1,356 research outputs found
Tuning the proximity effect in a superconductor-graphene-superconductor junction
We have tuned in situ the proximity effect in a single graphene layer coupled
to two Pt/Ta superconducting electrodes. An annealing current through the
device changed the transmission coefficient of the electrode/graphene
interface, increasing the probability of multiple Andreev reflections. Repeated
annealing steps improved the contact sufficiently for a Josephson current to be
induced in graphene.Comment: Accepted for publication in Phys. Rev.
Construction of equilibrium networks with an energy function
We construct equilibrium networks by introducing an energy function depending
on the degree of each node as well as the product of neighboring degrees. With
this topological energy function, networks constitute a canonical ensemble,
which follows the Boltzmann distribution for given temperature. It is observed
that the system undergoes a topological phase transition from a random network
to a star or a fully-connected network as the temperature is lowered. Both
mean-field analysis and numerical simulations reveal strong first-order phase
transitions at temperatures which decrease logarithmically with the system
size. Quantitative discrepancies of the simulation results from the mean-field
prediction are discussed in view of the strong first-order nature.Comment: To appear in J. Phys.
Superconducting diamagnetic fluctuations in ropes of carbon nanotubes
We report low-temperature magnetisation measurements on a large number of
purified ropes of single wall carbon nanotubes. In spite of a large
superparamagnetic contribution due to the small ferromagnetic catalytical
particles still present in the sample, at low temperature () and low
magnetic field (), a diamagnetic signal is detectable. This low
temperature diamagnetism can be interpreted as the Meissner effect in ropes of
carbon nanotubes which have previously been shown to exhibit superconductivity
from transport measurements.Comment: 10 pages 3 figure
Geometrical dependence of decoherence by electronic interactions in a GaAs/GaAlAs square network
We investigate weak localization in metallic networks etched in a two
dimensional electron gas between mK and mK when electron-electron
(e-e) interaction is the dominant phase breaking mechanism. We show that, at
the highest temperatures, the contributions arising from trajectories that wind
around the rings and trajectories that do not are governed by two different
length scales. This is achieved by analyzing separately the envelope and the
oscillating part of the magnetoconductance. For K we find
\Lphi^\mathrm{env}\propto{T}^{-1/3} for the envelope, and
\Lphi^\mathrm{osc}\propto{T}^{-1/2} for the oscillations, in agreement with
the prediction for a single ring \cite{LudMir04,TexMon05}. This is the first
experimental confirmation of the geometry dependence of decoherence due to e-e
interaction.Comment: LaTeX, 5 pages, 4 eps figure
Direct measurement of the phase coherence length in a GaAs/GaAlAs square network
The low temperature magnetoconductance of a large array of quantum
coherentloops exhibits Altshuler-Aronov-Spivak oscillations which
periodicitycorresponds to 1/2 flux quantum per loop.We show that the
measurement of the harmonics content in a square networkprovides an accurate
way to determine the electron phase coherence length in units of the
lattice length without any adjustableparameters.We use this method to determine
in a network realised from a 2Delectron gas (2DEG) in a GaAS/GaAlAs
heterojunction. The temperaturedependence follows a power law from
1.3 K to 25 mK with nosaturation, as expected for 1D diffusive electronic
motion andelectron-electron scattering as the main decoherence mechanism.Comment: Additional experimental data in version
Alteration of superconductivity of suspended carbon nanotubes by deposition of organic molecules
We have altered the superconductivity of a suspended rope of single walled
carbon nanotubes, by coating it with organic polymers. Upon coating, the normal
state resistance of the rope changes by less than 20 percent. But
superconductivity, which on the bare rope shows up as a substantial resistance
decrease below 300 mK, is gradualy suppressed. We correlate this to the
suppression of radial breathing modes, measured with Raman Spectroscopy on
suspended Single and Double-walled carbon nanotubes. This points to the
breathing phonon modes as being responsible for superconductivity in carbon
nanotubes
The consequence of excess configurational entropy on fragility: the case of a polymer/oligomer blend
By taking advantage of the molecular weight dependence of the glass
transition of polymers and their ability to form perfectly miscible blends, we
propose a way to modify the fragility of a system, from fragile to strong,
keeping the same glass properties, i.e. vibrational density of states,
mean-square displacement and local structure. Both slow and fast dynamics are
investigated by calorimetry and neutron scattering in an athermal
polystyrene/oligomer blend, and compared to those of a pure 17-mer polystyrene
considered to be a reference, of same Tg. Whereas the blend and the pure 17-mer
have the same heat capacity in the glass and in the liquid, their fragilities
differ strongly. This difference in fragility is related to an extra
configurational entropy created by the mixing process and acting at a scale
much larger than the interchain distance, without affecting the fast dynamics
and the structure of the glass
Assessing emission reduction targets with dynamic models: deriving target load functions for use in integrated assessment
International audienceInternational agreements to reduce the emission of acidifying sulphur (S) and nitrogen (N) compounds have been negotiated on the basis of an understanding of the link between acidification related changes in soil and surface water chemistry and terrestrial and aquatic biota. The quantification of this link is incorporated within the concept of critical loads. Critical loads are calculated using steady state models and give no indication of the time within which acidified ecosystems might be expected to recover. Dynamic models provide an opportunity to assess the timescale of recovery and can go further to provide outputs which can be used in future emission reduction strategies. In this respect, the Target Load Function (TLF) is proposed as a means of assessing the deposition load necessary to restore a damaged ecosystem to some pre-defined acceptable state by a certain time in the future. A target load represents the deposition of S and N in a defined year (implementation year) for which the critical limit is achieved in a defined time (target year). A TLF is constructed using an appropriate dynamic model to determine the value of a chemical criterion at a given point in time given a temporal pattern of S and N deposition loads. A TLF requires information regarding: (i) the chemical criterion required to protect the chosen biological receptor (i.e. the critical limit); (ii) the year in which the critical limit is required to be achieved; and (iii) time pattern of future emission reductions. In addition, the TLF can be assessed for whole regions to incorporate the effect of these three essentially ecosystem management decisions. Keywords: emission reduction, critical load, target load, dynamic model, recovery tim
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