664 research outputs found
Uniformity of the pseudomagnetic field in strained graphene
We present a study on the uniformity of the pseudomagnetic field in graphene
as a function of the relative orientation between the graphene lattice and
straining directions. For this, we strained a regular micron-sized graphene
hexagon by deforming it symmetrically by displacing three of its edges. By
simulations, we found that the pseudomagnetic field is strongest if the strain
is applied perpendicular to the armchair direction of graphene. For a hexagon
with a side length of 1 m, the pseudomagnetic field has a maximum of
1.2 T for an applied strain of 3.5% and it is uniform (variance %) within
a circle with a diameter of nm. This diameter is on the order of the
typical diameter of the laser spot in a state-of-the-art confocal Raman
spectroscopy setup, which suggests that observing the pseudomagnetic field in
measurements of shifted magneto-phonon resonance is feasible.Comment: 7 pages, 5 figure
Friction as Contrast Mechanism in Heterodyne Force Microscopy
The nondestructive imaging of subsurface structures on the nanometer scale
has been a long-standing desire in both science and industry. A few impressive
images were published so far that demonstrate the general feasibility by
combining ultrasound with an Atomic Force Microscope. From different excitation
schemes, Heterodyne Force Microscopy seems to be the most promising candidate
delivering the highest contrast and resolution. However, the physical contrast
mechanism is unknown, thereby preventing any quantitative analysis of samples.
Here we show that friction at material boundaries within the sample is
responsible for the contrast formation. This result is obtained by performing a
full quantitative analysis, in which we compare our experimentally observed
contrasts with simulations and calculations. Surprisingly, we can rule out all
other generally believed responsible mechanisms, like Rayleigh scattering,
sample (visco)elasticity, damping of the ultrasonic tip motion, and ultrasound
attenuation. Our analytical description paves the way for quantitative
SubSurface-AFM imaging.Comment: 7 pages main paper + 11 pages supplementary material
Interplay between nanometer-scale strain variations and externally applied strain in graphene
We present a molecular modeling study analyzing nanometer-scale strain
variations in graphene as a function of externally applied tensile strain. We
consider two different mechanisms that could underlie nanometer-scale strain
variations: static perturbations from lattice imperfections of an underlying
substrate and thermal fluctuations. For both cases we observe a decrease in the
out-of-plane atomic displacements with increasing strain, which is accompanied
by an increase in the in-plane displacements. Reflecting the non-linear elastic
properties of graphene, both trends together yield a non-monotonic variation of
the total displacements with increasing tensile strain. This variation allows
to test the role of nanometer-scale strain variations in limiting the carrier
mobility of high-quality graphene samples
Pulsar timing analysis in the presence of correlated noise
Pulsar timing observations are usually analysed with least-square-fitting
procedures under the assumption that the timing residuals are uncorrelated
(statistically "white"). Pulsar observers are well aware that this assumption
often breaks down and causes severe errors in estimating the parameters of the
timing model and their uncertainties. Ad hoc methods for minimizing these
errors have been developed, but we show that they are far from optimal.
Compensation for temporal correlation can be done optimally if the covariance
matrix of the residuals is known using a linear transformation that whitens
both the residuals and the timing model. We adopt a transformation based on the
Cholesky decomposition of the covariance matrix, but the transformation is not
unique. We show how to estimate the covariance matrix with sufficient accuracy
to optimize the pulsar timing analysis. We also show how to apply this
procedure to estimate the spectrum of any time series with a steep red
power-law spectrum, including those with irregular sampling and variable error
bars, which are otherwise very difficult to analyse.Comment: Accepted by MNRA
Fabrication of comb-drive actuators for straining nanostructured suspended graphene
We report on the fabrication and characterization of an optimized comb-drive
actuator design for strain-dependent transport measurements on suspended
graphene. We fabricate devices from highly p-doped silicon using deep reactive
ion etching with a chromium mask. Crucially, we implement a gold layer to
reduce the device resistance from k to
at room temperature in order to allow for
strain-dependent transport measurements. The graphene is integrated by
mechanically transferring it directly onto the actuator using a
polymethylmethacrylate membrane. Importantly, the integrated graphene can be
nanostructured afterwards to optimize device functionality. The minimum feature
size of the structured suspended graphene is 30 nm, which allows for
interesting device concepts such as mechanically-tunable nanoconstrictions.
