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Register a new userOrigin of the hysteresis in bilayer 2D systems in the quantum Hall regime
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The hysteresis observed in the magnetoresistance of bilayer 2D systems in the quantum Hall regime is generally attributed to the long time constant for charge transfer between the 2D systems due to the very low conductivity of the quantum Hall bulk states. We report electrometry measurements of a bilayer 2D system that demonstrate that the hysteresis is instead due to non-equilibrium induced current. This finding is consistent with magnetometry and electrometry measurements of single 2D systems, and has important ramifications for understanding hysteresis in bilayer 2D systems.
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We report simultaneous quasi-dc magnetotransport and high frequency surface acoustic wave measurements on bilayer two-dimensional electron systems in GaAs. Near strong integer quantized Hall states a strong magnetic field sweep hysteresis in the velocity of the acoustic waves is observed at low temperatures. This hysteresis indicates the presence of a metastable state with anomalously high conductivity in the interior of the sample. This non-equilibrium state is not revealed by conventional low frequency transport measurements which are dominated by dissipationless transport at the edge of the 2D system. We find that a field-cooling technique allows the equilibrium charge configuration within the interior of the sample to be established. A simple model for this behavior is discussed.
Measurements in GaAs hole bilayers with unequal layer densities reveal a pronounced magneto-resistance hysteresis at the magnetic field positions where either the majority or minority layer is at Landau level filling factor one. At a fixed field in the hysteretic regions, the resistance exhibits an unusual time dependence, consisting of random, bidirectional jumps followed by slow relaxations. These anomalies are apparently caused by instabilities in the charge distribution of the two layers.
We study the low energy edge states of bilayer graphene in a strong perpendicular magnetic field. Several possible simple boundaries geometries related to zigzag edges are considered. Tight-binding calculations reveal three types of edge state behaviors: weakly, strongly, and non-dispersive edge states. These three behaviors may all be understood within a continuum model, and related by non-linear transformations to the spectra of quantum Hall edge--states in a conventional two-dimensional electron system. In all cases, the edge states closest to zero energy include a hole-like edge state of one valley and a particle-like state of the other on the same edge, which may or may not cross depending on the boundary condition. Edge states with the same spin generically have anticrossings that complicate the spectra, but which may be understood within degenerate perturbation theory. The results demonstrate that the number of edge states crossing the Fermi level in clean, undoped bilayer graphene depends BOTH on boundary conditions and the energies of the bulk states.
We report magnetotransport measurements on bilayer GaAs hole systems with unequal hole concentrations in the two layers. At magnetic fields where one layer is in the integer quantum Hall state and the other has bulk extended states at the Fermi energy, the longitudinal and Hall resistances of the latter are hysteretic, in agreement with previous measurements. For a fixed magnetic field inside this region and at low temperatures ($Tle$ 350 mK), the time evolutions of the longitudinal and Hall resistances show pronounced jumps followed by slow relaxations, with no end to the sequence of jumps. Our measurements demonstrate that the jumps occur simultaneously in pairs of contacts 170 $mu$m apart, and appear to involve changes in the charge configuration of the bilayer. In addition, the jumps can occur with either random or regular periods, excluding thermal fluctuations as a possible origin for the jumps. Finally, while remaining at a fixed field, we warm the sample to above 350 mK, where the jumps disappear. Upon recooling the sample below this temperature, the jumps reappear, indicating that the jumps do not result from nearly dissipationless eddy currents either.
The condensation of excitons, bound electron-hole pairs in a solid, into a coherent collective electronic state was predicted over 50 years ago. Perhaps surprisingly, the phenomenon was first observed in a system consisting of two closely-spaced parallel two-dimensional electron gases in a semiconductor double quantum well. At an appropriate high magnetic field and low temperature, the bilayer electron system condenses into a state resembling a superconductor, only with the Cooper pairs replaced by excitons comprised of electrons in one layer bound to holes in the other. In spite of being charge neutral, the transport of excitons within the condensate gives rise to several spectacular electrical effects. This article describes these phenomena and examines how they inform our understanding of this unique phase of quantum electronic matter.
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