Pinning of high-magnetic-field Wigner solids of electrons and holes in ultralow-disorder GaAs quantum wells

We study the high-magnetic-field Wigner solid (WS) phase in a new generation [1] 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 (f_pk) measures the strength of disorder. We measure a series of samples differing only in the Al content of the Al_xGa_1-xAs barriers confining the quantum well (QW). Sharp, well-defined resonances below 2 GHz are observed with a clear trend between f_pk and Al content (x). Recent studies [2] point to alloy disorder in the barriers that define the QW as the scattering mechanism responsible for pinning the WS in n-type samples; the model used to relate f_pk to the probability a carrier is in the barriers is inadequate for the p-type samples. The 2DHS have strong dependence on x and on quantum well width (d): In varying x from 0.02 to 0.32, we see f_pk increase by a factor of 18; increasing d from 20 nm to 30 nm shows a decrease in f_pk from 145 MHz to 35 MHz. Currently, it is unclear whether holes are pinned in a way fundamentally different from electrons, or if the model of ref [2] for WS pinning can be reconciled with the data on 2DHS.