MAGNETIC PHASES NEAR THE VANHOVE-SINGULARITY IN S-BAND AND D-BAND HUBBARD-MODELS

We investigate the magnetic instabilities of the nondegenerate (s-band ) and a degenerate (d-band) Hubbard model in two dimensions using many -body effects due to particle-particle diagrams and Hund's rule local correlations. The density of states and the position of the Van Hove s ingularity change depending on the value of next-nearest-neighbor hopp ing t'. The Stoner parameter is strongly reduced in the s-band case, a nd ferromagnetism survives only if the electron density is small and t he band has flat regions. Due to next-nearest-neighbor hopping there a re flat regions in Gamma-X and Gamma-Y directions. In contrast, for th e d-band case the reduction of the Stoner parameter which follows from particle-particle correlations is much smaller and ferromagnetism sur vives to a large extent. Inclusion of local spin-spin correlations has a limited destabilizing effect on the magnetic states.