Magneto-optical evidence of many-body effects in a spin-polarized two-dimensional electron gas

Modulation-doped CdₓMn_1-xTe/CdyMg_1-yTe quantum wells are studied at 1.7 K by magnetoabsorption experiments performed near the fundamental heavy-hole--electron (HH₁-E₁) transition. In the investigated range of Mn content (x≈0.02) and electron concentration [nₑ≈(2--3)×10¹¹ cm^-2], the two-dimensional electron gas is fully spin-polarized at magnetic fields as low as $H=1.5$ T. The interband optical spectra exhibit unusual magnetic field dependence resulting from electron-hole interactions and phase-space-filling effects. An additional enhancement of the spin splitting of Landau levels found in this study is attributed to the electron-exchange interaction. A simplified theoretical model including the exchange energy and the electron-hole interaction provides a coherent quantitative description of the overall experimental features.