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 mínimo de repolho.

O presente Comunicado Técnico tem por objetivo apresentar as diferentes etapas relacionadas com o processamento mínimo do repolho.bitstream/item/42121/1/COT11001.pd

It’s 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 $2\times 2$ 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 für die Trockenseparierung von Mikroaggregaten mit unterschiedlicher Textur zur Messung der mechanischen Belastbarkeit und 3D-Porenstruktur

Die Bodenstruktur als Ausdruck der räumlichen 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 für Mikroorganismen. Die Bodenstruktur, deren einfachste Einheit die Aggregate bilden, befindet sich als labile Bodeneigenschaft in einem Zustand ständiger Veränderung. Die Eigenschaften der Aggregate werden durch viele Einflussfaktoren wie Textur, Alter, Quellung und Schrumpfung, sowie die biologische Aktivität gesteuert. Eines der Hauptprobleme bei der Untersuchung der Eigenschaften von Mikroaggregaten im Boden ist deren Separierung. Viele Separierungs-Methoden üben Spannungszustände aus, die die realen Bedingungen im Boden nur sehr bedingt abbilden. So werden z. B. bei Nasssiebungsverfahren hydraulische Spannungen erzeugt, die unter natürlichen Bedingungen nicht auftreten. Hierin liegt ein Risiko, dass Artefakte in den gewonnenen Aggregatfraktionen entstehen (z. B. durch Reaggregierung bei anschließender Trocknung) und die weitere Analyse von Eigenschaften dieser Aggregatfraktionen, bzw. deren Interpretation beeinflussen. Übergeordnetes Ziel unserer Untersuchungen ist die Erforschung der Genese von Mikroaggregaten und deren (Poren‑)Eigenschaften in Abhängigkeit von Texturunterschieden, sowie des Zusammenhangs von mikroskaligen Deformationsprozessen auf die Entwicklung der Bodenstruktur. Hierfür haben wir mit einem Verfahren der Trockenseparierung in drei Aggregatgrößen-Unterklassen (250-53, 53-20 und <20 µm) eine zuverlässige Methode zur Isolierung einzelner Mikroaggregate entwickelt, welche die Struktur der gewonnenen Aggregate selbst nicht beeinflusst. In einem nächsten Schritt wird die mechanische Belastbarkeit von Mikroaggregaten aus einer Toposequenz (mit unterschiedlichen Tongehalten) an einem Lastrahmen hochauflösend getestet, um die Hypothese zu überprüfen, dass die Stabilität 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 später mit gemessenen Gas- und Wasserflüssen in Verbindung bringen zu können