Spin-polarization instability in a tilted magnetic field of a two-dimensional electron gas with filled Landau levels

We have investigated the stability of a two-dimensional electron gas with two filled Landau levels (of opposite spin) in the high-field limit. The Zeeman energy can be increased by adding a component of the magnetic field parallel to the surface. The lowest-lying excitations can be described in terms of singlet and triplet excitons. Taking interaction effects into account, we have found that at a critical Zeeman energy (smaller than the cyclotron energy), there is a first-order transition to a fully spin-polarized state in which two Landau levels of equal spin are filled. Exotic intermediate states of the spin-density-wave type have been found not to occur in the simple case of nondegenerate bands.