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Scale-invariant large nonlocality in polycrystalline graphene

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      Abstract

      The observation of large nonlocal resistances near the Dirac point in graphene has been related to a variety of intrinsic Hall effects, where the spin or valley degrees of freedom are controlled by symmetry breaking mechanisms. Engineering strong spin or valley Hall signals on scalable graphene devices could stimulate further practical developments of spin- and valleytronics. Here we report on scale-invariant nonlocal transport in large-scale chemical vapor deposition graphene under an applied external magnetic field. Contrary to previously reported Zeeman spin Hall effect, our results are explained by field-induced spin-filtered edge states whose sensitivity to grain boundaries manifests in the nonlocal resistance. This phenomenon, related to the emergence of the quantum Hall regime, persists up to the millimeter scale, showing that polycrystalline morphology can be imprinted in nonlocal transport. This suggests that topological Hall effects in large-scale graphene materials are highly sensitive to the underlying structural morphology, limiting practical realizations.

      Abstract

      Nonlocal resistances in graphene Hall bars attributed to neutral current Hall effects have been mainly measured at the microscale. Here, the authors observe consistently strong nonlocal signals in Hall bars with channel length ranging from the micrometer up to the millimeter scale, and explain them by field-induced spin-split edge states.

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      Most cited references 55

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      Experimental Observation of Quantum Hall Effect and Berry's Phase in Graphene

      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 Berry's phase to these experiments is confirmed by the phase shift of magneto-oscillations, related to the exceptional topology of the graphene band structure.
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        Electronic spin transport and spin precession in single graphene layers at room temperature

        The specific band structure of graphene, with its unique valley structure and Dirac neutrality point separating hole states from electron states has led to the observation of new electronic transport phenomena such as anomalously quantized Hall effects, absence of weak localization and the existence of a minimum conductivity. In addition to dissipative transport also supercurrent transport has already been observed. It has also been suggested that graphene might be a promising material for spintronics and related applications, such as the realization of spin qubits, due to the low intrinsic spin orbit interaction, as well as the low hyperfine interaction of the electron spins with the carbon nuclei. As a first step in the direction of graphene spintronics and spin qubits we report the observation of spin transport, as well as Larmor spin precession over micrometer long distances using single graphene layer based field effect transistors. The non-local spin valve geometry was used, employing four terminal contact geometries with ferromagnetic cobalt electrodes, which make contact to the graphene sheet through a thin oxide layer. We observe clear bipolar (changing from positive to negative sign) spin signals which reflect the magnetization direction of all 4 electrodes, indicating that spin coherence extends underneath all 4 contacts. No significant changes in the spin signals occur between 4.2K, 77K and room temperature. From Hanle type spin precession measurements we extract a spin relaxation length between 1.5 and 2 micron at room temperature, only weakly dependent on charge density, which is varied from n~0 at the Dirac neutrality point to n = 3.6 10^16/m^2. The spin polarization of the ferromagnetic contacts is calculated from the measurements to be around 10%.
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          Room-Temperature Quantum Hall Effect in Graphene

          The quantum Hall effect (QHE), one example of a quantum phenomenon that occur on a truly macroscopic scale, has been attracting intense interest since its discovery in 1980 and has helped elucidate many important aspects of quantum physics. It has also led to the establishment of a new metrological standard, the resistance quantum. Disappointingly, however, the QHE could only have been observed at liquid-helium temperatures. Here, we show that in graphene - a single atomic layer of carbon - the QHE can reliably be measured even at room temperature, which is not only surprising and inspirational but also promises QHE resistance standards becoming available to a broader community, outside a few national institutions.
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            Author and article information

            Affiliations
            [1 ]ISNI 0000 0004 1761 1166, GRID grid.424265.3, CIC nanoGUNE, ; 20018 Donostia-San Sebastian, Basque Country Spain
            [2 ]GRID grid.473715.3, Catalan Institute of Nanoscience and Nanotechnology (ICN2), , CSIC and The Barcelona Institute of Science and Technology, ; Campus UAB, 08193 Bellaterra, Catalonia Spain
            [3 ]GRID grid.7080.f, Universitat Autònoma de Barcelona, ; 08193 Bellaterra, Catalonia Spain
            [4 ]ISNI 0000 0000 9601 989X, GRID grid.425902.8, ICREA—Institució Catalana de Recerca i Estudis Avançats, ; 08010 Barcelona, Catalonia Spain
            [5 ]ISNI 0000 0004 0467 2314, GRID grid.424810.b, IKERBASQUE, , Basque Foundation for Science, ; 48013 Bilbao, Basque Country Spain
            [6 ]ISNI 0000 0004 1784 4496, GRID grid.410720.0, Present Address: Center for Quantum Nanoscience, , Institute for Basic Science (IBS), ; Seoul, 03760 Republic of Korea
            Contributors
            ORCID: http://orcid.org/0000-0002-7918-8047, l.hueso@nanogune.eu
            ORCID: http://orcid.org/0000-0003-0316-2163, f.casanova@nanogune.eu
            Journal
            Nat Commun
            Nat Commun
            Nature Communications
            Nature Publishing Group UK (London )
            2041-1723
            19 December 2017
            19 December 2017
            2017
            : 8
            5736749 2346 10.1038/s41467-017-02346-x
            © The Author(s) 2017

            Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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