Large splitting of the cyclotron-resonance line inAlxGa1−xN/GaNheterostructures

Cyclotron-resonance (CR) measurements on two-dimensional (2D) electrons in ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}/\mathrm{G}\mathrm{a}\mathrm{N}$ heterojunctions reveal large splittings (up to 2 meV) of the CR line for all investigated densities, $n_{2D},$ from 1 to $4\times {10}^{12}{\mathrm{cm}}^{-2}$ over wide ranges of magnetic field. The features resemble a level anticrossing 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.