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  2. 鈴木 隆史

鈴木 隆史

スズキ タカフミ  (Takafumi Suzuki)

基本情報

所属
兵庫県立大学 大学院 工学研究科 准教授
学位
博士（工学）(2005年10月 大阪大学)
連絡先
takafumi-seng.u-hyogo.ac.jp
J-GLOBAL ID
201801021278662239
researchmap会員ID
B000310103
外部リンク

研究キーワード

 5

研究分野

 2

経歴

 2

学歴

 3

論文

 57
  • Huan-Kuang Wu, Takafumi Suzuki, Naoki Kawashima, Wei-Lin Tu
    Physical Review Research 6(2) 023297-023297 2024年6月20日  
  • Matthias Gohlke, Jose Carlos Pelayo, Takafumi Suzuki
    Physical Review B 109(22) L220410-L220410 2024年6月20日  査読有り責任著者
  • Hyun-Yong Lee, Takafumi Suzuki, Yong Baek Kim, Naoki Kawashima
    Physical Review B 104(2) 024417-024417 2021年7月13日  
  • Takafumi Suzuki, Takuto Yamada, Sei-ichiro Suga
    Physical Review B 103(22) 224425-224425 2021年6月21日  査読有り
  • Takuto Yamada, Takafumi Suzuki, Sei-ichiro Suga
    Physical Review B 102(2) 024415-024415 2020年7月13日  査読有り責任著者
  • Wei-Lin Tu, Huan-Kuang Wu, Takafumi Suzuki
    J. Phys.: Condens. Matter 32(45) 455401-455401 2020年7月7日  査読有り責任著者
  • Takafumi Suzuki, Youhei Yamaji
    Journal of the Physical Society of Japan 88(11) 115001-115001 2019年11月15日  
  • Takafumi Suzuki, Sei-ichiro Suga
    J. Phys. Soc. Jpn. 88(5) 053702 2019年4月  査読有り
  • Suzuki Takafumi, Suga Sei-ichiro
    PHYSICAL REVIEW B 98(18) 180406-180406 2018年11月  査読有り
  • Suzuki Takafumi, Suga Sei-ichiro
    AIP Advances 8(10) 101414 2018年9月  査読有り
  • Takafumi Suzuki, Youhei Yamaji
    Physica B: Condensed Matter 536 637-639 2018年5月1日  査読有り
    Temperature (T) dependence of heat capacity C(T) in the S=1/2 Kitaev honeycomb model shows a double-peak structure resulting from fractionalization of spins into two kinds of Majorana fermions. Recently it has been discussed that the double-peak structure in C(T) is also observed in magnetic ordered phases of the S=1/2 Kitaev-Heisenberg (KH) model on a honeycomb lattice when the system is located in the vicinity of the Kitaev's spin liquid phase. In addition to the S=1/2 spin case, similar double-peak structure has been confirmed in the KH honeycomb model for classical Heisenberg spins, where spin S is regarded as S→∞. We investigate spin-S dependence of C(T) for the KH honeycomb models by using thermal pure quantum state. We also perform classical Monte Carlo calculations to obtain C(T) for the classical KH model. From obtained results, we find that the origin of the high-temperature peak is different between the quantum spin case with small Ss and the classical Heisenberg spin case. Furthermore, the high-temperature peak in the quantum spin case, which is one of the clues for fractionalization of spins, disappears for S&gt 1.
  • Takafumi Suzuki, Sei-Ichiro Suga
    Physical Review B 97(13) 134424-134424 2018年4月25日  査読有り
    We use an exact numerical diagonalization method to calculate the dynamical spin structure factors of three ab initio models and one ab initio guided model for a honeycomb-lattice magnet α-RuCl3. We also use thermal pure quantum states to calculate the temperature dependence of the heat capacity, the nearest-neighbor spin-spin correlation function, and the static spin structure factor. From the results obtained from these four effective models, we find that, even when the magnetic order is stabilized at low temperature, the intensity at the Γ point in the dynamical spin structure factors increases with increasing nearest-neighbor spin correlation. In addition, we find that the four models fail to explain heat-capacity measurements whereas two of the four models succeed in explaining inelastic-neutron-scattering experiments. In the four models, when temperature decreases, the heat capacity shows a prominent peak at a high temperature where the nearest-neighbor spin-spin correlation function increases. However, the peak temperature in heat capacity is too low in comparison with that observed experimentally. To address these discrepancies, we propose an effective model that includes strong ferromagnetic Kitaev coupling, and we show that this model quantitatively reproduces both inelastic-neutron-scattering experiments and heat-capacity measurements. To further examine the adequacy of the proposed model, we calculate the field dependence of the polarized terahertz spectra, which reproduces the experimental results: the spin-gapped excitation survives up to an onset field where the magnetic order disappears and the response in the high-field region is almost linear. Based on these numerical results, we argue that the low-energy magnetic excitation in α-RuCl3 is mainly characterized by interactions such as off-diagonal interactions and weak Heisenberg interactions between nearest-neighbor pairs, rather than by the strong Kitaev interactions.
  • Takafumi Suzuki, Sei-Ichiro Suga
    Journal of Physics: Conference Series 969(1) 012123-012123 2018年4月19日  査読有り
  • Takuto Yamada, Takafumi Suzuki, Sei-ichiro Suga
    PHYSICA B-CONDENSED MATTER 525 91-93 2017年11月  査読有り
