We report measurements of the conductivity and Hall coefficient of insulating n-type CdSe with dopant concentrations near the critical concentration for the metal-insulator transition. In the temperature range 1.2–4.2 K, where the resistivity is consistent with variable-range hopping, the Hall coefficient is finite and observable and follows an analogous temperature dependence, $R_H$∼exp[$K_H$($T_0$/T${)}^n$]. We find n<1, so that the observed Hall coefficient is not due to carriers activated to extended states; the data are consistent instead with an exponent n=(1/4 or (1/2. We compare this result with previous experiments and with current theory.

• Received 19 October 1988

DOI:https://doi.org/10.1103/PhysRevB.39.10185