Yoji Kubota

Self-diffusion of oxygen in single crystal alumina

The self‐diffusion coefficient of oxygen in (polished slices of a Verneuil) single‐crystal alumina was determined in the temperature range 1500–1770 °C by means of the gas–solid isotope exchange technique. The results were represented by D=1.12×103 exp (−155×103/RT) cm2/s. The activation energy was interpreted to be for intrinsic diffusion. By comparison of the results with the oxygen self‐diffusion coefficients previously reported for crushed particles of a Verneuil alumina and a vapor‐grown alumina, the extrinsic diffusion exhibited by the crushed particles was confirmed to be due to a dislocation enhancement process.

Molecular dynamics study of fast dielectric relaxation of water around a molecular-sized ion

We have calculated the dielectric relaxation of water around an ion using molecular dynamics simulations. The collective motion of water near the ion showed fast relaxation, whereas the reorientational motion of individual water molecules does not have the fast component. The ratio of the relaxation time for the fast component and the bulk water was consistent with the experimental results, known as hyper-mobile water, for alkali halide aqueous solution.

Applicability of Site-Basis Time-Evolution Equation for Thermalization of Exciton States in a Quantum Dot Array

We verify the practical applicability of the conventional site-basis time-evolution equation to exciton transfer processes in a quantum-dot array model. The time-evolution equation has proved to work under the rather limited conditions of the zero temperature limit and/or a minimal two-dot system. The computed results dramatically change with the temperature, the number of quantum dots, and the intensity of transition rates between adjacent sites. This is due to the fact that the higher-order perturbation terms, which are neglected in deriving the site-basis equation, have a great influence on the exciton dynamics. We found that the thermal relaxation can be suppressed by controlling the dot size and interdot distance.

Magnetic Properties of Superconducting Tantalum-Niobium Alloys

The superconducting properties of Ta-rich Ta-Nb alloys have been investigated by magnetization measurements. The Ginzburg-Landau parameter κ of pure tantalum is estimated to be 0.42 and the critical value \(\kappa{=}1/\sqrt{2}\) is attained at a composition Ta–2.4 at. % Nb. The dependence of the thermodynamic critical field on temperature T follows the BCS prediction and all the alloys are considered to be weak-coupling superconductors. The lower critical field accords with the microscopic theory in the vicinity of the transition temperature T c , but the discrepancy between experiment and theory increases with decreasing T . The temperature dependence of the upper critical field and the generalized Ginzburg-Landau parameter κ 2 is stronger than the prediction and the disparity becomes large as the electron mean free path l increases. These properties may be ascribed to the effect of the anisotropy of the electronic structure in the state where the electron mean free path l is considerably larger than the...

Exciton transfer in quantum dot arrays: Comparison of eigenbasis and site basis representations

We discuss differences between eigenbasis and site basis representations for models of exciton transfers in an array of quantum dots. The exciton relaxation processes are well described by the master equation in the eigenbasis representation. The site basis evolution equation up to the second order of the interdot interaction is straightforwardly derived from the eigenbasis equation by using perturbation theory when the interaction is sufficiently small compared to the energy difference between the exciton states in each quantum dot. Although the higher order site basis equations can be derived similarly, the resultant equations are too complicated to use in the actual calculations. The master equation in the eigenbasis representation has several advantages over the site basis one: (i) the system described in terms of the eigenbasis representation can evolve into thermal equilibrium because the equation satisfies the detailed balance, (ii) the site basis equation does not reasonably describe the exciton state trapped in a local energy minimum at very low temperature, and (iii) it is computationally less demanding to carry out the eigenbasis evolution equation.

Exciton–polariton transmission in quantum dot waveguides and a new transmission path due to thermal relaxation

Exciton-polariton transmission in quantum dot waveguides is investigated with quantum time-evolution equations in Liouville space for exciton wave packet dynamics. The transmission efficiency of the exciton-polariton wave with the longitudinal and transverse mode transformations strongly depends on the geometric parameters (bending angle and interdot distance) of the waveguides and on configuration of an additional branch attached to the waveguide. We have numerically demonstrated that the transmission efficiency significantly improves by controlling these geometric parameters and the configuration of the branched waveguide. The optimal bending angle for efficient transmission with the longitudinal and transverse mode transformations deviates from the right angle owing to more than nearest-neighbor-site interactions through a shortcut. We have also found that existence of thermal relaxation enables to open a new transmission channel along which the exciton-polariton transmission through the Coulomb interaction is suppressed.

Assessment of three-dimensional distribution functions and triplet distribution functions for hard spheres calculated using a three-dimensional Ornstein-Zernike equation with hypernetted-chain closure

We calculated three-dimensional (3D) distribution functions around a contact dimer composed of hard spheres immersed in a fluid composed of same-sized hard spheres calculated using a three-dimensional Ornstein–Zernike equation with hypernetted-chain closure (3D-HNC-OZ theory). The results of the 3D-HNC-OZ theory were compared with those calculated using Monte Carlo simulations. Even though the packing fraction of solvent was high, such as in ambient water, the 3D-HNC-OZ theory gave semiquantitatively reasonable results. This means that the triplet distribution function was also calculated reasonably well, although the triplet distribution functions are not explicitly included in the equations of the 3D-HNC-OZ theory. However, the accuracy depended on the configuration of the solute. Our results are discussed in a biological context, such as molecular recognition and the stability of folded proteins.

Delay in a soliton transmission across an interface between two Toda lattices

The transmission of a single soliton is investigated numerically across an interface between two Toda lattices, which are connected by a harmonic lattice. The soliton transmission coefficient is used as a measure of transmission. When the spring constant (κ) of the harmonic spring is small and the number of harmonic springs is greater than or equal to 2, a delay in the transmission of the soliton is found for proper κ. It is shown that the delay in the soliton transmission is due to the existence of the quasi-localization of the wave in the harmonic lattice and the agreement of the time scale of the motion between the two springs.

Model Dependence of the Electrostatic Response to a Molecular-Sized Ion in Water

The electrostatic response to a molecular-sized ion in water was calculated using molecular dynamics simulations. In our previous study, we adopted the SPC/E model as a water molecule and a strong nonlinearity and fine structure were observed in the polarizability curves. In this study, various models of water molecules were adopted to calculate the electrostatic response of water. The model dependence of the water polarizability was small, and the interesting behavior observed in our previous study was also observed in this study for all water models.

An efficient numerical method for exciton states in quantum boxes

We have developed an efficient numerical method for exciton states in thin quantum boxes. In our numerical method, the exciton wave function is expanded in terms of coordinate eigenstates on each grid point. This method is found to be much more efficient and promising for calculating the exciton states than the standard configuration-interaction (CI) approach. In the CI approach, the wave function is expanded in terms of single-particle electron and hole eigenfunctions (φ and φ) [1,2]. Then, the computations of the matrix elements of the Hamiltonian (Coulomb potential) require multidimensional numerical integrals given by