1,136 research outputs found

    The medieval bronze doors of San Zeno, Verona: combining material analyses and art history

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    The bronze doors of the Basilica of San Zeno in Verona, Italy, are a special case in art history research. They were made by several workshops during the twelfth century: stylistically, two to three workshops were assumed to pro- duce the metal parts of the door. However, it is still unclear when exactly and if this interpretation can be supported by the chemical composition of the metal. In this research we aimed to verify the art history interpretation by iden- tifying the alloy composition of each individual metal plate. The composition of the supporting wooden structures are discussed. A portable ED-XRF instrument and optical microscopes were used to analyse and document the doors non-invasively. The doors were also photographed to produce high resolution orthophotos and 3D models. We can confirm that the metal parts of the doors were made of leaded tin-bronze as well as leaded brass and mounted on a wooden structure mainly made of spruce and oak wood. Chemically, two/three different groups of alloys have been identified, which can be associated with two or three different workshops, and which largely correspond to the stylistic interpretation

    LIPIcs, Volume 251, ITCS 2023, Complete Volume

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    LIPIcs, Volume 251, ITCS 2023, Complete Volum

    A Preliminary Study of the Effect of the Chirped Rotating Wall on a Positron Cloud

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    The density of the positron cloud is a crucial parameter in many applications ofaccumulated positrons. Previous work has shown that adjusting the frequency ofthe rotating wall potential following positron accumulation can be used to controlthe density of positron clouds. In this work, positron clouds were studied afterbeing compressed using a linear rotating wall frequency sweep under a selection ofrotating wall drive amplitudes and cooling gas pressures following an initial staticfrequency compression. This was performed for SF6, CF4, and briefly for CO. Theeffect of changing the cooling gas appears congruent to that shown by the staticfrequency case. The results are in qualitative agreement with previous work byDeller et al., and compare briefly but favourably to a simplistic numerical model

    Synthesis of Quasi-Freestanding Graphene Films Using Radical Species Formed in Cold Plasmas

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    For over a decade, the Stinespring laboratory has investigated scalable, plasma assisted synthesis (PAS) methods for the growth of graphene films on silicon carbide (SiC). These typically utilized CF4-based inductively coupled plasma (ICP) with reactive ion etching (RIE) to selectively etch silicon from the SiC lattice. This yielded a halogenated carbon-rich surface layer which was then annealed to produce the graphene layers. The thickness of the films was controlled by the plasma parameters, and overall, the process was readily scalable to the diameter of the SiC wafer. The PAS process reproducibly yielded two- to three-layer thick graphene films that were highly tethered to the underlying SiC substrate via an intermediate buffer layer. The buffer layer was compositionally similar to graphene. However, a significant number of graphene carbons were covalently bound to silicon atoms in the underlying substrate. This tethering lead to mixing of the film and substrate energy bands which degraded many of graphene’s most desirable electrical properties. The research described in this dissertation was aimed at improving graphene quality by reducing the extent of tethering using a fundamentally different plasma etching mechanism while maintaining scalability. In the ICP-RIE process, the etchant species include F and CFx (x = 1-3) radicals and their corresponding positive ions. These radicals are classified as “cold plasma species” in the sense that they are nominally in thermal equilibrium with the substrate and walls of the system. In contrast, the electrons exist at extremely high temperature (energy), and the ionic species are accelerated to energies on the order of several hundred electron volts by the plasma bias voltage that exists between the plasma and substrate. As a result, the ionic species create a directional, high rate etch that is dominated by physical etching characterized by energy and momentum transfer. In contrast, the neutral radicals chemically etch the surface at a much lower rate. In this work, the effects of physical etching due to high energy ions were eliminated by shielding the SiC substrate using a mask (e.g., quartz) supported by silicon posts. In this way, a microplasma consisting of chemically reactive cold plasma species was created in the small space between the substrate surface and the backside of the quartz mask. This process, referred to here as microplasma assisted synthesis (MPAS), was used to produce graphene films. A parametric investigation was conducted to determine the influence of MPAS operating parameters on graphene quality. The key parameters investigated included ICP power, RIE power, etch time, various mask materials, microreactor height, substrate cooling, initial surface morphology and SiC polytype. The resulting graphene films were characterized by x-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and atomic force microscopy (AFM). Following optimization of the MPAS process, some tethering of the graphene films remained. However, films produced by MPAS consistently exhibited significantly less tethering than those produced using the PAS process. Moreover, both XPS and Raman spectroscopy indicated that these films were quasi-free standing, and, in some cases, they approached free standing graphene. From a wide view, the results of these studies demonstrate the potential of MPAS as a technique for realizing the controlled synthesis of high-quality, lightly tethered mono-, and few-layer graphene films directly on an insulating substrate. On a more fundamental level, the results of these studies provide insight into the surface chemistry of radical species

