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Node-Level Performance of Adaptive Resolution in ls1 Mardyn
No full textIn this work we present a node-level performance analysis of an adaptive resolution scheme (AdResS) implemented in ls1 mardyn . This is relevant in simulations involving a very large number of particles or long timescales, because it lowers the computational effort required to calculate short-range interactions in molecular dynamics. An introduction to AdResS is given, together with an explanation of the coarsening technique used to obtain an effective potential for the coarse molecular model, i.e., the Iterative Boltzmann Inversion (IBI). This is accompanied by details of the implementation in our software package, as well as an algorithmic description of the IBI method and the simulation workflow used to generate results. This will be of interest for practitioners. Results are provided for a pure Lennard-Jones tetrahedral molecule coarsened to a single site, validated by verifying the correct reproduction of structural correlation functions, e.g. the radial distribution function. The performance analysis builds upon a literature-driven methodology, which provides a theoretical estimate for the speedup based on a reference simulation and the size of the full particle region. Additionally, a strong scaling study was performed at node level. In this sense, several configurations with vertical interfaces between the resolution regions are tested, where different resolution widths are benchmarked. A comparison between several linked cell traversal routines, which are provided in ls1 mardyn , was performed to showcase the effect of algorithmic aspects on the adaptive resolution simulation and on the estimated performance
Impact of Bulk-Phase Self-Assembly on Growth and Activation of Aqueous Surfactant Aerosol
No full textSurface active compounds (surfactants) have been found in atmospheric aerosols from many environments. In microscopic aqueous droplets, their presence may cause a variety of effects that are challenging to model comprehensively. In this work, we investigate the solute effects of atmospheric surfactant self-assembly to form micelles and small clusters in aqueous droplets. Several water activity models are employed in combination with Köhler theory to represent self-assembly phenomena within the droplet bulk while also accounting for surfactant bulk–surface partitioning. Our results show that surfactant self-assembly has only minor effects at the critical point of cloud droplet activation; however, very significant effects are observed during earlier stages of droplet growth at subsaturated ambient humidities. A major driver is the binding of free sodium counterions to the surfactant aggregates, strongly limiting the effect of dissolved sodium ions on droplet water activity. Variations in droplet equilibrium size at a given subsaturated humidity due to the presence of surfactant aggregates lead to different estimates of droplet water uptake and aerosol liquid water content. As a result, predictions of atmospheric processes involving surfactant aerosol could be highly sensitive to how surfactant self-assembly and related phenomena are taken into account
Probing the PeV region in the astrophysical neutrino spectrum using νμ from the Southern sky
No full textIceCube has observed a diffuse astrophysical neutrino flux over the energy region from a few TeV to a few PeV. At PeV energies, the spectral shape is not yet well measured due to the low statistics of the data. This analysis probes the gap between 1 and 10 PeV by using high-energy downgoing muon neutrinos. To reject the large atmospheric muon background, two complementary techniques are combined. The first technique selects events with high stochasticity to reject atmospheric muon bundles whose stochastic energy losses are smoothed due to high muon multiplicity. The second technique vetoes atmospheric muons with the IceTop surface array. Using 9 yrs of data, we found two neutrino candidate events in the signal region, consistent with expectation from background, each with relatively high signal probabilities. A joint maximum likelihood estimation is performed using this sample and an independent 9.5-yr sample of tracks to measure the neutrino spectrum. A likelihood ratio test is done to compare the single power-law (SPL) vs SPL+cutoff hypothesis; the SPL+cutoff model is not significantly better than the SPL. High-energy astrophysical objects from four source catalogs are also checked around the direction of the two events. No significant coincidence was found
Evidence for Neutrino Emission from X-ray Bright Active Galactic Nuclei with IceCube
No full textRecently, IceCube reported neutrino emission from the Seyfert galaxy NGC 1068. Using 13.1 years of IceCube data, we present a follow-up search for neutrino sources in the northern sky. NGC 1068 remains the most significant neutrino source among 110 preselected gamma-ray emitters while also being spatially compatible with the most significant location in the northern sky. Its energy spectrum is characterized by an unbroken power-law with spectral index . Consistent with previous results, the observed neutrino flux exceeds its gamma-ray counterpart by at least two orders of magnitude. Motivated by this disparity and the high X-ray luminosity of the source, we selected 47 X-ray bright Seyfert galaxies from the Swift/BAT spectroscopic survey that were not included in the list of gamma-ray emitters. When testing this collection for neutrino emission, we observe a 3.3 excess from an ensemble of 11 sources, with NGC 1068 excluded from the sample. Our results strengthen the evidence that X-ray bright cores of active galactic nuclei are neutrino emitters
Physics potential of the IceCube Upgrade for atmospheric neutrino oscillations
No full textThe IceCube Upgrade is an extension of the existing IceCube Neutrino Observatory and will be deployed in the 2025-2026 austral summer. It will significantly improve the sensitivity of the detector to atmospheric neutrino oscillations. The existing 86-string IceCube array contains a dense in-fill known as DeepCore which is optimized to measure neutrinos with energies down to a few GeV. The IceCube Upgrade will consist of seven new densely-instrumented strings placed within the DeepCore volume to further enhance the performance in the GeV energy range. The additional strings will feature new optical modules, each containing multiple PMTs, in contrast to the existing modules that each contain a single PMT. This will more than triple the number of PMT channels with respect to the current IceCube configuration, allowing for improved detection efficiency and reconstruction performance at GeV energies. We describe necessary updates to simulation, event selection, and reconstruction to accommodate the higher data rates observed by the upgraded detector and the addition of multi-PMT modules. We determine the expected sensitivity of the IceCube Upgrade to the atmospheric neutrino oscillation parameters sin and , the appearance of tau neutrinos and the neutrino mass ordering. The IceCube Upgrade will provide neutrino oscillation measurements that are of similar precision to those from accelerator experiments, while providing complementarity by probing higher energies and longer baselines, and with different sources of systematic uncertainties
