Observation of Roton Density of States in in Two-Dimensional Landau Level Excitiations.

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 upsilon >=1 we find in spectra of density of states the excitonic binding and roton behavior predicted by the Hartree-Fock approximation. The large critical point wavevectors, q>~ (h bar c/eB) sup (1/2) >~ 10 sup 6cm sup (-1), are probably accessible in resonant light scattering through the residual disorder that broadens the Landau levels.