Benjamin Lenz, Universität Göttingen
The variational cluster approximation (VCA) allows to study broken symmetry phases of various lattice models at zero temperature. We present VCA results for s-type superconductivity in the attractive two-dimensional Hubbard model after scaling to the limit of infinite cluster size. Interestingly, the superconducting order parameter in the large |U| limit shows an improvement in comparison to results from dynamical mean-field theory in combination with the numerical renormalization group method (DMFT+NRG). As an outlook, we present preliminary results for magnetism in a paradigmatic model for heavy fermions - the Kondo lattice model.