Monitoring Excitations of the N$=$1 Landau Level by Optical Emission at mK Temperatures

Optical emission experiments have proven to be powerful contactless probe of collective states of electrons in the second (N$=$1) Landau Level (LL) [1,2]. We report the emission spectrum from optical recombination in the N$=$0 and N$=$1 LL's the second LL. The 2DEG is confined in ultra-high-mobility GaAs quantum well structures. Optical emission red-shifted from the main luminescence of the N$=$0 and N$=$1 LL are interpreted as shakeup processes of quasiparticles in the N$=$1 LL. Results of two samples with different carrier densities measured in the temperature range of 42mK$\le $T$\le $650mK will be compared. The experimental observations will be discussed taking into account the striking quantum phases dominating the second LL. [1] Manfra, M. J. et al. Phys. Rev. B 57, R9467 (1998) [2] Gravier, L. et al. Phys. Rev. Lett. 80, 3344 (1998).