591 research outputs found
Efficient readout of micromechanical resonator arrays in ambient conditions
We present a method for efficient spectral readout of mechanical resonator
arrays in dissipative environments. Magnetomotive drive and detection is used
to drive double clamped resonators in the nonlinear regime. Resonators with
almost identical resonance frequencies can be tracked individually by sweeping
the drive power. Measurements are performed at room temperature and atmospheric
pressure. These conditions enable application in high throughput resonant
sensor arrays.Comment: 4 pages, 4 figure
Dark Matter Annihilation and the PAMELA, FERMI and ATIC Anomalies
If dark matter (DM) annihilation accounts for the tantalizing excess of
cosmic ray electron/positrons, as reported by the PAMELA, ATIC, HESS and FERMI
observatories, then the implied annihilation cross section must be relatively
large. This results, in the context of standard cosmological models, in very
small relic DM abundances that are incompatible with astrophysical
observations. We explore possible resolutions to this apparent conflict in
terms of non-standard cosmological scenarios; plausibly allowing for large
cross sections, while maintaining relic abundances in accord with current
observations.Comment: 13 pages, 3 figures; published for publication in Physical Review
Nanomechanical properties of few-layer graphene membranes
We have measured the mechanical properties of few-layer graphene and graphite
flakes that are suspended over circular holes. The spatial profile of the
flake's spring constant is measured with an atomic force microscope. The
bending rigidity of and the tension in the membranes are extracted by fitting a
continuum model to the data. For flakes down to eight graphene layers, both
parameters show a strong thickness-dependence. We predict fundamental resonance
frequencies of these nanodrums in the GHz range based on the measured bending
rigidity and tension.Comment: 9 pages, 3 figures, This article has been accepted by Appl. Phys.
Lett. After it is published, it will be found at http://apl.aip.org
Strongly coupled modes in a weakly driven micromechanical resonator
We demonstrate strong coupling between the flexural vibration modes of a
clamped-clamped micromechanical resonator vibrating at low amplitudes. This
coupling enables the direct measurement of the frequency response via
amplitude- and phase modulation schemes using the fundamental mode as a
mechanical detector. In the linear regime, a frequency shift of
is observed for a mode with a line width of
in vacuum. The measured response is well-described by the
analytical model based on the Euler-Bernoulli beam including tension.
Calculations predict an upper limit for the room-temperature Q-factor of
for our top-down fabricated micromechanical beam
resonators.Comment: 9 pages, 2 figure
Discrete-time quadrature feedback cooling of a radio-frequency mechanical resonator
We have employed a feedback cooling scheme, which combines high-frequency
mixing with digital signal processing. The frequency and damping rate of a 2
MHz micromechanical resonator embedded in a dc SQUID are adjusted with the
feedback, and active cooling to a temperature of 14.3 mK is demonstrated. This
technique can be applied to GHz resonators and allows for flexible control
strategies.Comment: To appear in Appl. Phys. Let
In-Chain Tunneling Through Charge-Density Wave Nanoconstrictions and Break-Junctions
We have fabricated longitudinal nanoconstrictions in the charge-density wave
conductor (CDW) NbSe using a focused ion beam and using a mechanically
controlled break-junction technique. Conductance peaks are observed below the
TK and TK CDW transitions, which correspond closely
with previous values of the full CDW gaps and
obtained from photo-emission. These results can be explained by assuming
CDW-CDW tunneling in the presence of an energy gap corrugation
comparable to , which eliminates expected peak at
. The nanometer length-scales our experiments imply
indicate that an alternative explanation based on tunneling through
back-to-back CDW-normal junctions is unlikely.Comment: 5 pages, 3 figures, submitted to physical review letter
Electric-field control of interfering transport pathways in a single-molecule anthraquinone transistor
It is understood that molecular conjugation plays an important role in charge
transport through single-molecule junctions. Here, we investigate electron
transport through an anthraquinone based single-molecule three-terminal device.
With the use of an electric-field induced by a gate electrode, the molecule is
reduced resulting into a ten-fold increase in the off-resonant differential
conductance. Theoretical calculations link the change in differential
conductance to a reduction-induced change in conjugation, thereby lifting
destructive interference of transport pathways.Comment: Nano Letters (2015
Nonlinear Viscous Vortex Motion in Two-Dimensional Josephson-Junction Arrays
When a vortex in a two-dimensional Josephson junction array is driven by a
constant external current it may move as a particle in a viscous medium. Here
we study the nature of this viscous motion. We model the junctions in a square
array as resistively and capacitively shunted Josephson junctions and carry out
numerical calculations of the current-voltage characteristics. We find that the
current-voltage characteristics in the damped regime are well described by a
model with a {\bf nonlinear} viscous force of the form , where is the vortex velocity,
is the velocity dependent viscosity and and are
constants for a fixed value of the Stewart-McCumber parameter. This result is
found to apply also for triangular lattices in the overdamped regime. Further
qualitative understanding of the nature of the nonlinear friction on the vortex
motion is obtained from a graphic analysis of the microscopic vortex dynamics
in the array. The consequences of having this type of nonlinear friction law
are discussed and compared to previous theoretical and experimental studies.Comment: 14 pages RevTex, 9 Postscript figure
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