1,968 research outputs found
Role of hexagonal boron nitride in protecting ferromagnetic nanostructures from oxidation
Ferromagnetic contacts are widely used to inject spin polarized currents into
non-magnetic materials such as semiconductors or 2-dimensional materials like
graphene. In these systems, oxidation of the ferromagnetic materials poses an
intrinsic limitation on device performance. Here we investigate the role of
ex-situ transferred chemical vapour deposited hexagonal boron nitride (hBN) as
an oxidation barrier for nanostructured cobalt and permalloy electrodes. The
chemical state of the ferromagnets was investigated using X-ray photoemission
electron microscopy owing to its high sensitivity and lateral resolution. We
have compared the oxide thickness formed on ferromagnetic nanostructures
covered by hBN to uncovered reference structures. Our results show that hBN
reduces the oxidation rate of ferromagnetic nanostructures suggesting that it
could be used as an ultra-thin protection layer in future spintronic devices.Comment: 7 pages, 6 figure
Resonant tunneling through a C60 molecular junction in liquid environment
We present electronic transport measurements through thiolated C
molecules in liquid environment. The molecules were placed within a
mechanically controllable break junction using a single anchoring group per
molecule. When varying the electrode separation of the C-modified
junctions, we observed a peak in the conductance traces. The shape of the
curves is strongly influenced by the environment of the junction as shown by
measurements in two distinct solvents. In the framework of a simple resonant
tunneling model, we can extract the electronic tunneling rates governing the
transport properties of the junctions.Comment: 13 pages, 4 figures. To appear in Nanotechnolog
Kondo effect and spin-active scattering in ferromagnet-superconductor junctions
We study the interplay of superconducting and ferromagnetic correlations on
charge transport in different geometries with a focus on both a quantum point
contact as well as a quantum dot in the even and the odd state with and without
spin-active scattering at the interface. In order to obtain a complete picture
of the charge transport we calculate the full counting statistics in all cases
and compare the results with experimental data. We show that spin-active
scattering is an essential ingredient in the description of quantum point
contacts. This holds also for quantum dots in an even charge state whereas it
is strongly suppressed in a typical Kondo situation. We explain this feature by
the strong asymmetry of the hybridisations with the quantum dot and show how
Kondo peak splitting in a magnetic field can be used for spin filtering. For
the quantum dot in the even state spin-active scattering allows for an
explanation of the experimentally observed mini-gap feature.Comment: 14 pages, 7 figures, accepted by PR
Competition between magnetic field dependent band structure and coherent backscattering in multiwall carbon nanotubes
Magnetotransport measurements in large diameter multiwall carbon nanotubes
(20-40 nm) demonstrate the competition of a magnetic-field dependent
bandstructure and Altshuler-Aronov-Spivak oscillations. By means of an
efficient capacitive coupling to a backgate electrode, the magnetoconductance
oscillations are explored as a function of Fermi level shift. Changing the
magnetic field orientation with respect to the tube axis and by ensemble
averaging, allows to identify the contributions of different Aharonov-Bohm
phases. The results are in qualitative agreement with numerical calculations of
the band structure and the conductance.Comment: 4 figures, 5 page
Magnetic field tuning and quantum interference in a Cooper pair splitter
Cooper pair splitting (CPS) is a process in which the electrons of naturally
occurring spin-singlet pairs in a superconductor are spatially separated using
two quantum dots. Here we investigate the evolution of the conductance
correlations in an InAs CPS device in the presence of an external magnetic
field. In our experiments the gate dependence of the signal that depends on
both quantum dots continuously evolves from a slightly asymmetric Lorentzian to
a strongly asymmetric Fano-type resonance with increasing field. These
experiments can be understood in a simple three - site model, which shows that
the nonlocal CPS leads to symmetric line shapes, while the local transport
processes can exhibit an asymmetric shape due to quantum interference. These
findings demonstrate that the electrons from a Cooper pair splitter can
propagate coherently after their emission from the superconductor and how a
magnetic field can be used to optimize the performance of a CPS device. In
addition, the model calculations suggest that the estimate of the CPS
efficiency in the experiments is a lower bound for the actual efficiency.Comment: 5 pages + 4 pages supplementary informatio
A Quantum Dot in the Kondo Regime Coupled to Superconductors
The Kondo effect and superconductivity are both prime examples of many-body
phenomena. Here we report transport measurements on a carbon nanotube quantum
dot coupled to superconducting leads that show a delicate interplay between
both effects. We demonstrate that the superconductivity of the leads does not
destroy the Kondo correlations on the quantum dot when the Kondo temperature,
which varies for different single-electron states, exceeds the superconducting
gap energy
Multi-shell gold nanowires under compression
Deformation properties of multi-wall gold nanowires under compressive loading
are studied. Nanowires are simulated using a realistic many-body potential.
