33,633 research outputs found
Effect of unitary impurities in non-STM-types of tunneling in high-T_c superconductors
Based on an extended Hubbard model, we present calculations of both the local
(i.e., single-site) and spatially-averaged differential tunneling conductance
in d-wave superconductors containing nonmagnetic impurities in the unitary
limit. Our results show that a random distribution of unitary impurities of any
concentration can at most give rise to a finite zero-bias conductance (with no
peak there) in spatially-averaged non-STM type of tunneling, in spite of the
fact that local tunneling in the immediate vicinity of an isolated impurity
does show a conductance peak at zero bias, whereas to give rise to even a small
zero-bias conductance peak in the former type of tunneling the impurities must
form dimers, trimers, etc. along the [110] directions. In addition, we find
that the most-recently-observed novel pattern of the tunneling conductance
around a single impurity by Pan et al. [Nature (London) 403,746 (2000)] can be
explained in terms of a realistic model of the tunneling configuration which
gives rise to the experimental results reported there. The key feature in this
model is the blocking effect of the BiO and SrO layers which exist between the
tunneling tip and the CuO_2 layer being probed.Comment: 9 pages, 7 ps-figures, to appear in Phys. Rev. B (Sep. 1, 2000);
typos corrected, references added, figure 6 changed to expand the explanation
on recent experimental measurements by S.H. Pan et al. [Nature (London) 403,
746 (2000)
Temperature enhanced effects of ozone on cardiovascular mortality in 95 large US communities, 1987-2000 - assessment using the NMMAPS data
A few studies examined interactive effects between air pollution and temperature on health outcomes. This study is to examine if temperature modified effects of ozone and cardiovascular mortality in 95 large US cities. A nonparametric and a parametric regression models were separately used to explore interactive effects of temperature and ozone on cardiovascular mortality during May and October, 1987-2000. A Bayesian meta-analysis was used to pool estimates. Both models illustrate that temperature enhanced the ozone effects on mortality in the northern region, but obviously in the southern region. A 10-ppb increment in ozone was associated with 0.41 % (95% posterior interval (PI): -0.19 %, 0.93 %), 0.27 % (95% PI: -0.44 %, 0.87 %) and 1.68 % (95% PI: 0.07 %, 3.26 %) increases in daily cardiovascular mortality corresponding to low, moderate and high levels of temperature, respectively. We concluded that temperature modified effects of ozone, particularly in the northern region
Collective Almost Synchronization in Complex Networks
This work introduces the phenomenon of Collective Almost Synchronization
(CAS), which describes a universal way of how patterns can appear in complex
networks even for small coupling strengths. The CAS phenomenon appears due to
the existence of an approximately constant local mean field and is
characterized by having nodes with trajectories evolving around periodic stable
orbits. Common notion based on statistical knowledge would lead one to
interpret the appearance of a local constant mean field as a consequence of the
fact that the behavior of each node is not correlated to the behaviors of the
others. Contrary to this common notion, we show that various well known weaker
forms of synchronization (almost, time-lag, phase synchronization, and
generalized synchronization) appear as a result of the onset of an almost
constant local mean field. If the memory is formed in a brain by minimising the
coupling strength among neurons and maximising the number of possible patterns,
then the CAS phenomenon is a plausible explanation for it.Comment: 3 figure
Comment on "Boson-fermion model beyond the mean-field approximation"
In a recent paper [A.S.Alexandrov, J.Phys.:Condens.Matter 8, 6923 (1996);
cond-mat/9603111], it has been suggested that there is no Cooper pairing in
boson-fermion models of superconductivity. We show that this conjecture is
based on an inconsistent approximation that violates an exact identity. Quite
generally, the divergence of the fermion t-matrix (the Thouless criterion) is
accompanied by the condensation of a boson mode.Comment: LaTeX, 5 pages, 2style files included, 4 embedded EPS figures,
submitted to J.Phys.:Condens.Matte
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