arXiv subjects

Yoshihiko Nonomura

Publications and source records attributed to Yoshihiko Nonomura.

At least 19 recordsLinked to original sources

arXiv2022

Phase diagram of the dipolar Ising ferromagnet on a kagome lattice

We study the field--temperature phase diagram of the two-dimensional dipolar Ising ferromagnet on a kagome lattice with a specific ratio between the exchange and dipolar constants, $δ= 1$. Using the stochastic cutoff (SCO) $O(N)$ Monte Carlo method, we calculated order parameters for stripe and bubble phases and other thermodynamical quantities. We find two kinds of stripe phases at low fields, where the arrangement of the branch spins neighboring the stripe frame varies, and two bubble phases at high fields, in which three-spin domains (bubbles) form a regular triangular lattice but the triangular array of bubbles changes on a kagome lattice. We also find that with increasing the field, there exist a disordered phase between the stripe and bubble phases and between the two bubble phases. We discuss the details of the features of these phases and phase transitions.

cond-mat.stat-mech↗

arXiv2021

Phase diagram of the two-dimensional dipolar Heisenberg model with the Dzyaloshinskii-Moriya interaction and the Ising anisotropy

We study phase transitions in the two-dimensional Heisenberg model with the Dzyaloshinskii-Moriya interaction, the Ising anisotropy ($η$), and the dipolar interaction under zero and finite magnetic fields ($H$). For three typical strengths (zero, weak, and strong) of the dipolar interaction, we present the $H$-$η$ phase diagrams by estimating order parameters for skyrmion-lattice and helical phases and in-plane magnetization by using a Monte Carlo method with an $O(N)$ algorithm. We find in the phase diagrams three types of skyrmion-lattice phases, i.e., two square lattices and a triangular lattice, helical phases with diagonal and vertical (or horizontal) stripes, canted ferromagnetic phase and polarized ferromagnetic phase. The effect of the dipolar interaction varies the types of the skyrmion and helical phases in a complex manner. The dipolar interaction also expands the regions of the ordered phases accompanying shifts of the phase boundaries to the positive $H$ and $η$ directions, and causes increase of the density of skyrmions and shortening of the pitch length (stripe width) of helical structures. We discuss the details of the features of the phase transitions.

cond-mat.stat-mech↗

arXiv2020

Temperature scaling in nonequilibrium relaxation in three-dimensional Heisenberg model in the Swendsen-Wang and Metropolis algorithms

Recently, the present authors proposed the nonequilibrium-to-equilibrium scaling (NE-ES) scheme for critical Monte Carlo relaxation process, which scales relaxation data in the whole simulation-time regions regardless of functional forms, namely both for the stretched-exponential critical relaxation in cluster algorithms and for the power-law critical relaxation in local-update algorithms. In the present study, we generalize this scheme to off-critical relaxation process, and scale relaxation data for various temperatures in the whole simulation-time regions. This is the first proposal of the off-critical scaling in cluster algorithms, which cannot be described by the dynamical finite-size scaling theory based on the power-law critical relaxation. As an example, we investigate the three-dimensional Heisenberg model previously analyzed with the NE-ES [Y. Nonomura and Y. Tomita, Phys. Rev. E 93, 012101 (2016)] in the Swendsen-Wang and Metropolis algorithms.

cond-mat.stat-mech↗

arXiv2019

Nonequilibrium-relaxation approach to quantum phase transitions: Nontrivial critical relaxation in cluster-update quantum Monte Carlo

Although the nonequilibrium relaxation (NER) method has been widely used in Monte Carlo studies on phase transitions in classical spin systems, such studies have been quite limited in quantum phase transitions. The reason is that relaxation process based on cluster-update quantum Monte Carlo (QMC) algorithms, which are now standards in Monte Carlo studies on quantum systems, has been considered "too fast" for such analyses. Recently the present authors revealed that the NER process in classical spin systems based on cluster-update algorithms is characterized by the stretched-exponential critical relaxation, rather than the conventional power-law one in local-update algorithms. In the present article we show that this is also the case in quantum phase transitions analyzed with the cluster-update QMC, and that advantages of NER analyses are available. As the simplest example of isotropic quantum spin models which exhibit quantum phase transitions, we investigate the Néel-dimer quantum phase transition in the two-dimensional $S=1/2$ columnar-dimerized antiferromagnetic Heisenberg model with the continuous-time loop algorithm.

