4,719 research outputs found
Electromagnetic response of high-Tc superconductors -- the slave-boson and doped-carrier theories
We evaluate the doping dependence of the quasiparticle current and low
temperature superfluid density in two slave-particle theories of the tt't''J
model -- the slave-boson theory and doped-carrier theory. In the slave-boson
theory, the nodal quasiparticle current renormalization factor
vanishes proportionally to the zero temperature superfluid density ;
however, we find that away from the limit displays a
much weaker doping dependence than . A similar conclusion applies to
the doped-carrier theory, which differentiates the nodal and antinodal regions
of momentum space. Due to its momentum space anisotropy, the doped-carrier
theory enhances the value of in the hole doped regime, bringing it to
quantitative agreement with experiments, and reproduces the asymmetry between
hole and electron doped cuprate superconductors. Finally, we use the
doped-carrier theory to predict a specific experimental signature of local
staggered spin correlations in doped Mott insulator superconductors which, we
propose, should be observed in STM measurements of underdoped high-Tc
compounds. This experimental signature distinguishes the doped-carrier theory
from other candidate mean-field theories of high-Tc superconductors, like the
slave-boson theory and the conventional BCS theory.Comment: 12 pages, RevTeX4, homepage http://dao.mit.edu/~we
Competing orders and inter-layer tunnelling in cuprate superconductors: A finite temperature Landau theory
We propose a finite temperature Landau theory that describes competing orders
and interlayer tunneling in cuprate superconductors as an important extension
to a corresponding theory at zero temperature [Nature {\bf 428}, 53 (2004)],
where the superconducting transition temperature is defined in three
possible ways as a function of the zero temperature order parameter. For given
parameters, our theory determines without any ambiguity. In mono- and
double-layer systems we discuss the relation between zero temperature order
parameter and the associated transition temperature in the presence of
competing orders, and draw a connection to the puzzling experimental fact that
the pseudo-gap temperature is much higher than the corresponding energy scale
near optimum doping. Applying the theory to multi-layer systems, we calculate
the layer-number dependence of . In a reasonable parameter space the
result turns out to be in agreement with experiments.Comment: 5 pages, 3 figure
A Variational Monte Carlo Study of the Current Carried by a Quasiparticle
With the use of Gutzwiller-projected variational states, we study the
renormalization of the current carried by the quasiparticles in
high-temperature superconductors and of the quasiparticle spectral weight. The
renormalization coefficients are computed by the variational Monte Carlo
technique, under the assumption that quasiparticle excitations may be described
by Gutzwiller-projected BCS quasiparticles. We find that the current
renormalization coefficient decreases with decreasing doping and tends to zero
at zero doping. The quasiparticle spectral weight Z_+ for adding an electron
shows an interesting structure in k space, which corresponds to a depression of
the occupation number k just outside the Fermi surface. The perturbative
corrections to those quantities in the Hubbard model are also discussed.Comment: 9 pages, 9 figure
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Supporting desistance through ambiguous practice: what can be learned from the first prison-based model of CoSA in England and Wales?
Circles of Support and Accountability (CoSA) are an initiative designed to support those previously convicted of sexual offences as they reintegrate back in to society, whilst still holding them accountable for their thoughts and behaviour (Cesaroni, 2002). The aim of the research was to explore the Core Member and volunteer experience of being involved in a CoSA that transitions from prison to community, with the objective being to focus upon what can be learnt from these initial experiences. The study included qualitative interviews with two separate groups of participants; Core Members (n=7) and volunteers (n=10) involved in the prison-model CoSA. Interpretative Phenomenological Analysis of the data was undertaken to consider the individual’s subjective experience of being involved in this initiative.
A superordinate theme of ambiguous practice was identified, whereby volunteers appeared to overlook the importance of expressive support, described a lack of commitment from other fellow volunteers and demonstrated a confusion surrounding the accountability aspect of their role. Despite the confusion highlighted, however, the accountability aspect of the volunteers’ role was identified through their indirect support of Core members’ desistance. The second superordinate theme outlined the volunteers’ encouragement of the Core Members new pro-social lifestyle, thus helping to reinforce their crime -free identity. This led to a concern, however, of what would happen once the CoSA journey had come to an end.
The learning derived from these findings can now be used to continue to develop and improve the use of prison-model CoSA across England and Wales
Faking and the Validity of Personality Tests: An Experimental Investigation Using Modern Forced Choice Measures
Despite the established validity of personality measures for personnel selection, their susceptibility to faking has been a persistent concern. However, the lack of studies that combine generalizability with experimental control makes it difficult to determine the effects of applicant faking. This study addressed this deficit in two ways. First, we compared a subtle incentive to fake with the explicit “fake-good” instructions used in most faking experiments. Second, we compared standard Likert scales to multidimensional forced choice (MFC) scales designed to resist deception, including more and less fakable versions of the same MFC inventory. MFC scales substantially reduced motivated score elevation but also appeared to elicit selective faking on work-relevant dimensions. Despite reducing the effectiveness of impression management attempts, MFC scales did not retain more validity than Likert scales when participants faked. However, results suggested that faking artificially bolstered the criterion-related validity of Likert scales while diminishing their construct validity
New Experimental limit on Optical Photon Coupling to Neutral, Scalar Bosons
We report on the first results of a sensitive search for scalar coupling of
photons to a light neutral boson in the mass range of approximately 1.0
milli-electron volts and coupling strength greater than 10 GeV using
optical photons. This was a photon regeneration experiment using the "light
shining through a wall" technique in which laser light was passed through a
strong magnetic field upstream of an optical beam dump; regenerated laser light
was then searched for downstream of a second magnetic field region optically
shielded from the former. Our results show no evidence for scalar coupling in
this region of parameter space.Comment: pdf-file, 10 pages, 4 figures, submitted to Physical Review Letter
Synchronization in small-world systems
We quantify the dynamical implications of the small-world phenomenon. We
consider the generic synchronization of oscillator networks of arbitrary
topology, and link the linear stability of the synchronous state to an
algebraic condition of the Laplacian of the graph. We show numerically that the
addition of random shortcuts produces improved network synchronizability.
Further, we use a perturbation analysis to place the synchronization threshold
in relation to the boundaries of the small-world region. Our results also show
that small-worlds synchronize as efficiently as random graphs and hypercubes,
and more so than standard constructive graphs
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