Anomalous temperature dependence of the cyclotron mass in n-type CdS at ultra-high magnetic fields

We have investigated the temperature dependence in detail of the cyclotron resonance in n-type CdS at ultra-high magnetic fields where the cyclotron resonance energy becomes larger than the LO phonon energy. Anomalous temperature dependence of the effective mass was observed near the resonant polaron region at wavelengths of 16.9, 23.1 and 28.0 μm. At low temperatures, we observed many distinct absorption peaks in the low-field side of the cyclotron resonance, which is ascribed to phonon-assisted harmonic transitions between the donor states. These phenomena are reminiscent of the phase transition of the polaron ground state energy, which was predicted by Peeters and Devreese for polar materials at high magnetic fields.