arXiv subjects

Seiji Miyashita

Publications and source records attributed to Seiji Miyashita.

At least 73 records · Page 4Linked to original sources

arXiv2009

Intrinsic effects of the boundary condition on switching processes in spin-crossover solids

We investigated domain growth in switching processes between the low-spin and high-spin phases in thermally induced hysteresis loops of spin-crossover (SC) solids. Elastic interactions among the molecules induce effective long-range interactions, and thus the boundary condition plays a significant role in the dynamics. In contrast to SC systems with periodic boundary conditions, where uniform configurations are maintained during the switching process, we found that domain structures appear with open boundary conditions. Unlike Ising-like models with short-range interactions, domains always grow from the corners of the system. The present clustering mechanism provides an insight into the switching dynamics of SC solids, in particular, in nano-scale systems.

cond-mat.mtrl-sci↗

arXiv2009

Comparison among Various Expressions of Complex Admittance for Quantum System in Contact with Heat Reservoir

Relation among various expressions of the complex admittance for quantum systems in contact with heat reservoir is studied. Exact expressions of the complex admittance are derived in various types of formulations of equations of motion under contact with heat reservoir. Namely, the complex admittance is studied in the relaxation method and the external-field method. In the former method, the admittance is calculated using the Kubo formula for quantum systems in contact with heat reservoir in no external driving fields, while in the latter method the admittance is directly calculated from equations of motion with external driving terms. In each method, two types of equation of motions are considered, i.e., the time-convolution (TC) equation and time-convolutionless (TCL) equation. That is, the full of the four cases are studied. It is turned out that the expression of the complex admittance obtained by using the relaxation method with the TC equation exactly coincides with that obtained by using the external-field method with the TC equation, while other two methods give different forms. It is also explicitly demonstrated that all the expressions of the complex admittance coincide with each other in the lowest Born approximation for the systemreservoir interaction. The formulae necessary for the higher order expansions in powers of the system-reservoir interaction are derived, and also the expressions of the admittance in the n-th order approximation are given. To characterize the TC and TCL methods, we study the expressions of the admittances of two exactly solvable models. Each exact form of admittance is compared with the results of the two methods in the lowest Born approximation. It is found that depending on the model, either of TC and TCL would be the better method.

cond-mat.stat-mech↗

arXiv2009

Nagaoka ferromagnetism in large-spin systems -Fermion and Boson systems--

We study magnetic properties of itinerant quantum magnetic particles described by a generalized Hubbard model with large spin ($S>1/2$) which may be realized in optical lattices of laser-cooled atom systems. In fermion systems (half-integer spins), an extended form of Nagaoka ferromagnetism may be realized. However, as novel aspects of the large spin cases, we found that the condition on the lattice connectivity is more stringent than in the case of $S=1/2$ particles, and that the system shows a peculiar degenerate structure of the ground statein which the ferromagnetic state is included. In contrast, it turns out that the ground state of itinerant bosonic systems (integer spins) has a degenerate structure similar to that of fermion system with $S>1/2$ regardless of the shape, connectivity or filling of the lattice, and that the state with the maximum total spin is always one of the ground states.

cond-mat.str-el↗

arXiv2009

Master equation approach to line shape in dissipative systems

We propose a formulation to obtain the line shape of a magnetic response with dissipative effects that directly reflects the nature of the environment. Making use of the fact that the time evolution of a response function is described by the same equation as the reduced density operator, we formulate a full description of the complex susceptibility. We describe the dynamics using the equation of motion for the reduced density operator, including the term for the initial correlation between the system and a thermal bath. In this formalism, we treat the full description of non-Markovian dynamics, including the initial correlation. We present an explicit and compact formula up to the second order of cumulants, which can be applied in a straightforward way to multiple spin systems. We also take into account the frequency shift by the system-bath interaction. We study the dependence of the line shape on the type of interaction between the system and the thermal bath. We demonstrate that the present formalism is a powerful tool for investigating various kinds of systems, and we show how it is applied to spin systems, including those with up to three spins. We distinguish the contributions of the initial correlation and the frequency shift, and make clear the role of each contribution in the Ohmic coupling spectral function. As examples of applications to multispin systems, we obtain the dependence of the line shape on the spatial orientation in relation to the direction of the static field (Nagata-Tazuke effect), including the effects of the thermal environment, in a two-spin system, along with the dependence on the arrangement of a triangle in a three-spin system.

