2021/12/12 更新

ニシヤマ　ヨシヒロ

NISHIYAMA Yoshihiro

学位

• 博士(理学) （ 東京大学 ）

• 相転移 多体問題

• 相転移 多体問題

研究分野

• 自然科学一般 / 数理物理、物性基礎

• 日本物理学会

論文

• Yoshihiro Nishiyama

The European Physical Journal B   94 ( 11 )   2021年11月

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掲載種別：研究論文（学術雑誌）   出版者・発行元：Springer Science and Business Media LLC

• Yoshihiro Nishiyama

Journal of Statistical Mechanics: Theory and Experiment   2021 ( 3 )   033103 - 033103   2021年3月

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掲載種別：研究論文（学術雑誌）   出版者・発行元：IOP Publishing

• Imaginary-field-driven phase transition for the 2D Ising antiferromagnet: A fidelity-susceptibility approach

Yoshihiro Nishiyama

Physica A: Statistical Mechanics and its Applications   555   124731 - 124731   2020年10月

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掲載種別：研究論文（学術雑誌）   出版者・発行元：Elsevier BV

• Yoshihiro Nishiyama

The European Physical Journal B   93 ( 9 )   2020年9月

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掲載種別：研究論文（学術雑誌）   出版者・発行元：Springer Science and Business Media LLC

• Criticality of the Higgs mass for the long-range quantum XY chain: Amplitude ratio between the Higgs and paramagnetic gaps

Yoshihiro Nishiyama

Physica A: Statistical Mechanics and its Applications   527   121395 - 121395   2019年8月

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掲載種別：研究論文（学術雑誌）   出版者・発行元：Elsevier BV

• Yoshihiro Nishiyama

The European Physical Journal B   92 ( 8 )   2019年8月

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掲載種別：研究論文（学術雑誌）   出版者・発行元：Springer Science and Business Media LLC

• Yoshihiro Nishiyama

The European Physical Journal B   91 ( 11 )   2018年11月

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掲載種別：研究論文（学術雑誌）   出版者・発行元：Springer Science and Business Media LLC

• Yoshihiro Nishiyama

The European Physical Journal B   91 ( 4 )   2018年4月

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掲載種別：研究論文（学術雑誌）   出版者・発行元：Springer Science and Business Media LLC

• Yoshihiro Nishiyama

The European Physical Journal B   90 ( 9 )   2017年9月

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掲載種別：研究論文（学術雑誌）   出版者・発行元：Springer Science and Business Media LLC

• Hierarchy of the low-lying excitations for the (2+1)-dimensional q=3 Potts model in the ordered phase

Yoshihiro Nishiyama

Nuclear Physics B   916   28 - 36   2017年3月

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掲載種別：研究論文（学術雑誌）   出版者・発行元：Elsevier BV

• (2+1)次元イジング模型のマグノン束縛状態のスペクトル系列

西山 由弘

日本物理学会講演概要集   71   2641 - 2641   2016年

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記述言語：日本語   出版者・発行元：一般社団法人 日本物理学会

<p>2次元横磁場イジング模型を、厳密対角化の方法でシミュレーションする。秩序相では、マグノン間には引力がはたらく。臨界点近傍で、普遍的なマグノンの束縛状態の質量を見つもる。</p>

MISC

• Magnon-bound-state hierarchy for the two-dimensional transverse-field Ising model in the ordered phase

Yoshihiro Nishiyama

PHYSICA A-STATISTICAL MECHANICS AND ITS APPLICATIONS   463   303 - 309   2016年12月

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記述言語：英語   出版者・発行元：ELSEVIER SCIENCE BV

In the ordered phase for an Ising ferromagnet, the magnons are attractive to form a series of bound states with the mass gaps, m(2) &lt; m(3) &lt; .... Each ratio m(2,3),.../m(1) (m(1): the single-magnon mass) is expected to be a universal constant in the vicinity of the critical point. In this paper, we devote ourselves to the (2 + 1)-dimensional counterpart, for which the universal hierarchical character remains unclear. We employed the exact diagonalization method, which enables us to calculate the dynamical susceptibility via the continued-fraction expansion. Thereby, we observe a variety of signals including m(2,3,4), and the spectrum is analyzed with the finite-size-scaling method to estimate the universal mass-gap ratios. (C) 2016 Elsevier B.V. All rights reserved.

• Universal scaled Higgs-mass gap for the bilayer Heisenberg model in the ordered phase

Yoshihiro Nishiyama

EUROPEAN PHYSICAL JOURNAL B   89 ( 2 )   1 - 5   2016年2月

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記述言語：英語   出版者・発行元：SPRINGER

The spectral properties for the bilayer quantum Heisenberg model were investigated with the numerical diagonalization method. In the ordered phase, there appears the massive Higgs excitation embedded in the continuum of the Goldstone excitations. Recently, it was claimed that the properly scaled Higgs mass is a universal constant in proximity to the critical point. Diagonalizing the finite-size cluster with N = 36 spins, we calculated the dynamical scalar susceptibility. chi(s)''(omega), which is rather insensitive to the Goldstone mode. A finite-size-scaling analysis of chi(s)'' (omega) is made, and the universal (properly scaled) Higgs mass is estimated.

