arXiv subjects

Kiyohide Nomura

Publications and source records attributed to Kiyohide Nomura.

At least 19 recordsLinked to original sources

arXiv2025

Gaussian fixed lines of $S=1/2$ XXZ chain with next-nearest neighbor interaction and $sl_2$ loop algebra

Spin systems are important to understand various physical properties in quantum many-body systems. We numerically study the Gaussian fixed lines (GFLs) of the $S=1/2$ XXZ chain with next-nearest neighbor (NNN) interaction in the XY phase. The GFLs are the set of points where the coefficient of the umklapp scattering vanishes. We show that the GFLs pass through the ``special points'', which are defined as the points where the $sl_2$ loop algebra symmetry governs in low-energy physics. In addition, we have discussed the Tomonaga-Luttinger parameter K, and the metamagnetism influences the shape of the GFLs.

cond-mat.stat-mech↗

arXiv2024

A multicritical point of $S = 1$ XXZ chain with single ion anisotropy

We research the ground state of the $S = 1$ XXZ spin chain with single ion anisotropy, focusing on XY1, XY2 (Spin Nematic), Neel, Haldane phases. Recently, it was reported that the four phases did not intersect at a single point using conventional numerical methods, which is inconsistent with the bosonization theory. In order to resolve this discrepancy, we propose a unified method to determine phase boundaries by combining conformal field theory (CFT) with numerical diagonalization. And we discuss the symmetry and universality class of the multicritical point.

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arXiv2023

Critical phenomena around the SU(3) symmetric tri-critical point of a spin-1 chain

We investigate critical phenomena of a spin-1 chain in the vicinity of the SU(3) symmetric critical point, which we already specified in the previous study. We numerically diagonalize a Hamiltonian combining the bilinear-biquadratic (BLBQ) Hamiltonian with the trimer Hamiltonian. We then discuss the numerical results based on the conformal field theory (CFT) and the renormalization group. As a result, we firstly verify that the critical point found in our previous study is the tri-critical point among the Haldane phase, trimer phase, and the trimer liquid (TL) phase. Secondly, with regard to the TL--trimer transition and TL--Haldane transition, we find that the critical phenomena around this tri-critical point belong to the Berezinskii--Kosterlitz--Thouless (BKT)-like universality class. We thirdly find the boundary between the Haldane phase and the trimer phase, which is illustrated by the massive self-dual sine-Gordon (SDSG) model.

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arXiv2022

Nematic Tomonaga-Luttinger Liquid Phase in an $S=1/2$ Ferromagnetic-Antiferromagnetic Bond-Alternating Chain

We numerically investigate the ground-state phase diagram of the $S=1/2$ ferromagnetic-antiferromagnetic bond-alternating chain, in which the ferromagnetic interactions are stronger than the antiferromagnetic ones, and the anisotropies of the former and latter interactions are of the Ising-type and the $XY$-type, respectively. We use various numerical methods, such as the level spectroscopy and phenomenological renormalization-group analyses of the numerical data obtained by the exact diagonalization method, and so on. The resultant phase diagrams contain the ferromagnetic, $XY$1, singlet-dimer, and up-up-down-down phases as well as the nematic Tomonaga-Luttinger liquid (nTLL) phase which appears in a wide region of the interaction parameters. Perturbation calculations from the strong limit of the ferromagnetic interactions reproduce fairly well the numerical results of the phase boundary lines associated with the nTLL phase in the phase diagrams.

cond-mat.str-el↗

arXiv2021

Phase transition of an SU(3) symmetric spin-1 chain

We investigate a phase transition in an SU(3) symmetric spin-1 chain. We identify the universality class of the critical trimer liquid phase, as well as the phase boundary between these two phases.

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arXiv2021

Universality Class around the SU(3) Symmetric Point of the Dimer-Trimer Spin-1 Chain

We study critical phenomena of the SU(3) symmetric spin-1 chains when adding the SU(3) asymmetric term. To investigate such system, we numerically diagonalize the Dimer-Trimer (DT) model Hamiltonian around the SU(3) symmetric point, named the pure trimer (PT) point. We analyze our numerical results with the conformal field theory (CFT). First of all, we discover soft modes at the wave number q = 0 and q = 2π/3 for the PT point, and then the system is critical. Secondly, we find that the system at the PT point belongs to the CFT with the central charge c = 2 and the scaling dimension x = 2/3. Finally, by investigating the eigenvalues of the Hamiltonian in the vicinity of the PT point, we find that there is a phase transition at the PT point from a massive phase to a massless phase. From these numerical results, the phase transition at the PT point belongs to the Berezinskii-Kosterlitz-Thouless (BKT)-like universality class that is explained by the level-1 SU(3) Wess-Zumino-Witten (SU(3) 1 WZW) model.

