arXiv subjects

Thomas Schmidt

Publications and source records attributed to Thomas Schmidt.

At least 37 records · Page 2Linked to original sources

arXiv2023

Nature and Nurture? Comparing Ly$α$ Detections in UV-Bright and Fainter [O III]+H$β$ Emitters at $z\sim8$ With Keck/MOSFIRE

The 100% detection rate of Ly$α$ emission in a sample of four luminous $z\sim8$ galaxies with red Spitzer/IRAC colors suggests objects with unusual ionizing capabilities that created early ionized bubbles in a neutral era. Whether such bubbles reflect enhanced ionizing properties (nature) or an overdense environment (nurture), however, remains unclear. Here we aim to distinguish between these hypotheses via a search for Ly$α$ emission in five fainter galaxies drawn from the CANDELS-GOODS fields using a similar IRAC excess and UV magnitudes that should reflect reduced clustering effects. Using Keck/MOSFIRE we tentatively detect $>4σ$ line emission in only two targets at redshifts $z_{\rm Lyα}$=7.1081 and $z_{\rm Lyα}$=7.9622 with rest-frame EWs of 16-17 A, $\sim$1.5$\times$ weaker compared to their brighter counterparts. Thus we find a reduced rate for Ly$α$ emission of $0.40^{+0.30}_{-0.25}$ compared to $1.00^{+0.00}_{-0.44}$ for more luminous examples. The lower rate agrees with predictions from simulations of a mostly neutral IGM and an intrinsic EW$_{0,\rm Lyα}$ distribution for $z\sim6$ galaxies. However, even with an extreme EW$_{0,\rm Lyα}$ model, it is challenging to match the detection rate for the luminous objects. SED-fitting of our fainter sample indicates young and star-forming systems, albeit with less extreme SFRs and ionization parameters compared to their luminous counterparts. The enhanced Ly$α$ rate in luminous galaxies is thus likely a byproduct of both extreme ionizing properties as well as environment effects. Further studies with JWST may be required to resolve the physical nature of this puzzling population.

astro-ph.GA↗

arXiv2022

Juggling too many balls at once: Qualitatively different effects when measuring priming and masking in single, dual, and triple tasks

Dissociation paradigms examine dissociations between indirect measures of prime processing and direct measures of prime awareness. It is debated whether direct measures should be objective or subjective, and whether these measures should be obtained on the same or separate trials. In two metacontrast experiments, we measured prime discrimination, PAS ratings, and response priming either separately or in multiple tasks. Single tasks show the fastest responses in priming and therefore most likely meet the assumption of feedforward processing as assumed under Rapid-Chase Theory. Similarly, dual tasks allow for a fast response activation by the prime; nevertheless, prolonged responses and slower errors occur more often. In contrast, triple tasks have a negative effect on response activation: responses are massively slowed and fast prime-locked errors are lost. Moreover, decreasing priming effects and prime identification performance result in a loss of a double dissociation. Here, a necessary condition for unconscious response priming, feedforward processing, is violated.

q-bio.NC↗

arXiv2022

DSME-LoRa: Seamless Long Range Communication Between Arbitrary Nodes in the Constrained IoT

Long range radio communication is preferred in many IoT deployments as it avoids the complexity of multi-hop wireless networks. LoRa is a popular, energy-efficient wireless modulation but its networking substrate LoRaWAN introduces severe limitations to its users. In this paper, we present and thoroughly analyze DSME-LoRa, a system design of LoRa with IEEE 802.15.4 DSME as a MAC layer. DSME-LoRa offers the advantage of seamless client-to-client communication beyond the pure gateway-centric transmission of LoRaWAN. We evaluate its feasibility via a full-stack implementation on the popular RIOT operating system, assess its steady-state packet flows in an analytical stochastic Markov model, and quantify its scalability in massive communication scenarios using large scale network simulations. Our findings indicate that DSME-LoRa is indeed a powerful approach that opens LoRa to standard network layers and outperforms LoRaWAN in many dimensions.

cs.NI↗

arXiv2022

Usable Security for an IoT OS: Integrating the Zoo of Embedded Crypto Components Below a Common API

