758 research outputs found
Environment of Er in a-Si:H: co-sputtering versus ion implantation
We report a comparative Extended X-Ray Fine Structure (EXAFS) study of Er in a-Si:H prepared by Er implantation in a-Si:H and by co-sputtering undergoing the same cumulative annealing processes. It was found that the Er environment in as-implanted samples is formed by Si atoms, which are replaced by oxygen under annealing. In the co-sputtered samples, the initial low coordination oxygen environment evolves under thermal treatment to an Er2O3 -like neighborhood.756759Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq
The Kindred Bonds of Mentally Ill Homeless Persons
While the unraveling of the kinship bond has long been suspected to play a role in the epidemiology of homelessness, the connection between kinship and homelessness has been little studied. Based on a normative analysis of the role of family structure in response to adversity, this article explores the impact of the amount and quality of kinship ties on episodes of homelessness experienced by discharged psychiatric patients in Ohio. Survey data derived from personal interviews with both former patients and their kin indicate more strain in relations with kin of the homeless than the nonhomeless. The strain in the kinship bond appears to emanate from a greater prevalence of chronic disabilities that undermine independent functioning and tax the resources of relatives who choose to remain involved. Consistent with this interpretation, patients with histories of homelessness reported more psychiatric symptoms, more deficits in daily living skills, and more contact with the criminal justice system. In general, patient variables were better able than family variables to differentiate the homeless from the nonhomeless. Nonetheless, the formulation of public policies for reducing the incidence and prevalence of homelessness will surely need to take account of the kinship bond and how it can be strengthened
Correlated theory of triplet photoinduced absorption in phenylene-vinylene chains
In this paper we present results of large-scale correlated calculations of
triplet photoinduced absorption (PA) spectrum of oligomers of
poly-(para)phenylenevinylene (PPV) containing up to five phenyl rings. In
particular, the high-energy features in the triplet PA spectrum of oligo-PPVs
are the focus of this study, which, so far, have not been investigated
theoretically, or experimentally. The calculations were performed using the
Pariser-Parr-Pople (PPP) model Hamiltonian, and many-body effects were taken
into account by means of multi-reference singles-doubles configuration
interaction procedure (MRSDCI), without neglecting any molecular orbitals. The
computed triplet PA spectrum of oligo-PPVs exhibits rich structure consisting
of alternating peaks of high and low intensities. The predicted higher energy
features of the triplet spectrum can be tested in future experiments.
Additionally, theoretical estimates of exciton binding energy are also
presented.Comment: To appear in Phys. Rev.
Mesoscopic Cooperative Emission From a Disordered System
We study theoretically the cooperative light emission from a system of classical oscillators confined within a volume with spatial scale, , much
smaller than the radiation wavelength, . We assume
that the oscillators frequencies are randomly distributed around a central
frequency, , with some characteristic width, . In
the absence of disorder, that is , the cooperative emission spectrum
is composed of a narrow subradiant peak superimposed on a wide superradiant
band. When , we demonstrate that if is large enough, the
subradiant peak is not simply broadened by the disorder but rather splits into
a system of random narrow peaks. We estimate the spectral width of these peaks
as a function of , and . We also estimate the
amplitude of this mesoscopic structure in the emission spectrum.Comment: 25 pages including 6 figure
Prediction of infrared light emission from pi-conjugated polymers: a diagrammatic exciton basis valence bond theory
There is currently a great need for solid state lasers that emit in the
infrared, as this is the operating wavelength regime for applications in
telecommunications. Existing --conjugated polymers all emit in the visible
or ultraviolet, and whether or not --conjugated polymers that emit in the
infrared can be designed is an interesting challenge. On the one hand, the
excited state ordering in trans-polyacetylene, the --conjugated polymer
with relatively small optical gap, is not conducive to light emission because
of electron-electron interaction effects. On the other hand, excited state
ordering opposite to that in trans-polyacetylene is usually obtained by
chemical modification that increases the effective bond-alternation, which in
turn increases the optical gap. We develop a theory of electron correlation
effects in a model -conjugated polymer that is obtained by replacing the
hydrogen atoms of trans-polyacetylene with transverse conjugated groups, and
show that the effective on-site correlation in this system is smaller than the
bare correlation in the unsubstituted system. An optical gap in the infrared as
well as excited state ordering conducive to light emission is thereby predicted
upon similar structural modifications.Comment: 15 pages, 15 figures, 1 tabl
Egypt’s 2011–2012 parliamentary elections: Voting for religious vs. secular democracy?
