1,075 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
Lutz-Kelker bias in pulsar parallax measurements
Lutz & Kelker showed that parallax measurements are systematically
overestimated because they do not properly account for the larger volume of
space that is sampled at smaller parallax values. We apply their analysis to
neutron stars, incorporating the bias introduced by the intrinsic radio
luminosity function and a realistic Galactic population model for neutron
stars. We estimate the bias for all published neutron star parallax
measurements and find that measurements with less than ~95% certainty, are
likely to be significantly biased. Through inspection of historic parallax
measurements, we confirm the described effects in optical and radio
measurements, as well as in distance estimates based on interstellar dispersion
measures. The potential impact on future tests of relativistic gravity through
pulsar timing and on X-ray--based estimates of neutron star radii is briefly
discussed.Comment: 9 pages, 3 tables, 1 figure. Accepted for publication in MNRA
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
Low-Frequency Spectral Turn-Overs in Millisecond Pulsars Studied from Imaging Observations
Measurements of pulsar flux densities are of great importance for
understanding the pulsar emission mechanism and for predictions of pulsar
survey yields and the pulsar population at large. Typically these flux
densities are determined from phase-averaged "pulse profiles", but this method
has limited applicability at low frequencies because the observed pulses can
easily be spread out by interstellar effects like scattering or dispersion,
leading to a non-pulsed continuum component that is necessarily ignored in this
type of analysis. In particular for the class of the millisecond pulsars (MSPs)
at frequencies below 200MHz, such interstellar effects can seriously compromise
de- tectability and measured flux densities. In this paper we investigate MSP
spectra based on a complementary approach, namely through investigation of
archival con- tinuum imaging data. Even though these images lose sensitivity to
pulsars since the on-pulse emission is averaged with off-pulse noise, they are
insensitive to effects from scattering and provide a reliable way to determine
the flux density and spectral indices of MSPs based on both pulsed and unpulsed
components. Using the 74MHz VLSSr as well as the 325MHz WENSS and 1.4GHz NVSS
catalogues, we investigate the imaging flux densities of MSPs and evaluate the
likelihood of spectral turn-overs in this population. We determine three new
MSP spectral indices and identify six new MSPs with likely spectral turn-overs.Comment: 10 pages, 4 figures, 3 tables, accepted for publication in MNRA
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
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
Assessing the Role of Spin Noise in the Precision Timing of Millisecond Pulsars
We investigate rotational spin noise (referred to as timing noise) in
non-accreting pulsars: millisecond pulsars, canonical pulsars, and magnetars.
Particular attention is placed on quantifying the strength and non-stationarity
of timing noise in millisecond pulsars because the long-term stability of these
objects is required to detect nanohertz gravitational radiation. We show that a
single scaling law is sufficient to characterize timing noise in millisecond
and canonical pulsars while the same scaling law underestimates the levels of
timing noise in magnetars. The scaling law, along with a detailed study of the
millisecond pulsar B1937+21, leads us to conclude that timing noise is latent
in most millisecond pulsars and will be measurable in many objects when better
arrival time estimates are obtained over long data spans. The sensitivity of a
pulsar timing array to gravitational radiation is strongly affected by any
timing noise. We conclude that detection of proposed gravitational wave
backgrounds will require the analysis of more objects than previously suggested
over data spans that depend on the spectra of both the gravitational wave
background and of the timing noise. It is imperative to find additional
millisecond pulsars in current and future surveys in order to reduce the
effects of timing noise.Comment: 16 pages and 6 figures. ApJ, accepte
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
Postpartum Health Services Requested by Mothers with Newborns Receiving Intensive Care
Objectives Our pilot study aimed to build knowledge of the postpartum health needs of mothers with infants in a newborn intensive care unit (NICU). Methods Between May 2008 and December 2009, a Certified Nurse Midwife was available during workday hours to provide health care services to mothers visiting their infants in the NICU at a large tertiary care center. Results A total of 424 health service encounters were recorded. Maternal requests for services covered a wide variety of needs, with primary care being the most common. Key health concerns included blood pressure monitoring, colds, coughs, sore throats, insomnia and migraines. Mothers also expressed a need for mental health assessment and support, obstetric care, treatment for sexually transmitted infections, tobacco cessation, breastfeeding assistance, postpartum visits, and provision of contraception. Conclusions Our study suggests that mothers with babies in the NICU have a host of health needs. We also found that women were receptive to receiving health services in a critical care pediatric setting. Intensive care nurseries could feasibly partner with in-patient mother-baby units and/or on-site obstetric clinics to increase access to health care for the mothers of the high-risk newborns in their units. Modifications should be made within health care systems that serve high-risk infants to better address the many needs of the mother/baby dyad in the postpartum period
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