768 research outputs found
Lowering the sintering temperature of barium strontium titanate bulk ceramics by barium strontium titanate-gel and BaCu(Bâ‚‚Oâ‚…)
In this paper the influence of barium strontium titanate (BST) xerogel as a sinter additive and BaCu(BO) (BCB) as a liquid phase sintering aid on the sintering behavior of BST bulk ceramics is investigated. BST as well as BCB powders were synthesized via a mixed oxide route and BST gel via a sol-gel process. Compared to pure BST bulk ceramics, BST gel reduces the sintering start (onset temperature) by up to 174°C and increases the density for a sintering temperature of 1200°C. By adding BCB to the BST powder the sintering was completed much faster and the onset temperatures were reduced by 281°C and 312°C for 1 mol. % and 2.5 mol. %, respectively. With 2.5 mol. % BCB, the highest density of 96 % (5.41 g∙cm) was achieved at 950°C
Improved limits on the coupling of ultralight bosonic dark matter to photons from optical atomic clock comparisons
We present improved constraints on the coupling of ultralight bosonic dark
matter to photons based on long-term measurements of two optical frequency
ratios. In these optical clock comparisons, we relate the frequency of the
electric-octupole (E3)
transition in Yb to that of the electric-quadrupole (E2) transition of the same ion, and
to that of the transition in Sr.
Measurements of the first frequency ratio are
performed via interleaved interrogation of both transitions in a single ion.
The comparison of the single-ion clock based on the E3 transition with a
strontium optical lattice clock yields the second frequency ratio
. By constraining oscillations of the
fine-structure constant with these measurement results, we improve
existing bounds on the scalar coupling of ultralight dark matter to
photons for dark matter masses in the range of about . These results constitute an improvement by
more than an order of magnitude over previous investigations for most of this
range. We also use the repeated measurements of
to improve existing limits on a linear
temporal drift of and its coupling to gravity.Comment: 7 pages, 5 figure
Coherent Excitation of the Highly Forbidden Electric Octupole Transition in Yb+ 172
We report on the first coherent excitation of the highly forbidden S21/2?F27/2 electric octupole (E3) transition in a single trapped Yb+172 ion, an isotope without nuclear spin. Using the transition in Yb+171 as a reference, we determine the transition frequency to be 642 116 784 950 887.6(2.4) Hz. We map out the magnetic field environment using the forbidden S21/2?D25/2 electric quadrupole (E2) transition and determine its frequency to be 729 476 867 027 206.8(4.4) Hz. Our results are a factor of 1×105 (3×105) more accurate for the E2 (E3) transition compared to previous measurements. The results open up the way to search for new physics via precise isotope shift measurements and improved tests of local Lorentz invariance using the metastable F27/2 state of Yb+. © 2020 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the "https://creativecommons.org/licenses/by/4.0/"Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI
Partisan Asymmetries in Online Political Activity
We examine partisan differences in the behavior, communication patterns and
social interactions of more than 18,000 politically-active Twitter users to
produce evidence that points to changing levels of partisan engagement with the
American online political landscape. Analysis of a network defined by the
communication activity of these users in proximity to the 2010 midterm
congressional elections reveals a highly segregated, well clustered partisan
community structure. Using cluster membership as a high-fidelity (87% accuracy)
proxy for political affiliation, we characterize a wide range of differences in
the behavior, communication and social connectivity of left- and right-leaning
Twitter users. We find that in contrast to the online political dynamics of the
2008 campaign, right-leaning Twitter users exhibit greater levels of political
activity, a more tightly interconnected social structure, and a communication
network topology that facilitates the rapid and broad dissemination of
political information.Comment: 17 pages, 10 figures, 6 table
Eigenvector localization as a tool to study small communities in online social networks
We present and discuss a mathematical procedure for identification of small
"communities" or segments within large bipartite networks. The procedure is
based on spectral analysis of the matrix encoding network structure. The
principal tool here is localization of eigenvectors of the matrix, by means of
which the relevant network segments become visible. We exemplified our approach
by analyzing the data related to product reviewing on Amazon.com. We found
several segments, a kind of hybrid communities of densely interlinked reviewers
and products, which we were able to meaningfully interpret in terms of the type
and thematic categorization of reviewed items. The method provides a
complementary approach to other ways of community detection, typically aiming
at identification of large network modules
Network segregation in a model of misinformation and fact checking
Misinformation under the form of rumor, hoaxes, and conspiracy theories
spreads on social media at alarming rates. One hypothesis is that, since social
media are shaped by homophily, belief in misinformation may be more likely to
thrive on those social circles that are segregated from the rest of the
network. One possible antidote is fact checking which, in some cases, is known
to stop rumors from spreading further. However, fact checking may also backfire
and reinforce the belief in a hoax. Here we take into account the combination
of network segregation, finite memory and attention, and fact-checking efforts.
We consider a compartmental model of two interacting epidemic processes over a
network that is segregated between gullible and skeptic users. Extensive
simulation and mean-field analysis show that a more segregated network
facilitates the spread of a hoax only at low forgetting rates, but has no
effect when agents forget at faster rates. This finding may inform the
development of mitigation techniques and overall inform on the risks of
uncontrolled misinformation online
Loosen Control Without Losing Control. Formalisation and Decentralisation within Commons-Based Peer Production
This study considers commons-based peer production (CBPP) by examining the organizational processes of the free/libre open-source software community, Drupal. It does so by exploring the sociotechnical systems that have emerged around both Drupal's development and its face-to-face communitarian events. There has been criticism of the simplistic nature of previous research into free software; this study addresses this by linking studies of CBPP with a qualitative study of Drupal's organizational processes. It focuses on the evolution of organizational structures, identifying the intertwined dynamics of formalization and decentralization, resulting in coexisting sociotechnical systems that vary in their degrees of organicity
Embodied Commons: Knowledge and Sharing in Delhi's Electronic Bazaars
The notion of the ‘sharing economy’ has recently received significant academic and non-academic attention. What the different debates have in common is an emphasis on how technologically mediated knowledge and specific social motivations enable practices of sharing. This article discusses knowledge and sharing in popular marketplaces. Based on an ethnography of Delhi’s electronic bazaars, Lajpat Rai market, Palika Bazaar and Nehru Place, this article suggests ways to think about knowledge that are embodied and practice-based: What is such embodied knowledge? How is it created and shared? The article argues that bazaars combine sociality established through face-to-face bargaining with informal trade arrangements to enable co-creation and collaboration around technological products. The resulting knowledge is tacit in nature and is mimetically transmitted between bodies. As a result, the bazaars feature a kind of sharing that is distinct from what is understood by most accounts of the sharing economy
- …