1,614 research outputs found
Evolution of Superconductivity in Electron-Doped Cuprates: Magneto-Raman Spectroscopy
The electron-doped cuprates Pr_{2-x}Ce_xCuO_4 and Nd_{2-x}Ce_xCuO_4 have been
studied by electronic Raman spectroscopy across the entire region of the
superconducting (SC) phase diagram. The SC pairing strength is found to be
consistent with a weak-coupling regime except in the under-doped region where
we observe an in-gap collective mode at 4.5 k_{B}T_c while the maximum
amplitude of the SC gap is ~8 k_{B}T_{c}. In the normal state, doped carriers
divide into coherent quasi-particles (QPs) and carriers that remain incoherent.
The coherent QPs mainly reside in the vicinity of (\pi/2, \pi/2) regions of the
Brillouin zone (BZ). We find that only coherent QPs contribute to the
superfluid density in the B_{2g} channel. The persistence of SC coherence peaks
in the B_{2g} channel for all dopings implies that superconductivity is mainly
governed by interactions between the hole-like coherent QPs in the vicinity of
(\pi/2, \pi/2) regions of the BZ. We establish that superconductivity in the
electron-doped cuprates occurs primarily due to pairing and condensation of
hole-like carriers. We have also studied the excitations across the SC gap by
Raman spectroscopy as a function of temperature (T) and magnetic field (H) for
several different cerium dopings (x). Effective upper critical field lines
H*_{c2}(T, x) at which the superfluid stiffness vanishes and
H^{2\Delta}_{c2}(T, x) at which the SC gap amplitude is suppressed by field
have been determined; H^{2\Delta}_{c2}(T, x) is larger than H*_{c2}(T, x) for
all doping concentrations. The difference between the two quantities suggests
the presence of phase fluctuations that increase for x< 0.15. It is found that
the magnetic field suppresses the magnitude of the SC gap linearly at
surprisingly small fields.Comment: 13 pages, 8 figures; submitted to Phys. Rev.
A Bayesian dose-response meta-analysis model: simulation study and application
Dose-response models express the effect of different dose or exposure levels
on a specific outcome. In meta-analysis, where aggregated-level data is
available, dose-response evidence is synthesized using either one-stage or
two-stage models in a frequentist setting. We propose a hierarchical
dose-response model implemented in a Bayesian framework. We present the model
with cubic dose-response shapes for a dichotomous outcome and take into account
heterogeneity due to variability in the dose-response shape. We develop our
Bayesian model assuming normal or binomial likelihood and accounting for
exposures grouped in clusters. We implement these models in R using JAGS and we
compare our approach to the one-stage dose-response meta-analysis model in a
simulation study. We found that the Bayesian dose-response model with binomial
likelihood has slightly lower bias than the Bayesian model with the normal
likelihood and the frequentist one-stage model. However, all three models
perform very well and give practically identical results. We also re-analyze
the data from 60 randomized controlled trials (15,984 participants) examining
the efficacy (response) of various doses of antidepressant drugs. All models
suggest that the dose-response curve increases between zero dose and 40 mg of
fluoxetine-equivalent dose, and thereafter is constant. We draw the same
conclusion when we take into account the fact that five different
antidepressants have been studied in the included trials. We show that
implementation of the hierarchical model in Bayesian framework has similar
performance to, but overcomes some of the limitations of the frequentist
approaches and offers maximum flexibility to accommodate features of the data
THE RELATIONSHIP BETWEEN INTRUSIVE COGNITIONS AND DEFENSE MECHANISMS IN HEALTHY AND CLINICAL POPULATIONS
Purpose: to examine the relationship between defense mechanisms and intrusive cognitions in normal healthy individuals and psychiatric patients.
Methodology: The study sample consists of a healthy group (n=60; 30 males & 30 females), whereas the clinical group (n=66; 34 males, 32 females) includes patients with major depressive disorder (12 patients, 5 males, 7 females), schizophrenia (31 patients; 14 males, 17 females), obsessive-compulsive disorder (23 patients; 15 males, 8 females). We used several scales to measure the following variables: intrusive cognitions, intrusive memories, and defense mechanisms.
Finding: The results show that there is a positive correlation between defense mechanisms and intrusive cognitions in healthy and clinical groups. Intrusive cognitions were more common in the patient than in a healthy group. Furthermore, there was no significant difference between males and females in measures of intrusive thoughts and memories in both groups.
Implications: These findings have implications for behavioral treatment. Treatments used for managing posttraumatic stress disorder can also be used for the treatment of a major depressive disorder, OCD, and schizophrenia.
Originality: This investigation the relationship between intrusive cognitions and defense mechanisms in healthy and clinical populations and its implication on the cue exposure therapy that can be the treatment of intrusive cognitions and thoughts in with major depressive disorder, OCD, and schizophrenia
Correlated Markov Quantum Walks
We consider the discrete time unitary dynamics given by a quantum walk on
performed by a particle with internal degree of freedom, called coin
state, according to the following iterated rule: a unitary update of the coin
state takes place, followed by a shift on the lattice, conditioned on the coin
state of the particle. We study the large time behavior of the quantum
mechanical probability distribution of the position observable in for
random updates of the coin states of the following form. The random sequences
of unitary updates are given by a site dependent function of a Markov chain in
time, with the following properties: on each site, they share the same
stationnary Markovian distribution and, for each fixed time, they form a
deterministic periodic pattern on the lattice.
We prove a Feynman-Kac formula to express the characteristic function of the
averaged distribution over the randomness at time in terms of the nth power
of an operator . By analyzing the spectrum of , we show that this
distribution posesses a drift proportional to the time and its centered
counterpart displays a diffusive behavior with a diffusion matrix we compute.
Moderate and large deviations principles are also proven to hold for the
averaged distribution and the limit of the suitably rescaled corresponding
characteristic function is shown to satisfy a diffusion equation.
An example of random updates for which the analysis of the distribution can
be performed without averaging is worked out. The random distribution displays
a deterministic drift proportional to time and its centered counterpart gives
rise to a random diffusion matrix whose law we compute. We complete the picture
by presenting an uncorrelated example.Comment: 37 pages. arXiv admin note: substantial text overlap with
arXiv:1010.400
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