We report the first observation of magnetophonon resonances in the two-dimensional electronic system in single interface GaAs-${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ heterojunctions and heterojunction superlattices. They result from inelastic scattering between Landau levels with resonant absorption of LO phonons of GaAs and yield a polaron mass ${m_p}^{*}=(0.077\mathrm{}\pm 0.004)m_e$.

• Received 3 December 1979

DOI:https://doi.org/10.1103/PhysRevLett.44.341