    We investigate ground state energies and low-energy excitations of the S = 1/2 Kitaev-Heisenberg model on honeycomb lattices by using dimer series expansions. We find that dimer series expansions can approach the close vicinity of the Kitaev limit, where the Heisenberg interaction is absent, in the lower order expansion than the Ising series expansion. When the system approaches the Kitaev limit, low-lying modes in the zigzag and Neel phases become flatter except for the Bragg wave numbers.
  • Youhei Yamaji, Takafumi Suzuki, Takuto Yamada, Sei-ichiro Suga, Naoki Kawashima, Masatoshi Imada
    PHYSICAL REVIEW B 93(17) 2016年1月21日  査読有り
    Contrary to the original expectation, Na$_2$IrO$_3$ is not a Kitaev's quantum spin liquid (QSL) but shows a zig-zag-type antiferromagnetic order in experiments. Here we propose experimental clues and criteria to measure how a material in hand is close to the Kitaev's QSL state. For this purpose, we systematically study thermal and spin excitations of a generalized Kitaev-Heisenberg model studied by Chaloupka $et$ $al$. in Phys. Rev. Lett. 110, 097204 (2013) and an effective ab initio Hamiltonian for Na$_2$IrO$_3$ proposed by Yamaji $et$ $al$. in Phys. Rev. Lett. 113, 107201 (2014), by employing a numerical diagonalization method. We reveal that closeness to the Kitaev's QSL is characterized by the following properties, besides trivial criteria such as reduction of magnetic ordered moments and Neel temperatures: (1) Two peaks in the temperature dependence of specific heat at $T_{\ell}$ and $T_h$ caused by the fractionalization of spin to two types of Majorana fermions. (2) In between the double peak, prominent plateau or shoulder pinned at $(R/2)\ln 2$ in the temperature dependence of entropy, where $R$ is the gas constant. (3) Failure of the linear spin wave approximation at the low-lying excitations of dynamical structure factors. (4) Small ratio $T_{\ell}/T_h$ close to or less than 0.03. According to the proposed criteria, Na$_2$IrO$_3$ is categorized to a compound close to the Kitaev's QSL, and is proven to be a promising candidate for the realization of the QSL if the relevant material parameters can further be tuned by making thin film of Na$_2$IrO$_3$ on various substrates or applying axial pressure perpendicular to the honeycomb networks of iridium ions. Applications of these characterization to (Na$_{1-x}$Li$_x$)$_2$IrO$_3$ and other related materials are also discussed.
  • Takafumi Suzuki, Takuto Yamada, Youhei Yamaji, Sei-ichiro Suga
    PHYSICAL REVIEW B 92(18) 2015年11月  査読有り
    We investigate the dynamical properties of Na2IrO3. For five effective models proposed for Na2IrO3, we numerically calculate dynamical structure factors (DSFs) with an exact diagonalization method. An effective model obtained from ab initio calculations explains inelastic neutron scattering experiments adequately. We further calculate excitation modes based on linearized spin-wave theory. The spin-wave excitation of the effective models obtained by ab initio calculations disagrees with the low-lying excitation of DSFs. We attribute this discrepancy to the location of Na2IrO3 in a parameter space close to the phase boundary with the Kitaev spin-liquid phase.
  • Takafumi Suzuki, Kenji Harada, Haruhiko Matsuo, Synge Todo, Naoki Kawashima
    PHYSICAL REVIEW B 91(9) 094414 2015年3月  査読有り
    We investigate thermal phase transitions to a valence-bond solid phase in SU(N) Heisenberg models with four-or six-body interactions on a square or honeycomb lattice, respectively. In both cases, a thermal phase transition occurs that is accompanied by rotational symmetry breaking of the lattice. We perform quantum Monte Carlo calculations in order to clarify the critical properties of the models. The estimated critical exponents indicate that the universality classes of the square- and honeycomb-lattice cases are identical to those of the classical XY model with a Z(4) symmetry-breaking field and the three-state Potts model, respectively. In the square-lattice case, the thermal exponent, nu, monotonically increases as the system approaches the quantum critical point, while the values of the critical exponents, eta and gamma/nu, remain constant. From a finite-size scaling analysis, we find that the system exhibits weak universality, because the Z(4) symmetry-breaking field is always marginal. In contrast,. in the honeycomb-lattice case exhibits a constant value, even in the vicinity of the quantum critical point, because the Z(3) field remains relevant in the SU(3) and SU(4) cases.
  • Takafumi Suzuki, Kenji Harada, Haruhiko Matsuo, Synge Todo, Naoki Kawashima
    INTERNATIONAL CONFERENCE ON STRONGLY CORRELATED ELECTRON SYSTEMS 2014 (SCES2014) 592 2015年  査読有り
    We study thermal transitions of the generalized SU(N) Heisenberg models with four-body interactions on a square lattice and with six-body interactions on a honeycomb lattice. In both models for the N=3 and 4 cases, a singlet-dimer state is stabilized at a very low temperature, where a rotational symmetry of lattice is broken spontaneously. We discuss the universality class of thermal transition to the singlet dimer phases, performing quantum Monte Carlo calculations. From the finite-size scaling analysis, we find that the criticality for the square lattice case is well explained by the 2D weak Ising universality, while the 2D three-state Potts universality is observed in the honeycomb lattice case.
