2,668 research outputs found
Identification of relaxation and diffusion mechanisms in amorphous silicon
The dynamics of amorphous silicon at low temperatures can be characterized by
a sequence of discrete activated events, through which the topological network
is locally reorganized. Using the activation-relaxation technique, we create
more than 8000 events, providing an extensive database of relaxation and
diffusion mechanisms. The generic properties of these events - size, number of
atoms involved, activation energy, etc. - are discussed and found to be
compatible with experimental data. We introduce a complete and unique
classification of defects based on their topological properties and apply it to
study of events involving only four-fold coordinated atoms. For these events,
we identify and present in detail three dominant mechanisms.Comment: 4 pages, three figures, submitted to PR
Statistical and Clinical Aspects of Hospital Outcomes Profiling
Hospital profiling involves a comparison of a health care provider's
structure, processes of care, or outcomes to a standard, often in the form of a
report card. Given the ubiquity of report cards and similar consumer ratings in
contemporary American culture, it is notable that these are a relatively recent
phenomenon in health care. Prior to the 1986 release of Medicare hospital
outcome data, little such information was publicly available. We review the
historical evolution of hospital profiling with special emphasis on outcomes;
present a detailed history of cardiac surgery report cards, the paradigm for
modern provider profiling; discuss the potential unintended negative
consequences of public report cards; and describe various statistical
methodologies for quantifying the relative performance of cardiac surgery
programs. Outstanding statistical issues are also described.Comment: Published in at http://dx.doi.org/10.1214/088342307000000096 the
Statistical Science (http://www.imstat.org/sts/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Magnon Dispersion in the Field-Induced Magnetically Ordered Phase of TlCuCl3
The magnetic properties of the interacting dimer system TlCuCl3 are
investigated within a bond-operator formulation. The observed field-induced
staggered magnetic order perpendicular to the field is described as a Bose
condensation of magnons which are linear combinations of dimer singlet and
triplet modes. This technique accounts for the magnetization curve and for the
field dependence of the magnon dispersion curves observed by high-field neutron
scattering measurements.Comment: 4 pages, 4 figures, REVTeX
Activated sampling in complex materials at finite temperature: the properly-obeying-probability activation-relaxation technique
While the dynamics of many complex systems is dominated by activated events,
there are very few simulation methods that take advantage of this fact. Most of
these procedures are restricted to relatively simple systems or, as with the
activation-relaxation technique (ART), sample the conformation space
efficiently at the cost of a correct thermodynamical description. We present
here an extension of ART, the properly-obeying-probability ART (POP-ART), that
obeys detailed balance and samples correctly the thermodynamic ensemble.
Testing POP-ART on two model systems, a vacancy and an interstitial in
crystalline silicon, we show that this method recovers the proper
thermodynamical weights associated with the various accessible states and is
significantly faster than MD in the diffusion of a vacancy below 700 K.Comment: 10 pages, 3 figure
Interplay between shear loading and structural aging in a physical gel
We show that the aging of the mechanical relaxation of a gelatin gel exhibits
the same scaling phenomenology as polymer and colloidal glasses. Besides,
gelatin is known to exhibit logarithmic structural aging (stiffening). We find
that stress accelerates this process. However, this effect is definitely
irreducible to a mere age shift with respect to natural aging. We suggest that
it is interpretable in terms of elastically-aided elementary (coilhelix)
local events whose dynamics gradually slows down as aging increases geometric
frustration
On possible superconductivity in the doped ladder compound La_(1-x)Sr_xCuO_2.5
LaCuO_2.5 is a system of coupled, two-chain, cuprate ladders which may be
doped systematically by Sr substitution. Motivated by the recent synthesis of
single crystals, we investigate theoretically the possibility of
superconductivity in this compound. We use a model of spin fluctuation-mediated
superconductivity, where the pairing potential is strongly peaked at \pi in the
ladder direction. We solve the coupled gap equations on the bonding and
antibonding ladder bands to find superconducting solutions across the range of
doping, and discuss their relevance to the real material.Comment: RevTex, 4 pages, 7 figure
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