1,284 research outputs found
Excitonic effects in the optical properties of CdSe nanowires
Using a first-principle approach beyond density functional theory we
calculate the electronic and optical properties of small diameter CdSe
nanowires.Our results demonstrate how some approximations commonly used in bulk
systems fail at this nano-scale level and how indispensable it is to include
crystal local fields and excitonic effects to predict the unique optical
properties of nanowires. From our results, we then construct a simple model
that describes the optical gap as a function of the diameter of the wire, that
turns out to be in excellent agreement with experiments for intermediate and
large diameters.Comment: submitte
Low-density silicon allotropes for photovoltaic applications
Silicon materials play a key role in many technologically relevant fields,
ranging from the electronic to the photovoltaic industry. A systematic search
for silicon allotropes was performed by employing a modified ab initio minima
hopping crystal structure prediction method. The algorithm was optimized to
specifically investigate the hitherto barely explored low-density regime of the
silicon phase diagram by imitating the guest-host concept of clathrate
compounds. In total 44 metastable phases are presented, of which 11 exhibit
direct or quasi-direct band-gaps in the range of 1.0-1.8 eV, close to
the optimal Shockley-Queisser limit of 1.4 eV, with a stronger overlap
of the absorption spectra with the solar spectrum compared to conventional
diamond silicon. Due to the structural resemblance to known clathrate compounds
it is expected that the predicted phases can be synthesized
Density-based mixing parameter for hybrid functionals
A very popular ab-initio scheme to calculate electronic properties in solids
is the use of hybrid functionals in density functional theory (DFT) that mixes
a portion of Fock exchange with DFT functionals. In spite of their success, a
major problem still remains, related to the use of one single mixing parameter
for all materials. Guided by physical arguments that connect the mixing
parameter to the dielectric properties of the solid, and ultimately to its band
gap, we propose a method to calculate this parameter from the electronic
density alone. This method is able to cut significantly the error of
traditional hybrid functionals for large and small gap materials, while
retaining a good description of structural properties. Moreover, its
implementation is simple and leads to a negligible increase of the
computational time.Comment: submitte
Knowledge Acquisition by Networks of Interacting Agents in the Presence of Observation Errors
In this work we investigate knowledge acquisition as performed by multiple
agents interacting as they infer, under the presence of observation errors,
respective models of a complex system. We focus the specific case in which, at
each time step, each agent takes into account its current observation as well
as the average of the models of its neighbors. The agents are connected by a
network of interaction of Erd\H{o}s-Renyi or Barabasi-Albert type. First we
investigate situations in which one of the agents has a different probability
of observation error (higher or lower). It is shown that the influence of this
special agent over the quality of the models inferred by the rest of the
network can be substantial, varying linearly with the respective degree of the
agent with different estimation error. In case the degree of this agent is
taken as a respective fitness parameter, the effect of the different estimation
error is even more pronounced, becoming superlinear. To complement our
analysis, we provide the analytical solution of the overall behavior of the
system. We also investigate the knowledge acquisition dynamic when the agents
are grouped into communities. We verify that the inclusion of edges between
agents (within a community) having higher probability of observation error
promotes the loss of quality in the estimation of the agents in the other
communities.Comment: 10 pages, 7 figures. A working manuscrip
On the calculation of the bandgap of periodic solids with MGGA functionals using the total energy
During the last few years, it has become more and more clear that functionals of the meta generalized gradient approximation (MGGA) are more accurate than GGA functionals for the geometry and energetics of electronic systems. However, MGGA functionals are also potentially more interesting for the electronic structure, in particular, when the potential is nonmultiplicative (i.e., when MGGAs are implemented in the generalized Kohn-Sham framework), which may help to get more accurate bandgaps. Here, we show that the calculation of bandgap of solids with MGGA functionals can also be done very accurately in a non-self-consistent manner. This scheme uses only the total energy and can, therefore, be very useful when the self-consistent implementation of a particular MGGA functional is not available. Since self-consistent MGGA calculations may be difficult to converge, the non-self-consistent scheme may also help to speed up the calculations. Furthermore, it can be applied to any other types of functionals, for which the implementation of the corresponding potential is not trivial
Efficient calculation of van der Waals dispersion coefficients with time-dependent density functional theory in real time: application to polycyclic aromatic hydrocarbons
The van der Waals dispersion coefficients of a set of polycyclic aromatic
hydrocarbons, ranging in size from the single-cycle benzene to circumovalene
(C66H20), are calculated with a real-time propagation approach to
time-dependent density functional theory (TDDFT). In the non-retarded regime,
the Casimir-Polder integral is employed to obtain C6, once the dynamic
polarizabilities have been computed at imaginary frequencies with TDDFT. On the
other hand, the numerical coefficient that characterizes the fully retarded
regime is obtained from the static polarizabilities. This ab initio strategy
has favorable scaling with the size of the system - as demonstrated by the size
of the reported molecules - and can be easily extended to obtain higher order
van der Waals coefficients.Comment: submitted to J. Chem. Phy
Occult Intraamniotic Infection at the Time of Midtrimester Genetic Amniocentesis: A Reassessment
Objective: The objective of this study was to reevaluate the incidence of occult early midtrimester
intraamniotic infection in asymptomatic patients at the time of genetic amniocentesis
Grapevine yellows diseases in Spain: eight years survey of disease spread and molecular characterization of phytoplasmas involved.