  • Akiko Masaki-Kato, Takafumi Suzuki, Kenji Harada, Synge Todo, Naoki Kawashima
    PHYSICAL REVIEW LETTERS 112(14) 2014年4月  査読有り
    Based on the worm algorithm in the path-integral representation, we propose a general quantum Monte Carlo algorithm suitable for parallelizing on a distributed-memory computer by domain decomposition. Of particular importance is its application to large lattice systems of bosons and spins. A large number of worms are introduced and its population is controlled by a fictitious transverse field. For a benchmark, we study the size dependence of the Bose-condensation order parameter of the hard-core Bose-Hubbard model with L x L x beta t = 10240 x 10240 x 16, using 3200 computing cores, which shows good parallelization efficiency.
  • Kenji Harada, Takafumi Suzuki, Tsuyoshi Okubo, Haruhiko Matsuo, Jie Lou, Hiroshi Watanabe, Synge Todo, Naoki Kawashima
    Physical Review B - Condensed Matter and Materials Physics 88(22) 2013年12月13日  査読有り
    To examine the validity of the scenario of the deconfined critical phenomena, we carry out a quantum Monte Carlo simulation for the SU(N) generalization of the Heisenberg model with four-body and six-body interactions. The quantum phase transition between the SU(N) Néel and valence-bond solid phases is characterized for N=2, 3, and 4 on the square and honeycomb lattices. While finite-size scaling analysis works well up to the maximum lattice size (L=256) and indicates the continuous nature of the phase transition, a clear systematic change towards the first-order transition is observed in the estimates of the critical exponent y≡1/ν as the system size increases. We also confirm the relevance of a squared valence-bond solid field Ψ2 for the SU(3) model. © 2013 American Physical Society.
  • Takahiro Ohgoe, Takafumi Suzuki, Naoki Kawashima
    JOURNAL OF LOW TEMPERATURE PHYSICS 171(3-4) 309-314 2013年5月  査読有り
    By performing exact quantum Monte Carlo simulations, we study the momentum distribution of the supersolid state in the two-dimensional extended Bose-Hubbard model with the nearest-neighbor repulsion. For strong nearest-neighbor repulsions, the supersolid state is stable in a broad region up to large hopping amplitudes and high particle densities. In the supersolid state, the momentum distribution shows a bimodal structure with two peaks related to the superfluidity and solidity respectively. By our calculations, we show that the bimodal structure becomes clearer as the chemical potential (or the particle density) is increased.
  • Toshiya Hikihara, Takafumi Suzuki
    Physical Review A - Atomic, Molecular, and Optical Physics 87(4) 042337 2013年4月29日  査読有り
    We investigate entanglement generation in one-dimensional quantum spin systems with the sinusoidal deformation. In the system, the energy scale of each local term in the Hamiltonian is modified according to a position-dependent function sinα[πN(x-12)], where x is the position of the local term and N is the length of the system. We show that at zero temperature the system with α≥2 is able to generate a sizable entanglement between two spins at open edges, even when the two spins are infinitely far apart. This long-distance entanglement is rather robust against thermal fluctuations and survives up to a temperature that decays with the system size slowly, in an algebraic form. © 2013 American Physical Society.
  • Takafumi Suzuki, Masahiro Sato
    PHYSICAL REVIEW B 86(22) 224411 2012年12月  査読有り
    We study the nature of edge states in extrinsically and spontaneously dimerized states of two-dimensional spin-1/2 antiferromagnets, by performing quantum Monte Carlo simulation. We show that a gapless edge mode emerges in the wide region of the dimerized phases, and the critical exponent of spin correlators along the edge deviates from the value of Tomonaga-Luttinger liquid (TLL) universality in large but finite systems at low temperatures. We also demonstrate that the gapless nature at edges is stable against several perturbations such as external magnetic field, easy-plane XXZ anisotropy, Dzyaloshinskii-Moriya interaction, and further-neighbor exchange interactions. The edge states exhibit non-TLL behavior, depending strongly on model parameters and kinds of perturbations. Possible ways of detecting these edge states are discussed. Properties of edge states we show in this paper could also be used as reference points to study other edge states of more exotic gapped magnetic phases such as spin liquids. DOI: 10.1103/PhysRevB.86.224411
  • Takahiro Ohgoe, Takafumi Suzuki, Naoki Kawashima
    PHYSICAL REVIEW A 86(6) 063635 2012年12月  査読有り
