312 research outputs found
Tunneling through a multigrain system: deducing the sample topology from the nonlinear conductance
We study a current transport through a system of a few grains connected with
tunneling links. The exact solution is given for an arbitrarily connected
double-grain system with a shared gate in the framework of the orthodox model.
The obtained result is generalized for multigrain systems with strongly
different tunneling resistances. We analyse the large-scale nonlinear
conductance and demonstrate how the sample topology can be unambiguously
deduced from the spectroscopy pattern (differential conductance versus
gate-bias plot). We present experimental data for a multigrain sample and
reconstruct the sample topology. A simple selection rule is formulated to
distinguish samples with spectral patterns free from spurious disturbance
caused by recharging of some grains nearby. As an example, we demonstrate
experimental data with additional peaks in the spectroscopy pattern, which can
not be attributed to coupling to additional grains. The described approach can
be used to judge the sample topology when it is not guaranteed by fabrication
and direct imaging is not possible.Comment: 13 pages (including 8 figures
Anomalous Hopping Exponents of Ultrathin Films of Metals
The temperature dependence of the resistance R(T) of ultrathin
quench-condensed films of Ag, Bi, Pb and Pd has been investigated. In the most
resistive films, R(T)=Roexp(To/T)^x, where x=0.75. Surprisingly, the exponent x
was found to be constant for a wide range of Ro and To in all four materials,
possibly implying a consistent underlying conduction mechanism. The results are
discussed in terms of several different models of hopping conduction.Comment: 6 pages, 5 figure
Dietary Salt Intake and Mortality in Patients With Type 2 Diabetes
OBJECTIVE: Many guidelines recommend that patients with type 2 diabetes should aim to reduce their intake of salt. However, the precise relationship between dietary salt intake and mortality in patients with type 2 diabetes has not been previously explored. RESEARCH DESIGN AND METHODS: Six hundred and thirty-eight patients attending a single diabetes clinic were followed in a prospective cohort study. Baseline sodium excretion was estimated from 24-h urinary collections (24hU(Na)). The predictors of all-cause and cardiovascular mortality were determined by Cox regression and competing risk modeling, respectively. RESULTS: The mean baseline 24hU(Na) was 184 ± 73 mmol/24 h, which remained consistent throughout the follow-up (intraindividual coefficient of variation [CV] 23 ± 11%). Over a median of 9.9 years, there were 175 deaths, 75 (43%) of which were secondary to cardiovascular events. All-cause mortality was inversely associated with 24hU(Na), after adjusting for other baseline risk factors (P < 0.001). For every 100 mmol rise in 24hU(Na), all-cause mortality was 28% lower (95% CI 6-45%, P = 0.02). After adjusting for the competing risk of noncardiovascular death and other predictors, 24hU(Na) was also significantly associated with cardiovascular mortality (sub-hazard ratio 0.65 [95% CI 0.44-0.95]; P = 0.03). CONCLUSIONS: In patients with type 2 diabetes, lower 24-h urinary sodium excretion was paradoxically associated with increased all-cause and cardiovascular mortality. Interventional studies are necessary to determine if dietary salt has a causative role in determining adverse outcomes in patients with type 2 diabetes and the appropriateness of guidelines advocating salt restriction in this setting
Infrared Studies of the Onset of Conductivity in Ultra-Thin Pb Films
In this paper we report the first experimental measurement of the infrared
conductivity of ultra-thin quenched-condensed Pb films. For dc sheet
resistances such that the ac conductance increases with
frequency but is in disagreement with the predictions of weak localization. We
attribute this behavior to the effects of an inhomogeneous granular structure
of these films, which is manifested at the very small probing scale of infrared
measurements. Our data are consistent with predictions of two-dimensional
percolation theory.Comment: Submitted to Physical Review Letter
Anisotropic Magnetoconductance in Quench-Condensed Ultrathin Beryllium Films
Near the superconductor-insulator (S-I) transition, quench-condensed
ultrathin Be films show a large magnetoconductance which is highly anisotropic
in the direction of the applied field. Film conductance can drop as much as
seven orders of magnitude in a weak perpendicular field (< 1 T), but is
insensitive to a parallel field in the same field range. We believe that this
