Atomic Physics
Long-lived metastable states in the 4f$^{13}$5d6s configuration of Yb$^+$
Z. E. D. Ackerman, A. Cadarso Quevedo, Ilango Maran et al. (+4 more)
We study the occurrence of long-lived metastable states in the 4f$^{13}$5d6s electron configuration of Yb$^+$. By optical pumping of a single trapped ion on the $^2F^\text{o}_{7/2}\rightarrow (7/2,0)_{7/2}$ transition at 377.5 nm, we prepare a wide range of metastable electronic states. We use a co-trapped control ion to sympathetically cool the spectroscopy ion, allowing us to accurately time its subsequent decay. We record a strong decay signal corresponding to a lifetime of 0.92(8) s, a weaker decay signal with lifetime 9.8(+2.9, -2.0) s, and find evidence for a much longer lifetime, $>$ 30 s. We identify the metastable states with these lifetimes qualitatively, and corroborate our results with atomic structure calculations that support the observed lifetimes and decay paths. These long-lived states provide new opportunities in
qubit and qudit state detection and optical clocks.
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All authors Z. E. D. Ackerman, A. Cadarso Quevedo, Ilango Maran, L. P. H. Gallagher, R. J. C. Spreeuw, J. C. Berengut, R. Gerritsma
Optics · Cross-listed from cond-mat.mes-hall
Progress on artificial flat band systems: classifying, perturbing, applying
Carlo Danieli and Sergej Flach
We highlight recent progress in the study of artificial flat band systems with a threefold focus. First, we discuss single-particle flat band physics, which has advanced through the design of various flat band generators. These generators rely on the classification of flat bands in terms of compact localized states - their fundamental building blocks. A related development is the complete real-space description of flat band projectors. Next, we review studies on perturbations of flat bands, which provide new insights into the effects of disorder and, more importantly, the intricate interplay between many-body interactions and flat band physics. Finally, we survey the growing number of experimental realizations of flat bands across diverse physical platforms.
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Fluid Dynamics
Optomicrofluidic measurement of particle-encapsulated droplet system
Kanimozhi Kumaresan, Thaipally Sujith, Anil Prabhakar et al. (+1 more)
Droplet microfluidics combined with optical detection has become a powerful approach for high-throughput single-
cell assays, but these systems often face limited sensitivity and signal heterogeneity due to optical and geometrical constraints. We investigate how key operating parameters influence the performance of a droplet-based optomicrofluidic platform. Experiments examine optical interactions between guided light and aqueous droplets containing fluorescent (FL) particles flowing in oil. Geometrical optics simulations model light-droplet interactions, while FL simulations quantify signal variations caused by particle size and position. Two refracted signals are observed experimentally: a droplet-refracted signal (DRS) that scales with droplet diameter and a particle-refracted signal (PRS) produced by light interaction with encapsulated particles. Both experiments and simulations show that PRS becomes prominent when the particle-to-droplet size ratio $D^*_\text{p}$ lies between 0.23-0.33, enabling label-free detection. Particles near the droplet center ($r^*_\text{p} < 0.4$) display reduced angular dependence and more uniform FL signals. Simulations further show that FL intensity increases with $D^*_\text{p}$, rising sharply from 0.33 to 0.5 and more gradually up to 0.66. Additionally, reducing the oil layer thickness enhances fluorescence by minimizing optical losses at the droplet-channel interface. These results demonstrate that controlling $D^*_\text{p}$, particle position, and oil layer thickness improves FL strength and uniformity, providing a framework for optimizing droplet-based fluorescence detection in microflow cytometry and single-
cell assays.
