2,047 research outputs found
Transverse fluctuations of grafted polymers
We study the statistical mechanics of grafted polymers of arbitrary stiffness
in a two-dimensional embedding space with Monte Carlo simulations. The
probability distribution function of the free end is found to be highly
anisotropic and non-Gaussian for typical semiflexible polymers. The reduced
distribution in the transverse direction, a Gaussian in the stiff and flexible
limits, shows a double peak structure at intermediate stiffnesses. We also
explore the response to a transverse force applied at the polymer free end. We
identify F-Actin as an ideal benchmark for the effects discussed.Comment: 10 pages, 4 figures, submitted to Physical Review
InAs nanowire hot-electron Josephson transistor
At a superconductor (S)-normal metal (N) junction pairing correlations can
"leak-out" into the N region. This proximity effect [1, 2] modifies the system
transport properties and can lead to supercurrent flow in SNS junctions [3].
Recent experimental works showed the potential of semiconductor nanowires (NWs)
as building blocks for nanometre-scale devices [4-7], also in combination with
superconducting elements [8-12]. Here, we demonstrate an InAs NW Josephson
transistor where supercurrent is controlled by hot-quasiparticle injection from
normal-metal electrodes. Operational principle is based on the modification of
NW electron-energy distribution [13-20] that can yield reduced dissipation and
high-switching speed. We shall argue that exploitation of this principle with
heterostructured semiconductor NWs opens the way to a host of
out-of-equilibrium hybrid-nanodevice concepts [7, 21].Comment: 6 pages, 6 color figure
In Vivo Determination of Fluctuating Forces during Endosome Trafficking Using a Combination of Active and Passive Microrheology
BACKGROUND: Regulation of intracellular trafficking is a central issue in cell biology. The forces acting on intracellular vesicles (endosomes) can be assessed in living cells by using a combination of active and passive microrheology. METHODOLOGY/PRINCIPAL FINDINGS: This dual approach is based on endosome labeling with magnetic nanoparticles. The resulting magnetic endosomes act both as probes that can be manipulated with external magnetic fields to infer the viscoelastic modulus of their surrounding microenvironment, and as biological vehicles that are trafficked along the microtubule network by means of forces generated by molecular motors. The intracellular viscoelastic modulus exhibits power law dependence with frequency, which is microtubule and actin-dependent. The mean square displacements of endosomes do not follow the predictions of the fluctuation-dissipation theorem, which offers evidence for active force generation. Microtubule disruption brings the intracellular medium closer to thermal equilibrium: active forces acting on the endosomes depend on microtubule-associated motors. The power spectra of these active forces, deduced through the use of a generalized Langevin equation, show a power law decrease with frequency and reveal an actin-dependent persistence of the force with time. Experimental spectra have been reproduced by a simple model consisting in a series of force steps power-law distributed in time. This model enlightens the role of the cytoskeleton dependent force exerted on endosomes to perform intracellular trafficking. CONCLUSIONS/SIGNIFICANCE: In this work, the influence of cytoskeleton components and molecular motors on intracellular viscoelasticity and transport is addressed. The use of an original probe, the magnetic endosome, allows retrieving the power spectrum of active forces on these organelles thanks to interrelated active and passive measures. Finally a computational model gives estimates of the force itself and hence of the number of the motors pulling on endosomes
The Milky Way's Circular Velocity Curve to 60 kpc and an Estimate of the Dark Matter Halo Mass from Kinematics of ~2400 SDSS Blue Horizontal Branch Stars
We derive new constraints on the mass of the Milky Way's dark matter halo,
based on a set of halo stars from SDSS as kinematic tracers. Our sample
comprises 2401 rigorously selected Blue Horizontal-Branch (BHB) halo stars
drawn from SDSS DR-6. To interpret these distributions, we compare them to
matched mock observations drawn from two different cosmological galaxy
formation simulations designed to resemble the Milky Way, which we presume to
have an appropriate orbital distribution of halo stars. We then determine which
value of brings the observed distribution into agreement with
the corresponding distributions from the simulations. This procedure results in
an estimate of the Milky Way's circular velocity curve to kpc, which
is found to be slightly falling from the adopted value of
at the Sun's location, and implies MM. The radial dependence of , derived in
statistically independent bins, is found to be consistent with the expectations
from an NFW dark matter halo with the established stellar mass components at
its center. If we assume an NFW halo profile of characteristic concentration
holds, we can use the observations to estimate the virial mass of the Milky
Way's dark matter halo, MM, which is lower than many previous estimates. This estimate
implies that nearly 40% of the baryons within the virial radius of the Milky
Way's dark matter halo reside in the stellar components of our Galaxy. A value