    By using an unbiased quantum Monte Carlo method, we investigate the hard-core Bose-Hubbard model on a square lattice with the anisotropic dipole-dipole interaction. To study the effect of the anisotropy, dipole moments are assumed to be polarized in the y direction on the two-dimensional xy plane. To perform efficient simulations of long-range interacting systems, we use the worm algorithm with an O(N) Monte Carlo method. We obtain the ground-state phase diagram that includes a superfluid phase and a striped solid phase at half-filling as two main phases. In addition to these two main phases, we find a small region where there are multiple plateaus in the particle density for small hopping amplitudes. In this region, the number of plateaus increases as the system size increases. This indicates the appearance of numerous competing ground states due to frustrated interactions. In our simulations, we find no evidence of a supersolid phase. DOI: 10.1103/PhysRevA.86.063635
  • Takahiro Ohgoe, Takafumi Suzuki, Naoki Kawashima
    PHYSICAL REVIEW B 86(5) 054520 2012年8月  査読有り
    We investigate the ground-state phase diagram of the soft-core Bose-Hubbard model with the nearest-neighbor repulsion on a square lattice by using an unbiased quantum Monte Carlo method. In contrast to a previous study [P. Sengupta et al., Phys. Rev. Lett. 94, 207202 (2005)], we present the ground-state phase diagrams up to high hopping amplitudes. As a result, in addition to the known supersolid above half-filling, we find a supersolid phase below and at half-filling for high hopping amplitudes. In addition, for a strong nearest-neighbor repulsion, we show that the supersolid phase occupies a remarkably broad region in the phase diagram. These results are in agreement with the results of the Gutzwiller mean-field approximation [M. Iskin, Phys. Rev. A 83, 051606(R) (2011); T. Kimura, Phys. Rev. A 84, 063630 (2011)]. However, it turns out that the regions of the supersolid phases are significantly smaller than the mean-field results.
  • Takahiro Ohgoe, Takafumi Suzuki, Naoki Kawashima
    PHYSICAL REVIEW LETTERS 108(18) 185302 2012年5月  査読有り
    Using an unbiased quantum Monte Carlo method, we obtain convincing evidence of the existence of a checkerboard supersolid at a commensurate filling factor 1/2 (a commensurate supersolid) in the soft-core Bose-Hubbard model with nearest-neighbor repulsions on a cubic lattice. In conventional cases, supersolids are realized at incommensurate filling factors by a doped-defect-condensation mechanism, where particles (holes) doped into a perfect crystal act as interstitials (vacancies) and delocalize in the crystal order. However, in the model, a supersolid state is stabilized even at the commensurate filling factor 1/2 without doping. By performing grand canonical simulations, we obtain a ground-state phase diagram that suggests the existence of a supersolid at a commensurate filling. To obtain direct evidence of the commensurate supersolid, we next perform simulations in canonical ensembles at a particle density rho = 1/2 and exclude the possibility of phase separation. From the obtained snapshots, we discuss its microscopic structure and observe that interstitial-vacancy pairs are unbound in the crystal order.
  • Toshihiro Sato, Yasuyuki Kato, Takafumi Suzuki, Naoki Kawashima
    PHYSICAL REVIEW E 85(5) 050105 2012年5月  査読有り
    We examine the validity of the projected Gross-Pitaevskii simulation by taking the two-dimensional homogeneous bosonic system as an example. The long-distance behaviors of the correlation function and equilibrium temperatures show good agreement with those of the quantum Monte Carlo calculations below temperatures near the Kosterlitz-Thouless transition. We find that in the projected Gross-Pitaevskii description, one needs to estimate the optimal wave-number cutoff in temperature. In the well-described region, the projected Gross-Pitaevskii equation presents reliable predictions for the long-wave bosonic components.
  • Takahiro Ohgoe, Takafumi Suzuki, Naoki Kawashima
    26TH INTERNATIONAL CONFERENCE ON LOW TEMPERATURE PHYSICS (LT26), PTS 1-5 400 2012年  査読有り
    We study the checkerboard supersolid of hard-core bosonic polar molecules in a 2D square lattice. This supersolid shows a novel double-peak structure in the momentum distribution of bosons. The double peaks indicate coexistence of superfluidity and solidility. The corresponding peaks can be measured by the time-of-flight experiment and will become a clear evidence for the supersolid state. By exact quantum Monte Carlo calculations, we reveal temperature dependence of the momentum distribution in the supersolid phase. As a result, we observe successive developments of the two peaks.
  • Shunsuke C. Furuya, Takafumi Suzuki, Shintaro Takayoshi, Yoshitaka Maeda, Masaki Oshikawa
    Physical Review B - Condensed Matter and Materials Physics 84(18) 2011年11月21日  査読有り
    We consider spin-1 Haldane chains with single-ion anisotropy, which exists in known Haldane chain materials. We develop a perturbation theory in terms of anisotropy, where the magnon-magnon interaction is important even in the low-temperature limit. The exact two-particle form factor in the O(3) nonlinear sigma model leads to quantitative predictions on several dynamical properties, including the dynamical structure factor and electron-spin-resonance frequency shift. These agree very well with numerical results, and with experimental data on the Haldane chain material Ni(C5H14N2) 2N3(PF6). © 2011 American Physical Society.
