Magnetotransport measurements performed on several well-characterized highly oriented pyrolitic graphite and single crystalline Kish graphite samples reveal a reentrant metallic behavior in the basal-plane resistance at high magnetic fields, when only the lowest Landau levels are occupied. The results suggest that the quantum Hall effect and Landau-level-quantization-induced superconducting correlations are relevant to understand the metalliclike state(s) in graphite in the quantum limit.PACS numbers: 71.30.+h, 72.20.My, 74.10.+v Conduction processes in two-dimensional (2D) electron (hole) systems, in particular the apparent metal-insulator transition (MIT) which takes place either varying the carrier concentration or applying a magnetic field H, have attracted a broad research interest [1]. Recently, a similar MIT driven by a magnetic field applied perpendicular to basal planes has been reported for graphite [2,3,4,5]. The quasi-particles (QP) in graphite behave as massless Dirac fermions (DF) with a linear dispersion relation, similar to the QP near the gap nodes in high-temperature superconductors. Theoretical analysis [6,7,8] suggests that the MIT in graphite is the condensed-matter realization of the magnetic catalysis (MC) phenomenon [9] known in relativistic theories of (2 + 1)-dimensional DF. According to this theory [6,7,8], the magnetic field H opens an insulating gap in the spectrum of DF of graphene, associated with the electron-hole (e-h) pairing, below a transition temperature T ce (H) which is an increasing function of field. However, at higher fields and at temperatures T < T max (H) an insulator-metal transition (IMT) occurs [2] indicating that additional physical processes may operate approaching the field H QL that pulls carriers into the lowest Landau level. The occurrence of superconducting correlations in the quantum limit (QL) [10,11] and below the temperature T max (H) has been proposed for graphite in Ref. [2]. On the other hand, authors of Ref. [8] argued that at high enough carrier concentration, the basal-plane resistance R b (H, T ) can decrease decreasing temperature below the e-h pairing temperature, and identified T max (H) with T ce (H). Other theoretical works predict the occurrence of the field-induced Luttinger liquid [12] and the integral quantum Hall effect (IQHE) [13] in graphite. All these indicate that understanding of the magnetic-field-induced insulating and metallic states in graphite is of importance and has an interdisciplinary interest. The aim of this Letter is to provide a fresh insight on the magnetotransport properties of graphite in the QL. We show that the IMT is generic to graphite with a sample-dependent

We report magnetization measurements performed on graphite-sulfur composites which demonstrate a clear superconducting behavior below the critical temperature T(c0) = 35 K. The Meissner-Ochsenfeld effect, screening supercurrents, and magnetization hysteresis loops characteristic of type-II superconductors were measured. The results indicate that the superconductivity occurs in a small sample fraction, possibly related to the sample surface.

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