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All authors Kanimozhi Kumaresan, Thaipally Sujith, Anil Prabhakar, Ashis Kumar Sen
Medical Physics
Feasibility of Continuous Ventricular Volumetric Quantification in Arrhythmias using Real-Time 3D CMR-MOTUS
Thomas E. Olausson, Maarten L. Terpstra, Rizwan Ahmad et al. (+7 more)
Conventional cardiovascular
magnetic resonance (CMR) cine imaging combines data across multiple heartbeats, an assumption that can fail in arrhythmia because beat-to-beat variation causes motion artifacts and obscures functional heterogeneity. Although 2D real-time cine resolves individual beats, stacked slices are suboptimal for capturing complex 3D cardiac dynamics. We investigated continuous beat-to-beat volumetric quantification in patients with premature ventricular contractions (PVCs) using free-running 3D real-time CMR. CMR-MOTUS was extended to jointly reconstruct time-resolved 3D motion fields and a motion-corrected reference image from continuously acquired data without breath-holding or ECG gating. Data were acquired with a variable-density Cartesian trajectory and either 3D spoiled gradient-echo or balanced steady-state free-precession imaging. Ventricular volumes were obtained by propagating one manual reference-image
segmentation through all reconstructed frames. The method was evaluated in a cardiac motion phantom with static ground-truth acquisitions, 10 healthy volunteers, and 10 patients with PVCs; all in vivo scans were performed after contrast administration. Phantom ejection fraction (EF) agreed closely with ground truth (17.86% versus 17.27%). Healthy volunteers showed narrow beat-to-beat EF distributions, whereas patients with PVCs showed broader and sometimes bimodal distributions. Simultaneous ECG recordings supported the temporal correspondence between volume irregularities and PVC episodes. Free-running joint 3D motion-field and image reconstruction enables continuous beat-to-beat volumetric assessment and can reveal functional heterogeneity obscured by gated or heartbeat-averaged methods. Larger studies are required to establish clinical validity and determine its role alongside standard 2D cine analysis.
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All authors Thomas E. Olausson, Maarten L. Terpstra, Rizwan Ahmad, Edwin Versteeg, Casper Beijst, Yuchi Han, Marco Guglielmo, Birgitta K. Velthuis, Cornelis van den Berg, Alessandro Sbrizzi
Fluid Dynamics · Cross-listed from math.NA
A Multi-Fidelity Parametric Framework for Reduced-Order Modeling using Optimal Transport-based Interpolation: Applications to Diffused-Interface Two-Phase Flows
Moaad Khamlich, Niccolò Tonicello, Federico Pichi et al. (+1 more)
This work introduces a data-driven, non-intrusive reduced-order modeling (ROM) framework that leverages Optimal Transport (OT) for multi-fidelity and parametric problems in two-phase flows modelling. Building upon the success of displacement interpolation for data augmentation in handling nonlinear dynamics, we extend its application to more complex and practical scenarios. The framework is designed to correct a computationally inexpensive low-fidelity (LF) model to match an accurate high-fidelity (HF) one by capturing its temporal
evolution via displacement interpolation while preserving the problem's physical consistency. The framework is further extended to address systems dependent on a physical parameter, for which we construct a surrogate model using a hierarchical, two-level interpolation strategy. First, it creates synthetic HF checkpoints via displacement interpolation in the parameter space. Second, the residual between these synthetic HF checkpoints and a true LF solution is interpolated in the time domain using the multi-fidelity OT-based methodology. This strategy provides a robust and efficient way to explore the parameter space and to obtain a refined description of the dynamical system. The potential of the method is discussed in the context of complex and computationally expensive diffuse-interface methods for two-phase flow simulations, which are characterized by moving interfaces and nonlinear
evolution, and challenging to be dealt with traditional ROM techniques.