  • Takahiro Ohgoe, Takafumi Suzuki, Naoki Kawashima
    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 80(11) 113001 2011年11月  査読有り
    We study the checkerboard supersolid of the hard-core Bose-Hubbard model with the dipole-dipole interaction. This supersolid is different from all other supersolids found in lattice models in the sense that superflow paths through which interstitials or vacancies can hop freely are absent in the crystal. By focusing on repulsive interactions between interstitials, we reveal that the long-range tail of the dipole-dipole interaction has the role of increasing the energy cost of domain wall formations. This effect produces the supersolid by the second-order hopping process of defects. We also perform exact quantum Monte Carlo simulations and observe a novel double peak structure in the momentum distribution of bosons, which is clear evidence of a supersolid. This can be measured by the time-of-flight experiment in optical lattice systems.
  • Shunsuke C. Furuya, Takafumi Suzuki, Shintaro Takayoshi, Yoshitaka Maeda, Masaki Oshikawa
    PHYSICAL REVIEW B 84(18) 180410 2011年11月  査読有り
    We consider spin-1 Haldane chains with single-ion anisotropy, which exists in known Haldane chain materials. We develop a perturbation theory in terms of anisotropy, where the magnon-magnon interaction is important even in the low-temperature limit. The exact two-particle form factor in the O(3) nonlinear sigma model leads to quantitative predictions on several dynamical properties, including the dynamical structure factor and electron-spin-resonance frequency shift. These agree very well with numerical results, and with experimental data on the Haldane chain material Ni(C5H14N2)(2)N-3(PF6).
  • Takafumi Suzuki, Yusuke Tomita, Naoki Kawashima, Pinaki Sengupta
    INTERNATIONAL CONFERENCE ON FRUSTRATION IN CONDENSED MATTER (ICFCM) 320 2011年  査読有り
    We study the finite-temperature transition to the 1/2 magnetization plateau in a model of interacting S = 1/2 spins with longer range interactions and strong exchange anisotropy on the geometrically frustrated Shastry-Sutherland lattice. In previous studies, it was obtained from Monte Carlo calculations that the transition to the plateau state occurs via two successive transitions with the two-dimensional Ising universality class, when the quantum exchange interactions are finite, while a single phase transition takes place in the purely Ising limit[1]. To understand these behaviors, we introduce the generalized four-state chiral clock model and perform Monte Carlo calculations for this model. By comparing the phase diagrams of the two models, we find that the topology of the thermal phase diagram is the same each other - the critically of the thermal phase transition in the original model can be well explained by that of the generalized four-state chiral clock model.
  • T. Suzuki, Y. Tomita, N. Kawashima
    PHYSICAL REVIEW B 82(21) 214404 2010年12月  査読有り
    We study the finite-temperature transition to the m=1/2 magnetization plateau in a model of interacting S=1/2 spins with longer-range interactions and strong exchange anisotropy on the geometrically frustrated Shastry-Sutherland lattice. This model was shown to capture the qualitative features of the field-induced magnetization plateaus in the rare-earth tetraboride, TmB4. Our results show that the transition to the plateau state occurs via two successive transitions with the two-dimensional Ising universality class, when the quantum exchange interactions are finite, whereas a single phase transition takes place in the purely Ising limit. To better understand these behaviors, we perform Monte Carlo simulations of the classical generalized four-state chiral clock model and compare the phase diagrams of the two models. Finally, we estimate a parameter set that can explain the magnetization curves observed in TmB4. The magnetic properties and critical behavior of the finite-temperature transition to the m=1/2 plateau state are also discussed.
  • Toshihiro Sato, Takafumi Suzuki, Naoki Kawashima
    PHYSICAL REVIEW A 81(2) 025601 2010年2月  査読有り
  • Takafumi Suzuki, Yusuke Tomita, Naoki Kawashima
    INTERNATIONAL CONFERENCE ON MAGNETISM (ICM 2009) 200 2010年  査読有り
    We study magnetic properties of the S=1/2 Ising-like XXZ model on the Shastry-Sutherland lattices considering the effect of long range interactions. By performing quantum Monte Carlo simulations, we find that magnetization plateau phases appear at one-half and one-third of the saturation magnetization. We also study the finite temperature transition to the magnetic plateau phases and discuss the universality class of the transition.
  • Takahumi Suzuki, Yusuke Tomita, Naoki Kawashima
    PHYSICAL REVIEW B 80(18) 180405 2009年11月  査読有り
