14 research outputs found
Temperature dependence of antiferromagnetic order in the Hubbard model
We suggest a method for an approximative solution of the two dimensional
Hubbard model close to half filling. It is based on partial bosonisation,
supplemented by an investigation of the functional renormalisation group flow.
The inclusion of both the fermionic and bosonic fluctuations leads in lowest
order to agreement with the Hartree-Fock result or Schwinger-Dyson equation and
cures the ambiguity of mean field theory . We compute the temperature
dependence of the antiferromagnetic order parameter and the gap below the
critical temperature. We argue that the Mermin-Wagner theorem is not
practically applicable for the spontaneous breaking of the continuous spin
symmetry in the antiferromagnetic state of the Hubbard model. The long distance
behavior close to and below the critical temperature is governed by the
renormalisation flow for the effective interactions of composite Goldstone
bosons and deviates strongly from the Hartree-Fock result.Comment: New section on critical behavior 31 pages,17 figure
Antiferromagnetic gap in the Hubbard model
We compute the temperature dependence of the antiferromagnetic order
parameter and the gap in the two dimensional Hubbard model at and close to half
filling. Our approach is based on truncations of an exact functional
renormalization group equation. The explicit use of composite bosonic degrees
of freedom permits a direct investigation of the ordered low temperature phase.
We show that the Mermin--Wagner theorem is not practically applicable for the
spontaneous breaking of the continuous spin symmetry in the antiferromagnetic
state. The critical behavior is dominated by the fluctuations of composite
Goldstone bosons.Comment: new discussion of critical behavior 4 pages,2 figures, LaTe
Spontaneous symmetry breaking in the colored Hubbard model
The Hubbard model is reformulated in terms of different ``colored'' fermion
species for the electrons or holes at different lattice sites.
Antiferromagnetic ordering or d-wave superconductivity can then be described in
terms of translationally invariant expectation values for colored composite
scalar fields. A suitable mean field approximation for the two dimensional
colored Hubbard model shows indeed phases with antiferromagnetic ordering or
d-wave superconductivity at low temperature. At low enough temperature the
transition to the antiferromagnetic phase is of first order. The present
formulation also allows an easy extension to more complicated microscopic
interactions.Comment: 19 pages, 5 figure