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Magneto-transport measurements in a clean two-dimensional electron system confined to a wide GaAs quantum well reveal that, when the electrons occupy two electric subbands, the sequences of fractional quantum Hall states observed at high fillings (ν > 2) are distinctly different from those of a single-subband system. Notably, when the Fermi energy lies in the ground state Landau level of either of the subbands, no quantum Hall states are seen at the even-denominator ν = 5/2 and 7/2 fillings; instead the observed states are at ν = (i + p/(2p ± 1)) where i = 2, 3, and p = 1, 2, 3, and include several new states at ν = 13/5, 17/5, 18/5, and 25/7. PACS numbers:The ground states of low-disorder two-dimensional electron systems (2DESs) at high Landau level (LL) fillings (ν > 2) have been enigmatic. Early experiments provided evidence for a unique fractional quantum Hall state (FQHS) at the even-denominator filling ν = 5/2 [1]. More recent measurements on the highest quality 2DESs have revealed a plethora of additional ground states including insulating and density-modulated phases [2][3][4][5][6][7][8]. But absent are clear sequences of odd-denominator FQHSs at ν = i + p/(2p ± 1) (where p = 1, 2, 3, ...) that are typically observed at lower fillings (i.e., when i = 0 or 1) [9]. It is believed that, in the higher LLs, the larger extent of the electron wavefunction (in the 2D plane), combined with the presence of extra nodes, leads to a modification of the (exchange-correlation) interaction effects and stabilizes the non-FQHSs at the expense of FQHSs.Meanwhile, the origin and the stability of the FQHSs at high fillings, especially those at ν = 5/2 and 12/5, have become the focus of renewed interest since these states might obey non-Abelian statistics and be useful for topological quantum computing [10]. In particular, it has been proposed that the ν =5/2 FQHS should be particularly stable in a "thick" 2DES confined to a relatively wide quantum well (QW) [11]. In a realistic, experimentally achievable system, of course, the electrons in a wide QW typically occupy two (or more) electric subbands [12]. Here we report measurements in such a system. Figure 1 highlights our main observations. In contrast to data taken in a narrow (30 nm) GaAs QW where only one electric subband is occupied ( Fig. 1(a)), data for the wider (56 nm) well (Figs. 1(b,c)) [13] do not exhibit even-denominator states at ν = 5/2 and 7/2. Instead, we observe FQHS sequences at ν = 2 + p/(2p ± 1) and 3+p/(2p±1), reminiscent of the usual composite Fermion (CF) sequences observed at lower ν around 1/2 and 3/2 (i.e., at ν = 0 + p/(2p ± 1) and ν = 1 + p/(2p ± 1)) [9]. The FQHSs we observe include states at ν = 7/3, 8/3, 12/5, 13/5, 10/3, 11/3, 17/5, 18/5, and 25/7, some of which have not been previously seen [6].Our samples were grown by molecular beam epitaxy and consist of GaAs QWs bounded on each side by undoped Al 0.24 Ga 0.76 As spacer layers and Si δ-doped layers. We studied several samples with well widths (w) ranging from 30 to 80 nm. Here we focus on...

At the low Landau filling factor termination of the fractional quantum Hall effect series, two-dimensional electron systems exhibit an insulating phase that is understood as a form of pinned Wigner solid. Here we use microwave spectroscopy to probe the transition to the insulator for a wide quantum well sample that can support single-layer or bilayer states depending on its overall carrier density. We find that the insulator exhibits a resonance which is characteristic of a bilayer solid. The resonance also reveals a pair of transitions within the solid, which are not accessible to dc transport measurements. As density is biased deeper into the bilayer solid regime, the resonance grows in specific intensity, and the transitions within the insulator disappear. These behaviours are suggestive of a picture of the insulating phase as an emulsion of liquid and solid components.

An interacting bilayer electron system provides an extended platform to study electron-electron interaction beyond single layers. We report here experiments demonstrating that the layer densities of an asymmetric bilayer electron system oscillate as a function of perpendicular magnetic field that quantizes the energy levels. At intermediate fields, this interlayer charge transfer can be well explained by the alignment of the Landau levels in the two layers. At the highest fields where both layers reach the extreme quantum limit, however, there is an anomalous, enhanced charge transfer to the majority layer. Surprisingly, when the minority layer becomes extremely dilute, this charge transfer slows down as the electrons in the minority layer condense into a Wigner crystal. Furthermore, by examining the quantum capacitance of the dilute layer at high fields, the screening induced by the composite fermions in an adjacent layer is unveiled. The results highlight the influence of strong interaction in interlayer charge transfer in the regime of very high fields and low Landau level filling factors.