negative magnetoconductance is due to the field de-phasing of the
superconducting pair wavefunction. This idea enables us to extract the finite
superconducting phase coherence length in nearly superconducting films. Our
data indicate that this local phase coherence persists even in highly
insulating films in the vicinity of the S-I transition.Comment: 4 pages, 4 figure RevTex, Typos Correcte
Electron pumping in graphene mechanical resonators
The combination of high frequency vibrations and metallic transport in
graphene makes it a unique material for nano-electromechanical devices. In this
letter, we show that graphene-based nano-electromechanical devices are
extremely well suited for charge pumping, due to the sensitivity of its
transport coefficients to perturbations in electrostatic potential and
mechanical deformations, with the potential for novel small scale devices with
useful applications
Performance of Monolayer Graphene Nanomechanical Resonators with Electrical Readout
The enormous stiffness and low density of graphene make it an ideal material
for nanoelectromechanical (NEMS) applications. We demonstrate fabrication and
electrical readout of monolayer graphene resonators, and test their response to
changes in mass and temperature. The devices show resonances in the MHz range.
The strong dependence of the resonant frequency on applied gate voltage can be
fit to a membrane model, which yields the mass density and built-in strain.
Upon removal and addition of mass, we observe changes in both the density and
the strain, indicating that adsorbates impart tension to the graphene. Upon
cooling, the frequency increases; the shift rate can be used to measure the
unusual negative thermal expansion coefficient of graphene. The quality factor
increases with decreasing temperature, reaching ~10,000 at 5 K. By establishing
many of the basic attributes of monolayer graphene resonators, these studies
lay the groundwork for applications, including high-sensitivity mass detectors
Nonlinear response of a driven vibrating nanobeam in the quantum regime
We analytically investigate the nonlinear response of a damped doubly clamped
nanomechanical beam under static longitudinal compression which is excited to
transverse vibrations. Starting from a continuous elasticity model for the
beam, we consider the dynamics of the beam close to the Euler buckling
instability. There, the fundamental transverse mode dominates and a quantum
mechanical time-dependent effective single particle Hamiltonian for its
amplitude can be derived. In addition, we include the influence of a
dissipative Ohmic or super-Ohmic environment. In the rotating frame, a
Markovian master equation is derived which includes also the effect of the
time-dependent driving in a non-trivial way. The quasienergies of the pure
system show multiple avoided level crossings corresponding to multiphonon
transitions in the resonator. Around the resonances, the master equation is
solved analytically using Van Vleck perturbation theory. Their lineshapes are
calculated resulting in simple expressions. We find the general solution for
the multiple multiphonon resonances and, most interestingly, a bath-induced
transition from a resonant to an antiresonant behavior of the nonlinear
response.Comment: 25 pages, 5 figures, submitted to NJ
Universal Vectorial and Ultrasensitive Nanomechanical Force Field Sensor
Miniaturization of force probes into nanomechanical oscillators enables
ultrasensitive investigations of forces on dimensions smaller than their
characteristic length scale. Meanwhile it also unravels the force field
vectorial character and how its topology impacts the measurement. Here we
expose an ultrasensitive method to image 2D vectorial force fields by
optomechanically following the bidimensional Brownian motion of a singly
clamped nanowire. This novel approach relies on angular and spectral tomography
of its quasi frequency-degenerated transverse mechanical polarizations:
immersing the nanoresonator in a vectorial force field does not only shift its
eigenfrequencies but also rotate eigenmodes orientation as a nano-compass. This
universal method is employed to map a tunable electrostatic force field whose
spatial gradients can even take precedence over the intrinsic nanowire
properties. Enabling vectorial force fields imaging with demonstrated
sensitivities of attonewton variations over the nanoprobe Brownian trajectory
will have strong impact on scientific exploration at the nanoscale
Anisotropy versus circular dichroism in second harmonic generation from fourfold symmetric arrays of G-shaped nanostructures
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