498 research outputs found
Collective modes at a disordered quantum phase transition
We study the collective excitations, i.e., the Goldstone (phase) mode and the
Higgs (amplitude) mode, near the superfluid--Mott glass quantum phase
transition in a two-dimensional system of disordered bosons. Using Monte Carlo
simulations as well as an inhomogeneous quantum mean-field theory with Gaussian
fluctuations, we show that the Higgs mode is strongly localized for all
energies, leading to a noncritical scalar response. In contrast, the
lowest-energy Goldstone mode undergoes a striking delocalization transition as
the system enters the superfluid phase. We discuss the generality of these
findings and experimental consequences, and we point out potential relations to
many-body localization.Comment: 5 pages + 7 pages supplement, 10 figures included. Final version as
publishe
Processamento mĂnimo de repolho.
O presente Comunicado TĂ©cnico tem por objetivo apresentar as diferentes etapas relacionadas com o processamento mĂnimo do repolho.bitstream/item/42121/1/COT11001.pd
Itâs all about information? The Following Behaviour of Professors and PhD Students on Twitter
In this paper we investigate the role of the academic status in the following behaviour of computer scientists on Twitter. Based on a uses and gratifications perspective, we focus on the activity of a Twitter account and the reciprocity of following relationships. We propose that the account activity addresses the users' information motive only, whereas the user's academic status relates to both the information motive and community development (as in peer networking or career planning). Variables were extracted from Twitter user data. We applied a biographical approach to correctly identify the academic status (professor versus PhD student). We calculated a MANOVA on the influence of the activity of the account and the academic status (on different groups of followers) to differentiate the influence of the information motive versus the motive for community development. Results suggest that for computer scientists Twitter is mainly an information network. However, we found significant effects in the sense of career planning, that is, the accounts of professors had even in the case of low activity a relatively high number of researcher followers -- both PhD followers as well as professor followers. Additionally, there was also some weak evidence for community development gratifications in the sense of peer-networking of professors. Overall, we conclude that the academic use of Twitter is not only about information, but also about career planning and networking
Two-dimensional solar spectropolarimetry with the KIS/IAA Visible Imaging Polarimeter
Spectropolarimetry at high spatial and spectral resolution is a basic tool to
characterize the magnetic properties of the solar atmosphere. We introduce the
KIS/IAA Visible Imaging Polarimeter (VIP), a new post-focus instrument that
upgrades the TESOS spectrometer at the German VTT into a full vector
polarimeter. VIP is a collaboration between the KIS and the IAA. We describe
the optical setup of VIP, the data acquisition procedure, and the calibration
of the spectropolarimetric measurements. We show examples of data taken between
2005 and 2008 to illustrate the potential of the instrument. VIP is capable of
measuring the four Stokes profiles of spectral lines in the range from 420 to
700 nm with a spatial resolution better than 0.5". Lines can be sampled at 40
wavelength positions in 60 s, achieving a noise level of about 2 x 10E-3 with
exposure times of 300 ms and pixel sizes of 0.17" x 0.17" (2 x 2 binning). The
polarization modulation is stable over periods of a few days, ensuring high
polarimetric accuracy. The excellent spectral resolution of TESOS allows the
use of sophisticated data analysis techniques such as Stokes inversions. One of
the first scientific results of VIP presented here is that the ribbon-like
magnetic structures of the network are associated with a distinct pattern of
net circular polarization away from disk center. VIP performs
spectropolarimetric measurements of solar magnetic fields at a spatial
resolution that is only slightly worse than that of the Hinode
spectropolarimeter, while providing a 2D field field of view and the
possibility to observe up to four spectral regions sequentially with high
cadence. VIP can be used as a stand-alone instrument or in combination with
other spectropolarimeters and imaging systems of the VTT for extended
wavelength coverage.Comment: 10 pages, 8 figures, accepted by Astronomy and Astrophysics v2:
figures updated with improved qualit
How Digital Are the Digital Humanities? An Analysis of Two Scholarly Blogging Platforms
