Microwave spectroscopic observation of phase transition between competing solids in wide quantum wells

Within a narrow range of Landau filling ($\nu$) near $\nu=1$, a resonance in the microwave spectrum in high mobility two-dimensional electron systems is known to occur [1]. The resonance, characterized by a peak frequency ($f_{pk}$), is a signature of a pinned Wigner solid in which quasiparticles oscillate about their pinned positions. In wide quantum wells, at sufficiently large density, we observe an abrupt shift in $f_{pk}$ vs $\nu$ as $\nu$ is decreased from 1. We interpret this jump to enhanced-$f_{pk}$ vs $\nu$ as a solid-solid phase transition. dc transport measurements reveal a reentrant integer quantum Hall effect (RIQHE) [2], which we show has the same origin as the enhanced-$f_{pk}$. \\[4pt] [1] Chen et al., Phys. Rev. Lett. 91, 016801 (2003).\\[0pt] [2] Liu et al., Phys. Rev. Lett. 109, 036801 (2012).