• Universal critical behavior of the two-magnon-bound-state mass gap for the (2+1)-dimensional Ising model

Yoshihiro Nishiyama

PHYSICA A-STATISTICAL MECHANICS AND ITS APPLICATIONS   413   577 - 582   2014年11月

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記述言語：英語   出版者・発行元：ELSEVIER SCIENCE BV

The two-magnon-bound-state mass gap m(2) for the two-dimensional quantum Ising model was investigated by means of the numerical diagonalization method; the low-lying spectrum is directly accessible via the numerical diagonalization method. It has been claimed that the ratio m(2)/m(1) (m(1): one-magnon mass gap) is a universal constant in the vicinity of the critical point. Aiming to suppress corrections to scaling (lattice artifact), we consider the spin-S = 1 Ising model with finely-adjusted extended interactions. The simulation result for the finite-size cluster with N &lt;= 20 spins indicates the mass-gap ratio m(2)/m(1) = 1.84(1). (C) 2014 Elsevier B.V. All rights reserved.

• 10aAY-13 (2+1)次元イジング模型の素励起対のユニバーサルな束縛エネルギー(10aAY 古典スピン系一般,領域11(物性基礎論・統計力学・流体物理・応用数学・社会経済物理))

西山 由弘

日本物理学会講演概要集   69 ( 2 )   183 - 183   2014年8月

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記述言語：日本語   出版者・発行元：一般社団法人日本物理学会

• 28pAK-13 空間異方的量子磁性体における脱閉じ込め臨界性のfidelity感受率による解析(28pAK 量子スピン系・ボソン系・冷却原子系,領域11(物性基礎論・統計力学・流体物理・応用数学・社会経済物理))

西山 由弘

日本物理学会講演概要集   69 ( 1 )   333 - 333   2014年3月

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記述言語：日本語   出版者・発行元：一般社団法人日本物理学会

• Critical behavior of the fidelity susceptibility for the d=2 transverse-field Ising model

Yoshihiro Nishiyama

PHYSICA A-STATISTICAL MECHANICS AND ITS APPLICATIONS   392 ( 19 )   4345 - 4350   2013年10月

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記述言語：英語   出版者・発行元：ELSEVIER SCIENCE BV

The overlap (inner product) between the ground-state eigenvectors with proximate interaction parameters, the so-called fidelity, plays a significant role in the quantum-information theory. In this paper, the critical behavior of the fidelity susceptibility is investigated for a two-dimensional transverse-field (quantum) Ising model by means of the numerical diagonalization method. In order to treat a variety of system sizes N = 12, 14, ... , 32, we adopt the screw-boundary condition. Finite-size artifacts (scaling corrections) of the fidelity susceptibility appear to be suppressed, as compared to those of the Binder parameter. As a result, we estimate the fidelity susceptibility critical exponent as alpha(F) = 0.715(20). (C) 2013 Elsevier B.V. All rights reserved.

• 27pKG-12 二次元横磁場イジング模型の縦及び横成分fidelity感受率の臨界性(古典フラストレート系・古典スピン系一般,領域11(統計力学,物性基礎論,応用数学,力学,流体物理))

西山 由弘

日本物理学会講演概要集   68 ( 2 )   260 - 260   2013年8月

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記述言語：日本語   出版者・発行元：一般社団法人日本物理学会

• 29pXZC-9 二次元横磁場イジング模型のfidelity感受率の臨界性(29pXZC 古典フラストレート系・古典スピン系一般,領域11(統計力学,物性基礎論,応用数学,力学,流体物理))

西山 由弘

日本物理学会講演概要集   68 ( 1 )   414 - 414   2013年3月

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記述言語：日本語   出版者・発行元：一般社団法人日本物理学会

• Criticalities of the transverse- and longitudinal-field fidelity susceptibilities for the d=2 quantum Ising model

Physical Review E   88   012129-5   2013年

• 20pAC-12 空間異方的三角格子上S=1模型における脱閉じ込め臨界現象(20pAC 量子スピン系2・ボゾン系,領域11(統計力学,物性基礎論,応用数学,力学,流体物理))

西山 由弘

日本物理学会講演概要集   67 ( 2 )   292 - 292   2012年8月

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記述言語：日本語   出版者・発行元：一般社団法人日本物理学会

• 26pAG-5 四体相互作用のあるJ_1-J_2模型のネール-VBS相境界の概形(26pAG 量子スピン系・冷却原子系,領域11(統計力学,物性基礎論,応用数学,力学,流体物理))

西山 由弘

日本物理学会講演概要集   67 ( 1 )   350 - 350   2012年3月

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記述言語：日本語   出版者・発行元：一般社団法人日本物理学会

• Numerical diagonalization analysis of the criticality of the (2+1) -dimensional XY model: Off-diagonal Novotny's method

Yoshihiro Nishiyama

Physical Review E - Statistical, Nonlinear, and Soft Matter Physics   78 ( 2 )   021135-1-021135-7   2008年8月

