Akira Yanase

Band Structure in the High Temperature Phase of Fe3O4

Band structure of the ferrimagnetic Fe 3 O 4 was calculated in the high temperature cubic phase by means of self-consistent APW method. Though the obtained band structure corresponds closely with the ionic Neel model, it was shown that the itinerant electron model is more adequate than the localized electron model. The minority spin d e bands of B site iron have an electron Fermi surface at the \( \varGamma \) point and hole surfaces around the W point. Experimental data including photoelectron emission, optical reflectivity, neutron scattering and transport phenomena were discussed qualitatively in relation to the calculated band structure.

Crystal Electric Fields for Cubic Point Groups

Crystal electric fields for the cubic point groups are reexamined and it is found that the new term O 6 2 - O 6 6 is nonvanishing for the cubic point groups T h and T due to the lack of Umklappung and fourfold symmetry axis of the point group O h . The eigenfunctions and eigenvalues of the crystal electric field for 4 f n configurations are labeled by the irreducible representations of the point group T h . The degeneracy of each sublevel does not change as compared with those of the point group O h , but some eigenfunctions and eigenvalues are affected by the new term. Thus inelastic neutron scattering spectra for T and T h are different from those for O h due to different transition probabilities and selection rules.

Mechanisms for the Anomalous Properties of Eu-Chalcogenides Alloys. I.

Various anomalous magneto-optic and magnetic properties of Eu 1- x Gd x Se are explained on the magnetic exciton and the magnetic impurity state model. It is shown that the optically active magnetic exciton with the lowest energy is of the type of (5 d –5 d ), that is, 5 d character on the central Eu 3+ and also 5 d on the nearest neighbour Eu 2+ . The origin of the anomalous magneto-optic properties is attributed to the d – f exchange interaction on the nearest neighbour Eu 2+ ions. While, for the magnetic impurity state, (6 s –6 s ) type state has the lowest energy. At low temperature, a giant spin molecule ( S =46) composed of the spin of an impurity electron and the 4 f spin on the central Gd 3+ ion and the n.n.Eu 2+ ion is constructed through the s – f exchage interaction among them. However, this giant spin molecule disappears rather quickly as temperature increases exceeding J 1/κ , where J 1 is the s – f exchange constant at n.n. Eu 2+ and is estimated as 25κ. The anomalous magnetic properties of ...

New Class of Diluted Ferromagnetic Semiconductors based on CaO without Transition Metal Elements

We propose a new class of diluted magnetic semiconductors based on CaO without transition metal elements. The electronic structure and the magnetic properties of B-, C- or N-doped CaO are calculated by using the Korringa-Kohn-Rostoker method within the local spin density approximation. The substitutional and magnetic disorder is taken into account by the coherent potential approximation. It is found that B, C and N impurities show finite local magnetic moments in CaO at the oxygen-substitutional site. Moreover, these C- and N-doped CaO show the room-temperature ferromagnetism with half-metallic density of states.

Materials Design of Transparent and Half-Metallic Ferromagnets of MgO, SrO and BaO without Magnetic Elements

We propose materials design to fabricate a new class of diluted magnetic semiconductors (DMSs) based on MgO, SrO and BaO without magnetic elements. Replacing the oxygen atoms by other atoms such as C atoms leads to the induction of a deep-impurity band. These materials show ferromagnetic behaviour if the impurity-band width ( W ) and the electron-correlation energy ( U ) satisfy the conditions of a highly correlated electron system ( U > W ). Based on first-principles calculations, we demonstrate that these materials could satisfy Stoners condition, and could be new candidates for transparent and half-metallic ferromagnetic DMSs without any magnetic elements.

Electronic structure in high temperature phase of Fe3O4

Band structure of the ferrimagnetic Fe 3 O 4 was calculated in the high temperature cubic phase by means of the full-potential linearized augmented-plane-wave method with the local spin density approximation. In the majority spin bands, the Fermi level stays in the band gap and 3d states of A site Fe are unoccupied while 3d states of B site Fe are completely occupied. In the minority spin bands, 3d states of A site are occupied and the Fermi level stays in de bands of B site Fe. The obtained Fermi surfaces well explain the mechanism of the structural phase transition in Fe 3 O 4 . The calculated charge density has good correspondence to the recent experimental results.

Energy Band Structures of Gd-Pnictides

Energy band structures of Gd-pnictides, GdX(X=N, P, As and Sb), known to show a transition from a semiconductor to a metal in the series from GdN to GdSb, are calculated using the APW method. It is found that the energy bands depend sensitively on a choice of the one-electron potential. Then, the calculation is carried out self-consistently. The calculated bands account qualitatively well for the observed trend of the conducting properties through the pnictides. The density of states agrees reasonably well with experimental results. The Fermi surface of metallic pnictides is calculated both for paramagnetic and ferromagnetic states. It consists of simple closed electron and hole surfaces. Relativistic effects are investigated using first-order perturbation theory. They do not affect seriously nonrelativistic bands.

Implications of Band-Structure Calculations for High-Tc Related Oxides

We have performed band calculatons for the high- T c related oxides, La 2 CuO 4 , YBa 2 Cu 3 O y ( y =6 or 7), La 2 CaCu 2 O 6 , CuO, and also NiO with the full-potential linear augmented plane wave method in order to derive information about the characteristic aspects of these materials. We pay particular attention to the energy separation between the oxygen p and the copper d levels, the assignment of a p orbital for the extra hole accommodation, the significance of the p - p hopping, the occupancy in the two-dimensional Cu-O bands, and the Cu valency.

Single High-Tc Phase Region of the Bi-Pb-Sr-Ca-Cu-O System

The phase diagram in the superconducting Bi1.68PbvSrxCavCuzOw(v=0.28, 0.32) system was investigated by means of X-ray diffraction and electrical resistivity for samples calcined in air. The results revealed that the phase was classified into four regions, a single 110 K-phase region, two types of two-phase regions (110 K phase+Ca2CuO3 and 110 K phase+80 K phase) and a three-phase region (110 K phase+Ca2CuO3+80 K phase). The single 110 K phase with a Tc of about 106–108 K exists in a fairly wide composition range, especially in the Bi1.68Pb0.32Sr1.75CayCuzOw system, 1.75y1.85 and 2.65z2.85. This means that the single 110 K phase can be easily obtained, even by the usual calcination in air in the Bi-Pb-Sr-Ca-Cu-O system.

Electronic Structure of La2CuO4 by APW Method

The self-consistent APW band calculation for the body centered tetragonal structure La2CuO4, which is a proper reference material for the study of the origin of high-temperature superconductivity in (La1-xMx)2CuO4 compounds, has been done using the local density approximation. The bonding-antibonding splitting of the Cu 3d(x2-y2) and O 2p states results in a pair of subbands of about 0.58 Ry width. The half-filled antibonding band has the two dimensional property and nearly square Fermi surface.