1,042 research outputs found
Distinguishing Topical and Social Groups Based on Common Identity and Bond Theory
Social groups play a crucial role in social media platforms because they form
the basis for user participation and engagement. Groups are created explicitly
by members of the community, but also form organically as members interact. Due
to their importance, they have been studied widely (e.g., community detection,
evolution, activity, etc.). One of the key questions for understanding how such
groups evolve is whether there are different types of groups and how they
differ. In Sociology, theories have been proposed to help explain how such
groups form. In particular, the common identity and common bond theory states
that people join groups based on identity (i.e., interest in the topics
discussed) or bond attachment (i.e., social relationships). The theory has been
applied qualitatively to small groups to classify them as either topical or
social. We use the identity and bond theory to define a set of features to
classify groups into those two categories. Using a dataset from Flickr, we
extract user-defined groups and automatically-detected groups, obtained from a
community detection algorithm. We discuss the process of manual labeling of
groups into social or topical and present results of predicting the group label
based on the defined features. We directly validate the predictions of the
theory showing that the metrics are able to forecast the group type with high
accuracy. In addition, we present a comparison between declared and detected
groups along topicality and sociality dimensions.Comment: 10 pages, 6 figures, 2 table
Optically controlled orbital angular momentum generation in a polaritonic quantum fluid
Applications of the orbital angular momentum (OAM) of light range from the
next generation of optical communication systems to optical imaging and optical
manipulation of particles. Here we propose a micron-sized semiconductor source
which emits light with pre-defined OAM components. This source is based on a
polaritonic quantum fluid. We show how in this system modulational
instabilities can be controlled and harnessed for the spontaneous formation of
OAM components not present in the pump laser source. Once created, the OAM
states exhibit exotic flow patterns in the quantum fluid, characterized by
generation-annihilation pairs. These can only occur in open systems, not in
equilibrium condensates, in contrast to well-established vortex-antivortex
pairs
Bulk electronic structure of non-centrosymmetric EuTGe3 (T= Co, Ni, Rh, Ir) studied by hard x-ray photoelectron spectroscopy
Non-centrosymmetric EuTGe3 (T=Co, Ni, Rh, and Ir) possesses magnetic Eu2+
ions and antiferromagnetic ordering appears at low temperatures. Transition
metal substitution leads to changes of the unit cell volume and of the magnetic
ordering. However, the magnetic ordering temperature does not scale with the
volume change and the Eu valence is expected to remain divalent. Here we study
the bulk electronic structure of non-centrosymmetric EuTGe3 (T=Co, Ni, Rh, and
Ir) by hard x-ray photoelectron spectroscopy. The Eu 3d core level spectrum
confirms the robust Eu2+ valence state against the transition metal
substitution with a small contribution from Eu3+. The estimated Eu mean-valence
is around 2.1 in these compounds as confirmed by multiplet calculations. In
contrast, the Ge 2p spectrum shifts to higher binding energy upon changing the
transition metal from 3d to 4d to 5d elements, hinting of a change in the Ge-T
bonding strength. The valence bands of the different compounds are found to be
well reproduced by ab initio band structure calculations
Anharmonicity due to Electron-Phonon Coupling in Magnetite
We present the results of inelastic x-ray scattering for magnetite and
analyze the energies and spectral widths of the phonon modes with different
symmetries in a broad range of temperature 125<T<293 K. The phonon modes with
X_4 and Delta_5 symmetries broaden in a nonlinear way with decreasing
temperature when the Verwey transition is approached. It is found that the
maxima of phonon widths occur away from high-symmetry points which indicates
the incommensurate character of critical fluctuations. Strong phonon
anharmonicity induced by electron-phonon coupling is discovered within ab
initio calculations which take into account local Coulomb interactions at Fe
ions. It (i) explains observed anomalous phonon broadening, and (ii)
demonstrates that the Verwey transition is a cooperative phenomenon which
involves a wide spectrum of phonons coupled to charge fluctuations condensing
in the low-symmetry phase.Comment: 5 pages, 5 figures, accepted in Physical Review Letter
Short-Range Correlations in Magnetite above the Verwey Temperature