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記述言語：英語

The criticality of the (2+1) -dimensional XY model is investigated with the numerical diagonalization method. So far, it has been considered that the diagonalization method would not be very suitable for analyzing the criticality in large dimensions (d≥3)
in fact, the tractable system size with the diagonalization method is severely restricted. In this paper, we employ Novotny's method, which enables us to treat a variety of system sizes N=6,8,...,20 (N is the number of spins constituting a cluster). For that purpose, we develop an off-diagonal version of Novotny's method to adopt the off-diagonal (quantum-mechanical XY) interaction. Moreover, in order to improve the finite-size-scaling behavior, we tune the coupling-constant parameters to a scale-invariant point. As a result, we estimate the critical indices as ν=0.675 (20) and γν=1.97 (10). © 2008 The American Physical Society.

• Bound-state energy of the three-dimensional Ising model in the broken-symmetry phase: Suppressed finite-size corrections

Yoshihiro Nishiyama

Phys. Rev. E   78   02135-1-7   2008年

• Finite-size scaling of the d=5 Ising model embedded in a cylindrical geometry:The influence of hyperscaling violation

Yoshihiro Nishiyama

Phys. Rev. E   75   011106001-011106007   2007年

• Multicriticality of the (2+1)-dimensional gonihedric model:A realization of the (d,m)=(3,2) Lifshitz point

Yoshihiro Nishiyama

Phys. Rev. E   75   051116001-051116008   2007年

• Transfer-matrix approach to the three-dimensional bond percolation:An application of Novotny's formalism

Yoshihiro Nishiyama

Phys. Rev. E   2006年

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• Eliminated corrections to scaling around a renormalization-group fixed point: Transfer-matrix simulation of an extended d=3 Ising model

Yoshihiro Nishiyama

Physical Review E - Statistical, Nonlinear, and Soft Matter Physics   74 ( 1 )   2006年

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記述言語：英語

Extending the parameter space of the three-dimensional (d=3) Ising model, we search for a regime of eliminated corrections to finite-size scaling. For that purpose, we consider a real-space renormalization group (RSRG) with respect to a couple of clusters simulated with the transfer-matrix (TM) method. Imposing a criterion of "scale invariance," we determine a location of the nontrivial RSRG fixed point. Subsequent large-scale TM simulation around the fixed point reveals eliminated corrections to finite-size scaling. As anticipated, such an elimination of corrections admits systematic finite-size-scaling analysis. We obtained the estimates for the critical indices as ν=0.6245 (28) and yh =2.4709 (73). As demonstrated, with the aid of the preliminary RSRG survey, the transfer-matrix simulation provides rather reliable information on criticality even for d=3, where the tractable system size is restricted severely. © 2006 The American Physical Society.

• Finite-size-scaling analysis of the XY universality class between two and three dimensions: An application of Novotny's transfer-matrix method

Yoshihiro Nishiyama

Phys. Rev. E   2005年

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• Folding of the triangular lattice in a discrete three-dimensional space: Crumpling transitions in the negative-bending-rigidity regime

Yoshihiro Nishiyama

Phys. Rev. E   2005年

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• Synchrotron X-ray diffraction studies of the incommensurate phase of a spin-Peierls system CuGeO3 in strong magnetic fields

Y Narumi, K Katsumata, Y Tabata, S Kimura, Y Tanaka, T Nakamura, S Shimomura, M Matsuda, Harada, I, Y Nishiyama, T Ishikawa, H Kitamura, T Hara, T Tanaka, K Tamasaku, M Yabashi, S Goto, H Ohashi, K Takeshita, T Ohata, T Matsushita, T Bizen

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   73 ( 10 )   2650 - 2653   2004年10月

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記述言語：英語   出版者・発行元：PHYSICAL SOC JAPAN

Synchrotron X-ray diffraction measurements on a spin-Peierls material CuGeO3 in applied magnetic fields, H, up to 15 T are made. We find that the temperature, T, dependence of the incommensurate Bragg peak at a lower H is quite different from that at a higher H. At sufficiently high fields, we find that the lattice incommensurability, deltal, is almost independent of T, while at H slightly above the critical field = 12.25 T for the commensurate to incommensurate transition, deltal decreases with increasing T. We interpret that this finding is due to a stabilization of the incommensurate state by a strong magnetic field which suppresses thermal fluctuations.

• Multicriticality of the three-dimensional Ising model with plaquette interactions: An extension of Novotny's transfer-matrix formalism

Yoshihiro Nishiyama

Physical Review E - Statistical, Nonlinear, and Soft Matter Physics   70 ( 2 )   1 - 26120   2004年8月

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記述言語：英語

A three-dimensional Ising model was numerically investigated with the plaquette interactions. It was found that the interfacial energy lacks the surface tension term and thermally activated undulations must be promoted significantly because of the absence of the surface tension. It was noticed that a phase transition occurs at a considerably low temperature quite reminiscent of that of the two-dimensional Ising model. It was also confirmed that the phase transition belongs to the three-dimensional Ising university class.