IoT devices differ widely in crypto-supporting hardware, ranging from no hardware support to powerful accelerators supporting numerous of operations including protected key storage. An operating system should provide uniform access to these heterogeneous hardware features, which is a particular challenge in the resource constrained IoT. Effective security is tied to the usability of cryptographic interfaces. A thoughtful API design is challenging, and it is beneficial to re-use such an interface and to share the knowledge of programming embedded security widely. In this paper, we integrate an emerging cryptographic interface into usable system-level calls for the IoT operating system RIOT, which runs on more than 240 platforms. This interface supports ID-based key handling to access key material in protected storage without exposing it to anyone. Our design foresees hardware acceleration on all available variants; our implementation integrates diverse cryptographic hardware and software backends via the uniform interface. Our performance measurements show that the overhead of the uniform API with integrated key management is negligible compared to the individual crypto operation. Our approach enhances the usability, portability, and flexibility of cryptographic support in the IoT.

cs.CR↗

arXiv2022

X-ray performance of critical-angle transmission grating prototypes for the Arcus mission

Arcus is a proposed soft x-ray grating spectrometer Explorer. It aims to explore cosmic feedback by mapping hot gases within and between galaxies and galaxy clusters and characterizing jets and winds from supermassive black holes, and to investigate the dynamics of protoplanetary discs and stellar accretion. Arcus features 12 m-focal-length grazing-incidence silicon pore optics (SPO) developed for the Athena mission. Critical-angle transmission (CAT) gratings efficiently disperse high diffraction orders onto CCDs. We report new and improved x-ray performance results for Arcus-like CAT gratings, including record resolving power for two co-aligned CAT gratings. Multiple Arcus prototype grating facets were illuminated by an SPO at the PANTER facility. The facets consist of $32\times32.5$ mm$^2$ patterned silicon membranes, bonded to metal frames. The bonding angle is adjusted according to the measured average tilt angle of the grating bars in the membrane. Two simultaneously illuminated facets show minor broadening of the Al-K$_α$ doublet in 18$^{\rm th}$ and 21$^{\rm st}$ orders with a best fit record effective resolving power of $R_G \approx 1.3^{+\infty}_{-0.5}\times10^4$ ($3σ$), about 3-4 times the Arcus requirement. We measured the diffraction efficiency of quasi-fully illuminated gratings at O-K wavelengths in orders 4-7 in an Arcus-like configuration and compare results with synchrotron spot measurements. After corrections for geometrical effects and bremsstrahlung continuum we find agreement between full and spot illumination at the two different facilities, as well as with the models used for Arcus effective area predictions. We find that these flight-like gratings meet diffraction efficiency and greatly exceed resolving power Arcus requirements.

astro-ph.IM↗

arXiv2022

The VMC survey -- XLV. Proper motion of the outer LMC and the impact of the SMC

The Large Magellanic Cloud (LMC) is the most luminous satellite galaxy of the Milky Way and owing to its companion, the Small Magellanic Cloud (SMC), represents an excellent laboratory to study the interaction of dwarf galaxies. The aim of this study is to investigate the kinematics of the outer regions of the LMC by using stellar proper motions to understand the impact of interactions, e.g. with the SMC about 250 Myr ago. {We calculate proper motions using multi-epoch $K_\mathrm{s}$-band images from the VISTA survey of the Magellanic Clouds system (VMC). Observations span a time baseline of 2$-$5 yr. We combine the VMC data with data from the Gaia early Data Release 3 and introduce a new method to distinguish between Magellanic and Milky Way stars based on a machine learning algorithm. This new technique enables a larger and cleaner sample selection of fainter sources as it reaches below the red clump of the LMC. We investigate the impact of the SMC on the rotational field of the LMC and find hints of stripped SMC debris. The south east region of the LMC shows a slow rotational speed compared to the overall rotation. $N$-body simulations suggest that this could be caused by a fraction of stripped SMC stars, located in that particular region, that move opposite to the expected rotation.

astro-ph.GA↗

arXiv2022

Ostwald ripening in an oxide-on-metal system

Ostwald ripening is a well-known physicochemical phenomenon in which smaller particles, characterized by high surface energy, dissolve and feed the bigger ones that are thermodynamically more stable. The effect is commonly observed in solid and liquid solutions, as well as in systems consisting of supported metal clusters or liquid droplets. Here, we provide the first evidence for the occurrence of Ostwald ripening in an oxide-on-metal system which, in our case, consists of ultrathin iron monoxide (FeO) islands grown on Ru(0001) single-crystal support. The results reveal that the thermally-driven sintering of islands allows altering their fine structural characteristics, including size, perimeter length, defect density and stoichiometry, which are crucial, e.g., from the point of view of heterogeneous catalysis.

physics.chem-ph↗

arXiv2021

Point-spread function reconstruction of adaptive-optics imaging: Meeting the astrometric requirements for time-delay cosmography