    Measuring the impact of COVID-19 on hospital care pathways

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    Care pathways in hospitals around the world reported significant disruption during the recent COVID-19 pandemic but measuring the actual impact is more problematic. Process mining can be useful for hospital management to measure the conformance of real-life care to what might be considered normal operations. In this study, we aim to demonstrate that process mining can be used to investigate process changes associated with complex disruptive events. We studied perturbations to accident and emergency (A &E) and maternity pathways in a UK public hospital during the COVID-19 pandemic. Co-incidentally the hospital had implemented a Command Centre approach for patient-flow management affording an opportunity to study both the planned improvement and the disruption due to the pandemic. Our study proposes and demonstrates a method for measuring and investigating the impact of such planned and unplanned disruptions affecting hospital care pathways. We found that during the pandemic, both A &E and maternity pathways had measurable reductions in the mean length of stay and a measurable drop in the percentage of pathways conforming to normative models. There were no distinctive patterns of monthly mean values of length of stay nor conformance throughout the phases of the installation of the hospital’s new Command Centre approach. Due to a deficit in the available A &E data, the findings for A &E pathways could not be interpreted

    The development of efficient hemi-autotrophic carbon fixation in Escherichia Coli

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    Carbon fixation is a process vital to any life and as by far its most prevalent variant, the Calvin Benson Bassham (CBB) cycle is vital to virtually all known terrestrial life. Mostly occurring in plants, it uses light energy to sequester atmospheric carbon dioxide (CO2) and convert it into biomass. As the most inefficient natural carboxylation process and source of most biomass documented, even a small increase of its performance could have vast downstream effects. Such a development could assimilate the abundantly available atmospheric CO2 while generating minimal amounts of waste for any biosynthesized product. The Escherichia coli bacterium was previously shown to functionally express the CBB cycle upon the addition of phosphoribulokinase (PRK) and ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBisCO). Further knock-outs severed its energetic metabolism from the carbon metabolism resulted CO2-dependent biomass accumulation. This carbon fixation is driven by the energy independently generated in the TCA cycle from a supply of pyruvate. This unique, split metabolism was dubbed hemi-autotrophy. The hemi-autotrophic strain of E. coli serves as a model organism for the CBB cycle, but lacking any of the difficulties of light-dependent or multi-cellular organisms. A pyrophosphate-dependent 6-phosphofructokinase (PFP) originating from Methylococcus capsulatus Bath was characterised as catalyzing three reactions of the typical CBB cycle. Where PRK completes its catalysis with a dependency on energy-carrier adenosine triphosphate (ATP), PFP was shown to complete this reaction with the less energetic pyrophosphate (PPi) that is partially generated in its FBPase and SBPase-equivalent reactions. Successful integration of this synthetic CBB cycle would conserve 33% of all ATP expended in the native CBB cycle. The hemi-autrophic E. coli strain’s unique culturing requirements proved challenging but methods with increased dependability were established. Transformations without the relief of these conditions remain elusive, requiring pre-cultures in rich media and heterotrophic metabolism. The consecutive sub-culturing of the strain to increase its hampered growth characteristics resulted in mild improvements. Despite observing modest culturing characteristic and a relatively high chromosomal mutation rate, the strain did not demonstrated an increase in transformation efficiency. The attempted replacements of the plasmid-encoded prkA by pfp did not result in hemi-autotrophic growth in any of its constructs, despite modulation of their expression. Troubled by high mutation rates, it remains unknown whether the expression range of the significantly less efficient PFP was sufficient or if the cytoplasmic availability of PPi remained below its functionally required concentration. The putative H+-pyrophosphatase pump (HPP), natively expressed as the second gene in the pfp-hpp operon, remains uncharacterised but its co-expression did not manage to compensate for this deficiency either. Though native fbp was successfully knocked-out, the essential inorganic pyrophosphatase gene of E. coli remains. Thorough analysis of the components in the CBB system led to several design improvements and pathway modelling indicates the proposed synthetic CBB cycle is a viable alternative to its natural variant. Thermodynamic feasibility of the synthetic pathway was confirmed and kinetic analysis also predicted it to perform at reduced efficiencies while still indicating culture viability. Growth rates approximating those of the hemi-autotrophic strain were produced in a kinetic model of the central carbon metabolism while incorporating minimal assumptions. Modifying it to support the synthetic CBB cycle suggested its viability at a nominal reduction of growth, while suggesting further directions of research for the system

    A Region-Shrinking-Based Acceleration for Classification-Based Derivative-Free Optimization

