78 research outputs found
Charge Gap in the One-Dimensional Extended Hubbard Model at Quarter Filling
We propose a new combined approach of the exact diagonalization, the
renormalization group and the Bethe ansatz for precise estimates of the charge
gap in the one-dimensional extended Hubbard model with the onsite and
the nearest-neighbor interactions and at quarter filling. This approach
enables us to obtain the absolute value of including the prefactor
without ambiguity even in the critical regime of the metal-insulator transition
(MIT) where is exponentially small, beyond usual renormalization group
methods and/or finite size scaling approaches. The detailed results of
down to of order of near the MIT are shown as contour lines on the
- plane.Comment: 4 pages, 4 figure
Combined Analysis of Numerical Diagonalization and Renormalization Group methods for the One-Dimensional - Model at Quarter filling
The one-dimensional extended Hubbard model with both the on-site and the
nearest neighbor interactions at quarter filling is studied by using a
novel finite size scaling. We diagonalize finite size systems numerically and
calculate the Luttinger-liquid parameter which is substituted into
the renormalization group equation as an initial condition. It leads
in the infinite size system and the result agrees very well with the available
exact result with . This approach also yields the charge gap in the
insulating state near the metal-insulator transition where the characteristic
energy becomes exponentially small and the usual finite size scaling is not
applicable.Comment: 7 pages, 8 figures,submitted to PR
First-principles Study of the RKKY Interaction and the Quadrupole Order in the Pr 1-2-20 systems PrT2Al20 (T=Ti, V)
Electronic states and quadrupole orders in the Pr 1-2-20 systems PrT2Al20
(T=Ti, V) are investigated on the basis of the first-principles calculations.
The effective 196 orbital model is derived to reproduce the first-principles
electronic structures of LaT2Al20 (T=Ti, V) without contribution from the Pr 4f
electrons which are considered to be well localized and is employed to
calculate the Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions between
quadrupole and octupole moments of the Pr ions. Within the random phase
approximation for the RKKY Hamiltonian, the most divergent susceptibility is
found to be the quadrupole one for the wave vector Q = (0,0,0) in the case of
PrTi2Al20 while that for Q = (pi/a,0,pi/a) in the case of PrV2Al20 as
consistent with experimental observations in the both cases which exhibit the
ferro-quadrupole (FQ) and the antiferro-quadrupole (AFQ) orders, respectively.
We also discuss the ordered states using the mean-field approximation and find
that, in the case of PrTi2Al20, the 1st-order phase transition to the O20 FQ
order with a tiny discontinuity takes place as predicted by the Landau theory.
In the case of PrV2Al20, the system exhibits two distinct O22 AFQ orders, AFQ-I
and AFQ-II, and shows subsequent two phase transitions, the 2nd-order one from
normal to AFQ-I and the 1st-order one from AFQ-I to AFQ-II, that may be
responsible for the double transitions observed by specific heat measurements.Comment: 6 pages, 6 figure
Phase diagram and dynamic response functions of the Holstein-Hubbard model
We present the phase diagram and dynamical correlation functions for the
Holstein-Hubbard model at half filling and at zero temperature. The
calculations are based on the Dynamical Mean Field Theory. The effective
impurity model is solved using Exact Diagonalization and the Numerical
Renormalization Group. Excluding long-range order, we find three different
paramagnetic phases, metallic, bipolaronic and Mott insulating, depending on
the Hubbard interaction U and the electron-phonon coupling g. We present the
behaviour of the one-electron spectral functions and phonon spectra close to
the metal insulator transitions.Comment: contribution to the SCES04 conferenc
Electronic State of Na_xCoO_2 Based on the Two Dimensional Triangular Lattice d-p Model
The electronic state in a CoO_2 plane of the layered cobalt oxides
Na_{x}CoO_2 is investigated by using the 11 band d-p model on a two-dimensional
triangular lattice, where the tight-binding parameters are determined so as to
fit the LDA band structure. Effects of the Coulomb interaction at a Co site:
the intra- and inter-orbital direct terms U and U', the exchange coupling J and
the pair-transfer J', are treated within the Hartree-Fock approximation. We
also consider the effect of the Na order at x=0.5, where Na ions form one
dimensional chains, by taking into account of an effective one-dimensional
potential Delta epsilon_{d} on the CoO_2 plane. It is found that the Na order
enhances the Fermi surface nesting resulting in the antiferromagnetism (AFM)
which is suppressed due to the frustration effect in the case without the Na
order. When U and Delta epsilon_{d} are varied, we observe three types of the
