1,356 research outputs found
Nonadiabatic quantum pumping in mesoscopic nanostructures
We consider a nonadiabatic quantum pumping phenomena in a ballistic narrow
constriction. The pumping is induced by a potential that has both spatial and
temporal periodicity characterized by and . In the zero frequency
() limit, the transmission through narrow constriction exhibits
valley structures due to the opening up of energy gaps in the pumping region --
a consequence of the periodicity. These valley structures remain robust in
the regime of finite , while their energies of occurrence are shifted
by about . The direction of these energy shifts depend on the
directions of both the phase-velocity of the pumping potential and the
transmitting electrons. This frequency dependent feature of the valley
structures gives rise to both the asymmetry in the transmission coefficients
and the pumping current. An experimental setup is suggested for a possible
observation of our nonadiabatic quantum pumping findings.Comment: 4 pages, 2 figure
Differential conductance of a saddle-point constriction with a time-modulated gate-voltage
The effect of a time-modulated gate-voltage on the differential conductance
of a saddle-point constriction is studied. The constriction is modeled by a
symmetric saddle-point potential and the time-modulated gate-voltage is
represented by a potential of the form . For less than half of the transverse subband energy
level spacing, gate-voltage-assisted (suppressed) feature occurs when the
chemical potential is less (greater) than but close to the threshold
energy of a subband. As increases, is found to exhibit,
alternatively, the assisted and the suppressed feature. For larger
, these two features may overlap with one another. Dip structures
are found in the suppressed regime. Mini-steps are found in the assisted regime
only when the gate-voltage covers region far enough away from the center of the
constriction.Comment: 8 pages, 6 figure
International collaboration in science: The global map and the network
The network of international co-authorship relations has been dominated by
certain European nations and the USA, but this network is rapidly expanding at
the global level. Between 40 and 50 countries appear in the center of the
international network in 2011, and almost all (201) nations are nowadays
involved in international collaboration. In this brief communication, we
present both a global map with the functionality of a Google Map (zooming,
etc.) and network maps based on normalized relations. These maps reveal
complementary aspects of the network. International collaboration in the
generation of knowledge claims (that is, the context of discovery) changes the
structural layering of the sciences. Previously, validation was at the global
level and discovery more dependent on local contexts. This changing
relationship between the geographical and intellectual dimensions of the
sciences also has implications for national science policies.Comment: El Profesional de la Informaci\'on (2013, in press
Measure of National Return in International Science Cooperation
In the decades since science and technology measures were crafted and adopted by governments, R&D has increasingly taken place across national boundaries. This leaves a gap for policy makers in how to account for the benefits to national governments of supporting international collaboration in science. This article seeks to address this gap by suggesting a measure for the impact of international collaboration in science using fractionalized field-weighted citations and analysing these in relationship to public spending and researcher mobility
Transport spectroscopy in a time-modulated open quantum dot
We have investigated the time-modulated coherent quantum transport phenomena
in a ballistic open quantum dot. The conductance and the electron dwell
time in the dots are calculated by a time-dependent mode-matching method. Under
high-frequency modulation, the traversing electrons are found to exhibit three
types of resonant scatterings. They are intersideband scatterings: into
quasibound states in the dots, into true bound states in the dots, and into
quasibound states just beneath the subband threshold in the leads. Dip
structures or fano structures in are their signatures. Our results show
structures due to 2 intersideband processes. At the above
scattering resonances, we have estimated, according to our dwell time
calculation, the number of round-trip scatterings that the traversing electrons
undertake between the two dot openings.Comment: 8 pages, 5 figure
Coherent quantum transport in narrow constrictions in the presence of a finite-range longitudinally polarized time-dependent field
We have studied the quantum transport in a narrow constriction acted upon by
a finite-range longitudinally polarized time-dependent electric field. The
electric field induces coherent inelastic scatterings which involve both
intra-subband and inter-sideband transitions. Subsequently, the dc conductance
G is found to exhibit suppressed features. These features are recognized as the
quasi-bound-state (QBS) features which are associated with electrons making
transitions to the vicinity of a subband bottom, of which the density of states
is singular. Having valley-like instead of dip-like structures, these QBS
features are different from the G characteristics for constrictions acted upon
by a finite-range time-modulated potential. In addition, the subband bottoms in
the time-dependent electric field region are shifted upward by an energy
proportional to the square of the electric field and inversely proportional to
the square of the frequency. This effective potential barrier is originated
from the square of the vector potential and it leads to the interesting
field-sensitive QBS features. An experimental set-up is proposed for the
observation of these features.Comment: 8 pages, 4 figure