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All authors Moaad Khamlich, Niccolò Tonicello, Federico Pichi, Gianluigi Rozza
Chemical Physics
False Metallization in Short-Ranged Machine Learned Interatomic Potentials
Isaac J. Parker, Mandy J. Hoffmann, William J. Baldwin et al. (+7 more)
Machine learned interatomic potentials (MLIPs) have enabled atomistic simulations with ab initio accuracy for a fraction of the computational cost. However, many widely used MLIPs are short-ranged and do not accurately capture long-ranged electrostatic interactions. At interfaces with polar solvents, such as water, this deficiency can drive unphysical long-distance dipolar
alignment far away from the interface. Here we reveal that neglecting long-ranged physics leads to spurious metallization of the water layer due to artificially large fluctuations of the total solvent dipole, similar to the electron rearrangement observed to prevent polar catastrophes at polar interfaces. This metallization is eliminated in MLIPs that explicitly include long-ranged electrostatics. Our results showcase a fundamental flaw of short-ranged MLIPs, highlighting that long-ranged electrostatics are essential for studying systems with a polar-
liquid component, especially if one is interested in electronic properties.
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All authors Isaac J. Parker, Mandy J. Hoffmann, William J. Baldwin, Shuang Han, Srishti Gupta, Kara D. Fong, Angelos Michaelides, Christoph Schran, Sandip De, Gábor Csányi
Physics and Society
Graphs as focal interaction systems in higher-order dynamics
Elkaïoum M. Moutuou
Graphs and hypergraphs are commonly distinguished by the order of the relations they represent, with graphs encoding pairwise relations and hypergraphs encoding interactions among arbitrary groups. We show that, for node-update dynamics, a more natural viewpoint is to regard a graph as a collection of focal neighbourhoods, each centered on a vertex and consisting of the vertex and its neighbours. From this perspective, hypergraphs naturally generalise graph neighbourhoods by allowing arbitrary interaction domains without requiring a distinguished focal node. This viewpoint also clarifies the relationship between graph and hypergraph dynamical models. Although sufficiently general graph-based node functions can reproduce the dynamics of higher-order interactions, such dynamical equivalence does not imply equivalence of the underlying interaction structures. Graph representations may encode higher-order organisation implicitly in their functions while losing the explicit grouping, roles, and
symmetries carried by hyperedges. We therefore distinguish dynamical expressivity from structural representation and show that hypergraphs provide a generalisation of graph-based interaction domains even when their dynamics can be emulated by graph models.
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Optics
Design of a monolithic source of photon pairs comprising a semiconductor laser and a Bragg reflection waveguide
Thomas Tenzler, Jan-Philipp Koester, Hans Wenzel et al. (+4 more)
We propose a monolithic, electrically driven source of
photon pairs based on a non-linear AlGaAs Bragg reflection waveguide and a
laser structure stacked on top. By introducing lateral tapers, the fundamental mode of the lasing waveguide is vertically coupled into a higher order mode of the Bragg reflection waveguide (Bragg mode) such that
photon pairs can be generated through a type-II spontaneous parametric down conversion process. According to numerical simulations, a coupling efficiency of 28% is achieved between both modes. Phase matching the Bragg mode with two fundamental modes at 1550 nm results in a
photon pair rate of 1.7*10^8 pairs/s for a 2 mm long device assuming 1 mW of power in the Bragg mode. Since the Bragg reflection waveguide does not require doping for this vertically coupled structure, free-carrier absorption losses and parasitic luminescence are avoided.
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All authors Thomas Tenzler, Jan-Philipp Koester, Hans Wenzel, Thorsten Passow, Quankui Yang, Marko Haertelt, Andrea Knigge
Accelerator Physics
Collimation
Nuria Fuster Martínez
Collimation systems are essential in particle accelerators to safely and efficiently manage unavoidable beam losses during operation. These systems rely on collimators, which are specially designed movable jaws or absorbers positioned close to the beam envelope to intercept and localize beam losses. Their role is particularly critical in high-intensity
hadron machines, where uncontrolled losses can lead to equipment damage or operational downtime. While the specific requirements vary across accelerator types, circular accelerators, especially present and future high-energy
colliders, cannot operate safely without a well-optimized collimation system. This lecture offers an overview of the fundamental principles, design challenges and operational strategies of beam collimation, with emphasis on high-intensity
hadron accelerators. The Large
Hadron Collider, the most advanced example to date, will serve as the main reference for illustrating state-of-the-art collimation approaches and technologies.
