110 research outputs found
Near-edge x-ray absorption fine structure investigation of graphene
We report the near-edge x-ray absorption fine structure (NEXAFS) spectrum of
a single layer of graphite (graphene) obtained by micromechanical cleavage of
Highly Ordered Pyrolytic Graphite (HOPG) on a SiO2 substrate. We utilized a
PhotoEmission Electron Microscope (PEEM) to separately study single- double-
and few-layers graphene (FLG) samples. In single-layer graphene we observe a
splitting of the pi* resonance and a clear signature of the predicted
interlayer state. The NEXAFS data illustrate the rapid evolution of the
electronic structure with the increased number of layers.Comment: 5 pages, 4 figure
Self-aligned nanoscale SQUID on a tip
A nanometer-sized superconducting quantum interference device (nanoSQUID) is
fabricated on the apex of a sharp quartz tip and integrated into a scanning
SQUID microscope. A simple self-aligned fabrication method results in
nanoSQUIDs with diameters down to 100 nm with no lithographic processing. An
aluminum nanoSQUID with an effective area of 0.034 m displays flux
sensitivity of 1.8 \mu_B/\mathrm{Hz}^{1/2}$ and high bandwidth, the SQUID on a tip is a highly
promising probe for nanoscale magnetic imaging and spectroscopy.Comment: 14 manuscript pages, 5 figure
Ballistic Josephson junctions in edge-contacted graphene
Hybrid graphene-superconductor devices have attracted much attention since
the early days of graphene research. So far, these studies have been limited to
the case of diffusive transport through graphene with poorly defined and modest
quality graphene-superconductor interfaces, usually combined with small
critical magnetic fields of the superconducting electrodes. Here we report
graphene based Josephson junctions with one-dimensional edge contacts of
Molybdenum Rhenium. The contacts exhibit a well defined, transparent interface
to the graphene, have a critical magnetic field of 8 Tesla at 4 Kelvin and the
graphene has a high quality due to its encapsulation in hexagonal boron
nitride. This allows us to study and exploit graphene Josephson junctions in a
new regime, characterized by ballistic transport. We find that the critical
current oscillates with the carrier density due to phase coherent interference
of the electrons and holes that carry the supercurrent caused by the formation
of a Fabry-P\'{e}rot cavity. Furthermore, relatively large supercurrents are
observed over unprecedented long distances of up to 1.5 m. Finally, in the
quantum Hall regime we observe broken symmetry states while the contacts remain
superconducting. These achievements open up new avenues to exploit the Dirac
nature of graphene in interaction with the superconducting state.Comment: Updated version after peer review. Includes supplementary material
and ancillary file with source code for tight binding simulation
Transverse Electronic Transport through DNA Nucleotides with Functionalized Graphene Electrodes
Graphene nanogaps and nanopores show potential for the purpose of electrical
DNA sequencing, in particular because single-base resolution appears to be
readily achievable. Here, we evaluated from first principles the advantages of
a nanogap setup with functionalized graphene edges. To this end, we employed
density functional theory and the non-equilibrium Green's function method to
investigate the transverse conductance properties of the four nucleotides
occurring in DNA when located between the opposing functionalized graphene
electrodes. In particular, we determined the electrical tunneling current
variation as a function of the applied bias and the associated differential
conductance at a voltage which appears suitable to distinguish between the four
nucleotides. Intriguingly, we observe for one of the nucleotides a negative
differential resistance effect.Comment: 19 pages, 7 figure
UNISOR on-line nuclear orientation facility (UNISOR/NOF)
The UNISOR on-line nuclear orientation facility (UNISOR/NOF) consists of a3He-4He dilution refrigerator on line to the isotope separator. Nuclei are implanted directly into a target foil which is soldered to the bottom accessed cold finger of the refrigerator. A 1.5 T superconducting magnet polarizes the ferromagnetic target foils and determines the axis of symmetry. Up to eight gamma detectors can be positioned around the refrigerator, each 9 cm from the target. A unique feature of this system is that the k=4 term in the directional distribution function can be directly and unambigously deduced so that a single solution for the mixing ratio can be found. The first on-line experiment at this facility reported here was a study of the decay of the191Hg and193Hg isotopes. © 1998 J.C. Baltzer A.G., Scientific Publishing Company
