5,323 research outputs found
Mixing with the radiofrequency single-electron transistor
By configuring a radio-frequency single-electron transistor as a mixer, we
demonstrate a unique implementation of this device, that achieves good charge
sensitivity with large bandwidth about a tunable center frequency. In our
implementation we achieve a measurement bandwidth of 16 MHz, with a tunable
center frequency from 0 to 1.2 GHz, demonstrated with the transistor operating
at 300 mK. Ultimately this device is limited in center frequency by the RC time
of the transistor's center island, which for our device is ~ 1.6 GHz, close to
the measured value. The measurement bandwidth is determined by the quality
factor of the readout tank circuit.Comment: Submitted to APL september 200
High-Frequency Nanofluidics: An Experimental Study using Nanomechanical Resonators
Here we apply nanomechanical resonators to the study of oscillatory fluid
dynamics. A high-resonance-frequency nanomechanical resonator generates a
rapidly oscillating flow in a surrounding gaseous environment; the nature of
the flow is studied through the flow-resonator interaction. Over the broad
frequency and pressure range explored, we observe signs of a transition from
Newtonian to non-Newtonian flow at , where is a
properly defined fluid relaxation time. The obtained experimental data appears
to be in close quantitative agreement with a theory that predicts purely
elastic fluid response as
The relation between repeated 6-minute walk test performance and outcome in patients with chronic heart failure
Objective: To assess the prognostic implications of the 6-minute walk test (6-MWT) distance measured twice, one year apart, in a large sample of patients with chronic heart failure (CHF) followed for an extended period ( > . 8. years from baseline). Material and methods: Patients undertook a 6-MWT at baseline and at one year, and were followed up for 8. years from baseline. Results: Six hundred patients (median [inter-quartile range, IQR]) (age 78 [72-84] years; 75% males; body mass index 27 [25-31] kg·m -2 ; left ventricular ejection fraction 34 [26-38] %) were included. At baseline, median 6-MWT distance was 232 (60-386) m. There was no significant change in 6-MWT distance at one year (change -12m; P=0.533). During a median follow-up of 8.0years in survivors, 396patients had died (66%). Four variables were independent predictors of all-cause mortality in a multivariable Cox model (adjusted for body mass index, age, QRS duration, left ventricular ejection fraction); increasing NT pro-BNP, decreasing 6-MWT distance at 1year, decreasing haemoglobin, and increasing urea. Conclusions: Distance walked during the 6-MWT is an independent predictor of all-cause mortality in patients with CHF. In survivors, the 6-MWT distance is stable at 1 year. The 6-MWT distance at 1 year carries similar prognostic information. © 2014 Elsevier Masson SAS
Hot electrons in low-dimensional phonon systems
A simple bulk model of electron-phonon coupling in metals has been
surprisingly successful in explaining experiments on metal films that actually
involve surface- or other low-dimensional phonons. However, by an exact
application of this standard model to a semi-infinite substrate with a free
surface, making use of the actual vibrational modes of the substrate, we show
that such agreement is fortuitous, and that the model actually predicts a
low-temperature crossover from the familiar T^5 temperature dependence to a
stronger T^6 log T scaling. Comparison with existing experiments suggests a
widespread breakdown of the standard model of electron-phonon thermalization in
metals
Quantum Effects in the Mechanical Properties of Suspended Nanomechanical Systems
We explore the quantum aspects of an elastic bar supported at both ends and
subject to compression. If strain rather than stress is held fixed, the system
remains stable beyond the buckling instability, supporting two potential
minima. The classical equilibrium transverse displacement is analogous to a
Ginsburg-Landau order parameter, with strain playing the role of temperature.
We calculate the quantum fluctuations about the classical value as a function
of strain. Excitation energies and quantum fluctuation amplitudes are compared
for silicon beams and carbon nanotubes.Comment: RevTeX4. 5 pages, 3 eps figures. Submitted to Physical Review Letter
The Effect of Mechanical Resonance on Josephson Dynamics
We study theoretically dynamics in a Josephson junction coupled to a
mechanical resonator looking at the signatures of the resonance in d.c.
electrical response of the junction. Such a system can be realized
experimentally as a suspended ultra-clean carbon nanotube brought in contact
with two superconducting leads. A nearby gate electrode can be used to tune the
junction parameters and to excite mechanical motion. We augment theoretical
estimations with the values of setup parameters measured in the samples
fabricated.
We show that charging effects in the junction give rise to a mechanical force
that depends on the superconducting phase difference. The force can excite the
resonant mode provided the superconducting current in the junction has
oscillating components with a frequency matching the resonant frequency of the
mechanical resonator. We develop a model that encompasses the coupling of
electrical and mechanical dynamics. We compute the mechanical response (the
effect of mechanical motion) in the regime of phase bias and d.c. voltage bias.
We thoroughly investigate the regime of combined a.c. and d.c. bias where
Shapiro steps are developed and reveal several distinct regimes characteristic
for this effect. Our results can be immediately applied in the context of
experimental detection of the mechanical motion in realistic superconducting
nano-mechanical devices.Comment: 18 pages, 11 figure
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
Heat capacity of a thin membrane at very low temperature
We calculate the dependence of heat capacity of a free standing thin membrane
on its thickness and temperature. A remarkable fact is that for a given
temperature there exists a minimum in the dependence of the heat capacity on
the thickness. The ratio of the heat capacity to its minimal value for a given
temperature is a universal function of the ratio of the thickness to its value
corresponding to the minimum. The minimal value of the heat capacitance for
given temperature is proportional to the temperature squared. Our analysis can
be used, in particular, for optimizing support membranes for microbolometers
Diffusion-induced dephasing in nanomechanical resonators
We study resonant response of an underdamped nanomechanical resonator with
fluctuating frequency. The fluctuations are due to diffusion of molecules or
microparticles along the resonator. They lead to broadening and change of shape
of the oscillator spectrum. The spectrum is found for the diffusion confined to
a small part of the resonator and where it occurs along the whole nanobeam. The
analysis is based on extending to the continuous limit, and appropriately
modifying, the method of interfering partial spectra. We establish the
conditions of applicability of the fluctuation-dissipation relations between
the susceptibility and the power spectrum. We also find where the effect of
frequency fluctuations can be described by a convolution of the spectra without
these fluctuations and with them as the only source of the spectral broadening.Comment: 10 page
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