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Stanford, California 94305 (unpublished). 5 The yield has been corrected for reflection so that it is in terms of electrons per photon absorbed.We have observed a discontinuity in the magnetic-field dependence of the cyclotron-resonance energy in InSb which can be attributed to polaron effects. Because the transitions studied in the present experiment are between conduction-band Landau levels, the polaron theory developed earlier 1 can be expected to apply directly and quantitatively to the data. In contrast, quantitative interpretation of the interband magneto absorption data obtained previously 2 is complicated by valence-band and exciton effects. Our present data, however, confirm the essential correctness of attributing the interband anomalies to polaron effects and constitute the first direct observation of polaron effects on Landau levels.It has been previously suggested 3 that the experimental investigation of cyclotron resonance in polar materials for frequencies above and below the Reststrahl frequency could be expected to show characteristic self-energy effects associated with the electron-LO-phonon interaction. Our recent theoretical examination of the polaron in a magnetic field has shown that (1) the experimental cyclotron-resonance energy, hvQ^9which is a linear function of magnetic field at low fields, should saturate at the value of the LO phonon energy, fioo 0 , as the magnetic field increases; (2) a second resonance, whose energy, ^l , rvi2 9 * s never less than fiu) 0 , should appear at higher magnetic fields; (3) /^CR2 snou ld lie above the position expected from the extrapolation of h VQUI from the low-field region.Experimentally, we have examined the variation with magnetic field of the cyclotron-resonance energy of electrons in InSb for photon 6 The general features of this band structure are taken from the work of M. L. Cohen and T. K. Bergstresser, Phys. Rev. 141, 789 (1966). 7 M. L. Cohen and J. C. Phillips, Phys. Rev. 139, A912 (1965). energies above and below Hoo 0 . The separation of ^CR2 ^r om tne extrapolation of ^CRl * s clarly demonstrated. In addition, our measurements are consistent with predictions (1) and (2) above. The observation of cyclotron resonance for photon energies arbitrarily close to H(JO 0 is hindered greatly by lattice absorption. The results of the present experiment are consistent with our previous study of the corresponding energy levels using interband magnetoabsorption, 1 ? 2 where the lattice absorption problem does not arise. The latter results show more clearly effects corresponding to (1) and (2) but contain complications due to excitons and to valence band behavior.Consider the predictions of Frohlich's polaron theory. The energy of a conduction electron in the presence of weak coupling to the longitudinal optical phonons and in the limit of small k can be calculated using simple perturbation theory and is given by 4where m e is the electron effective mass, and a is the coupling constant (defined in Ref. 4) with characterizes the strength of the inte...

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