quant-ph↗

arXiv2009

Mechanism of Slow Relaxation due to Screening Effect in a Frustrated System

We study a slow relaxation process in a frustrated spin system in which a type of screening effect due to a frustrated environment plays an important role. This screening effect is attributed to the highly degenerate configurations of the frustrated environment. This slow relaxation is due to an entropy effect and is different from those due to the energy barrier observed in systems such as random ferromagnets. In the present system, even if there is no energy gap, the slow relaxation still takes place. Thus, we call this phenomenon ``entropic slowing down''. Here, we study the mechanism of entropic slowing down quantitatively in an Ising spin model with the so-called decorated bonds. The spins included in decorated bonds (decoration spins) cause a peculiar density of states, which causes on entropy-induced screening effect. We analytically estimate the time scale of the system that increases exponentially with the number of decoration spins. We demonstrate the scaling of relaxation processes using the time scale at all temperatures including the critical point.

cond-mat.stat-mech↗

arXiv2009

Quantum Annealing for Clustering

This paper studies quantum annealing (QA) for clustering, which can be seen as an extension of simulated annealing (SA). We derive a QA algorithm for clustering and propose an annealing schedule, which is crucial in practice. Experiments show the proposed QA algorithm finds better clustering assignments than SA. Furthermore, QA is as easy as SA to implement.

cond-mat.dis-nn↗

arXiv2009

Quantum Annealing for Variational Bayes Inference

This paper presents studies on a deterministic annealing algorithm based on quantum annealing for variational Bayes (QAVB) inference, which can be seen as an extension of the simulated annealing for variational Bayes (SAVB) inference. QAVB is as easy as SAVB to implement. Experiments revealed QAVB finds a better local optimum than SAVB in terms of the variational free energy in latent Dirichlet allocation (LDA).

cond-mat.dis-nn↗

arXiv2009

Structure of energy level degeneracy of a single-spin model from a view point of symmetry of the spin anisotropy and its nontrivial spin($S$)-dependence on the higher order anisotropy

We study structure of the gapless points (diabolical points) at zero magnetic field ($H_z=0$) of single-spin models with spin anisotropies. Nontrivial appearance of diabolical points at finite transverse field $H_x$ has been studied from the view point of interference of the Berry phase, and related phenomena have been experimentally found in the single molecular magnet Fe$_8$. We study effects of the orthorhombic single-ion anisotropy $E(S_+^2+S_-^2)$ and find a symmetry associated with the degeneracy, which provides a clear picture of the global structure of energy level diagram including the excited states. Moreover, we study effects of the higher order anisotropy $C(S_+^4+S_-^4)$, and find that, in contrast to the semiclassical limit $(S\to\infty)$, location of a pair annihilation of the diabolical point does not coincides with a point at which a pair of diabolical points appears in nonzero $H_y$ space(bifurcation points). Distance between the annihilation and bifurcation points vanishes when $S\to\infty$, which restores the semiclassical result. We obtain a complete structure of the diabolical points in the $(C,H_x)$ plane.

cond-mat.mes-hall↗

arXiv2009

Threshold Phenomena under Photo Excitation of Spin-crossover Materials with Cooperativity due to Elastic Interactions

Photo-induced switching from the low-spin state to the high-spin state is studied in a model of spin-crossover materials, in which long-range interactions are induced by elastic distortions due to different molecular sizes the two spin states. At a threshold value of the light intensity we observe nonequilibrium critical behavior corresponding to a mean-field spinodal point. Finite-size scaling of the divergence of the relaxation time is revealed by analysis of kinetic Monte Carlo simulations.