    We study magnetic properties of the spin-1/2 Ising-like XXZ model on the Shastry-Sutherland lattices with long-range interactions, using the quantum Monte Carlo method. This model shows magnetization plateau phases at one-half and one-third of the saturation magnetization when additional couplings are considered. We investigate the finite temperature transition to one-half and one-third plateau phases. The obtained results suggest that the former case is of the first order and the latter case is of the second order. We also find that the system undergoes two successive transitions with the two-dimensional Ising model universality although there is a single phase transition in the Ising limit case. Finally, we estimate the coupling ratio to explain the magnetization process observed in TmB(4).
  • Toshihiro Sato, Takafumi Suzuki, Naoki Kawashima
    25TH INTERNATIONAL CONFERENCE ON LOW TEMPERATURE PHYSICS (LT25), PART 3 150 2009年  査読有り
    We study the finite-temperature transition of the quasi-2D Bose gas in an uniaxially-compressed harmonic trap by numerically solving the projected Gross-Pitaevskii equation. Gradual emergence of superfluidity is confirmed by calculating the moment of inertia when a temperature decreases. By investigating the long-distance behavior of a phase correlation function, superfluid density gradually increases reflecting the development of the phase correlation around the center of the system. From these results, we obtain the evidence for the emergence of superfluidity in this system directly.
  • Takafumi Suzuki, Naoki Kawashima, Kouichi Okunishi
    25TH INTERNATIONAL CONFERENCE ON LOW TEMPERATURE PHYSICS (LT25), PART 4 150 2009年  査読有り
    We study the ordered state of the quasi-one dimensional S = 1/2 XXZ spin model in magnetic fields by using quantum Monte Carlo simulations based on the directed-loop algorithm. The H-T phase diagram is illustrated and we find that an incommensurate long-range-ordered state is stable in a certain low-magnetized region. The result of the finite-size scaling analysis indicates that the finite temperature transition to the incommensurate order is of the first order.
  • Kouichi Okunishi, Takahumi Suzuki
    PHYSICAL REVIEW B 76(22) 224411 2007年12月  査読有り
    We investigate phase transitions of the quasi-one-dimensional S=1/2 XXZ model in a magnetic field, using bosonization combined with a mean-field treatment of the interchain interaction. We then find that the field-induced incommensurate order is certainly realized in the low-field region, while transverse staggered order appears in the high-field region. On the basis of the result, we discuss the field-induced phase transition recently observed for BaCo2V2O8.
  • Takafumi Suzuki, Naoki Kawashima, Kohichi Okunishi
    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 76(12) 123707 2007年12月  査読有り
    We investigate the ordered state in S = 1/2 XXZ spin chains with weak interchain couplings in a magnetic field by using quantum Monte Carlo simulations based on the directed-loop algorithm. The HT phase diagram is illustrated for a square lattice and we find that an exotic spin-liquid state with quasi long-range incommensurate correlations in both the spin-chain and interchain directions may appear in the low-magnetization region without a finite-temperature transition. We also show that the incommensurate long-range-ordered state is stable for a cubic lattice.
  • M. Hagiwara, H. Tsujit, C. R. Rotundu, B. Andraka, Y. Takano, T. Suzuki, S. Suga
    MODERN PHYSICS LETTERS B 21(16) 965-976 2007年7月  査読有り
    We review the results of specific-heat experiments on the S = 1 quasi-one-dimensional (quasi-1D) bond-alternating antiferromagnet Ni(C9H24N4)(NO2)ClO4, alias NTENP. At low temperatures above the transition temperature of a field-induced long-range order, the magnetic specific heat (C-mag) of this compound becomes proportional to temperature (T), when a magnetic field along the spin chains exceeds the critical field H-c at which the energy gap vanishes. The ratio C-mag / T, which increases as the magnetic field approaches H-c from above, is in good quantitative agreement with a prediction of conformal field theory combined with the field-dependent velocity of the excitations calculated by the Lanczos method. This result is the first conclusive evidence for a Tomonaga-Luttinger liquid in a gapped quasi-1D antiferromagnet.
  • Yasuyuki Kato, Takafumi Suzuki, Naoki Kawashima
    PHYSICAL REVIEW E 75(6) 066703 2007年6月  査読有り
    We modify the directed-loop algorithm (DLA) to solve the problem that typically arises from large on-site interaction. The large on-site interaction is inevitable when one tries to simulate a Bose gas system in continuum by discretizing the space with small lattice spacings. While the efficiency of a straightforward application of DLA decreases as the mesh becomes finer, the performance of the new method does not depend on it except for the trivial factor due to the increase in the number of lattice points.
  • Takahumi Suzuki, Naoki Kawashima
    PHYSICAL REVIEW B 75(18) 180502 2007年5月  査読有り
