Hiroshi Kamimura

On the Absorption Spectra of Complex Ions IV. The Effect of the Spin-orbit Interaction and the Field of Lower Symmetry on d electrons in Cubic Field

It is shown that the matrix elements of the crystalline field of lower symmetry than cubic and of the spin-orbit interaction for d -electrons in octahedral complex ions can be easily evaluated by the method of tensor operators, and the tables necessary to evaluate these elements by this method are given. It is also pointed out that the effective Hamiltonian which plays the same role as the spin Hamiltonian for the non-degenerate states can be constructed for the degenerate orbital states and will be useful for the analysis of line spectra of these complexes.

Electronic Structure of Potassium-Graphite Intercalation Compound: C8K

The electronic band structure of the first-stage potassium-graphite intercalation compound C 8 K was calculated by a semi-empirical tight-binding scheme. The calculated Fermi surfaces can be classified into two distinct types. One is potassium-like and nearly isotropic; the other is carbon-like and of cylindrical shape. In addition, the cylindrical portions show nesting property, which is likely to induce charge-density-wave instability. The isotropic portions of the Fermi surfaces are responsible for the large reduction of anisotropy in conductivity of C 8 K relative to graphite. The calculated density of states has a peak around the Fermi level and is in good agreement with the observed density of states derived from the specific heat measurements.

Self-Consistent Calculation of the Band Structure of C8K Including the Charge Transfer Effect

The method of the band structure calculation for layer-type materials is presented with use of atomic pseudopotentials and the Madelung-type potential due to the charge transfer. A simple physical model is proposed in order to estimate the amount of charge transfer, which has been a matter of controversy in graphite intercalation compounds. The band structure of C 8 K is calculated self-consistently by the present method and the amount of the charge transfer is determined non-empirically to be 0.6. The obtained band structure is almost the same as that calculated by the tight binding method.

Band Structures and Optical Properties of Semiconducting Layer Compounds GaS and GaSe

The band structures of GaS and GaSe near the Fermi levels are derived in a semiempirical way. In view of the anisotropic structures, the π band structures are mainly studied. The valence band consists of a π band with the heavy effective masses. The conduction band at the center of the Brillouin zone has the light effective masses for the both directions parallel and perpendicular to the layers, while its minimum at the zone edge has a two-dimensional character. The characteristics of the optical properties of these compounds are the existence of the sharp peaks associated with the nearly two-dimensional pair bands at the saddle points and further the limited spatial extension of an exciton along the c -axis. Most of the structures in reflectivities in GaS and GaSe and the binding energy of an exciton in GaSe are explained fairly well.

Mechanisms of High Temperature Superconductivity

The focal points of the symposium presented in this book were the following: For the high temperature copper oxide superconductors, the general consensus was that electron correlation effects are of essential importance. Based on this understanding a number of discussions took place: how to include the correlation effects and on the relative importance of various interactions; whether the ground state of a normal state is Fermi-liquid-like or not; where the doped holes mainly exist and on the nature of these holes; the value of the ratio of the energy gap to the critical temperature; whether the mechanics s are magnetic, or charge fluctuation, or excitonic, or very unusual ones quite different from the Fermi-liquid picture; whether a normal phase is normal or abnormal, etc. For the Bi-O superconductors, such as the BaKBiO system, the discussion concentrated on whether or not the mechanism of superconductivity is the same as that of the high temperature copper oxide superconductors.

Correlation Effects on Variable Range Hopping Conduction and the Magnetoresistance

Effects of intra-state correlation on the variable range hopping are investigated in the absence and presence of a magnetic field. In the absence of a magnetic field, Motts expression for the resistivity, ρ∝exp [ T 0 / T ] 1/4 , still holds in its temperature dependence. However, the prefactor T 0 now depends on two types of localization lengths brought about by the intra-state correlation. Magnetoresistance is found to be positive due to the intra-state correlation. It shows a linear dependence on a magnetic field in lower magnetic fields and saturates above a certain magnetic field.

Band structure of semiconductor superlattices with ultrathin layers (GaAs)n/(AlAs)n with n=1,2,3,4

The band structures of ultrathin layered superlattices (GaAs) n /(AlAs) n with n =1, 2, 3 and 4 are calculated by the self-consistent pseudopotential method. In this calculalion a value of α in the Xα approximation for the exchange-correlation potential is adjusted so as to reproduce both band gaps of GaAs and AlAs. This reflects an effect of the self-energy correction. The resulting band structures and charge densities show that upper valence band states of (GaAs) n /(AlAs) n with n =1 to 4 are approximately obtained by folding the valence bands of GaAs and AlAs, while that lower conduction band states correspond to strong hybridization of folded bands of GaAs and AlAs. It is shown that the band gap of (GaAs) 1 /(AlAs) 1 is indirect, while those for n =2 to 4 are direct and decrease with increasing n. This trend is consistent with recent experimental results.

Spin-Orbit Coupling in Ionic Crystals with Zincblende and Wurtzite Structures

The spin-orbit couplings in the valence bands of the zincblende and wurtzite structures such as cuprous halides and ZnO are investivigated in the tight-binding approximation. It is shown that the d-orbital of the cation contributes a negative term to the spin-orbit coupling in the valence band at k =0 in these compounds and, as a result of it, the spin-orbit splittings in the valence bands of CuCl and ZnO are inverted.

Magneto-optical properties of ferromagnetic chromium trihalides

Abstract Very large rotations of the axis of linear polarization may be observed in visible light passing through ferromagnetic chromium trihalides. The greatest effects are apparently associated with an absorption band edge which, for CrBr3, is located in the green. This paper presents the spectral variation of that rotation and of the corresponding absorption band edge. We consider a model consisting of a Cr3+ ion in an octahedron of Br− ions, subjected to a molecular field. It is shown that the absorption band edge in CrBr3 corresponds to charge transfer transitions in which an electron in a bromine orbital is promoted into an obital localized on the central chromium ion. The rotation arises as a consequence of the splitting of the excited energy levels by the spin-orbit coupling and the trigonal crystal field.

1A1g to 3B1g conversion at the onset of superconductivity in La2-xSrxCuO4 due to the apical oxygen effect

By the first-principles variational calculations we show that, with the increase of Sr concentration, the ground state of CuO 6 octahedron in La 2- x Sr x CuO 4 changes from 1 A 1 g to 3 B 1 g near the observed onset concentration of superconductivity while the ground state of the CuO 4 cluster in Nd 2- x Ce x CuO 4 is always 3 B 1 g in the presence of dopant electrons. Based on this result, two kinds of phase diagrams are predicted for hole- and electron-doped superconductors. These results support the spin polaron mechanism proposed by Kamimura et al .