3,223 research outputs found
A Population Density Model of the Driven LGN/PGN
The interaction of two populations of integrate-and-fire-or-burst neurons representing thalamocortical cells from the dorsal lateral geniculate nucleus (dLGN) and thalamic reticular cells from the perigeniculate nucleus (PGN) is studied using a population density approach. A two-dimensional probability density function that evolves according to a time-dependent advection-reaction equation gives the distribution of cells in each population over the membrane potential and de-inactivation level of a low-threshold calcium current. In the absence of retinal drive, the population density network model exhibits rhythmic bursting. In the presence of constant retinal input, the aroused LGN/PGN population density model displays a wide range of responses depending on cellular parameters and network connectivity.https://scholarworks.wm.edu/asbookchapters/1130/thumbnail.jp
Ca2+ Alternans in a Cardiac Myocyte Model that Uses Moment Equations to Represent Heterogeneous Junctional SR Ca2+
AbstractMultiscale whole-cell models that accurately represent local control of Ca2+-induced Ca2+ release in cardiac myocytes can reproduce high-gain Ca2+ release that is graded with changes in membrane potential. Using a recently introduced formalism that represents heterogeneous local Ca2+ using moment equations, we present a model of cardiac myocyte Ca2+ cycling that exhibits alternating sarcoplasmic reticulum (SR) Ca2+ release when periodically stimulated by depolarizing voltage pulses. The model predicts that the distribution of junctional SR [Ca2+] across a large population of Ca2+ release units is distinct on alternating cycles. Load-release and release-uptake functions computed from this model give insight into how Ca2+ fluxes and stimulation frequency combine to determine the presence or absence of Ca2+ alternans. Our results show that the conditions for the onset of Ca2+ alternans cannot be explained solely by the steepness of the load-release function, but that changes in the release-uptake process also play an important role. We analyze the effect of the junctional SR refilling time constant on Ca2+ alternans and conclude that physiologically realistic models of defective Ca2+ cycling must represent the dynamics of heterogeneous junctional SR [Ca2+] without assuming rapid equilibration of junctional and network SR [Ca2+]
Proximity effect-assisted absorption of spin currents in superconductors
The injection of pure spin current into superconductors by the dynamics of a
ferromagnetic contact is studied theoretically. Taking into account suppression
of the order parameter at the interfaces (inverse proximity effect) and the
energy-dependence of spin-flip scattering, we determine the
temperature-dependent ferromagnetic resonance linewidth broadening. Our results
agree with recent experiments in Nb|permalloy bilayers [C. Bell et al.,
arXiv:cond-mat/0702461].Comment: 4 page
A robust morphological classification of high-redshift galaxies using support vector machines on seeing limited images. II. Quantifying morphological k-correction in the COSMOS field at 1<z<2: Ks band vs. I band
We quantify the effects of \emph{morphological k-correction} at by
comparing morphologies measured in the K and I-bands in the COSMOS area.
Ks-band data have indeed the advantage of probing old stellar populations for
, enabling a determination of galaxy morphological types unaffected by
recent star formation. In paper I we presented a new non-parametric method to
quantify morphologies of galaxies on seeing limited images based on support
vector machines. Here we use this method to classify
selected galaxies in the COSMOS area observed with WIRCam at CFHT. The obtained
classification is used to investigate the redshift distributions and number
counts per morphological type up to and to compare to the results
obtained with HST/ACS in the I-band on the same objects from other works. We
associate to every galaxy with and a probability between 0 and
1 of being late-type or early-type. The classification is found to be reliable
up to . The mean probability is . It decreases with redshift
and with size, especially for the early-type population but remains above
. The classification is globally in good agreement with the one
obtained using HST/ACS for . Above , the I-band classification
tends to find less early-type galaxies than the Ks-band one by a factor
1.5 which might be a consequence of morphological k-correction effects.
