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Register a new userObservation of chiral photocurrent transport in the quantum Hall regime in graphene
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Optical excitation provides a powerful tool to investigate non-equilibrium physics in quantum Hall systems. Moreover, the length scale associated with photo-excited charge carries lies between that of local probes and global transport measurements. Here, we investigate non-equilibrium physics of optically-excited charge carriers in graphene through photocurrent measurements in the integer quantum Hall regime. We observe that the photocurrent oscillates as a function of Fermi level, revealing the Landau-level quantization, and that the photocurrent oscillations are different for Fermi levels near and distant from the Dirac point. Our observation qualitatively agrees with a model that assumes the photocurrent is dominated by chiral edge transport of non-equilibrium carriers. Our experimental results are consistent with electron and hole chiralities being the same when the Fermi level is distant from the Dirac point, and opposite when near the Dirac point.
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We have studied the breakdown of the integer quantum Hall (QH) effect with fully broken symmetry, in an ultra-high mobility graphene device sandwiched between two single crystal hexagonal boron nitride substrates. The evolution and stabilities of the QH states are studied quantitatively through the nonlinear transport with dc Hall voltage bias. The mechanism of the QH breakdown in graphene and the movement of the Fermi energy with the electrical Hall field are discussed. This is the first study in which the stabilities of fully symmetry broken QH states are probed all together. Our results raise the possibility that the v=6 states might be a better target for the quantum resistance standard.
When electrons are confined in two-dimensional (2D) materials, quantum mechanically enhanced transport phenomena, as exemplified by the quantum Hall effects (QHE), can be observed. Graphene, an isolated single atomic layer of graphite, is an ideal realization of such a 2D system. Here, we report an experimental investigation of magneto transport in a high mobility single layer of graphene. Adjusting the chemical potential using the electric field effect, we observe an unusual half integer QHE for both electron and hole carriers in graphene. Vanishing effective carrier masses is observed at Dirac point in the temperature dependent Shubnikov de Haas oscillations, which probe the relativistic Dirac particle-like dispersion. The relevance of Berrys phase to these experiments is confirmed by the phase shift of magneto-oscillations, related to the exceptional topology of the graphene band structure.
Using high quality graphene pnp junctions, we observe prominent conductance fluctuations on transitions between quantum Hall (QH) plateaus as the top gate voltage Vtg is varied. In the Vtg-B plane, the fluctuations form crisscrossing lines that are parallel to those of the adjacent plateaus, with different temperature dependences for the conductance peaks and valleys. These fluctuations arise from Coulomb-induced charging of electron- or hole-doped localized states when the device bulk is delocalized, underscoring the importance of electronic interactions in graphene in the QH regime.
Owing to a linear and gapless band structure and a tunability of the charge carrier type, graphene offers a unique system to investigate transport of Dirac Fermions at p-n junctions (PNJs). In a magnetic field, combination of quantum Hall physics and the characteristic transport across PNJs leads to a fractionally quantized conductance associated with the mixing of electron-like and hole-like modes and their subsequent partitioning. The mixing and partitioning suggest that a PNJ could be used as an electronic beam-splitter. Here we report the shot noise study of the mode mixing process and demonstrate the crucial role of the PNJ length. For short PNJs, the amplitude of the noise is consistent with an electronic beam-splitter behavior, whereas, for longer PNJs, it is reduced by the energy relaxation. Remarkably, the relaxation length is much larger than typical size of mesoscopic devices, encouraging using graphene for electron quantum optics and quantum information processing.
Using a periodic train of Lorentzian voltage pulses, which generates soliton-like electronic excitations called Levitons, we investigate the charge density backscattered off a quantum point contact in the fractional quantum Hall regime. We find a regular pattern of peaks and valleys, reminiscent of analogous self-organization recently observed for optical solitons in non-linear environments. This crystallization phenomenon is confirmed by additional side dips in the Hong-Ou-Mandel noise, a feature that can be observed in nowadays electron quantum optics experiments.
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