Evidence for a Spectral Break or Curvature in the Spectrum of Astrophysical Neutrinos from 5 TeV--10 PeV
No full textWe report improved measurements of the all flavor astrophysical neutrino spectrum with IceCube by combining complementary neutrino samples in two independent analyses. Both analyses show evidence of a harder spectrum at energies below 30~TeV compared to higher energies where the spectrum is well characterized by a power law. The spectrum is better described by a log parabola or a broken power law, the latter being the preferred model. Both, however, reject a single power law over an energy range 5~TeV-10~PeV with a significance , providing new constraints on properties of cosmic neutrino sources
A stress test of global PDF fits: closure testing the MSHT PDFs and a first direct comparison to the neural net approach
No full textWe present a first global closure test of the fixed parameterisation (MSHT) approach to PDF fitting. We find that the default MSHT20 parameterisation can reproduce the features of the input set in such a closure test to well within the textbook uncertainties. This provides strong evidence that parameterisation inflexibility in the MSHT20 fit is not a significant issue in the data region. We also present the first completely like-for-like comparison between two global PDF fits, namely MSHT and NNPDF, where the only difference is guaranteed to be due to the fitting methodology. To achieve this, we present a fit to the NNPDF4.0 data and theory inputs, but with the MSHT fixed parameterisation. We find that this gives a moderately, but noticeably, better fit quality than the central NNPDF4.0 fits, both with perturbative and fitted charm, and that this difference persists at the level of the PDFs and benchmark cross sections. The NNPDF4.0 uncertainties are found to be broadly in line with the MSHT results if a textbook tolerance is applied, but to be significantly smaller if a tolerance typical of the MSHT20 fit is applied. This points to an inherent inconsistency between these approaches. We discuss the need for an enlarged tolerance criterion in global PDF fits in detail, and demonstrate the impact of data/theory inconsistencies in the closure test setting; namely, these do not lead to any increase in the PDF uncertainty. We also investigate the impact of restricting the PDF parameterisation to have fewer free parameters than the default MSHT20 case, and find this can be significant at the level of both closure tests and the full fit
High-Pressure MgCl Synthesized in a Diamond Anvil Cell as a Polar Metal with Second-Harmonic Generation
No full textPolar metals have attracted growing interest due to both their significance in fundamental science and their potential functionalities. Here, we report the discovery of a novel polar metal, magnesium chloride MgCl, in which the metallicity of the polar structure is uniquely driven by attractive halogen interactions. MgCl was synthesized in laser-heated diamond anvil cells and observed at pressures of 28(2)–93(3) GPa. Synchrotron single-crystal X-ray diffraction revealed that the structure of the new compound has polar hexagonal space group P6mc, representing an example of a previously unknown anti-ThFe structure type. Measurements of the physical properties have shown that the material is a metallic conductor capable of emitting second-harmonic generation light. Ab initio calculations support experimental findings and reveal complex halogen–halogen interactions, anionic metallicity, anisotropic electronic structure, and the presence of Dirac and Weyl points at the Fermi level. Our findings broaden the family of polar metals, provide new insight into halogen bonding under extreme conditions, and offer a platform for further exploration of materials’ unconventional electronic behavior
High-Pressure Synthesis and Characterization of the Novel Potassium Superhydride
No full textThrough high-pressure diamond anvil cell experiments, we report the synthesis of two novel potassium superhydrides (KH9-I and KH9-II) and investigate their structural and vibrational properties via synchrotron X-ray powder diffraction and Raman spectroscopy, complemented by density functional theory (DFT) calculations. Above 17 GPa at room temperature, KH-II and H2 react to form KH9-I; this reaction can be accelerated with temperature. KH9-I possesses a face-centered-cubic (fcc) potassium sublattice with a slight rhombohedral distortion (space group R3̅m). Compression above 78 GPa converts KH9-I to another polymorph, KH9-II, which adopts a primitive simple hexagonal potassium sublattice (space group P6/mmm) and remains stable up to at least 100 GPa. Both KH9 polymorphs exhibit ionic character, comprising K+ and H− ions, along with quasi-molecular H2 units, resulting in rich Raman activity
Structural and electrochemical behaviour of bilayer manganite cathode for all-solid-state fluoride ion batteries
No full textIn this study, we explore the potential of the RP-type bilayer manganite LaSr2Mn2O6.96 as an intercalation-based cathode material for all-solid-state fluoride ion batteries (FIBs). Structural changes of LaSr2Mn2O6.96 during fluoride intercalation and de-intercalation were analyzed via ex-situ X-ray diffraction, revealing that F- insertion induces the formation of three distinct tetragonal phases. To understand the complex behavior of these phases, we examined the changes in the Mn oxidation state and coordination environment using X-ray absorption spectroscopy and magnetic measurements. Under stack pressure (20 kN), electrochemical cycling of LaSr2Mn2O6.96 in the potential range of 1 V to -1 V exhibited a continuous increase in specific capacity from capacity of ∼ 30 mAh/g to ∼ 68 mAh/g over 200 cycles, with ∼99% coulombic efficiency and no signs of capacity fading. This makes the bilayer manganite LaSr2Mn2O6.96 a promising candidate for a cycling stable cathode for all-solid-state FIBs, especially under the application of stack pressure