Superconducting density of states at the border of an amorphous thin film grown by focused-ion-beam

We present very low temperature Scanning Tunneling Microscopy and Spectroscopy (STM/S) measurements of a W based amorphous thin film grown with focused-ion-beam. In particular, we address the superconducting properties close to the border, where the thickness of the superconducting film decreases, and the Au substrate emerges. When approaching the Au substrate, the superconducting tunneling conductance strongly increases around the Fermi level, and the quasiparticle peaks do not significantly change its position. Under magnetic fields, the vortex lattice is observed, with vortices positioned very close to the Au substrate.Comment: To appear in Journal of Physics: Conference serie

Pinning and switching of magnetic moments in bilayer graphene

We examine the magnetic properties of the localized states induced by lattice vacancies in bilayer graphene with an unrestricted Hartree-Fock calculation. We show that with realistic values of the parameters and for experimentally accessible gate voltages we can have a magnetic switching between an unpolarized and a fully polarized system.Comment: 9 pages, 4 figure

Evidence of Josephson-coupled superconducting regions at the interfaces of Highly Oriented Pyrolytic Graphite

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, reaching zero resistance at low enough temperatures. The overall behavior indicates the existence of superconducting regions with critical temperatures above 100 K at the internal interfaces of oriented pyrolytic graphite.Comment: 6 Figures, 5 page

Nanoscale superconducting properties of amorphous W-based deposits grown with focused-ion-beam

We present very low temperature Scanning Tunneling Microscopy and Spectroscopy (STM/S) measurements in W-based amorphous superconducting nanodeposits grown using a metal-organic precursor and focused-ion-beam. The superconducting gap closely follows s-wave BCS theory, and STS images under magnetic fields show a hexagonal vortex lattice whose orientation is related to features observed in the topography through STM. Our results demonstrate that the superconducting properties at the surface of these deposits are very homogeneous, down to atomic scale. This, combined with the huge nanofabrication possibilities of the focused-ion-beam technique, paves the way to use focused-ion-beam to make superconducting circuitry of many different geometries

Quantum Andreev Oscillations in normal-superconducting-normal nanostructures

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.Comment: 5 pages and 4 figure

Untersuchung mittels Ferromagnetischer Resonanz an Kobalt-, Nickel- und Eisen-Filmen aus elektrochemischer und molekularepitaktischer Abscheidung

Kobalt- und Nickel-Schichten aus elektrochemischer Abscheidung wurden mittels frequenz- und winkelabhaengiger Ferromagnetischer Resonanz (FMR) untersucht. Durch die Bestimmung der magnetischen Anisotropiekonstanten konnte Aussagen ueber ihre magnetische Struktur gewonnen und diese, durch den Vergleich mit strukturellen Untersuchungen, auf die Wachstumsbedingungen zurueckgefuehrt werden. Hierzu wurden FMR-Messungen waehrend des elektrochemischen Wachstums und, mit Hilfe eines Rastertunnelmikroskop, mit hoher lateraler Aufloesung durchgefuehrt. Durch frequenzabhaengige FMR-Messungen konnten die magnetische Daempfung und der g-Faktor von Molekularstrahl epitaktisch gewachsenen Eisen Schichten auf Halbleitersubstraten bestimmt werden. Dabei konnten Vorzugsrichtungen innerhalb der Probenebene erkannt werden, in denen die Daempfung erniedrigt bzw. der g-Faktor erhoeht ist. Diese Abweichung zum erwarteten richtungsunabhaengigen Verhalten kann durch in situ Wachstumsuntersuchungen erlaeutert werden.Cobalt and nickel layers from elektrochemical deposition were studied by means of frequency and angle-dependent ferromagnetic resonance (FMR). By the determination of the magnetic anisotropy constant statements on their magnetic structure could be obtained and these by the comparison with structural studies be related to the growth conditions. For this FMR measurements were performed during the elctrochemical growth and by means of a scanning tunnel microsope with high lateral resolution. By frequency-dependent FMR-measurement the magnetic damping and the g-faktor of molecular-beam epitactically grown iron layers on semiconductor substrates could be determined. Thereby preferential directions within the sample plane could be detected, in which the damping is lowered respectively the g-factor increased. This deviation from the expected direction-independent behaviour can be explained by in situ growth studies.SIGLEAvailable from: http://deposit.ddb.de/cgi-bin/dokserv?idn=972652663 / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

From Ohmic to Ballistic Transport in Oriented Graphite

28 pages, 8 figures. To be published in Phys. Rev. B (2008).-- Submitted on 14 Mar 2008 (v1), last revised 28 May 2008 (this version, v2).-- PACS Nr.: 72.15.Lh, 72.80.Cw, 73.23.Ad, 81.05.UwIn this work we show that for a quasi-2D system of size $\Omega$ and thickness $t$ the resistance goes as $(2\rho/\pi t)\ln(\Omega/W)$, 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.This work was done with the support of a HBFG-grant no. 036-371, the Spanish CACyT and Ministerio de Educación y Ciencia, the Deutsche Forschungsgesellschaft under DFG ES 86/16-1, the European Union project "Ferrocarbon" and the DAAD under D/07/13369 “Acciones Integradas Hispano-Alemanas”.Peer reviewe