Magnetophonon resonance is studied in the resistivity of high mobility GaAs-GaA1As heterojunctions. In contrast to previous studies which show an exponentially damped series of oscillations, we find this only at low magnetic fields. At high fields the oscillation amplitude decreases, leading to collapse of the fundamental resonance. Fully self-consistent calculations, including all scattering processes, show that Landau level width oscillates in field and explain this unusual behavior. PACS

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... ers: 73.40.Kp Magnetophonon resonance (MPR) leads to a series of exponentially damped oscillations in the resistivity p"" of semiconductors due to the resonant absorption and emission of longitudinal optic (LO) phonons. It has been used to extract information about the band structure of bulk and reduced dimensional semiconductors for the past three decades and has been reviewed several times [1 -3].