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1630 Graphene – Synthesis, Characterization, Properties anWdill-bAe-psept-lbiyc-IaN-tTiEoCnHs In addition, it must be stressed that when the field is parallel to the graphene planes no effect is observed, suggesting that the superconducting regions or patches are rather parallel to the graphene layers. Quantum resonances in the MR were also observed, see Fig. 9(b), and ascribed to Andreev’s reflections between localized superconducting patches connected by semiconducting regions in which Cooper pairs may flow relatively large distances (Esquinazi et al., 2008). From the amplitude and period of the oscillations in field, the distance between superconducting granular domains was estimated to be 1 μm (Esquinazi et al., 2008). (a) Low field hysteresis (b) Quantum oscillations Fig. 9. (a) Weak hysteresis in the resistance of a thin graphite sample vs. magnetic field, similar to that observed in superconductors with Josephson-coupled grains near zero field, measured in a multigraphene sample (after Esquinazi et al. (2008)). (b) Oscillations in the resistance of a multigraphene sample as function of the magnetic field at 2 K after subtraction of a linear field background. The continuous line corresponds to a Fourier fit. Adapted from Esquinazi et al. (2008). 4.4 Behavior of the magnetoresistance of multigraphene samples with micro-constrictions The novel method of using thin mesoscopic samples with micro-constrictions not only allows us to obtain basic parameters like the mean free path and carrier density, see Section 3, but provides also the possibility to increase the sensitivity of the voltage measurement to the superconducting regions that may be at the constriction region. We mean that the expected superconducting patches inside the constriction area might be better "detected" by usual voltage measurement at two contacts on either side of the constriction. Dusari, Barzola-Quiquia & Esquinazi (2011) verified this effect and found an increase in the range of temperature where the previously reported (Esquinazi et al., 2008) anomalous hysteresis loops can be observed, see Fig. 10. All the experimental results indicate that, in case that the superconducting regions exist, they should be localized mainly at the interfaces between the crystalline regions of slightly different orientation observed by TEM, see Fig. 2. At these interfaces the density of carriers should be high enough to achieve high critical temperatures, provided the quasi-two dimensionality remains (García & Esquinazi, 2009). The sample can be modeled as the sum of superconducting and normal in series and in parallel circuit paths. A coherent superconducting state at low enough temperatures within the superconducting patches isPDF Image | GRAPHENE SYNTHESIS CHARACTERIZATION PROPERTIES
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