On-site interaction effects on localization: Dominance of nonuniversal contributions

The influence of on-site (Hubbard) electron-electron interactions on disorder-induced localization is studied in order to clarify the role of electronic spin. The motivation is based on the recent experimental indications of a “metal-insulator” transition in two-dimensional systems. We use both analytical and numerical techniques, addressing the limit of weak short-range interactions. The analytical calculation is based on random matrix theory (RMT). We demonstrate that, at least in the diffusive regime, delocalization can indeed be induced by electron-electron interactions and that an in-plane magnetic field has a strong influenece on this effect. It is found that although RMT gives a qualitative explanation of the numerical results, it is quantitatively incorrect. This is due to an exact cancellation of short-range and long-range correlations in RMT, which does not occur in the nonuniversal corrections to RMT. An estimate for these contributions is given.