Indications for intrinsic superconductivity in highly oriented pyrolytic graphite

High-resolution magnetoresistance data in highly oriented pyrolytic graphite thin samples manifest nonhomogenous superconductivity with critical temperature T_c∼25 K and higher temperature. Our claim is based mainly in the observation of anomalous hysteresis loops of resistance versus magnetic field that cannot be assigned to magnetic irreversibility but indicates the existence of Josephson-coupled superconducting grains. In addition we observe quantum resonances that can be assigned to Andreev reflections and the absence of Schubnikov de Haas oscillations. The results indicate that graphite is a system with nonpercolative superconducting domains immersed in a semiconductinglike matrix. As possible origin of the superconductivity in graphite we discuss interior-gap superconductivity when two very different type of carriers with different masses are present.