cond-mat.stat-mech↗

arXiv2019

Anisotropy-temperature phase diagram for the two-dimensional dipolar Heisenberg model with and without magnetic field

We investigate phase transitions in the two-dimensional dipolar Heisenberg model with uniaxial anisotropy with a specific ratio between the exchange and dipolar constants, $δ=1$. We obtain the $η$--$T$ (anisotropy vs. temperature) phase diagrams for typical values of magnetic field by a Monte Carlo method with an $O(N)$ algorithm. We find that at lower fields, the $η$--$T$ phase diagram consists of the planar ferromagnetic (F), (perpendicular) stripe-ordered (SO), and paramagnetic (P) phases, and is characterized by the triple point. In the SO phase realized at larger $η$ and smaller $T$, the SO pattern changes depending on the field. On the other hand, we find that at higher fields, the SO phase does not exist, while the planer F phase robustly remains. We study the properties of the phase boundaries by employing finite-size-scaling analyses. We find that the slope of the spin-reorientation-transition line is positive with and without field, i.e., $\frac{dη}{dT}>0$, which implies that the planar F phase changes to the SO phase with lowering temperature. In the phase diagrams we observe a characteristic shape of the P--planer F phase-transition line, whose maximum point of $η$ is located at an intermediate temperature. This structure leads to the temperature-induced reentrant transition associated with P and planar F phases, which appears in successive phase transitions with lowering temperature: P $\rightarrow$ planar F $\rightarrow$ P $\rightarrow$ SO phase at lower fields and P $\rightarrow$ planar F $\rightarrow$ P phases at higher fields.

cond-mat.stat-mech↗

arXiv2018

Critical Nonequilibrium Cluster-flip Relaxations in Ising Models

We investigate nonequilibrium relaxations of Ising models at the critical point by using a cluster update. While preceding studies imply that nonequilibrium cluster-flip dynamics at the critical point are universally described by the stretched-exponential function, we find that the dynamics changes from the stretched-exponential to the power function as the dimensionality is increased: The two-, three-, four-, and infinite-dimensional Ising models are numerically studied, and the four- and infinite-dimensional Ising models exhibit the power-law relaxation. We also show that the finite-size scaling analysis using the normalized correlation length is markedly effective for the analysis of relaxational processes rather than the direct use of the Monte Carlo step.

cond-mat.stat-mech↗

arXiv2018

Temperature-field phase diagram of the two-dimensional dipolar Ising ferromagnet

We study field-induced phase transitions in the two-dimensional dipolar Ising ferromagnet with a specific ratio between the exchange and dipolar constants, $δ=1$, which exhibits a stripe-ordered phase with the width of one lattice unit at low temperatures without magnetic field. By using a mean-field (MF) approximation and a Monte Caro (MC) method with the stochastic-cutoff algorithm, which is an $O(N)$ simulation method, we show the temperature-field phase diagrams. In the MC study the orientational order and the structure factor are evaluated. Second-order transition points are determined by a finite-size-scaling analysis and first-order transition points are identified by the analysis of the energy histogram. Although both the MF and MC phase diagrams consist of wide regions of several stripe-ordered phases and narrow regions between them characterized by complicated stripe patterns, they show qualitative and quantitative differences in possible phases and phase boundaries. In the MF phase diagram, three main stripe-ordered phases exhibit a nesting structure, while in the MC phase diagram, two main stripe-ordered phases are located separately, which causes a characteristic field-induced reentrant transition of the orientational order.

cond-mat.stat-mech↗

arXiv2018

Thermal algebraic-decay charge liquid driven by competing short-range Coulomb repulsion

We explore the possibility of a Berezinskii-Kosterlitz-Thouless-like critical phase for the charge degrees of freedom in the intermediate-temperature regime between the charge-ordered and disordered phases in two-dimensional systems with competing short-range Coulomb repulsion. As the simplest example, we investigate the extended Hubbard model with on-site and nearest-neighbor Coulomb interactions on a triangular lattice at half filling in the atomic limit by using a classical Monte Carlo method, and find a critical phase, characterized by algebraic decay of the charge correlation function, belonging to the universality class of the two-dimensional XY model with a $\mathbb{Z}_6$ anisotropy. Based on the results, we discuss possible conditions for the critical phase in materials.

