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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Magnetization M(H,T) measurements have been performed on microporous carbon (MC) with a three-dimensional nano-array structure corresponding to that of a zeolite Y supercage. The obtained results unambiguously demonstrate the occurrence of high-temperature ferromagnetism in MC, probably originating from a topological disorder associated with curved graphene sheets. The results provide evidence that the ferromagnetic behavior of MC is governed by isolated clusters in a broad temperature range, and suggest the occurrence of percolative-type transition with the temperature lowering. A comparative analysis of the results obtained on MC and related materials is given.

We report conduction electron spin resonance measurements performed on highly oriented pyrolitic graphite samples between 10 K and 300 K using S (ν = 4 GHz), X (ν = 9.4 GHz), and Q (ν = 34.4 GHz) microwave bands for the external dc-magnetic field applied parallel (H c) and perpendicular (H ⊥ c) to the sample hexagonal c − axis. The results obtained in the H c geometry are interpreted in terms of the presence of an effective internal ferromagnetic-like field, H ef f int (T, H), that increases as the temperature decreases and the applied dc-magnetic field increases. We associate the occurrence of the H ef f int (T, H) with the field-induced metal-insulator transition in graphite and discuss its origin in the light of relevant theoretical models.