Evidence for Two Different Solid Phases of Two-Dimensional Electrons in High Magnetic Fields

We have observed two different rf resonances in the frequency dependent real diagonal conductivity of very high quality two-dimensional electron systems in the high magnetic field insulating phase and interpret them as coming from two different pinned electron solid phases (labeled as “A” and “B”). The “A” resonance is observable for Landau level filling ν<2/9 [reentrant around the ν=1/5 fractional quantum Hall effect (FQHE)] and then crosses over to the different “B” resonance which dominates at sufficiently low ν. Moreover, the “A” resonance is found to show dispersion with respect to the size of the transmission line, indicating that the “A” phase has a large correlation length. We suggest that quantum correlations such as those responsible for FQHE may play an important role in giving rise to such different solids.