Kazuo Kitahara

Gas kinetic approach for electron mobility in dense media

The motion of electrons in nonpolar dense media is studied in the framework of gas kinetic transport theory. The observed external electric field dependence of electron mobility is explained by means of the solution of Boltzmann‐type transport equation. Numerical calculations of the mobility, mean energy, and energy distribution function of electrons in liquid Ar and in liquid CH4 are presented and a good agreement with experimental data is obtained. Also the cases of gaseous Ar are studied.

Superradiance and dipole ordering of an N two-level system interacting with optical near fields

A model is presented for a system of N two-level excitons interacting with each other via optical nearfields represented as localized photons. In a low exciton density limit, quantum dynamics of the dipolemoments or quantum coherence between any two energy levels is linear. As the exciton density becomeshigher, the dynamics becomes nonlinear, and the system has several kinds of quasi-steady states of thedipole distribution depending on the system parameters. These quasi-steady states are classified with thehelp of the effective Hamiltonian that is derived from the renormalization of degrees of freedom oflocalized photons with a unitary transformation. Among them there exist a ‘‘ferromagnetic’’ state(dipole-ordered state), in which all electric dipoles are aligned in the same direction, and an ‘‘anti-ferromagnetic’’ state, where all dipoles alternatingly change the direction. In addition, we show that anarbitrary state can be transformed into a dipole-ordered state by manipulating initial values of thepopulation differences appropriately. For example, if we initially prepare a dipole-forbidden state, whichis similar to the ‘‘anti-ferromagnetic’’ state and cannot be coupled with propagating far fields, and if wemanipulate the distribution of the population differences properly, the initial state evolves into a dipole-ordered state. The radiation property of such dipole-ordered states is examined in detail. Neglectingenergy dissipation by radiation, we find that some of the ordered states show strong radiation equivalentto Dicke’s superradiance. Then by introducing a radiation reservoir, the dissipative master equation isderived. Solving the equation with and without quantum correlations, we numerically show that multiplepeaks in the radiation profile can survive in both cases. The mechanism of this phenomenon is discussed,and a brief comment on an application to photonic devices on a nanometer scale is given.

A Theory of Domain- and Filament-Formation due to Carrier-Density Fluctuations in Conductors with Negative Differential Resistance

Fluctuations of the carrier density causes instabilities of the electrically uniform state in conductors with a negative differential resistance. Transitions to electrically non-uniform states are studied, based on evolution criterions derived after Glansdorff and Prigogine. The method enables us to analyze the phenomena in an analogous way as we discuss the thermodynamics of the usual two-phase separations.

Spin Depolarization of Particles in Tunnelling States

The spin depolarization of a particle in quantum states is considered using a model of two sites, having different transverse magnetic fields, and between which the particle can tunnel. The ensemble average is performed with random distributions of the magnetic fields. A Gaussian decay law is obtained with the exponent reduced by a factor 2, for all times when the system is prepared in suitable eigenstates and for longtimes with arbitrary preparation. The apparent contradiction to van Vlecks theorem is resolved.

Excitation dynamics in a three-quantum dot system driven by optical near-field interaction: towards a nanometric photonic device

Using density operator formalism, we discuss interdot excitation energy transfer dynamics driven by the optical near‐field and phonon bath reservoir, as well as coherent excitation dynamics of a quantum dot system. As an effective interaction between quantum dots induced by the optical near‐field, the projection operator method gives a renormalized dipole interaction, which is expressed as a sum of the Yukawa functions and is used as the optical near‐field coupling of quantum dots. We examine one‐ and two‐exciton dynamics of a three‐quantum dot system suggesting a nanometric photonic switch, and numerically obtain a transfer time comparable with the recent experimental results for CuCl quantum dots.

Nonequilibrium thermodynamics of multicomponent systems

A general formalism of nonequilibrium thermodynamics of multicomponent fluids is given. Reversible parts of balance equations for the extensive variables are constructed in such a way that the growth rate of the extensive variables are related to their conjugate intensive parameters in an antisymmetric manner. This enables us to give the balance equation for the diffusional flow which plays an important role in the study of superfluidity and complex fluids.

Gas Kinetic Approach for Electron-Ion Recombination Rate Constant in Dense Media

We present a statistical mechanical analysis of electron-ion recombination processes in weakly ionized nonpolar dense media, being based on the model which can explain the external electric field dependence of electron mobility in these media. We derive a diffusion equation by a projection technique from the phase space to the coordinate space where the stationary distribution of electron momentum in an external electric field has been realized. Then we estimate the recombination rate constant from this equation. Here we deal with the system classically as a three-body recombination process.

On the Spatial Correlations in Nonequilibrium Systems

The conditions for the emergence of spatial correlations in a nonequilibrium steady state, proposed by Nicolis and Malek Mansour, are examined and generalized to systems involving many reversible chemical reactions. It is found that, by appropriately choosing controllable parameters, we can obtain a nonequilibrium steady state without spatial correlations.

Imaginary noise and parity conservation in the reaction A+A⇌0

The master equation for the reversible reaction A+A 0 is considered in Poisson representation, where it is equivalent to a Langevin equation with imaginary noise for a complex stochastic variable \phi. Such Langevin equations appear quite generally in field-theoretic treatments of reaction-diffusion problems. For this example we study the probability flow in the complex \phi plane both analytically and by simulation. We show that this flow has various curious features that must be expected to occur similarly in other Langevin equations associated with reaction-diffusion problems.

Vacancy-Assisted Diffusion in a Honeycomb Lattice and in a Diamond Lattice

Vacancy-assisted diffusion in a crystalline solid can be modeled by means of many particles jumping stochastically to their respective nearest-neighbor lattice-sites with double occupancy forbidden. The diffusion coefficient of a tagged particle, defined in terms of its mean square displacement, depends not only on the transition rate but also on the particle concentration. Nakazato and Kitahara [Prog. Theor. Phys. 64 (1980) 2261] devised a projection operator method to calculate its approximate expression interpolating between the low- and high-concentration limits for a square lattice in any dimension. In this paper, we apply their method to a honeycomb lattice and a diamond lattice, in each of which a set of the nearest-neighbor vectors depends on a site from which they originate. Compared with simulation results, our explicit expression is found to give a good interpolation in each lattice unless the host particles migrate more slowly than the tagged particle.