Simulations start from cylindrical fcc(111) structures at T=0 K. After
annealing cycles axial compression is applied on multi-shell nanowires for a
number of radii and lengths at T=300 K. Several types of deformation are found,
such as large buckling distortions and progressive crushing. Compressed
nanowires are found to recover their initial lengths and radii even after
severe structural deformations. However, in contrast to carbon nanotubes
irreversible local atomic rearrangements occur even under small compressions.Comment: 1 gif figure, 5 ps figure
Controlled formation of metallic nanowires via Au nanoparticle ac trapping
Applying ac voltages, we trapped gold nanoparticles between microfabricated
electrodes under well-defined conditions. We demonstrate that the nanoparticles
can be controllably fused together to form homogeneous gold nanowires with
pre-defined diameters and conductance values. Whereas electromigration is known
to form a gap when a dc voltage is applied, this ac technique achieves the
opposite, thereby completing the toolkit for the fabrication of nanoscale
junctions.Comment: Nanotechnology 18, 235202 (2007
Superconductivity enhanced conductance fluctuations in few layer graphene nanoribbons
We investigate the mesoscopic disorder induced rms conductance variance
in a few layer graphene nanoribbon (FGNR) contacted by two
superconducting (S) Ti/Al contacts. By sweeping the back-gate voltage, we
observe pronounced conductance fluctuations superimposed on a linear background
of the two terminal conductance G. The linear gate-voltage induced response can
be modeled by a set of inter-layer and intra-layer capacitances.
depends on temperature T and source-drain voltage .
increases with decreasing T and . When lowering , a
pronounced cross-over at a voltage corresponding to the superconducting energy
gap is observed. For |V_{sd}|\ltequiv \Delta the fluctuations are
markedly enhanced. Expressed in the conductance variance of one
graphene-superconducutor (G-S) interface, values of 0.58 e^2/h are obtained at
the base temperature of 230 mK. The conductance variance in the sub-gap region
are larger by up to a factor of 1.4-1.8 compared to the normal state. The
observed strong enhancement is due to phase coherent charge transfer caused by
Andreev reflection at the nanoribbon-superconductor interface.Comment: 15 pages, 5 figure
The Amplitude of Non-Equilibrium Quantum Interference in Metallic Mesoscopic Systems
We study the influence of a DC bias voltage V on quantum interference
corrections to the measured differential conductance in metallic mesoscopic
wires and rings. The amplitude of both universal conductance fluctuations (UCF)
and Aharonov-Bohm effect (ABE) is enhanced several times for voltages larger
than the Thouless energy. The enhancement persists even in the presence of
inelastic electron-electron scattering up to V ~ 1 mV. For larger voltages
electron-phonon collisions lead to the amplitude decaying as a power law for
the UCF and exponentially for the ABE. We obtain good agreement of the
experimental data with a model which takes into account the decrease of the
electron phase-coherence length due to electron-electron and electron-phonon
scattering.Comment: New title, refined analysis. 7 pages, 3 figures, to be published in
Europhysics Letter
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