Magnetite, FeO, is the first magnetic material discovered and
utilized by mankind in Ancient Greece, yet it still attracts attention due to
its puzzling properties. This is largely due to the quest for a full and
coherent understanding of the Verwey transition that occurs at K and
is associated with a drop of electric conductivity and a complex structural
phase transition. A recent detailed analysis of the structure, based on single
crystal diffraction, suggests that the electron localization pattern contains
linear three-Fe-site units, the so-called trimerons. Here we show that whatever
the electron localization pattern is, it partially survives up to room
temperature as short-range correlations in the high-temperature cubic phase,
easily discernible by diffuse scattering. Additionally, {\it ab initio}
electronic structure calculations reveal that characteristic features in these
diffuse scattering patterns can be correlated with the Fermi surface topology.Comment: 7 pages, 6 figure
Directional optical switching and transistor functionality using optical parametric oscillation in a spinor polariton fluid
Over the past decade, spontaneously emerging patterns in the density of
polaritons in semiconductor microcavities were found to be a promising
candidate for all-optical switching. But recent approaches were mostly
restricted to scalar fields, did not benefit from the polariton's unique
spin-dependent properties, and utilized switching based on hexagon far-field
patterns with 60{\deg} beam switching (i.e. in the far field the beam
propagation direction is switched by 60{\deg}). Since hexagon far-field
patterns are challenging, we present here an approach for a linearly polarized
spinor field, that allows for a transistor-like (e.g., crucial for
cascadability) orthogonal beam switching, i.e. in the far field the beam is
switched by 90{\deg}. We show that switching specifications such as
amplification and speed can be adjusted using only optical means
Climate pattern-scaling set for an ensemble of 22 GCMs – adding uncertainty to the IMOGEN version 2.0 impact system
Global circulation models (GCMs) are the best tool to understand climate change, as they attempt to represent all the important Earth system processes, including anthropogenic perturbation through fossil fuel burning. However, GCMs are computationally very expensive, which limits the number of simulations that can be made. Pattern scaling is an emulation technique that takes advantage of the fact that local and seasonal changes in surface climate are often approximately linear in the rate of warming over land and across the globe. This allows interpolation away from a limited number of available GCM simulations, to assess alternative future emissions scenarios. In this paper, we present a climate pattern-scaling set consisting of spatial climate change patterns along with parameters for an energy-balance model that calculates the amount of global warming. The set, available for download, is derived from 22 GCMs of the WCRP CMIP3 database, setting the basis for similar eventual pattern development for the CMIP5 and forthcoming CMIP6 ensemble. Critically, it extends the use of the IMOGEN (Integrated Model Of Global Effects of climatic aNomalies) framework to enable scanning across full uncertainty in GCMs for impact studies. Across models, the presented climate patterns represent consistent global mean trends, with a maximum of 4 (out of 22) GCMs exhibiting the opposite sign to the global trend per variable (relative humidity). The described new climate regimes are generally warmer, wetter (but with less snowfall), cloudier and windier, and have decreased relative humidity. Overall, when averaging individual performance across all variables, and without considering co-variance, the patterns explain one-third of regional change in decadal averages (mean percentage variance explained, PVE, 34.25 ± 5.21), but the signal in some models exhibits much more linearity (e.g. MIROC3.2(hires): 41.53) than in others (GISS_ER: 22.67). The two most often considered variables, near-surface temperature and precipitation, have a PVE of 85.44 ± 4.37 and 14.98 ± 4.61, respectively. We also provide an example assessment of a terrestrial impact (changes in mean runoff) and compare projections by the IMOGEN system, which has one land surface model, against direct GCM outputs, which all have alternative representations of land functioning. The latter is noted as an additional source of uncertainty. Finally, current and potential future applications of the IMOGEN version 2.0 modelling system in the areas of ecosystem modelling and climate change impact assessment are presented and discussed
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