Astrometric precision and knowledge of the point spread function are key ingredients for a wide range of astrophysical studies including time-delay cosmography in which strongly lensed quasar systems are used to determine the Hubble constant and other cosmological parameters. Astrometric uncertainty on the positions of the multiply-imaged point sources contributes to the overall uncertainty in inferred distances and therefore the Hubble constant. Similarly, knowledge of the wings of the points spread function (PSF) is necessary to disentangle light from the background sources and the foreground deflector. We analyze adaptive optics (AO) images of the strong lens system J0659+1629 obtained with the W. M. Keck Observatory using the laser guide star AO system. We show that by using a reconstructed point spread function we can i) obtain astrometric precision of $< 1$ milliarcsecond (mas), which is more than sufficient for time-delay cosmography; and ii) subtract all point-like images resulting in residuals consistent with the noise level. The method we have developed is not limited to strong lensing, and is generally applicable to a wide range of scientific cases that have multiple point sources nearby.

astro-ph.IM↗

arXiv2021

lenstronomy II: A gravitational lensing software ecosystem

lenstronomy is an Astropy-affiliated Python package for gravitational lensing simulations and analyses. lenstronomy was introduced by Birrer and Amara (2018) and is based on the linear basis set approach by Birrer et a. (2015). The user and developer base of lenstronomy has substantially grown since then, and the software has become an integral part of a wide range of recent analyses, such as measuring the Hubble constant with time-delay strong lensing or constraining the nature of dark matter from resolved and unresolved small scale lensing distortion statistics. The modular design has allowed the community to incorporate innovative new methods, as well as to develop enhanced software and wrappers with more specific aims on top of the lenstronomy API. Through community engagement and involvement, lenstronomy has become a foundation of an ecosystem of affiliated packages extending the original scope of the software and proving its robustness and applicability at the forefront of the strong gravitational lensing community in an open source and reproducible manner.

astro-ph.CO↗

arXiv2021

Is response priming based on surface color? Response to Skrzypulec (2021)

Skrzypulec (2021) raises the question whether motor activation by masked color primes is based on the same type of color representation as conscious vision. He postulates that the literature on color processing without awareness makes an implicit assumption that "conscious" and "unconscious" color representations have the same properties, in which case priming by masked color stimuli would indeed indicate that the same complex representation of surface color can be conscious as well as unconscious. I review some evidence from response priming by lightness stimuli in the context of a visual illusion that alters the perceived lightness of the primes (Schmidt et al., 2010). Those results clearly show that response priming is not driven by color-constant information but instead by local image contrast, making it unlikely that rapid response activation by color primes is based on a color-constant representation of surface color.

q-bio.NC↗

arXiv2021

Fully-staggered-array bulk Re-Ba-Cu-O short-period undulator: large-scale 3D electromagnetic modelling and design optimization using A-V and H-formulation methods

The development of a new hard x-ray beamline I-TOMCAT equipped with a 1-meter-long short-period bulk high-temperature superconductor undulator (BHTSU) has been scheduled for the upgrade of the Swiss Light Source (SLS 2.0) at the Paul Scherrer Institute (PSI). The very hard x-ray source generated by the BHTSU will increase the brilliance at the beamline by over one order of magnitude in comparison to other state-of-the-art undulator technologies and allow experiments to be carried out with photon energies in excess of 60 keV. One of the key challenges for designing a 1-meter-long (100 periods) BHTSU is the large-scale simulation of the magnetization currents inside 200 staggered-array bulk superconductors. A feasible approach to simplify the electromagnetic model is to retain five periods from both ends of the 1-meter-long BHTSU, reducing the number of degrees of freedom (DOFs) to the scale of millions. In this paper, the theory of the recently-proposed 2D A-V formulation-based backward computation method is extended to calculate the critical state magnetization currents in the ten-period staggered-array BHTSU in 3D. The simulation results of the magnetization currents and the associated undulator field along the electron beam axis are compared with the well-known 3D H-formulation and the highly efficient 3D H-ϕ formulation method, all methods showing excellent agreement with each other as well as with experimental results. The mixed H-ϕ formulation avoids computing the eddy currents in the air subdomain and is significantly faster than the full H-formulation method, but is slower in comparison to the A-V formulation-based backward computation. Finally, the fastest and the most efficient A-V formulation in ANSYS 2020R1 Academic is adopted to optimize the integrals of the undulator field along the electron beam axis by optimizing the sizes of the end bulks.

physics.acc-ph↗

arXiv2021

Stellar substructures in the periphery of the Magellanic Clouds with the VISTA Hemisphere Survey from the red clump and other tracers

