45,413 research outputs found
Party membership and campaign activity in Britain: The impact of electoral performance
The article examines the impact of electoral results on party membership and activity. Previous studies have focused on the long-term effects of electoral success or failure, suggesting that they may produce a spiral of demobilization or mobilization. The article shows that the dramatic change of electoral fortunes experienced by British parties at the 1997 general election broke this spiral, with the outcome leading to significant changes in the health and activity of local parties. It is concluded that dramatic election results can have significant implications for party organization
Fractal Droplets in Two Dimensional Spin Glasses
The two-dimensional Edwards-Anderson model with Gaussian bond distribution is
investigated at T=0 with a numerical method. Droplet excitations are directly
observed. It turns out that the averaged volume of droplets is proportional to
l^D with D = 1.80(2) where l is the spanning length of droplets, revealing
their fractal nature. The exponent characterizing the l dependence of the
droplet excitation energy is estimated to be -0.42(4), clearly different from
the stiffness exponent for domain wall excitations.Comment: 4 pages 4 figure
Quantum critical phenomena of long-range interacting bosons in a time-dependent random potential
We study the superfluid-insulator transition of a particle-hole symmetric
system of long-range interacting bosons in a time-dependent random potential in
two dimensions, using the momentum-shell renormalization-group method. We find
a new stable fixed point with non-zero values of the parameters representing
the short- and long-range interactions and disorder when the interaction is
asymptotically logarithmic. This is contrasted to the non-random case with a
logarithmic interaction, where the transition is argued to be first-order, and
to the Coulomb interaction case, where either a first-order transition or
an XY-like transition is possible depending on the parameters. We propose that
our model may be relevant in studying the vortex liquid-vortex glass transition
of interacting vortex lines in point-disordered type-II superconductors.Comment: 10 pages, 3 figure
Integration of Technology in Math and Science Education â A Model for Teaching Elementary and Middle School Pre-Service Teachers
This paper describes the development and implementation of a course, Integration of Technology in Math and Science Education, to introduce elementary and middle school pre-service teachers to real technology skills that they can use in their future classrooms. Activities allowed the students to learn technology skills while using the Internet to enrich their content skills and share information with their fellow students. The course was designed to allow students to master a variety of technology skills, and see how these skills can be used appropriately in their future classrooms, while also increasing their comfort level to use the technology and reduce their resistance and anxiety to use it later in their real-time classrooms. During the class hands-on activities, the students became ïŹuent at using the Internet for enrichment and communication, and at developing strategies for using their new skills to present SOL-relevant lesson plans. Students enter this course with very little in the way of educational technology skills, but leave with a teaching toolbox ïŹlled with new skills
Distributions of gaps and end-to-end correlations in random transverse-field Ising spin chains
A previously introduced real space renormalization-group treatment of the
random transverse-field Ising spin chain is extended to provide detailed
information on the distribution of the energy gap and the end-to-end
correlation function for long chains with free boundary conditions. Numerical
data, using the mapping of the problem to free fermions, are found to be in
good agreement with the analytic finite size scaling predictions.Comment: 12 pages revtex, 10 figures, submitted to Phys. Rev.
Fluctuating loops and glassy dynamics of a pinned line in two dimensions
We represent the slow, glassy equilibrium dynamics of a line in a
two-dimensional random potential landscape as driven by an array of
asymptotically independent two-state systems, or loops, fluctuating on all
length scales. The assumption of independence enables a fairly complete
analytic description. We obtain good agreement with Monte Carlo simulations
when the free energy barriers separating the two sides of a loop of size L are
drawn from a distribution whose width and mean scale as L^(1/3), in agreement
with recent results for scaling of such barriers.Comment: 11 pages, 4 Postscript figure
Anomalous relaxation kinetics and charge density wave correlations in underdoped BaPb1-xBixO3
Superconductivity often emerges in proximity of other symmetry-breaking
ground states, such as antiferromagnetism or charge-density-wave (CDW) order.
