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We report a new correlated phase of two-dimensional charged carriers in high magnetic fields, manifested by an anisotropic insulating behavior at low temperatures. It appears in a large range of low Landau level fillings $1/3\ensuremath{\lesssim}\ensuremath{\nu}\ensuremath{\lesssim}2/3$ in hole systems confined to wide GaAs quantum wells when the sample is tilted in magnetic field to an intermediate angle. The parallel field component (${B}_{\ensuremath{\parallel}}$) leads to a crossing of the lowest two Landau levels, and an elongated hole wave function in the direction of ${B}_{\ensuremath{\parallel}}$. Under these conditions, the in-plane resistance exhibits an insulating behavior, with the resistance along ${B}_{\ensuremath{\parallel}}$ about 10 times smaller than the resistance perpendicular to ${B}_{\ensuremath{\parallel}}$. We interpret this anisotropic insulating phase as a two-component, striped Wigner crystal.

We study the role of anisotropy on the transport properties of composite fermions near Landau level filling factor $\ensuremath{\nu}=1/2$ in two-dimensional holes confined to a GaAs quantum well. By applying a parallel magnetic field, we tune the composite fermion Fermi sea anisotropy and monitor the relative change of the transport scattering time at $\ensuremath{\nu}=1/2$ along the principal directions. Interpreted in a simple Drude model, our results suggest that the scattering time is longer along the longitudinal direction of the composite fermion Fermi sea. Furthermore, the measured energy gap for the fractional quantum Hall state at $\ensuremath{\nu}=2/3$ decreases when anisotropy becomes significant. The decrease, however, might partly stem from the charge distribution becoming bilayerlike at very large parallel magnetic fields.

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.

The fractional quantum Hall effect (FQHE), observed in two-dimensional (2D) charged particles at high magnetic fields, is one of the most fascinating, macroscopic manifestations of a many-body state stabilized by the strong Coulomb interaction. It occurs when the filling factor ($\ensuremath{\nu}$) of the quantized Landau levels (LLs) is a fraction, which, with very few exceptions, has an odd denominator. In 2D systems with additional degrees of freedom it is possible to cause a crossing of the LLs at the Fermi level. At and near these crossings, the FQHE states are often weakened or destroyed. Here we report the observation of an unusual crossing of the two lowest-energy LLs in high-mobility GaAs 2D hole systems, which brings to life a new even-denominator FQHE at $\ensuremath{\nu}=1/2$.

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.