We study the transport property of symmetry-induced zero-gap semiconductors, such as gray tin, under uniaxial tensile stress and at absolute zero temperature. It is shown that there is a negative-differential-resistance region in the current-voltage characteristics for $p$-type samples with small hole concentrations. This calculation confirms an earlier speculation by Liu.

• Received 29 August 1974

DOI:https://doi.org/10.1103/PhysRevLett.33.1145