89 research outputs found

### Cumulant expansion of the periodic Anderson model in infinite dimension

The diagrammatic cumulant expansion for the periodic Anderson model with
infinite Coulomb repulsion ($U=\infty$) is considered here for an hypercubic
lattice of infinite dimension ($d=\infty$). The same type of simplifications
obtained by Metzner for the cumulant expansion of the Hubbard model in the
limit of $d=\infty$, are shown to be also valid for the periodic Anderson
model.Comment: 13 pages, 7 figures.ps. To be published in J. Phys. A: Mathematical
and General (1997

### Electronic State and Magnetic Susceptibility in Orbitally Degenerate (J=5/2) Periodic Anderson Model

Magnetic susceptibility in a heavy fermion systemis composed of the Pauli
term (\chi_P) and the Van-Vleck term (\chi_V). The latter comes from the
interband excitation, where f-orbital degeneracy is essential. In this work, we
study \chi_P and \chi_V in the orbitally degenerate (J=5/2) periodic Anderson
model for both the metallic and insulating cases. The effect of the correlation
between f-electrons is investigated using the self-consistent second-order
perturbation theory. The main results are as follows. (i) Sixfold degenerate
model: both \chi_P and \chi_V are enhanced by a factor of 1/z (z is the
renormalization constant). (ii) Nondegenerate model: only \chi_P is enhanced by
1/z. Thus, orbital degeneracy is indispensable for enhancement of \chi_V.
Moreover, orbital degeneracy reduces the Wilson ratio and stabilizes a
nonmagnetic Fermi liquid state.Comment: 4 pages, revtex, to be published in J. Phys. Soc. Jpn. (No.8

### Optical conductivity of the Kondo insulator YbB_12: Gap formation and low-energy excitations

Optical reflectivity experiments have been conducted on single crystals of
the Kondo insulator YbB_12 in order to obtain its optical conductivity,
\sigma(\omega). Upon cooling below 70 K, a strong supression of \sigma(\omega)
is seen in the far-infrared region, indicating the opening of an energy gap of
~ 25 meV. This gap development is coincident with a rapid decrease in the
magnetic susceptibility, which shows that the gap opening has significant
influence on magnetic properties. A narrow, asymmetric peak is observed at ~40
meV in \sigma(\omega), which is attributed to optical transitions between the
Yb 4f-derived states across the gap. In addition, a broad peak is observed at
~0.25 eV. This peak is attributed to transitions between Yb 4f-derived states
and p-d band, and is reminiscent of similar peaks previously observed for
rare-earth hexaborides.Comment: 4 pages, 4 figure

### Calculation of Optical Conductivity, Resistivity and Thermopower of Filled Skutterudite CeRu$_4$Sb$_{12}$ based on a Realistic Tight-binding Model with Strong Correlation

The filled-skutterudite compound CeRu$_4$Sb$_{12}$ shows a pseudo-gap
structure in the optical conductivity spectra similar to the Kondo insulators,
but metallic behavior below 80 K. The resistivity shows a large peak at 80 K,
and the Seebeck coefficient is positive and also shows a large peak at nearly
the same temperature. In order to explain all these features, a simplified
tight-binding model, which captures the essential features of the band
calculation, is proposed. Using this model and introducing the correlation
effect within the framework of the dynamical mean field approximation and the
iterative perturbation theory, the temperature dependences of the optical
conductivity, resistivity and the Seebeck coefficient are calculated, which can
explain the experiments.Comment: 4 pages, 6 figure

### Anomalous Metal-Insulator Transition in Filled Skutterudite CeOs$_4$Sb$_{12}$

Anomalous metal-insulator transition observed in filled skutterudite
CeOs$_4$Sb$_{12}$ 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 $\Gamma$ and H points, a spin density wave
transition with the ordering vector $\mathbf{Q}=(1,0,0)$ occurs due to the
nesting property of the Fermi surfaces. Magnetic field enhances this phase in
accord with the experiments.Comment: 4 pages, 4 figure

### Field-induced phase transitions in a Kondo insulator

We study the magnetic-field effect on a Kondo insulator by exploiting the
periodic Anderson model with the Zeeman term. The analysis using dynamical mean
field theory combined with quantum Monte Carlo simulations determines the
detailed phase diagram at finite temperatures. At low temperatures, the
magnetic field drives the Kondo insulator to a transverse antiferromagnetic
phase, which further enters a polarized metallic phase at higher fields. The
antiferromagnetic transition temperature $T_c$ takes a maximum when the Zeeman
energy is nearly equal to the quasi-particle gap. In the paramagnetic phase
above $T_c$, we find that the electron mass gets largest around the field where
the quasi-particle gap is closed. It is also shown that the induced moment of
conduction electrons changes its direction from antiparallel to parallel to the
field.Comment: 7 pages, 6 figure

### Nuclear spin relaxation rates in two-leg spin ladders

Using the transfer-matrix DMRG method, we study the nuclear spin relaxation
rate 1/T_1 in the two-leg s=1/2 ladder as function of the inter-chain
(J_{\perp}) and intra-chain (J_{|}) couplings. In particular, we separate the
q_y=0 and \pi contributions and show that the later contribute significantly to
the copper relaxation rate ^{63}(1/T_1) in the experimentally relevant coupling
and temperature range. We compare our results to both theoretical predictions
and experimental measures on ladder materials.Comment: Few modifications from the previous version 4 pages, 5 figures,
accepted for publication in PR

### Periodic Anderson model with correlated conduction electrons

We investigate a periodic Anderson model with interacting conduction
electrons which are described by a Hubbard-type interaction of strength U_c.
Within dynamical mean-field theory the total Hamiltonian is mapped onto an
impurity model, which is solved by an extended non-crossing approximation. We
consider the particle-hole symmetric case at half-filling. Similar to the case
U_c=0, the low-energy behavior of the conduction electrons at high temperatures
is essentially unaffected by the f-electrons and for small U_c a quasiparticle
peak corresponding to the Hubbard model evolves first. These quasiparticles
screen the f-moments when the temperature is reduced further, and the system
turns into an insulator with a tiny gap and flat bands. The formation of the
quasiparticle peak is impeded by increasing either U_c or the c-f
hybridization. Nevertheless almost dispersionless bands emerge at low
temperature with an increased gap, even in the case of initially insulating
host electrons. The size of the gap in the one-particle spectral density at low
temperatures provides an estimate for the low-energy scale and increases as U_c
increases.Comment: 11 pages RevTeX with 13 ps figures, accepted by PR

### Laparoscopic findings in patients with nonalcoholic steatohepatitis

ArticleLIVER INTERNATIONAL. 26(1): 32-38 (2006)journal articl

### A Renormalization Group Method for Quasi One-dimensional Quantum Hamiltonians

A density-matrix renormalization group (DMRG) method for highly anisotropic
two-dimensional systems is presented. The method consists in applying the usual
DMRG in two steps. In the first step, a pure one dimensional calculation along
the longitudinal direction is made in order to generate a low energy
Hamiltonian. In the second step, the anisotropic 2D lattice is obtained by
coupling in the transverse direction the 1D Hamiltonians. The method is applied
to the anisotropic quantum spin half Heisenberg model on a square lattice.Comment: 4 pages, 4 figure

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