Macoto Kikuchi

MICROPHASE SEPARATION IN THIN FILMS OF THE SYMMETRIC DIBLOCK-COPOLYMER MELT

By means of Monte Carlo simulations of a lattice model, microphase separation transition of symmetric diblock‐copolymer melts confined in the thin‐film geometry between parallel hard walls is studied. We impose a surface field which acts repulsively only on one of the two species, to stabilize lamellar order parallel to the surfaces. However, interplay between two characteristic lengths, that is, the natural thickness of the lamellar l and the thickness of the film D, causes complicated behavior. In case that the two lengths are compatible with each other, clear lamellar order parallel to the surfaces is observed at temperatures lower than the bulk transition temperature, as expected. On the other hand, tilted or deformed lamellar structure, or even coexistence of lamellae in different orientations are found in cases of strong conflict. In both cases, lamellae are fully established. Even at temperature higher than the bulk transition temperature, weak order is induced by the surface field, and a gradual t...

MULTI-SELF-OVERLAP ENSEMBLE FOR PROTEIN FOLDING : GROUND STATE SEARCH AND THERMODYNAMICS

Long chains of the HP lattice protein model are studied by the Multi-Self-Overlap Ensemble(MSOE) Monte Carlo method, which was developed recently by the authors. MSOE successfully finds the lowest energy states reported before for sequences of the chain length

Simulation of Lattice Polymers with Multi-Self-Overlap Ensemble

N=42\sim 100

Coupled-Map Modeling of One-Dimensional Traffic Flow

in two and three dimensions. Moreover, MSOE realizes the lowest energy state that ever found in a case of N=100. Finite-temperature properties of these sequences are also investigated by MSOE. Two successive transitions are observed between the native and random coil states. Thermodynamic analysis suggests that the ground state degeneracy is relevant to the order of the transitions in the HP model.

Coupled Map Traffic Flow Simulator Based on Optimal Velocity Functions

A novel family of dynamical Monte Carlo algorithms for lattice polymers is proposed. Our central idea is to simulate an extended ensemble in which the self-avoiding condition is systematically weakened. The degree of self-overlap is controlled in a similar manner as the multicanonical ensemble. As a consequence, the ensemble – the multi-self-overlap ensemble – contains adequate portions of self-overlapping conformations as well as higher energy ones. It is shown that the multi-self-overlap ensemble algorithm correctly reproduces the canonical averages at finite temperatures of the HP model of lattice proteins. Moreover, it is superior in performance to the standard multicanonical algorithm when applied to a complicated problem of a polymer with eight-stickers. An alternative algorithm based on the exchange Monte Carlo method is also discussed.

Quantum Monte Carlo Simulation of the Spin 1/2 XXZ Model on the Square Lattice

We propose a new model of one-dimensional traffic flow using a coupled map lattice. In the model, each vehicle is assigned a map and changes its velocity according to it. A single map is designed so as to represent the motion of a vehicle properly, and the maps are coupled to each other through the headway distance. By simulating the model, we obtain a plot of the flow against the concentration similar to the observed data in real traffic flows. Realistic traffic jam regions are observed in space-time trajectories.

Monte Carlo Study of Thin Films of the Symmetric Diblock-Copolymer Melt

A coupled map traffic flow model is introduced, based on optimal velocity functions. The model is simulated under open boundary conditions. Effects of noises in velocity are investigated. The average car density increases with the noise level. The high throughput flow is realized when the noise level is sufficiently large, and power law behavior appears in temporal spectra of density fluctuations at the same time. By introducing traffic bottlenecks, a hysteresis loop, which indicates the emergence of traffic jams, is observed in the headway-velocity plane. Temporal spectra of density fluctuations also obey a power law in this case, whose exponent is independent of the noise level.

Metastability in the formation of an experimental traffic jam

We systematically study the spin 1/2 quantum XXZ model on the square lattice using a quantum Monte Carlo method. The temperature dependence of the energy, specific heat and order parameter is calculated for the systems of sizes up to 16×16. Investigating the size dependence carefully, we exarnine the ground-state energy and the existence of the long-range order.

Density Fluctuations in Traffic flow

Microphase separation transition of symmetric diblock-copolymer melts confined in the thin-film geometry between parallel hard walls is studied by means of Monte Carlo simulations of a lattice model. Under a surface field which acts repulsively only on one of the two species, we investigate the order parameter profiles. Due to the competition between the two characteristic lengths, that is the natural thickness of the lamellar l and the thickness of the film D, the ordered structure depends strongly on D. In case that the two lengths do not conflict strongly, the transition from the surface-induced order at high temperatures to the bulk order at low temperatures is observed. In the opposite case, the lamellar order parallel to the surface is suppressed.

Phase transition in traffic jam experiment on a circuit

We show detailed data about the process of jam formation in a traffic experiment on a circuit without any bottlenecks. The experiment was carried out using a circular road on a flat ground. At the initial stage, vehicles are running homogeneously distributed on the circuit with the same velocity, but roughly 10 min later a traffic jam emerges spontaneously on the circuit. In the process of the jam formation, we found a homogeneous flow with large velocity is temporarily realized before a jam cluster appears. The instability of such a homogeneous flow is the key to understanding jam formation.