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Experimental Study of the Intrinsic and Exxpetriminenstaicl StTudryaonf tshepInotrintsiPc arnodpExetrintsieic sTraonsfpoGrt rParopehrtietseofaGnradphiMte aundltMigurltiagrpaphenne SeamSpalesmples 13273 conducting behaviors in bulk graphite surfaces. From this newly acquired knowledge the observed behaviors appear compatible with the existence of non-percolative partially highly conducting, partially superconducting regions coupled by Josephson-coupling through graphene planes. The interfaces observed by TEM may have enough carrier density to trigger quasi-two dimensional superconductivity. Theoretical work that deals with superconductivity in graphite as well as in graphene has been published in recent years. For example, p-type superconductivity has been predicted to occur in inhomogeneous regions of the graphite structure (González et al., 2001) or d−wave high-Tc superconductivity based on resonance valence bonds (Black-Schaffer & Doniach, 2007). Following a BCS approach in two dimensions critical temperatures Tc ∼ 60 K have been obtained if the density of conduction electrons per graphene plane increases to n ∼ 1014 cm−2, a density that might be induced by defects and/or hydrogen ad-atoms (García & Esquinazi, 2009). Further predictions for superconductivity in graphene support the premise that n > 1013 cm−2 in order to reach Tc > 1 K (Kopnin & Sonin, 2008; Uchoa & Neto, 2007). The interfaces observed by TEM might then be the regions where enough carrier density exists to trigger quasi-two dimensional superconductivity. In contrast to the basically 3D superconductivity in intercalated graphitic compounds (Csányi et al., 2005) we expect that superconductivity at quasi-2D graphite interfaces as well as at doped surfaces (Han et al., 2010) may exist at much higher temperatures, partially because of the role of the high-energy phonons in the 2D graphite structure itself (García & Esquinazi, 2009). Room temperature superconductivity with a d + id pairing symmetry has been predicted to occur in doped graphene with a carrier concentration n 1014 cm−2 (Pathak et al., 2010). We note that if we take into account the density of interfaces then a measured carrier density in bulk graphite samples of 5 × 1012 cm−2 would mean an effective density ∼ 2 × 1014 cm−2 at the interfaces. And last but not least we refer to a recent theoretical work from Kopnin et al. (2011) where the authors emphasize that a topological protected flat band in semimetals may promote superconductivity at very high temperatures. 6. References Ambegaokar, V. & Halperin, B. I. (1969). Voltage due to thermal noise in the DC Josephson effect, Phys. Rev. Lett. 22: 1364–1366. Arndt, A., Spoddig, D., Esquinazi, P., Barzola-Quiquia, J., Dusari, S. & Butz, T. (2009). Electric carrier concentration in graphite: Dependence of electrical resistivity and magnetoresistance on defect concentration, Phys. Rev. B 80: 195402. Barzola-Quiquia, J., Dusari, S., Bridoux, G., Bern, F., Molle, A. & Esquinazi, P. (2010). The influence of Ga+ irradiation on the transport properties of mesoscopic conducting thin films, Nanotechnology 21: 145306. Barzola-Quiquia, J. & Esquinazi, P. (2010). Ferromagnetic- and superconducting-like behavior of the electrical resistance of an inhomogeneous graphite flake, J Supercond Nov Magn 23: 451–455. Barzola-Quiquia, J., Esquinazi, P., Rothermel, M., Spemann, D., Butz, T. & García, N. (2007). Experimental evidence for two-dimensional magnetic order in proton bombarded graphite, Phys. Rev. B 76: 161403(R). Barzola-Quiquia, J., Yao, J.-L., Rödiger, P., Schindler, K. & Esquinazi, P. (2008). Sample size effects on the transport properties of mesoscopic graphite samples, phys. stat. sol. (a) 205: 2924–2933.PDF Image | GRAPHENE SYNTHESIS CHARACTERIZATION PROPERTIES
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