Low-Temperature Transport in the Hopping Regime: Evidence for Correlations Due to Exchange

Activated conduction, ρ=${\mathrm{\rho }}_0$exp[(${\mathrm{T}}_0^{\mathrm{″}}$/T${)}^{\mathrm{x}}$], with x equal to or close eto 1 is observed for insulating Si:B at low temperatures, indicating the presence of a “hard” gap in the density of states. A magnetic field suppresses this unexpectedly strong temperature dependence, changing it to the variable-range-hopping form ρ(H)=${\mathrm{\rho }}_0$(H)exp{[${\mathrm{T}}_0^{\prime }$(H)/T${]}^{1/2}$}expected for a “soft” parabolic Coulomb gap. This suggests that the density of states is determined by electron correlations due to spin as well as charge. © 1992 The American Physical Society.