arXiv subjects

Koji Nemoto

Publications and source records attributed to Koji Nemoto.

28 records · Page 2Linked to original sources

arXiv2008

Spatio-temporal scaling for out-of-equilibrium relaxation dynamics of an elastic manifold in random media: crossover between the Larkin regime and thermally activated regime

We study relaxation dynamics of a three dimensional elastic manifold in random potential from a uniform initial condition by numerically solving the Langevin equation.We observe growth of roughness of the system up to larger wavelengths with time.We analyze structure factor in detail and find a compact scaling ansatz describing two distinct time regimes and crossover between them. We find short time regime corresponding to length scale smaller than the Larkin length $L_c$ is well described by the Larkin model which predicts a power law growth of domain size $L(t)$. Longer time behavior exhibits the random manifold regime with slower growth of $L(t)$.

cond-mat.dis-nn↗

arXiv2006

Nonequilibrium relaxation analysis of a quasi-one-dimensional frustrated XY model for charge-density waves in ring-shaped crystals

We propose a model for charge density waves in ring shaped crystals, which depicts frustration between intra- and inter-chain couplings coming from cylindrical bending. It is then mapped to a three dimensional uniformly frustrated XY model with one dimensional anisotropy in connectivity. The nonequilibrium relaxation dynamics is investigated by Monte Carlo simulations to find a phase transition which is quite different from that of usual whisker crystal. We also find that the low temperature state is a three dimensional phase vortex lattice with a two dimensional phase coherence in a cylindrical shell and the system shows power law relaxation in the ordered phase.

cond-mat.stat-mech↗

arXiv2004

Ordered Phase of the Dipolar Spin Ice under [110]-Magnetic Fields

We find that the true ground state of the dipolar spin ice system under [110]-magnetic fields is the ``Q=X'' structure, which is consistent with both experiments and Monte Carlo simulations. We then perform a Monte Carlo simulation to confirm that there exists a first order phase transition under the [110]-field. In particular this result indicates the existence of the first order phase transition to the ``Q=X'' phase in the field above 0.35 T for Dy2Ti2O7. We also show the magnetic field-temperature phase diagram to summarize the ordered states of this system.

cond-mat.stat-mech↗

arXiv2003

Symmetry, complexity and multicritical point of the two-dimensional spin glass

We analyze models of spin glasses on the two-dimensional square lattice by exploiting symmetry arguments. The replicated partition functions of the Ising and related spin glasses are shown to have many remarkable symmetry properties as functions of the edge Boltzmann factors. It is shown that the applications of homogeneous and Hadamard inverses to the edge Boltzmann matrix indicate reduced complexities when the elements of the matrix satisfy certain conditions, suggesting that the system has special simplicities under such conditions. Using these duality and symmetry arguments we present a conjecture on the exact location of the multicritical point in the phase diagram.

cond-mat.stat-mech↗

arXiv2002

Duality and Multicritical Point of Two-Dimensional Spin Glasses

Determination of the precise location of the multicritical point and phase boundary is a target of active current research in the theory of spin glasses. In this short note we develop a duality argument to predict the location of the multicritical point and the shape of the phase boundary in models of spin glasses on the square lattice.

cond-mat.dis-nn↗

arXiv2000

Numerical Study of Aging in the Generalized Random Energy Model

Magnetizations are introduced to the Generalized Random Energy Model (GREM) and numerical simulations on ac susceptibility is made for direct comparison with experiments in glassy materials. Prominent dynamical natures of spin glasses, {\it i.e.}, {\em memory} effect and {\em reinitialization}, are reproduced well in the GREM. The existence of many layers causing continuous transitions is very important for the two natures. Results of experiments in other glassy materials such as polymers, supercooled glycerol and orientational glasses, which are contrast to those in spin glasses, are interpreted well by the Single-layer Random Energy Model.

cond-mat.dis-nn↗

arXiv2000

Analysis on Aging in the Generalized Random Energy Model

A new dynamics more natural than that proposed by Bouchaud and Dean is introduced to the Generalized Random Energy Model, and the master equation for the dynamics is solved exactly to calculate the time correlation function. Although our results are very similar to those obtained by Bouchaud and Dean qualitatively, the exponents for power law relaxation are different. The Zero-Field-Cooled magnetization is also calculated with a relation between the correlation function and the response function which holds even if the relaxation is non-equilibrium. The validity of these analytic results are confirmed by numerical simulations.

cond-mat.dis-nn↗

arXiv2000

Memory Effect, Rejuvenation and Chaos Effect in the Multi-layer Random Energy Model

We introduce magnetization to the Multi-layer Random Energy Model which has a hierarchical structure, and perform Monte Carlo simulation to observe the behavior of ac-susceptibility. We find that this model is able to reproduce three prominent features of spin glasses, i.e., memory effect, rejuvenation and chaos effect, which were found recently by various experiments on aging phenomena with temperature variations.

cond-mat.dis-nn↗

arXiv2000

Scaling Law and Aging Phenomena in the Random Energy Model

We study the effect of temperature shift on aging phenomena in the Random Energy Model (REM). From calculation on the correlation function and simulation on the Zero-Field-Cooled magnetization, we find that the REM satisfies a scaling relation even if temperature is shifted. Furthermore, this scaling property naturally leads to results obtained in experiment and the droplet theory.

cond-mat.dis-nn↗

arXiv1995

Exchange Monte Carlo Method and Application to Spin Glass Simulations

We propose an efficient Monte Carlo algorithm for simulating a ``hardly-relaxing" system, in which many replicas with different temperatures are simultaneously simulated and a virtual process exchanging configurations of these replica is introduced. This exchange process is expected to let the system at low temperatures escape from a local minimum. By using this algorithm the three-dimensional $\pm J$ Ising spin glass model is studied. The ergodicity time in this method is found much smaller than that of the multi-canonical method. In particular the time correlation function almost follows an exponential decay whose relaxation time is comparable to the ergodicity time at low temperatures. It suggests that the system relaxes very rapidly through the exchange process even in the low temperature phase.

cond-mat↗