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    Derivative-free optimization algorithms play an important role in scientific and engineering design optimization problems, especially when derivative information is not accessible. In this paper, we study the framework of classification-based derivative-free optimization algorithms. By introducing a concept called hypothesis-target shattering rate, we revisit the computational complexity upper bound of this type of algorithms. Inspired by the revisited upper bound, we propose an algorithm named "RACE-CARS", which adds a random region-shrinking step compared with "SRACOS" (Hu et al., 2017).. We further establish a theorem showing the acceleration of region-shrinking. Experiments on the synthetic functions as well as black-box tuning for language-model-as-a-service demonstrate empirically the efficiency of "RACE-CARS". An ablation experiment on the introduced hyperparameters is also conducted, revealing the mechanism of "RACE-CARS" and putting forward an empirical hyperparameter-tuning guidance

    Mesozoic-Cenozoic Topographic Evolution of the South Tianshan (NW China): Insights from Detrital Apatite Geo-Thermochronological and Geochemical Analyses

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    The present-day topography of Tianshan is the product of repeated phases of Meso-Cenozoic intracontinental deformation and reactivation, whereas the long-term Mesozoic topographic evolution and the timing of the onset of Cenozoic deformation remain debated. New insights into the Meso-Cenozoic geodynamic evolution and related basin-range interactions in the Tianshan were obtained based on new detrital single-grain apatite U-Pb, fission-track, and trace-element provenance data from Mesozoic sedimentary sequences on the northern margin of the Tarim Basin. Detrital apatite U-Pb age data from Early-Middle Triassic clastic rocks show two prominent age populations at 500–390 Ma and 330–260 Ma, with a paucity of ages between 390 and 330 Ma, suggesting that sediment source is predominantly from the northern Tarim and South Tianshan. From the Late Triassic to Early Jurassic, the first appearance of populations in the 390–330 Ma and 260–220 age ranges indicates that the Central Tianshan-Yili Block and Western Kunlun Orogen were source regions for the northern margin of Tarim Basin. In the Cretaceous strata, south-directed paleocurrents combined with the decrease in the 390–330 Ma age population from the Central Tianshan-Yili Block imply that South Tianshan was uplifted and again became the main source region to the Baicheng-Kuqa depression during the Cretaceous. Our new apatite fission-track data from the southern Chinese Tianshan suggest that rapid cooling commenced at c. 30 Ma along the southern margin, and the Early Mesozoic strata exposed on the southern flank of the Tianshan underwent c. 4–5 km of late Cenozoic exhumation during this period. This age is approximately synchronous with the onset of exhumation/deformation not only in the whole Tianshan but also in the interior of the Tibetan Plateau and its margins. It suggests that far-field, N-directed shortening resulting from the India-Asia collision was transmitted to the Tianshan at that time

    When is the average number of saddle points typical?

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    A common measure of a function's complexity is the count of its stationary points. For complicated functions, this count grows exponentially with the volume and dimension of their domain. In practice, the count is averaged over a class of functions (the annealed average), but the large numbers involved can produce averages biased by extremely rare samples. Typical counts are reliably found by taking the average of the logarithm (the quenched average), which is more difficult and not often done in practice. When most stationary points are uncorrelated with each other, quenched and annealed averages are equal. Equilibrium heuristics can guarantee when most of the lowest minima will be uncorrelated. We show that these equilibrium heuristics cannot be used to draw conclusions about other minima and saddles by producing examples among Gaussian-correlated functions on the hypersphere where the count of certain saddles and minima has different quenched and annealed averages, despite being guaranteed "safe" in the equilibrium setting. We determine conditions for the emergence of non-trivial correlations between saddles, and discuss the implications for the geometry of those functions and what out-of-equilibrium settings might be affected

    Strong Topological Trivialization of Multi-Species Spherical Spin Glasses

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    We study the landscapes of multi-species spherical spin glasses. Our results determine the phase boundary for annealed trivialization of the number of critical points, and establish its equivalence with a quenched \emph{strong topological trivialization} property. Namely in the "trivial" regime, the number of critical points is constant, all are well-conditioned, and all approximate critical points are close to a true critical point. As a consequence, we deduce that Langevin dynamics at sufficiently low temperature has logarithmic mixing time. Our approach begins with the Kac--Rice formula. We derive closed form expressions for some asymptotic determinants studied in (Ben Arous-Bourgade-McKenna 2023, McKenna 2021), and characterize the annealed trivialization phase by explicitly solving a suitable multi-dimensional variational problem. To obtain more precise quenched results, we develop general purpose techniques to avoid sub-exponential correction factors and show non-existence of \emph{approximate} critical points. Many of the results are new even in the 11-species case.Comment: 57 pages, 4 figures. Updated reference
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