• Folding of the triangular lattice in a discrete three-dimensional space: Density-matrix renormalization-group study

Yoshihiro Nishiyama

Physical Review E - Statistical, Nonlinear, and Soft Matter Physics   70 ( 1 )   1 - 16101   2004年7月

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記述言語：英語

The three-dimensional discrete folding by means of the density-matrix renormalization group was investigated. The technique was found to be successful even in the problem of soft materials such as fluid membranes. The bulk properties of discrete folding was investigated in detail taking the advantage that the technique can allow to treat very large system sizes. The folding entropy which appears to lie within the best analytical bound obtained through combinatorics arguments was also calculated.

• Asymptotic behavior of the emptiness formation probability in the critical phase of XXZ spin chain

VE Korepin, S Lukyanov, Y Nishiyama, M Shiroishi

PHYSICS LETTERS A   312 ( 1-2 )   21 - 26   2003年6月

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記述言語：英語   出版者・発行元：ELSEVIER SCIENCE BV

We study the emptiness formation probability (EFP) for the spin 1/2 XXZ spin chain. EFP P(n) detects a formation of ferromagnetic string of the length n in the ground state. It is expected that EFP decays in a Gaussian way for large strings P(n) similar to n(-gamma) C-n&lt;^&gt;2. Here, we propose the explicit expressions for the rate of Gaussian decay C as well as for the exponent gamma. In order to confirm the validity of our formulas, we employed an ab initio simulation technique of the density-matrix renormalization group to simulate XXZ spin chain of sufficient length. Furthermore, we performed Monte Carlo integration of the Jimbo-Miwa multiple integral for P (n). Those numerical results for P (n) support our formulas fairly definitely. (C) 2003 Elsevier Science B.V. All rights reserved.

• Third-neighbor correlators of a one-dimensional spin-1/2 Heisenberg antiferromagnet

K Sakai, M Shiroishi, Y Nishiyama, M Takahashi

PHYSICAL REVIEW E   67 ( 6 )   2003年6月

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記述言語：英語   出版者・発行元：AMER PHYSICAL SOC

We exactly evaluate the third-neighbor correlator &lt;S(j)(z)S(j+3)(z)&gt; and all the possible nonzero correlators &lt;S(j)(alpha)S(j+1)(beta)S(j+2)(gamma)S(j+3)(delta)&gt; of the one-dimensional spin-1/2 Heisenberg XXX antiferromagnet in the ground state without magnetic field. All the correlators are expressed in terms of certain combinations of logarithm ln 2, the Riemann zeta function zeta(3), zeta(5) with rational coefficients. The results accurately coincide with the numerical ones obtained by the density-matrix renormalization group method and the numerical diagonalization.

• Direct observation of the effective bending moduli of a fluid membrane: free-energy cost due to the reference-plane defomations

y nishiyama

Physical Review E 68   68 031901   2003年

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• Direct observation of the effective bending moduli of a fluid membrane: Free-energy cost due to the reference-plane deformations

Yoshihiro Nishiyama

Phys. Rev. E   2003年

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• Quantum-fluctuation-induced collisions and subsequent excitation gap of an elastic string between walls

Y Nishiyama

PHYSICAL REVIEW B   66 ( 18 )   2002年11月

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記述言語：英語   出版者・発行元：AMERICAN PHYSICAL SOC

An elastic string embedded between rigid walls is simulated by means of the density-matrix renormalization group. The string collides against the walls owing to the quantum-mechanical zero-point fluctuations. Such a "quantum entropic" interaction has come under thorough theoretical investigation in the context of the stripe phase observed experimentally in doped cuprates. We found that the excitation gap opens in the form of an exponential singularity DeltaEsimilar toexp(-Ad(sigma)) (d=wall spacing) with the exponent sigma=0.6(3), which is substantially smaller than the mean-field value sigma=2. That is, the excitation gap is much larger than that anticipated from mean field, suggesting that the string is subjected to a robust pinning potential due to the quantum collisions. This feature supports Zaanen's "order out of disorder" mechanism, which would be responsible for the stabilization of the stripe phase.

• Field-induced long-range ordering in a quasi-one-dimensional Haldane system

JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS   63 ( 6-8 )   1439 - 1442   2002年6月

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記述言語：英語   出版者・発行元：PERGAMON-ELSEVIER SCIENCE LTD

Quantum S = 1 spin chain with single-ion anisotropy in the presence of an external magnetic field is investigated by means of the finite-temperature density-matrix renormalization group. Emphasis is laid on the point that the underlying physics differs significantly with respect to magnetic-field directions. There appear various phases, which are absent in the classical counterpart studied extensively in the past. We argue possible implications of our numerical results in the field-induced magnetic long-range ordering observed experimentally in NDMAP. (C) 2002 Elsevier Science Ltd. All rights reserved.

