Electron-electron and electron-phonon interactions in Si/SiGe and CdTe/CdMgTe quantum wells probed by high field cyclotron resonance

We have observed unusual field-dependent cyclotron masses in a Si/SiGe quantum well and a CdTe/CdMgTe quantum well in high magnetic fields up to 33 T. In a high mobility Si/SiGe quantum well sample (well-thickness = 10 nm, n = 2.0 x 10^11 cm^-2, μ = 5.0 x 10^4 cm^2/Vs at 20 K), a sharp cyclotron resonance peak was observed, corresponding to an effective mass of 0.191 - 0.194 m_0, depending on the magnetic field. The effective mass shows a maximum at a filling factor ν = 1 (8.3 T) followed by an almost linear increase at higher fields above 15 T. The mass increase between 15 T and 30 T reached 1.6 %, which is much larger than what is expected from band non-parabolicity, suggesting the importance of the electron-electron interaction. In a CdTe/CdMgTe quantum well (well-thickness ∼10 nm, n = 6.8 x 10^11cm^-2), a large polaron pinning effect was observed near the LO phonon energy. In the gap between the two modes due to the anti-crossing, two unidentified new absorption lines were observed.