in the high-temperature paramagnetic phase. However, it is unclear whether preformed orbital molecules are dynamic or static. Here, we provide clear experimental evidence of the slow dynamics of disordered orbital molecules realized in the high-temperature paramagnetic phase of LiVS2, which exhibits vanadium trimerization upon cooling below 314 K. Unexpectedly, the preformed orbital molecules appear as a disordered zigzag chain that fluctuate in both time and space under electron irradiation. Our findings should advance studies on soft matter physics realized in an inorganic material due to disordered orbital molecules.

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Light-induced switching of magnetic order in the anisotropic triangular-lattice Hubbard model

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

Physical Review B （Physical Review B） 103 （ 16 ） 2021年04月

責任著者, 査読有り, ISSN 24699950

The time-dependent exact-diagonalization method is used to study the light-induced phase transition of magnetic orders in the anisotropic triangular-lattice Hubbard model. Calculating the spin correlation function, we confirm that the phase transition from the 120° order to the Néel order can take place due to high-frequency periodic fields. We show that the effective Heisenberg-model Hamiltonian derived from the high-frequency expansion by the Floquet theory describes the present system very well and that the ratio of the exchange interactions expressed in terms of the frequency and amplitude of the external field determines the type of the magnetic orders. Our results demonstrate the controllability of the magnetic orders by tuning the external field.

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Successive magnetic transitions in the heavy-fermion superconductor Ce3PtIn11 studied by in 115 nuclear quadrupole resonance

Fukazawa H., Kumeda K., Shioda N., Lee Y., Kohori Y., Sugimoto K., Das D., Bławat J., Kaczorowski D.

Physical Review B （Physical Review B） 102 （ 16 ） 2020年10月

査読有り, ISSN 24699950

© 2020 American Physical Society. Nuclear quadrupole resonance (NQR) measurements were performed on the heavy-fermion superconductor Ce3PtIn11 with Tc=0.32K. The temperature dependence of both spin-lattice relaxation rate 1/T1 and NQR spectra evidences the occurrence of two successive magnetic transitions with TN1≃2.2K and TN2≃2.0K. In successive magnetic transitions, even though the magnetic moment at the Ce(2) site plays a major role, the magnetic moment at the Ce(1) site also contributes to some extent. While a commensurate antiferromagnetic ordered state appears for TN2<T<TN1, a partially incommensurate antiferromagnetic ordered state is suggested for T<TN2.

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Successive magnetic transitions in the heavy-fermion superconductor Ce3PtIn11 studied by 115In nuclear quadrupole resonance

Hideto Fukazawa, Kazuki Kumeda, Naoki Shioda, Yongsun Lee, Yoh Kohori, Koudai Sugimoto, Debarchan Das, Joanna Bławat, Dariusz Kaczorowski

Physical Review B 102 165124 2020年10月

研究論文（学術雑誌）, 共著, 査読有り

Superconductivity and charge density wave under a time-dependent periodic field in the one-dimensional attractive Hubbard model

Fujiuchi R., Kaneko T., Sugimoto K., Yunoki S., Ohta Y.

Physical Review B （Physical Review B） 101 （ 23 ） 235122 2020年06月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 24699950

© 2020 American Physical Society. We investigate the competition between superconductivity (SC) and charge density wave (CDW) under a time-dependent periodic field in the attractive Hubbard model. By employing the time-dependent exact diagonalization method, we show that the driving frequency and amplitude of the external field can control the enhancement of either the superconducting pair or the CDW correlation in the system, for which SC and CDW are degenerate in the ground state of the half-filled attractive Hubbard model in the absence of the field. In the strong-coupling limit of the attractive Hubbard interaction, the controllability is characterized by the anisotropic interaction of the effective model. The anisotropy is induced by the external field and lifts the degeneracy of SC and CDW. We find that the enhancement or suppression of the superconducting pair and CDW correlations in the periodically driven attractive Hubbard model can be well interpreted by the quench dynamics of the effective model derived in the strong-coupling limit.

