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 show that the resistivity perpendicular ρc and parallel ρa to the basal planes of different graphite samples show similar magnetic-field-driven metal-insulator-transitions at a field Bc ∼ 0.1 T applied parallel to the c−axis. Our results demonstrate the universality of the recently found scaling in ρa of graphite and indicate that the metallic-like temperature dependence of ρc is directly correlated to that of ρa. The similar magnetoresistance found for both resistivities, the violation of Kohler's rule and the field-induced transition indicate that the semiclassical transport theory is inadecuate to understand the transport properties of graphite.Although a considerable amount of studies has been performed on graphite and related compounds, their transport properties are still not well understood. The scientific interest on graphite has been recently renewed by new magnetization [1] and transport [2] results on highly oriented pyrolytic graphite (HOPG). These show irreversible magnetization [1] that, upon sample, its previous thermal treatment and magnetic field orientation, resembles that of a superconducting loop even at room temperature suggesting the existence of localized superconducting domains at high temperatures in topological disordered regions [3]. This result is indirectly supported by the recently found magnetic-field-driven superconductor-insulator-type transition (SIT) in the inplane resistivity of a HOPG sample [2]. Remarkably, this field-driven transition showed a similar scaling as found for two-dimensional (2D) disordered superconductors as well as for Si MOSFETs [4].

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A magnetic-field-driven transition from metallic-to semiconducting-type behavior in the basal-plane resistance takes place in highly oriented pyrolytic graphite at a field H c ∼ 1 kOe applied along the hexagonal c-axis. The analysis of the data reveals a striking similarity between this transition and that measured in thinfilm superconductors and Si MOSFET's. However, in contrast to those materials, the transition in graphite is observable at almost two orders of magnitude higher temperatures.