Observability of counterpropagating modes at fractional quantum Hall edges

When the bulk filling factor is nu=1-1/m with m odd, at. least one counterp ropagating chiral collective mode occurs simultaneously with magnetoplasmon s at the edge of Fractional quantum Hall samples. Initial experimental sear ches for an additional mode were unsuccessful. In this paper, we address co nditions under which its observation should be expected in experiments wher e the electronic system is excited and probed by capacitive coupling. We de rive realistic expressions for the velocity of the slow counterpropagating mode, starting from a microscopic calculation which is simplified by a Land au-Silin-like separation between long-range Hartree and residual interactio ns. The microscopic calculation determines the stiffness of the edge to lon g-wavelength neutral excitations, which fixes the slow-mode velocity, and t he effective width of the edge region, which influences the magnetoplasmon dispersion. [S0163-1829(98)02243-7].