In this work we investigated correlations between the internal microstructure and sample size (lateral as well as thickness) of mesoscopic, tens of nanometer thick graphite (multigraphene) samples and the temperature (T ) and field (B) dependence of their electrical resistivity ρ(T, B). Low energy transmission electron microscopy reveals that the original highly oriented pyrolytic graphite material -from which the multigraphene samples were obtained by exfoliation -is composed of a stack of ∼ 50 nm thick and micrometer long crystalline regions separated by interfaces running parallel to the graphene planes. We found a qualitative and quantitative change in the behavior of ρ(T, B) upon thickness of the multigraphene samples, indicating that their internal microstructure is important.The overall results indicate that the metallic-like behavior of ρ(T ) at zero field measured for bulk graphite samples is not intrinsic of ideal graphite. The results suggest that the interfaces between crystalline regions may be responsible for the superconducting-like properties observed in graphite. Our transport measurements also show that reducing the sample lateral size as well as the length between voltage electrodes decreases the magnetoresistance, in agreement with recently published results. The magnetoresistance of the multigraphene samples shows a scaling of the form ((R(B) − R(0))/R(0))/T α = f (B/T ) with a sample dependent exponent α ∼ 1, which applies in the whole temperature 2 K ≤ T ≤ 270 K and magnetic field range B ≤ 8 T.

We have studied the resistance of a large number of highly oriented graphite samples with areas ranging from several mm 2 to a few µm 2 and thickness from ∼10 nm to several tens of micrometers. The measured resistance can be explained by the parallel contribution of semiconducting graphene layers with low carrier density < 10 9 cm −2 and the one from metallic-like internal interfaces. The results indicate that ideal graphite with Bernal stacking structure is a semiconductor with a narrow band gap E g ∼ 40 meV. Contents

Transport properties of a few hundreds of nanometers thick (in the graphene plane direction) lamellae of highly oriented pyrolytic graphite (HOPG) have been investigated. Current-voltage characteristics as well as the temperature dependence of the voltage at different fixed input currents provide evidence for Josephson-coupled superconducting regions embedded in the internal two-dimensional interfaces of HOPG, reaching zero resistance at low enough temperatures. 13 References 13

We have prepared magnetic graphite samples bombarded by protons at low temperatures and low fluences to attenuate the large thermal annealing produced during irradiation. An overall optimization of sample handling allowed us to find Curie temperatures Tc 350 K at the used fluences. The magnetization versus temperature shows unequivocally a linear dependence, which can be interpreted as due to excitations of spin waves in a two dimensional Heisenberg model with a weak uniaxial anisotropy. PACS numbers: 75.50.Pp,75.30.Ds Recent advances to develop nanographitic systems have led to a renewed interest on their electrical properties worldwide [1]. A single layer of graphite, the twodimensional (2D) graphene, appears to have quantum properties at room temperature[2] as well as rectifying electronic properties [3,4]. On the other hand, some of those properties were already observed in highly oriented pyrolytic graphite (HOPG) of low mosaicity, as the quantum Hall effect[5] and de Haas -van Halphen quantum oscillations even at room temperature [6]. The twodimensional properties of the graphene planes in graphite open up the possibility of using nanometer to micron size regions of graphite in new integrated devices with spintronic properties either through the use of ferromagnetic electrodes, e.g. spin-valves, and/or making graphite itself magnetic. In fact this has been a topic of study in the last years and reports exist showing magnetic hysteresis in blank graphite [7] but especially in proton bombarded graphite [8]. Severe limitations in the sensitivity and reproducibility of standard magnetometers added to annealing effects during bombardment, hindered the identification of a critical temperature T c as well as the characteristics and dimensionality of the ferromagnetic signals. The aim of this work is to show that specially prepared highly oriented pyrolytic graphite (HOPG) samples show ferromagnetic order with T c 350 K and the magnetization temperature dependence is in good agreement with a 2D anisotropic Heisenberg model (2DHM) and the presence of spin waves excitations [9,10,11].For the experiments we used four pieces of a HOPG sample grade ZYA, samples 1 to 4 (mass: 12.8, 12.5, 10.1, and 6 mg respectively) irradiated by a 2.25 MeV proton micro-beam (sample 4: 2.0 MeV, 0.8 mm broad beam) perpendicular to the graphite planes. With the micro-beam we produced several thousands of spots of ∼ 2 µm diameter each and separated by 5 µm (sample 1) or 10 µm (samples 2 and 3) distance, similarly to the procedure used in Ref. 12. Samples 1 and 2 were irradiated at 110 K whereas samples 3 and 4 at room temperature. Further irradiation parameters for sam-ple 1 (2,3,4) were: 51375 (25600,25600,6) spots, fluence: 0.124 (0.08,0.13,0.3) nC/µm 2 , total irradiated charge 46.9 (44.8,37.4,900) µC, and 1 nA proton current (100 nA for sample 4). The pieces we have irradiated showed an iron concentration (the only detected magnetic impurity) within the first 35 µm of ∼ (0.4±0.04) µg/g (< 0.1 ppm).Previous experiments [8] showed ferr...

