Magneto-optics of the fractional Quantum Hall effect.

We have carried out a magneto-optical investigation of the 2D electron gas in the extreme quantum limit. In the optical emission arising from recombination of the 2D electron gas with holes in the ground valence subband states we observe in three samples a well-defined doublet structure at magnetic fields where simultaneous transport measurements show fractional quantization (nu = 2/3). The high energy component of the luminescence doublet shows a striking temperature dependence: disappearing above T = 2.5K, and dominating the optical emission at nu = 2/3 below T = 0.5K. Associated features are identified in luminescence excitation spectra. These results indicate the assignment of the high energy peak to recombination from a state involving the fractional quantum Hall effect.