The Wigner glass and conductance oscillations in silicon inversion layers

The authors show that on changing the nature of the background random potential the inversion layer of the Si MOSFET exhibits the conductance oscillations previously observed in both GaAs, the source and drain regions of Si MOSFETS and MOSFETS with a very high concentration of Na+ ions at the Si-SiO2 interface. Measurements of the temperature dependence of conductance show that oscillations are found when conductance is by an excitation process as well as hopping. The oscillations arise from an oscillating activation energy which is due to either an oscillating interaction contribution to the activation energy, or a negative effective density of states at certain values of carrier concentration. This appears due to electron ordering in a small current limiting region and is contrasted with the case where the oscillations are absent, although localisation is due to both background disorder and the random field of localised electrons. It is shown that near the transition, localisation is due almost entirely to the random field of the localised electrons, and the system is a strongly interacting Fermi glass (or Wigner glass), even though the oscillations are not apparent.