arXiv subjects

Shin-ichi Sawada

Publications and source records attributed to Shin-ichi Sawada.

10 recordsLinked to original sources

arXiv2018

Thermodynamic transports in a circular system with a temperature difference

Thermodynamic transport phenomena in the system consisting of many hard-disks confined in a circular tube with a temperature difference are discussed. Here, temperatures on parts of the walls of the tube are imposed by stochastic boundary conditions for particles to contact with these thermal walls. In this system, the temperature difference induces, not only energy currents, but also a circulating particle current, inside the tube. Transport properties of these steady currents are discussed in various values of system parameters, such as the temperature difference, the particle density, the width of the tube, and the positions of the thermal walls.

cond-mat.stat-mech↗

arXiv2017

Stochastic boundary approaches to many-particle systems coupled to a particle reservoir

Stochastic boundary conditions for interactions with a particle reservoir are discussed in many-particle systems. We introduce the boundary conditions with the injection rate and the momentum distribution of particles coming from a particle reservoir in terms of the pressure and the temperature of the reservoir. It is shown that equilibrium ideal gases and hard-disk systems with these boundary conditions reproduce statistical-mechanical properties based on the corresponding grand canonical distributions. We also apply the stochastic boundary conditions to a hard-disk model with a steady particle current escaping from a particle reservoir in an open tube, and discuss its nonequilibrium properties such as a chemical potential dependence of the current and deviations from the local equilibrium hypothesis.

cond-mat.stat-mech↗

arXiv2016

On the origin of atomistic mechanism of rapid diffusion in alkali halide nanoclusters

To elucidate the atomistic diffusion mechanism responsible for the rapid diffusion in alkali halide nano particles, called Spontaneous Mixing, we execute molecular dynamics simulations with empirical models for KCl-KBr, NaCl-NaBr, RbCl-RbBr and KBr-KI. We successfully reproduce essential features of the rapid diffusion phenomenon. It is numerically confirmed that the rate of the diffusion clearly depends on the size and temperature of the clusters, which is consistent with experiments. A quite conspicuous feature is that the surface melting and collective motions of ions are inhibited in alkali halide clusters. This result indicates that the Surface Peeling Mechanism, which is responsible for the spontaneous alloying of binary metals, does not play a dominant role for the spontaneous mixing in alkali halide nanoclusters. Detailed analysis of atomic motion inside the clusters reveals that the Vacancy Mechanism is the most important mechanism for the rapid diffusion in alkali halide clusters. This is also confirmed by evaluation of the vacancy formation energy: the formation energy notably decreases with the cluster size, which makes vacancy formation easier and diffusion more rapid in small alkali halide clusters.

cond-mat.mes-hall↗

arXiv2014

Escape Dynamics of Many Hard Disks

Many-particle effects in escapes of hard disks from a square box via a hole are discussed in a viewpoint of dynamical systems. Starting from $N$ disks in the box at the initial time, we calculate the probability $P_{n}(t)$ for at least $n$ disks to remain inside the box at time $t$ for $n=1,2,\cdots,N$. At early times the probabilities $P_{n}(t)$, $n=2,3,\cdots,N-1$, are described by superpositions of exponential decay functions. On the other hand, after a long time the probability $P_{n}(t)$ shows a power-law decay $\sim t^{-2n}$ for $n\neq 1$, in contrast to the fact that it decays with a different power law $\sim t^{-n}$ for cases without any disk-disk collision. Chaotic or non-chaotic properties of the escape systems are discussed by the dynamics of a finite time largest Lyapunov exponent, whose decay properties are related with those of the probability $P_{n}(t)$.

cond-mat.stat-mech↗

arXiv2012

Quantum particle escape from a time-dependent confining potential

Quantum escape of a particle via a time-dependent confining potential in a semi-infinite one-dimensional space is discussed. We describe the time-evolution of escape states in terms of scattering states of the quantum open system, and calculate the probability $P(t)$ for a particle to remain in the confined region at time $t$ in the case of a delta-function potential with a time-oscillating magnitude. The probability $P(t)$ decays exponentially in time at early times, then decays as a power later, along with a time-oscillation in itself. We show that a larger time-oscillation amplitude of the confining potential leads to a faster exponential decay of the probability $P(t)$, while it can rather enhance the probability $P(t)$ decaying as a power. These contrastive behaviors of the probability $P(t)$ in different types of decay are discussed quantitatively by using the decay time and the power decay magnitude of the probability $P(t)$.

