Masanori Itaba

Computer Experiments on Scattering of Atomic Excitations by Defects in Model Crystals

A molecular dynamics computer simulation has been carried out for a monatomic, anharmonic, and two-dimensional hexagonal model crystal. Central forces between the nearest neighbor atoms and anharmonic forces up to the third order are considered. Pulse input displacements are applied to the line of atoms at one end of a rectangular crystal, and the atomic excitations propagating in the crystal are observed. The excitations are shown to be phonons or solitons when the applied pulse is small or large, respectively. A mass defect – an atom of which the mass is lighter or heavier than that of the lattice atoms – is placed near the center of the crystal. The excitation is scattered by the defect, and the simulations are made for various cases of large and small inputs, and light and heavy mass defects. The obtained results are represented using two-dimensional maps. The intention of the study and the significance of the results are briefly discussed.

Computer Experiments on Soliton Production for Realizing Laboratory Experiments

Molecular dynamics computer experiments have been carried out for two-dimensional hexagonal mass-spring model crystals. Anharmonic forces up to the third order were taken into account, and central forces were considered between the nearest neighbor atoms. Input pulse displacements were given to atoms on the crystal surface, and induced displacements and velocities of all atoms in the crystal were computed. Solitons were produced as the atomic excitation, and the propagation velocity and the strength of solitons were enumerated. The dependences of these two quantities on the pulse application direction and on two parameters, the magnitudes of the input pulse and the higher order force constant, were investigated. Two crystallographic directions were taken for the input pulse direction, and values of the two parameters were widely varied. On the basis of the results of the simulation, a possible laboratory experiment of soliton production was considered.

Computer Simulation of the Film Growth Process on the Two-Dimensional Penrose Pattern

Abstract We performed computer simulation of the film growth process on the two-dimensional (2D) Penrose pattern, which is considered a typical structural model of quasicrystal. The atomistic structure of the deposited atoms was obtained as a function of time under various conditions of atomic binding energy, temperature and deposition rate. The Monte Carlo method based upon the solid-on-solid model was utilized for the present calculation. We found a geometrical restriction on the growth front of grains in the 2D Penrose pattern, which is also expected for the growth of an actual quasicrystal.

Computer Simulation of Film Growth Process on the Two-Dimensional Penrose Pattern

We performed computer simulation of the film growth process on the two-dimensional (2D) Penrose pattern, which is considered a typical structural model of quasicrystal. The atomistic structure of the deposited atoms was calculated as a function of time under various conditions of atomic binding energy, temperature and deposition rate. The Monte Carlo method based upon the solid-on-solid model was utilized for the present calculation. We found a geometrical restriction on the growth front of grains in the 2D Penrose pattern, which is also expected for the growth of an actual quasicrystal.

A simple optical audio signal transmitting system using Maximum Length Sequence

In the previous paper, we reported a communication method of audio signal using LED or laser diode. However the interference occurs easily in the receiver side when multiple transmitters exist. In this paper, we propose a new modulation technique which does not produce interference. In this method, the random number signal is generated using Maximum Length Sequence (MLS) by a microcomputer. An audio signal and random number are compared. If the audio signal is larger than the random number, a square wave of AM broadcasting modulates a optical source. If the audio signal is smaller than the random number, the optical source is turned on and off alternately. We change the period of the random number signal and examine the characteristic and the effectiveness of this method by some experiments.

Molecular Dynamics Simulation of Nucleation and Growth of a Binary Quasicrystal

Abstract The quasicrystal structure is considered to be a new type of ordered phase because its Fourier transform has Laue spots with icosahedral symmetry, which is inconsistent with crystal structure. Computer simulation of the formation process of a quasicrystal was performed by the molecular dynamics method. On the basis of the Strandburg type of quasicrystal model, we developed an algorithm of the formation process of binary quasicrystal reflecting the procedure as realistically as possible. The Fourier transform of some of the obtained structures has shown decagonal symmetry although the spots are rather diffused. It has been shown that the potential parameter and experimental condition should be limited to produce a perfect quasicrystal structure.

Modeling of personal thinking and its application to studies of group thinking over the Internet

Authors mathematical model of thinking is modified so that the model covers the non-logical thinking as well as the logical thinking. A computer network system works as a secondary brain in the case of group thinking. It has been shown that the refined model has opened the way to studying group thinking over the Internet, where the non-logical factors sometimes play an important role in obtaining conclusion.

Velocity Measurement by Microcomputer Using Supersonic Wave for Physics Education

In this paper, we propose a velocity measurement method using period of supersonic wave for physics education. We separate a transmitter and a receiver in this method. The transmitter is put on the movement body and transmits the supersonic wave of 40kHz continuously. The receiver converts the supersonic wave into a digital square wave. A microcomputer with two timer counters is used in this method. The timer counters count the square wave and the system clock. The period of supersonic wave is calculated by these values of counters and the velocity measurement of the movement body becomes possible by the Doppler effects. We examine the characteristic and the effectiveness of this method by fundamental experiment.

Computer experiments on phonons and solitons in two-dimensional hexagonal crystals

A molecular dynamics computer simulation was carried out for two-dimensional anharmonic mass-spring model crystals. Instead of the square lattice crystals in our previous study, hexagonal lattice crystals were chosen. Interactions between the nearest neighbor and the next nearest neighbor atoms were taken into account. Mechanical excitations produced in the lattice by an input pulse displacement applied on the edge of the crystal were studied. Phonons and solitons were, respectively, produced when the input was small and not small. Our interest was focused on anticipating probable laboratory experiment of soliton production in real crystals.

Computer experiments of fracture of two-dimensional Henley-Elser type quasicrystals

Abstract Fracture processes in two-dimensional Henley-Elser type quasicrystals were studied by the molecular dynamics method (MD). The model structures were ternary systems and were prerelaxed for the subsequent deformation process. The deformation process was simulated by structural relaxation by MD while the calculation cell was extended along a uniaxial direction by a certain factor after a certain time interval. The system temperature was kept constant by the scaling method during the calculation. The system temperature and potential energy were monitored to detect microscopic bond breaking in the sample. We found a large variance in the change in potential energy when the deformation process moved into plasticity from elasticity, corresponding to the creation of voids in the specimen. By changing the strength of the model potential, different types of deformation process were observed. The stability of the pentagonal clusters, which are considered to be the unit structure of the quasicrystal, was changed drastically according to the strength of the heterogenous interaction in the pentagonal cluster. Consequently, different types of fracture, i.e. interpentagonal and intrapentagonal cluster, occured depending on the strength of the heterogeneous interaction.