1,962 research outputs found
Charge and Spin Gap Formation in Exactly Solvable Hubbard Chains with Long-Rang Hopping
We discuss the transition from a metal to charge or spin insulating phases
characterized by the opening of a gap in the charge or spin excitation spectra,
respectively. These transitions are addressed within the context of two exactly
solvable Hubbard and tJ chains with long range, hopping. We discuss the
specific heat, compressibility, and magnetic susceptibility of these models as
a function of temperature, band filling, and interaction strength. We then use
conformal field theory techniques to extract ground state correlation
functions. Finally, by employing the -ology analysis we show that the charge
insulator transition is accompanied by an infinite discontinuity in the Drude
weight of the electrical conductivity. While the magnetic properties of these
models reflect the genuine features of strongly correlated electron systems,
the charge transport properties, especially near the Mott-Hubbard transition,
display a non-generic behavior.Comment: 47 pages, REVTEX 3.0, 14 postscript figures available form
[email protected] (submitted using the figures-command
The Anderson impurity model with a narrow-band host: from orbital physics to the Kondo effect
A particle-hole symmetric Anderson impurity model with a metallic host of
narrow bandwidth is studied within the framework of the local moment approach.
The resultant single-particle spectra are compared to unrestricted
Hartree-Fock, second order perturbation theory about the noninteracting limit,
and Lanczos spectra by Hofstetter and Kehrein. Rather accurate analytical
results explain the spectral evolution over almost the entire range of
interactions. These encompass, in particular, a rationale for the four-peak
structure observed in the low-energy sector of the Lanczos spectra in the
moderate-coupling regime. In weak coupling, the spectral evolution is governed
by orbital effects, while in the strong coupling Kondo limit, the model is
shown to connect smoothly to the generic Anderson impurity with a flat and
infinitely wide hybridization band.Comment: 17 pages, 7 figure
Perturbation theory for optical excitations in the one-dimensional extended Peierls--Hubbard model
For the one-dimensional, extended Peierls--Hubbard model we calculate
analytically the ground-state energy and the single-particle gap to second
order in the Coulomb interaction for a given lattice dimerization. The
comparison with numerically exact data from the Density-Matrix Renormalization
Group shows that the ground-state energy is quantitatively reliable for Coulomb
parameters as large as the band width. The single-particle gap can almost
triple from its bare Peierls value before substantial deviations appear. For
the calculation of the dominant optical excitations, we follow two approaches.
In Wannier theory, we perturb the Wannier exciton states to second order. In
two-step perturbation theory, similar in spirit to the GW-BSE approach, we form
excitons from dressed electron-hole excitations. We find the Wannier approach
to be superior to the two-step perturbation theory. For singlet excitons,
Wannier theory is applicable up to Coulomb parameters as large as half band
width. For triplet excitons, second-order perturbation theory quickly fails
completely.Comment: 32 pages, 12 figures, submtted to JSTA
Quantum Antiferromagnetism of Fermions in Optical Lattices with Half-filled p-band
We study Fermi gases in a three-dimensional optical lattice with five
fermions per site, i.e. the s-band is completely filled and the p-band with
three-fold degeneracy is half filled. We show that, for repulsive interaction
between fermions, the system will exhibit spin-3/2 antiferromagnetic order at
low temperature. This conclusion is obtained in strong interaction regime by
strong coupling expansion which yields an isotropic spin-3/2 Heisenberg model,
and also in weak interaction regime by Hatree-Fock mean-field theory and
analysis of Fermi surface nesting. We show that the critical temperature for
this antiferromagnetism of a p-band Mott insulator is about two orders of
magnitudes higher than that of an -band Mott insulator, which is close to
the lowest temperature attainable nowadays
New Comparisons for Local Quantities of the Two-Dimensional Hubbard Model
We have compared the results of our approximation scheme, the composite
operator method, for the double occupancy and the internal energy of the
two-dimensional Hubbard model with numerical data obtained by means of the