cond-mat.stat-mech↗

arXiv2009

Scaling properties of the relaxation time near the mean-field spinodal

We study the relaxation processes of the infinitely long-range interaction model (the Husimi-Temperley model) near the spinodal point. We propose a unified finite-size scaling function near the spinodal point, including the metastable region, the spinodal point, and the unstable region. We explicitly adopt the Glauber dynamics, derive a master equation for the probability distribution of the total magnetization, and perform the so-called van Kampen Omega expansion (an expansion in terms of the inverse of the systems size), which leads to a Fokker-Planck equation. We analyze the scaling properties of the Fokker-Planck equation and confirm the obtained scaling plot by direct numerical solution of the original master equation, and by kinetic Monte Carlo simulation of the stochastic decay process.

cond-mat.stat-mech↗

arXiv2009

Quantum Spinodal Phenomena

We study the dynamical magnetization process in the ordered ground-state phase of the transverse Ising model under sweeps of magnetic field with constant velocities. In the case of very slow sweeps and for small systems studied previously (Phys. Rev. B 56, 11761 (1997)), non-adiabatic transitions at avoided level-crossing points give the dominant contribution to the shape of magnetization process. In contrast, in the ordered phase of this model and for fast sweeps, we find significant, size-independent jumps in the magnetization process. We study this phenomenon in analogy to the spinodal decomposition in classical ordered state and investigate its properties and its dependence on the system parameters. An attempt to understand the magnetization dynamics under field sweep in terms of the energy-level structure is made. We discuss a microscopic mechanism of magnetization dynamics from a viewpoint of local cluster flips and show that this provides a picture that explains the size independence. The magnetization dynamics in the fast-sweep regime is studied by perturbation theory and we introduce a perturbation scheme based on interacting Landau-Zener type processes to describe the local cluster flip dynamics.

cond-mat.other↗

arXiv2009

Quantum response to time-dependent external fields

Recently, explicit real time dynamics has been studied in various systems. These quantum mechanical dynamics could provide new recipes in information processing. We study quantum dynamics under time dependent external fields, and explore how to control the quantum state, and also how to bring the state into a target state. Here, we investigate a pure quantum mechanical dynamics, dynamics in quantum Monte Carlo simulation and also in quantum master equation. For the control magnetic states, operators which do not commute with magnetization are important. We study case of the transverse Ising model, in which we compare natures of thermal and quantum fluctuations. We also study the cases of the Dzyaloshinsky-Moriya interaction, where we find a peculiar energy level structure. Moreover we study the case of itinerant magnetic state, where we study the change from the Mott insulator to the Nagaoka ferromagnetic state. Effects of dissipation are also discussed.

cond-mat.stat-mech↗

arXiv2008

Dynamics of the Density Matrix in Contact with a Thermal Bath and the Quantum Master Equation

We study the structure of the time evolution of the density matrix in contact with a thermal bath in a standard projection operator sheme. The reduced density matrix of the system in the steady state is obtained by tracing out the degree of freedom of the thermal bath from the equilibrium density matrix of the total system. This reduced density matrix is modified by the interaction, and is different from that of the equilibrium of the system alone. We explicitly calculate the contribution of each term in quantum master equation to the realization of the steady state density matrix, and make clear roles of each term. By making use of the role of each term, the properties of the commonly used quantum master equation are examined.

cond-mat.stat-mech↗

arXiv2008

Successive phase transitions at finite temperatures of the supersolid in the three-dimensional extended Bose-Hubbard model

We study the finite temperature properties of the extended Bose-Hubbard model on a cubic lattice. This model exhibits the so-called supersolid state. To start with, we investigate ordering processes by quantum Monte Carlo simulations, and find successive superfluid and solid phase transitions. There, we find that the two order parameters compete with each other. We obtain the finite temperature phase diagram, which contains the superfluid, the solid, the supersolid and the disordered phase. We develop a mean-field theory to analyze the ordering processes and compare the result with that obtained by simulations, and discuss the mechanism of the competition of these two orders. We also study how the supersolid region shrinks as the on-site repulsion becomes strong.

