Kondo--attractive-Hubbard model for the ordering of local magnetic moments in superconductors

We consider local magnetic moments coupled to conduction electrons with on-site attraction, in order to discuss the interplay between pairing and magnetic order. We probe the ground state properties of this model on a one-dimensional lattice through pair binding energies and several correlation functions, calculated by means of density-matrix renormalization group. A phase diagram is obtained (for fixed electron density 1/3), from which we infer that coexistence between magnetic order and superconductivity is robust, at the expense of a continuous distortion of the magnetic arrangement of the local moments, as evidenced by a strong dependence of the characteristic wave vector k* with the coupling constants. This allows us to understand some trends of the coexistence, such as the influence of the rare earth on k*, as observed experimentally in the borocarbides.