374 research outputs found
Comparison of non-crossing perturbative approach and generalized projection method for strongly coupled spin-fermion systems at low doping
We analyze the two-dimensional spin-fermion model in the strong coupling
regime relevant to underdoped cuprates. We recall the set of general sumrules
that relate moments of spectral density and the imaginary part of fermion
self-energy with static correlation functions. We show that two-pole
approximation of projection method satisfies the sumrules for first four
moments of spectral density and gives an exact upper bound for quasiparticle
energy near the band bottom. We prove that non-crossing approximation that is
often made in perturbative consideration of the model violates the sumrule for
third moment of spectral density. This leads to wrong position of lowest
quasiparticle band. On the other hand, the projection method is inadequate in
weak coupling limit because of approximate treatment of kinetic energy term. We
propose a generalization of projection method that overcomes this default and
give the fermion self-energy that correctly behaves both in weak and strong
coupling limits.Comment: 9 pages, 4 EPS figures, RevTe
Non-rigid hole band in the extended t-J model
The dispersion of one hole in an extended - model with additional
hopping terms to second and third nearest neighbours and a frustration term in
the exchange part has been investigated. Two methods, a Green's function
projection technique describing a magnetic polaron of minimal size and the
exact diagonalization of a lattice, have been applied, showing reasonable
agreement among each other. Using additional hopping integrals which are
characteristic for the CuO plane in cuprates we find in the nonfrustrated
case an isotropic minimum of the dispersion at the point in
-space in good coincidence with recent angle-resolved photoemission results
for the insulating compound SrCuOCl. Including frustration or
finite temperature which shall simulate the effect of doping, the dispersion is
drastically changed such that a flat region and an extended saddle point may be
observed between and in agreement with experimental
results for the optimally doped cuprates.Comment: 14 pages, LaTeX, 6 figures on request, submitted to Zeitschrift fuer
Physi
d-Wave Pairing in an Ensemble of Spin Polaron Quasiparticles in the Spin-Fermion Model of the Electronic Structure of the CuO2 Plane
It is demonstrated for the first time that the strong coupling between spin
moments of copper ions and oxygen holes, which arises upon hybridazation mixing
of two hole subsystems in the Emery model, not only affects the formation of
spin polaron quasiparticles but also ensures effective attraction between them
via the exchange interaction. This results in the Cooper instability with
d-wave pairing in a 2D ensemble of spin polaron quasiparticles. The T-x-phase
diagram obtained using this approach agrees well with the available
experimental data
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