We have experimentally studied the nonequilibrium transport in low-density clean two-dimensional (2D) electron systems at mesoscopic length scales. At zero magnetic field ($B$), a double-peak structure in the nonlinear conductance was observed close to the Fermi energy in the localized regime. From the behavior of these peaks at nonzero $B$, we could associate them with the opposite spin states of the system, indicating a spontaneous spin polarization at $B=0$. Detailed temperature and disorder dependence of the structure shows that such a splitting is a ground-state property of low-density 2D systems.

• Received 6 October 2003

DOI:https://doi.org/10.1103/PhysRevLett.92.116601