cond-mat.str-el↗

arXiv2018

Cluster nonequilibrium relaxation in Ising models observed with the Binder ratio

The Binder ratios exhibit discrepancy from the Gaussian behavior of the magnetic cumulants, and their size independence at the critical point has been widely utilized in numerical studies of critical phenomena. In the present article we reformulate the nonequilibrium relaxation (NER) analysis in cluster algorithms using the $(2,1)$-Binder ratio, and apply this scheme to the two- and three-dimensional Ising models. Although the stretched-exponential relaxation behavior at the critical point is not explicitly observed in this quantity, we find that there exists a logarithmic finite-size scaling formula which can be related with a similar formula recently derived in cluster NER of the correlation length, and that the formula enables precise evaluation of the critical point and the stretched-exponential relaxation exponent $σ$. Physical background of this novel behavior is explained by the simulation-time dependence of the distribution function of magnetization in two dimensions and temperature dependence of $σ$ obtained from magnetization in three dimensions.

cond-mat.stat-mech↗

arXiv2015

Critical nonequilibrium relaxation in the Swendsen-Wang algorithm in the Berezinsky-Kosterlitz-Thouless and weak first-order phase transitions

Recently we showed that the critical nonequilibrium relaxation in the Swendsen-Wang algorithm is widely described by the stretched-exponential relaxation of physical quantities in the Ising or Heisenberg models. Here we make a similar analysis in the Berezinsky-Kosterlitz-Thouless phase transition in the two-dimensional (2D) XY model and in the first-order phase transition in the 2D $q=5$ Potts model, and find that these phase transitions are described by the simple exponential relaxation and power-law relaxation of physical quantities, respectively. We compare the relaxation behaviors of these phase transitions with those of the second-order phase transition in the 3D and 4D XY models and in the 2D $q$-state Potts models for $2 \le q \le 4$, and show that the species of phase transitions can be clearly characterized by the present analysis. We also compare the size dependence of relaxation behaviors of the first-order phase transition in the 2D $q=5$ and $6$ Potts models, and propose a quantitative criterion on "weakness" of the first-order phase transition.

cond-mat.stat-mech↗

arXiv2015

Nonequilibrium behaviors of 3D Heisenberg model in the Swendsen-Wang algorithm

Recently Y. N. showed that the nonequilibrium critical relaxation of the 2D Ising model from the perfectly-ordered state in the Wolff algorithm is described by the stretched-exponential decay, and found a universal scaling scheme to connect nonequilibrium and equilibrium behaviors. In the present study we extend these findings to vector spin models, and the 3D Heisenberg model could be a typical example. In order to evaluate the critical temperature and critical exponents precisely with the above scaling scheme, we calculate the nonequilibrium ordering from the perfectly-disordered state in the Swendsen-Wamg algorithm, and find that the critical ordering process is described by the stretched-exponential growth with the comparable exponent to that of the 3D XY model. The critical exponents evaluated in the present study are consistent with those in previous studies.

cond-mat.stat-mech↗

arXiv2014

New Nonequilibrium-to-Equilibrium Dynamical Scaling and Stretched-Exponential Critical Relaxation in Cluster Algorithms

Nonequilibrium relaxation behaviors in the Ising model on a square lattice based on the Wolff algorithm are totally different from those based on local-update algorithms. In particular, the critical relaxation is described by the stretched-exponential decay. We propose a novel scaling procedure to connect nonequilibrium and equilibrium behaviors continuously, and find that the stretched-exponential scaling region in the Wolff algorithm is as wide as the power-law scaling region in local-update algorithms. We also find that relaxation to the spontaneous magnetization in the ordered phase is characterized by the exponential decay, not the stretched-exponential decay based on local-update algorithms.

cond-mat.stat-mech↗

arXiv2012

Nontrivial in-plane-magnetic-field dependence of THz wave emission from intrinsic Josephson junctions controlled by surface impedance