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arXiv2020

Method to Observe Anomaly of Magnetic Susceptibility for Quantum Spin Systems

We propose a new method for studying the anomaly of magnetic susceptibility $χ$ that indicates a phase transition for quantum spin systems. In addition, we introduce the fourth derivative $A$ of the lowest energy eigenvalue per site with respect to magnetization, that is, the second derivative of $χ^{-1}$. To verify the validity of this method, we apply it to an $S=1/2$ XXZ antiferromagnetic chain. The lowest energy of the chain is calculated by numerical diagonalization. As a result, the anomaly of $χ$ and $A$ exists at zero magnetization. That of $A$ is easier to observe than that of $χ$, which indicates that the observation of $A$ is a more efficient method to evaluate an anomaly than that of $χ$. The observation of $A$ reveals an anomaly that shows the high order phase transition, namely, the fourth order phase transition. Our method is helpful for analyzing critical phenomena.

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arXiv2017

Discrete symmetries and Lieb-Schultz-Mattis theorem

In this study, we consider one-dimension (1D) quantum spin systems with the translation and discrete symmetries (spin reversal, space inversion and time reversal symmetries). By combining the continuous U(1) symmetry with the discrete symmetries and using the extended Lieb-Schultz-Mattis theorem \cite{Lieb-Schultz-Mattis-1961}\cite{Nomura-Morishige-Isoyama-2015}, we investigate the relation between the ground states, energy spectra and symmetries. For half-integer spin cases, we generalize the dimer and Néel concepts using the discrete symmetries, and we can reconcile the LSM theorem with the dimer or Néel states, since there was a subtle dilemma. Furthermore, a part of discrete symmetries is enough to classify possible phases. Thus we can deepen our understanding of the relation between the LSM theorem and the discrete symmetries.

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arXiv2017

Extension of the Lieb-Schultz-Mattis and Kolb theorem

The theorem of Lieb, Schultz and Mattis (LSM), which states that the S=1/2 XXZ spin chain has gapless or degenerate ground states, can be applied to broader models. Independently, Kolb considered the relation between the wave number $q$ and the twisting boundary condition, and he obtained a similar result as LSM. However, in frustrating cases it is known that there exist several exceptions for the assumption of the unique lowest state for the finite size, which is important in the traditional LSM theorem. In our previous paper, without the assumption of the uniqueness, we have extended the LSMK theorem for frustrating and non-symmetric cases. However, there remains a complexity in the proof of continuity. In this paper, we will simplify the proof than the previous work.

cond-mat.stat-mech↗

arXiv2015

Extension of the Lieb-Schultz-Mattis theorem

Lieb, Schultz and Mattis (LSM) studied the S=1/2 XXZ spin chain. Theorems of LSM's paper can be applied to broader models. In the original LSM theorem it was assumed the nonfrustrating system. However, reconsidering the LSM theorem, we can extend the LSM theorem for frustrating systems. Next, several researchers have tried to extend the LSM theorem for excited states. In the cases $S^{z}_{T} = \pm 1,\pm 2 \cdots$, the lowest energy eigenvalues are continuous for wave number $q$. But we find that their proofs are insufficient, and we improve them. In addition, we can prove the LSM theory without the assumption of the discrete symmetry, which means that the LSM type theorems are applicable for Dzyaloshinskii-Moriya type interactions or other nonsymmetric models.

cond-mat.stat-mech↗

arXiv2011

How to distinguish the Haldane/Large-D state and the intermediate-D state in an S=2 quantum spin chain with the XXZ and on-site anisotropies

We numerically investigate the ground-state phase diagram of an S=2 quantum spin chain with the $XXZ$ and on-site anisotropies described by ${\mathcal H}=\sum_j (S_j^x S_{j+1}^x+S_j^y S_{j+1}^y+ΔS_j^z S_{j+1}^z) + D \sum_j (S_j^z)^2$, where $Δ$ denotes the XXZ anisotropy parameter of the nearest-neighbor interactions and $D$ the on-site anisotropy parameter. We restrict ourselves to the $Δ>0$ and $D>0$ case for simplicity. Our main purpose is to obtain the definite conclusion whether there exists or not the intermediate-$D$ (ID) phase, which was proposed by Oshikawa in 1992 and has been believed to be absent since the DMRG studies in the latter half of 1990's. In the phase diagram with $Δ>0$ and $D>0$ there appear the XY state, the Haldane state, the ID state, the large-$D$ (LD) state and the Néel state. In the analysis of the numerical data it is important to distinguish three gapped states; the Haldane state, the ID state and the LD state. We give a physical and intuitive explanation for our level spectroscopy method how to distinguish these three phases.

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arXiv2011

Haldane, Large-D and Intermediate-D States in an S=2 Quantum Spin Chain with On-Site and XXZ Anisotropies

Using mainly numerical methods, we investigate the ground-state phase diagram of the S=2 quantum spin chain described by $H = \sum_j (S_j^x S_{j+1}^x + S_j^y S_{j+1}^y + ΔS_j^z S_{j+1}^z) + D \sum_j (S_j^z)^2$, where $Δ$ denotes the $XXZ$ anisotropy parameter of the nearest-neighbor interactions and $D$ the on-site anisotropy parameter. We restrict ourselves to the case with $Δ\ge 0$ and $D \ge 0$ for simplicity. Each of the phase boundary lines is determined by the level spectroscopy or the phenomenological renormalization analysis of numerical results of exact-diagonalization calculations. The resulting phase diagram on the $Δ$-$D$ plane consists of four phases; the XY 1 phase, the Haldane/large-$D$ phase, the intermediate-$D$ phase and the Néel phase. The remarkable natures of the phase diagram are: (1) the Haldane state and the large-$D$ state belong to the same phase; (2) there exists the intermediate-$D$ phase which was predicted by Oshikawa in 1992; (3) the shape of the phase diagram on the $Δ$-$D$ plane is different from that believed so far. We note that this is the first report of the observation of the intermediate-$D$ phase.

