Many electron, spin-orbit and resonant polaron effects in the THz cyclotron resonance of modulation-doped CdMnTe/CdMgTe Quantum Wells.

We present low temperature THz cyclotron resonance studies of modulation-doped CdMnTe/CdMgTe quantum wells with Mn compositions x = 1-3.6% in magnetic fields up to 17T. Electron densities n_s are (1 - 4) x 10¹¹ cm^-2. We focus on B-regions where the giant Zeeman splitting of Landau levels (LLs) leads to the 1↓ level crossing 0↑ at a field B_c. LLs are resonantly coupled and split by the spin-orbit interaction, and a gap due to many-electron exchange can also occur (Quantum Hall Ferromagnetism) when B_c is close to the field for filling-factor (FF) 2 at low enough temperatures. For x=2.85% B_c lies at 8.65T, and CR is below our Ge:Ga detector onset. With exchange corrected LLs B_c should occur at ~10T, but no splitting is observed. Between 9 and 15T, FF<1.67 so only spin-orbit coupling is important. Two samples do show a clear splitting of CR into two lines near 15T; strong TO phonon absorption blocks CR transmission at higher B. The origin of this splitting is not understood. For low x samples B_c lies between 3 and 4.5 T, depending upon temperature and sp-d exchange. No LL splittings are observed at 4.2 K, but at 1.5K there is a small splitting near 4.5T. A strong resonant magnetopolaron effect is also observed at high B in all samples. Effect of e-e exchange will be presented.