  Cyclotron-resonance (CR) measurements on two-dimensional (2D) electrons in
AlGaN/GaN heterojunctions reveal large splittings (up to 2 meV) of the CR line
for all investigated densities, $n_{2D}$, from 1 to $4\times 10^{12}cm^{-2}$
over wide ranges of magnetic field. The features resemble a level anti-crossing
and imply a strong interaction with an unknown excitation of the solid. The
critical energy of the splitting varies from 5 to 12 meV and as
$\sqrt{n_{2D}}$. The phenomenon resembles data from AlGaAs/GaAs whose origin
remains unresolved. It highlights a lack of basic understanding of a very
elementary resonance in solids.
