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.

High-resolution magnetoresistance data in highly oriented pyrolytic graphite thin samples manifest nonhomogenous superconductivity with critical temperature T c ϳ 25 K and higher temperature. Our claim is based mainly in the observation of anomalous hysteresis loops of resistance versus magnetic field that cannot be assigned to magnetic irreversibility but indicates the existence of Josephson-coupled superconducting grains. In addition we observe quantum resonances that can be assigned to Andreev reflections and the absence of Schubnikov de Haas oscillations. The results indicate that graphite is a system with nonpercolative superconducting domains immersed in a semiconductinglike matrix. As possible origin of the superconductivity in graphite we discuss interior-gap superconductivity when two very different type of carriers with different masses are present.

In this work we investigate the electrical transport properties and growth conditions of tungsten carbon (WC) and palladium carbon (PdC) nanostructures on Si substrates using a focused ion beam and scanning electron microscope. In situ energy dispersive x-ray (EDX) characterizations reveal that electron-beam-induced WC and PdC nanostructure depositions (EBID) show a lower metal concentration (below 3% atomic percentage) than in ion-beam-induced deposition (IBID) (above 20%). In the case of PdC the growth pattern and the Pd/C content were optimized by adjusting the deposition temperature of the precursor material. In situ measurements of the resistivity of the nanostructures as a function of thickness reveal a minimum at a thickness approximately 200 nm. The lowest resistivity obtained for the PdC and WC structures is two orders of magnitude higher than the corresponding bulk values for pure Pd and W. The EBID samples show a non-metallic behaviour due to the low metal content. The temperature and magnetic field dependence of the IBID structures reveal a behaviour similar to disordered or granular conductors. The upper critical field and critical current density of the WC structures were measured below the superconducting critical temperature of approximately 5 K.

We show that the voltage drop of specially prepared normal-superconducting-normal nanostructures show quantum Andreev oscillations as a function of magnetic field or input current. These oscillations are due to the interference of the electron wave function between the normal parts of the structure that act as reflective interfaces, i.e. our devices behave as a Fabry-Perot interferometer for conduction electrons. The observed oscillations and field periods are well explained by theory.The possibilities of exploiting quantum mechanical effects -with all the interferences and other phenomena occurring in real nano-devices -find new expectations that may lead to the fabrication of small devices with applications in new fields of technology as ballistic electronics and spintronics and flux devices combining normal and superconducting materials. Earlier work in semiconductors and STM experiments demonstrate the existence of quantum oscillations [1,2]. Recently, spin-polarized resonant tunneling in magnetic tunneling junctions showed large changes in the magnetoresistance due to the interference of the carrier wave function [3]. In this work we seek after quantum mechanical interference effects in the magnetoresistance using normal-superconducting structures without tunneling. Because one of the characteristics needed is ballistic transport our experiments have to be done at low temperatures and the systems should be designed to have Fermi wavelength of the order or larger than their relevant size.Assume a strip with a lateral structure M1/M2/M1, where M1 and M2 are two different materials with different Fermi energies E F and lengths L 1 and L 2 and where the electrical current passes through them. Because of the different E F 's between M1 and M2, the one particle potential can be described by a barrier U 2 of length L 2 in M2 that acts as a potential well where the wave function behaves coherently having multireflections, i.e. a kind of Fabry-Perot interferometer for electrons, a problem studied recently for the case of fluctuations in the magnetoresistance of graphene [4]. Inset in Fig. 1(a) shows the one-dimensional geometry of the system with the barrier U 2 (x) depending on the potential drop between the two ends of the trilayer. The transmittivity along such structure is [4,5]