Observation of Reentrant Phases Induced by Short-Range Disorder in the Lowest Landau Level ofAlxGa1−xAs/Al0.32Ga0.68As

Abstract

We report the observation of a reentrant quantum Hall effect in the lowest Landau level between filling factors of 2/3 and 3/5 in a Al{x}Ga{1-x}As/Al{0.32}Ga{0.68}As heterostructure sample with x=0.85%. A reentrant insulating phase is also observed between filling factors of 2/5 and 1/3, demonstrating particle-hole symmetry between these phases. A sample with x=0.21% shows much weaker reentrant features, indicating that increased short-range scattering, due to the Al alloy in the conduction channel, aids in the formation of these phases.

ν=5/2Fractional Quantum Hall State in the Presence of Alloy Disorder

“…[42]. However, in these samples [2,42,43] the ν ¼ 5=2 FQHS has not been observed. In contrast, our samples have several essential features which are optimized for a strong ν ¼ 5=2 FQHS even in the presence of the disorder.…”

Pinning modes of high-magnetic-field Wigner solids with controlled alloy disorder

“…The characteristics of these samples have been well established in earlier work on these wafers [16][17][18][19][20] . In particular, study 16 of the mobility vs x has shown that for x ≤ 0.85% the Al is randomly distributed in the channel, and has allowed modeling the potential around each Al atom as a spherical square well with known radius and potential-depth parameters.…”

Microwave pinning modes near Landau fillingν=1in two-dimensional electron systems with alloy disorder

“…We find resonances of the strongly overdamped type in both our samples at the fillings where the RIQHE was located in ref. 32. Possibly because of sample processing, variation within the wafer, or cooldown procedure, we do not find a 2/3 FQHE suppressing the resonance completely in the present x = 0.85% sample.…”

“…An explanation 46 of the extended ν range is that in the competition between solid and FQHE liquids, the solid can be more stable in larger disorder due its pinning energy, which the solid gains as carrier positions adjust to the disorder potential. This is also the explanation for the transition to insulator at the low ν termination of the FQHE series occurring around ν = 1/3 in Al alloy-disordered samples 32,37 (as opposed to near 1/5 in ultralow disorder samples 8 ). The resonance also sets in as ν goes below 1/3 in low-density samples 47 or in which the pinning energy is presumably more important relative to the carrier interaction.…”

“…Samples from the same wafers were used in earlier work in dc transport [32][33][34][35][36] . Characterization 33 of the samples at One sample has x = 0.33%, density n ≈ 2.1 × 10 11 cm −2 and as-cooled mobility µ about 2.3 × 10 6 cm 2 /V-s; the other sample has x = 0.85%, n ≈ 2.5 × 10 11 cm −2 and µ ≈ 1.4 × 10 6 cm 2 /V-s. A previous paper 37 showed that samples from this series exhibit pinning mode resonances at the low ν termination of the FQHE series, which for these disordered samples occurred for ν below that of the 1/3 FQHE.…”