Melting Temperatures of 2D Electron Solids in the Lowest Landau Level from Microwave Spectroscopy

We studied the temperature($T$) dependence of the microwave conductivity spectra of two dimensional electron systems in the high magnetic field ($B$) insulating phase (HBIP) for Landau filling factor $\nu$$<\sim$1/5. Such an insulating phase, believed to be a pinned electron solid, supports a characteristic pinning resonance in the conductivity spectrum. Two samples were studied. Sample 1 is a heterojunction with density $n$$\sim$7$\times$10$^{10} $cm$^ {-2}$ and mobility $\mu$$\sim$5$\times$10$^{6}$cm$^2$/Vs and has a single resonance in the HBIP. Sample 2 is a 65nm-wide QW with $n$$\sim$6$\times$10 $^{10}$cm$^{-2}$ and $\mu$$\sim$10$\times$10$^{6}$cm$^2$/V and was recently found to have two resonances in the HBIP, interpreted as corresponding to two different solid phases, with one crossing over to the other as $\nu$ is reduced [1]. We studied the higher-$T$ behavior of the resonances at many different combinations of $n$ (through backgating) and $B$, and measured the characteristic temperatures $T_c$ at which the resonances disappear. We found$T_c$ is a non-increasing function of $\nu$ for either sample, although the function differs significantly for both samples. We interpret $T_c$ as the melting temperature of the electron solid(s) to a quantum liquid, for which $\nu$ captures the importance of inter-electron quantum correlation. [1] Y.P. Chen et al., Phys.Rev.Lett. 93, 206805 (2004)