781 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
Optimal Path to Epigenetic Switching
We use large deviation methods to calculate rates of noise-induced
transitions between states in multistable genetic networks. We analyze a
synthetic biochemical circuit, the toggle switch, and compare the results to
those obtained from a numerical solution of the master equation.Comment: 5 pages. 2 figures, uses revtex 4. PR-E reviewed for publicatio
Improved Mean-Field Scheme for the Hubbard Model
Ground state energies and on-site density-density correlations are calculated
for the 1-D Hubbard model using a linear combination of the Hubbard projection
operators. The mean-field coefficients in the resulting linearized Equations of
Motion (EOM) depend on both one-particle static expectation values as well as
static two-particle correlations. To test the model, the one particle
expectation values are determined self-consistently while using Lanczos
determined values for the two particle correlation terms. Ground state energies
and on-site density-density correlations are then compared as a function of
to the corresponding Lanczos values on a 12 site Hubbard chain for 1/2 and 5/12
fillings. To further demonstrate the validity of the technique, the static
correlation functions are also calculated using a similar EOM approach, which
ignores the effective vertex corrections for this problem, and compares those
results as well for a 1/2 filled chain. These results show marked improvement
over standard mean-field techniques.Comment: 10 pages, 3 figures, text and figures as one postscript file -- does
not need to be "TeX-ed". LA-UR-94-294
Low-Temperature Spin Diffusion in a Spin-Polarized Fermi Gas
We present a finite temperature calculation of the transverse spin-diffusion
coefficient, , in a dilute degenerate Fermi gas in the presence of a
small external magnetic field, . While the longitudinal diffusion
coefficient displays the conventional low-temperature Fermi-liquid behavior,
, the corresponding results for show three
separate regimes: (a) for ; (b) , for and large spin-rotation
parameter , and (c) for and . Our results are qualitatively consistent with the available
experimental data in weakly spin-polarized and mixtures.Comment: 13 pages, REVTEX, 3 figures available upon request, RU-94-4
Correlation Induced Insulator to Metal Transitions
We study a spinless two-band model at half-filling in the limit of infinite
dimensions. The ground state of this model in the non-interacting limit is a
band-insulator. We identify transitions to a metal and to a charge-Mott
insulator, using a combination of analytical, Quantum Monte Carlo, and zero
temperature recursion methods. The metallic phase is a non-Fermi liquid state
with algebraic local correlation functions with universal exponents over a
range of parameters.Comment: 12 pages, REVTE
Spin-Charge Decoupling and Orthofermi Quantum Statistics
Currently Gutzwiller projection technique and nested Bethe ansatz are two
main methods used to handle electronic systems in the infinity limit. We
demonstrate that these two approaches describe two distinct physical systems.
In the nested Bethe ansatz solutions, there is a decoupling between the spin
and charge degrees of freedom. Such a decoupling is absent in the Gutzwiller
projection technique. Whereas in the Gutzwiller approach, the usual
antisymmetry of space and spin coordinates is maintained, we show that the
Bethe ansatz wave function is compatible with a new form of quantum statistics,
viz., orthofermi statistics. In this statistics, the wave function is
antisymmetric in spatial coordinates alone. This feature ultimately leads to
spin-charge decoupling.Comment: 12 pages, LaTex Journal_ref: A slightly abridged version of this
paper has appeared as a brief report in Phys. Rev. B, Vol. 63, 132405 (2001
Effect of three-particle correlations in low dimensional Hubbard models
A simple approximation which captures some non-perturbative aspects of the
one electron Green function of strongly interacting Fermion systems is
developed. It provides a way to go one step beyond the usual dilute limit since
particle-particle as well as particle-hole scattering are treated on the same
footing. Intermediate states are constrained to contain only one particle-hole
excitation besides the incoming particle. The Faddeev equations resulting from
an exact treatment of this three-body problem are investigated. In one
dimension the method is able to show spin and charge decoupling, but does not
reproduce the exact nature of power-law singularities. Hey dudes, check out the
analytical solution in section III!Comment: 21 pages plus six figures (appended as postscript files) in RevTeX
v.
Band Crossing and Novel Low-Energy Behaviour in a Mean Field Theory of a Three-Band Model on a Cu--O lattice
We study correlation effects in a three-band extended Hubbard model of Cu --
O planes within the 1/N mean field approach, in the infinite U limit. We
investigate the emerging phase diagram and discuss the low energy scales
associated with each region. With increasing direct overlap between oxygen
orbitals, , the solution displays a band crossing which, for an
extended range of parameters, lies close to the Fermi level. In turn this leads
to the nearly nested character of the Fermi surface and the resulting linear
temperature dependence of the quasi-particle relaxation rate for sufficiently
large T. We also discuss the effect of band crossing on the optical
conductivity and comment on the possible experimental relevance of our
findings.Comment: 12 pages, Latex-Revtex, 6 PostScript figures. Submitted to Phys. Rev.
Longitudinal spin waves in a dilute Bose gas
We present a kinetic theory for a dilute noncondensed Bose gas of two-level
atoms that predicts the transient spin segregation observed in a recent
experiment. The underlying mechanism driving spin currents in the gas is due to
a mean field effect arising from the quantum interference between the direct
and exchange scattering of atoms in different spin states. We numerically solve
the spin Boltzmann equation, using a one dimensional model, and find excellent
agreement with experimental data.Comment: 4.5 pages, 3 embedded color figure
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