16,905 research outputs found
Fluorescence intermittency in blinking quantum dots: renewal or slow modulation?
We study time series produced by the blinking quantum dots, by means of an
aging experiment, and we examine the results of this experiment in the light of
two distinct approaches to complexity, renewal and slow modulation. We find
that the renewal approach fits the result of the aging experiment, while the
slow modulation perspective does not. We make also an attempt at establishing
the existence of an intermediate condition.Comment: 27 pages, 8 figures. Accepted for pubblication on Journal of Chemical
Physic
Social Support and the Perception of Geographical Slant.
The visual perception of geographical slant is influenced by physiological resources, such as physical fitness, age, and being physically refreshed. In two studies we tested whether a psychosocial resource, social support, can also affect the visual perception of slants. Participants accompanied by a friend estimated a hill to be less steep when compared to participants who were alone (Study 1). Similarly, participants who thought of a supportive friend during an imagery task saw a hill as less steep than participants who either thought of a neutral person or a disliked person (Study 2). In both studies, the effects of social relationships on visual perception appear to be mediated by relationship quality (i.e., relationship duration, interpersonal closeness, warmth). Artifacts such as mood, social desirability, and social facilitation did not account for these effects. This research demonstrates that an interpersonal phenomenon, social support, can influence visual perception
Towards the graviton from spinfoams: higher order corrections in the 3d toy model
We consider the recent calculation gr-qc/0508124 of the graviton propagator
in the spinfoam formalism. Within the 3d toy model introduced in gr-qc/0512102,
we test how the spinfoam formalism can be used to construct the perturbative
expansion of graviton amplitudes. Although the 3d graviton is a pure gauge, one
can choose to work in a gauge where it is not zero and thus reproduce the
structure of the 4d perturbative calculations. We compute explicitly the next
to leading and next to next to leading orders, corresponding to one-loop and
two-loop corrections. We show that while the first arises entirely from the
expansion of the Regge action around the flat background, the latter receives
contributions from the microscopic, non Regge-like, quantum geometry.
Surprisingly, this new contribution reduces the magnitude of the next to next
to leading order. It thus appears that the spinfoam formalism is likely to
substantially modify the conventional perturbative expansion at higher orders.
This result supports the interest in this approach. We then address a number
of open issues in the rest of the paper. First, we discuss the boundary state
ansatz, which is a key ingredient in the whole construction. We propose a way
to enhance the ansatz in order to make the edge lengths and dihedral angles
conjugate variables in a mathematically well-defined way. Second, we show that
the leading order is stable against different choices of the face weights of
the spinfoam model; the next to leading order, on the other hand, is changed in
a simple way, and we show that the topological face weight minimizes it.
Finally, we extend the leading order result to the case of a regular, but not
equilateral, tetrahedron.Comment: 24 pages, many figure
First-principles investigation of Ag-Cu alloy surfaces in an oxidizing environment
In this paper we investigate by means of first-principles density functional
theory calculations the (111) surface of the Ag-Cu alloy under varying
conditions of pressure of the surrounding oxygen atmosphere and temperature.
This alloy has been recently proposed as a catalyst with improved selectivity
for ethylene epoxidation with respect to pure silver, the catalyst commonly
used in industrial applications. Here we show that the presence of oxygen leads
to copper segregation to the surface. Considering the surface free energy as a
function of the surface composition, we construct the convex hull to
investigate the stability of various surface structures. By including the
dependence of the free surface energy on the oxygen chemical potential, we are
able compute the phase diagram of the alloy as a function of temperature,
pressure and surface composition. We find that, at temperature and pressure
typically used in ethylene epoxidation, a number of structures can be present
on the surface of the alloy, including clean Ag(111), thin layers of copper
oxide and thick oxide-like structures. These results are consistent with, and
help explain, recent experimental results.Comment: 10 pages, 6 figure
Measured performance of the new University of California gamma ray telescope
The design of the new medium energy balloon-borne gamma ray telescope is discussed. This telescope is sensitive to 1-30 MeV gamma rays. The results of the initial calibration are described. The position and energy resolutions of 32 plastic and NaI(Tl) scintillator bars, each 100 cm long are discussed. The telescope's measured angular and energy resolutions as a function of incident angle are compared with detailed Monte Carlo calculations at 1.37, 2.75 and 6.13 MeV. The expected resolutions are 5 deg FHWM and 8% at 2.75 MeV. The expected area-efficiency is 250 cm
