We report magneto-transport measurements for the fractional quantum Hall state at filling factor ν = 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. Remarkably, the resistance 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.

We study the critical spin-polarization energy (αC) above which fractional quantum Hall states in two-dimensional electron systems confined to symmetric GaAs quantum wells become fully spinpolarized. We find a significant decrease of αC as we increase the well-width. In systems with comparable electron layer thickness, αC for fractional states near Landau level filling ν = 3/2 is about twice larger than those near ν = 1/2, suggesting a broken particle-hole symmetry. Theoretical calculations, which incorporate Landau level mixing through an effective three-body interaction, and finite layer thickness, capture certain qualitative features of the experimental results.

In two-dimensional electron systems confined to GaAs quantum wells, as a function of either tilting the sample in magnetic field or increasing density, we observe multiple transitions of the fractional quantum Hall states (FQHSs) near filling factors ν = 3/4 and 5/4. The data reveal that these are spin-polarization transitions of interacting two-flux composite Fermions, which form their own FQHSs at these fillings. 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 FQHS strongly links the reentrant phase to a pinned ferromagnetic Wigner crystal of two-flux composite Fermions.Fractional quantum Hall states (FQHSs) are among the most fundamental hallmarks of ultra-clean interacting two-dimensional electron systems (2DESs) at a large perpendicular magnetic field (B ⊥ ) [1]. These incompressible quantum liquid phases, signaled by the vanishing of the longitudinal resistance (R xx ) and the quantization of the Hall resistance (R xy ), can be explained by mapping the interacting electrons to a system of essentially noninteracting, 2p-flux composite Fermions ( 2p CFs), each formed by attaching 2p magnetic flux quanta to an electron (p is an integer). The 2p CFs have discrete energy levels, the so-called Λ-levels, and the FQHSs of electrons seen around Landau level (LL) filling factor ν = 1/2 (1/4) would correspond to the integer quantum Hall states of 2 CFs (. In state-of-the-art, highmobility 2DESs, FQHSs also develop around ν = 3/4, and are usually understood as the particle-hole counterparts of the FQHSs near ν = 1/4 through the relation. Alternatively, these states might also be the FQHSs of interacting 2 CFs at ν CF = ν/(1 − 2ν). For example, the ν = 4/5 state is the ν CF = −4/3 FQHS of 2p CFs, and has the same origin as the unconventional FQHS seen at ν = 4/11 (ν CF = 4/3) [5,6].Another hallmark of clean 2DESs is an insulating phase that terminates the series of FQHSs at low fillings, near ν = 1/5, [7,8]. This insulating phase is generally believed to be an electron Wigner crystal, pinned by the small but ubiquitous disorder potential [9]. Recently, an insulating phase was observed near ν = 4/5 in clean 2DESs [10,11]. This phase, which is signaled by a reentrant integer quantum Hall state (RIQHS) near ν = 1, was interpreted as the particle-hole symmetric state of the Wigner crystal seen at very small ν [10,11]. In this picture, the holes, unoccupied states in the lowest LL, have filling factor ν h ∼ 1/5 (= 1 − 4/5) and form a liquid phase when the short-range interaction is strong; see the left panel of Fig. 1(a). They turn into a solid phase when the thickness of the 2DES increases and the long-range interaction dominates (right panel of Fig. 1(a)). This interpretation is plausible, since the RIQHS only appears when the well-width (W ) is more than five times larger than the magnetic length. However, it does not predict or allow for any transitions of the ν = 4/5 FQHS, which is always seen just before the RIQHS de...

We observe the fractional quantum Hall effect (FQHE) at the even-denominator Landau level filling factor ν=1/2 in two-dimensional hole systems confined to GaAs quantum wells of width 30 to 50 nm and having bilayerlike charge distributions. The ν=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 ν=1/2 FQHE to be consistent with a two-component, Halperin-Laughlin (Ψ331) state.

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 (ν) 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 ν = 1/2. arXiv:1401.7742v1 [cond-mat.mes-hall]