Transport phenomena in the Co-based zero-gap semimagnetic semiconductor:Hg1−xCoxSe

Transport experiments are performed on semiconducting compound ${\mathrm{Hg}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Co}}_{\mathit{x}}$Se with 0<x≤0.0087. Two kinds of phenomena are observed: an enhancement of the mobility μ(T) when x increases up to 0.0045, followed by a decrease of μ for x between 0.0045 and 0.0087. This behavior, previously seen in ${\mathrm{Hg}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Fe}}_{\mathit{x}}$Se, could be explained by the energy position of the uppermost occupied cobalt d level in the conduction band. This leads to a resonance effect and a damping of the conduction electrons via the hybridization of the d level with the s conduction band, and which changes mobility. Near the solubility limit (x=0.01), a dip in the mobility curve appears between 10 and 80 K, corresponding to two phase transitions. The dip is probably due to the presence of macroscopic clusters of a cobalt compound, which undergo magnetic phase transitions at these temperatures.