Resonant Magnetophonon Emission by Supersonic Electrons in Ultrahigh-Mobility Two-Dimensional Systems

We investigate resonant acoustic phonon scattering in the magnetoresistivity of an ultrahigh-mobility two-dimensional electron gas system subject to DC current in the temperature range 10 mK to 3.9 K. For a DC current density of ∼1.1  A/m, the induced carrier drift velocity v_{drift} becomes equal to the speed of sound s∼3  km/s. When v_{drift}≳s very strong resonant features with only weak temperature dependence are observed and identified as phonon-induced resistance oscillations at and above the "sound barrier." Their behavior contrasts with that in the subsonic regime (v_{drift}