Finally, we characterize the fabricated devices by measuring the Raman spectrum
as well as the a mechanical resonance frequency of an integrated graphene sheet
for different strain values.Comment: 10 pages, 9 figure
High speed collision and reconnection of Abelian Higgs strings in the deep type-II regime
We study high speed collision and reconnection of cosmic strings in the
type-II regime (scalar-to-gauge mass ratios larger than one) of the Abelian
Higgs model. New phenomena such as multiple reconnections and clustering of
small scale structure have been observed and reported in a previous paper, as
well as the fact that the previously observed loop that mediates the second
intercommutation is only a loop for sufficiently large beta =
m_scalar^2/m_gauge^2. Here we give a more detailed account of our study,
involving 3D numerical simulations with beta in the range 1 to 64, the largest
value simulated to date, as well as 2D simulations of vortex-antivortex (v-av)
collisions to understand the possible relation to the new 3D phenomena. Our
simulations give further support to the idea that Abelian Higgs strings never
pass through each other, unless this is the result of a double reconnection;
and that the critical velocity (v_c) for double reconnection goes down with
increasing mass ratio, but energy conservation suggests a lower bound around
0.77c. We discuss the qualitative change in the intermediate state observed for
large mass ratios. We relate it to a similar change in the outcome of 2D v-av
collisions in the form of radiating bound states. In the deep type-II regime
the angular dependence of v_c for double reconnection does not seem to conform
to semi-analytic predictions based on the Nambu-Goto approximation. We model
the high angle collisions reasonably well by incorporating the effect of core
interactions, and the torque they produce on the approaching strings, into the
Nambu-Goto description of the collision. An interesting, counterintuitive
aspect is that the effective collision angle is smaller because of the torque.
Our results suggest differences in network evolution and radiation output with
respect to the predictions based on Nambu-Goto or beta = 1 Abelian Higgs
dynamics.Comment: 13 pages, 7 figures Send For Publication in Physics Review
Tunable mechanical coupling between driven microelectromechanical resonators
We present a microelectromechanical system, in which a silicon beam is
attached to a comb-drive actuator, that is used to tune the tension in the
silicon beam, and thus its resonance frequency. By measuring the resonance
frequencies of the system, we show that the comb-drive actuator and the silicon
beam behave as two strongly coupled resonators. Interestingly, the effective
coupling rate (~ 1.5 MHz) is tunable with the comb-drive actuator (+10%) as
well as with a side-gate (-10%) placed close to the silicon beam. In contrast,
the effective spring constant of the system is insensitive to either of them
and changes only by 0.5%. Finally, we show that the comb-drive actuator
can be used to switch between different coupling rates with a frequency of at
least 10 kHz.Comment: 5 pages, 4 figures, 1 tabl
Tuning dissipation dilution in 2D material resonators by MEMS-induced tension
Resonators based on two-dimensional (2D) materials have exceptional
properties for application as nanomechanical sensors, which allows them to
operate at high frequencies with high sensitivity. However, their performance
as nanomechanical sensors is currently limited by their low quality
()-factor. Here, we make use of micro-electromechanical systems (MEMS) to
apply pure in-plane mechanical strain, enhancing both their resonance frequency
and Q-factor. In contrast to earlier work, the 2D material resonators are
fabricated on the MEMS actuators without any wet processing steps, using a
dry-transfer method. A platinum clamp, that is deposited by electron
beam-induced deposition, is shown to be effective in fixing the 2D membrane to
the MEMS and preventing slippage. By in-plane straining the membranes in a
purely mechanical fashion, we increase the tensile energy, thereby diluting
dissipation. This way, we show how dissipation dilution can increase the
-factor of 2D material resonators by 91\%. The presented MEMS actuated
dissipation dilution method does not only pave the way towards higher
-factors in resonators based on 2D materials, but also provides a route
toward studies of the intrinsic loss mechanisms of 2D materials in the
monolayer limit.Comment: 20 pages, 15 figure
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