715 research outputs found
Image-charge induced localization of molecular orbitals at metal-molecule interfaces: Self-consistent GW calculations
Quasiparticle (QP) wave functions, also known as Dyson orbitals, extend the
concept of single-particle states to interacting electron systems. Here we
employ many-body perturbation theory in the GW approximation to calculate the
QP wave functions for a semi-empirical model describing a -conjugated
molecular wire in contact with a metal surface. We find that image charge
effects pull the frontier molecular orbitals toward the metal surface while
orbitals with higher or lower energy are pushed away. This affects both the
size of the energetic image charge shifts and the coupling of the individual
orbitals to the metal substrate. Full diagonalization of the QP equation and,
to some extent, self-consistency in the GW self-energy, is important to
describe the effect which is not captured by standard density functional theory
or Hartree-Fock. These results should be important for the understanding and
theoretical modeling of electron transport across metal-molecule interfaces.Comment: 7 pages, 6 figure
Electron-electron interaction effects on optical excitations in semiconducting single-walled carbon nanotubes
We report correlated-electron calculations of optically excited states in ten
semiconducting single-walled carbon nanotubes with a wide range of diameters.
Optical excitation occurs to excitons whose binding energies decrease with the
increasing nanotube diameter, and are smaller than the binding energy of an
isolated strand of poly-(paraphenylene vinylene). The ratio of the energy of
the second optical exciton polarized along the nanotube axis to that of the
lowest exciton is smaller than the value predicted within single-particle
theory. The experimentally observed weak photoluminescence is an intrinsic
feature of semiconducting nanotubes, and is consequence of dipole-forbidden
excitons occurring below the optical exciton.Comment: 5 pages, 3 figures, To appear in PR
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Algorithms, Automation, and News
This special issue examines the growing importance of algorithms and automation in the gathering, composition, and distribution of news. It connects a long line of research on journalism and computation with scholarly and professional terrain yet to be explored. Taken as a whole, these articles share some of the noble ambitions of the pioneering publications on ‘reporting algorithms’, such as a desire to see computing help journalists in their watchdog role by holding power to account. However, they also go further, firstly by addressing the fuller range of technologies that computational journalism now consists of: from chatbots and recommender systems, to artificial intelligence and atomised journalism. Secondly, they advance the literature by demonstrating the increased variety of uses for these technologies, including engaging underserved audiences, selling subscriptions, and recombining and re-using content. Thirdly, they problematize computational journalism by, for example, pointing out some of the challenges inherent in applying AI to investigative journalism and in trying to preserve public service values. Fourthly, they offer suggestions for future research and practice, including by presenting a framework for developing democratic news recommenders and another that may help us think about computational journalism in a more integrated, structured manner
Data Portraits and Intermediary Topics: Encouraging Exploration of Politically Diverse Profiles
In micro-blogging platforms, people connect and interact with others.
However, due to cognitive biases, they tend to interact with like-minded people
and read agreeable information only. Many efforts to make people connect with
those who think differently have not worked well. In this paper, we
hypothesize, first, that previous approaches have not worked because they have
been direct -- they have tried to explicitly connect people with those having
opposing views on sensitive issues. Second, that neither recommendation or
presentation of information by themselves are enough to encourage behavioral
change. We propose a platform that mixes a recommender algorithm and a
visualization-based user interface to explore recommendations. It recommends
politically diverse profiles in terms of distance of latent topics, and
displays those recommendations in a visual representation of each user's
personal content. We performed an "in the wild" evaluation of this platform,
and found that people explored more recommendations when using a biased
algorithm instead of ours. In line with our hypothesis, we also found that the
mixture of our recommender algorithm and our user interface, allowed
politically interested users to exhibit an unbiased exploration of the
recommended profiles. Finally, our results contribute insights in two aspects:
first, which individual differences are important when designing platforms
aimed at behavioral change; and second, which algorithms and user interfaces
should be mixed to help users avoid cognitive mechanisms that lead to biased
behavior.Comment: 12 pages, 7 figures. To be presented at ACM Intelligent User
Interfaces 201
Evidence for Excimer Photoexcitations in an Ordered {\pi}-Conjugated Polymer Film
We report pressure-dependent transient picosecond and continuous-wave
photomodulation studies of disordered and ordered films of
2-methoxy-5-(2-ethylhexyloxy) poly(para-phenylenevinylene). Photoinduced
absorption (PA) bands in the disordered film exhibit very weak pressure
dependence and are assigned to intrachain excitons and polarons. In contrast,
the ordered film exhibits two additional transient PA bands in the midinfrared
that blueshift dramatically with pressure. Based on high-order configuration
interaction calculations we ascribe the PA bands in the ordered film to
excimers. Our work brings insight to the exciton binding energy in ordered
films versus disordered films and solutions. The reduced exciton binding energy
in ordered films is due to new energy states appearing below the continuum band
threshold of the single strand.Comment: 5.5 pages, 5 figure
Density-matrix functional theory of the Hubbard model: An exact numerical study
A density functional theory for many-body lattice models is considered in
which the single-particle density matrix is the basic variable. Eigenvalue
equations are derived for solving Levy's constrained search of the interaction
energy functional W, which is expressed as the sum of Hartree-Fock energy and
the correlation energy E_C. Exact results are obtained for E_C of the Hubbard
model on various periodic lattices. The functional dependence of E_C is
analyzed by varying the number of sites, band filling and lattice structure.