for M of only M also (re-)opens the
question of whether all of the Milky Way's satellite galaxies are on bound
orbits.Comment: 42 pages, 17 figures and 3 table. Accepted by AP
Identification of Residues in the Heme Domain of Soluble Guanylyl Cyclase that are Important for Basal and Stimulated Catalytic Activity
Nitric oxide signals through activation of soluble guanylyl cyclase (sGC), a heme-containing heterodimer. NO binds to the heme domain located in the N-terminal part of the β subunit of sGC resulting in increased production of cGMP in the catalytic domain located at the C-terminal part of sGC. Little is known about the mechanism by which the NO signaling is propagated from the receptor domain (heme domain) to the effector domain (catalytic domain), in particular events subsequent to the breakage of the bond between the heme iron and Histidine 105 (H105) of the β subunit. Our modeling of the heme-binding domain as well as previous homologous heme domain structures in different states point to two regions that could be critical for propagation of the NO activation signal. Structure-based mutational analysis of these regions revealed that residues T110 and R116 in the αF helix-β1 strand, and residues I41 and R40 in the αB-αC loop mediate propagation of activation between the heme domain and the catalytic domain. Biochemical analysis of these heme mutants allows refinement of the map of the residues that are critical for heme stability and propagation of the NO/YC-1 activation signal in sGC
Systematic theoretical study of the spin and orbital magnetic moments of 4d and 5d interfaces with Fe films
Results of ab initio calculations using the relativistic Local Spin Density
theory are presented for the magnetic moments of periodic 5d and 4d transition
metal interfaces with bcc Fe(001). In this systematic study we calculated the
layer-resolved spin and orbital magnetic moments over the entire series. For
the Fe/W(001) system, the Fe spin moment is reduced whilst its orbital moment
is strongly enhanced. In the W layers a spin moment is induced, which is
antiparallel to that of Fe in the first and fourth W layers but parallel to Fe
in the second and third W layers. The W orbital moment does not follow the spin
moment. It is aligned antiparallel to Fe in the first two W layers and changes
sign in the third and fourth W layers. Therefore, Hund's third rule is violated
in the first and third W layers, but not in the second and fourth W layers. The
trend in the spin and orbital moments over the 4d and 5d series for multilayers
is quite similar to previous impurity calculations. These observations strongly
suggest that these effects can be seen as a consequence of the hybridization
between 5d (4d) and Fe which is mostly due to band filling, and to a lesser
extent geometrical effects of either single impurity or interface
Field Blue Stragglers and Related Mass Transfer Issues
This chapter contains my impressions and perspectives about the current state
of knowledge about field blue stragglers (FBS) stars, drawn from an extensive
literature that I searched. I conclude my review of issues that attend FBS and
mass transfer, by a brief enumeration of a few mildly disquieting observational
facts.Comment: Chapter 4, in Ecology of Blue Straggler Stars, H.M.J. Boffin, G.
Carraro & G. Beccari (Eds), Astrophysics and Space Science Library, Springe
Many-Body Physics with Ultracold Gases
This article reviews recent experimental and theoretical progress on
many-body phenomena in dilute, ultracold gases. Its focus are effects beyond
standard weak-coupling descriptions, like the Mott-Hubbard-transition in
optical lattices, strongly interacting gases in one and two dimensions or
lowest Landau level physics in quasi two-dimensional gases in fast rotation.
Strong correlations in fermionic gases are discussed in optical lattices or
near Feshbach resonances in the BCS-BEC crossover.Comment: revised version, accepted for publication in Rev. Mod. Phy
Deep Broadband Observations of the Distant Gamma-ray Blazar PKS 1424+240
We present deep VERITAS observations of the blazar PKS 1424+240, along with
contemporaneous Fermi Large Area Telescope, Swift X-ray Telescope and Swift UV
Optical Telescope data between 2009 February 19 and 2013 June 8. This blazar
resides at a redshift of , displaying a significantly attenuated
gamma-ray flux above 100 GeV due to photon absorption via pair-production with
the extragalactic background light. We present more than 100 hours of VERITAS
observations from three years, a multiwavelength light curve and the
contemporaneous spectral energy distributions. The source shows a higher flux
of (2.1) ph ms above 120 GeV in 2009 and
2011 as compared to the flux measured in 2013, corresponding to
(1.02) ph ms above 120 GeV. The measured
differential very high energy (VHE; GeV) spectral indices are
3.80.3, 4.30.6 and 4.50.2 in 2009, 2011 and 2013,
respectively. No significant spectral change across the observation epochs is
detected. We find no evidence for variability at gamma-ray opacities of greater
than , where it is postulated that any variability would be small and
occur on longer than year timescales if hadronic cosmic-ray interactions with
extragalactic photon fields provide a secondary VHE photon flux. The data
cannot rule out such variability due to low statistics.Comment: ApJL accepted March 17, 201
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