Nonperturbative saddle point for the effective action of disordered and interacting electrons in 2D

We, find a nonperturbative saddle-point solution for the nonlinear sigma mo del proposed by Finkelstein for interacting and disordered electronic syste ms. Spin rotation symmetry, present in the original saddlepoint solution, i s spontaneously broken at one loop, as in the Coleman-Weinberg mechanism. T he new solution is singular in both the disorder and triplet interaction st rengths, and it also explicitly demonstrates that a nontrivial ferromagneti c state appears in a theory where the disorder average is carried out from the outset.