    We investigate a supersolid state in hardcore boson models on the face-centered-cubic (fcc) lattice. The supersolid state is characterized by a coexistence of crystalline order and superfluidity. Using a quantum Monte Carlo method based on the directed-loop algorithm, we calculate static structure factors and superfluid density at finite temperature, from which we obtain the phase diagram. The supersolid phase exists at intermediate fillings between a three-quarter-filled solid phase and a half-filled solid phase. We also discuss the mechanism of the supersolid state on the fcc lattice.
  • Sei-Ichiro Suga, Takahumi Suzuki
    JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS 310(2) E381-E383 2007年3月  査読有り
    We investigate NMR relaxation rates 1/T-1 of gapped quantum spin chains in magnetic fields. Universal properties for the divergence behavior of 1/T-1 with decreasing temperature are studied in a viewpoint of the Tomonaga-Luttinger liquid. On the basis of the results, 1/T-1 for some typical gapped spin chains in the magnetic field is discussed in connection with experiments. (c) 2006 Elsevier B.V. All rights reserved.
  • Takafumi Suzuki, Sei-Ichiro Suga
    JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS 310(2) 1197-1199 2007年3月  査読有り
    We calculate dynamical structure factors of the S = 1 bond-alternating spin chain in transverse magnetic fields. The numerical results successfully explain the field dependence of the peaks observed in recent inelastic neutron-scattering experiments on NTENP. (c) 2006 Elsevier B.V. All rights reserved.
  • Takahumi Suzuki, Sei-ichiro Suga
    PHYSICAL REVIEW B 74(17) 172410 2006年11月  査読有り
    We investigate NMR relaxation rates 1/T-1 of quantum spin chains in magnetic fields. Universal properties for the divergence behavior of 1/T-1 are obtained in the Tomonaga-Luttinger-liquid state. The results are discussed in comparison with experimental results.
  • M Hagiwara, H Tsujii, CR Rotundu, B Andraka, Y Takano, N Tateiwa, TC Kobayashi, T Suzuki, S Suga
    PHYSICAL REVIEW LETTERS 96(14) 147203-147203 2006年4月  査読有り
    Specific-heat experiments on single crystals of the S=1 quasi-one-dimensional bond-alternating antiferromagnet Ni(C9H24N4)(NO2)ClO4 (NTENP) have been performed in magnetic fields applied both parallel and perpendicular to the spin chains. We have found for the parallel field configuration that the magnetic specific heat (C-mag) is proportional to temperature (T) above a critical field H-c, at which the energy gap vanishes, in a temperature region above that of the long-range ordered state. The ratio C-mag/T increases as the magnetic field approaches H-c from above. The data are in good quantitative agreement with the prediction of the c=1 conformal field theory in conjunction with the velocity of the excitations calculated by a numerical diagonalization, providing conclusive evidence for a Tomonaga-Luttinger liquid.
  • T. Suzuki, S. Suga
    YAMADA CONFERENCE LX ON RESEARCH IN HIGH MAGNETIC FIELDS 51(51) 175-+ 2006年  査読有り
    We investigate dynamical properties of the half- magnetization plateau in S=1 bond-alternating spin chains. Dynamical structure factors calculated by the exact diagonalization method are compared with the dispersion curves of low-lying excitations obtained by a cluster expansion technique. We find that in the half-magnetization plateau the quintet excitation forms the lowest-lying excitation. The field dependence of the critical exponents of the spin correlation functions are further investigated. On the basis of the results, characteristics of the half- magnetization plateau are discussed.
  • T Suzuki, S Suga
    PHYSICS LETTERS A 343(6) 462-468 2005年8月  査読有り
    Dynamical structure factors of the S = 1 bond-afternating spin chains in the dimer phase are calculated at finite temperature, using the pair dynamical correlated-effective-field approximation. At T = 0, the delta-function-type peak of the one-magnon mode appears. When temperature is increased, such a sharp peak is broadened and the additional weak peak caused by the excitation from the triplet state to the quintet state emerges in the higher energy region. The results are discussed in comparison with those obtained by the exact diagonalization method. (c) 2005 Elsevier B.V. All rights reserved.
  • T Suzuki, S Suga
    PHYSICAL REVIEW B 72(1) 014434 2005年7月  査読有り
    We calculate dynamical structure factors of the S=1 bond-alternating Heisenberg chain with a single-ion anisotropy in transverse magnetic fields, using a continued fraction method based on the Lanczos algorithm. In the Haldane-gap phase and the dimer phase, dynamical structure factors show characteristic field dependence. Possible interpretations are discussed in the viewpoint of the field dependence of the excitation continuum. The numerical results are in qualitative agreement with recent results for inelastic neutron-scattering experiments on the S=1 bond-alternating Heisenberg-chain compound Ni(C9D24N4)(NO2)ClO4 and the S=1 Haldane-gap compound Ni(C5D14N2)(2)N-3(PF6) in transverse magnetic fields.