• Quantum correlations and number theory

HE Boos, VE Korepin, Y Nishiyama, M Shiroishi

JOURNAL OF PHYSICS A-MATHEMATICAL AND GENERAL   35 ( 20 )   4443 - 4451   2002年5月

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記述言語：英語   出版者・発行元：IOP PUBLISHING LTD

We study the spin-1/2 Heisenberg XXX antiferromagnet for which the spectrum of the Hamiltonian was found by Bethe in 1931. We study the probability of the formation of ferromagnetic string in the antiferromagnetic ground state, which we call emptiness formation probability P(n). This is the most fundamental correlation function. We prove that, for short strings, it can be expressed in terms of the Riemann zeta function with odd arguments, logarithm In 2 and rational coefficients. This adds yet another link between statistical mechanics and number theory. We have obtained an analytical formula for P(5) for the first time. We have also calculated P(n) numerically by the density matrix renormalization group. The results agree quite well with the analytical results. Furthermore, we study the asymptotic behaviour of P (n) at finite temperature by quantum Monte Carlo simulation. This also agrees with our previous analytical results.

• 26pXY-1 DMRGと強結合展開による1+1Dφ^4格子模型の(偽)真空の研究(26pXY 量子スピン系,領域11(統計力学,物性基礎論,応用数学,力学,流体物理分野))

西山 由弘

日本物理学会講演概要集   57 ( 1 )   271 - 271   2002年3月

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記述言語：日本語   出版者・発行元：一般社団法人日本物理学会

• 量子スピン鎖における新しい相関関数─emptiness formation probability─に関する最近の発展

城石正弘, 西山由弘

物性研究   Vol. 79 No.1 43--64   2002年

• Magnetic phases and long-range ordering of the Haldane system in high magnetic fields

PROGRESS OF THEORETICAL PHYSICS SUPPLEMENT   ( 145 )   107 - 112   2002年

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記述言語：英語   出版者・発行元：PROGRESS THEORETICAL PHYSICS PUBLICATION OFFICE

Critical properties of field-induced magnetic phases are investigated in the one-dimensional S = 1 Haldane system with planar anisotropy, using the finite-temperature density-matrix renormalization group method. We point out that the underlying physics of the properties differs significantly between cases of different field directions: The magnetic phase is characterized as the gapless Tomonaga-Luttinger regime in one case and as the gapful soliton regime in the other. In connection with the information obtained for the one-dimensional spin chain, possible implications for the field-induced magnetic long-range ordering are discussed.

• Quantum-entropic interaction of an elastic string between walls

Y Nishiyama

PROGRESS OF THEORETICAL PHYSICS SUPPLEMENT   ( 145 )   229 - 232   2002年

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記述言語：英語   出版者・発行元：PROGRESS THEORETICAL PHYSICS PUBLICATION OFFICE

Recently, there arose a problem of 'quantum string,' which is a linelike object meandering quantum-mechanically and undergoes entropy-reducing collisions with adjacent neighbors. In this paper, we overview underlying physics of the quantum string, and report our simulation result on the collision-induced entropy loss of a quantum string between walls by means of the density-matrix renormalization group.

• Stiffening of fluid membranes due to thermal undulations: density matrix renormalization group study

Y. Nishiyama

Phys. Rev. E   2002年

• Emptiness formation probability for the one-dimensional isotropic XY model

M Shiroishi, M Takahashi, Y Nishiyama

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   70 ( 12 )   3535 - 3543   2001年12月

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記述言語：英語   出版者・発行元：PHYSICAL SOC JAPAN

We study a correlation function for the one-dimensional isotropic Xy model (XXO model), which is called the Emptiness Formation Probability (EFP). It is the probability of the formation of a ferromagnetic string in the anti-ferromagnetic ground state. Using the expression of the EFP as a Toeplitz determinant, we discuss its asymptotic behaviors. We also compare the analytical results with numerical calculations as the density-matrix renormalization group and the quantum Monte-Carlo method.

• Strong-coupling-expansion analysis of the false-vacuum decay rate of the lattice phi(4) model in 1+1 dimensions

Y Nishiyama

JOURNAL OF PHYSICS A-MATHEMATICAL AND GENERAL   34 ( 50 )   11215 - 11223   2001年12月

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記述言語：英語   出版者・発行元：IOP PUBLISHING LTD

Strong-coupling expansion is performed for the lattice phi(4) model in 1 + 1 dimensions. Because the strong-coupling limit itself is not solvable, we employed numerical calculations so as to set up unperturbed eigensystems. Restricting the number of Hilbert-space bases, we performed linked-cluster expansion up to 11th order. We carried out an alternative simulation by means of the density-matrix renormalization group. Thereby, we confirmed that our series-expansion data with a convergence-acceleration trick are in good agreement with the simulation result. Through the analytic continuation to the domain of negative biquadratic interaction, we obtain the false-vacuum decay rate. Contrary to common belief that the tunnelling phenomenon lies out of perturbative treatments, our series expansion reproduces the instanton-theory behaviour for a high potential barrier. For a shallow barrier, in contrast, our result tells us that the relaxation is no longer described by an instanton, but the decay rate acquires notable enhancement.

