Faculty Profiles - NISHIYAMA Yoshihiro

Papers - NISHIYAMA Yoshihiro

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Two-dimensional transverse-field <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si55.svg" display="inline" id="d1e2320"><mml:mrow><mml:mi>X</mml:mi><mml:mi>Y</mml:mi></mml:mrow></mml:math> model with the in-plane anisotropy and Dzyaloshinskii–Moriya interaction: Anisotropy-driven transition

Yoshihiro Nishiyama

Physica A: Statistical Mechanics and its Applications 664 130444 - 130444 2025.4

XY-VBS phase boundary for the square-lattice $$J_1$$-$$J_2$$ XXZ model with the ring exchange

Yoshihiro Nishiyama

The European Physical Journal B 97 ( 10 ) 2024.10

Publishing type：Research paper (scientific journal) Publisher：Springer Science and Business Media LLC

Abstract

The square-lattice $$J_1$$-$$J_2$$XXZ model with the ring-exchange interaction K was investigated numerically. As for the hard-core-boson model with the nearest-neighbor hopping $$J_1/2$$, namely, the $$J_1$$-KXY model, it has been reported that the ring exchange leads to a variety of exotic phases such as the valence-bond-solid (VBS) phase. In this paper, we extend the parameter space to investigate the phase boundary between the XY (superfluid) and VBS phases. A notable feature is that the phase boundary terminates at the fully frustrated point, $$J_2/J_1 \rightarrow 0.5^-$$. As a scaling parameter for the multi-criticality, the distance from the multi-critical point $$\delta (\ge 0)$$ is introduced. To detect the phase transition, we employed the high-order fidelity susceptibility $$\chi ^{(3)}_F$$, which is readily evaluated via the exact-diagonalization scheme. As a demonstration, for a fixed value of $$\delta $$, the XY-VBS criticality was analyzed by the probe $$\chi ^{(3)}_F$$. Thereby, with properly scaling $$\delta $$, the $$\chi ^{(3)}_F$$ data were cast into the crossover-scaling formula to determine the multi-criticality.

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DOI： 10.1140/epjb/s10051-024-00793-2

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Other Link： https://link.springer.com/article/10.1140/epjb/s10051-024-00793-2/fulltext.html

Transverse-field XY spin chain with the competing long-range interactions: multi-criticality around the XX-symmetric point

Yoshihiro Nishiyama

Journal of Statistical Mechanics: Theory and Experiment 2023 ( 8 ) 083102 - 083102 2023.8

Publishing type：Research paper (scientific journal) Publisher：IOP Publishing

Abstract

The transverse-field XY spin chain with competing antiferromagnetic long-range interactions, $J_r \propto 1/r^\alpha$ (r: distance between spins), and the exponent α was investigated numerically. The main concern is to clarify the character of the transverse-field-driven phase transition for the small-α regime around the XX-symmetric point, η = 0 (η: XY-anisotropy parameter). To cope with the negative-sign problem, we employed the exact-diagonalization method, which enables us to evaluate the fidelity susceptibility χ_F. Because the fidelity susceptibility does not assume any order parameter after a phase transition, it detects the multi-criticality around the XX-symmetric point in a systematic manner. As a preliminary survey, with η = 0.5 and α = 2 fixed, we made the scaling analysis of χ_F. The scaling behavior of χ_F shows that the transverse-field-driven phase transition belongs to the 2D-Ising universality class. Thereby, with the properly crossover-scaled η, the χ_F data is cast into the crossover scaling formula for the small-α regime. The multi-critical exponents fed into the scaling formula are argued through referring to related studies.

DOI： 10.1088/1742-5468/aceb55

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Other Link： https://iopscience.iop.org/article/10.1088/1742-5468/aceb55/pdf

End-point singularity of the XY-paramagnetic phase boundary for the $$(2+1)$$D $$S=1$$ square-lattice $$J_1$$–$$J_2$$ XY model with the single-ion anisotropy

Yoshihiro Nishiyama

The European Physical Journal B 96 ( 7 ) 2023.7

Criticality of the excess energy cost due to the unit-flux-quantum external field for the (2 + 1)D superfluid–insulator transition

Yoshihiro Nishiyama

Journal of Statistical Mechanics: Theory and Experiment 2022 ( 3 ) 033102 - 033102 2022.3

Publishing type：Research paper (scientific journal) Publisher：IOP Publishing

Abstract

The two-dimensional (2D) spin-S = 1 XY model was investigated numerically as a realization of the (2 + 1)D superfluid–Mott-insulator (SF–MI) transition. The interaction parameters are extended so as to suppress corrections to finite-size scaling. Thereby, the external field of a unit flux quantum (Φ = 2π) is applied to the 2D cluster by incorporating the phase factor ${\text{e } }^{\text{i}{\phi }_{ij } }$ (ϕ_ij: gauge angle between the i and j sites) into the hopping amplitudes. Taking the advantage in that the exact-diagonalization method allows us to treat such a complex-valued matrix element, we evaluated the excess energy cost ΔE(2π) due to the magnetic flux Φ = 2π explicitly in the SF (XY) phase. As a result, we found that the amplitude ratio ρ_s/ΔE(2π) (ρ_s: spin stiffness) makes sense in proximity to the critical point, exhibiting a notable plateau in the SF-phase side. The plateau height is estimated, and compared to the related studies.

DOI： 10.1088/1742-5468/ac5cb2

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Other Link： https://iopscience.iop.org/article/10.1088/1742-5468/ac5cb2/pdf

Fidelity-mediated analysis of the transverse-field XY chain with the long-range interactions: anisotropy-driven multi-criticality

Yoshihiro Nishiyama

The European Physical Journal B 94 ( 11 ) 2021.11