8,923 research outputs found
The evaluation of Education Maintenance Allowance Pilots: three years' evidence: a quantitative evaluation
This is the third report of the longitudinal quantitative evaluation of Education Maintenance Allowance (EMA) pilots and the first since the government announced that EMA is to be rolled out nationally from 2004. The evaluation was commissioned in 1999, by the Department for Education and Skills (DfES) from a consortium of research organisations, led by the Centre for Research in Social Policy (CRSP) and including the National Centre for Social Research, the Institute for Fiscal Studies (IFS) and the National Institute for Careers Education and Counselling (NICEC).
The statistical evaluation design is a longitudinal cohort study involving large random sample surveys of young people (and their parents) in 10 EMA pilot areas and eleven control areas. Two cohorts of young people were selected from Child Benefit records. The first cohort of young people left compulsory schooling in the summer of 1999 and they, and their parents, were interviewed between October 1999 and April 2000 (Year 12 interview). A second interview was carried out with these young people between October 2000 and April 2001 (Year 13 interview). The second cohort left compulsory education the following summer of 2000 and young people, and their parents, were first interviewed between October 2000 and April 2001.
The report uses both propensity score matching (PSM) and descriptive techniques, each of which brings their own particular strengths to the analysis
Full capacitance matrix of coupled quantum dot arrays: static and dynamical effects
We numerically calculated the full capacitance matrices for both
one-dimensional (1D) and two-dimensional (2D) quantum-dot arrays. We found it
is necessary to use the full capacitance matrix in modeling coupled quantum dot
arrays due to weaker screening in these systems in comparison with arrays of
normal metal tunnel junctions. The static soliton potential distributions in
both 1D and 2D arrays are well approximated by the unscreened (1/r) coulomb
potential, instead of the exponential fall-off expected from the often used
nearest neighbor approximation. The Coulomb potential approximation also
provides a simple expression for the full inverse capacitance matrix of uniform
quantum dot arrays. In terms of dynamics, we compare the current-voltage (I-V)
characteristics of voltage biased 1D arrays using either the full capacitance
matrix or its nearest neighbor approximation. The I-V curves show clear
differences and the differences become more pronounced when larger arrays are
considered.Comment: 8 pages preprint format, 3 PostScript figure
Percolation of satisfiability in finite dimensions
The satisfiability and optimization of finite-dimensional Boolean formulas
are studied using percolation theory, rare region arguments, and boundary
effects. In contrast with mean-field results, there is no satisfiability
transition, though there is a logical connectivity transition. In part of the
disconnected phase, rare regions lead to a divergent running time for
optimization algorithms. The thermodynamic ground state for the NP-hard
two-dimensional maximum-satisfiability problem is typically unique. These
results have implications for the computational study of disordered materials.Comment: 4 pages, 4 fig
Dislocations in the ground state of the solid-on-solid model on a disordered substrate
We investigate the effects of topological defects (dislocations) to the
ground state of the solid-on-solid (SOS) model on a simple cubic disordered
substrate utilizing the min-cost-flow algorithm from combinatorial
optimization. The dislocations are found to destabilize and destroy the elastic
phase, particularly when the defects are placed only in partially optimized
positions. For multi defect pairs their density decreases exponentially with
the vortex core energy. Their mean distance has a maximum depending on the
vortex core energy and system size, which gives a fractal dimension of . The maximal mean distances correspond to special vortex core
energies for which the scaling behavior of the density of dislocations change
from a pure exponential decay to a stretched one. Furthermore, an extra
introduced vortex pair is screened due to the disorder-induced defects and its
energy is linear in the vortex core energy.Comment: 6 pages RevTeX, eps figures include
Measuring functional renormalization group fixed-point functions for pinned manifolds
Exact numerical minimization of interface energies is used to test the
functional renormalization group (FRG) analysis for interfaces pinned by
quenched disorder. The fixed-point function R(u) (the correlator of the
coarse-grained disorder) is computed. In dimensions D=d+1, a linear cusp in
R''(u) is confirmed for random bond (d=1,2,3), random field (d=0,2,3), and
periodic (d=2,3) disorders. The functional shocks that lead to this cusp are
seen. Small, but significant, deviations from 1-loop FRG results are compared
to 2-loop corrections. The cross-correlation for two copies of disorder is
compared with a recent FRG study of chaos.Comment: 4 pages, 4 figure
Simulation of the Zero Temperature Behavior of a 3-Dimensional Elastic Medium
We have performed numerical simulation of a 3-dimensional elastic medium,
with scalar displacements, subject to quenched disorder. We applied an
efficient combinatorial optimization algorithm to generate exact ground states
for an interface representation. Our results indicate that this Bragg glass is
characterized by power law divergences in the structure factor . We have found numerically consistent values of the coefficient for
two lattice discretizations of the medium, supporting universality for in
the isotropic systems considered here. We also examine the response of the
ground state to the change in boundary conditions that corresponds to
introducing a single dislocation loop encircling the system. Our results
indicate that the domain walls formed by this change are highly convoluted,
with a fractal dimension . We also discuss the implications of the
domain wall energetics for the stability of the Bragg glass phase. As in other
disordered systems, perturbations of relative strength introduce a new
length scale beyond which the perturbed ground
state becomes uncorrelated with the reference (unperturbed) ground state. We
have performed scaling analysis of the response of the ground state to the
perturbations and obtain . This value is consistent with the
scaling relation , where characterizes the
scaling of the energy fluctuations of low energy excitations.Comment: 20 pages, 13 figure
Einstein Gravity on a Brane in 5D Non-compact Flat Spacetime -DGP model revisited-
We revisit the 5D gravity model by Dvali, Gabadadze, and Porrati (DGP).
