Results 1 - 10 of 13
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[en] Thermoelectric power of the Kondo insulators is analyzed theoretically. It is pointed out that Kelvin's relation found in 1851 must be modified for insulators at low temperatures. A realistic tight-binding model is constructed for the description of the conduction bands of the most typical Kondo insulator YbB12. It will be a good starting point for the development of a realistic theory of the Kondo insulators with strong correlation
[en] It is shown that the conduction band of typical Kondo insulator YbB12 can be expressed rather well by a simple tight-binding model of 5dε orbitals on Yb. The (ddσ) overlapping integral is considered to be produced by the hopping through B12 clusters. The density of states is also calculated. If the mixing with the f-states is properly taken into account, this model can be a useful starting point for incorporating the strong correlation effect, and can consistently explain all the thermal, thermoelectric, transport and magnetic properties of YbB12
[en] We propose a modified scheme for calculating the single-particle excitation spectrum of the impurity Anderson model. It is based on the second order perturbation theory, but modifies the self-energy so as to reproduce the correct atomic limit and to fulfill the Friedel sum rule. Therefore, it offers a simple scheme valid over wide range of excitation energy and parameters, and would be useful also for potential application to the lattice problems. (author)
[en] A new scheme of the iterative perturbation theory is proposed for the strongly correlated electron systems with orbital degeneracy. The method is based on the modified self-energy of Yeyati et al (Phys. Rev. Lett. 1999 83 600) which interpolates between the weak and the strong correlation limits, but here a much simpler scheme is proposed which is useful in the case of the strong correlation with orbital degeneracy. It will be also useful in the study of the electronic structures combined with the band calculations. (author). Letter-to-the-editor
[en] Anomalous metal-insulator transition observed in filled skutterudite CeOs4Sb12 is investigated by constructing the effective tight-binding model with the Coulomb repulsion between f electrons. By using the mean field approximation, magnetic susceptibilities are calculated and the phase diagram is obtained. When the band structure has a semimetallic character with small electron and hole pockets at Γ and H points, a spin density wave transition with an ordering vector Q=(1, 0, 0) occurs due to the nesting property of the Fermi surfaces. Magnetic field enhances this phase in accord with the experiments. (author)
[en] Effects of strong correlation and magnetic field on Kondo insulators are investigated. Using the dynamical mean-field theory and the self-consistent perturbation theory, a magnetic-field-induced first-order insulator-to-metal transition is predicted, which was experimentally found in YbB12 later. The transition metal compound FeSi is also investigated from the standpoint of strong correlation, and the anomalous behavior in temperature- and frequency-dependences of optical conductivity is qualitatively understood. A new method is proposed to incorporate spin fluctuations into the dynamical mean-field theory. (author)
[en] By using recently proposed effective model for heavy fermion metallic systems, the temperature dependence of the specific heat coefficient of typical heavy fermion material YbAl3 is investigated. The Hamiltonian of the band term consists of the conduction electrons described with nearly free electron method, the localized 4f electrons of Yb ions and the hybridization term between the conduction and 4f electrons. In order to take account of the correlation effect, we reconstruct the low-energy effective Hamiltonian where the low-energy bands near the Fermi level are extracted. The self-consistent perturbation theory with local approximation is applied to this Hamiltonian, so that some physical quantities are calculated. The obtained temperature dependence of the specific heat divided by temperature shows two peak structures, which are in agreement with the experimental results of YbAl3. We show that one peak structure at the very low temperature region originates from the correlation effect and the other peak structure results from the band structure.
[en] To investigate the magnetic properties of strongly correlated electron systems, the magnetization curve and the magnetic susceptibility of the infinite-dimensional Hubbard model are calculated using the quantum Monte Carlo method for solving the single-impurity part. For the Bethe lattice, it is found that the magnetization curve is similar to that obtained by the Gutzwiller approximation: the curve rises steeply towards the saturation value near the metal-insulator (MI) transition, in contrast with the result obtained by Georges and Krauth for the simple hypercubic lattice. The susceptibility is consistent with χ similar (Uc-U)-1 except near the MI transition, and rather close to the Gutzwiller result. ((orig.))
[en] Based on the recently proposed effective Hamiltonian for typical heavy fermion compound YbAl3, the correlation effect is systematically investigated in the physical properties. The band part of the Hamiltonian consists of conduction bands described by nearly free electron method, hybridization between the conduction and 4f-electrons, and the localized 4f-states of Yb ions on the lattice site. The correlation effect is considered by using the self-consistent perturbation theory with local approximation. The temperature dependence of the specific heat coefficient is calculated, and the anomalous behaviors are found in the low temperature region in accord with the experiments. We show that the anomalies may originate from the correlation effect and the structure of the non-interacting density of states.