AFM: (1) the metallic AFM with large density of states N_F at the Fermi level
for small values of U and Delta epsilon_{d}, (2) the semimetallic AFM with tiny
N_F for large U with small Delta epsilon_{d} and (3) the insulating AFM with a
finite energy gap for large values of U and Delta epsilon_{d}.Comment: 6 pages, 4 figure
Effect of the spin-orbit interaction and the electron phonon coupling on the electronic state in a silicon vacancy
The electronic state around a single vacancy in silicon crystal is
investigated by using the Green's function approach. The triply degenerate
charge states are found to be widely extended and account for extremely large
elastic softening at low temperature as observed in recent ultrasonic
experiments. When we include the LS coupling on each Si
atom, the 6-fold spin-orbital degeneracy for the state with the valence
+1 and spin 1/2 splits into doublet groundstates and
quartet excited states with a reduced excited energy of . We also consider the effect of couplings between electrons and
Jahn-Teller phonons in the dangling bonds within the second order perturbation
and find that the groundstate becomes quartet which is responsible
for the magnetic-field suppression of the softening in B-doped silicon.Comment: 4 pages, 2 figure
Mott transitions in correlated electron systems with orbital degrees of freedom
Mott metal-insulator transitions in an M-fold orbitally degenerate Hubbard
model are studied by means of a generalization of the linearized dynamical
mean-field theory. The method allows for an efficient and reliable
determination of the critical interaction U_c for any integer filling n and
different M at zero temperature. For half-filling a linear dependence of U_c on
M is found. Inclusion of the (full) Hund's rule exchange J results in a strong
reduction of U_c. The transition turns out to change qualitatively from
continuous for J=0 to discontinuous for any finite J
Crystalline-Electric-Field Effect on the Resistivity of Ce-based Heavy Fermion Systems
The behavior of the resistivity of Ce-based heavy fermion systems is studied
using a 1/-expansion method a la Nagoya, where is the spin-orbital
degeneracy of f-electrons. The 1/-expansion is performed in terms of the
auxiliary particles, and a strict requirement of the local constraints is
fulfilled for each order of 1/N. The physical quantities can be calculated over
the entire temperature range by solving the coupled Dyson equations for the
Green functions self-consistently at each temperature. This 1/N-expansion
method is known to provide asymptotically exact results for the behavior of
physical quantities in both low- and high-energy regions when it is applied to
a single orbital periodic Anderson model (PAM). On the basis of a generalized
PAM including crystalline-electric-field splitting with a single conduction
band, the pressure dependence of the resistivity is calculated by
parameterizing the effect of pressure as the variation of the hybridization
parameter between the conduction electrons and f-electrons. The main result of
the present study is that the double-peak structure of the -dependence of
the resistivity is shown to merge into a single-peak structure with increasing
pressure.Comment: 37 pages, 22 figure
Field dependent effective masses in YbAl
We show for the intermediate valence compound YbAl that the high field
(40 60T) effective masses measured by the de Haas-van
Alphen experiment for field along the direction are smaller by
approximately a factor of two than the low field masses. The field
40T for this reduction is much smaller than the Kondo field ( 670K) but is comparable to the field
where 40K is the temperature for the onset
of Fermi liquid coherence. This suggests that the field scale does not
arise from 4 polarization but is connected with the removal of the anomalies
that are known to occur in the Fermi liquid state of this compound.Comment: 7 pages plus 3 figures Submitted to PRL 9/12/0
Periodic Anderson model with degenerate orbitals: linearized dynamical mean field theory approach
We investigate a multi-orbital extension of the periodic Anderson model with
particular emphasis on electron correlations including orbital fluctuations. By
means of a linearized version of the dynamical mean-field theory, we compute
the renormalization factor, the density of states, the spectral gap and the
local correlation functions for a given set of the intra- and inter-orbital
Coulomb interactions as well as the Hund coupling. It is found that when a
certain condition is met for the intra- and inter-orbital interactions for
electrons, orbital fluctuations are enhanced, thereby enlarging the Kondo
insulating gap. This effect is suppressed in the presence of the Hund coupling.
We also clarify how the Kondo insulator is continuously changed to the Mott
insulator when electron correlations among conduction electrons are increased.Comment: 7 pages, 10 figure
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