Granular superconductivity in powders of small graphite grains (several tens of micrometers) is demonstrated after treatment with pure water. The temperature, magnetic field and time dependence of the magnetic moment of the treated graphite powder provides evidence for the existence of superconducting vortices with some similarities to high-temperature granular superconducting oxides but even at temperatures above 300 K. Room temperature superconductivity in doped graphite or at its interfaces appears to be possible.

Measuring with high precision the electrical resistance of highly ordered natural graphite samples from a Brazil mine, we have identified a transition at ∼350K with ∼40K transition width. The steplike change in temperature of the resistance, its magnetic irreversibility and time dependence after a field change, consistent with trapped flux and flux creep, and the partial magnetic flux expulsion obtained by magnetization measurements, suggest the existence of granular superconductivity below 350K. The zero-field virgin state can only be reached again after zero field cooling the sample from above the transition. Paradoxically, the extraordinarily high transition temperature we found for this and several other graphite samples is the reason why this transition remained undetected so far. The existence of well ordered rhombohedral graphite phase in all measured samples has been proved by x-rays diffraction measurements, suggesting its interfaces with the Bernal phase as a possible origin for the high-temperature superconductivity, as theoretical studies predicted. The localization of the granular superconductivity at these two dimensional interfaces prevents the observation of a zero resistance state or of a full Meissner state.

The electrical, in-plane resistance as a function of temperature R(T ) of bulk and mesoscopic thin graphite flakes obtained from the same batch was investigated. Samples thicker than ∼ 30 nm show metalliclike contribution in a temperature range that increases with the sample thickness, whereas a semiconductinglike behavior was observed for thinner samples. The temperature dependence of the in-plane resistance of all measured samples and several others from literature can be very well explained between 2 K and 1100 K assuming three contributions in parallel: a metalliclike conducting path at the interfaces between crystalline regions, composed of two semiconducting phases, i.e. Bernal and rhombohedral stacking. From the fits of R(T ) we obtain a semiconducting energy gap of 110 ± 20 meV for the rhombohedral and 38 ± 8 meV for the Bernal phase. The presence of these crystalline phases was confirmed by x-ray diffraction measurements. We review similar experimental data from literature of the last 33 years and two more theoretical models used to fit R(T ).

In this work we show that the spreading Ohmic resistance of a quasi-two-dimensional system of size ⍀, thickness t, and with a constriction of size W connecting two half-parts of resistivity goes as ͑2 / t͒ln͑⍀ / W͒, diverging logarithmically with the size. Measurements in highly oriented pyrolytic graphite ͑HOPG͒ as well as numerical simulations confirm this relation. Furthermore, we present an experimental method that allows us to obtain the carriers' mean-free path ᐉ͑T͒, the Fermi wavelength ͑T͒, and the mobility ͑T͒ directly from experiments without adjustable parameters. Measuring the electrical resistance through microfabricated constrictions in HOPG and observing the transition from Ohmic to ballistic regime, we obtain that 0.2 m Շ ᐉ Շ 10 m, 0.1 m ՇՇ2 m, and a mobility 5 ϫ 10 4 cm 2 / V sՇ Շ 4 ϫ 10 7 cm 2 / V s when the temperature T decreases from 270 to 3 K. A comparison of these results with those from literature indicates that conventional, multiband Boltzmann-Drude approaches are inadequate for oriented graphite. The upper value obtained for the mobility is much larger than that for the mobility in graphene samples of micrometer size can have.