58,442 research outputs found
Low-density series expansions for directed percolation III. Some two-dimensional lattices
We use very efficient algorithms to calculate low-density series for bond and
site percolation on the directed triangular, honeycomb, kagom\'e, and
lattices. Analysis of the series yields accurate estimates of the critical
point and various critical exponents. The exponent estimates differ only
in the digit, thus providing strong numerical evidence for the
expected universality of the critical exponents for directed percolation
problems. In addition we also study the non-physical singularities of the
series.Comment: 20 pages, 8 figure
Absence of orbital-selective Mott transition in Ca_2-xSr_xRuO4
Quasi-particle spectra of the layer perovskite SrRuO are calculated
within Dynamical Mean Field Theory for increasing values of the on-site Coulomb
energy . At small the planar geometry splits the bands near
into a wide, two-dimensional band and two narrow, nearly
one-dimensional bands. At larger , however, the spectral
distribution of these states exhibit similar correlation features, suggesting a
common metal-insulator transition for all bands at the same critical
.Comment: 4 pages, 4 figure
Low-density series expansions for directed percolation II: The square lattice with a wall
A new algorithm for the derivation of low-density expansions has been used to
greatly extend the series for moments of the pair-connectedness on the directed
square lattice near an impenetrable wall. Analysis of the series yields very
accurate estimates for the critical point and exponents. In particular, the
estimate for the exponent characterizing the average cluster length near the
wall, , appears to exclude the conjecture . The
critical point and the exponents and have the
same values as for the bulk problem.Comment: 8 pages, 1 figur
1/z-renormalization of the mean-field behavior of the dipole-coupled singlet-singlet system HoF_3
The two main characteristics of the holmium ions in HoF_3 are that their
local electronic properties are dominated by two singlet states lying well
below the remaining 4f-levels, and that the classical dipole-coupling is an
order of magnitude larger than any other two-ion interactions between the
Ho-moments. This combination makes the system particularly suitable for testing
refinements of the mean-field theory. There are four Ho-ions per unit cell and
the hyperfine coupled electronic and nuclear moments on the Ho-ions order in a
ferrimagnetic structure at T_C=0.53 K. The corrections to the mean-field
behavior of holmium triflouride, both in the paramagnetic and ferrimagnetic
phase, have been calculated to first order in the high-density 1/z-expansion.
The effective medium theory, which includes the effects of the single-site
fluctuations, leads to a substantially improved description of the magnetic
properties of HoF_3, in comparison with that based on the mean-field
approximation.Comment: 26pp, plain-TeX, JJ
Directed percolation near a wall
Series expansion methods are used to study directed bond percolation clusters
on the square lattice whose lateral growth is restricted by a wall parallel to
the growth direction. The percolation threshold is found to be the same
as that for the bulk. However the values of the critical exponents for the
percolation probability and mean cluster size are quite different from those
for the bulk and are estimated by and respectively. On the other hand the exponent
characterising the scale of the cluster size
distribution is found to be unchanged by the presence of the wall.
The parallel connectedness length, which is the scale for the cluster length
distribution, has an exponent which we estimate to be and is also unchanged. The exponent of the mean
cluster length is related to and by the scaling
relation and using the above estimates
yields to within the accuracy of our results. We conjecture that
this value of is exact and further support for the conjecture is
provided by the direct series expansion estimate .Comment: 12pages LaTeX, ioplppt.sty, to appear in J. Phys.
Computation of spectroscopic factors with the coupled-cluster method
We present a calculation of spectroscopic factors within coupled-cluster
theory. Our derivation of algebraic equations for the one-body overlap
functions are based on coupled-cluster equation-of-motion solutions for the
ground and excited states of the doubly magic nucleus with mass number and
the odd-mass neighbor with mass . As a proof-of-principle calculation, we
consider O and the odd neighbors O and N, and compute the
spectroscopic factor for nucleon removal from O. We employ a
renormalized low-momentum interaction of the type derived
from a chiral interaction at next-to-next-to-next-to-leading order. We study
the sensitivity of our results by variation of the momentum cutoff, and then
discuss the treatment of the center of mass.Comment: 8 pages, 6 figures, 3 table
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