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Accelerator Physics
A Unified Approach for Coupled Beam Optics in Accelerators
Onur Gilanliogullari, Brahim Mustapha and Pavel Snopok
Coupled beam optics can be geometrically described in terms of invariant eigenmode planes of a stable
symplectic “one-turn” map $\mathcal M\in Sp(4)$. We show that the non-uniqueness of symplectically normalized bases within each eigenmode plane constitutes an in-plane gauge freedom $Sp(2)\times Sp(2)$, and that many coupled-optics parametrizations differ primarily by gauge choice. Building on this fact, we identify basis-independent descriptors of
lattice and beam optics and introduce bounded, gauge-invariant coupling parameters or fractions $u_{k,\mathrm{inv}}$ computed from orthogonal projectors onto the eigenmode planes. To obtain smooth $s$-dependent optics functions and consistent mode labeling, we present a unifying and practical approach based on an $SO(2)$ continuity gauge (Procrustes
alignment), together with diagnostics for stability and invariance. We further relate Edwards–Teng, Mais–Ripken, Lebedev–Bogacz, Wolski, and Sagan–Rubin parametrizations as gauge-equivalent representations within the respective $Sp(2)\times Sp(2)$ gauge freedom. Numerical examples of coupled
lattices and beam optics illustrate the proposed invariants and show how representation-dependent scalar coupling parameters (e.g.\ in the Lebedev–Bogacz gauge) can leave their nominal bounds while $u_{k,\mathrm{inv}}$, defined here, remain bounded and physically interpretable.
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Biological Physics
Cell-Cell Adhesion as a Double-Edged Sword in Tissue Fluidity
Anh Q. Nguyen, Pradip K. Bera, Jacob Notbohm et al. (+1 more)
Cell migration plays a fundamental role in numerous physiological processes, including embryonic development, wound healing, and cancer metastasis. While
cell-
cell adhesion is known to regulate motion by shaping
cell morphology and intercellular force balance, its dynamic, rate-dependent contributions to tissue behavior remain poorly understood. In this study, we examine how the dissipative nature of
cell-
cell adhesion influences tissue dynamics and collective migration using an extended vertex model with explicit junctional viscosity. Our findings reveal a nontrivial interplay between two distinct components of adhesion: an interfacial adhesion energy (energetic, rate-independent) contribution, which sets the effective junctional tension, and a dissipative (rate-dependent) contribution, which controls resistance to relative motion during
cell rearrangements. We show that increasing the energetic component promotes migration by modifying
cell shape and lowering the barrier to neighbor exchanges, whereas strengthening the dissipative component induces jamming and suppresses
cell motion. Linear rheological analysis further demonstrates that, in the unjammed regime, vertex-model tissues exhibit power-law viscoelastic behavior, with adhesion modulating the power-law exponent and thereby controlling the spread of relaxation timescales. Together, these findings clarify the dual role of adhesion in governing tissue mechanics and rheology and provide a mechanistic framework for understanding the balance between fluidity and
rigidity in epithelial monolayers.
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All authors Anh Q. Nguyen, Pradip K. Bera, Jacob Notbohm, Dapeng Bi
History and Philosophy of Physics
Observational Indistinguishability and the Beginning of the Universe
Daniel Linford
Can we infer whether all of physical reality began to exist? Several novel results are offered suggesting a negative verdict. First, two necessary (but not necessarily sufficient) conditions are offered for a
cosmic beginning. Second, I discuss three
theorems which show that, in a wide range of spacetimes, observers cannot infer the global structure of their spacetime; one of the three shows that spacetimes satisfying a condition necessary for a
cosmic beginning are observationally indistinguishable from spacetimes which do not satisfy that condition. Next, I offer two extensions to the aforementioned
theorems. The two extensions show that, in an equally wide range of spacetimes, observers cannot collect sufficient data to determine whether the application conditions for the class singularity
theorems are satisfied or whether their spacetime satisfies the two necessary conditions for a
cosmic beginning. Lastly, a reply is offered to the objection that the skeptical consequences of the aforementioned
theorems can be overcome with induction. Importantly, all past singular
dust FLRW spacetimes are observationally indistinguishable from counterparts which, while sharing a number of important local properties, either do not include a singularity to the past of every point or else do not have the sort of time ordering intuitively required for a
cosmic beginning.
