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In two-dimensional hole systems confined to wide GaAs quantum wells, where the heavy- and light-hole states are close in energy, we observe a very unusual crossing of the lowest two Landau levels as the sample is tilted in a magnetic field. At a magic tilt angle $\ensuremath{\theta}\ensuremath{\simeq}{34}^{\ensuremath{\circ}}$, which surprisingly is independent of the well width or hole density, in a large filling factor range near $\ensuremath{\nu}=1$, the lowest two levels are nearly degenerate as evidenced by the presence of two-component quantum Hall states. Remarkably, a quantum Hall state is seen at $\ensuremath{\nu}=1$, consistent with a correlated ${\mathrm{\ensuremath{\Psi}}}_{111}$ state.

In wide GaAs quantum wells where two electric subbands are occupied we apply a parallel magnetic field or increase the electron density to cause a crossing of the two N = 0 Landau levels of these subbands and with opposite spins. Near the crossing, the fractional quantum Hall states in the filling factor range 1 < ν < 3 exhibit a remarkable sequence of pseudospin polarization transitions resulting from the interplay between the spin and subband degrees of freedom. The field positions of the transitions yield a new and quantitative measure of the composite Fermions’ discrete energy level separations. Surprisingly, the separations are smaller when the electrons have higher spin-polarization. An interacting two-dimensional electron system (2DES) exhibits numerous fractional quantum Hall states (FQHSs) when it is subjected to a large perpendicular magnetic field (B⊥) and electrons occupy the lowest (N = 0) Landau levels (LLs) [1, 2]. These incompressible many-body states can be explained by composite Fermions (CFs), quasiparticles formed by attaching an even number (2p) of quantized flux quanta to each electron [2–4]. When B⊥ deviates from half-integer fillings, the CFs form their own discrete energy levels, the so

In two-dimensional electron systems confined to GaAs quantum wells, as a function of either tilting the sample in a magnetic field or increasing density, we observe multiple spin-polarization transitions of the fractional quantum Hall states at filling factors ν=4/5 and 5/7. The number of observed transitions provides evidence that these are fractional quantum Hall states of interacting two-flux composite fermions. Moreover, the fact that the reentrant integer quantum Hall effect near ν=4/5 always develops following the transition to full spin polarization of the ν=4/5 fractional quantum Hall state links the reentrant phase to a pinned ferromagnetic Wigner crystal of composite fermions.

We observe the fractional quantum Hall effect (FQHE) at the even-denominator Landau level filling factor $\ensuremath{\nu}=1/2$ in two-dimensional hole systems confined to GaAs quantum wells of width 30 to 50 nm and having bilayerlike charge distributions. The $\ensuremath{\nu}=1/2$ FQHE is stable when the charge distribution is symmetric and only in a range of intermediate densities, qualitatively similar to what is seen in two-dimensional electron systems confined to approximately twice wider GaAs quantum wells. Despite the complexity of the hole Landau level structure, originating from the coexistence and mixing of the heavy- and light-hole states, we find the hole $\ensuremath{\nu}=1/2$ FQHE to be consistent with a two-component, Halperin-Laughlin (${\mathrm{\ensuremath{\Psi}}}_{331}$) state.

We report magnetotransport measurements for the fractional quantum Hall state at filling factor $\ensuremath{\nu}=$ 5/2 as a function of applied parallel magnetic field (${B}_{||}$). As ${B}_{||}$ is increased, the 5/2 state becomes increasingly anisotropic, with the in-plane resistance along the direction of ${B}_{||}$ becoming more than 30 times larger than in the perpendicular direction. Despite the very large resistance anisotropy, the anisotropy ratio remains constant over a relatively large temperature range, yielding an energy gap which is the same for both directions. Our data are qualitatively consistent with a fractional quantum Hall nematic phase.