Among grapevine yellows phytoplasma diseases in Europe,
flavescence dor\ue9e (FD) is the most devastating and in the last
decade has reached Spanish vineyards, mainly in Catalonia. An
eight-year survey was carried out in the areas where the disease
has spread (Alt Empord\ue0, Catalonia, Northern Spain) and in the
remaining vine-growing areas of Catalonia. Sequence analyses
of a portion of the 16S-23S ribosomal DNA cistron, from selected
grapevine samples from Catalonia, showed that the phytoplasmas
involved in grapevine yellows belong to 16S ribosomal
subgroups V-D (flavescence dor\ue9e, FD) and XII-A (bois noir, BN).
A set of Spanish FD isolates collected during these years were
further studied by RFLP analyses of the 16S-23S ribosomal DNA
fragment, as well as the rpS3 and SecY genes. All the FD phytoplasma
strains studied were related to phytoplasmas belonging
to ribosomal protein subgroup rp-E
Detection and nudge-intervention on sensitive information in social networks
[EN] Detecting sensitive information considering privacy is a relevant issue on Online Social Networks (OSNs). It is often difficult for users to manage the privacy associated with their posts on social networks taking into account all the possible consequences. The aim of this work is to provide information about the sensitivity of the content of a publication when a user is going to share it in OSN. For this purpose, we developed a privacy-assistant agent that detects sensitive information. Based on this information, the agent provides a message through a nudge mechanism warning about the possible risks of sharing the message. To avoid being annoying, the agent also considers the user's previous behaviour (e.g. if he previously ignored certain nudges) and adapts the messages it sends to give more relevance to those categories that are more important to the user from the point of view of the privacy risk. This agent was integrated into the social network Pesedia. We analysed the performance of different models to detect a set of sensitive categories (i.e. location, medical, drug/alcohol, emotion, personal attacks, stereotyping, family and association details, personal details and personally identifiable information) in a dataset of tweets in Spanish. The model that obtained the best results (i.e. F1 and accuracy) and that was finally integrated into the privacy-assistant agent was transformer-based.This work is supported by the Spanish Government project TIN2017-89156-R.Alemany, J.; Botti-Cebriá, V.; Del Val Noguera, E.; García-Fornes, A. (2022). Detection and nudge-intervention on sensitive information in social networks. Logic Journal of IGPL. 30(6):942-953. https://doi.org/10.1093/jigpal/jzac00494295330
High-throughput search of ternary chalcogenides for p-type transparent electrodes
Delafossite crystals are fascinating ternary oxides that have demonstrated transparent conductivity and ambipolar doping. Here we use a high-throughput approach based on density functional theory to find delafossite and related layered phases of composition ABX(2), where A and B are elements of the periodic table, and X is a chalcogen (O, S, Se, and Te). From the 15 624 compounds studied in the trigonal delafossite prototype structure, 285 are within 50 meV/atom from the convex hull of stability. These compounds are further investigated using global structural prediction methods to obtain their lowest-energy crystal structure. We find 79 systems not present in the materials project database that are thermodynamically stable and crystallize in the delafossite or in closely related structures. These novel phases are then characterized by calculating their band gaps and hole effective masses. This characterization unveils a large diversity of properties, ranging from normal metals, magnetic metals, and some candidate compounds for p-type transparent electrodes
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