Lanczos and quantum Monte Carlo schemes. The agreement is very good at both
half-filling and away from it showing how reliable is the approximation scheme.Comment: 6 pages, 3 figure
Orbital-selective Mott-Hubbard transition in the two-band Hubbard model
Recent advances in the field of quantum Monte Carlo simulations for impurity
problems allow --within dynamical mean field theory-- for a more thorough
investigation of the two-band Hubbard model with narrow/wide band and
SU(2)-symmetric Hund's exchange. The nature of this transition has been
controversial, and we establish that an orbital-selective Mott-Hubbard
transition exists. Thereby, the wide band still shows metallic behavior after
the narrow band became insulating -not a pseudogap as for an Ising Hund's
exchange. The coexistence of two solutions with metallic wide band and
insulating or metallic narrow band indicates, in general, first-order
transitions.Comment: 4 pages, 3 figures; 2nd version as published in Phys. Rev. B (R);
minor corrections, putting more emphasis on differences in spectra when
comparing SU(2) and Ising Hund's exchang
Strong-coupling approach to the Mott--Hubbard insulator on a Bethe lattice in Dynamical Mean-Field Theory
We calculate the Hubbard bands for the half-filled Hubbard model on a Bethe
lattice with infinite coordination number up to and including third order in
the inverse Hubbard interaction. We employ the Kato--Takahashi perturbation
theory to solve the self-consistency equation of the Dynamical Mean-Field
Theory analytically for the single-impurity Anderson model in multi-chain
geometry. The weight of the secondary Hubbard sub-bands is of fourth order so
that the two-chain geometry is sufficient for our study. Even close to the
Mott--Hubbard transition, our results for the Mott--Hubbard gap agree very well
with those from numerical Dynamical Density-Matrix Renormalization Group
(DDMRG) calculations. The density of states of the lower Hubbard band also
agrees very well with DDMRG data, apart from a resonance contribution at the
upper band edge which cannot be reproduced in low-order perturbation theory.Comment: 40 pages, 7 figure
Equation of state for the two component Van der Waals gas with relativistic excluded volumes
A canonical partition function for the two-component excluded volume model is derived, leading to two di erent van der Waals approximations. The one is known as the Lorentz-Berthelot mixture and the other has been proposed recently. Both models are analysed in the canonical and grand canonical ensemble. In comparison with the one-component van der Waals excluded volume model the suppression of particle densities is reduced in these two-component formulations, but in two essentially di erent ways. Presently used multi-component models have no such reduction. They are shown to be not correct when used for components with di erent hard-core radii. For high temperatures the excluded volume interaction is refined by accounting for the Lorentz contraction of the spherical excluded volumes, which leads to a distinct enhancement of lighter particles. The resulting e ects on pion yield ratios are studied for AGS and SPS data
Decentralized Taxation and the Size of Government: Evidence from Swiss State and Local Governments
According to the Leviathan-Model, fiscal federalism is seen as a binding constraint on a revenue-maximizing government. The competitive pressure of fiscal federalism is supposed to reduce public sector size as compared to unitary states. However, empirical results concerning the Leviathan hypothesis are mixed. This study uses a state and local-level panel data set of Swiss cantons from 1980 to 1998 to empirically analyze the effect of different federalist institutions on the size and structure of government revenue. Because of the considerable tax autonomy of sub-national Swiss governments, it is possible to investigate different mechanisms by which fiscal federalism may influence government size. The results indicate that tax exporting has a revenue expanding effect whereas tax competition favors a smaller size of government. Fragmentation has essentially no effect on the size of government revenue for Swiss cantons. The overall effect of revenue decentralization leads to fewer tax revenue but higher user charges. Thus, revenue decentralization favors a smaller size of government revenue and shifts government revenue from taxes to user charges.federalism, government revenue, tax competition, tax exporting
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