Superconducting and normal state properties of Niobium nanofilms have been systematically investigated as a function of film thickness, on different substrates. The width of the superconducting-to-normal transition for all films is remarkably narrow, confirming their high quality. The superconducting critical current density exhibits a pronounced maximum for thickness around 25 nm, marking the 3D-to-2D crossover. The magnetic penetration depth shows a sizeable enhancement for the thinnest films. Additional amplification effects of the superconducting properties have been obtained with sapphire substrates or squeezing the lateral size of the nanofilms. For thickness close to 20 nm we measured a doubled perpendicular critical magnetic field compared to its large thickness value, indicating shortening of the correlation length and the formation of small Cooper pairs. Our data analysis indicates an exciting interplay between quantum-size and proximity effects together with strong-coupling effects and the importance of disorder in the thinnest films, placing these nanofilms close to the BCS-BEC crossover regime.

Covalent bond-forming reactions can be used to tailor the properties of graphene, aiming at electronic band structure engineering and surface functionalization. We present a novel and easy method for the production of chemically modified monolayer graphene based on the electrochemical intercalation of graphite, that could be used for adding various functional groups to the graphene lattice. Oxy-fluorinated graphene layers have been produced and fully characterized in terms of their chemical composition and functionalization. Moreover, Raman spectroscopy allows ready discrimination between monolayers and few-layers, and field-effect devices have been fabricated in order to study the transport properties of monolayer graphene oxyfluoride. Interesting conduction mechanisms such as two dimensional Mott variable range hopping and colossal negative magneto-resistance are observed, making this novel material suitable for both fundamental research and graphene-based applications

We report low-temperature measurements of current-voltage characteristics for highly conductive Nb/Al-AlO x -Nb junctions with thicknesses of the Al interlayer ranging from 40 to 150 nm and ultrathin barriers formed by diffusive oxidation of the Al surface. In a superconducting state these devices have revealed a strong subgap current leakage. Analyzing Cooper-pair and quasiparticle currents across the devices, we conclude that the strong suppression of the subgap resistance compared with conventional tunnel junctions is not related to technologically derived pinholes in the barrier but rather has more fundamental grounds. We argue that it originates from a universal bimodal distribution of transparencies across the aluminum oxide barrier proposed earlier by Schep and Bauer (1997 Phys. Rev. Lett. 78 3015). We suggest a simple physical explanation of its source in the nanometer-thick oxide films relating it to strong local barrier-height fluctuations in the nearest to conducting electrode layers of the insulator which are generated by oxygen vacancies in thin aluminum oxide tunnel barriers formed by thermal oxidation.

No abstract

No abstract