Millimeterwave photoconductivity in a high-mobility $\mathrm{GaAs}-{\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ two-dimensional electron gas exhibits giant amplitude oscillations in a weak magnetic field. These oscillations resemble the Shubnikov–de Haas effect but their period is determined by $\omega /{\omega }_C,$ where $\omega$ and ${\omega }_C$ are the millimeterwave and cyclotron frequencies, respectively. The major observations can be explained in terms of Landau-level transitions between spatially shifted oscillators. A ${2k}_F$ momentum transfer is accompanying the transition where $k_F$ is the electron Fermi wave number.

• Received 27 April 2001

DOI:https://doi.org/10.1103/PhysRevB.64.201311