• Numerical analyses of the nonequilibrium electron transport through the Kondo impurity beside the Toulouse point

Y Nishiyama

PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES   11 ( 4 )   293 - 302   2001年11月

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記述言語：英語   出版者・発行元：ELSEVIER SCIENCE BV

Nonequilibrium electron transport through the Kondo impurity is investigated numerically for the system with 20 conduction-electron levels. The electron current under finite voltage drop is calculated in terms of the `conductance viewed as transmission' picture proposed by Landauer. Here, we take into account the full transmission processes of both the many-body correlation and the hybridization amplitude up to infinite order. Our results demonstrate, for instance, how the exact solution of the differential conductance by Schiller and Hershfield obtained at the Toulouse point becomes deformed by more realistic interactions. The differential-conductance-peak height is suppressed below e(2)/h with the width hardly changed through reducing the Kondo coupling from the Toulouse point, whereas it is kept unchanged by further increase of the coupling. We calculated the nonequilibrium local Green function as well. This clarifies the spectral property of the Kondo impurity driven far from equilibrium. (C) 2001 Elsevier Science B.V. All rights reserved.

• Quantum-fluctuation-induced repulsive interaction of a quantum string between walls

Y Nishiyama

PHYSICAL REVIEW B   64 ( 6 )   2001年8月

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記述言語：英語   出版者・発行元：AMER PHYSICAL SOC

A quantum string, which was brought into discussion recently as a model for the stripe phase in doped cuprates, is simulated by means of the density-matrix-renormalization-group method. String collides with adjacent neighbors, as it wanders, owing to quantum zero-point fluctuations. The energy cost due to the collisions is our main concern. Embedding a quantum string between rigid walls with separation d, we found that for sufficiently large d, collision-induced energy cost obeys the formula similar to exp(-Ad(alpha)) with alpha =0.808(1), and the, string's mean fluctuation width grows logarithmically similar to log d. Those results are not understood in terms of a conventional picture that the string is "disordered," and only short-wavelength fluctuations contribute to collisions. Rather, our results support a recent proposal that owing to collisions, short-wavelength fluctuations are suppressed, but instead, long-wavelength fluctuations become significant. This mechanism would be responsible for stabilizing the stripe phase.

• Numerical analysis of the magnetic-field-tuned superconductor-insulator transition in two dimensions

Y Nishiyama

PHYSICA C   353 ( 1-2 )   147 - 155   2001年5月

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記述言語：英語   出版者・発行元：ELSEVIER SCIENCE BV

Ground state of the two-dimensional hard-core-boson model subjected to external magnetic field and quenched random chemical potential is studied numerically. In experiments, magnetic-field-tuned superconductor-insulator transition has already come under through investigation, whereas in computer simulation, only randomness-driven localization (with zero magnetic field) has been studied so far: The external magnetic field brings about a difficulty that the hopping amplitude becomes complex number (through the gauge twist), for which the quantum Monte-Carlo simulation fails. Here, we employ the exact diagonalization method, with which we demonstrate that the model does exhibit field-tuned localization transition at a certain critical magnetic field. At the critical point, we found that the DC conductivity is not universal, but is substantially larger than that of the randomness-driven localization transition at zero magnetic field. Our result supports recent experiment by Markovic et al. reporting an increase of the critical conductivity with magnetic field strengthened. (C) 2001 Elsevier Science B.V. All rights reserved.

• Field-induced long-range ordering in an S=1 quasi-one-dimensional Heisenberg antiferromagnet

Z Honda, K Katsumata, Y Nishiyama, Harada, I

PHYSICAL REVIEW B   63 ( 6 )   2001年2月

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記述言語：英語   出版者・発行元：AMER PHYSICAL SOC

Detailed results of heat capacity and magnetization measurements are reported on a single crystal sample of the spin S=1 quasi-one-dimensional Heisenbrrg antiferromagnet Ni(C5H14N2)(2)N-3(PF6), abbreviated NDMAP. From these results, we constructed the magnetic field (H) versus temperature phase diagram of this compound which exhibits the quantum disordered Haldane, field-induced long-range-ordered (LRO) and thermally disordered paramagnetic phases. The phase boundary curve separating the paramagnetic and LRO phases is anisotropic; the increase of the Neel temperature (T-N) with H along the a and b axes is more rapid than that along the c axis. We calculated T-N as a function of H by taking into account the interchain coupling as the form of a mean held. The calculation of the staggered susceptibility of the S=1 one-dimensional antiferromagnet with an easy-plane anisotropy shows quite different behavior in the different field directions, resulting in the anisotropic phase boundary curve. A good agreement is obtained between theory and experiment using the exchange and anisotropy constants obtained from the neutron scattering experiment.

• Nonequilibrium-current-induced corrections to the one-particle-correlation function in a wire

Y Nishiyama

PHYSICS LETTERS A   274 ( 3-4 )   143 - 151   2000年9月

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記述言語：英語   出版者・発行元：ELSEVIER SCIENCE BV

Electron gas in a wire connected to two terminals with potential drop is studied with the Schwinger-Keldysh formalism. Recent studies, where the current is enforced to flow with a Lagrange-multiplier term, demonstrated that the current enhances the one-particle-correlation function. We report that in our model, such enhancement is not guaranteed to occur, but conditional both on the potential drop and the positions where we observe the correlation function. That is, under a certain condition, spatially modulated pattern is formed in the wire owing to the nonequilibrium current. (C) 2000 Published by Elsevier Science B.V.