In THz wave emission from intrinsic Josephson junctions in in-plane magnetic fields, emission intensity strongly depends on the surface impedance $Z$ similarly to the case without external magnetic fields. Cavity resonance modes are stabilized for $Z \ge 3$, and the fundamental mode gives the strongest emission. As the in-plane magnetic field increases for a fixed number of junctions, dynamical phase transitions seem to occur between the $π$-phase-kink state, various incommensurate phase-kink states, and in-phase state. As $Z$ varies, a crossover of the field profile of maximum intensity takes place for $Z \approx 50$ between characteristic peaks for smaller $Z$ (typically $Z \approx 30$) and monotonic decrease for larger $Z$ (typically $Z \approx 70$). The double-peak structure reported in experiments can be explained for Z=30 by finite-size analysis with respect to number of junctions.

cond-mat.supr-con↗

arXiv2012

Hysteresis behavior in current-driven stationary resonance induced by nonlinearity in the coupled sine-Gordon equation

Recently novel current-driven resonant states characterized by the $π$-phase kinks were proposed in the coupled sine-Gordon equation. In these states hysteresis behavior is observed with respect to the application process of current, and such behavior is due to nonlinearity in the sine term. Varying strength of the sine term, there exists a critical strength for the hysteresis behavior and the amplitude of the sine term coincides with the applied current at the critical strength.

cond-mat.other↗

arXiv2009

Stationary phase-kink states and dynamical phase transitions controlled by surface impedance in THz wave emission from intrinsic Josephson junctions

As possible states to characterize THz wave emission from intrinsic Josephson junctions without external fields, the McCumber-like state and $π$-phase-kink state have been proposed. In the present article it is numerically shown that both states are stationary according to the bias current $J$ and surface impedance $Z$. The McCumber-like state is stable for low $J$ and small $Z$. For higher $J$, the $π$-phase-kink state accompanied with symmetry breaking along the c axis is stable even for Z=1, though strong emission in the vicinity of cavity resonance points only takes place for larger $Z$. Different emission behaviors for Z=1 and 10 are precisely compared. The dynamical phase diagram for $1 \le Z \le 10$ and the optimal value of $Z$ for the strongest emission are also evaluated.

cond-mat.supr-con↗

arXiv2006

Energy landscape and shear modulus of interlayer Josephson vortex systems

The ground state of interlayer Josephson vortex systems is investigated on the basis of a simplified Lawrence-Doniach model in which spatial dependence of the gauge field and the amplitude of superconducting order parameter is not taken into account. Energy landscape is drawn with respect to the in-plane field, the period of insulating layers including Josephson vortices, and the shift from the aligned vortex lattice. The energy landscape has a multi-valley structure and ground-state configurations correspond to bifurcation points of the valleys. In the high-field region, the shear modulus becomes independent of field and its anisotropy dependence is given by $c_{66}\propto γ^{-4}$.

cond-mat.supr-con↗

arXiv2004

Reply to "Comment on "Effects of Point Defects on the Phase Diagram of Vortex States in High-Tc Superconductors in the B//c Axis""

We reply to a recent Comment on our previous article [Phys. Rev. Lett. 86, 5140 (2001)] by Olsson and Teitel [cond-mat/0404473]. We point out that their scaling argument to question the possible vortex slush phase in the frustrated XY model with point defects cannot be sufficiently justified by the provided numerical data, and that their data are indeed consistent with our previous article.

cond-mat.supr-con↗

arXiv2003

Crossover behaviors in liquid region of vortex states above a critical point caused by point defects

Vortex states in high-$T_{\rm c}$ superconductors with point defects are studied by large-scale Monte Carlo simulations of the three-dimensional frustrated XY model. A critical point is observed on the first-order phase boundary between the vortex slush and vortex liquid phases. A step-like anomaly of the specific heat is detected in simulations of finite systems, similar to an experimental observation [F.~Bouquet {\it et al.}, Nature (London) {\bf 411}, 448 (2001)]. However, it diminishes with increasing system size, and the number and size distribution of thermally-excited vortex loops show continuous behaviors around this anomaly. Therefore, the present study suggests a crossover rather than a thermodynamic phase transition above the critical point.

cond-mat.supr-con↗