Experimental Evidence for an Inhomogeneous State of the Correlated Two-Dimensional Electron System in the Vicinity of a Metal–Insulator Transition

From measurements of the oscillatory magnetoresistance in weak perpendicular magnetic fields, we found that the quantum oscillations in two-dimensional electron systems in Si-MOS structures are observed down to the critical carrier density n c of the transition to the strongly localized state. For such low densities, the oscillations exhibit an anticipated period, phase, and amplitude, even though the conductivity becomes less than e 2 / h , and, hence, the mean free path becomes less than the Fermi wavelength λ F . We believe that this apparent contradiction with the Ioffe-Regel criterion for diffusive transport is caused by emergence of an inhomogeneous state of the 2D system, in which the regions of diffusive and hopping conduction are spatially separated.