• Finite-size-scaling analyses of the chiral order in the Josephson-junction ladder with half a flux quantum per plaquette

Y Nishiyama

EUROPEAN PHYSICAL JOURNAL B   17 ( 2 )   295 - 299   2000年9月

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記述言語：英語   出版者・発行元：SPRINGER-VERLAG

Chiral order of the Josephson-junction ladder with half a flux quantum per plaquette is studied by means of the exact diagonalization method. We consider an extreme quantum limit where each superconductor grain (order parameter) is represented by S = 1/2 spin. So far, the semi-classical S -&gt; infinity case, where each spin reduces to a plane rotator: has been considered extensively. We found that in the case of S = 1/2, owing to the strong quantum fluctuations, the chiral (vortex lattice) order becomes dissolved except in a region, where attractive intrachain and, to our surprise, repulsive interchain interactions both exist. On the contrary, for considerably wide range of parameters, the superconductor (XY) order is kept critical. The present results are regarded as a demonstration of the critical phase accompanying chiral-symmetry breaking predicted for frustrated XXZ chain field-theoretically.

• Spin correlation in the one-dimensional Heisenberg antiferromagnet with competing interactions

Harada, I, Y Nishiyama, Y Aoyama, S Mori

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   69   339 - 346   2000年5月

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記述言語：英語   出版者・発行元：PHYSICAL SOC JAPAN

The systematic trend of spin correlation in the S = 1/2 quantum antiferromagnetic chain with the next nearest neighbor exchange interaction, which competes with the alternating nearest neighbor interaction, is studied at zero temperature as well as at finite temperatures, focusing our attention on their incommensurate nature. We find that, at zero temperature, the dynamical structure factor exhibits a minimal gap at a commensurate or at an incommensurate wave number, which is explained as a consequence of a domain-wall like excitation due to the structure of the dimer order. At finite temperatures, the static structure factor is shown to exhibit a peak at a commensurate or at an incommensurate wave number, depending on thr temperature. The boundary between them, called the Lifshitz temperature, is determined. The behavior is discussed in connection with the experimental aspect of the recent neutron inelastic scattering in CuGeO3.

• Numerical analysis of the dissipative two-state system with the density-matrix Hilbert-space-reduction algorithm

Y Nishiyama

EUROPEAN PHYSICAL JOURNAL B   12 ( 4 )   547 - 554   1999年12月

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記述言語：英語   出版者・発行元：SPRINGER VERLAG

Ground state of the dissipative two-state system is investigated by means of the Lanczos diagonalization method. We adopted the Hilbert-space-reduction scheme proposed by Zhang, Jeckelmann and White so as to reduce the overwhelming reservoir Hilbert space to being tractable in computers. Both the implementation of the algorithm and the precision are reported in detail. We evaluate the dynamical susceptibility (resolvent) with the continued-fraction-expansion formula. Through analysing the resolvent over a frequency range, which is often called "interesting" frequency, we obtain the damping rate and the oscillation frequency. Our results agree with those of a recent quantum Monte-Carlo study, which concludes that the critical dissipation from oscillatory to over-damped behavior decreases as the tunneling amplitude is strengthened.

• Numerical diagonalization analysis of the ground-state superfluid-localization transition in two dimensions

Y Nishiyama

EUROPEAN PHYSICAL JOURNAL B   9 ( 2 )   215 - 223   1999年5月

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記述言語：英語   出版者・発行元：SPRINGER VERLAG

Ground state of the two-dimensional hard-core-boson system in the presence of the quenched random chemical potential is investigated by means of the exact-diagonalization method for the system sizes up to L = 5. The criticality and the DC conductivity at the superfluid-localization transition have been controversial so far. We estimate, with the finite-size scaling analysis, the correlation-length and the dynamical critical exponents as nu = 2.3 +/- 0.6 and z = 2, respectively. The AC conductivity is computed with the Gagliano-Balseiro formula, with which the resolvent (dynamical response function) is expressed in terms of the continued-fraction form consisting of Lanczos tri-diagonal elements. Thereby, we estimate the universal DC conductivity as sigma(c)(omega --&gt; 0) = 0.135 +/- 0.01((2e)(2)/h).

