A combined excition-cyclotron resonance line is found in photoluminescence excitation and refectivity spectra of semiconductor quantum wells with electron gas of low density. This line appears in a magnetic field due to the combine transition when an incident photon creates an exciton in the ground state and simultaneously excites an electron beteen Landau levels. An inelastic process of exciton-electron scattering is found in quantum wells under external magnetic fields. The process is spin-dependent and is determined by the exchange interaction of excitions with free electrons. A theoretical model is developed to describe these effects.
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