rf spectroscopy of Wigner solids in ultralow-disorder GaAs hosted two-dimensional hole systems in high magnetic fields

We study the high-magnetic-field Wigner solid (WS) phase in a new generation [1,2] of 2D hole systems (2DHS) in GaAs. The WS are invariably pinned by disorder in the semiconductor host and exhibit rf pinning modes, whose frequency measures the strength of disorder. In one sample, a 320MHz resonance is observed, significantly lower than in older-generation 2DHS with nearly identical quantum well width and density. Previous DC transport studies [3] demonstrated that 2DHS of similar density show a WS phase reentrant in the narrow region, 1/3 < ν < 2/5. We observe a pinning mode in this reentrant range, a feature not seen in the older, more disordered samples. Effects of higher-order fractional quantum Hall effect (FQHE) states on the WS are striking, with the pinning mode strongly depressed in frequency and amplitude at ν = 2/7. Partial suppression of the pinning mode due to a FQHE state is seen also for ν = 1/5, though only at elevated temperature.