Microwave resonances in low-filling insulating phases of two-dimensional electron and hole systems

The conductivity of the high magnetic field insulating phases of two-dimensional electron and hole systems (2DES and 2DHS) of sufficiently low disorder exhibits a resonance at microwave frequency. We present and compare results on this resonance in a moderate disorder 2DES and in a high quality 2DHS. The 2DES exhibits the resonance even though the high- insulator is not reentrant around the fractional quantum Hall effect. The resonance in the 2DHS is much sharper than that in the 2DES, and appears to be inconsistent with the standard model of a two-dimensional harmonic oscillator in a magnetic field.