4,241 research outputs found
Current measurement by real-time counting of single electrons
The fact that electrical current is carried by individual charges has been
known for over 100 years, yet this discreteness has not been directly observed
so far. Almost all current measurements involve measuring the voltage drop
across a resistor, using Ohm's law, in which the discrete nature of charge does
not come into play. However, by sending a direct current through a
microelectronic circuit with a chain of islands connected by small tunnel
junctions, the individual electrons can be observed one by one. The quantum
mechanical tunnelling of single charges in this one-dimensional array is time
correlated, and consequently the detected signal has the average frequency
f=I/e, where I is the current and e is the electron charge. Here we report a
direct observation of these time-correlated single-electron tunnelling
oscillations, and show electron counting in the range 5 fA-1 pA. This
represents a fundamentally new way to measure extremely small currents, without
offset or drift. Moreover, our current measurement, which is based on electron
counting, is self-calibrated, as the measured frequency is related to the
current only by a natural constant.Comment: 9 pages, 4 figures; v2: minor revisions, 2 refs added, words added to
title, typos correcte
Random attractors for degenerate stochastic partial differential equations
We prove the existence of random attractors for a large class of degenerate
stochastic partial differential equations (SPDE) perturbed by joint additive
Wiener noise and real, linear multiplicative Brownian noise, assuming only the
standard assumptions of the variational approach to SPDE with compact
embeddings in the associated Gelfand triple. This allows spatially much rougher
noise than in known results. The approach is based on a construction of
strictly stationary solutions to related strongly monotone SPDE. Applications
include stochastic generalized porous media equations, stochastic generalized
degenerate p-Laplace equations and stochastic reaction diffusion equations. For
perturbed, degenerate p-Laplace equations we prove that the deterministic,
infinite dimensional attractor collapses to a single random point if enough
noise is added.Comment: 34 pages; The final publication is available at
http://link.springer.com/article/10.1007%2Fs10884-013-9294-
Gender trends in authorships and publication impact in Academic Radiology—a 10-year perspective
Objectives: To analyze the development of publication numbers of female authors in high-, medium-, and low-impact radiological journals.
Methods: In this bibliometric analysis, gender of the first (FA) and senior author (SA) was assigned to all original research articles and reviews, published in 10 high-, medium-, and low-impact radiological journals in 2007/8 and 2017/18. The adjusted event rate (AER) and adjusted odds ratio (AOR) were calculated using mixed logistic and multinomial logistic regression models to assess and compare female publications according to impact factor, journal, author position, and combination.
Results: The proportion of female FA and female SA in N = 6979 (2007/2008) and N = 7383 (2017/2018) articles increased to 29.1% and 16.1% in 2017/2018, respectively. While most female authorships were continuously observed in medium-impact journals, the strongest increase occurred for both female FA (AOR 2.0; p < .0001) and SA (AOR 2.1; p < .0001) in low-impact journals. Female SA published significantly more often in a low- (AOR 1.5) or medium- (AOR 1.8) than in a high-ranking journal. Among the high-ranking journals, female FA published most frequently in European Radiology (32.4%; 95% CI [29.3-35.8]; p < .0001), female SA in Investigative Radiology (15.9%; 95% CI [13.7-18.4]; p < .0001). Male same-sex authorships decreased (AOR 0.9), but remained at least twice as common as all-female or mixed authorships.
Conclusion: The increase in female authorship is reflected in all impact areas. Female FA and SA increased most in low-ranking journals but are most common in medium-ranking journals. Female SA remain rare, especially in high impact journals
Applying scale-free mass estimators to the Local Group in Constrained Local Universe Simulations
We use the recently proposed scale-free mass estimators to determine the
masses of the Milky Way (MW) and Andromeda (M31) galaxy in a dark matter only
Constrained Local UniversE Simulation (CLUES). While these mass estimators work
rather well for isolated spherical host systems, we examine here their
applicability to a simulated binary system with a unique satellite population
similar to the observed satellites of MW and M31. We confirm that the
scale-free estimators work also very well in our simulated Local Group galaxies
with the right number of satellites which follow the observed radial
distribution. In the isotropic case and under the assumption that the
satellites are tracking the total gravitating mass, the power-law index of the
radial satellite distribution is directly related
to the host's mass profile as .
The use of this relation for any given leads to highly accurate mass
estimations which is a crucial point for observer, since they do not know a
priori the mass profile of the MW and M31 haloes. We discuss possible bias in
the mass estimators and conclude that the scale-free mass estimators can be
satisfactorily applied to the real MW and M31 system.Comment: 14 pages, 6 figures, 6 tables. Accepted in MNRAS 2012 March 29.
