Microwave spectroscopic observation of multiple phase transitions in the bilayer electron solid in wide quantum wells

The termination of the low Landau filling factor ($\nu$) fractional quantum Hall series for a single layer two dimensional system results in the formation of a pinned Wigner solid for $\nu<1/5$ [1]. In a wide quantum well the system can support a bilayer state in which interlayer and intralayer interactions become comparable, which is measured in traditional transport as an insulating state for $\nu<1/2$ [2]. We perform microwave spectroscopic studies of this bilayer state and observe that this insulator exhibits a resonance, a signature of a solid phase. Additionally, we find that as we increase the density of the well at fixed $\nu$ this bilayer solid exhibits multiple sharp reductions in the resonance amplitude vs $\nu$. This behavior is characteristic of multiple phase transitions, which remain hidden from dc transport measurements. \\[4pt] [1] Lozovik and Yudson, JETP Lett. 22, 11 (1975).\\[0pt] [2] Manoharan et al., Phys. Rev. Lett. 77, 1813 1996).