However, the subtle inter-relation of these phases remains poorly understood,
and in some cases even the existence of short-range correlations for
superconducting compositions is uncertain. In such circumstances, ultrafast
experiments can provide new insights, by tracking the relaxation kinetics
following excitation at frequencies related to the broken symmetry state. Here,
we investigate the transient terahertz conductivity of BaPb1-xBixO3 - a
material for which superconductivity is adjacent to a competing CDW phase -
after optical excitation tuned to the CDW absorption band. In insulating BaBiO3
we observed an increase in conductivity and a subsequent relaxation, which are
consistent with quasiparticles injection across a rigid semiconducting gap. In
the doped compound BaPb0.72Bi0.28O3 (superconducting below Tc=7K), a similar
response was also found immediately above Tc. This observation evidences the
presence of a robust gap up to T=40 K, which is presumably associated with
short-range CDW correlations. A qualitatively different behaviour was observed
in the same material fo T>40 K. Here, the photo-conductivity was dominated by
an enhancement in carrier mobility at constant density, suggestive of melting
of the CDW correlations rather than excitation across an optical gap. The
relaxation displayed a temperature dependent, Arrhenius-like kinetics,
suggestive of the crossing of a free-energy barrier between two phases. These
results support the existence of short-range CDW correlations above Tc in
underdoped BaPb1-xBixO3, and provide new information on the dynamical interplay
between superconductivity and charge order.Comment: 19 pages, 4 figure
Absence of a Phase Transition in a Three--Dimensional Vortex Glass Model with Screening
We study the gauge glass model for the vortex glass transition in type--II
superconductors, including screening of the interaction between vortices.
{}From the size dependence of the domain wall energy we find that, in
two--dimensions, the transition is at both with and without screening but
the exponents are different in the two cases. In three-dimensions, we find that
screening destroys the finite temperature transition found earlier when
screening was neglected.Comment: 11 pages plus LaTeX with Revtex macros, 3 postscript figures,
uuencoded and compressse
Phase diagram of vortex matter in layered superconductors with random point pinning
We study the phase diagram of the superconducting vortex system in layered
high-temperature superconductors in the presence of a magnetic field
perpendicular to the layers and of random atomic scale point pinning centers.
We consider the highly anisotropic limit where the pancake vortices on
different layer are coupled only by their electromagnetic interaction. The free
energy of the vortex system is then represented as a Ramakrishnan-Yussouff free
energy functional of the time averaged vortex density. We numerically minimize
this functional and examine the properties of the resulting phases. We find
that, in the temperature () -- pinning strength () plane at constant
magnetic induction, the equilibrium phase at low and is a Bragg glass.
As one increases or a first order phase transition occurs to another
phase that we characterize as a pinned vortex liquid. The weakly pinned vortex
liquid obtained for high and small smoothly crosses over to the
strongly pinned vortex liquid as is decreased or increased -- we do not
find evidence for the existence, in thermodynamic equilibrium, of a distinct
vortex glass phase in the range of pinning parameters considered here. %cdr We
present results for the density correlation functions, the density and defect
distributions, and the local field distribution accessible via SR
experiments. These results are compared with those of existing theoretical,
numerical and experimental studies.Comment: 15 pages, including figures. Higher resolution files for Figs 3a and
11 available from author
Nodal Liquid Theory of the Pseudo-Gap Phase of High-Tc Superconductors
We introduce and study the nodal liquid, a novel zero-temperature quantum
phase obtained by quantum-disordering a d-wave superconductor. It has numerous
remarkable properties which lead us to suggest it as an explanation of the
pseudo-gap state in underdoped high-temperature superconductors. In the absence
of impurities, these include power-law magnetic order, a T-linear spin
susceptibility, non-trivial thermal conductivity, and two- and one-particle
charge gaps, the latter evidenced, e.g. in transport and electron photoemission
(which exhibits pronounced fourfold anisotropy inherited from the d-wave
quasiparticles). We use a 2+1-dimensional duality transformation to derive an
effective field theory for this phase. The theory is comprised of gapless
neutral Dirac particles living at the former d-wave nodes, weakly coupled to
the fluctuating gauge field of a dual Ginzburg-Landau theory. The nodal liquid
interpolates naturally between the d-wave superconductor and the insulating
antiferromagnet, and our effective field theory is powerful enough to permit a
detailed analysis of a panoply of interesting phenomena, including charge
ordering, antiferromagnetism, and d-wave superconductivity. We also discuss the
zero-temperature quantum phase transitions which separate the nodal liquid from
various ordered phases.Comment: 19 pages, 4 figure
- âŠ