Investigating the coexistence of magnetism and superconductivity in the Kondo-Heisenberg model

We perform a finite-size cluster study of the Kondo-Heisenberg model, describing a system of itinerant electrons locally coupled via a Kondo-type interaction to a set of magnetic ions. The ionic moments are assumed to have antiferromagnetic order, due to a nearest-neighbor Heisenberg coupling. By consecutive application of displacement and squeezing transformations, a spin polaron effective Hamiltonian is derived, which is exactly solved on a 8-site closed ring. By analyzing the behavior of suitably defined correlation functions in the variationally optimized ground state, special attention is devoted to the conditions under which a superconducting phase coexisting with spin order can be established.