27,644 research outputs found
The cosmological constant and the relaxed universe
We study the role of the cosmological constant (CC) as a component of dark
energy (DE). It is argued that the cosmological term is in general unavoidable
and it should not be ignored even when dynamical DE sources are considered.
From the theoretical point of view quantum zero-point energy and phase
transitions suggest a CC of large magnitude in contrast to its tiny observed
value. Simply relieving this disaccord with a counterterm requires extreme
fine-tuning which is referred to as the old CC problem. To avoid it, we discuss
some recent approaches for neutralising a large CC dynamically without adding a
fine-tuned counterterm. This can be realised by an effective DE component which
relaxes the cosmic expansion by counteracting the effect of the large CC.
Alternatively, a CC filter is constructed by modifying gravity to make it
insensitive to vacuum energy.Comment: 6 pages, no figures, based on a talk presented at PASCOS 201
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The effect of network topology on optimal exploration strategies and the evolution of cooperation in a mobile population
We model a mobile population interacting over an underlying spatial structure using a Markov movement model. Interactions take the form of public goods games, and can feature an arbitrary group size. Individuals choose strategically to remain at their current location or to move to a neighbouring location, depending upon their exploration strategy and the current composition of their group. This builds upon previous work where the underlying structure was a complete graph (i.e. there was effectively no structure). Here, we consider alternative network structures and a wider variety of, mainly larger, populations. Previously, we had found when cooperation could evolve, depending upon the values of a range of population parameters. In our current work, we see that the complete graph considered before promotes stability, with populations of cooperators or defectors being relatively hard to replace. By contrast, the star graph promotes instability, and often neither type of population can resist replacement. We discuss potential reasons for this in terms of network topology
Low-Energy Structures in Strong Field Ionization Revealed by Quantum Orbits
Experiments on atoms in intense laser pulses and the corresponding exact ab
initio solutions of the time-dependent Schr\"odinger equation (TDSE) yield
photoelectron spectra with low-energy features that are not reproduced by the
otherwise successful work horse of strong field laser physics: the "strong
field approximation" (SFA). In the semi-classical limit, the SFA possesses an
appealing interpretation in terms of interfering quantum trajectories. It is
shown that a conceptually simple extension towards the inclusion of Coulomb
effects yields very good agreement with exact TDSE results. Moreover, the
Coulomb quantum orbits allow for a physically intuitive interpretation and
detailed analysis of all low-energy features in the semi-classical regime, in
particular the recently discovered "low-energy structure" [C.I. Blaga et al.,
Nature Physics 5, 335 (2009) and W. Quan et al., Phys. Rev. Lett. 103, 093001
(2009)].Comment: 4 pages, 3 figures, REVTe
Soft-Collinear Messengers: A New Mode in Soft-Collinear Effective Theory
It is argued that soft-collinear effective theory for processes involving
both soft and collinear partons, such as exclusive B-meson decays, should
include a new mode in addition to soft and collinear fields. These
"soft-collinear messengers" can interact with both soft and collinear particles
without taking them far off-shell. They thus can communicate between the soft
and collinear sectors of the theory. The relevance of the new mode is
demonstrated with an explicit example, and the formalism incorporating the
corresponding quark and gluon fields into the effective Lagrangian is
developed.Comment: 22 pages, 5 figures. Extended Section 6, clarifying the relevance of
different types of soft-collinear interaction
Non-equilibrium spin accumulation in ferromagnetic single-electron transistors
We study transport in ferromagnetic single-electron transistors. The non-
equilibrium spin accumulation on the island caused by a finite current through
the system is described by a generalized theory of the Coulomb blockade. It
enhances the tunnel magnetoresistance and has a drastic effect on the time-
dependent transport properties. A transient decay of the spin accumulation may
reverse the electric current on time scales of the order of the spin-flip
relaxation time. This can be used as an experimental signature of the non-
equilibrium spin accumulation.Comment: 9 postscript figures, to appear in The European Physical Journal
A proof of factorization for B -> D pi
We prove that the matrix elements of four fermion operators mediating the
decay B^0 -> D^+ \pi^- and B^- -> D^0 \pi^- factor into the product of a form
factor describing the B -> D transition and a convolution of a short distance
coefficient with the nonperturbative pion light-cone wave function. This is
shown to all orders in alpha_s, up to corrections suppressed by factors of
1/mb, 1/mc, and 1/E_pi. It is not necessary to assume that the pion state is
dominated by the q-qbar Fock state.Comment: 4 pages, 3 figs, PRL versio
The Euphrosyne family's contribution to the low albedo near-Earth asteroids
The Euphrosyne asteroid family is uniquely situated at high inclination in
the outer Main Belt, bisected by the nu_6 secular resonance. This large, low
albedo family may thus be an important contributor to specific subpopulations
of the near-Earth objects. We present simulations of the orbital evolution of
Euphrosyne family members from the time of breakup to the present day, focusing
on those members that move into near-Earth orbits. We find that family members
typically evolve into a specific region of orbital element-space, with
semimajor axes near ~3 AU, high inclinations, very large eccentricities, and
Tisserand parameters similar to Jupiter family comets. Filtering all known NEOs
with our derived orbital element limits, we find that the population of
candidate objects is significantly lower in albedo than the overall NEO
population, although many of our candidates are also darker than the Euphrosyne
family, and may have properties more similar to comet nuclei. Followup
characterization of these candidates will enable us to compare them to known
family properties, and confirm which ones originated with the breakup of (31)
Euphrosyne.Comment: Accepted for publication in Ap
Interface resistance of disordered magnetic multilayers
We study the effect of interface disorder on the spin-dependent interface
resistances of Co/Cu, Fe/Cr and Au/Ag multilayers using a newly developed
method for calculating transmission matrices from first-principles. The
efficient implementation using tight-binding linear-muffin-tin orbitals allows
us to model interface disorder using large lateral supercells whereby specular
and diffuse scattering are treated on an equal footing. Without introducing any
free parameters, quantitative agreement with experiment is obtained. We predict
that disorder {\it reduces} the majority-spin interface resistance of
Fe/Cr(100) multilayers by a factor 3.Comment: 5 pages, 2 figures, submitted to PR
Two-channel point-contact tunneling theory of superconductors
We introduce a two-channel tunneling model to generalize the widely used BTK
theory of point-contact conductance between a normal metal contact and
superconductor. Tunneling of electrons can occur via localized surface states
or directly, resulting in a Fano resonance in the differential conductance
. We present an analysis of within the two-channel model when
applied to soft point-contacts between normal metallic silver particles and
prototypical heavy-fermion superconductors CeCoIn and CeRhIn at high
pressures. In the normal state the Fano line shape of the measured is well
described by a model with two tunneling channels and a large
temperature-independent background conductance. In the superconducting state a
strongly suppressed Andreev reflection signal is explained by the presence of
the background conductance. We report Andreev signal in CeCoIn consistent
with standard -wave pairing, assuming an equal mixture of
tunneling into [100] and [110] crystallographic interfaces. Whereas in
CeRhIn at 1.8 and 2.0 GPa the signal is described by a -wave
gap with reduced nodal region, i.e., increased slope of the gap opening on the
Fermi surface. A possibility is that the shape of the high-pressure Andreev
signal is affected by the proximity of a line of quantum critical points that
extends from 1.75 to 2.3 GPa, which is not accounted for in our description of
the heavy-fermion superconductor.Comment: 13 pages, 13 figure
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