Search

In two-dimensional electron systems confined to wide GaAs quantum wells we observe a remarkable sequence of transitions of the fractional quantum Hah states in the filling factor range 1 < v < 3 near the crossing of two N = 0 Landau levels from different subbands and with opposite spins. The transitions attest to the interplay between the spin and subband degrees of freedom and can be explained as pseudospin polarization transitions where the pseudospins are the two crossing levels. The magnetic field positions of the transitions yield a quantitative measure of the transition energies or, equivalently, the composite fermions' discrete energy level separations. Surprisingly, these energies are much larger than those reported for other systems with SU(2) symmetry; moreover, they are larger when the electron system is less spin polarized. [ABSTRACT FROM AUTHOR]

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 θ ≃ 34°, which surprisingly is independent of the well width or hole density, in a large filling factor range near ν = 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 ν = 1, consistent with a correlated Ψ111 state. [ABSTRACT FROM AUTHOR]

Magnetotransport measurements on two-dimensional electrons confined to wide GaAs quantum wells reveal a remarkable evolution of the ground state at filling factor v = 1/2 as we tilt the sample in the magnetic field. Starting with a compressible state at zero tilt angle, a strong v = 1/2 fractional quantum Hall state appears at intermediate angles. At higher angles an insulating phase surrounds this state and eventually engulfs it at the highest angles. This evolution occurs because the parallel component of the field renders the charge distribution increasingly bilayer-like. The evolution is qualitatively similar to the one seen, in the absence of parallel field, as a function of increasing the electron density in the quantum well, but there are some notable differences. [ABSTRACT FROM AUTHOR]

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 spin polarized. We find a significant decrease of a c as we increase the well width. In systems with comparable electron layer thickness, a c for fractional states near Landau level filling v = 3/2 is about twice larger than those near v = 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. [ABSTRACT FROM AUTHOR]

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≲ν≲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|) leads to a crossing of the lowest two Landau levels, and an elongated hole wave function in the direction of B|. Under these conditions, the in-plane resistance exhibits an insulating behavior, with the resistance along B| about 10 times smaller than the resistance perpendicular to B|. We interpret this anisotropic insulating phase as a two-component, striped Wigner crystal. [ABSTRACT FROM AUTHOR]

We observe geometric resonance features of composite fermions on the flanks of the even-denominator ν = 1/2 fractional quantum Hall state in high-mobility two-dimensional electron and hole systems confined to wide GaAs quantum wells and subjected to a weak, strain-induced, unidirectional periodic potential modulation. The features provide a measure of how close to ν = 1/2 the system stays singlecomponent and supports a composite fermion Fermi sea before transitioning into a ν = 1/2 fractional quantum Hall state, presumably, the two-component Ψ331 state. [ABSTRACT FROM AUTHOR]

We observe the fractional quantum Hall effect (FQHE) at the even-denominator Landau level filling factor v=1/2 in two-dimensional hole systems confined to GaAs quantum wells of width 30 to 50 nm and having bilayerlike charge distributions. The v=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 v=1/2 FQHE to be consistent with a two-component, Halperin-Laughlin (ψ331) state. [ABSTRACT FROM AUTHOR]