We argue therefore that studies based on I-band HST/ACS classifications at
could be underestimating the elliptical population. [abridged]Comment: accepted for publication in A&A, updated with referee comments, 12
pages, 10 figure
UBQLN4 Represses Homologous Recombination and Is Overexpressed in Aggressive Tumors
Genomic instability can be a hallmark of both human genetic disease and cancer. We identify a deleterious UBQLN4 mutation in families with an autosomal recessive syndrome reminiscent of genome instability disorders. UBQLN4 deficiency leads to increased sensitivity to genotoxic stress and delayed DNA double-strand break (DSB) repair. The proteasomal shuttle factor UBQLN4 is phosphorylated by ATM and interacts with ubiquitylated MRE11 to mediate early steps of homologous recombination-mediated DSB repair (HRR). Loss of UBQLN4 leads to chromatin retention of MRE11, promoting non-physiological HRR activity in vitro and in vivo. Conversely, UBQLN4 overexpression represses HRR and favors non-homologous end joining. Moreover, we find UBQLN4 overexpressed in aggressive tumors. In line with an HRR defect in these tumors, UBQLN4 overexpression is associated with PARP1 inhibitor sensitivity. UBQLN4 therefore curtails HRR activity through removal of MRE11 from damaged chromatin and thus offers a therapeutic window for PARP1 inhibitor treatment in UBQLN4-overexpressing tumors
Dynamic exchange coupling and Gilbert damping in magnetic multilayers
We theoretically study dynamic properties of thin ferromagnetic films in
contact with normal metals. Moving magnetizations cause a flow of spins into
adjacent conductors, which relax by spin flip, scatter back into the
ferromagnet, or are absorbed by another ferromagnet. Relaxation of spins
outside the moving magnetization enhances the overall damping of the
magnetization dynamics in accordance with the Gilbert phenomenology. Transfer
of spins between different ferromagnets by these nonequilibrium spin currents
leads to a long-ranged dynamic exchange interaction and novel collective
excitation modes. Our predictions agree well with recent
ferromagnetic-resonance experiments on ultrathin magnetic films.Comment: 15 pages, 3 figures, for MMM'02 proceeding
Spin-dependent boundary conditions for isotropic superconducting Green's functions
The quasiclassical theory of superconductivity provides the most successful
description of diffusive heterostructures comprising superconducting elements,
namely, the Usadel equations for isotropic Green's functions. Since the
quasiclassical and isotropic approximations break down close to interfaces, the
Usadel equations have to be supplemented with boundary conditions for isotropic
Green's functions (BCIGF), which are not derivable within the quasiclassical
description. For a long time, the BCIGF were available only for spin-degenerate
tunnel contacts, which posed a serious limitation on the applicability of the
Usadel description to modern structures containing ferromagnetic elements. In
this article, we close this gap and derive spin-dependent BCIGF for a contact
encompassing superconducting and ferromagnetic correlations. This finally
justifies several simplified versions of the spin-dependent BCIGF, which have
been used in the literature so far. In the general case, our BCIGF are valid as
soon as the quasiclassical isotropic approximation can be performed. However,
their use require the knowledge of the full scattering matrix of the contact,
an information usually not available for realistic interfaces. In the case of a
weakly polarized tunnel interface, the BCIGF can be expressed in terms of a few
parameters, i.e. the tunnel conductance of the interface and five
conductance-like parameters accounting for the spin-dependence of the interface
scattering amplitudes. In the case of a contact with a ferromagnetic insulator,
it is possible to find explicit BCIGF also for stronger polarizations. The
BCIGF derived in this article are sufficienly general to describe a variety of
physical situations and may serve as a basis for modelling realistic
nanostructures.Comment: This paper presents an improvement of arXiv:cond-mat/0204116. The
present version takes into account corrections from the erratum Phys. Rev. B
83, 139901 (2011
An ultraviolet spectral study of fullerene-rich planetary nebulae
Several planetary nebulae (PNe) have been found to contain both polycyclic
aromatic hydrocarbon (PAH-like) species and fullerenes (C) distinguished
