O. Sakai

A New Interpretation of NMR in Quadrupolar Ordering Phase of CeB6 - Consistency with Neutron Scattering -

The hyperfine coupling between the multipolar moments of the Ce ion and the nucleus of B ion is discussed phenomenologically for the antiferro-quadrupolar ordering (AFQ) phase of CeB 6 . The longstanding mutual inconsistency between neutron diffraction and NMR in phase II can be resolved by considering the induced octupolar moment with the T x y z type. The experimental facts indicate that the AFQ of the Γ 5 + symmetry (i.e. O x y , O y z and O z x types) is realized in CeB 6 .

New interpretation of the de Haas-van Alphen signals of LaB6

Abstract A new model for the Fermi surface of LaB6 is proposed. It consists of spherical balls at X points connected by the short necks, instead of the slender necks previously considered, and the small ellipsoid-electron-pockets which are elongated along the Σ axes and overlap on the necks. Angular variations of frequencies and intensities of the de Haas-van Alphen signals are calculated. Accumulated data including recent acoustic measurement are explained consistently. The model is obtained by displacing the unoccupied 4f levels upward from those of the self-consistent band calculation. This indicates the importance of the correction to the local density approximation for the 4f states.

Electronic structures in cerium monopnictides

Abstract Among Ce-monopnictides, CeBi and CeSb show very interesting magnetic properties, competing and coexisting with the dense Kondo state. The p-f mixing model was used successfully to explain all the anomalous magnetic properties as well as the photoemission spectra. For a better understanding of their electronic states, a more crucial measurement of the de Haas-van Alphen effect has been done and compared with the band calculation. LMTO method was used because it is convenient to select a special character in a muffin-tin sphere and it really can fit the experimental results satisfactorily even if it has no adjustable parameters. However, still some controversies remain. It is pointed out that the d-f Coulomb screening effect for a 4f hole is crucially important to enhance the Kondo temperature in semimetals such as Ce-monopnictides.

Kondo Effect in Single Quantum Dot Systems – Study with Numerical Renormalization Group Method –

The tunneling conductance is calculated as a function of the gate voltage in wide temperature range for the single quantum dot systems with Coulomb interaction. We assume that two orbitals are active for the tunneling process. We show that the Kondo temperature for each orbital channel can be largely different. The tunneling through the Kondo resonance almost fully develops in the region (T lesssim 0.1 T_{rm K}^{*} sim 0.2 T_{rm K}^{*}), where T K * is the lowest Kondo temperature when the gate voltage is varied. At high temperatures the conductance changes to the usual Coulomb oscillations type. In the intermediate temperature region, the degree of the coherency of each orbital channel is different, so strange behaviors of the conductance can appear. For example, the conductance once increases and then decreases with temperature decreasing when it is suppressed at T =0 by the interference cancellation between different channels. The interaction effects in the quantum dot systems lead the sensitivit...

Excitation spectra of two impurity Anderson model

Abstract Magnetic and single particle excitation spectra of the two impurity Anderson model are calculated based on the numerical renormalization method. The direct f-f hoping or the realistic c-f hybridization terms cause the splitted Kondo peaks corresponding to the even and odd parity channels. The antiferromagnetic fluctuation of local spin pair shows softening when it has weak antiferromagnetic coupling. This softening, and the divergence of the staggered susceptibility previously pointed out by Jones et al. are suppressed by the parity splitting.

4f-band of Ce-compounds in photoexcitation and in low energy phenomena

Abstract Energy band structures in the ferromagnetically orbital-ordered state are calculated for CeSb. A self-energy correction term due to the scattering with magnon like modes is calculated, and shown to have a part representing the relaxation process through the c-f hybridization. The location of the f-band well corresponds to the deep 4f peak in PES. The same calculation also gives a probable explanation of the Fermi surface structures. The possibility of observing a shallow f-band is pointed out.

Effective 4ƒ energy level in virtual excitation and in photoemission processes in Ce-pnictides

Effects of relaxation of occupied band electrons to the ƒ-hole state through the hybridization between ƒ and band states are studied based on a detailed model for Ce-monopnictides. The effective 4ƒ level is shifted about 1–2 eV to shallow energy side from the unrenormalized bare level in processes in which the 4ƒ electron is only virtually excited, such as in excitation to the vacant p band states through the p-ƒ mixing. Photoemission spectra show two peaks, one near the Fermi energy and the other about 3 eV below it. The latter is shifted to deep energy side about 0.5–1 eV from the bare level when it lies near the bottom of the valence band. The discrepancy between the 4ƒ level estimated from the low energy phenomena and that from photoemission is resolved.

Numerical Renormalization-Group Study of Particle-Hole Symmetry Breaking in Two-Channel Kondo Problem : Effect of Repulsion among Conduction Electrons and Potential Scattering

Particle-hole symmetry breaking perturbation in the two-channel pseudospin Kondo problem is studied by the numerical renormalization-group method. It is shown that the repulsion among conduction electrons at the impurity site and the single particle potential are relevant perturbations against the conventional non-Fermi-liquid fixed point. Although the repulsion (potential) with realistic strength prevents the overscreening of the pseudospin, it induces in turn a real spin, which is also overscreened again. Thus the real spin susceptibility becomes anomalous, contrary to the conventional two-channel Kondo problem.

Application of the numerical renormalization group method to the hubbard model in infinite dimensions

Abstract We present a solution of the infinite dimensional Hubbard model obtained by the numerical renormalization group method. This method gives directly the dynamical response in real frequency at zero temperature, and also gives reliable estimation of static quantities such as the electronic specific heat near absolute zero. We note technical points which need due care in applying this method.

Excitation spectra of the impurity Anderson model calculated by the numerical renormalization group method

A method to calculate the excitation spectra of the impurity Anderson model based on the numerical renormalization group technique is reviewed. The single particle and the magnetic excitation spectra are calculated for the cases of various magnitudes of the 1-1 Coulomb interaction to compare them with experimentally observed broad PES-BIS spectra in U-compounds. Excitation spectra in the Kondo limit are also examined in detail. The Kondo effect due to the magnetic ions with complex multiplet structures, such as the Sm- and the Tm·like ions, is studied. The excitation spectra of the two impurity Anderson model are calculated and roles of the parity splitting are discussed. In this paper we review our recent. calculation for the excitation spectra of the impurity Ande:r;son model. The method of calculation is developed based on the numerical renormalization group (NRG) technique which has been originally introduced by Wilson to study the thermodynamic properties of the Kondo problem. 1 >,z> The purposes of the calculation are three folds. The first one is to study the single particle excitation (SPE) spectra of the impurity Anderson model in wide energy range. 3 > 4 > We study roles of the atomic/-/ Coulomb interaction in the origin of the broad PES-BIS spectra of U-compounds, in which the atomic interactions and the c-/ hybridization are expected to have comparable magnitude. The second one is to study the dynamical excitation spectra of the Kondo problem. Several approximation schemes have been developed to study the dynamixad cal properties of the problem, but exact excitation spectra at very low temperature have not been obtained before our calculation. We calculate the spectra of SPE, magnetic and charge excitations in the low energy region of the order of the Kondo · temperature, TK., carefully. 5 >