  The performance of a two-phase Ar avalanche detector in a single electron
counting mode was studied, with regard to potential application in coherent
neutrino-nucleus scattering and dark matter search experiments. The detector
comprised of a 1 cm thick liquid Ar layer and a triple-GEM multiplier operated
in the saturated vapour above the liquid phase. Successful operation of the
detector in single electron counting mode, in the gain range from 6000 to
40000, has for the first time been demonstrated.
