  We report on the far-infrared magnetospectroscopy of HgTe quantum wells with
inverted band ordering at different electron concentrations. We particularly
focus on optical transitions from zero-mode Landau levels, which split from the
edges of electron-like and hole-like bands. We observe a pronounced dependence
of the transition energies on the electron concentration varied by persistent
photoconductivity effect. This is striking evidence that in addition to the
already well-documented crystalline and interface asymmetries,
electron-electron interactions also have a significant impact on the usual
behavior of the optical transitions from zero mode Landau levels.
