Title: A mean-field approach to Kondo-attractive-Hubbard model,
Abstract: With the purpose of investigating coexistence between magnetic order and
superconductivity, we consider a model in which conduction electrons interact
with each other, via an attractive Hubbard on-site coupling $U$, and with local
moments on every site, via a Kondo-like coupling, $J$. The model is solved on a
simple cubic lattice through a Hartree-Fock approximation, within a
`semi-classical' framework which allows spiral magnetic modes to be stabilized.
For a fixed electronic density, $n_c$, the small $J$ region of the ground state
($T=0$) phase diagram displays spiral antiferromagnetic (SAFM) states for small
$U$. Upon increasing $U$, a state with coexistence between superconductivity
(SC) and SAFM sets in; further increase in $U$ turns the spiral mode into a
Néel antiferromagnet. The large $J$ region is a (singlet) Kondo phase. At
finite temperatures, and in the region of coexistence, thermal fluctuations
suppress the different ordered phases in succession: the SAFM phase at lower
temperatures and SC at higher temperatures; also, reentrant behaviour is found
to be induced by temperature. Our results provide a qualitative description of
the competition between local moment magnetism and superconductivity in the
borocarbides family.