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Register a new userPuddle-induced resistance oscillations in the breakdown of the graphene quantum Hall effect
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We report on the stability of the quantum Hall plateau in wide Hall bars made from a chemically gated graphene film grown on SiC. The $ u=2$ quantized plateau appears from fields $B simeq 5$ T and persists up to $B simeq 80$ T. At high current density, in the breakdown regime, the longitudinal resistance oscillates with a $1/B$ periodicity and an anomalous phase, which we relate to the presence of additional electron reservoirs. The high field experimental data suggest that these reservoirs induce a continuous increase of the carrier density up to the highest available magnetic field, thus enlarging the quantum plateaus. These in-plane inhomogeneities, in the form of high carrier density graphene pockets, modulate the quantum Hall effect breakdown and decrease the breakdown current.
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The quantum anomalous Hall effect (QAHE) realizes dissipationless longitudinal resistivity and quantized Hall resistance without the need of an external magnetic field. However, when reducing the device dimensions or increasing the current density, an abrupt breakdown of the dissipationless state occurs with a relatively small critical current, limiting the applications of the QAHE. We investigate the mechanism of this breakdown by studying multi-terminal devices and identified that the electric field created between opposing chiral edge states lies at the origin. We propose that electric-field-driven percolation of two-dimensional charge puddles in the gapped surface states of compensated topological-insulator films is the most likely cause of the breakdown.
Puzzling results obtained from torque magnetometry in the quantum Hall effect (QHE) regime are presented, and a theory is proposed for their explanation. Magnetic moment saturation, which is usually attributed to the QHE breakdown, is shown to be related to the charge redistribution across the sample.
We report on Hall field-induced resistance oscillations (HIRO) in a 60 nm-wide GaAs/AlGaAs quantum well with an emph{in situ} grown back gate, which allows tuning the carrier density $n$. At low $n$, when all electrons are confined to the lowest subband (SB1), the HIRO frequency, proportional to the product of the cyclotron diameter and the Hall field, scales with $n^{-1/2}$, as expected. Remarkably, population of the second subband (SB2) significantly enhances HIRO, while their frequency now scales as $n^{-1}$. We demonstrate that in this two-subband regime HIRO still originate solely from backscattering of SB1 electrons. The unusual density dependence occurs because the population of SB2 steadily increases, while that of SB1 remains essentially unchanged. The enhancement of HIRO manifests an unexpected, step-like increase of the quantum lifetime of SB1 electrons, which reaches a record value of 52 ps in the two-subband regime.
We study the effect of a uniform pseudomagnetic field, induced by a strain in a monolayer and double layer of gapped graphene, acting on excitons. For our analysis it is crucial that the pseudomagnetic field acts on the charges of the constituent particles of the excitons, i.e., the electrons and holes, the same way in contrast to a magnetic field. Moreover, using a circularly polarized laser field, the electrons and the holes can be excited only in one valley of the honeycomb lattice of gapped graphene. This breaks the time-reversal symmetry and provides the possibility to observe the various Quantum Hall phenomena in this pseudomagnetoexciton system. Our study poses a fundamental problem of the quantum Hall effect for composite particles and paves the way for quantum Hall physics of pseudomagnetoexcitons.
We investigate the transport properties of high-quality single-layer graphene, epitaxially grown on a 6H-SiC(0001) substrate. We have measured transport properties, in particular charge carrier density, mobility, conductivity and magnetoconductance of large samples as well as submicrometer-sized Hall bars which are entirely lying on atomically flat substrate terraces. The results display high mobilities, independent of sample size and a Shubnikov-de Haas effect with a Landau level spectrum of single-layer graphene. When gated close to the Dirac point, the mobility increases substantially, and the graphene-like quantum Hall effect occurs. This proves that epitaxial graphene is ruled by the same pseudo-relativistic physics observed previously in exfoliated graphene.
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