Magnetic and Superconducting Instabilities of the Hubbard Model at the Van Hove Filling

We use a novel temperature-flow renormalization group technique to analyze magnetic and superconducting instabilities in the two-dimensional t-t^' Hubbard model for particle densities close to the Van Hove filling as a function of the next-nearest-neighbor hopping t^'. In the one-loop flow at the Van Hove filling, the characteristic temperature for the flow to strong coupling is suppressed drastically around tc^'≈-0.33t, suggesting a quantum critical point between d-wave pairing at moderate t^'>tc^' and ferromagnetism for t^'<tc^'. Upon increasing the particle density in the latter regime, the leading instability occurs in the triplet pairing channel.