cond-mat.stat-mech↗

arXiv2007

Event-based computer simulation model of Aspect-type experiments strictly satisfying Einstein's locality conditions

Inspired by Einstein-Podolsky-Rosen-Bohm experiments with photons, we construct an event-based simulation model in which every essential element in the ideal experiment has a counterpart. The model satisfies Einstein's criteria of local causality and does not rely on concepts of quantum and probability theory. We consider experiments in which the averages correspond to those of a singlet and product state of a system of two $S=1/2$ particles. The data is analyzed according to the experimental procedure, employing a time window to identify pairs. We study how the time window and the passage time of the photons, which depends on the relative angle between their polarization and the polarizer's direction, influences the correlations, demonstrating that the properties of the optical elements in the observation stations affect the correlations although the stations are separated spatially and temporarily. We show that the model can reproduce results which are considered to be intrinsically quantum mechanical.

quant-ph↗

arXiv2007

Realization of the mean-field universality class in spin-crossover materials

In spin-crossover materials, the volume of a molecule changes depending on whether it is in the high-spin (HS) or low-spin (LS) state. This change causes distortion of the lattice. Elastic interactions among these distortions play an important role for the cooperative properties of spin-transition phenomena. We find that the critical behavior caused by this elastic interaction belongs to the mean-field universality class, in which the critical exponents for the spontaneous magnetization and the susceptibility are $β= 1/2$ and $γ= 1$, respectively. Furthermore, the spin-spin correlation function is a constant at long distances, and it does not show an exponential decay in contrast to short-range models. The value of the correlation function at long distances shows different size-dependences: $O(1/N)$, $O(1/\sqrt{N})$, and constant for temperatures above, at, and below the critical temperature, respectively. The model does not exhibit clusters, even near the critical point. We also found that cluster growth is suppressed in the present model and that there is no critical opalescence in the coexistence region. During the relaxation process from a metastable state at the end of a hysteresis loop, nucleation phenomena are not observed, and spatially uniform configurations are maintained during the change of the fraction of HS and LS. These characteristics of the mean-field model are expected to be found not only in spin-crossover materials, but also generally in systems where elastic distortion mediates the interaction among local states.

cond-mat.mtrl-sci↗

arXiv2007

Slow Relaxation of Spin Structure in Exotic Ferromagnetic Phase of Ising-like Heisenberg Kagome Antiferromagnets

In the corner-sharing lattice, magnetic frustration causes macroscopic degeneracy in the ground state, which prevents systems from ordering. However, if the ensemble of the degenerate configuration has some global structure, the system can have a symmetry breaking phenomenon and thus posses a finite temperature phase transition. As a typical example of such cases, the magnetic phase transition of the Ising-like Heisenberg antiferromagnetic model on the kagome lattice has been studied. There, a phase transition of the two-dimensional ferromagnetic Ising universality class occurs accompanying with the uniform spontaneous magnetization. Because of the macroscopic degeneracy in the ordered phase, the system is found to show an entropy-driven ordering process, which is quantitatively characterized by the number of ``weathervane loop''. We investigate this novel type of slow relaxation in regularly frustrated system.

cond-mat.stat-mech↗

arXiv2007

Non-monotonic Relaxation in Systems with Reentrant Type Interaction

Recently, interesting non-monotonic time evolution has been pointed out in the experiments by Jönsson, {\it et al.} and Jonsson {\it et.al.} and also in the numerical simulation by Takayama and Hukushima where the magnetic susceptibility does not monotonically relax to the equilibrium value, but moves to the opposite side. We study mechanism of this puzzling non-monotonic dynamical property in a frustrated Ising model in which the equilibrium correlation exhibits non-monotonic temperature dependence (reentrant type). We study the time evolution of spin correlation function after sudden change of temperature. There, we find that the value of the correlation function shows non-monotonic relaxation, and analyze mechanisms of the non-monotonicity. We also point out that competition between different configurations widely causes non-monotonic relaxation.

cond-mat.stat-mech↗