Probind the Coulomb Gap in Insulating n-Type CdSe

We report the first observation of a crossover with decreasing temperature of the resistivity of a doped semiconductor from the ${\mathrm{expT}}_0$/T${)}^{1/4}$ behavior characteristic of Mott variable-range hopping to the exp(${\mathrm{T}}_0^{\prime }$/T${)}^{1/2}$ form expected in the presence of a Coulomb gap. Simultaneous determination of ${\mathrm{T}}_0$ and ${\mathrm{T}}_0^{\prime }$ for several dopant concentrations yields the empirical relation ${\mathrm{T}}_0^{\prime }$∝${\mathrm{T}}_0^{\mathrm{p}}$, with p≈23. If current theory is valid and applicable, this has unexpected implications regarding the critical behavior of the localization length and the dielectric constant at the metal-insulator transition. © 1990 The American Physical Society