Received 2012 March 29; in original form 2011 September 2
Coarsening Dynamics of Domains in Lipid Membranes
We investigate isothermal diffusion and growth of micron-scale liquid domains within membranes of free-floating giant unilamellar vesicles with diameters between 80 and 250 Am. Domains appear after a rapid temperature quench, when the membrane is cooled through a miscibility phase transition such that coexisting liquid phases form. In membranes quenched far from a miscibility critical point, circular domains nucleate and then progress within seconds to late stage coarsening in which domains grow via two mechanisms 1), collision and coalescence of liquid domains, and 2), Ostwald ripening. Both mechanisms are expected to yield the same growth exponent, alpha = 1/3, where domain radius grows as time(alpha). We measure alpha = 0.28 +/- 0.05, in excellent agreement. In membranes close to a miscibility critical point, the two liquid phases in the membrane are bicontinuous. A quench near the critical composition results in rapid changes in morphology of elongated domains. In this case, we measure alpha = 0.50 +/- 0.16, consistent with theory and simulation
The stellar content of the Hamburg/ESO survey VI. The metallicity distribution of main-sequence turnoff stars in the Galactic halo
We determine the metallicity distribution function (MDF) of the Galactic halo
based on metal-poor main-sequence turnoff-stars (MSTO) which were selected from
the Hamburg/ESO objective-prism survey (HES) database. Corresponding follow-up
moderateresolution observations (R ~ 2000) of some 682 stars (among which 617
were accepted program stars) were carried out with the 2.3m telescope at the
Siding Spring Observatory (SSO). Corrections for the survey volume covered by
the sample stars were quantitatively estimated and applied to the observed MDF.
The corrections are quite small, when compared with those for a previously
studied sample of metal-poor giants. The corrected observational MDF of the
turnoff sample was then compared with that of the giants, as well as with a
number of theoretical predictions of Galactic chemical evolution, including the
mass-loss modified Simple Model. Although the survey-volume corrected MDFs of
the metal-poor turnoff and the halo giants notably differ in the region of
[Fe/H] > -2.0, below [Fe/H] ~ -2.0, (the region we scientifically focus on
most) both MDFs show a sharp drop at [Fe/H] ~ -3.6 and present rather similar
distributions in the low-metallicity tail. Theoretical models can fit some
parts of the observed MDF, but none is found to simultaneously reproduce the
peak as well as the features in the metal-poor region with [Fe/H] between -2.0
to -3.6. Among the tested models only the GAMETE model, when normalized to the
tail of the observed MDF below [Fe/H] ~ -3.0, and with Z_{cr} =
10^{-3.4}Z_{\odot}, is able to predict the sharp drop at [Fe/H] ~ -3.6.Comment: 10 pages, 11 figures, accepted for publication in A&
Energy- and flux-budget (EFB) turbulence closure model for the stably stratified flows. Part I: Steady-state, homogeneous regimes
We propose a new turbulence closure model based on the budget equations for
the key second moments: turbulent kinetic and potential energies: TKE and TPE
(comprising the turbulent total energy: TTE = TKE + TPE) and vertical turbulent
fluxes of momentum and buoyancy (proportional to potential temperature).
Besides the concept of TTE, we take into account the non-gradient correction to
the traditional buoyancy flux formulation. The proposed model grants the
existence of turbulence at any gradient Richardson number, Ri. Instead of its
critical value separating - as usually assumed - the turbulent and the laminar
regimes, it reveals a transition interval, 0.1< Ri <1, which separates two
regimes of essentially different nature but both turbulent: strong turbulence
at Ri<<1; and weak turbulence, capable of transporting momentum but much less
efficient in transporting heat, at Ri>1. Predictions from this model are
consistent with available data from atmospheric and lab experiments, direct
numerical simulation (DNS) and large-eddy simulation (LES).Comment: 40 pages, 6 figures, Boundary-layer Meteorology, resubmitted, revised
versio
Light Curve Templates and Galactic Distribution of RR Lyrae Stars from Sloan Digital Sky Survey Stripe 82
We present an improved analysis of halo substructure traced by RR Lyrae stars
in the SDSS stripe 82 region. With the addition of SDSS-II data, a revised
selection method based on new ugriz light curve templates results in a sample
of 483 RR Lyrae stars that is essentially free of contamination. The main
result from our first study persists: the spatial distribution of halo stars at
galactocentric distances 5--100 kpc is highly inhomogeneous. At least 20% of
halo stars within 30 kpc from the Galactic center can be statistically
associated with substructure. We present strong direct evidence, based on both
RR Lyrae stars and main sequence stars, that the halo stellar number density
profile significantly steepens beyond a Galactocentric distance of ~30 kpc, and
a larger fraction of the stars are associated with substructure. By using a
novel method that simultaneously combines data for RR Lyrae and main sequence
stars, and using photometric metallicity estimates for main sequence stars
derived from deep co-added u-band data, we measure the metallicity of the
Sagittarius dSph tidal stream (trailing arm) towards R.A.2h-3h and Dec~0 deg to
be 0.3 dex higher ([Fe/H]=-1.2) than that of surrounding halo field stars.
Together with a similar result for another major halo substructure, the
Monoceros stream, these results support theoretical predictions that an early
forming, smooth inner halo, is metal poor compared to high surface brightness
material that have been accreted onto a later-forming outer halo. The mean
metallicity of stars in the outer halo that are not associated with detectable
clumps may still be more metal-poor than the bulk of inner-halo stars, as has
been argued from other data sets.Comment: Submitted to ApJ, 68 pages, 26 figures, supplemental material (light
curves, templates, animation) can be downloaded from
http://www.astro.washington.edu/bsesar/S82_RRLyr.htm
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