We present the first observation of the magnetophonon effect in the frequency-dependent conductivity of electrons in a polar semiconductor. The cyclotron-resonance position and linewidth oscillate as a function of magnetic field and show resonances for ${\mathrm{\omega }}_{\mathrm{LO}}$≊N${\mathrm{\omega }}_{\mathit{c}}$. The resonances grow rapidly with increasing temperature, giving a direct measure of the optic-phonon scattering rate. Good agreement is found with a one-electron theory of the absorption spectrum, showing that the oscillations have the same origin as magnetophonon-resonance oscillations in the resistivity.

• Received 5 September 1990

DOI:https://doi.org/10.1103/PhysRevLett.66.794