MISC

 20
  • Youhei Yamaji, Takafumi Suzuki, Mitsuaki Kawamura
    2018年2月8日  
    A numerical algorithm to calculate exact finite-temperature spectra of<br /> many-body lattice Hamiltonians is formulated by combining the typicality<br /> approach and the shifted Krylov subspace method. The combined algorithm, which<br /> we name finite-temperature shifted Krylov subspace method for simulating<br /> spectra (FTK$\omega$), efficiently reproduces the canonical-ensemble<br /> probability distribution at finite temperatures with the computational cost<br /> proportional to the Fock space dimension. The present FTK$\omega$ enables us to<br /> exactly calculate finite-temperature spectra of many-body systems whose system<br /> sizes are twice larger than those handled by the canonical ensemble average and<br /> allows us to access the frequency domain without sequential real-time evolution<br /> often used in previous studies. By employing the reweighting method with the<br /> present algorithm, we obtain significant reduction of the numerical costs for<br /> temperature sweeps. Application to the Kiteav-Heisenberg model (KHM) on a<br /> honeycomb lattice demonstrates the capability of the FTK$\omega$. The KHM shows<br /> quantum phase transitions from the quantum spin liquid (QSL) phase to<br /> magnetically ordered phases when the finite Heisenberg exchange coupling is<br /> introduced. We examine temperature dependence of dynamical spin structure<br /> factors of the KHM in proximity to the QSL. It is clarified that the crossover<br /> from a spin-excitation continuum, which is a characteristic of the QSL, to a<br /> damped high-energy magnon mode occurs at temperatures higher than the energy<br /> scale of the Heisenberg couplings or the spin gap that is a signature of the<br /> QSL at zero temperature. The crossover and the closeness to the Kitaev&#039;s QSL<br /> are quantitatively measured by the width of the excitation continuum or the<br /> magnon spectrum. The present results shed new light on analysis of neutron<br /> scattering and other spectroscopy measurements on QSL candidates.
  • Takafumi Suzuki, Masahiro Sato
    2015年5月25日  
    We study the boundary nature of trapped bosonic Mott insulators in optical<br /> square lattices, by performing quantum Monte Carlo simulation. We show that a<br /> finite superfluid density generally emerges in the incommensurate-filling (IC)<br /> boundary region around the bulk Mott state, irrespectively of the width of the<br /> IC region. Both off-diagonal and density correlation functions in the IC<br /> boundary region exhibit a nearly power-law decay. The power-law behavior and<br /> superfluidity are well developed below a characteristic temperature. These<br /> results indicate that a gapless boundary mode always emerges in any atomic Mott<br /> insulators on optical lattices. This further implies that if we consider a<br /> topological insulating state in Bose or Fermi atomic systems, its boundary<br /> possesses at least two gapless modes (or coupled modes) of an above IC edge<br /> state and the intrinsic topologically-protected edge state.
  • 大久保 毅, 原田 健自, 鈴木 隆史, 松尾 春彦, 渡辺 宙志, 藤堂 眞治, 川島 直輝
    日本物理学会講演概要集 69.1.2 333 2014年  
  • 原田 健自, 鈴木 隆史, 松尾 春彦, 大久保 毅, 正木 晶子, 渡辺 宙志, 藤堂 眞治, 川島 直輝
    日本物理学会講演概要集 68.1.2 404 2013年  
  • 原田 健自, 鈴木 隆史, 大久保 毅, 松尾 春彦, 渡辺 宙志, 藤堂 眞治, 川島 直輝
    日本物理学会講演概要集 68.2.2 217 2013年  
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