Control and Characterization of Individual Grains and Grain Boundaries in Graphene Grown by Chemical Vapor Deposition
The strong interest in graphene has motivated the scalable production of high
quality graphene and graphene devices. Since large-scale graphene films
synthesized to date are typically polycrystalline, it is important to
characterize and control grain boundaries, generally believed to degrade
graphene quality. Here we study single-crystal graphene grains synthesized by
ambient CVD on polycrystalline Cu, and show how individual boundaries between
coalescing grains affect graphene's electronic properties. The graphene grains
show no definite epitaxial relationship with the Cu substrate, and can cross Cu
grain boundaries. The edges of these grains are found to be predominantly
parallel to zigzag directions. We show that grain boundaries give a significant
Raman "D" peak, impede electrical transport, and induce prominent weak
localization indicative of intervalley scattering in graphene. Finally, we
demonstrate an approach using pre-patterned growth seeds to control graphene
nucleation, opening a route towards scalable fabrication of single-crystal
graphene devices without grain boundaries.Comment: New version with additional data. Accepted by Nature Material
Recommended from our members
UNISOR on-Line Nuclear Orientation Facility (UNISOR/NOF)
The UNISOR on-line nuclear orientation facility (UNISOR/NOF) consists of a /sup 3/He--/sup 4/He dilution refrigerator on line to the isotope separator. Nuclei are implanted directly into a target foil which is soldered to the bottom accessed cold finger of the refrigerator. A 1.5 T superconducting magnet polarizes the ferromagnetic target foils and determines the axis of symmetry. Up to eight gamma detectors can be positioned around the refrigerator, each 9 cm from the target. A unique feature of this system is that the k = 4 term in the directional distribution function can be directly and unambiguously deduced so that a single solution for the mixing ratio can be found. The first on-line experiment at this facility reported here was a study of the decay of the /sup 191/Hg and /sup 193/Hg isotopes. 5 refs., 4 figs., 1 tab
Giant Phonon-induced Conductance in Scanning Tunneling Spectroscopy of Gate-tunable Graphene
The honeycomb lattice of graphene is a unique two-dimensional (2D) system
where the quantum mechanics of electrons is equivalent to that of relativistic
Dirac fermions. Novel nanometer-scale behavior in this material, including
electronic scattering, spin-based phenomena, and collective excitations, is
predicted to be sensitive to charge carrier density. In order to probe local,
carrier-density dependent properties in graphene we have performed
atomically-resolved scanning tunneling spectroscopy measurements on
mechanically cleaved graphene flake devices equipped with tunable back-gate
electrodes. We observe an unexpected gap-like feature in the graphene tunneling
spectrum which remains pinned to the Fermi level (E_F) regardless of graphene
electron density. This gap is found to arise from a suppression of electronic
tunneling to graphene states near E_F and a simultaneous giant enhancement of
electronic tunneling at higher energies due to a phonon-mediated inelastic
channel. Phonons thus act as a "floodgate" that controls the flow of tunneling
electrons in graphene. This work reveals important new tunneling processes in
gate-tunable graphitic layers
A Tunable Phonon-Exciton Fano System in Bilayer Graphene
Interference between different possible paths lies at the heart of quantum
physics. Such interference between coupled discrete and continuum states of a
system can profoundly change its interaction with light as seen in Fano
resonance. Here we present a unique many-body Fano system composed of a
discrete phonon vibration and continuous electron-hole pair transitions in
bilayer graphene. Mediated by the electron-phonon interactions, the excited
state is described by new quanta of elementary excitations of hybrid
phonon-exciton nature. Infrared absorption of the hybrid states exhibit
characteristic Fano lineshapes with parameters renormalized by many-body
interactions. Remarkably, the Fano resonance in bilayer graphene is
continuously tunable through electrical gating. Further control of the
phonon-exciton coupling may be achieved with an optical field exploiting the
excited state infrared activity. This tunable phonon-exciton system also offers
the intriguing possibility of a 'phonon laser' with stimulated phonon
amplification generated by population inversion of band-edge electrons.Comment: 21 pages, 3 figure
- …