Within their framework it was shown that even in 5D non-compact Minkowski space
, the Newtonian gravity can emerge on a brane at short distances by
introducing a brane-localized 4D Einstein-Hilbert term
in the action. Based on this idea,
we construct simple setups in which graviton standing waves can arise, and we
introduce brane-localized derivative terms as a correction to
. We show that the gravity potential
of brane matter becomes at {\it long} distances, because the
brane-localized derivative terms allow only a smooth graviton wave function
near the brane. Since the bulk gravity coupling may be arbitrarily small,
strongly interacting modes from the 5D graviton do not appear. We note that the
brane metric utilized to construct
can be relatively different from the bulk metric by a conformal factor, and
show that the graviton tensor structure that the 4D Einstein gravity predicts
are reproduced in DGP type models.Comment: 1+12 pages, no figure, to appear in JHE
Avalanches and the Renormalization Group for Pinned Charge-Density Waves
The critical behavior of charge-density waves (CDWs) in the pinned phase is
studied for applied fields increasing toward the threshold field, using
recently developed renormalization group techniques and simulations of
automaton models. Despite the existence of many metastable states in the pinned
state of the CDW, the renormalization group treatment can be used successfully
to find the divergences in the polarization and the correlation length, and, to
first order in an expansion, the diverging time scale. The
automaton models studied are a charge-density wave model and a ``sandpile''
model with periodic boundary conditions; these models are found to have the
same critical behavior, associated with diverging avalanche sizes. The
numerical results for the polarization and the diverging length and time scales
in dimensions are in agreement with the analytical treatment. These
results clarify the connections between the behaviour above and below
threshold: the characteristic correlation lengths on both sides of the
transition diverge with different exponents. The scaling of the distribution of
avalanches on the approach to threshold is found to be different for automaton
and continuous-variable models.Comment: 29 pages, 11 postscript figures included, REVTEX v3.0 (dvi and PS
files also available by anonymous ftp from external.nj.nec.com in directory
/pub/alan/cdwfigs
Critical slowing down in polynomial time algorithms
Combinatorial optimization algorithms which compute exact ground state
configurations in disordered magnets are seen to exhibit critical slowing down
at zero temperature phase transitions. Using arguments based on the physical
picture of the model, including vanishing stiffness on scales beyond the
correlation length and the ground state degeneracy, the number of operations
carried out by one such algorithm, the push-relabel algorithm for the random
field Ising model, can be estimated. Some scaling can also be predicted for the
2D spin glass.Comment: 4 pp., 3 fig
Computational Complexity of Determining the Barriers to Interface Motion in Random Systems
The low-temperature driven or thermally activated motion of several condensed
matter systems is often modeled by the dynamics of interfaces (co-dimension-1
elastic manifolds) subject to a random potential. Two characteristic
quantitative features of the energy landscape of such a many-degree-of-freedom
system are the ground-state energy and the magnitude of the energy barriers
between given configurations. While the numerical determination of the former
can be accomplished in time polynomial in the system size, it is shown here
that the problem of determining the latter quantity is NP-complete. Exact
computation of barriers is therefore (almost certainly) much more difficult
than determining the exact ground states of interfaces.Comment: 8 pages, figures included, to appear in Phys. Rev.
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