A large anisotropy in the electrical conductivity of CdS appearing below 30°K is reported. This anisotropy casts serious doubt on the customary view that impurity hopping supplies the low-temperature conductivity. We present a new model for this conductivity based upon the electronic screening of the piezoelectric polarization around dislocations. Numerical solutions of the highly nonlinear Poisson equation quantitatively justify this model.

• Received 7 November 1980

DOI:https://doi.org/10.1103/PhysRevLett.46.1159