34,187 research outputs found
Vanishing of Gravitational Particle Production in the Formation of Cosmic Strings
We consider the gravitationally induced particle production from the quantum
vacuum which is defined by a free, massless and minimally coupled scalar field
during the formation of a gauge cosmic string. Previous discussions of this
topic estimate the power output per unit length along the string to be of the
order of ergs/sec/cm in the s-channel. We find that this production
may be completely suppressed. A similar result is also expected to hold for the
number of produced photons.Comment: 10 pages, Plain LaTex. Minor improvements. To appear in PR
Phonon-induced quadrupolar ordering of the magnetic superconductor TmNiBC
We present synchrotron x-ray diffraction studies revealing that the lattice
of thulium borocarbide is distorted below T_Q = 13.5 K at zero field. T_Q
increases and the amplitude of the displacements is drastically enhanced, by a
factor of 10 at 60 kOe, when a magnetic field is applied along [100]. The
distortion occurs at the same wave vector as the antiferromagnetic ordering
induced by the a-axis field. A model is presented that accounts for the
properties of the quadrupolar phase and explains the peculiar behavior of the
antiferromagnetic ordering previously observed in this compound.Comment: submitted to PR
High order Fuchsian equations for the square lattice Ising model:
This paper deals with , the six-particle contribution to
the magnetic susceptibility of the square lattice Ising model. We have
generated, modulo a prime, series coefficients for . The
length of the series is sufficient to produce the corresponding Fuchsian linear
differential equation (modulo a prime). We obtain the Fuchsian linear
differential equation that annihilates the "depleted" series
. The factorization of the corresponding differential
operator is performed using a method of factorization modulo a prime introduced
in a previous paper. The "depleted" differential operator is shown to have a
structure similar to the corresponding operator for . It
splits into factors of smaller orders, with the left-most factor of order six
being equivalent to the symmetric fifth power of the linear differential
operator corresponding to the elliptic integral . The right-most factor has
a direct sum structure, and using series calculated modulo several primes, all
the factors in the direct sum have been reconstructed in exact arithmetics.Comment: 23 page
First-principles study of the energetics of charge and cation mixing in U_{1-x} Ce_x O_2
The formalism of electronic density-functional-theory, with Hubbard-U
corrections (DFT+U), is employed in a computational study of the energetics of
U_{1-x} Ce_x O_2 mixtures. The computational approach makes use of a procedure
which facilitates convergence of the calculations to multiple self-consistent
DFT+U solutions for a given cation arrangement, corresponding to different
charge states for the U and Ce ions in several prototypical cation
arrangements. Results indicate a significant dependence of the structural and
energetic properties on the nature of both charge and cation ordering. With the
effective Hubbard-U parameters that reproduce well the measured
oxidation-reduction energies for urania and ceria, we find that charge transfer
between U(IV) and Ce(IV) ions, leading to the formation of U(V) and Ce(III),
gives rise to an increase in the mixing energy in the range of 4-14 kJ/mol of
formula unit, depending on the nature of the cation ordering. The results
suggest that although charge transfer between uranium and cerium ions is
disfavored energetically, it is likely to be entropically stabilized at the
high temperatures relevant to the processing and service of urania-based solid
solutions.Comment: 8 pages, 6 figure
Quantum Geometry and Diffusion
We study the diffusion equation in two-dimensional quantum gravity, and show
that the spectral dimension is two despite the fact that the intrinsic
Hausdorff dimension of the ensemble of two-dimensional geometries is very
different from two. We determine the scaling properties of the quantum gravity
averaged diffusion kernel.Comment: latex2e, 10 pages, 4 figure
Response of the large-scale subglacial drainage system of Northeast Greenland to surface elevation changes
The influence of subglacial water on the dynamics of ice flow has been the
object of increasing interest in the past decade. In this study we focus on
large-scale, long-term changes in surface elevation over Northeast
Greenland and the corresponding changes in subglacial water routeways. Our
results show that over timescales ranging from decades to millennia the area
may experience redistribution of and fluctuation in subglacial water outflux
under the main glacier outlets. The fluctuations in subglacial water routing
occur even in the absence of external forcing. Based on these results we
conclude that changes in the subglacial water routeways are an intrinsic part
of the drainage basin dynamics, where the subglacial system is likely always
in a transient state. The results also imply that fluctuations at the margins
observed at present might originate from changes several hundred kilometres
upstream. Since surface elevation changes may propagate upstream over
timescales much longer than the observational period, the cause of the
fluctuations may not be present in current observational records
Perimeter Generating Functions For The Mean-Squared Radius Of Gyration Of Convex Polygons
We have derived long series expansions for the perimeter generating functions
of the radius of gyration of various polygons with a convexity constraint.
Using the series we numerically find simple (algebraic) exact solutions for the
generating functions. In all cases the size exponent .Comment: 8 pages, 1 figur
The Tully-Fisher relation of intermediate redshift field and cluster galaxies from Subaru spectroscopy
We have carried out spectroscopic observations in 4 cluster fields using
Subaru's FOCAS multi-slit spectrograph and obtained spectra for 103 bright disk
field and cluster galaxies at . Seventy-seven of these
show emission lines, and 33 provide reasonably-secure determinations of the
galaxies' rotation velocity. The rotation velocities, luminosities, colours and
emission-line properties of these galaxies are used to study the possible
effects of the cluster environment on the star-formation history of the
galaxies. Comparing the Tully-Fisher relations of cluster and field galaxies at
similar reshifts we find no measurable difference in rest-frame -band
luminosity at a given rotation velocity (the formal difference is mag). The colours of the cluster emission line galaxies are only marginally
redder in rest-frame (by mag) than the field galaxies in
our sample. Taken at face value, these results seem to indicate that bright
star-forming cluster spirals are similar to their field counterparts in their
star-formation properties. However, we find that the fraction of disk galaxies
with absorption-line spectra (i.e., with no current star formation) is larger
in clusters than in the field by a factor of --5. This suggests that the
cluster environment has the overall effect of switching off star formation in
(at least) some spiral galaxies. To interpret these observational results, we
carry out simulations of the possible effects of the cluster environment on the
star-formation history of disk galaxies and thus their photometric and
spectroscopic properties. Finally, we evaluate the evolution of the rest-frame
absolute -band magnitude per unit redshift at fixed rotation velocity.Comment: 21 pages, 13 figures, accepted for publication in MNRA
A new transfer-matrix algorithm for exact enumerations: Self-avoiding polygons on the square lattice
We present a new and more efficient implementation of transfer-matrix methods
for exact enumerations of lattice objects. The new method is illustrated by an
application to the enumeration of self-avoiding polygons on the square lattice.
A detailed comparison with the previous best algorithm shows significant
improvement in the running time of the algorithm. The new algorithm is used to
extend the enumeration of polygons to length 130 from the previous record of
110.Comment: 17 pages, 8 figures, IoP style file
Self-avoiding walks and polygons on the triangular lattice
We use new algorithms, based on the finite lattice method of series
expansion, to extend the enumeration of self-avoiding walks and polygons on the
triangular lattice to length 40 and 60, respectively. For self-avoiding walks
to length 40 we also calculate series for the metric properties of mean-square
end-to-end distance, mean-square radius of gyration and the mean-square
distance of a monomer from the end points. For self-avoiding polygons to length
58 we calculate series for the mean-square radius of gyration and the first 10
moments of the area. Analysis of the series yields accurate estimates for the
connective constant of triangular self-avoiding walks, ,
and confirms to a high degree of accuracy several theoretical predictions for
universal critical exponents and amplitude combinations.Comment: 24 pages, 6 figure
- …