27,544 research outputs found
Collective Diffusion and a Random Energy Landscape
Starting from a master equation in a quantum Hamiltonian form and a coupling
to a heat bath we derive an evolution equation for a collective hopping process
under the influence of a stochastic energy landscape. There results different
equations in case of an arbitrary occupation number per lattice site or in a
system under exclusion. Based on scaling arguments it will be demonstrated that
both systems belong below the critical dimension to the same universality
class leading to anomalous diffusion in the long time limit. The dynamical
exponent can be calculated by an expansion. Above the
critical dimension we discuss the differences in the diffusion constant for
sufficient high temperatures. For a random potential we find a higher mobility
for systems with exclusion.Comment: 15 pages, no figure
Two harmonically coupled Brownian particles in random media
We study the behaviour of two Brownian particles coupled by an elastic
harmonic force in a quenched disordered medium. We found that to first order in
disorder strength, the relative motion weakens (with respect to the reference
state of a Brownian particle with the double mass) the effect of the quenched
forces on the centre of mass motion of the Brownian particles, so that the
motion will become less subdiffusive (superdiffusive) for potential
(solenoidal) disorder. The mean-square relative distance between the particles
behaves in a different way depending of whether the particles are free to move
or one particle is anchored in the space. While the effect of nonpotential
disorder consists in increasing the mean-square distance in both cases, the
potential disorder decreases the mean-square distance, when the particles are
free to move, and increases it when one particle is anchored in the space.Comment: 8 pages, 3 figure
Differential Emission Measure Determination of Collisionally Ionized Plasma: II. Application to Hot Stars
In a previous paper we have described a technique to derive constraints on
the differential emission measure (DEM) distribution, a measure of the
temperature distribution, of collisionally ionized hot plasmas from their X-ray
emission line spectra. We apply this technique to the Chandra/HETG spectra of
all of the nine hot stars available to us at the time this project was
initiated. We find that DEM distributions of six of the seven O stars in our
sample are very similar but that theta Ori has an X-ray spectrum characterized
by higher temperatures. The DEM distributions of both of B stars in our sample
have lower magnitudes than those of the O stars and one, tau Sco, is
characterized by higher temperatures than the other, beta Cru. These results
confirm previous work in which high temperatures have been found for theta Ori
and tau Sco and taken as evidence for channeling of the wind in magnetic
fields, the existence of which are related to the stars' youth. Our results
demonstrate the utility of our method for deriving temperature information for
large samples of X-ray emission line spectra.Comment: The contents of this paper were formerly part of astro-ph/0403603
which was split into two paper
Differential effects of age on involuntary and voluntary autobiographical memory
"This article may not exactly replicate the final version published in the APA journal. It is not the copy of record." Original article can be found at: http://psycnet.apa.org/journals/pag/24/2/397/ Copyright American Psychological Association. DOI: 10.1037/a0015785Research on aging and autobiographical memory has focused almost exclusively on voluntary autobiographical memory. However, in everyday life, autobiographical memories often come to mind spontaneously without deliberate attempt to retrieve anything. In the present study, diary and word-cue methods were used to compare the involuntary and voluntary memories of 44 young and 38 older adults. The results showed that older adults reported fewer involuntary and voluntary memories than did younger adults. Additionally, the life span distribution of involuntary and voluntary memories did not differ in young adults (a clear recency effect) or in older adults (a recency effect and a reminiscence bump). Despite these similarities between involuntary and voluntary memories, there were also important differences in terms of the effects of age on some memory characteristics. Thus, older adults’ voluntary memories were less specific and were recalled more slowly than those of young adults, but there were no reliable age differences in the specificity of involuntary memories. Moreover, older adults rated their involuntary memories as more positive than did young adults, but this positivity effect was not found for voluntary memories. Theoretical implications of these findings for research on autobiographical memory and cognitive aging are discussedPeer reviewe
Systematic search for high-energy gamma-ray emission from bow shocks of runaway stars
Context. It has been suggested that the bow shocks of runaway stars are
sources of high-energy gamma rays (E > 100 MeV). Theoretical models predicting
high-energy gamma-ray emission from these sources were followed by the first
detection of non-thermal radio emission from the bow shock of BD+43^\deg 3654
and non-thermal X-ray emission from the bow shock of AE Aurigae. Aims. We
perform the first systematic search for MeV and GeV emission from 27 bow shocks
of runaway stars using data collected by the Large Area Telescope (LAT) onboard
the Fermi Gamma-ray Space Telescope (Fermi). Methods. We analysed 57 months of
Fermi-LAT data at the positions of 27 bow shocks of runaway stars extracted
from the Extensive stellar BOw Shock Survey catalogue (E-BOSS). A likelihood
analysis was performed to search for gamma-ray emission that is not compatible
with diffuse background or emission from neighbouring sources and that could be
associated with the bow shocks. Results. None of the bow shock candidates is
detected significantly in the Fermi-LAT energy range. We therefore present
upper limits on the high-energy emission in the energy range from 100 MeV to
300 GeV for 27 bow shocks of runaway stars in four energy bands. For the three
cases where models of the high-energy emission are published we compare our
upper limits to the modelled spectra. Our limits exclude the model predictions
for Zeta Ophiuchi by a factor 5.Comment: 5 pages, 5 figures, 1 table, accepted by A&
Universal behavior of the IMS domain formation in superconducting niobium
In the intermediate mixed state (IMS) of type-II/1 superconductors, vortex
lattice (VL) and Meissner state domains coexist due to a partially attractive
vortex interaction. Using a neutron-based multiscale approach combined with
magnetization measurements, we study the continuous decomposition of a
homogeneous VL into increasingly dense domains in the IMS in bulk niobium
samples of varying purity. We find a universal temperature dependence of the
vortex spacing, closely related to the London penetration depth and independent
of the external magnetic field. The rearrangement of vortices occurs even in
the presence of a flux freezing transition, i.e. pronounced pinning, indicating
a breakdown of pinning at the onset of the vortex attraction
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