129 research outputs found
Spintronics-based mesoscopic heat engine
We consider a nanowire suspended on two spin-polarized leads and subject to a
nonuniform magnetic field. We show that in such a system a temperature drop
between leads can significantly affect the nanowire dynamics. In particular, it
is demonstrated that under certain conditions the stationary distribution of
the mechanical subsystem has Boltzmann form with effective temperature which is
smaller than the temperature of the "cold" lead; this seems rather
counterintuitive. We also find that the change of the direction of the
temperature gradient results in generation of mechanical vibrations rather than
heating of the mechanical subsystem.Comment: 5 pages, 3 figure
Mass-loading induced dephasing in nanomechanical resonators
I study dephasing of an underdamped nanomechanical resonator subject to
random mass loading of small particles. I present a frequency noise model which
describes dephasing due to attachment and detachment of particles at random
points and particle diffusion along the resonator. This situation is commonly
encountered in current mass measurement experiments using NEM resonators. I
discuss the conditions which can lead to inhomogeneous broadening and fine
structure in the modes absorption spectra. I also show that the spectra of the
higher order cumulants of the (complex) vibrational mode amplitude are
sensitive to the parameters characterizing the frequency noise process. Hence,
measurement of these cumulants can provide information not only about the mass
but also about other parameters of the particles (diffusion coefficient and
attachment-detachment rates.)Comment: 7 pages, 4 figure
Diffusion-induced bistability of driven nanomechanical resonators
We study nanomechanical resonators with frequency fluctuations due to
diffusion of absorbed particles. The diffusion depends on the vibration
amplitude through inertial effect. We find that, if the diffusion coefficient
is sufficiently large, the resonator response to periodic driving displays
bistability. The lifetime of the coexisting vibrational states scales
exponentially with the diffusion coefficient. It also displays a characteristic
scaling dependence on the distance to bifurcation points.Comment: 4 pages, 3 figure
Enhancing the Mass Sensitivity of Graphene Nanoresonators Via Nonlinear Oscillations: The Effective Strain Mechanism
We perform classical molecular dynamics simulations to investigate the
enhancement of the mass sensitivity and resonant frequency of graphene
nanomechanical resonators that is achieved by driving them into the nonlinear
oscillation regime. The mass sensitivity as measured by the resonant frequency
shift is found to triple if the actuation energy is about 2.5 times the initial
kinetic energy of the nanoresonator. The mechanism underlying the enhanced mass
sensitivity is found to be the effective strain that is induced in the
nanoresonator due to the nonlinear oscillations, where we obtain an analytic
relationship between the induced effective strain and the actuation energy that
is applied to the graphene nanoresonator. An important implication of this work
is that there is no need for experimentalists to apply tensile strain to the
resonators before actuation in order to enhance the mass sensitivity. Instead,
enhanced mass sensitivity can be obtained by the far simpler technique of
actuating nonlinear oscillations of an existing graphene nanoresonator.Comment: published versio
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
A Mechanical Mass Sensor with Yoctogram Resolution
Nanoelectromechanical systems (NEMS) have generated considerable interest as
inertial mass sensors. NEMS resonators have been used to weigh cells,
biomolecules, and gas molecules, creating many new possibilities for biological
and chemical analysis [1-4]. Recently, NEMS-based mass sensors have been
employed as a new tool in surface science in order to study e.g. the phase
transitions or the diffusion of adsorbed atoms on nanoscale objects [5-7]. A
key point in all these experiments is the ability to resolve small masses. Here
we report on mass sensing experiments with a resolution of 1.7 yg (1 yg =
10^-24 g), which corresponds to the mass of one proton, or one hydrogen atom.
The resonator is made of a ~150 nm long carbon nanotube resonator vibrating at
nearly 2 GHz. The unprecedented level of sensitivity allows us to detect
adsorption events of naphthalene molecules (C10H8) and to measure the binding
energy of a Xe atom on the nanotube surface (131 meV). These ultrasensitive
nanotube resonators offer new opportunities for mass spectrometry,
magnetometry, and adsorption experiments.Comment: submitted version of the manuscrip
Determination of the Bending Rigidity of Graphene via Electrostatic Actuation of Buckled Membranes
The small mass and atomic-scale thickness of graphene membranes make them
highly suitable for nanoelectromechanical devices such as e.g. mass sensors,
high frequency resonators or memory elements. Although only atomically thick,
many of the mechanical properties of graphene membranes can be described by
classical continuum mechanics. An important parameter for predicting the
performance and linearity of graphene nanoelectromechanical devices as well as
for describing ripple formation and other properties such as electron
scattering mechanisms, is the bending rigidity, {\kappa}. In spite of the
importance of this parameter it has so far only been estimated indirectly for
monolayer graphene from the phonon spectrum of graphite, estimated from AFM
measurements or predicted from ab initio calculations or bond-order potential
models. Here, we employ a new approach to the experimental determination of
{\kappa} by exploiting the snap-through instability in pre-buckled graphene
membranes. We demonstrate the reproducible fabrication of convex buckled
graphene membranes by controlling the thermal stress during the fabrication
procedure and show the abrupt switching from convex to concave geometry that
occurs when electrostatic pressure is applied via an underlying gate electrode.
The bending rigidity of bilayer graphene membranes under ambient conditions was
determined to be eV. Monolayers have significantly lower
{\kappa} than bilayers
Loss of neuronal 3d chromatin organization causes transcriptional and behavioural deficits related to serotonergic dysfunction
The interior of the neuronal cell nucleus is a highly organized three-dimensional (3D) structure where regions of the genome that are linearly millions of bases apart establish sub-structures with specialized functions. To investigate neuronal chromatin organization and dynamics in vivo, we generated bitransgenic mice expressing GFP-tagged histone H2B in principal neurons of the forebrain. Surprisingly, the expression of this chimeric histone in mature neurons caused chromocenter declustering and disrupted the association of heterochromatin with the nuclear lamina. The loss of these structures did not affect neuronal viability but was associated with specific transcriptional and behavioural deficits related to serotonergic dysfunction. Overall, our results demonstrate that the 3D organization of chromatin within neuronal cells provides an additional level of epigenetic regulation of gene expression that critically impacts neuronal function. This in turn suggests that some loci associated with neuropsychiatric disorders may be particularly sensitive to changes in chromatin architecture
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
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