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A new effect has been observed at low temperatures when conduction is restricted to two dimensions in an n-type GaAs impurity band. When the carrier concentration is less than 10" cm-2 the conductance is found to oscillate as a function of carrier concentration. The oscillations can be resolved into simple series with the minima periodic functions of the mean electron separation. Suppression of the main series and enhancement of a subsidiary series is achieved by increasing the electric field responsible for transport; this results in differential negative resistance at certain carrier concentrations. The effect may possibly arise from, an ordering of the electrons by Coulomb repulsion, although it is difficult to envisage how this can happen in a random array of donors and we have at present no model to explain the effect.

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