by their mid-infrared emission. Previous laboratory and astronomical studies
suggest that the formation of both species could be related to the
decomposition, by photochemical processing, of hydrogenated amorphous carbon
(HAC) grains. Then, HACs and, seemingly, big-fullerene related species (e.g.,
carbon onions) have been suggested as potential carriers of the UV bump at
2175{\AA} and the far-UV rise common to interstellar extinction curves. Our
goal is to investigate the UV bump with the possible presence of a HAC
extinction component in the International Ultraviolet Explorer (IUE) spectra of
C-rich PNe; both with detected and non-detected fullerenes. The considered
sample includes three C-PNe (Tc 1, IC 418, and IC 2501) and the
non-C-PN Hen 2-5. Independently of the presence of C in their
circumstellar envelopes, we found that the UV bump in all sample PNe is well
explained by interstellar extinction, suggesting that species different from
those of the foreground insterstellar medium, e.g., large fullerene-related
species like carbon onions, are not the carrier. Interestingly, we found that
PNe Tc 1 and Hen 2-5 show an absorption in the FUV rise. Their IUE continuum
spectra may be very well reproduced by including the extinction curve of
HAC-like very small grains (VSG). The possible presence of both species,
HAC-like grains and fullerenes (C), in Tc 1 could support the HAC
photochemical processing scenario for the formation of fullerenes in the
complex circumstellar envelopes of PNe.Comment: 9 pages, 8 figures. Accepted for publication in MNRA
Hydrogenated amorphous carbon grains as an alternative carrier of the 913m plateau feature in the fullerene planetary nebula Tc 1
Fullerenes have been observed in several astronomical objects since the
discovery of C in the mid-infrared (mid-IR) spectrum of the planetary
nebula (PN) Tc 1. It has been suggested that the carriers of the broad
unidentified infrared (UIR) plateau features, such as the 913m emission
feature (12m hereafter), may be related to the formation of fullerenes. In
particular, their carriers have been suggested to be mixed aromatic or
aliphatic hydrocarbons such as hydrogenated amorphous carbon (HAC-like
hereafter) grains. For this study, we modeled the mid-IR emission of the
C-PN Tc 1 with a photoionization code, including for the first time the
laboratory optical constants ( and indices) of HAC-like dust at 300 K.
Interestingly, we find that the broad 12m plateau feature in Tc 1 is well
reproduced by using a distribution of canonical HAC grains, while at the same
time they provide an important fraction of the IR dust continuum emission and
are consistent with the other UIR features observed (e.g., the broad
69m plateau feature). This finding suggests that HAC-like grains may be
possible carriers of the 12m plateau feature, being likely related to the
fullerene formation mechanism in PNe. More laboratory experiments, to obtain
the optical constants of HAC-like dust with several structures or a composition
at different physical conditions, are strongly encouraged -- that is, in order
to extend this pilot study to more fullerene PNe, and to unveil the details of
fullerene formation and of the potential carriers of the elusive UIR plateau
features.Comment: Published in Astronomy and Astrophysics Letters. 7 pages, 3 figure
Engineering applications using CAD based application programming interface
Automating the design process of a product or a system can provide engineers and designers with many benefits. As such, repeatable tasks that are time consuming can be handled automatically and with minimal human attention. This is achieved by using the API (Application Programmable Interface) of CAD systems in order to create macros or even develop software applications. The present paper deals with an application that has been developed with the API of a general purposes CAD system. This application automates the design process of a standard pneumatic double acting cylinder based on the appropriate inserted parameters (ISO 15552).The design process begins with the creation of a series of components developed as solids, and extends to the extraction of basic attributes and properties from the complete mechanical assembly. Finally, the assembled models and the extracted data can be used to further study the design of the pneumatic double acting cylinder. It is expected in the future to expand the features of the presented application in order to automate the design process of other related mechanical systems
- …