The carrier density in tens of nanometers thick graphite samples (multi-layer-graphene, MLG) has been modified by applying a gate voltage ($V_g$) perpendicular to the graphene planes. Surface potential microscopy shows inhomogeneities in the carrier
density ($n$) in the sample near surface region and under different values of $V_g$ at room temperature. Transport measurements on different MLG samples reveal that under a large enough applied electric field these regions undergo a superconducting-like transition at $T lesssim 17$ K. A magnetic field applied parallel or normal to the graphene layers suppresses the transition without changing appreciably the transition temperature.
We report temperature dependent Andreev reflection measurements of Co/ Y$_{1}$Ba$_{2}$Cu$_{3}$O$_{7-delta}$ (YBCO) heterostructure samples with junction areas of 1 $mu$m diameter. Modelling of the 5-70 K conductivity data according to a modified Blon
der-Tinkham-Klapwijk theory yields a spin polarization in Co film amounting to 34% which is almost constant up to 70 K. The YBCO films have been grown by pulsed laser deposition on sapphire substrates. The Co films are deposited by thermal evaporation on YBCO. The film is characterized by powder X-ray diffraction measurements which shows YBCO is grown in (001) direction.The critical current density, 5 x 10$^{6}$ A/cm$^{2}$, in YBCO remains nearly constant after deposition of Co at zero field and 77 K.
We discuss recently obtained data using different experimental methods including magnetoresistance measurements that indicate the existence of metal-free high-temperature magnetic order in graphite. Intrinsic as well as extrinsic difficulties to trig
ger magnetic order by irradiation of graphite are discussed in view of recently published theoretical work.
In this work we show that for a quasi-2D system of size $Omega$ and thickness $t$ the resistance goes as $(2rho/pi t)ln(Omega/W)$, diverging logarithmically with the size. Measurements in highly oriented pyrolytic graphite (HOPG) as well as numerical
simulations confirm this relation. Furthermore, we present an experimental method that allows us to obtain the carriers mean free path $l(T)$, the Fermi wavelength $lambda(T)$ and the mobility $mu(T)$ 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 $0.2 mu$m $lesssim l lesssim 10 mu$m, $0.1 mu$m $lesssim lambda lesssim 2 mu$m and a mobility $5 times 10^4$ cm$^2$/Vs $ lesssim mu lesssim 4 times 10^7$ cm$^2$/Vs when the temperature decreases from 270K to 3K. 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 the mobility graphene samples of micrometer size can have.
High resolution magnetoresistance data in highly oriented pyrolytic graphite thin samples manifest non-homogenous superconductivity with critical temperature $T_c sim 25 $K. These data exhibit: i) hysteretic loops of resistance versus magnetic field
similar to Josephson-coupled grains, ii) quantum Andreevs resonances and iii) absence of the Schubnikov-de Haas oscillations. The results indicate that graphite is a system with non-percolative superconducting domains immersed in a semiconducting-like matrix. As possible origin of the superconductivity in graphite we discuss interior-gap superconductivity when two very different electronic masses are present.
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 temperature
s $ T_c gtrsim 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.