In this paper we compare two academic networking platforms, HASTAC and Hypotheses, to show the distinct ways in which they serve specific communities in the Digital Humanities (DH) in different national and disciplinary contexts. After providing background information on both platforms, we apply co-word analysis and topic modeling to show thematic similarities and differences between the two sites, focusing particularly on how they frame DH as a new paradigm in humanities research. We encounter a much higher ratio of posts using humanities-related terms compared to their digital counterparts, suggesting a one-way dependency of digital humanities-related terms on the corresponding unprefixed labels. The results also show that the terms digital archive, digital literacy, and digital pedagogy are relatively independent from the respective unprefixed terms, and that digital publishing, digital libraries, and digital media show considerable cross-pollination between the specialization and the general noun. The topic modeling reproduces these findings and reveals further differences between the two platforms. Our findings also indicate local differences in how the emerging field of DH is conceptualized and show dynamic topical shifts inside these respective contexts
The GREGOR Fabry-P\'erot Interferometer
The GREGOR Fabry-P\'erot Interferometer (GFPI) is one of three first-light
instruments of the German 1.5-meter GREGOR solar telescope at the Observatorio
del Teide, Tenerife, Spain. The GFPI uses two tunable etalons in collimated
mounting. Thanks to its large-format, high-cadence CCD detectors with
sophisticated computer hard- and software it is capable of scanning spectral
lines with a cadence that is sufficient to capture the dynamic evolution of the
solar atmosphere. The field-of-view (FOV) of 50" x 38" is well suited for quiet
Sun and sunspot observations. However, in the vector spectropolarimetric mode
the FOV reduces to 25" x 38". The spectral coverage in the spectroscopic mode
extends from 530-860 nm with a theoretical spectral resolution R of about
250,000, whereas in the vector spectropolarimetric mode the wavelength range is
at present limited to 580-660 nm. The combination of fast narrow-band imaging
and post-factum image restoration has the potential for discovery science
concerning the dynamic Sun and its magnetic field at spatial scales down to
about 50 km on the solar surface.Comment: 14 pages, 17 figures, 4 tables; pre-print of AN 333, p.880-893, 2012
(AN special issue to GREGOR
Ein neuer Ansatz fĂŒr die Trockenseparierung von Mikroaggregaten mit unterschiedlicher Textur zur Messung der mechanischen Belastbarkeit und 3D-Porenstruktur
Die Bodenstruktur als Ausdruck der rĂ€umlichen Anordnung mineralischer und organischer Bodenbestandteile ist eine zentrale Charakteristik des Bodens. Sie steuert viele wichtige biologische, physikalische und geochemische Prozesse, wie die Rolle des Bodens als Kohlenstoffspeicher oder die Ausbildung bzw. Verteilung von Habitaten fĂŒr Mikroorganismen. Die Bodenstruktur, deren einfachste Einheit die Aggregate bilden, befindet sich als labile Bodeneigenschaft in einem Zustand stĂ€ndiger VerĂ€nderung. Die Eigenschaften der Aggregate werden durch viele Einflussfaktoren wie Textur, Alter, Quellung und Schrumpfung, sowie die biologische AktivitĂ€t gesteuert. Eines der Hauptprobleme bei der Untersuchung der Eigenschaften von Mikroaggregaten im Boden ist deren Separierung. Viele Separierungs-Methoden ĂŒben SpannungszustĂ€nde aus, die die realen Bedingungen im Boden nur sehr bedingt abbilden. So werden z. B. bei Nasssiebungsverfahren hydraulische Spannungen erzeugt, die unter natĂŒrlichen Bedingungen nicht auftreten. Hierin liegt ein Risiko, dass Artefakte in den gewonnenen Aggregatfraktionen entstehen (z. B. durch Reaggregierung bei anschlieĂender Trocknung) und die weitere Analyse von Eigenschaften dieser Aggregatfraktionen, bzw. deren Interpretation beeinflussen.
Ăbergeordnetes Ziel unserer Untersuchungen ist die Erforschung der Genese von Mikroaggregaten und deren (Porenâ)Eigenschaften in AbhĂ€ngigkeit von Texturunterschieden, sowie des Zusammenhangs von mikroskaligen Deformationsprozessen auf die Entwicklung der Bodenstruktur. HierfĂŒr haben wir mit einem Verfahren der Trockenseparierung in drei AggregatgröĂen-Unterklassen (250-53, 53-20 und <20 ”m) eine zuverlĂ€ssige Methode zur Isolierung einzelner Mikroaggregate entwickelt, welche die Struktur der gewonnenen Aggregate selbst nicht beeinflusst. In einem nĂ€chsten Schritt wird die mechanische Belastbarkeit von Mikroaggregaten aus einer Toposequenz (mit unterschiedlichen Tongehalten) an einem Lastrahmen hochauflösend getestet, um die Hypothese zu ĂŒberprĂŒfen, dass die StabilitĂ€t mit abnehmender struktureller Entropie (d. h. zunehmendem Grad an Strukturierung) zunimmt.
Des Weiteren wird die Geometrie des Porennetzwerkes der Mikroaggregate mit unterschiedlichen Tongehalten mittels hochauflösender Computertomographie untersucht, um diese spĂ€ter mit gemessenen Gas- und WasserflĂŒssen in Verbindung bringen zu können
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