Absence of a Coulomb gap in a two-dimensional impurity band

The temperature dependence (30K>T>400 mK) and magnetic field dependence (H<50 kG) of hopping conduction have been measured as a function of impurity concentration and surface electric field in a quasi-two-dimensional impurity band formed in the inversion layer of a sodium-doped Si metal-oxide-semiconductor field-effect transistor. We find that our observations can be accommodated by noninteracting, single-particle hopping models based on percolation theory in which the effect of Coulomb interactions between electrons on different sites is ignored. Our observations are not consistent with the existence of a Coulomb gap in the single-particle excitation spectrum, although the gap was expected to determine the conductivity under the conditions examined in these experiments. © 1986 The American Physical Society.