Anomalous cyclotron-resonance line splitting of two-dimensional holes in (311)A ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As/GaAs heterojunctions

We have studied the far-infrared cyclotron resonance (CR) of high-mobility two-dimensional holes in ${\mathrm{Al}}_{\mathit{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As/GaAs single interface heterojunctions grown on (311)A GaAs substrates. An anomalous anticrossing behavior was observed in the CR spectra for Landau-level filling factor ν<1. By comparing the CR spectra with the intersubband transition spectra measured at zero magnetic field, we have successfully identified the anomalous CR line splitting as a crossing of the Landau levels associated with the heavy-hole and light-hole subbands.