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Measurements of the basal-plane resistivity ρ a (T,H) performed on highly oriented pyrolitic graphite, with magnetic field H || 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. 2 The magnetic field -temperature (H-T) phase diagram of conventional type-II superconductors is well known. In the Meissner state, the surface currents screen the applied magnetic field. Above the lower critical field H c1 (T), the field penetrates the superconductor in the form of a lattice of vortices (Abrikosov lattice). Superconductivity persists up to the upper critical field H c2 (T), described by the Abrikosov-Gor'kov theory [1,2]. On the other hand, it has been proposed [3] that the superconducting state can appear (or reappear) under application to an electron system of high enough magnetic field, such that the Landau quantization of the energy spectrum is important. In particular, when all electrons are in the lowest Landau level, the superconducting transition temperature T c (H) is expected to increase with field increasing, opposite to the T c (H) dependence in the classical low-field-limit [3]. However, superconductivity in the quantum regime has not been identified in experiments so far, remaining the subject of theoretical investigations only.In the present work, we report the results of basal-plane resistivity ρ a (T,H) measurements performed on highly oriented pyrolitic graphite (HOPG), which provide evidence for the occurrence of superconducting correlations in both quantum and classical limits. Besides, magnetization measurements M(T,H) suggest an interplay between superconducting and other Fermi surface instabilities, possibly spin-densitywave (SDW) or charge-density-wave (CDW) type.The HOPG sample was obtained from the Research Institute "GRAPHITE" (Moscow). X-ray diffraction (Θ -2Θ) measurements give the crystal lattice parameters a = 2.48 Å and c = 6.71 Å. The high degree of crystallites orientation along the hexagonal c-axis was confirmed from x-ray rocking curves (FWHM = 1.4 o ). The geometrical sample density was 2.26 ± 0.01 g/cm 3 . A cylindrical specimen with diameter of 5.2 mm, and thickness 3.14 mm, and a parallelepiped 4.9 x 4.3 x 2.5 mm 3 , both made from the 3 same piece of HOPG, were used for magnetization and transport measurements, respectively. The c-axis was along the smallest size of the sample. The studies were performed for H || c-axis. M(T,H) dc magnetization was measured in fields up to 5 T and temperatures between 2 and 300 K by means of SQUID magnetometer MPMS5 (Qunatum Design). Low-frequency (f = 1 Hz) standard four-probe resistance measurements were performed in fields up to 8 T, in the same temperature interval, with PPMS (Physical Properties Measurement System, Quantum Design).Low-temperature portions of the basal-plan...

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Superconducting characteristics such as the Meissner-Ochsenfeld state, screening supercurrents and hysteresis loops of type-II superconductors were observed from the temperature and magnetic field dependences of the magnetic moment, m(T, H), in graphite powders reacted with sulfur for temperatures below 9.0 K. The temperature dependence of the lower critical field H c1 (T) was determined and the zero-temperature penetration depth, λ(0), was estimated (λ(0) = 2270 Å). The superconductivity was observed to be highly anisotropic and to coexist with a ferromagnetic state that has a Curie temperature well above room temperature. A continuous transition from the superconducting state to the ferromagnetic state could be achieved by simply increasing the applied magnetic field.

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