Anatomía comparada de los fascioláridos Pustulatirus ogum y Hemipolygona beckyae de Brasil (Gastropoda: Buccinoidea: Peristerniinae)
The Brazilian species Pustulatirus ogum and Hemipolygona beckyae were examined, and certain morphological characters were described. Both species were originally assigned to the genus Latirus, considered as a heterogeneous complex. The radulae of both species are like that which characterizes Latirus, in which the innermost cusp of the rachidian tooth is well developed but always smaller than the other cusps. This feature differs from Leucozonia, in which this cusp is reduced or absent. The penis tapers terminally, and the tapered part may be long (more than half the total penis length), as in H. beckyae, or very short (less than half the total penis length), as in P. ogum. The anatomical data observed in both species are discussed under the framework of fasciolariid systematics and they appear to be widespread among other fasciolariid species. For this reason, to date, the soft-part features here provided and those known from previously studied species of Latirus are not useful for delineating precise generic diagnoses.Se describe la morfología y anatomía de las especies brasileñas Pustulatirus ogum y Hemipolygona beckyae. Ambas especies han sido tradicionalmente asignadas al género Latirus, que en la actualidad se considera que agrupa a un conjunto heterogéneo de especies. Las rádulas de ambas especies son como la que caracteriza a Latirus, con la cúspide más interna del diente raquídeo bien desarrollada y siempre menor que las otras cúspides. Esta característica difiere de la que presentan las especies del género Leucozonia, en las que esta cúspide está reducida o ausente. El pene se estrecha en su parte terminal, y la parte cónica puede ser larga (más de la mitad de la longitud total del pene), como en H. beckyae, o muy corta (menos de la mitad de la longitud total del pene), como en P. ogum. Los caracteres morfológicos observados en ambas especies se discuten en el marco de la sistemática de los Fasciolariidae y parecen estar ampliamente distribuidos en otras especies de fascioláridos. Por ello, hasta la fecha, las características de las partes blandas del animal aquí descritas y las conocidas previamente de otras especies de Latirus no se consideran de utilidad para la diagnosis de los géneros en esta familia
Rangeland Ecosystem Services: Improving Decisions with a Systematic Approach
Delivering ecosystem services from rangelands represents a unique challenge. While social, ecological and economic complexity and diversity often lend stability to rangeland systems, the broad array of services, users and connections makes the process of identifying what services to manage for, which management practices are most effective and how to deliver them challenging. In addition, lag times between management changes and responses, climatic variability and changes in demand can further complicate decision-making. We propose a structured process that includes: (1) inventory of existing conditions; (2) identification of relevant scenarios; (3) stakeholder involvement; and (4) monitoring for verification based on the unique nature of rangelands as complex socio-ecological systems. Our objectives are to improve the quality of management planning and implementation by land managers, better inform the policies and programs that assist managers and to enhance the credibility of delivery systems. The goal of this approach is to improve sustainability by expanding the mix of ecosystem services rangelands can deliver and stabilizing income to support people who depend on rangelands
Theory of continuum percolation II. Mean field theory
I use a previously introduced mapping between the continuum percolation model
and the Potts fluid to derive a mean field theory of continuum percolation
systems. This is done by introducing a new variational principle, the basis of
which has to be taken, for now, as heuristic. The critical exponents obtained
are , and , which are identical with the mean
field exponents of lattice percolation. The critical density in this
approximation is \rho_c = 1/\ve where \ve = \int d \x \, p(\x) \{ \exp [-
v(\x)/kT] - 1 \}. p(\x) is the binding probability of two particles
separated by \x and v(\x) is their interaction potential.Comment: 25 pages, Late
The Hyperfine Splitting in Charmonium: Lattice Computations Using the Wilson and Clover Fermion Actions
We compute the hyperfine splitting on the lattice,
using both the Wilson and -improved (clover) actions for quenched quarks.
The computations are performed on a lattice at ,
using the same set of 18 gluon configurations for both fermion actions. We find
that the splitting is 1.83\err{13}{15} times larger with the clover action than
with the Wilson action, demonstrating the sensitivity of the spin-splitting to
the magnetic moment term which is present in the clover action. However, even
with the clover action the result is less than half of the physical
mass-splitting. We also compute the decay constants and
, both of which are considerably larger when computed using
the clover action than with the Wilson action. For example for the ratio
we find 0.32\err{1}{2} with the Wilson action
and with the clover action (the physical value is 0.44(2)).Comment: LaTeX file, 8 pages and two postscript figures. Southampton Preprint:
SHEP 91/92-27 Edinburgh Preprint: 92/51
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