Katsuhiko Takegahara

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.

Fermi surface instability in Pr-based skutterudites

Abstract The Fermi surfaces are calculated for PrFe 4 P 12 , PrRu 4 P 12 and PrRu4Sb12, by using an FLAPW and the LDA+U method within Im = 3 structure, where Pr3+(4f2) is singlet. The result reveals that the instability of the Fermi surfaces is responsible to the structural phase transition observed in PrFe4P12 and PrRu4P12.

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.

Conduction bands in the filled skutterudites

Some filled skutterudite compounds have recently been found to exhibit very interesting properties: a metal–insulator transition (PrRu4P12), an antiferroquadrupole ordering and heavy-fermion behaviour under magnetic fields (PrFe4P12) and a new class of heavy-fermion superconductivity (PrOs4Sb12). Such varied and interesting physical properties are thought to reflect the Fermi surface properties. Band structure calculations have revealed the characteristics of the conduction bands. The main conduction band consisting of p orbitals of pnictogen surrounding the rare-earth ions has a nesting property, and strongly hybridizes with one of 4f electrons. One of the other conduction bands does not have mixing matrix elements with 4f electrons, resulting in the unique band structure in CeOs4Sb12.

X-dependence of electronic bandstructures for LaFe4X12(X=P,As,Sb)

Abstract The bandstructures and Fermi surfaces are calculated for LaFe4P12, LaFe4As12 and LaFe4Sb12, by using an FLAPW method. The density of states at the Fermi level increases from P to Sb, due to the rising of the relative position of the Fe-d(t2g) bands, because of the increase of the interatomic distances.

Electronic Band Structure of the Filled Skutterudite YbFe4Sb12

The electronic band structures are calculated for YbFe 4 Sb 12 and its reference compound LaFe 4 Sb 12 , by using an FLAPW method. The result reveals that the Yb ions have an intermediate valence but the calculated Sommerfeld coefficient is a half of the observed value indicating the strongly correlated electron effects.

APW Band Structure of Cubic BaPb1-xBixO3

The self-consistent APW band calculations for the materials of the ideal perovskite structure, BaPbO 3 and BaBiO 3 and the Nail type super-cell structure BaPb 0.5 Bi 0.5 O 3 have been done using the local density approximation. In both BaPbO 3 and BaBiO 3 , the bonding-antibonding splitting of the (Pb, Bi) 6s and O 2p states makes a pair of wide bands of about 15 eV width. At the center of these s-p bands, there are non-bonding O 2p bands with about 4 eV width. The character of these bands is substantially different from the previously reported results of LAPW method. In BaPb 0.5 Bi 0.5 O 3 , due to the potential difference between Pb and Bi sites, each bonding and antibonding s-p band splits into two subbands but the split antibonding bands overlap each other slightly. This result refuses the possibility of the gap formation in the Bi-rich alloys due to the charge density wave because the ordered BaPb 0.5 Bi 0.5 O 3 offers the upper limit of the charge density in the present system. Then the origin of th...

Electronic Structure of CeRh3B2:A Large f-f Direct Mixing System

The standard self-consistent APW band calculations for LaRh 3 B 2 and CeRh 3 B 2 , which crystallize in the hexagonal CeCo 3 B 2 type structure, have been done using the local density approximation. Among the 4 f states, the magnetic quantum number l z =0 state has the large mixing with the conduction bands and thus is the ground state of the crystal field. For CeRh 3 B 2 , the band calculation, in which the orbital degeneracy of 4 f states is lifted by introducing the l z dependent pseudo-potential, is also done for the ferromagnetic alignment of the 4 f state with l z =0. Results show that, due to the large ( f f σ), this 4 f state has strong dispersion along the c -direction creating 4 f holes on the \(\varGamma\)- M - K plane in the Brillouin zone. This 4 f hole formation is the origin of the anomalous magnetic properties and various anomalous properties are explained consistently on this model.

Electronic band structures of Ce3Pt3Sb4 and Ce3Pt3Bi4

One-electron energy band structures for Ce 3 Pt 3 X 4 (X=Sb and Bi), which belong to the valence fluctuation regime with an energy gap, are calculated by a self-consistent LAPW method with the local density approximation. The valence bands consist of the X p and the Pt 5 d states and the conduction bands are derived from the Ce 5 d states. With depressing the valence bands, the empty Ce 4 f bands are located between these bands and thus a narrow band gap appears at the Fermi level. This result explains reasonably the semiconductor-like property of these compounds.

Electronic band structure of the Th3Ni3Sb4 and Th3X4 (X=P, As, Sb)

In order to clarify the origin of the semiconductor like behavior of U 3 Ni 3 Sb 4 , one-electron energy band structures for the isostructural Th 3 Ni 3 Sb 4 and the Th 3 P 4 type Th 3 X 4 (X=P, As, Sb), which belong to the same space group, are calculated by the self-consistent APW method with the local density approximation. The calculated results for Th 3 X 4 show that the compounds are the narrow gap semiconductor or semimetal; the valence bands are derived from the X p states and the conduction bands from the Th 6 d states. Th 3 Ni 3 Sb 4 is also the semiconductor and the valence bands consist of the Ni 3 d and Sb 5 p states. Due to the mixing between the Ni 3 d and Th 6 d states, the empty Th 6 d conduction band is shifted up resulting a gap of 0.36 eV. This result explains reasonably why U 3 X 4 is semimetal but U 3 Ni 3 Sb 4 is semiconductor with a gap of 0.2 eV.