For a series of samples with 2D electron systems in dilute Al_x Ga_{1-x}As, with varying x from 0 to 0.8 %, we survey the pinning mode resonances of Wigner solids at the low Landau filling termination of the fractional quantum Hall effect (FQHE) seri
es. For all x studied, the pinning modes are present with frequencies, f_pk, that are consistent with collective weak pinning. For x>=0.22% we find f_pk vs B exhibits a rapid increase that is not present for x=0. We find the observed f_pk is much smaller than values calculated with a simple Wigner solid model which neglects the effects of the disorder on the charge distribution of a carrier.
Using a 50-nm width, ultra-clean GaAs/AlGaAs quantum well, we have studied the Landau level filling factor $ u = 5/2$ fractional quantum Hall effect in a perpendicular magnetic field $B sim$ 1.7 T and determined its dependence on tilted magnetic fiel
ds. Contrary to all previous results, the 5/2 resistance minimum and the Hall plateau are found to strengthen continuously under an increasing tilt angle $0 < theta < 25^circ$ (corresponding to an in-plane magnetic field 0 $<$ $B_parallel$ $< 0.8$ T). In the same range of $theta$ the activation gaps of both the 7/3 and the 8/3 states are found to increase with tilt. The 5/2 state transforms into a compressible Fermi liquid upon tilt angle $theta > 60^circ$, and the composite fermion series [2+$p/(2ppm1)$], $p =$ 1, 2 can be identified. Based on our results, we discuss the relevance of a Skyrmion spin texture at $ u = 5/2$ associated with small Zeeman energy in wide quantum wells, as proposed by W$acute{text o}$js $et$ $al$., Phys. Rev. Lett. 104, 086801 (2010).
We investigate effects of lateral confinement on spin splitting of energy levels in 2D hole gases grown on [311] GaAs. We found that lateral confinement enhances anisotropy of spin splitting relative to the 2D gas for both confining directions. Unexp
ectedly, the effective $g$-factor does not depend on the 1D energy level number $N$ for $B|[0bar{1}1]$ while it has strong $N$-dependence for $B|[bar{2}33]$. Apart from quantitative difference in the spin splitting of energy levels for the two orthogonal confinement directions we also report qualitative differences in the appearance of spin-split plateaus, with non-quantized plateaus observed only for the confinement in $[0bar{1}1]$ direction. In our samples we can clearly associate the difference with anisotropy of spin-orbit interactions.
The zero-field temperature-dependence of the resistivity of two-dimensional holes are observed to exhibit two qualitatively different characteristics for a fixed carrier density for which only the metallic behavior of the so-called metal-insulator tr
ansition is anticipated. As $T$ is lowered from 150 mK to 0.5 mK, the sign of the derivative of the resistivity with respect to $T$ changes from being positive to negative when the temperature is lowered below $sim$30 mK and the resistivity continuously rises with cooling down to 0.5 mK, suggesting a crossover from being metal-like to insulator-like.