The infinite one-dimensional chain and one-, two-, or three-dimensional finite
clusters with periodic boundary conditions are considered. The properties of
E_C are discussed in the limits of weak and strong electronic correlations, as
well as in the crossover region. Using an appropriate scaling we observe a
pseudo-universal behavior which suggests that the correlation energy of
extended systems could be obtained quite accurately from finite cluster
calculations. Finally, the behavior of E_C for repulsive (U>0) and attractive
(U<0) interactions are contrasted.Comment: Phys. Rev. B (1999), in pres
Interaction energy functional for lattice density functional theory: Applications to one-, two- and three-dimensional Hubbard models
The Hubbard model is investigated in the framework of lattice density
functional theory (LDFT). The single-particle density matrix with
respect the lattice sites is considered as the basic variable of the many-body
problem. A new approximation to the interaction-energy functional
is proposed which is based on its scaling properties and which recovers exactly
the limit of strong electron correlations at half-band filling. In this way, a
more accurate description of is obtained throughout the domain of
representability of , including the crossover from weak to strong
correlations. As examples of applications results are given for the
ground-state energy, charge-excitation gap, and charge susceptibility of the
Hubbard model in one-, two-, and three-dimensional lattices. The performance of
the method is demonstrated by comparison with available exact solutions, with
numerical calculations, and with LDFT using a simpler dimer ansatz for .
Goals and limitations of the different approximations are discussed.Comment: 25 pages and 8 figures, submitted to Phys. Rev.
Using an Electronic Monitoring System to Link Offspring Provisioning and Foraging Behavior of a Wild Passerine
Although the costs of parental care are at the foundations of optimal-parental-investment theory, our understanding of the nature of the underlying costs is limited by the difficulty of measuring variation in foraging effort. We simultaneously measured parental provisioning and foraging behavior in a free-living population of Zebra Finches (Taeniopygia guttata) using an electronic monitoring system. We fitted 145 adults with a passive transponder tag and remotely recorded their visits to nest boxes and feeders continuously over a 2-month period. After validating the accuracy of this monitoring system, we studied how provisioning and foraging activities varied through time (day and breeding cycle) and influenced the benefits (food received by the offspring) and costs (interclutch interval) of parental care. The provisioning rates of wild Zebra Finches were surprisingly low, with an average of only one visit per hour throughout the day. This was significantly lower than those reported for this model species in captivity and for most other passerines in the wild. Nest visitation rate only partially explained the amount of food received by the young, with parental foraging activity, including the minimum distance covered on foraging trips, being better predictors. Parents that sustained higher foraging activity and covered more distance during the first breeding attempt took longer to renest. These results demonstrate that in some species matching foraging activity with offspring provisioning may provide a better estimate of the true investment that individuals commit to a reproductive attempt
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My friends, editors, algorithms, and I: Examining audience attitudes to news selection
Prompted by the ongoing development of content personalization by social networks and mainstream news brands, and recent debates about balancing algorithmic and editorial selection, this study explores what audiences think about news selection mechanisms and why. Analysing data from a 26-country survey (N=53,314), we report the extent to which audiences believe story selection by editors and story selection by algorithms are good ways to get news online and, using multi-level models, explore the relationships that exist between individuals’ characteristics and those beliefs. The results show that, collectively, audiences believe algorithmic selection guided by a user’s past consumption behaviour is a better way to get news than editorial curation. There are, however, significant variations in these beliefs at the individual level. Age, trust in news, concerns about privacy, mobile news access, paying for news, and six other variables had effects. Our results are partly in line with current general theory on algorithmic appreciation, but diverge in our findings on the relative appreciation of algorithms and experts, and in how the appreciation of algorithms can differ according to the data that drive them. We believe this divergence is partly due to our study’s focus on news, showing algorithmic appreciation has context-specific characteristics
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
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