We report the observation of a colossal, narrow resistance peak that arises in ultraclean (mobility $\sim 3\times {10}^7{\mathrm{cm}}^2/\mathrm{V}\mathrm{s}$) $\mathrm{GaAs}/\mathrm{AlGaAs}$ quantum wells (QWs) under millimeter wave irradiation and a weak magnetic field. Such a spike is superposed on the 2nd harmonic microwave-induced resistance oscillations (MIRO) but having an amplitude $>300\%$ of the MIRO, and a typical FWHM $\sim 50\mathrm{mK}$, comparable with the Landau level width. Systematic studies show a correlation between the spike and a pronounced negative magnetoresistance in these QWs, suggesting a mechanism based on the interplay of strong scatterers and smooth disorder. Alternatively, the spike may be interpreted as a manifestation of quantum interference between the quadrupole resonance and the higher-order cyclotron transition in well-separated Landau levels.

• Received 10 July 2010

DOI:https://doi.org/10.1103/PhysRevLett.105.246802