Hiroshi Ajiki

Electronic and magnetic properties of nanographite ribbons

Electronic and magnetic properties of ribbon-shaped nanographite systems with zigzag and armchair edges in a magnetic field are investigated by using a tight-binding model. One of the most remarkable features of these systems is the appearance of edge states, strongly localized near zigzag edges. The edge state in a magnetic field, generating a rational fraction of the magnetic flux ( f5 p/q) in each hexagonal plaquette of the graphite plane, behaves like a zero-field edge state with q internal degrees of freedom. The orbital diamagnetic susceptibility strongly depends on the edge shapes. The reason is found in the analysis of the ring currents, which are very sensitive to the lattice topology near the edge. Moreover, the orbital diamagnetic susceptibility is scaled as a function of the temperature, Fermi energy, and ribbon width. Because the edge states lead to a sharp peak in the density of states at the Fermi level, the graphite ribbons with zigzag edges show Curie-like temperature dependence of the Pauli paramagnetic susceptibility. Hence, there is a crossover from hightemperature diamagnetic to low-temperature paramagnetic behavior in the magnetic susceptibility of nanographite ribbons with zigzag edges. @S0163-1829~99!02111-6#

Magnetic properties of nano-graphites at low temperature

Abstract Both orbital diamagnetic and Pauli paramagnetic contributions to the magnetic response in ribbon-shaped nanographite systems with zigzag and armchair edges are discussed. These systems possess edge states, strongly localized near zigzag edges. The edge states lead to a sharp peak in the density of states at the Fermi level and generate a Pauli spin susceptibility with a nearly Curie-like temperature dependence. In nanographite systems this paramagnetic contribution of the edge states can compete with the diamagnetic orbital magnetic signal of the bulk. At high temperature the system is diamagnetic and at low temperature the paramagnetic behavior dominates.

Exciton states and optical properties of carbon nanotubes

Exciton states and related optical properties of a single-walled carbon nanotube are reviewed, primarily from a theoretical viewpoint. The energies and wavefunctions of excitons are discussed using a screened Hartree-Fock approximation with an effective-mass or k·p approximation. The close relationship between a long-range electron-hole exchange interaction and a depolarization effect is clarified. I discuss optical properties including the radiative lifetime of excitons, absorption spectra and radiation force. To describe these properties in a unified scheme, a self-consistent method is introduced for calculating the scattering light and induced current density due to excitons. I also briefly review experimental results on the Aharonov-Bohm effect in excitons and quasi-dark excitons excited by light polarized perpendicular to the tube axis.

Parameters of a driven Jaynes-Cummings Hamiltonian

Each parameter in a driven Jaynes–Cummings (JC) model of a cavity system is specified in terms of experimental parameters, such as the transmission coefficient of the mirrors, the length of the cavity, and the intensity of the input coherent field. For this purpose, linear and the third-order nonlinear spectra are calculated from the driven JC model, and the results are compared with those from the universe-mode approach which enables us to treat experimental parameters in a straightforward manner. Although the relationships of the parameters are obtained in the high-Q cavity regime, they are also applicable in the low-Q cavity regime.

Branching ratio of light incident on a photonic crystal in a multibranch dispersion region

Abstract In a frequency region where dispersion relation shows multibranch structure, the branching ratio of the Poynting vectors for various transmitted and reflected modes has been determined via the consideration of boundary conditions at the surface. For this purpose, it is required to obtain the field amplitudes of many eigenmodes with real and complex wave vectors, which numerically is a rather demanding task. An example is given for a simple cubic photonic crystal with intersecting square rods. It is found that a different choice of (0 0 1) surface in a unit cell leads to a remarkable change in the branching ratio, or reflectance curve.

Size dependence of exciton–light interaction in a spherical semiconductor from quantum dot to bulk limit

Abstract Exciton–light coupled modes in a spherical crystal are studied as a function of radius. The coupled mode has complex eigenenergy, and its imaginary part corresponds to the radiative width in optical spectra. The radiative width of each mode shows resonant enhancement as a function of crystal size due to the cavity effect. The maximum radiative width increases with radius, and it seems to be inconsistent with the fact that the coupled mode becomes a polariton in a bulk limit. We discuss graphically and qualitatively how coupled mode approaches polariton state.

Simulation method for resonant light scattering of exciton confined to arbitrary geometry.

We develop an electromagnetic (EM) simulation method based on a finite-element method (FEM) for an exciton confined to a semiconductor nanostructure. The EM field inside the semiconductor excites two transverse exciton polariton and a single longitudinal exciton at a given frequency. Established EM simulation methods cannot be applied directly to semiconductor nanostructures because of this multimode excitation; however, the present method overcomes this difficulty by introducing an additional boundary condition. To avoid spurious solutions and enhance the precision, we propose a hybrid edge-nodal element formulation in which edge and nodal elements are employed to represent the transverse and longitudinal polarizations, respectively. We apply the developed method to the EM-field scattering and distributions of exciton polarizations of spherical and hexagonal-disk quantum dots.

Magneto-optical spectra of carbon nanotubes: effect of Aharonov-Bohm flux on depolarization effect

Abstract Optical absorption spectra of carbon nanotubes in a magnetic field perpendicular to the tube axis, are calculated for parallel polarization. A depolarization field is induced by Aharonov–Bohm flux, which reduces the lowest peak dramatically.

Exciton–Phonon Interaction in a Spherical Quantum Dot: Effect of Electron–Hole Exchange Interaction

Exciton-LO-phonon interactions in a spherical quantum dot (QD) are studied theoretically in the weak confinement regime. The e-h exchange interaction makes it possible to lift the degeneracy of the lowest exciton levels via exciton-phonon interaction as the Jahn-Teller effect. The Huang-Rhys factor on the levels having maximum oscillator strength changes much slower than a -5 which is the radius a dependence in a large-size limit studied in a previous work.

Optical Response of a Spherically Confined Exciton with the Effects of e–h Exchange and Image Charge Density

A thorough theoretical study has been made on the optical response of an exciton weakly confined in a sphere over a wide range of size, based on the previous work on the exciton level scheme which takes account of electron-hole (e-h) exchange interaction, size quantized center-of-mass (CM) motion, and the effect of background polarization. In the presence of transverse EM field, the cross section of elastically scattered light has been calculated according to a modified microscopic nonlocal response theory. The results allow us a systematic understanding of resonant spectral structure in a wide range of sample size, in relation with the corresponding level scheme, radiative shift and width, and the longitudinal and transverse character of each exciton level.