We study the morphology of the stellar periphery of the Magellanic Clouds in search of substructure using near-infrared imaging data from the VISTA Hemisphere Survey (VHS). Based on the selection of different stellar populations using the ($J-K_\mathrm{s}$, $K_\mathrm{s}$) colour-magnitude diagram, we confirm the presence of substructures related to the interaction history of the Clouds and find new substructures on the easter side of the LMC disc which may be owing to the influence of the Milky Way, and on the northern side of the SMC, which is probably associated to the ellipsoidal structure of the galaxy. We also study the luminosity function of red clump stars in the SMC and confirm the presence of a bi-modal distance distribution, in the form of a foreground population. We find that this bi-modality is still detectable in the eastern regions of the galaxy out to a 10 deg distance from its centre. Additionally, a background structure is detected in the North between 7 and 10 deg from the centre which might belong to the Counter Bridge, and a foreground structure is detected in the South between 6 and 8 deg from the centre which might be linked to the Old Bridge.

astro-ph.GA↗

arXiv2020

Highly Active Nanoperovskite Catalysts for Oxygen Evolution Reaction: Insights into Activity and Stability of Ba0.5Sr0.5Co0.8Fe0.2O3 and PrBaCo2O6

It is shown that producing PrBaCo2O5 and Ba0.5Sr0.5Co0.8Fe0.2O3 nanoparticle by a scalable synthesis method leads to high mass activities for the oxygen evolution reaction with outstanding improvements by 10 and 50 times, respectively, compared to those prepared via the state of the art synthesis method. Here, detailed comparisons at both laboratory and industrial scales show that Ba0.5Sr0.5Co0.8Fe0.2O3 appears to be the most active and stable perovskite catalyst under alkaline conditions, while PrBaCo2O6 reveals thermodynamic instability described by the density functional theory based Pourbaix diagrams highlighting cation dissolution under oxygen evolution conditions. Operando Xray absorption spectroscopy is used in parallel to monitor electronic and structural changes of the catalysts during oxygen evolution reaction. The exceptional BSCF functional stability can be correlated to its thermodynamic metastability under oxygen evolution conditions as highlighted by Pourbaix diagram analysis. BSCF is able to dynamically self reconstruct its surface, leading to formation of Co based oxyhydroxide layers while retaining its structural stability. Differently, PBCO demonstrates a high initial oxygen evolution reaction activity while it undergoes a degradation process considering its thermodynamic instability under oxygen evolution conditions as anticipated by its Pourbaix diagram. Overall, this work demonstrates a synergetic approach of using both experimental and theoretical studies to understand the behavior of perovskite catalysts.

cond-mat.mtrl-sci↗

arXiv2020

Low Temperature Growth of Graphene on Semiconductor

The industrial realization of graphene has so far been limited by challenges related to the quality, reproducibility, and high process temperatures required to manufacture graphene on suitable substrates. We demonstrate that epitaxial graphene can be grown on transition metal treated 6H-SiC(0001) surfaces, with an onset of graphitization starting around $450-500^\circ\text{C}$. From the chemical reaction between SiC and thin films of Fe or Ru, $\text{sp}^{3}$ carbon is liberated from the SiC crystal and converted to $\text{sp}^{2}$ carbon at the surface. The quality of the graphene is demonstrated using angle-resolved photoemission spectroscopy and low-energy electron diffraction. Furthermore, the orientation and placement of the graphene layers relative to the SiC substrate is verified using angle-resolved absorption spectroscopy and energy-dependent photoelectron spectroscopy, respectively. With subsequent thermal treatments to higher temperatures, a steerable diffusion of the metal layers into the bulk SiC is achieved. The result is graphene supported on magnetic silicide or optionally, directly on semiconductor, at temperatures ideal for further large-scale processing into graphene based device structures.

cond-mat.mtrl-sci↗

arXiv2020

Fast and efficient critical state modelling of field-cooled bulk high-temperature superconductors using a backward computation method

A backward computation method has been developed to accelerate modelling of the critical state magnetization current in a staggered-array bulk high-temperature superconducting (HTS) undulator. The key concept is as follows: i) a large magnetization current is first generated on the surface of the HTS bulks after rapid field-cooling (FC) magnetization; ii) the magnetization current then relaxes inwards step-by-step obeying the critical state model; iii) after tens of backward iterations the magnetization current reaches a steady state. The simulation results show excellent agreement with the H-formulation method for both the electromagnetic and electromagnetic-mechanical coupled analyses, but with significantly faster computation speed. Solving the FEA model with 1.8 million degrees of freedom (DOFs), the backward computation method takes less than 1.4 hours, an order of magnitude or higher faster than other state-of-the-art numerical methods. Finally, the models are used to investigate the influence of the mechanical stress on the distribution of the critical state magnetization current and the undulator field along the central axis.