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Chemical Physics
Structure-resolved free energy estimation of the 38-atom Lennard Jones cluster via population annealing
Akie Kowaguchi and Koji Hukushima
We systematically investigate the thermodynamic landscape of the 38-atom Lennard–Jones cluster LJ$_{38}$ using Population Annealing (PA), a method suited for systems with challenging double-funnel energy landscapes. By employing an adaptive temperature schedule, we demonstrate that thermodynamic observables, such as internal energy and heat capacity, converge robustly when the population size is sufficiently large. To gain deeper insights into the competing basins, we introduce an integrated framework that combines PA reweighting factors with structure-resolved analysis. Using quenched configurations characterized by potential energy and Steinhardt's bond-orientational order parameters, we identify three structural basins, FCC-like, icosahedral, and
liquid-like, via dimensionality reduction and clustering. This framework enables the direct computation of structure-resolved free energy differences from population fractions, providing a quantitative mapping of the thermodynamic competition between the funnels. The resulting structural crossovers are consistent with the heat-capacity peak, demonstrating PA as a promising and scalable framework for structure-resolved thermodynamics in complex molecular systems.
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Computational Physics · Cross-listed from cond-mat.mtrl-sci
Machine-learned interatomic potential for titanium carbide MXenes: Application to ion irradiation simulations
Jesper Byggmästar
A computationally efficient and accurate machine-learned (ML) interatomic potential is developed for bare Ti$_{n+1}$C$_n$ MXenes. With a diverse set of structures computed with density functional theory, the trained ML potential demonstrates good accuracy and robustness to a wide range of bond distances and environments, making it a useful tool for molecular dynamics simulations of MXenes subjected to mechanical load or irradiation. The ML potential is applied to simulations of light and heavy ion irradiation, gathering insight into the statistics and probabilities of sputtering, reflection, defect creation, and implantation into bare Ti$_{n+1}$C$_n$ MXene sheets. The results provide guidelines for defect engineering of MXenes through ion irradiation and implantation. Additionally, the ML potential development provides a landmark recipe for enabling machine-learning-driven atomistic simulations of other MXenes.
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Computational Physics · Cross-listed from cond-mat.mtrl-sci
Nine-element machine-learned interatomic potentials for multiphase refractory alloys
Jesper Byggmästar, Tiago Lopes, Zheyong Fan et al. (+1 more)
New refractory
alloys are being continuously designed and characterised for applications requiring good high-temperature mechanical properties and stability. Computational design from atomistic simulations is limited by interatomic potentials missing key elements, being too inaccurate, or computationally too slow for large-scale simulations. Here we present development of a refractory
alloy database and two computationally efficient and general-purpose machine-learned potentials (tabGAP and NEP). We also design a cross-sampling strategy for effective sampling of training data using predictions from two potentials with completely different underlying architecture. The potentials support arbitrary
alloy compositions of elements in groups four to six in the periodic table (Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W). The database is diverse yet multitargeted to enable simulations of refractory metals and
alloys across different pure-metal, solid-solution, intermetallic, and glassy phases. We demonstrate the usefulness of the potentials by reproducing known pressure-, temperature-, and solute-induced phase transitions, grain boundary segregation, and simulations of radiation damage in the WTaCrVHf metallic glass.
