3,269 research outputs found
Nature of correlations in the atomic limit of the boson fermion model
Using the equation of motion technique for Green's functions we derive the
exact solution of the boson fermion model in the atomic limit. Both (fermion
and boson) subsystems are characterised by the effective three level excitation
spectra. We compute the spectral weights of these states and analyse them in
detail with respect to all possible parameters.
Although in the atomic limit there is no true phase transition, we notice
that upon decreasing temperature some pairing correlations start to appear.
Their intensity is found to be proportional to the depleted amount of the
fermion nonbonding state. We notice that pairing correlations behave in a
fashion observed for the optimally doped and underdoped high
superconductors. We try to identify which parameter of the boson fermion model
can possibly correspond to the actual doping level. This study c larifies the
origin of pairing correlations within the boson fermion model and may elucidate
how to apply it for interpretation of experimental data.Comment: 5 pages, 3 figures; to appear in Eur. Phys. J.
Continuous canonical transformation for the double exchange model
The method of continuous canonical transformation is applied to the double
exchange model with a purpose to eliminate the interaction term responsible for
non conservation of magnon number. Set of differential equations for the
effective Hamiltonian parameters is derived. Within the lowest order
(approximate) solution we reproduce results of the standard (single step)
canonical transformation. Results of the selfconsistent numerical treatment are
compared with the other known studies for this model.Comment: 21 pages, 9 figures. Eur. Phys. J B (accepted for publication
Quasiparticle states driven by a scattering on the preformed electron pairs
We analyze evolution of the single particle excitation spectrum of the
underdoped cuprate superconductors near the anti-nodal region, considering
temperatures below and and above the phase transition. We inspect the
phenomenological self-energy that reproduces the
angle-resolved-photoemission-spectroscopy (ARPES) data and we show that above
the critical temperature, such procedure implies a transfer of the spectral
weight from the Bogoliubov-type quasiparticles towards the in-gap damped
states. We also discuss some possible microscopic arguments explaining this
process.Comment: 11 pages, 7 figure
Estimating Mixed Logit Recreation Demand Models With Large Choice Sets
Discrete choice models are widely used in studies of recreation demand. They have proven valuable when modeling situations where decision makers face large choice sets and site substitution is important. However, when the choice set faced by the individual becomes very large (on the order of hundreds or thousands of alternatives), computational limitations make estimation with the full choice set intractable. Sampling of alternatives in a conditional logit framework is an effective method to limit computational burdens while still producing consistent estimates. This method is allowed by the existence of the independence of irrelevant alternatives (IIA) assumption. More advanced mixed logit models account for unobserved preference heterogeneity and overcome the behavioral limitations of the IIA assumption, however in doing so, prohibit sampling of alternatives. A method is developed where a latent class (finite mixture) model is estimated via the expectations-maximization algorithm and in doing so, allows consistent sampling of alternatives in a mixed logit model. The method is tested and applied to a recreational demand Wisconsin fishing survey.Sampling of alternatives, discrete choice, mixed logit, conditional logit, recreational demand, Wisconsin, fishing, microeconometrics, Environmental Economics and Policy, Research Methods/ Statistical Methods,
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