Spin-flip excitations in the fractional-quantum-Hall-effect regime studied by polarized photoluminescence of charged excitons

We report on a low-temperature magneto-optical study of the spin-singlet charged exciton ${(X}_s^{-})$ transitions in ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{0.1}{\mathrm{Ga}}_{0.9}\mathrm{As}$ modulation-doped multiple quantum wells with an optically tuned two-dimensional electron gas (2DEG) density corresponding to a filling factor in the range $0<\mathrm{}\nu <~1.$ We find strong evidence for the effect of 2DEG magnetic correlations on the $X_s^{-}$ spin-resolved transitions. The energy difference between the ${\sigma }^{-}$- and ${\sigma }^{+}$-polarized $X_s^{-}$ photoluminescence (PL) intensity maxima is found to oscillate with varying ν. The numerical derivative of these oscillations with respect to ν shows minima at odd denominator rational fractions and maxima at even denominator fractions, similar to the longitudinal resistivity measured in the fractional quantum Hall effect. At $\nu =1,$ the observed ${\sigma }^{-}$-polarized low-energy PL tail is interpreted (by a line-shape analysis) to be due to a finite-k 2DEG spin-wave emission coupled to the optical recombination of the $X_s^{-}.$