144 research outputs found

    Experimental Study of the Intrinsic and Extrinsic Transport Properties of Graphite and Multigraphene Samples

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    This work deals with the intrinsic and extrinsic properties of the graphene layers inside the graphite structure, in particular the influence of defects and interfaces. We discuss the evidence for ballistic transport found in mesoscopic graphite samples and the possibility to obtain the intrinsic carrier density of graphite, without the need of free parameters or arbitrary assumptions. The influence of internal interfaces on the transport properties of bulk graphite is described in detail. We show that in specially prepared multigraphene samples the transport properties show clear signs for the existence of granular superconductivity within the graphite interfaces. We argue that the superconducting-insulator or metal-insulator transition (MIT) reported in the literature for bulk graphite is not intrinsic of the graphite structure but it is due to the influence of these interfaces. Current-Voltage characteristics curves reveal Josephson-like behavior at the interfaces with superconducting critical temperatures above 150K.Comment: 26 pages, 15 figures. To be published in "Graphene, Book 2" by Intech, Open Access Publisher 2011, ISBN: 979-953-307-180-

    Ballistic transport at room temperature in micrometer size multigraphene

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    The intrinsic values of the carriers mobility and density of the graphene layers inside graphite, the well known structure built on these layers in the Bernal stacking configuration, are not well known mainly because most of the research was done in rather bulk samples where lattice defects hide their intrinsic values. By measuring the electrical resistance through microfabricated constrictions in micrometer small graphite flakes of a few tens of nanometers thickness we studied the ballistic behavior of the carriers. We found that the carriers' mean free path is micrometer large with a mobility μ≃6×106\mu \simeq 6 \times 10^6 cm2^2/Vs and a carrier density n≃7×108n \simeq 7 \times 10^8 cm−2^{-2} per graphene layer at room temperature. These distinctive transport and ballistic properties have important implications for understanding the values obtained in single graphene and in graphite as well as for implementing this last in nanoelectronic devices.Comment: 6 pages, 6 figure

    Uncompensated magnetization and exchange-bias field in La0.7_{0.7}Sr0.3_{0.3}MnO3_3/YMnO3_3 bilayers: The influence of the ferromagnetic layer

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    We studied the magnetic behavior of bilayers of multiferroic and nominally antiferromagnetic o-YMnO3_3 (375~nm thick) and ferromagnetic La0.7_{0.7}Sr0.3_{0.3}MnO3_3 and La0.67_{0.67}Ca0.33_{0.33}MnO3_3 (8…225 8 \ldots 225~nm), in particular the vertical magnetization shift MEM_E and exchange bias field HEH_E for different thickness and magnetic dilution of the ferromagnetic layer at different temperatures and cooling fields. We have found very large MEM_E shifts equivalent to up to 100\% of the saturation value of the o-YMO layer alone. The overall behavior indicates that the properties of the ferromagnetic layer contribute substantially to the MEM_E shift and that this does not correlate straightforwardly with the measured exchange bias field HEH_E.Comment: 10 figures, 8 page
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