cond-mat.str-el↗

arXiv2008

Critical intermediate phase and phase transitions in a triangular-lattice three-spin interaction model: Level-spectroscopy approach

We investigate infinite-order phase transitions like the Berezinskii-Kosterlitz-Thouless transition observed in a triangular-lattice three-spin interaction model. Based on a field theoretical description and the operator-production-expansion technique, we perform the renormalization-group analysis, and then clarify properties of marginal operators near the phase transition points. The results are utilized to establish criteria to determine the transition points and some universal relations among excitation levels to characterize the transitions. We verify these predictions via the numerical analysis on eigenvalue structures of the transfer matrix. Also, we discuss an enhancement of symmetry at the end points of a critical intermediate phase in connection with a transition observed in the ground state of the bilinear-biquadratic spin-1 chain.

cond-mat.stat-mech↗

arXiv2006

Cancellation of oscillatory behaviors in incommensurate region

In several frustrating systems incommensurate behaviors are often observed. For the S=1 bilinear-biquadratic model, we show that the main oscillatory behavior, which is proportional to the free edge spins, is eliminated in the incommensurate subphase, considering the average of triplet and singlet energy spectra under open boundary conditions. In the same way, the $π$-mode oscillation is also removed in the commensurate subphase. Moreover, we find that higher order corrections are exponentially decaying from an analysis of small size data.

cond-mat.str-el↗

arXiv2006

Global phase diagram and six-state clock universality behavior in the triangular antiferromagnetic Ising model with anisotropic next-nearest-neighbor coupling: Level-spectroscopy approach

We investigate the triangular-lattice antiferromagnetic Ising model with a spatially anisotropic next-nearest-neighbor ferromagnetic coupling, which was first discussed by Kitatani and Oguchi. By employing the effective geometric factor, we analyze the scaling dimensions of the operators around the Berezinskii-Kosterlitz-Thouless (BKT) transition lines, and determine the global phase diagram. Our numerical data exhibit that two types of BKT-transition lines separate the intermediate critical region from the ordered and disordered phases, and they do not merge into a single curve in the antiferromagnetic region. We also estimate the central charge and perform some consistency checks among scaling dimensions in order to provide the evidence of the six-state clock universality. Further, we provide an analysis of the shapes of boundaries based on the crossover argument.

cond-mat.stat-mech↗

arXiv2006

Incommensurability and edge states in the one-dimensional S=1 bilinear-biquadratic model

Commensurate-incommensurate change on the one-dimensional S=1 bilinear-biquadratic model (${\cal H}(α)=\sum_i \{{\bf S}_i\cdot {\bf S}_{i+1} +α({\bf S}_i\cdot{\bf S}_{i+1})^2\}$) is examined. The gapped Haldane phase has two subphases (the commensurate Haldane subphase and the incommensurate Haldane subphase) and the commensurate-incommensurate change point (the Affleck-Kennedy-Lieb-Tasaki point, $α=1/3$). There have been two different analytical predictions about the static structure factor in the neighborhood of this point. By using the Sørensen-Affleck prescription, these static structure factors are related to the Green functions, and also to the energy gap behaviors. Numerical calculations support one of the predictions. Accordingly, the commensurate-incommensurate change is recognized as a motion of a pair of poles in the complex plane.

cond-mat.str-el↗

arXiv2005

Berezinskii-Kosterlitz-Thouless transitions in the six-state clock model

Classical 2D clock model is known to have a critical phase with Berezinskii-Kosterlitz-Thouless(BKT) transitions. These transitions have logarithmic corrections which make numerical analysis difficult. In order to resolve this difficulty, one of the authors has proposed the method called level spectroscopy, which is based on the conformal field theory. We extend this method to the multi-degenerated case. As an example, we study the classical 2D 6-clock model which can be mapped to the quantum self-dual 1D 6-clock model. Additionally, we confirm that the self-dual point has a precise numerical agreement with the analytical result, and we argue the degeneracy of the excitation states at the self-dual point from the effective field theoretical point of view.

cond-mat.stat-mech↗

arXiv2005

Level spectroscopy of the square-lattice three-state Potts model with a ferromagnetic next-nearest-neighbor coupling

We study the square-lattice three-state Potts model with the ferromagnetic next-nearest-neighbor coupling at finite temperature. Using the level-spectroscopy method, we numerically analyze the excitation spectrum of the transfer matrices and precisely determine the global phase diagram. Then we find that, contrary to a previous result based on the finite-size scaling, the massless region continues up to the decoupling point with ${\bf Z}_3\times{\bf Z}_3$ criticality in the antiferromagnetic region. We also check the universal relations among excitation levels to provide the reliability of our result.

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