Theory of Spin-Split Cyclotron Resonance in the Extreme Quantum Limit
  We present an interpretation of recent cyclotron resonance experiments on the two-dimensional electron gas in GaAs/AlGaAs heterostructures. We show that the observed dependence of the resonance spectrum on Landau level occupancy and temperature arises from the interplay of three factors: spin-splitting of the cyclotron frequency; thermal population of the two spin states; and coupling of the resonances for each spin orientation by Coulomb interactions. In addition, we derive an $f$-sum rule which allows spin polarisation to be determined directly from resonance spectra. 
N.R. Cooper and J.T. Chalker