Microwave Conductivity Resonance of Two-Dimensional Hole System

We have measured microwave conductivity, Re(σₓₓ), 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 ν<0.3, Re(σₓₓ) vs f shows a well-defined peak at frequency fₚₖ. As B is increased, the maximal Re(σₓₓ) increases. The resonance Q ( fₚₖ divided by full line width at half maximum) increases linearly with B. fₚₖ 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 fₚₖ are inconsistent with the conventional (Fukuyama-Lee) interpretation of the resonance as a Wigner crystal pinning mode.