Observation of roton density of states in two-dimensional Landau-level excitations

Inelastic light scattering by inter-Landau-level excitations of the 2D electron gas in high-mobility GaAs structures in a perpendicular magnetic field was observed at the energies of the critical points in the mode dispersions. For Landau-level filling factors ν≥1, structure in the spectra indicates the excitonic binding and roton behavior predicted by the Hartree-Fock approximation. The large critical-point wave vectors, q≳(ħc/eB${)}^{\mathrm{-}1/2}$≳${10}^6$ ${\mathrm{cm}}^{\mathrm{-}1}$, are probably accessible in resonant light scattering through the residual disorder that broadens the Landau levels.