Discovery and Observations of ASASSN-13db, an EX Lupi-Type Accretion Event on a Low-Mass T Tauri Star
We discuss ASASSN-13db, an EX Lupi-type ("EXor") accretion event on the young
stellar object (YSO) SDSS J051011.01032826.2 (hereafter SDSSJ0510)
discovered by the All-Sky Automated Survey for SuperNovae (ASAS-SN). Using
archival photometric data of SDSSJ0510 we construct a pre-outburst spectral
energy distribution (SED) and find that it is consistent with a low-mass class
II YSO near the Orion star forming region ( pc). We present
follow-up photometric and spectroscopic observations of the source after the
5.4 magnitude outburst that began in September 2013 and ended
in early 2014. These data indicate an increase in temperature and luminosity
consistent with an accretion rate of yr,
three or more orders of magnitude greater than in quiescence. Spectroscopic
observations show a forest of narrow emission lines dominated by neutral
metallic lines from Fe I and some low-ionization lines. The properties of
ASASSN-13db are similar to those of the EXor prototype EX Lupi during its
strongest observed outburst in late 2008.Comment: 14 pages, 4 figures, 1 table. Updated May 2014 to reflect changes in
the final version published in ApJL. Photometric data presented in this
submission are included as ancillary files. For a brief video explaining this
paper, see http://youtu.be/yRCCrNJnvt
Early activation of the interleukin-23-17 axis in a murine model of oropharyngeal candidiasis
P>Candida albicans is an oral commensal yeast that causes oropharyngeal candidiasis (OPC) in immunocompromised individuals. The immunological pathways involved in OPC have been revisited after the interleukin-17 (IL-17) pathway was implicated in fungal immunity. We studied immediate (< 24 h) and adaptive (3-6 day) IL-12 and IL-23-17 pathway activation in naive p40-/- mice, which lack IL-12 and IL-23 and develop severe, chronic OPC upon oral inoculation with C. albicans. Macrophages from p40-/- mice were less efficient than C57BL/6J controls at killing C. albicans in vitro but very low numbers in the oral mucosae of infected C57BL/6J mice suggest that they are not critical in vivo, at least in this strain. Migration of macrophages to regional lymph nodes of infected p40-/- mice was impaired; however, dendritic cell migration was not affected. Recombinant IL-12 therapy provided only temporary relief from OPC, suggesting that IL-23 is required for full protection. In C57BL/6J mice, but not p40-/- mice, messenger RNAs encoding IL-23p19 and IL-17 were induced in the oral mucosa within 24 h of infection (6 +/- 0.6 and 12 +/- 2.7-fold). By day 6 of infection in C57BL/6J mice, IL-17A messenger RNA level had increased 5.1 +/- 1.8 and 83 +/- 21-fold in regional lymph nodes and oral tissues respectively. Ablation of p40 was associated with delayed or abrogated induction of IL-17A pathway targets (monocyte chemoattractant protein-1, IL-6 and macrophage inflammatory protein-2), and a lack of organized recruitment of neutrophils to the infected oral mucosa. Overall our data show that the IL-23-17A axis is activated early in the oral mucosae of immunologically naive mice with OPC
Optical and X-ray correlations during the 2015 outburst of the black hole V404 Cyg
We present a serendipitous multiwavelength campaign of optical photometry simultaneous with Integral X-ray monitoring of the 2015 outburst of the black hole V404 Cyg. Large-amplitude optical variability is generally correlated with X-rays, with lags of order a minute or less compatible with binary light travel time-scales or jet ejections. Rapid optical flaring on time-scales of seconds or less is incompatible with binary light-travel time-scales and has instead been associated with synchrotron emission from a jet. Both this rapid jet response and the lagged and smeared one can be present simultaneously. The optical brightness is not uniquely determined by the X-ray brightness, but the X-ray/optical relationship is bounded by a lower envelope such that at any given optical brightness there is a maximum X-ray brightness seen. This lower envelope traces out a Fopt∝F0.54X relation that can be approximately extrapolated back to quiescence. Rapid optical variability is only seen near this envelope, and these periods correspond to the hardest hard X-ray colours. This correlation between hard X-ray colour and optical variability (and anticorrelation with optical brightness) is a novel finding of this campaign, and apparently a facet of the outburst behaviour in V404 Cyg. It is likely that these correlations are driven by changes in the central accretion rate and geometry
Particle acceleration in three-dimensional tearing configurations
In three-dimensional electromagnetic configurations that result from unstable
resistive tearing modes particles can efficiently be accelerated to
relativistic energies. To prove this resistive magnetohydrodynamic simulations
are used as input configurations for successive test particle simulations. The
simulations show the capability of three-dimensional non-linearly evolved
tearing modes to accelerate particles perpendicular to the plane of the
reconnecting magnetic field components. The simulations differ considerably
from analytical approaches by involving a realistic three-dimensional electric
field with a non-homogenous component parallel to the current direction. The
resulting particle spectra exhibit strong pitch-angle anisotropies. Typically,
about 5-8 % of an initially Maxwellian distribution is accelerated to the
maximum energy levels given by the macroscopic generalized electric potential
structure. Results are shown for both, non-relativistic particle acceleration
that is of interest, e.g., in the context of auroral arcs and solar flares, and
relativistic particle energization that is relevant, e.g., in the context of
active galactic nuclei.Comment: Physics of Plasmas, in prin
- …