cond-mat.stat-mech↗

arXiv2012

Quantum and classical chaos of a two-electron system in a quantum wire

We study classical and quantum dynamics of two spinless particles confined in a quantum wire with repulsive or attractive Coulomb interaction. The interaction induces irregular dynamics in classical mechanics, which reflects on the quantum properties of the system in the energy level statistics (the signatures of quantum chaos). We investigate especially closer correspondence between the classical and quantum chaos. The present classical dynamics has some scaling property, which the quantum counterpart does not have. However, we demonstrate that the energy level statistics implies the existence of the corresponding scaling property even in the quantum system. Instead of ordinary maximum Lyapunov exponent (MLE), we introduce a novel kind of MLE, which is shown to be suitable measure of chaotic irregularity for the present classical system. We show that tendency of the energy dependence of the Brody parameter, which characterizes the energy level statistics in the quantum system, is consistent with that of the novel kind of MLE.

cond-mat.mes-hall↗

arXiv2011

Particle escapes in an open quantum network via multiple leads

Quantum escapes of a particle from an end of a one-dimensional finite region to $N$ number of semi-infinite leads are discussed by a scattering theoretical approach. Depending on a potential barrier amplitude at the junction, the probability $P(t)$ for a particle to remain in the finite region at time $t$ shows two different decay behaviors after a long time; one is proportional to $N^{2}/t^{3}$ and another is proportional to $1/(N^{2}t)$. In addition, the velocity $V(t)$ for a particle to leave from the finite region, defined from a probability current of the particle position, decays in power $\sim 1/t$ asymptotically in time, independently of the number $N$ of leads and the initial wave function, etc. For a finite time, the probability $P(t)$ decays exponentially in time with a smaller decay rate for more number $N$ of leads, and the velocity $V(t)$ shows a time-oscillation whose amplitude is larger for more number $N$ of leads. Particle escapes from the both ends of a finite region to multiple leads are also discussed by using a different boundary condition.

cond-mat.stat-mech↗

arXiv2010

Escape Behavior of Quantum Two-Particle Systems with Coulomb Interactions

Quantum escapes of two particles with Coulomb interactions from a confined one-dimensional region to a semi-infinite lead are discussed by the probability of particles remaining in the confined region, i.e. the survival probability, in comparison with one or two free particles. For free-particle systems the survival probability decays asymptotically in power as a function of time. On the other hand, for two-particle systems with Coulomb interactions it shows an exponential decay in time. A difference of escape behaviors between Bosons and Fermions is considered as quantum effects of identical two particles such as the Pauli exclusion principle. The exponential decay in the survival probability of interacting two particles is also discussed in a viewpoint of quantum chaos based on a distribution of energy level spacings.

cond-mat.stat-mech↗

arXiv2002

On a relationship between the collective migration of surface atoms in microclusters and the saddle points on the potential energy surface

Plenty of saddles on a multidimensional potential energy surface(PES) of two-dimensional microclusters, where atoms are interacting via Morse potential, are numerically located. The reaction paths emanating from the two types of the local minima, which represent the compact and the non-compact shape of Morse clusters, to their neighboring saddles on PES are elucidated. By associating the reaction path crossing these saddles with the atomic rearrangements,we evaluate the barrier height corresponding to various characteristic atomic motion accompanied by the {\it floaters} (i.e. surface atoms popped out of the cluster surface). Our findings are summarized as: (i)The saddle points implying the {\it gliding motion} of a single {\it floater} over the cluster surface yields extremely small values of the energy barriers regardless of the shapes of clusters. In particular, the {\it gliding motion} of a train composed of a few surface atoms also appears as the low-lying saddles. As a result, the barrier height corresponding to the {\it simultaneous gliding motion}, which is a manifestation of the reaction path crossing the higher-index saddles on PES, is significantly low. (ii)A surface rearrangement, where {\it floaters} are created or annihilated, implies relatively high barrier energy which is still accessible below melting point. (iii)On the other hand, the atomic motion, where atoms located deep inside of the clusters are rearranged as well as surface atoms, yields extremely high barrier energies. Some relations between these results and the recent experimental study of the surface cluster diffusion are also pointed out.

cond-mat.mtrl-sci↗

arXiv2001

Spontaneous alloying in binary metal microclusters - A molecular dynamics study -

Microcanonical molecular dynamics study of the spontaneous alloying(SA), which is a manifestation of fast atomic diffusion in a nano-sized metal cluster, is done in terms of a simple two dimensional binary Morse model. Important features observed by Yasuda and Mori are well reproduced in our simulation. The temperature dependence and size dependence of the SA phenomena are extensively explored by examining long time dynamics. The dominant role of negative heat of solution in completing the SA is also discussed. We point out that a presence of melting surface induces the diffusion of core atoms even if they are solid-like. In other words, the {\it surface melting} at substantially low temperature plays a key role in attaining the SA.

cond-mat.mtrl-sci↗