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Finite-temperature properties of excitonic condensation in the extended Falicov–Kimball model: Cluster mean-field-theory approach

Kadosawa M., Nishimoto S., Sugimoto K., Ohta Y.

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan） 89 （ 5 ） 053706-1 - 053706-5 2020年05月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 00319015

©2020 The Physical Society of Japan We study the electron–hole pair (or excitonic) condensation in the extended Falicov–Kimball model at finite temperatures based on the cluster mean-field-theory approach, where we make the grand canonical exact-diagonalization analysis of small clusters using the sine-square deformation function. We thus calculate the ground-state and finite-temperature phase diagrams of the model, as well as its optical conductivity and single-particle spectra, thereby clarifying how the preformed pair states appear in the strong-coupling regime of excitonic insulators. We compare our results with experiments on Ta2NiSe5,.

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Observation of a novel phase transition in Sr7Re4O19

Ueda Y., Murase M., Kobayashi Y., Itoh M., Yajima T., Yamaguchi T., Takahashi R., Sugimoto K., Ohta Y.

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan） 89 （ 5 ） 054703-1 - 054703-5 2020年05月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 00319015

© 2020 The Physical Society of Japan. We have synthesized good-quality powder samples of Sr7Re4O19 and found a phase transition at 206 K accompanied by some changes in both electronic properties and structure. The values and the temperature dependence of magnetic susceptibility indicate the existence of a Pauli paramagnetic state above 206 K and a nonmagnetic state below 206 K. The electrical resistivity of the sintered sample gradually increases with decreasing temperature below room temperature and sharply increases below 206 K, which is followed by insulating behavior below 160 K. In addition, the structure changes from monoclinic to triclinic at the transition. Sr7Re4O19 has a quasi-one-dimensional structure comprising the coupled zigzag chain formed by corner-shared ReO6 octahedra. Further, a long=short alternation of the Re–Re distance along the zigzag chain is observed in the low-temperature phase. Quasi-one-dimensional Fermi surfaces and band dispersions are also identified by the electronic structure calculations. These results suggest that the phase transition is a possible metal–insulator transition caused by the Peierls instability.

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Emergence of pure spin current in doped excitonic magnets

Yamamoto S., Sugimoto K., Ohta Y.

Physical Review B （Physical Review B） 101 （ 17 ） 174428 2020年05月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 24699950

© 2020 American Physical Society. ©2020 American Physical Society. An excitonic magnet hosts a condensate of spin-triplet excitons composed of conduction-band electrons and valence-band holes and may be described by the two-orbital Hubbard model. When the Hamiltonian has the nearest-neighbor interorbital hopping integrals with d-wave symmetry and the number of electrons is slightly away from half filling, the k-space spin texture appears in the excitonic phase with a broken time-reversal symmetry. We then show that, applying electric field to this doped excitonic magnet along a particular direction, a pure spin current emerges along its orthogonal direction. We discuss possible experimental realization of this type of the pure spin current in actual materials.

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Pressure-Induced Restoration of the Reversed Crystal-Field Splitting in α-Sr2CrO4

Ryo Takahashi, Tomoki Yamaguchi, Koudai Sugimoto, Touru Yamauchi, Hiroya Sakurai, Yukinori Ohta

Proceedings of the International Conference on Strongly Correlated Electron Systems (SCES2019) （Journal of the Physical Society of Japan） 2020年03月

査読有り

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Pressure-Induced Restoration of the Reversed Crystal-Field Splitting in α-Sr2CrO4

Ryo Takahashi, Tomoki Yamaguchi, Koudai Sugimoto, Touru Yamauchi, Hiroya Sakurai, Yukinori Ohta

JPS Conference Proceedings 30 011026-1 - 011026-4 2020年03月

研究論文（国際会議プロシーディングス）, 共著, 査読有り

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 Ta2NiSe5

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.