Interaction coupled cyclotron transitions of two-dimensional electron systems in GaAs at high temperatures.

We report here the first observation of the cyclotron resonance Landau splitting for low density electron inversion layers in ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{A}\mathrm{s}\ensuremath{-}\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}$ heterojunctions. Even at temperatures close to 100 K electron-electron interactions couple the electrical dipole transitions from the ground and first excited Landau levels, with a coupling strength comparable to the one found at liquid helium temperatures. The experiment can be explained only in a single-particle approximation at sufficiently low densities and/or magnetic field strengths where the resonant polaron effect is important.