Pinning modes of solid phases in wide quantum wells near $\nu =$ 1

Near filling factor $\nu =$ 1 the microwave spectra of sufficiently low disorder two-dimensional electron systems (2DESs) exhibit a resonance [1], understood as a Wigner solid pinning mode, in which quasiparticles or quasiholes oscillate about their pinned positions. For 2DESs in a wide quantum well of thickness 54 nm and density n$=$2.4 x 10$^{\mathrm{11}}$ cm$^{\mathrm{-2}}$, we find that the resonance frequency, f$_{\mathrm{pk}}$, is comparatively enhanced for $\nu $\textless 0.88, and interpret this as a phase transition between Wigner solids, as shown by the reentrant integer quantum Hall effect (RIQHE) recently observed in wide wells under similar conditions [2]. As n is increased by gating, the transition to enhanced f$_{\mathrm{pk}}$ moves closer to $\nu =$ 1, similar to the RIQHE in [2]. [1] Chen et al., Phys. Rev. Lett. \textbf{91},016801 (2003). [2] Liu et al., Phys. Rev. Lett. \textbf{109}, 036801 (2012).