cond-mat.supr-con↗

arXiv2020

The VMC survey -- XXXVIII. Proper motion of the Magellanic Bridge

The Magellanic Clouds are a nearby pair of interacting dwarf galaxies and satellites of the Milky Way. Studying their kinematic properties is essential to understanding their origin and dynamical evolution. They have prominent tidal features and the kinematics of these features can give hints about the formation of tidal dwarfs, galaxy merging and the stripping of gas. In addition they are an example of dwarf galaxies that are in the process of merging with a massive galaxy. The goal of this study is to investigate the kinematics of the Magellanic Bridge, a tidal feature connecting the Magellanic Clouds, using stellar proper motions to understand their most recent interaction. We calculated proper motions based on multi-epoch $K_{s}$-band aperture photometry, which were obtained with the Visible and Infrared Survey Telescope for Astronomy (VISTA), spanning a time of 1-3 yr, and we compared them with $Gaia$ Data Release 2 (DR2) proper motions. We tested two methods for removing Milky Way foreground stars using $Gaia$~DR2 parallaxes in combination with VISTA photometry or using distances based on Bayesian inference. We obtained proper motions for a total of 576,411 unique sources over an area of $23$ deg$^{2}$ covering the Magellanic Bridge including mainly Milky Way foreground stars, background galaxies, and a small population of possible Magellanic Bridge stars ($<$15,000). The first proper motion measurement of the Magellanic Bridge centre is $1.80\pm0.25$ mas yr$^{-1}$ in right ascension and $-0.72\pm0.13$ mas yr$^{-1}$ in declination. The proper motion measurements confirm a flow motion from the Small to the Large Magellanic Cloud. This flow can now be measured all across the entire length of the Magellanic Bridge. Our measurements indicate that the Magellanic Bridge is stretching.

astro-ph.GA↗

arXiv2020

Magnetization Current Simulation of High Temperature Bulk Superconductors Using A-V-A Formulation and Iterative Algorithm Method: Critical State Model and Flux Creep Model

In this work we will introduce the A-V-A formulation based iterative algorithm method (IAM) for simulating the magnetization current of high temperature superconductors. This new method embedded in ANSYS can simulate the critical state model by forcing the trapped current density to the critical current density Jc for all meshed superconducting elements after each iterative load step, as well as simulate the flux creep model by updating the E-J power law based resistivity values. The simulation results of a disk-shaped ReBCO bulk during zero field cooling (ZFC) or field cooling (FC) magnetization agree well with the simulation results from using the H-formulation in COMSOL. The computation time is shortened by using the A-V formulation in superconductor areas and the A-formulation in non-superconductor areas. This iterative method is further proved friendly for adding ferromagnetic materials into the FEA model or taking into account the magnetic field-dependent or mechanical strain-related critical current density of the superconductors. The influence factors for the magnetization simulation, including the specified iterative load steps, the initial resistivity, the ramping time and the updating coefficient, are discussed in detail. The A-V-A formulation based IAM, implemented in ANSYS, shows its unique advantages in adjustable computation time, multi-frame restart analysis and easy-convergence.

cond-mat.supr-con↗

arXiv2019

Mechanical interplay between cell shape and actin cytoskeleton organization

We investigate the mechanical interplay between the spatial organization of the actin cytoskeleton and the shape of animal cells adhering on micropillar arrays. Using a combination of analytical work, computer simulations and in vitro experiments, we demonstrate that the orientation of the stress fibers strongly influences the geometry of the cell edge. In the presence of a uniformly aligned cytoskeleton, the cell edge can be well approximated by elliptical arcs, whose eccentricity reflects the degree of anisotropy of the cell's internal stresses. Upon modeling the actin cytoskeleton as a nematic liquid crystal, we further show that the geometry of the cell edge feeds back on the organization of the stress fibers by altering the length scale at which these are confined. This feedback mechanism is controlled by a dimensionless number, the anchoring number, representing the relative weight of surface-anchoring and bulk-aligning torques. Our model allows to predict both cellular shape and the internal structure of the actin cytoskeleton and is in good quantitative agreement with experiments on fibroblastoid (GD$β$1,GD$β$3) and epithelioid (GE$β$1, GE$β$3) cells.

physics.bio-ph↗