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Register a new userMagnetothermal Conductivity of Highly Oriented Pyrolytic Graphite in the Quantum Limit
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We report on the magnetic field (0T$ le B le 9$T) dependence of the longitudinal thermal conductivity $kappa(T,B)$ of highly oriented pyrolytic graphite in the temperature range 5 K $le Tle$ 20 K for fields parallel to the $c-$axis. We show that $kappa(T,B)$ shows large oscillations in the high-field region (B > 2 T) where clear signs of the Quantum-Hall effect are observed in the Hall resistance. With the measured longitudinal electrical resistivity we show that the Wiedemann-Franz law is violated in the high-field regime.
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We present a systematic investigation of the magnetoreflectance of highly oriented pyrolytic graphite in magnetic field B up to 18 T . From these measurements, we report the determination of lifetimes tau associated with the lowest Landau levels in the quantum limit. We find a linear field dependence for inverse lifetime 1/tau(B) of the lowest Landau levels, which is consistent with the hypothesis of a three-dimensional (3D) to 1D crossover in an anisotropic 3D metal in the quantum limit. This enigmatic result uncovers the origin of the anomalous linear in-plane magnetoresistance observed both in bulk graphite and recently in mesoscopic graphite samples.
High resolution magnetoresistance data in highly oriented pyrolytic graphite thin samples manifest non-homogenous superconductivity with critical temperature $T_c sim 25 $K. These data exhibit: i) hysteretic loops of resistance versus magnetic field similar to Josephson-coupled grains, ii) quantum Andreevs resonances and iii) absence of the Schubnikov-de Haas oscillations. The results indicate that graphite is a system with non-percolative superconducting domains immersed in a semiconducting-like matrix. As possible origin of the superconductivity in graphite we discuss interior-gap superconductivity when two very different electronic masses are present.
A simple and effective stepwise-method has been developed to remove defects from the top graphene layers of highly orientated pyrolytic graphite. Using a combination of ozone exposure and moderately high temperature we have shown that a defect-rich graphite surface can be modified to generate a graphene-like surface containing a negligible amount of oxygen, hydrogen and sp3 carbon. We report definitive x-ray photoelectron and x-ray absorption spectroscopy analysis after each stage of the process, suggest a mechanism by which the modification occurs and propose it as a route towards the preparation or manipulation of pristine graphene samples.
Graphite surfaces interact weakly with molecules compared to other conducting surfaces bringing the molecule-molecule interaction to the foreground. C$_{60}$ on highly oriented pyrolytic graphite is a model system for studying the molecular self-assembly on surfaces. Our scanning tunneling microscopy measurements at liquid nitrogen temperatures confirm the previously observed island growth mode. Our results indicate that there is an epitaxial relationship of the molecular islands and the substrate with three possible orientations of the islands. For one of these orientations, we determine this epitaxial relationship by analyzing in detail an image taken across a C$_{60}$ island step edge. In this image we have obtained high resolution on both the molecular island and the substrate. The result of this analysis is confirmed by two-dimensional Fourier analysis.
Measurements of the basal-plane resistivity rho_a(T,H) performed on highly oriented pyrolitic graphite, with magnetic field H parallel to the c-axis in the temperature interval 2 - 300 K and fields up to 8 T, provide evidence for the occurrence of both field - induced and zero-field superconducting instabilities. Additionally, magnetization M(T,H) measurements suggest the occurrence of Fermi surface instabilities which compete with the superconducting correlations.
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