Long-term memory magnetic correlations in the Hubbard model: A dynamical mean-field theory analysis

We investigate the onset of a not-decaying asymptotic behavior of temporal magnetic correlations in the Hubbard model in infinite dimensions. This long-term memory feature of dynamical spin correlations can be precisely quantified by computing the difference between the zero-frequency limit of the Kubo susceptibility and the corresponding static isothermal one. Here, we present a procedure for reliably evaluating this difference starting from imaginary time-axis data, and apply it to the testbed case of the Mott-Hubbard metal-insulator transition (MIT). At low temperatures, we find long-term memory effects in the entire Mott regime, abruptly ending at the first order MIT. This directly reflects the underlying local moment physics and the associated degeneracy in the many-electron spectrum. At higher temperatures, a more gradual onset of an infinitely-long time-decay of magnetic correlations occurs in the crossover regime, not too far from the Widom line emerging from the critical point of the MIT. Our work has relevant algorithmic implications for the analytical continuation of dynamical susceptibilities in strongly correlated regimes and offers a new perspective for unveiling fundamental properties of the many-particle spectrum of the problem under scrutiny.Comment: 36 pages, 14 figures, resubmission to SciPos

Comparative study regarding the sputtering yield of nanocolumnar tungsten surfaces under Ar+ irradiation

Nanostructured tungsten has been proposed as a promising option for plasma facing materials in future fusion reactors, because compared to conventional tungsten it shows advantages such as a better radiation resistance and, in particular, a retardation of tungsten-fuzz growth. Besides these aspects, the sputtering yield of nanostructured tungsten under ion bombardment is of interest, since it would affect the atomic density of tungsten emitted into the fusion plasma, which leads to radiative heat losses. In this work, we present a multiscale approach for investigating the sputtering yield of nanocolumnar tungsten surfaces under 1 keV and 2 keV Ar irradiation. Our results cover experiments and also computational simulations, which operate either on the basis of the binary collision approximation and ray tracing or use a full molecular dynamics implementation. In our studied case, both computational approaches can predict the sputtering yield of nanocolumnar tungsten surfaces very well. In comparison to flat W, we observe a much reduced dependence on the ion incidence angle, similar as reported for conventional rough surfaces in literature. However, an additional global reduction of the sputtering yield was identified, which can be attributed to geometrical redeposition effects between the separated nanocolumns. These results support the applicability of nanocolumnar tungsten as a first wall coating.Peer reviewe

Methods for non-proportional hazards in clinical trials: A systematic review

For the analysis of time-to-event data, frequently used methods such as the log-rank test or the Cox proportional hazards model are based on the proportional hazards assumption, which is often debatable. Although a wide range of parametric and non-parametric methods for non-proportional hazards (NPH) has been proposed, there is no consensus on the best approaches. To close this gap, we conducted a systematic literature search to identify statistical methods and software appropriate under NPH. Our literature search identified 907 abstracts, out of which we included 211 articles, mostly methodological ones. Review articles and applications were less frequently identified. The articles discuss effect measures, effect estimation and regression approaches, hypothesis tests, and sample size calculation approaches, which are often tailored to specific NPH situations. Using a unified notation, we provide an overview of methods available. Furthermore, we derive some guidance from the identified articles. We summarized the contents from the literature review in a concise way in the main text and provide more detailed explanations in the supplement (page 29)

Wertfalle KMU für mittelständische Banken im Hinblick auf Basel II

von Martina FellingerKlagenfurt, Alpen-Adria-Univ., Dipl.-Arb., 2008KB2008 07(VLID)241284

Investigation of Sputtering Properties of Tungsten Model Systems Relevant for Nuclear Fusion Devices

