Figure 4: Electron density dependence of the transition.
From: Microwave spectroscopy of the low-filling-factor bilayer electron solid in a wide quantum well
Plots of the microwave spectra, Re (σxx), in the (ν, f) plane at various densities, n. σmax is the largest Re (σxx) on the colour scale. Vertical lines mark ν=1/3 and the solid white area at low ν is outside the magnetic field measurement range. (a) For n=1.18, the FQHE around ν=1/3 appears as a region with a flat spectrum and vanishing Re (σxx)=0. A resonance is seen only for ν≤0.31, just below the FQHE state. At ν=0.285, the resonance peak (σpk) in Re (σxx) has a sharp minimum, interpreted as a transition within the solid. Possibly coincidentally, this ν is within error of 2/7. (b) On increasing to n=1.22, the resonance is re-entrant around the 1/3 FQHE and a minimum in σpk is located at ν=0.300. (c) For n=1.26, a resonance has replaced the FQHE at ν=1/3, while for ν=0.315 and 0.340 there are σpk minima, which are accompanied by jumps in fpk. (d,e) Data for n=1.32 and n=1.38, respectively. Each plot shows two minima in σpk with jumps in fpk. The jump in fpk is larger for the higher-ν (ν2) minimum in σpk. For ν above ν2 the resonance is broader with lower σpk. The jump in fpk for the lower-ν (ν1) transition at n=1.38 is much weaker than it is for lower n. (f) Data for n=1.52, at which both transitions have disappeared, and fpk and the resonance amplitude vary only marginally throughout the measurement range.
