K. Takegahara

Experimental study of lattice dynamics in LaB6 and YbB6

Abstract Lattice dynamics of La 11 B 6 was studied on the neutron triple axis spectrometers at HFIR. The acoustic and optical modes were measured in the high symmetry directions. The rotary motion of the B 6 molecule was measured only near the zone boundaries but shows no softening. Instead, very flat low frequency La modes were observed. The agreement with the elastic constants and the temperature dependence of specific heat is satisfactory. In YbB 6 , the elastic constants and specific heat were measured. Both results give the quite different feature from those of LaB 6 , no softening of C 12 and more softened flat low frequency Yb modes, due to the lack of the metallic bonds.

Anisotropic p−ƒ mixing mechanism explaining anomalous magnetic properties in Ce monopnictides

Abstract Anomalously small crystalline field splitting in the paramagnetic region and extremely strong magnetic anisotropy in the ordered phases in CeSb and CeBi are explained based on the anisotropic mixing mechanism between the 4ƒ states and the valence bands. In the paramagnetic region, the mixing gives the effective crystalline field splitting which is estimated to cancel the splitting of the point charge model in good agreement with experiment. The anisotropy energy of CeSb calculated by using the realistic valence bands is consistent with the strong anisotropy observed experimentally.

Mechanisms for anomalous properties in SmB6

Abstract The present situation in SmB 6 is reviewed. Properties of the gap in the 4f pseudo-band and its mechanism are studied on the new experimental results for transport, ultrasonic and NMR. They are interpreted on the model of the amorphous Wigner crystallization with an opening gap consistently.

Theoretical study of lattice dynamics in LaB6 and YbB6

Abstract Elastic constants and phonon dispersion curves of LaB6 and YbB6 are calculated theoretically. In LaB6, a small value of the elastic constant C12 is explained by the volume dependent force acting on La atoms. Furthermore, introducing the long range force between La atoms, we get the phonon dispersion curves in good agreement with the measured ones. In YbB6, too, the calculated phonon dispersion curves are consistent with the measured elastic constants and temperature dependence of specific heat. The origin of the volume dependent and long range forces is considered.

Slater-Koster tables for f electrons

By representing f orbitals by cubic harmonics, overlap (or energy) integrals for f electrons have been calculated in terms of two-centre integrals. Results of s-f, p-f, d-f and f-f integrals are useful in making the tight-binding calculation as an interpolation method for substances with rare-earth or actinide elements.

Semiconducting properties of the isomorphous compounds, Ce3Au3Sb4 and Ce3Pt3Sb4

Abstract Lattice parameters, electrical resistivity, Hall constant, magnetic susceptibility and specific heat measurements have been made on R 3 Au 3 Sb 4 ( R = La, Ce, Pr) and R 3 Pt 3 Sb 4 (R = Ce, Pr). It is found that La 3 Au 3 Sb 4 and Ce 3 Au 3 Sb 4 show semi-metallic or semiconducting behaviors, while Pr 3 Pt 3 Sb 4 shows metallic one. Furthermore, it is revealed that the value of the valence of Ce ions in Ce 3 Au 3 Sb 4 is well-defined 3+. These results indicate that R 3 3+ Au 3 Sb 4 and R 3 3+ Pt 3 Sb 4 are narrow-gap semiconductors and metals, respectively, in agreement with our band calculation on La 3 Au 3 Sb 4 . As for the Ce 3 Pt 3 Sb 4 , it also shows semiconducting property and the Ce ions is in a mixed valence state, similar to that in CeO 2 .

The magnetic phase transitions in Ce-monopnictides, strong p-f mixing effect

Specific heat of CeBi and CeSb, as well as LaBi and LaSb, was measured from 1.7 to 60 K under magnetic fields of up to 10 T. A Kondo like peak persists in CeBi in whole the studied region. In CeSb the effect of magnetic order is more significant but the similar situation exists, even though crossover from the Γ 7 dominated to Γ 8 dominated Kondo states appears. The γ-value is about 20 mJ/mol K 2 in both materials with weak field dependence. Phonon softening effect was also observed. It was shown that the extended ANNNI model including non-linear interactions can explain the essential feature of the complicated phase diagram in CeSb fairly satisfactorily.

Systematics of valence fluctuating states in f-electron systems

Abstract Results of our theoretical and experimental studies on the anomalous Kondo ferromagnet CeRh3B2 based on the f0 (lz = 0) band and the d0 screening of the 4f hole are shown. Gap formation in the unique compounds YbB12 and Ce(Pd1−xCux)3 is described by comparing with SmB6. Gap creation on the Kondo lattice is thereby established. Finally, a dense Kondo state in samarium compounds is shown in our recent experiments compared with that in cerium compounds.

Electronic band structure of ThNiSn

Abstract One-electron energy band structure for ThNiSn, which is a proper reference material for the study of the anomalous physical properties of UNiSn, is calculated by a self-consistent APW method with the local density approximation. To clarify the feature of band structure, the band calculation is performed for the isostructural ThRhSb and the NaCl type ThSb, too. The valence bands consist of the Sn 5p and Ni 3d states. Due to the strong mixing of Sn 5p and Ni 3d states, the bonding and anti-bonding bands are formed and the non-bonding Ni 3d bands locate between them. The conduction bands are derived from the Th 6d states and are raised by the mixing of Th 6d and Ni 3d states. A band gap appears between the occupied valence bands and the empty conduction bands. The mechanism of anomalous properties of UNiSn is considered.

Electronic band structures in cubic perovskite-type oxides: bismuthates and transition metal oxides

Abstract A review is given of the electronic band structures of BaPb1-xBixO3 and Ba1-xKxBiO3. On the basis of calculated band structures, the origin of gap formation in BaBiO3 is discussed. Recent results of APW band calculations for cubic perovskite-type transition metal oxides are summarized. Energy band structures and densities of states are shown for SrTO3 (where T is Ti, V, Cr, Fe), CaTO3 (where T is V, Nb), and LaTO3 (where T is Co, Cu). The various band parameters are quantitatively estimated.