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All authors Jesper Byggmästar, Tiago Lopes, Zheyong Fan, Tapio Ala-Nissila
Optics
Atomic-scale Stark-shift spectroscopy and microscopy of organic molecules
Xabier Arrieta, Sofia Canola, Ruben Esteban et al. (+2 more)
In conventional optical Stark-shift spectroscopy, molecules are exposed to spatially homogeneous static electric fields that shift the energies of their spectral lines. These shifts are attributed to the molecular electronic properties, such as variation of dipolar moment and polarizability of the molecule associated with photo(de)excitation. In realistic environments containing structural defects and nanoscale heterogeneities, however, molecules experience internal electric fields that vary strongly on the molecular scale, rendering the standard Stark selection rules inapplicable. Here we develop an extended theory of atomic-scale Stark shift, addressing such scenarios. Specifically, we present a detailed theoretical analysis of an experimentally relevant configuration where the atomically sharp tip of a light-assisted scanning tunneling microscope is used to controllably apply inhomogeneous electrostatic fields to representative molecular dyes spanning several molecular families. We decompose the total Stark shift into linear and quadratic contributions and show that they contain different information about the molecular properties. Concretely, spatial variations of the linear Stark shift as the tip scans across the molecule enable subnanometric mapping of the charge redistribution between ground and excited electronic states, with high sensitivity to molecular composition and chemical functionalization. The quadratic Stark contribution, in contrast, reflects changes in the conventional dipolar polarizability upon excitation. Together, these results establish nanoscale Stark-shift spectroscopy as a powerful tool for resolving excited-state charge dynamics in single molecules under realistic, strongly inhomogeneous electric fields.
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All authors Xabier Arrieta, Sofia Canola, Ruben Esteban, Javier Aizpurua, Tomáš Neuman
Fluid Dynamics
Evaluation of the performance of an analytical-numerical coupled method for droplet impacts on soft material surfaces
Hao Hao, Antonis Sergis, Alex M. K. P. Taylor et al. (+2 more)
Impacts between droplets and solid surfaces can commonly cause erosion problem in Engineering applications, including aircraft surface erosion, wind blade leading-edge erosion and steam turbine blade erosion. In practice, the impacted solid surfaces have varied material softness, ranging from stiff metallic coatings to soft materials. An analytical-numerical coupled model (ANCM) for simulating droplet impacts on surfaces, and corresponding material analysis, has been developed in the literature. However, the analytical impact pressure solution of the ANCM model has been derived assuming rigid solid surface. In the current study, we investigate the performance of the ANCM model for droplet impacts on soft materials made of urethane gel phantom, by comparing the ANCM computations to lab-based experiments and numerical simulations based on Smoothed Particle Hydrodynamics (SPH). Parametric studies explore the applicability limit of the ANCM model for droplet impacts on very soft materials at low Young's modulus. It was found that for materials at Young's modulus of $47,400$ Pa or stiffer, which covers most engineering applications, the developed ANCM model performs as expected for an assumed rigid surface. For softer solid materials, the SPH-modeled
liquid interacts with the evolving surface geometry and mitigates impact intensity as deformation occurs. The analytical impact loads estimated by ANCM are independent of surface geometry, and hence provide conserved impact impulse in a non-physical way. Results show a critical value of Young's modulus at $E=10,000$ pa for the ANCM model, below which the model exhibits overshoot in total contact force and surface deformation, leading to the formation of steep wall craters.
