5,271 research outputs found
Nonlinear modal interactions in clamped-clamped mechanical resonators
A theoretical and experimental investigation is presented on the intermodal
coupling between the flexural vibration modes of a single clamped-clamped beam.
Nonlinear coupling allows an arbitrary flexural mode to be used as a
self-detector for the amplitude of another mode, presenting a method to measure
the energy stored in a specific resonance mode. Experimentally observed complex
nonlinear dynamics of the coupled modes are quantitatively captured by a model
which couples the modes via the beam extension; the same mechanism is
responsible for the well-known Duffing nonlinearity in clamped-clamped beams.Comment: 5 pages, 3 figure
Parametric resonances in electrostatically interacting carbon nanotube arrays
We study, numerically and analytically, a model of a one-dimensional array of
carbon nanotube resonators in a two-terminal configuration. The system is
brought into resonance upon application of an AC-signal superimposed on a
DC-bias voltage. When the tubes in the array are close to each other,
electrostatic interactions between tubes become important for the array
dynamics. We show that both transverse and longitudinal parametric resonances
can be excited in addition to primary resonances. The intertube electrostatic
interactions couple modes in orthogonal directions and affect the mode
stability.Comment: 11 pages, 12 figures, RevTeX
Noise-enabled precision measurements of a Duffing nanomechanical resonator
We report quantitative experimental measurements of the nonlinear response of
a radiofrequency mechanical resonator, with very high quality factor, driven by
a large swept-frequency force. We directly measure the noise-free transition
dynamics between the two basins of attraction that appear in the nonlinear
regime, and find good agreement with those predicted by the one-dimensional
Duffing equation of motion. We then measure the response of the transition
rates to controlled levels of white noise, and extract the activation energy
from each basin. The measurements of the noise-induced transitions allow us to
obtain precise values for the critical frequencies, the natural resonance
frequency, and the cubic nonlinear parameter in the Duffing oscillator, with
direct applications to high sensitivity parametric sensors based on these
resonators.Comment: 5 pages, 5 figure
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
Dissipation in graphene and nanotube resonators
Different damping mechanisms in graphene nanoresonators are studied: charges
in the substrate, ohmic losses in the substrate and the graphene sheet,
breaking and healing of surface bonds (Velcro effect), two level systems,
attachment losses, and thermoelastic losses. We find that, for realistic
structures and contrary to semiconductor resonators, dissipation is dominated
by ohmic losses in the graphene layer and metallic gate. An extension of this
study to carbon nanotube-based resonators is presented.Comment: Published version with updated reference
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
Telemonitoring after discharge from hospital with heart failure: cost-effectiveness modelling of alternative service designs.
Objectives To estimate the cost-effectiveness of remote monitoring strategies versus usual care for adults recently discharged after a heart failure (HF) exacerbation.
Design Decision analysis modelling of cost-effectiveness using secondary data sources.
Setting Acute hospitals in the UK.
Patients Patients recently discharged (within 28 days) after a HF exacerbation.
Interventions Structured telephone support (STS) via human to machine (STS HM) interface, (2) STS via human to human (STS HH) contact and (3) home telemonitoring (TM), compared with (4) usual care.
Main outcome measures The incremental cost per quality-adjusted life year (QALY) gained by each strategy compared to the next most effective alternative and the probability of each strategy being cost-effective at varying willingness to pay per QALY gained.
Results TM was the most cost-effective strategy in the scenario using these base case costs. Compared with usual care, TM had an estimated incremental cost effectiveness ratio (ICER) of £11 873/QALY, whereas STS HH had an ICER of £228 035/QALY against TM. STS HM was dominated by usual care. Threshold analysis suggested that the monthly cost of TM has to be higher than £390 to have an ICER greater than £20 000/QALY against STS HH. Scenario analyses performed using higher costs of usual care, higher costs of STS HH and lower costs of TM do not substantially change the conclusions.
Conclusions Cost-effectiveness analyses suggest that TM was an optimal strategy in most scenarios, but there is considerable uncertainty in relation to clear descriptions of the interventions and robust estimation of costs
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