734 research outputs found
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
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
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
Limits on the Mass, Velocity and Orbit of PSR J19336211
We present a high-precision timing analysis of PSR J19336211, a
millisecond pulsar (MSP) with a 3.5-ms spin period and a white dwarf (WD)
companion, using data from the Parkes radio telescope. Since we have accurately
measured the polarization properties of this pulsar we have applied the matrix
template matching approach in which the times of arrival are measured using
full polarimetric information. We achieved a weighted root-mean-square timing
residuals (rms) of the timing residuals of 1.23 , 15.5
improvement compared to the total intensity timing analysis. After studying the
scintillation properties of this pulsar we put constraints on the inclination
angle of the system. Based on these measurements and on mapping we put
a 2- upper limit on the companion mass (0.44 M). Since this
mass limit cannot reveal the nature of the companion we further investigate the
possibility of the companion to be a He WD. Applying the orbital period-mass
relation for such WDs, we conclude that the mass of a He WD companion would be
about 0.260.01 M which, combined with the measured mass function
and orbital inclination limits, would lead to a light pulsar mass
1.0 M. This result seems unlikely based on current neutron star
formation models and we therefore conclude that PSR J19336211 most likely
has a CO WD companion, which allows for a solution with a more massive pulsar
Field theory of the photon self-energy in a medium with a magnetic field and the Faraday effect
A convenient and general decomposition of the photon self-energy in a
magnetized, but otherwise isotropic, medium is given in terms of the minimal
set of tensors consistent with the transversality condition. As we show, the
self-energy in such a medium is completely parametrized in terms of nine
independent form factors, and they reduce to three in the long wavelength
limit. We consider in detail an electron gas with a background magnetic field,
and using finite temperature field theory methods, we obtain the one-loop
formulas for the form factors, which are exact to all orders in the magnetic
field. Explicit results are derived for a variety of physical conditions. In
the appropriate limits, we recover the well-known semi-classical results for
the photon dispersion relations and the Faraday effect. In more general cases,
where the semi-classical treatment or the linear approximation (weak field
limit) are not applicable, our formulas provide a consistent and systematic way
for computing the self-energy form factors and, from them, the photon
dispersion relations.Comment: Revtex, 27 page
Collective learning in schools described: building collective learning capacity
Processes of collective learning are expected to increase the professionalism of teachers and school leaders. Little is known about the processes of collective learning which take place in schools and about the way in which those processes may be improved. This paper describes a research into processes of collective learning at three primary schools. Processes of collective learning are described which took place in small teams in these schools. It is also pointed out which attempts can be made in order to reinforce these processes in the schools mentioned
Extremely high precision VLBI astrometry of PSR J0437-4715 and implications for theories of gravity
Using the recently upgraded Long Baseline Array, we have measured the
trigonometric parallax of PSR J0437-4715 to better than 1% precision, the most
precise pulsar distance determination made to date. Comparing this VLBI
distance measurement to the kinematic distance obtained from pulsar timing,
which is calculated from the pulsar's proper motion and apparent rate of change
of orbital period, gives a precise limit on the unmodeled relative acceleration
between the Solar System and PSR J0437-4715, which can be used in a variety of
applications. Firstly, it shows that Newton's gravitational constant G is
stable with time (\dot{G}/G = (-5 +- 26) x 10^{-13} yr^{-1}, 95% confidence).
Secondly, if a stochastic gravitational wave background existed at the
currently quoted limit, this null result would fail ~50% of the time. Thirdly,
it excludes Jupiter-mass planets within 226 AU of the Sun in 50% of the sky
(95% confidence). Finally, the ~1% agreement of the parallax and orbital period
derivative distances provides a fundamental confirmation of the parallax
distance method upon which all astronomical distances are based.Comment: 11 pages, 1 Figure, submitted to ApJ
Direct evidence of the failure of electric-dipole approximation in second-harmonic generation from a chiral polymer film
Second-harmonic generation from Langmuir-Blodgett films of a polythiophene is strongly influenced by the chirality of the polymer. The polarization dependence of the process cannot be explained in the elec.-dipole approxn. Evidence of contributions beyond elec. dipoles is obtained directly from individual second-harmonic signal
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