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 polarization to be determined directly from resonance spectra.