Microwave conductivity resonance of two-dimensional hole system

We have measured microwave conductivity, Re(σ_xx), of a high quality two-dimensional hole system (2DHS) in the high magnetic field (B) insulating phase for frequency (f) between 0.2 and 9 GHz. For V < 0.3, Re(σ_xx) vs f shows a well-defined peak at frequency f_pk. As B is increased, the maximal Re(σ_xx) increases. The resonance Q (f_pk divided by full line width at half maximum) increases linearly with B. f_pk vs B increases, but is nearly flat, at 1.25 GHz, for B≥10T. Oscillator strength, S, measured by integrating the spectrum, is likewise flat for B≥10T. The simultaneously B-independent S and f_pk are inconsistent with the conventional (Fukuyama-Lee) interpretation of the resonance as a Wigner crystal pinning mode.