596 research outputs found
Electron transport through a metal-molecule-metal junction
Molecules of bisthiolterthiophene have been adsorbed on the two facing gold
electrodes of a mechanically controllable break junction in order to form
metal-molecule(s)-metal junctions. Current-voltage (I-V) characteristics have
been recorded at room temperature. Zero bias conductances were measured in the
10-100 nS range and different kinds of non-linear I-V curves with step-like
features were reproducibly obtained. Switching between different kinds of I-V
curves could be induced by varying the distance between the two metallic
electrodes. The experimental results are discussed within the framework of
tunneling transport models explicitly taking into account the discrete nature
of the electronic spectrum of the molecule.Comment: 12 pages, 12 figures to appear in Phys. Rev. B 59(19) 199
Electron transport through rectifying self-assembled monolayer diodes on silicon: Fermi level pinning at the molecule-metal interface
We report the synthesis and characterization of molecular rectifying diodes
on silicon using sequential grafting of self-assembled monolayers of alkyl
chains bearing a pi group at their outer end (Si/sigma-pi/metal junctions). We
investigate the structure-performance relationships of these molecular devices
and we examine to what extent the nature of the pi end-group (change in the
energy position of their molecular orbitals) drives the properties of these
molecular diodes. For all the pi-groups investigated here, we observe
rectification behavior. These results extend our preliminary work using phenyl
and thiophene groups (S. Lenfant et al., Nano Letters 3, 741 (2003)).The
experimental current-voltage curves are analyzed with a simple analytical
model, from which we extract the energy position of the molecular orbital of
the pi-group in resonance with the Fermi energy of the electrodes. We report
the experimental studies of the band lineup in these silicon/alkyl-pi
conjugated molecule/metal junctions. We conclude that Fermi level pinning at
the pi-group/metal interface is mainly responsible for the observed absence of
dependence of the rectification effect on the nature of the pi-groups, even
though they were chosen to have significant variations in their electronic
molecular orbitalsComment: To be published in J. Phys. Chem.
Clustering of vector nulls in homogeneous isotropic turbulence
We analyze the vector nulls of velocity, Lagrangian acceleration, and
vorticity, coming from direct numerical simulations of forced homogeneous
isotropic turbulence at . We show that the clustering
of velocity nulls is much stronger than those of acceleration and vorticity
nulls. These acceleration and vorticity nulls, however, are denser than the
velocity nulls. We study the scaling of clusters of these null points with
and with characteristic turbulence lengthscales. We also analyze
datasets of point inertial particles with Stokes numbers , 3, and 6,
at . Inertial particles display preferential concentration
with a degree of clustering that resembles some properties of the clustering of
the Lagrangian acceleration nulls, in agreement with the proposed sweep-stick
mechanism of clustering formation
First-principles calculation of intrinsic defect formation volumes in silicon
We present an extensive first-principles study of the pressure dependence of
the formation enthalpies of all the know vacancy and self-interstitial
configurations in silicon, in each charge state from -2 through +2. The neutral
vacancy is found to have a formation volume that varies markedly with pressure,
leading to a remarkably large negative value (-0.68 atomic volumes) for the
zero-pressure formation volume of a Frenkel pair (V + I). The interaction of
volume and charge was examined, leading to pressure--Fermi level stability
diagrams of the defects. Finally, we quantify the anisotropic nature of the
lattice relaxation around the neutral defects.Comment: 9 pages, 9 figure
Clustering of vector nulls in homogeneous isotropic turbulence
We analyze the vector nulls of velocity, Lagrangian acceleration, and vorticity, coming from direct numerical simulations of forced homogeneous isotropic turbulence at ReλâO([40-600]). We show that the clustering of velocity nulls is much stronger than those of acceleration and vorticity nulls. These acceleration and vorticity nulls, however, are denser than the velocity nulls. We study the scaling of clusters of these null points with Reλ and with characteristic turbulence lengthscales. We also analyze datasets of point inertial particles with Stokes numbers St=0.5, 3, and 6, at Reλ=240. Inertial particles display preferential concentration with a degree of clustering that resembles some properties of the clustering of the Lagrangian acceleration nulls, in agreement with the proposed sweep-stick mechanism of clustering formation.Fil: Mora, D. O.. University of Washington; Estados Unidos. Universite Grenoble Alpes; FranciaFil: Bourgoin, M.. Centre National de la Recherche Scientifique; FranciaFil: Mininni, Pablo Daniel. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Oficina de CoordinaciĂłn Administrativa Ciudad Universitaria. Instituto de FĂsica de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de FĂsica de Buenos Aires; ArgentinaFil: Obligado, M.. Universite Grenoble Alpes; Franci
Scaling Law in Carbon Nanotube Electromechanical Devices
We report a method for probing electromechanical properties of multiwalled
carbon nanotubes(CNTs). This method is based on AFM measurements on a doubly
clamped suspended CNT electrostatically deflected by a gate electrode. We
measure the maximum deflection as a function of the applied gate voltage. Data
from different CNTs scale into an universal curve within the experimental
accuracy, in agreement with a continuum model prediction. This method and the
general validity of the scaling law constitute a very useful tool for designing
actuators and in general conducting nanowire-based NEMS.Comment: 12 pages, 4 figures. To be published in Phys. Rev. Let
Measurements of the magnetic field induced by a turbulent flow of liquid metal
Initial results from the Madison Dynamo Experiment provide details of the
inductive response of a turbulent flow of liquid sodium to an applied magnetic
field. The magnetic field structure is reconstructed from both internal and
external measurements. A mean toroidal magnetic field is induced by the flow
when an axial field is applied, thereby demonstrating the omega effect.
Poloidal magnetic flux is expelled from the fluid by the poloidal flow.
Small-scale magnetic field structures are generated by turbulence in the flow.
The resulting magnetic power spectrum exhibits a power-law scaling consistent
with the equipartition of the magnetic field with a turbulent velocity field.
The magnetic power spectrum has an apparent knee at the resistive dissipation
scale. Large-scale eddies in the flow cause significant changes to the
instantaneous flow profile resulting in intermittent bursts of non-axisymmetric
magnetic fields, demonstrating that the transition to a dynamo is not smooth
for a turbulent flow.Comment: 9 pages, 11 figures, invited talk by C. B. Forest at 2005 APS DPP
meeting, resubmitted to Physics of Plasma
Markov property of Lagrangian turbulence
Based on direct numerical simulations with point-like inertial particles
transported by homogeneous and isotropic turbulent flows, we present evidence
for the existence of Markov property in Lagrangian turbulence. We show that the
Markov property is valid for a finite step size larger than a Stokes
number-dependent Einstein-Markov memory length. This enables the description of
multi-scale statistics of Lagrangian particles by Fokker-Planck equations,
which can be embedded in an interdisciplinary approach linking the statistical
description of turbulence with fluctuation theorems of non-equilibrium
stochastic thermodynamics and fluctuation theorems, and local flow structures.Comment: submitted to PRL, 5 pages, 4 figure
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