Yukio Osaka

Theory of the Anomalous Magnetoresistance in Amorphous Si and Ge

Anomalous magnetoresistance in amorphous Si and Ge is calculated by taking into account the hopping motion with spin flip of localized electrons. Magnetoresistance is deduced to be the results of the modification of the spin-flip relaxation time of the electrons by external magnetic field. The simplified theory can account for experiments.

Polaron Mobility at Finite Temperature (Weak Coupling Limit)

On the basis of the Feynman model for polarons, the drift mobility of an electron in a polar crystal is studied for the weak coupling limit of the electron-phonon interaction. The expression for the polaron mobility at the low temperature limit, which was previously obtained by the author, is extended to the case of the finite temperature. The analysis of the experiments with AgCl and AgBr showed that the expression thus obtained gives a smaller band mass than other methods of analysis of the drift mobility (Van Heyningen and Garcia-Moliner) do.

Collective Excited States and Dielectric Constant in Insulators

The electron interaction in insulators is studied with special attentions focused on the dielectric properties and the exciton states. Use is made of Hubbards formalism and of the propagator formalism for an electron-hole pair. The following results are obtained: (a) the relations between the internal dielectric constant and the external one are elucidated, (b) an expression of the internal dielectric constant is given in a convenient form to see local field corrections due to lattice periodicity, (c) conditions for applicability of the effective mass equation (EME) to shallow excitons are made clear, (d) the screening constant appearing in EME is the same as the uniform static dielectric constant, (e) exchange correction for EME is not so small even for shallow excitons in silicon and germanium as is usually expected.

On the Reflection and Transmission Coefficients Associated with a Lowest Exciton Level (in the Case of Weak Coupling between Polariton and Phonon)

With use of the photon Greens function, the reflection and transmission coefficient associated with a lowest exciton level based on the “polariton” picture was calculated. To consider the effects of the spatial dispersion due to the translational motion of excitons, the “specular” and “diffuse” boundary conditions are adopted. The calculation based on a simple model for polaritons shows that the minimum of the transmission coefficient arises in the neighbourhood of the transverse exciton energy, and the hump may be expected at the longitudinal exciton energy.

Renormalization Constant of Electron Gas

An evaluation is carried out for the renormalization constant Z of an interacting electron gas. It is shown that the lowest approximation with respect to effective interaction between electrons leads to Daniel and Voskos result for Z , while higher order correction for Z modifies their result appreciably and their result is not correct even in the limit of high density.

Model Calculation of Interface State in a MOS Structure

In order to understand physical origin of interface states in a MOS structure, model calculations of the interface states are carried out. The model of the ideal Si-SiO 2 interface is introduced. This model represents that the Si-SiO 2 interface is given as the boundary between two bonding-type materials and the intrinsic interface states is absent. It will be assumed that observed interface state is caused by the difference between real interface and the ideal one. Model calculations show the following results. The gate metal is effective on interface state formation only thinner SiO 2 film (the film width is less than 20 A). The density of interface state nearly independent of the type of metals.

Formal Theory of an Inhomogeneous Electron Gas

Hohenberg and Kohn have dealt with the ground state of an interacting electron gas in an external potential and show that there exists a universal functional of the electron density n ( r ), G [ n ( r )], independent of an external potential. This functional plays a central role in a theory of inhomogeneous electron gas. In this paper, a formal theory for deriving the form of this functional is developed. The method of this derivation is based on the use of a nonlinear response theory. Our analysis shows that G [ n ] can be expressed entirely in terms of nonlinear response functions. We derive the full expression of G [ n ] which is written explicitly by the density n ( r ). Furthermore, we find the generalized form of “compressibility sum rule”.

Exciton Structure in Alkali-Halide Crystals

Exciton states in alkali halides are analysed on the basis of a “transfer” model. Numerical calculation is made for the exciton state in KCl. The Hartree-Fock solutions of the 4 s electron in a free potassium atom and of the 3 p electron in a free chlorine atom are used for the wave functions of the transferred electron and the hole, respectively. The result obtained for KCl is employed to discuss exciton structures in KBr and KI qualitatively. The qualitative behaviors of exciton structures obtained for alkali halide crystals with fcc structure are similar to the conjectures of Eby, Teegarden and Dutton based on the analysis of their experiments.

Interface States Induced by Amorphous SiO2 in MOS Structures

In order to understand physical origin of interface states in MOS structures, model calculations of the interface states are carried out. The model of the ideal Si–SiO 2 , interface is introduced. This model represents that the Si–SiO 2 interface is given as the boundary between two bonding-type materials and the intrinsic interface state is absent. It will be assumed that observed interface state is caused by the difference between real interface and the ideal one. The real boundary of MOS structures may be described approximately in terms of the crystalline Si and the amorphous SiO 2 with an ideal glassy structure. Based on the simple model of amorphous SiO 2 , the density of interface states is calculated semi-qualitatively. The obtained results suggest that the interface states induced by the amorphous SiO 2 do not give the observed density of the interface states and the amorphous state of SiO may take into account this density.

Ultrasonic Attenuation in Normal Metals

A general treatment of ultrasonic attenuation of both longitudinal and transverse wave in normal metals, valid for an arbitary mean free path, is given by means of Greens function formalism. When the mean free path of the Fermi electron is shorter than the wave length of the ultrasonic wave, the result is not equivalent to that obtained by Pippard. The result shows that by choosing a value of mean free path of experiments so as to give the best fit to Pippards formulae, longitudinal attenuation constants of experiments are well in agreement with Pippards formulae for an arbitary mean free path. Quantitative discussions for transverse attenuation are carried out only in the case of screened impurity potential.