This study investigates whether individuals’ attitudes towards democracy and
secular politics have any influence on voting behavior in Egypt. Based on data
from a survey conducted immediately after the Egyptian parliamentary elections
in January 2012, this study finds that Egyptians’ attitudes towards democratic
governance were quite negative around the parliamentary elections, yet Egyptians
still endorsed democracy as the ideal political system for their country. However,
empirical findings suggest that support for democracy has a limited impact on
electoral results. On the other hand, the main division in Egyptian society around
the first free and fair parliamentary elections was the religious-secular cleavage. As
people support secular politics more, they become significantly less likely to vote
for Islamist parties. These results illustrate that preferences in regard to the type
of democracy – either a liberal and secular or a religious democracy – were
the main determinant of the historic 2012 elections in Egypt
Relationship between photonic band structure and emission characteristics of a polymer distributed feedback laser
G. A. Turnbull, P. Andrew, M. J. Jory, William L. Barnes, and I. D. W. Samuel, Physical Review B, Vol. 64, article 125122 (2001). "Copyright © 2001 by the American Physical Society."We present an experimental study of the emission characteristics and photonic band structure of a distributed feedback polymer laser, based on the material poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene]. We use measurements of the photonic band dispersion to explain how the substrate microstructure modifies both spontaneous and stimulated emission. The lasing structure exhibits a one-dimensional photonic band gap around 610 nm, with lasing occurring at one of the two associated band edges. The band edge (frequency) selection mechanism is found to be a difference in the level of output coupling of the modes associated with the two band edges. This is a feature of the second-order distributed feedback mechanism we have employed and is clearly evident in the measured photonic band structur
Religious Fundamentalism as an Obstacle to Peace in the Middle East: Under What Conditions Might Pragmatism Prevail?
Religious fundamentalists have in several instances served as barriers to the peaceful conclusion of disputes in the Middle East, especially in the framework of the peace process in the Israeli-Palestinian conflict. As a consequence and irrespective of the specific definition of fundamentalism, these groups can constitute in various countries a substantial hurdle for any arms control initiative such as the upcoming Middle East Conference (MEC). This gathering is to deal with the establishment of a zone free of all kinds of weapons of mass destruction (WMD) and their delivery vehicles (DVs). Achieving a successful and sustainable outcome at the MEC could be hampered by a host of problems, one of which is the diffi cult role that religious fundamentalism could possibly play in undermining this initiative. Hence, this POLICY BRIEF provides decision makers and practitioners with information on these presumably ‘negative’ actors, drawn from case studies covering various fundamentalist groups within the Middle East and beyond. These recommendations are based on the key finding that, contrary to mainstream expectations, the major players can behave pragmatically, provided they are acting in favorable circumstances
Theory of excited state absorptions in phenylene-based -conjugated polymers
Within a rigid-band correlated electron model for oligomers of
poly-(paraphenylene) (PPP) and poly-(paraphenylenevinylene) (PPV), we show that
there exist two fundamentally different classes of two-photon A states in
these systems to which photoinduced absorption (PA) can occur. At relatively
lower energies there occur A states which are superpositions of one
electron - one hole (1e--1h) and two electron -- two hole (2e--2h) excitations,
that are both comprised of the highest delocalized valence band and the lowest
delocalized conduction band states only. The dominant PA is to one specific
member of this class of states (the mA). In addition to the above class of
A states, PA can also occur to a higher energy kA state whose 2e--2h
component is {\em different} and has significant contributions from excitations
involving both delocalized and localized bands. Our calculated scaled energies
of the mA and the kA agree reasonably well to the experimentally
observed low and high energy PAs in PPV. The calculated relative intensities of
the two PAs are also in qualitative agreement with experiment. In the case of
ladder-type PPP and its oligomers, we predict from our theoretical work a new
intense PA at an energy considerably lower than the region where PA have been
observed currently. Based on earlier work that showed that efficient
charge--carrier generation occurs upon excitation to odd--parity states that
involve both delocalized and localized bands, we speculate that it is the
characteristic electronic nature of the kA that leads to charge generation
subsequent to excitation to this state, as found experimentally.Comment: Revtex4 style, 2 figures inserted in the text, three tables, 10 page
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