Isaaki Yokota

Collective motion in superionic conductors

Abstract A theory of collective motion in superionic conductors is described by the use of a model of a crystalline cage immersed in a viscous liquid. The viscoelastic force and the interionic Coulomb force are considered as the cage—liquid interaction. The density-correlation functions and the frequency-dependent conductivity are calculated. The calculated conductivities for α-AgI are in good agreement with experiments. It is concluded that the structure in a.c. conductivity experimentally observed for α-AgI at frequencies below 10 cm -1 can be ascribed to acoustic phonons.

Magnetoplasma Oscillation of a Layered Electron Gas

The plasma dispersion of a layered electron gas in a magnetic field is calculated in the random phase approximation for the Voigt configuration. We found that the plasmon gradually changes its character from the two-dimensional to the three-dimensional ones as the spacing between the planes of the layer decreases.

Collective Motion in Superionic Conductors : Transverse Conductivity and Polaritons

The frequency and wave number dependent transverse conductivity of α-AgI type superionic conductors is calculated by using a continuum model. The anion cage is supposed to be immersed in the cation liquid. A short-range viscoelastic force as well as the Coulomb interaction is taken into account as the anion-cation interaction. The calculated conductivity, which satisfies the sum rule, reproduces the main features of the experimental results. The spectral density function and the dispersion relation of polaritons are also computed. It is shown that the latter is much similar to that in ordinary ionic crystals except that phonons are much damped.

Conduction and diffusion in ionic-electronic conductors

Abstract The phenomenological theory of ionic-electronic conduction, or mixed conduction, is reviewed. Recent developments in the physics of mixed conduction are also discussed, particularly with reference to experiments by Miyatani on cross conductivities and ambipolar diffusion.

Softening of the LO—phonon frequencies in SmS

Abstract The zone-boundary LO—phonon frequencies of SmS show a “softening” in the semiconducting as well as in the valence-fluctuating metallic phase. This has been interpreted as a renormalization of the phonon frequencies due to the phonon-induced on-site ƒ- d hybridization interaction. Renormalized phonon frequencies, which are calculated as a function of the ƒ- d energy gap, show a larger “softening” in the semiconducting phase than in the metallic phase, being in qualitative agreement with experiment.

Cyclotron Waves in a Layered Electron Gas

The dispersion relation of the cyclotron waves for a layered electron gas is calculated numerically using the semiclassical magnetoconductivity tensor for the two-dimensional electron gas. The Voigt configuration is assumed with a static magnetic field perpendicular to the planes.

Raman scattering theory in superionic conductors

Abstract A theory of Raman Scattering in a superionic conductor is presented. The motion of ions is described by a continuum model. For obtaining Raman Scattering at Stokes frequency, a phenomenological nonlinear force term is introduced into the equations of motion. The theory is applied to two phases of AgI, α and molten phases.

Transverse conductivity and polaritons in superionic conductors

Abstract A theory of transverse response in α-AgI-type superionic conductors is described by the use of a model of a crystalline cage (immobile ion) immersed in a viscous liquid (mobile ion). The short-range viscoelastic force and the long-range Coulomb force are considered as the cage-liquid interaction. The frequency- and wave-number-dependent transverse conductivity and the dispersion of phonon-polaritons are calculated for α-AgI. The calculated conductivity satisfies the f-sum rule and is in good agreement with the observed value. The calculated dispersion curve of polaritons is similar to that in ordinary ionic crystals except that the optical phonons are considerably damped and the portion of the lower branch whose wave number is less than a critical value is over-damped.

de Haas-Shubnikov Like Quantum Oscillation in Raman Spectra of Coupled LO Phonon-Plasmon Mode in Polar Semiconductors

Longitudinal optical phonons coupled with plasmons in a polar semiconductor located in a magnetic field is studied theoretically in detail for the Faraday configuration and is numerically analyzed for n-GaAs. The theory predicts that the de Haas-Shubnikov like quantum oscillations in phonon spectral lines will be observed experimentally. It is also shown that the broadening of the phonon spectral density function takes place in the single particle excitation region.

A Theory of Geometric Resonance Observed in Electromagnetic Generation of Transverse Sound in Metals

The amplitude of transverse acoustic waves generated by electromagnetic radiation in metals is calculated when a magnetic field is applied parallel to the surface. Our treatment is similar to the existing theory of geometric resonance in ultrasonic attenuation. The calculated relative generation amplitude is compared with experimental results reported on silver by Gaerttner et al. , showing fairly good agreement.