講演・口頭発表等

• 二次元反強磁性イジング模型における虚数磁場フィデリティ感受率の計算

西山由弘

日本物理学会第74回年次大会  2020年3月18日

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会議種別：口頭発表（一般）

• 長距離相互作用量子イジング鎖におけるマグノン束縛状態の臨界現象

西山由弘

日本物理学会2018年秋季大会  2018年9月12日

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会議種別：口頭発表（一般）

• 二次元量子ポッツ模型の秩序相の素励起ヒエラルキー

西山由弘

日本物理学会第72回年次大会  2017年3月20日

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会議種別：口頭発表（一般）

• (d,m)=(3,2)型リフシッツ点の臨界性:2+1次元gonihedric模型

日本物理学会第62回年次大会  2007年

• d=5イジング模型の有限サイズスケーリング:ハイパースケーリングの破れの影響

日本物理学会2007年春季大会  2007年

• 三次元ボンドパーコレーションの転送行列の構成と応用

日本物理学会第61回年次大会  2006年

• 繰り込み固定点付近での有限サイズスケーリング補正の軽減

日本ん物理学会 ２００６年秋季大会  2006年

• 2と3次元の間のXY型臨界現象の解析: Novotny法の応用

日本物理学会台60回年次大会  2005年

• DMRGの三角格子の折りたたみ問題への応用としわくちゃ転移の解析II

日本物理学会2005年秋季大会  2005年

• DMRGの三角格子の折りたたみ問題への応用とcrumpling転移の解析

日本物理学会、第59回年次大会  2004年

• 相互作用する曲面のガス(ゴニヘドリック)模型の多重臨界性

日本物理学会2004年秋季大会、  2004年

• Discrete Folding in three dimensions: Density-Matrix-Renormalization-Group Study

STATPHYS 22  2004年

• 4量体化されたS=1/2梯子の磁化過程

日本物理学会、第59回年次大会  2004年

• DMRGの膜の熱力学への応用と実効曲げ弾性係数の解析

日本物理学会、第58回年次大会  2003年

• DMRGの膜の熱力学への応用と実効曲げ弾性係数の解析II

日本物理学会2003年秋季大会  2003年

• スピン1/2-XXZ 鎖の Emptiness Formation Probability の漸近的ふるまい

日本物理学会、第58回年次大会  2003年

• DMRGと強結合展開による$1+1$D$\phi^4$格子模型の(偽)真空の研究

日本物理学会 第57回年次大会  2002年

• 1次元ハイゼンベルグ XXX 模型の Emptiness Formation Probability

日本物理学会2002年秋季大会  2002年

• 剛体壁間の紐の量子揺らぎによる衝突エントロピーII

日本物理学会2002年秋季大会  2002年

• 磁場下のハルデン系と磁場誘起長距離秩序

物性研短期研究会「量子スピン系の磁場誘起現象とフラストレーションの効果」  2001年

• 剛体壁間の紐の量子揺らぎによる衝突エントロピー

日本物理学会2001年秋季大会  2001年

• ジョセフソン梯子の磁束格子秩序の有限サイズスケーリング解析

日本物理学会第56回年次大会  2001年

• 1次元等方的 XY 模型の Emptiness Formation Probability

日本物理学会2001年秋季大会  2001年

• ハルデン系の磁場誘起長距離秩序

日本物理学会第56回年次大会  2001年

• 磁場によるハルデン相の破壊と3次元ネール秩序

日本物理学会春の分科会  2000年

• Toulouse点近傍の近藤不純物を流れる非平衡電流の数値計算

日本物理学会第55回年次大会  2000年

• Toulouse点近傍の近藤不純物を流れる非平衡電流の数値計算、

日本物理学会第55回年次大会  2000年

• DMRGによる散逸二準位系のシミュレーション

日本物理学会第春の分科会  2000年

• 磁場によるハルデン相の破壊と3次元ネール秩序

日本物理学会春の分科会  2000年

• DMRGによる散逸二準位系のシミュレーション

日本物理学会第春の分科会  2000年

• 磁場下における1次元競合スピン(S=1/2)鎖の動的スピン関数、

物性研短期研究会「低次元磁性体の磁化過程に見られる新しい量子現象」  1999年

• 磁場による二次元超流動-アンダーソン局在転移のシミュレーション

物性研短期研究会「低次元磁性体の磁化過程に見られる新しい量子現象」  1999年

• 3Dイジング模型の秩序相における素励起対のユニバーサルな束縛エネルギー

日本物理学会第63回年次大会  0020年

担当授業科目

• 熱力学 （2021年度） ３・４学期  - 水1,水2

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• 量子力学演習１ （2021年度） １・２学期  - 金3,金4

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• 量子力学演習１B （2021年度） 第２学期  - 金3,金4

• 量子力学演習２ （2021年度） ３・４学期  - 金5,金6

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• 量子力学演習２B （2021年度） 第４学期  - 金5,金6

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• 熱力学A （2020年度） 第３学期  - 水1,水2

• 熱力学B （2020年度） 第４学期  - 水1,水2

• 物質科学基礎論ＩＩ （2020年度） 前期  - 木3,木4

• 量子力学演習１ （2020年度） １・２学期  - 金3,金4

• 量子力学演習１A （2020年度） 第１学期  - 金3,金4

• 量子力学演習１B （2020年度） 第２学期  - 金3,金4

• 量子力学演習２ （2020年度） ３・４学期  - 金5,金6

• 量子力学演習２A （2020年度） 第３学期  - 金5,金6

• 量子力学演習２B （2020年度） 第４学期  - 金5,金6