Arbeit an der Bibliothek noch nicht eingelangt - Daten nicht geprüftAbweichender Titel nach Übersetzung der Verfasserin/des VerfassersDie Entwicklung einer Möglichkeit die immense nukleare Energie von Fusionsprozessen technisch nutzbar zu machen ist ein ambitioniertes Unterfangen, dessen Umsetzung seit Jahrzehnten von Wissenschaft und Technik angestrebt wird. Zukünftige Reaktoren werden stetig geplant und weiterentwickelt. Neueste Erkenntnisse bei der Entwicklung und Testung möglicher Operationsmodi haben gezeigt, dass die Wissenschaft mit heuti- gen Mitteln so nah an der technischen Realisierung der Kernfusionsenergie als sichere, effiziente und kohlenstoffarme Energiequelle steht wie noch nie.Allerdings gibt es vor dem tatsächlichen Bau eines konventionell nutzbaren Fusionsreaktors noch Aspekte, die weitere wissenschaftliche Untersuchungen benötigen. Ein bisher noch nicht vollständig untersuchtes Gebiet beschäftigt sich mit dem Verhalten von Wandmaterialen, mit denen die Innenseiten des Reaktors ausgekleidet werden. Sowohl Erosions- als auch Implantationsprozesse werden in zukünftigen Reaktoren dynamisch auftreten. Die Eigenschaften der verwendeten Stoffe müssen dabei kritischen Auswahlkriterien für potentielle Wandmaterialien genügen, da ins Plasma erodierte Teilchen die Effektivität des Reaktors stark drosseln.Aus diesem Grund wurde im Zuge dieser Diplomarbeit das Zerstäubungsverhalten von reinen Wolfram (W) Schichten sowie von redeponierten W Schichten untersucht. Dazu wurden zwei präzise Quarz-Kristall-Mikrowaagen (QCM) gemeinsam mit einem zweckoptimierten Aufbau am IAP TU Wien verwendet. Das experimentelle Setup wurde im Zuge dieser Arbeit erweitert und mit Motoren bestückt, sodass nun vollautomatische Messprozedere durchgeführt werden können. Zusätzlich zur experimentellen Arbeit wurden ergänzende Simulationen durchgeführt, um einen Vergleich mit den Messdaten zu ermöglichen.Ergebnisse haben gezeigt, dass die Absolutwerte der Zerstäubungsausbeuten für die verwendeten Schichten aus reinem W sowie redeponiertem W vergleichbar sind. Eine detaillierte Analyse der Abhängigkeit der Zerstäubungsausbeute vom Ioneneinfallswinkel lässt außerdem auf interessante physikalische Konsequenzen, nämlich mögliche Kristall- strukturen auf einer den verwendeten Proben, schließen.Die Resultate aus dieser Arbeit tragen zum Fortschritt des europäischen Konsortiums für die Entwicklung von Fusionsenergie, EUROFusion, im Zuge des Projekts ”Plasma- Wall Interaction & Exhaust (PWIE) - Subproject SP B” bei.An implementation utilising the huge amounts of nuclear energy released in the pro- cess of fusing light nuclei to heavier ones is an ambitious project, whose realisation has by now been pursued by science and technology over several decades. Future fusion reactors are presently being planned and recent results obtained at test beds for such devices revealed promising data, suggesting that fusion as a safe, efficient and lowcarbon energy supply is on its way to make expectations meet.However, various issues concerning this proposal still need profound analyses. One aspect that is tackled in the course of this project, are properties of potential first wall materials directly facing the plasma in fusion devices. Erosion as well as implanta- tion processes will occur dynamically due to plasma wall interactions inside the reactor. Hence, sputtering behaviours of relevant wall coating materials and additionally redeposited layers thereof are of great interest for fusion research. This concern arises from the fact that properties like erosion form crucial selection criteria for first wall coatings, since eroded particles drastically reduce the reactor efficiency.In this work, a precise twofold quartz crystal microbalance (QCM) measurement tech- nique combined with a dedicated sputtering setup at IAP TU Wien was successfully employed in order to measure sputtering properties of pure tungsten (W) as well as redeposited W layers. The experimental setup has been upgraded in the course of this thesis, enabling to perform sophisticated measurement routines entirely remotely. Ad- ditionally, numerical simulations were performed for comparison.Results have shown that the absolute values of sputter yields as well as the angular distribution of sputtered particles show comparable values for both the pure W target and the sample hosting an additional layer containing redeposited W. However, a more detailed look onto the dependence of the sputter yields on ion incidence angles has revealed interesting physical behaviour, suggesting specific crystal structures on one of the utilised samples.The outcome of this project contributed to the scientific progress in the work package ”Plasma Wall Interaction & Exhaust (PWIE) - Subproject SP B” of the European consor- tium for the development of fusion energy, EUROfusion.5