Seiji Mizuno

Phonon dispersion of nano-graphite ribbons

Based on the classical force constant models, we study numerically and analytically out-of-plane phonon-dispersion relations for two types of nano-graphite ribbons confined by the armchair- and zigzag-shaped edges. We demonstrate for two ribbons how the edge-localized vibrational modes, i.e., edge phonon modes, behave depending on edge environment by means of altering the effective mass of the edge sites. Special emphasis is placed on the qualitative difference in the phonon modes which are localized near two types of edges.

Acoustic phonon modes and dispersion relations of nanowire superlattices

We study theoretically acoustic phonon modes in nanowire superlattices (NWSLs) composed of cubic materials. We classify the acoustic phonon modes in rectangular and square cross-section NWSLs, based on group theory. For NWSLs consisting of GaAs and AlAs, we calculate numerically the dispersion relations of each phonon mode and corresponding displacement fields. We examine the effects of both the lateral confinement and superlattice modulation along the wire axis. The results suggest that peculiar electron-phonon interactions occur because the vibrations of both the lateral and longitudinal confining potentials induce scattering potential in addition to the deformation and piezoelectric potentials.

A New Self-Consistent Band Structure of C8K

The electronic structure of C 8 K is investigated by using the self-consistent numerical-basis-set LCAO method within the local-density-functional formalism. It is found that the graphite π * bands are split in C 8 K and some of them go down below the Fermi energy. As a result there exist three-dimensional Fermi surfaces originating from graphite π * bands in the center of the Brillouin zone. This is due to the charge unbalance between environmentally non-equivalent carbon atoms in a graphite layer, which is produced by the potassium atoms. The new band structure is shown consistent with recent experiments of optical spectra.

Time Advance and Delay of Phonon Packets Scattered off Superlattice Systems

We study numerically the time development of the phonon packets propagating through the single- and double-superlattice systems. In a single superlattice system, we find the time advance and delay for the transmitted and reflected phonon packets, respectively. In a double superlattice system in which the resonant transmission occurs, we find large time delays for both the transmitted and reflected packets when the spectrum of the incident packet has a large component at the resonance frequency. These results are well understood in terms of the phase time defined by the stationary phase analysis.

Electronic states of graphitic heterocompounds of carbon, boron and nitrogen

Summary form only given, as follows. The electronic structures of the graphitic heteromaterials B/C, C/N and B/C/N including BC3 and BC2N are studied theoretically by performing the band calculations based on a localdensity- functional formalism with use of self-consistent numerical-basis-set linear combindion of atomic-orbitals method. We show the electronic band structures and give the estimations of the amount of charge transfers among individual atoms. In these heteromaterials, large charge transfers occur from boron to carbon and nitrogen, and from carbon to nitrogen suggesting the ionic properties. Further we exhibit the character of orbitals for various arrangements of heteroatoms at the top of valence bands and at the bottom of conduction band, which should correspond to the images of STM measurements.

Transmission of sub-THz acoustic phonons in a Kronig-Penny system for phonons

Based on the transfer matrix method, we study the transmission characteristics of sub-THz acoustic phonons in multiple barrier systems for phonons consisting of alternate stacking of bulk layers and periodic superlattices. Specifically, we consider a frequency window within a frequency gap of the periodic superlattice and examine the behavior of phonon transmission as the number of the barriers increases. The systems we consider are analogous to the Kronig-Penny potential structure for electrons.

Phononic bandgaps peculiar to solid–fluid superlattices

We theoretically study the phonons propagating through a superlattice consisting of alternating layers of an elastic solid and a fluid. In this structure, there exist phononic bandgaps not originating from Bragg reflections. We examine the origin of these non-Bragg gaps and show that they are peculiar to the solid–fluid superlattices, where the number of allowed modes varies periodically. Even a single solid layer immersed in fluid contains discrete frequencies at which incident waves are perfectly reflected. We demonstrate the resonant reflection process at these frequencies. In the multilayered structure, these transmission zeros are gathered and form a bandgap. This is similar to the relation between atomic levels and an electronic energy band, though the allowed and forbidden states are interchanged. This non-Bragg gap introduces novel degrees of freedom to the design of phononic bandgap structures.

Localized Torsional Modes in a Nanowire Superlattice with a Defect Layer

We study theoretically the localized vibrational modes in a nanowire superlattice with a defect layer. We focus on azimuthally symmetric torsional modes and calculate the eigenfrequencies of the localized modes. The localized modes are generated within frequency gaps determined by the periodicity of the nanowire superlattice. Furthermore, the radial confinement of vibration generates a critical frequency. When the frequency is lower than this critical frequency, a wave number defined in the defect layer becomes an imaginary number. We show that the localized modes can be generated only above the critical frequency.

Electronic structures of potassium-oxygen-graphite ternary intercalation compounds

Abstract The electronic structures of potassium-oxygen-graphite ternary intercalation compounds are studied theoretically. We construct several simple structural models for stage-1 and -2 compounds based on experimental results, and carry out the self-consistent band-structure calculation. For all models, we show that the oxygen 2 p band with small dispersion intersects the Fermi level. We also estimate the amount of charge transfer among constituent atoms, and show that the 4 s charges of potassium are perfectly transferred to oxygen and carbon. Our results show that the present GICs are donor types.

Acoustic phonon modes and phononic bandgaps in GaN/AlN nanowire superlattices.

We study numerically the phonon dispersion relations and corresponding displacement fields for a circular cross-section nanowire superlattices consisting of anisotropic GaN and AlN. We determine a set of parameters which gives complete phononic bandgaps. The results suggest the potential for manipulating phonons in the micro/nano electromechanical systems.