| A new experimental technique, the Millimeterwave Photoconductivity Spectroscopy, has been developed and subsequently employed in studies of magneto-excitations near the Fermi surface of a high-mobility GaAs-AlGaAs two-dimensional electron system (2DES). Such low-energy magneto-excitations include high-order cyclotron modes and magneto-plasmon modes in a 2DES subjected to a weak perpendicular magnetic field at low temperature.; Higher-order cyclotron modes, forbidden in zeroth-order approximation, reveal themselves as giant amplitude oscillations in millimeterwave photoresistance. It is shown that qualitatively the effect can be explained in terms of beats of Shubnikov-de Haas oscillations, as if there were two Fermi surfaces separated by a radiation quantum. The scattering time tauCR derived within the framework of this model is shown to well exceed Shubnikov-de Haas scattering time tauSdH. This result is attributed to the different roles played by the electron density inhomogeneity in these two regimes. We also emphasize that regular transmission measurements reveal only the cyclotron resonance at the fundamental frequency with no features corresponding to high-order transitions.; In addition, a distinct peak detected in photoresistance is shown to obey the dispersion law of low-frequency magneto-plasmon modes. The finite momentum transfer q needed to excite the 2D plasmon corresponds to a cut-off wavelength given by the width of the sample. It is demonstrated that magneto-plasmon modes can be viewed as standing waves in a 2D waveguide defined by the Hall bar mesa. |
