Yositaka Onodera

Excitons in Alkali Halides

Exciton states in alkali halides are analyzed on the basis of the energy band picture. First the excitons at the point \(\varGamma\) are discussed. A simple treatment in which the spin-orbit and the electron-hole exchange interactions are taken into account indicates that the intensity ratio of the halogen doublet is fairly sensitive to the exchange energy, while the doublet splitting is not very sensitive. It is shown that a pure triplet exciton state exists below the first allowed exciton state. Although such a state has not yet been detected, lifetime broadening of the first peak due to the existence of such a hidden state is conceived to make the magnitude of the width of the first peak comparable with that of the second peak. The pronounced structure observed in K and Rb halides in the spectral region just above the step is ascribed to d e exciton formed at the point X . Multiplicities of the observed absorption peaks are explained in terms of this model. Semiquantitative analysis of the triplet stru...

Relativistic Electronic Structure of KI Crystal

Energy bands in KI are calculated by means of the relativistic Greens function method. In setting up the potential, one parameter is used so that the gap energy agrees with the observed value. It is found that the highest filled band is completely split off into a couple of bands on account of the strong spin-orbit interaction. Both the top of the valence band and the bottom of the conduction band are located at the point k =0. The d bands which originate from the atomic 3 d states in the K + ions lie just above the lowest s -like conduction band. The results are discussed with reference to recent experimental information. It is suggested that the d bands centered on the alkali ions are responsible for the extra excitons observed in potassium and rubidium halides.

Soliton Model for Halogen-Bridged Mixed-Valence Platinum Complexes

The quasi-one-dimensional chains of halogen-bridged mixed-valence platinum complexes (HMPC) have characteristic twofold degeneracy similar to the trans isomer of polyacetylene, which strongly suggests the presence of soliton-like excitations in HMPC. It is shown that the microscopic Hamiltonian proposed by Ichinose for HMPC, which naturally is different from the Su-Schrieffer-Heeger model of trans -polyacetylene, leads to the Takayama-Lin-Liu-Maki Hamiltonian when the continuum limit is taken. This theoretical result implies that we can expect various soliton processes found in trans -polyacetylene to take place in HMPC as well, though such experimental evidences are scarce as yet.

Two-Polaron Solution and Its Stability in the Continuum Model of Polyacetylene

Explicit two-polaron solution is found in the continuum model of trans -polyacetylene. The solution is explored to examine stability of two polarons. It is found that two free polarons attract each other with a long range force until they form a single polaron with two carriers therein, which then dissociates into charged kink-antikink pair owing to a short range repulsion. Thus the reaction mechanism of two polarons into two charged kinks is clarified.

Breakdown of Debye's Model for Dielectric Relaxation in High Frequencies

Although Debyes relaxation model has been extensively used in analyzing experimental data of dielectric dispersion and absorption, the model has a fault to violate the sum rule for dielectric function. A simple and tractable modification to Debyes model is proposed which is free from the above-mentioned fault. The resulting Cole-Cole diagram is shown to deviate from an exact circle.

Two Kinds of Dipole Moments in Ferroelectric Phase Transitions in Terms of a Coupled Anharmonic-Oscillator Model

Motivated by Tokunagas proposal that ratio of the paraelectric dipole moment µ c to the saturation dipole moment µ s is correlated with whether a ferroelectric substance belongs to the displacive or order-disorder type, we calculated the two dipole moments for the classical coupled an harmonic-oscillator model in the independent-site approximation. It is found that \(\mu_{\text{c}}/\mu_{\text{s}}{\gtrapprox}1\) in the displacive type and µ c /µ s ≈1 in the order-disorder type. Thus, the ratio indeed has some correlation with the types of ferroelectric phase transitions, but its magnitude alone is insufficient in discriminating the two types. In addition, our theory predicts a certain relation between µ c /µ s and the discontinuity of the specific heal at T c , which BaTiO 3 is shown to satisfy strikingly well.

Proton Tunneling Model for the Ferroelectric Phase Transition in CsH2PO4

To understand the isotope effect in the ferroelectric phase transition of CsH 2 PO 4 and CsD 2 PO 4 , a quasi-one-dimensional model is proposed. In contrast to the existing models which consider only either the protons degree of freedom or the polarization dipoles, both of them are regarded as essential elements of the system, while the tunneling protons are assumed to have nothing to do with dielectric polarization. The static dielectric constant, calculated on the assumption of no tunneling for CsD 2 PO 4 and a finite tunneling frequency for CsH 2 PO 4 , reproduces well the experimental data by using model parameters consistent with the saturated polarization data. The model predicts absence of soft modes despite proton tunneling, in agreement with the order-disorder behavior of the phase transition as has been confirmed by experiments.

Correlation between Transition Entropy and the Rhodes-Wohlfarth Ratio in Ferroelectric Materials

A systematic correlation is discovered for a number of ferroelectrics between transition entropy Δ S and the Rhodes-Wohlfarth ratio p c / p s , where p c is the effective dipole moment determined from the Curie-Weiss constant assuming the existence of permanent dipoles, while p s is the saturation moment. The unified theoretical model of ferroelectrics, which is capable of describing essential features of both displacive-type and order-disorder-type ferroelectrics, is shown to reproduce the above correlation strikingly well.

Doublet Structure of the L2,3 Absorption Edge Singularity of Na Metal

The singularity of the L 2,3 X-ray absorption edge of light metals possesses a doublet structure owing to the spin-orbit splitting of the core p level. The observed intensity ratio of the doublet is deviated from the expected 2:1. The deviation can be explained by introducing the electron-hole exchange scattering. A line shape formula is obtained which takes account of the exchange and spin-orbit interactions. It is used to analyse the Na spectra with success.

k-Dependence of Exciton Longitudinal-Transverse Splitting

In the existing theory of Wannier exciton, the dependence of longitudinal-transverse splitting on exciton wave vector k is understood to arise from the dipole-dipole interaction. A theory is developed which, on the basis of the standard k · p perturbation theory, yields a much stronger k -dependence of the splitting, accounting for the recently observed exciton dispersion relation in CuCl.