Filling-factor-dependent electron correlations observed in cyclotron resonance

Cyclotron resonances of electron space-charge layers in GaAs were studied at 3.He temperatures covering a density regime from 2 to 13 x 10(exp 11) cm(exp -2). For densities higher than about 6 x 10(exp 11) cm(exp -2) many-body influences are sufficiently reduced such that line splittings due to band-structure influences were involved. At integer filling factors the spin-up and spin-down electrons are not completely decoupled as predicted by theory. Inter Landau-level optical gaps are renormalized at odd fillings, and there is a correlation gap between spin- and charge-density excitations in the long-wavelength limit.