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Experimental Study of the Intrinsic and Exxpetriminenstaicl StTudryaonf tshepInotrintsiPc arnodpExetrintsieic sTraonsfpoGrt rParopehrtietseofaGnradphiMte aundltMigurltiagrpaphenne SeamSpalesmples 12195 see Section 2. In this Section we are interested in particular on the influence of the internal interfaces on the transport. 4.2 Bulk graphite Starting with the bulk material, Kempa et al. (2000) studied the magnetoresistance in the in-plane direction in HOPG bulk samples where a magnetic-field-driven transition from metallic- to semiconducting-type behavior of the basal-plane resistance ρa was found. This was reproduced in later publications by other groups (Du et al., 2005; Tokumoto et al., 2004). Note, however, that depending on the thickness of the graphite samples and their quality, metallic or semiconducting behavior in graphite can be observed if no magnetic field is applied (Barzola-Quiquia et al., 2008). There is a clear suppression of the metallic-like phase (in case it appears) by a magnetic field applied perpendicular to the graphene planes (i.e. parallel to the c-axis) (Kempa et al., 2000). Later work found a field-induced metal-insulator transition (MIT) also in the c-axis resistivity of graphite ρc (Kempa et al., 2002). The metallic-like behavior of ρc has been related to the one found in the longitudinal resistivity due to a conduction-path mixing mechanism, i.e. the conduction path of the carriers along the c-axis is in part short circuited by lattice defects and/or impurities. If non-percolative superconducting grains are located at interfaces running mostly parallel to the graphene planes, the largest increase in the resistance with magnetic field applied parallel to the c-axis is expected in the temperature region where either the resistance decreases decreasing temperature or it levels off at zero field applied. This is expected since at a temperature below the maximum or leveling off one speculates that the coupling between superconducting grains starts to be observable. Kopelevich et al. (1999) showed a sensitive change in the temperature dependence of the resistivity in bulk HOPG samples under an applied magnetic field parallel to the c-axis. Additional studies done by Kempa et al. (2003) show the absence of a MIT in graphite if the magnetic field is applied parallel to the graphene planes, a fact that indicates that its origin is related to regions running parallel to the graphene layers. 4.3 Anomalous hysteresis loops and quantum resonances in the magnetoresistance In percolative homogenous superconducting samples, one can ascribe superconductivity by observing a screening of the external magnetic field (Meissner effect) below a critical field and/or by measuring the drop of resistance to practically zero below a critical temperature. Inhomogeneous granular superconducting materials (in which the superconducting phase could be found in small parts of the whole sample) require other way of testing, as resistance does not drop necessarily to zero and the Meissner effect is probably too small to be measured. Esquinazi et al. (2008) found anomalous hysteresis loops in the MR similar to those observed in granular superconductors with Josephson-coupled grains, see Fig. 9(a). Esquinazi et al. (2008) and Dusari, Barzola-Quiquia & Esquinazi (2011) show that the minima in the MR are located in the same quadrant field from which one starts the field sweeping as it was reported in conventional high Tc granular superconductors (Ji et al., 1993). This can be explained based on a two-level critical-state model where pinned fluxons exist inside the Josephson-coupled superconducting grains but also between them. These last ones are usually much less pinned and therefore can strongly influence the MR behavior, especially at low enough fields. The fact that the minimum of the MR is rather flat can be interpreted as due to non-uniform superconducting grain size.PDF Image | GRAPHENE SYNTHESIS CHARACTERIZATION PROPERTIES
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