8 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
Spin polaron damping in the spin-fermion model for cuprate superconductors
A self-consistent, spin rotational invariant Green's function procedure has
been developed to calculate the spectral function of carrier excitations in the
spin-fermion model for the CuO2 plane. We start from the mean field description
of a spin polaron in the Mori-Zwanzig projection method. In order to determine
the spin polaron lifetime in the self-consistent Born approximation, the
self-energy is expressed by an irreducible Green's function. Both, spin polaron
and bare hole spectral functions are calculated. The numerical results show a
well pronounced quasiparticle peak near the bottom of the dispersion at
(pi/2,pi/2), the absence of the quasiparticle at the Gamma-point, a rather
large damping away from the minimum and an asymmetry of the spectral function
with respect to the antiferromagnetic Brillouin zone. These findings are in
qualitative agreement with photoemission data for undoped cuprates. The direct
oxygen-oxygen hopping is responsible for a more isotropic minimum at
(pi/2,pi/2).Comment: 18 pages, 13 figure