Low-energy excitations in multiple modulation-doped CdTe/(CdMg)Te quantum wells

Low-energy excitations of a two-dimensional electron gas (2DEG) in modulation-doped multiple (ten) quantum wells (QWs) was studied using far-infrared magnetotransmission technique at liquid helium temperatures. A large distance between neighboring QWs of 54 nm excluded a direct interaction of electron wave functions confined in the wells. In four samples which differed in the spacer width and the level of doping with iodine donors, supplied with a metallic grid coupler, a uniform picture of excitations of the 2DEG was observed. These involved the cyclotron resonance (CR), its second harmonic (2CR), and magnetoplasmon modes (MPMs). MPMs with a small amplitude originated from excitations of the 2DEG in a single QW and these with a high amplitude resulted from a coherent excitation of the 2DEG in all wells. A polaron effect resulting from the interaction of the CR, 2CR, and MPMs with an optical phonon was observed and described with appropriate models. Both types of MPMs exhibited gaps in the disperion relations at the frequency close to the 2CR, leading to Bernstein modes, which was described with an appropriate (nonlocal) theoretical model.