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All authors Hao Hao, Antonis Sergis, Alex M. K. P. Taylor, Yannis Hardalupas, Maria N. Charalambides
Instrumentation and Detectors
Irradiation Studies of TGC Electronics Components for the ATLAS Experiment at High-Luminosity LHC
Yuya Ohsumi, Daisuke Hashimoto, Yasuyuki Horii et al. (+23 more)
This paper evaluates the radiation tolerance of commercial off-the-shelf (COTS) electronics components for use in the Thin Gap Chamber (TGC) frontend electronics of the ATLAS experiment at the High-Luminosity LHC (HL-LHC). The ATLAS experiment has accumulated more than 450 fb^-1 of data as of 2025. Its luminosity upgrade, the HL-LHC scheduled to begin operation in 2030, will deliver 3000-4000 fb^-1 over ten years and lead to substantially higher radiation levels in detector electronics. The radiation levels for the TGC frontend electronics are estimated to be 4.1-7.3 Gy in terms of Total Ionizing Dose (TID) and 1.1-2.2 x 10^11 n_1MeV cm^-2 in terms of Non-Ionizing Energy Loss (NIEL). To evaluate component suitability under these conditions, TID tests were conducted using Cobalt-60 gamma rays at Nagoya University, and NIEL tests were performed with the Tandem Accelerator at Kobe University. Various COTS components, including SFP+ optical transceivers, clock jitter cleaners, optical fibers, voltage references, operational amplifiers, analog-to-digital converters, digital-to-analog converters, SD cards, flash
memories, and low-dropout regulators, were tested and evaluated against the required radiation levels. The results demonstrate that all evaluated components meet the TID and NIEL tolerance requirements for application in the TGC frontend electronics at the HL-LHC.
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All authors Yuya Ohsumi, Daisuke Hashimoto, Yasuyuki Horii, Takumi Aoki, Haruka Asada, Kazumasa Hashizume, Hayato Inaguma, Masaya Ishino, Miyuki Kikuchi, Shota Kondo, Reita Maeno, Airu Makita, Masaki Minakawa, Yuki Mitsumori, Yuki Nabeyama, Ren Nagasaka, Takumi Nakajima, Yoshifumi Narukawa, Atsuhiko Ochi, Yasuyuki Okumura, Osamu Sasaki, Aoto Tanaka, Akira Taniike, Makoto Tomoto, Arisa Wada, Erika Yamashita
Fluid Dynamics
An analytical-numerical coupled model of liquid droplet impact on solid material surfaces
Hao Hao, Maria N. Charalambides, Yannis Hardalupas et al. (+2 more)
Impacts of
liquid droplets on wind turbine blade surfaces, for example sea sprays, can result in material damage through erosion. In this study, we derive an explicit, closed-form analytical approximation for droplet impact and subsequent spreading on a solid surface in inertia-dominated regimes of large Reynolds and Weber numbers. The formulation extends an existing theoretical framework based on inviscid potential flow for a rising expanding disk in an infinite
liquid domain. The modified solution provides full spatio-temporal pressure distributions and impact force histories on the impact surface over the entire impact duration, capturing both the early-time self-similar flow and the inertia-driven lamella spreading following the peak impact force. The predicted pressure and force profiles show good agreement with analytical, numerical and experimental results reported in the literature, including accurate reproduction of the well-known ring-shaped pressure distribution. Key quantities, such as the radial location and magnitude of peak pressure, as well as the timing and magnitude of the peak impact force, are predicted analytically with reasonable accuracy. To enable solid material erosion analysis, the analytical
liquid-phase solution is coupled with a finite-element (FE) simulation for the solid response. This analytical-numerical coupled method (ANCM) eliminates the need to explicitly simulate droplet fluid dynamics, which is conventionally performed using smoothed particle hydrodynamics (SPH). As a result, for the purpose of material response analysis, the proposed approach achieves grid independence at substantially lower mesh resolutions and reduces computational cost by more than 97% compared to SPH-based simulations, while maintaining or improving numerical accuracy.
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All authors Hao Hao, Maria N. Charalambides, Yannis Hardalupas, Antonis Sergis, Alex M. K. P. Taylor
Instrumentation and Detectors
The CMS ME0 Upgrade: Enhancing Forward Muon Reconstruction at the HL-LHC
Anureet Kaur
The CMS
muon system is undergoing substantial upgrades to meet the challenges of the High-Luminosity LHC (HL-LHC), including the installation of the new
Muon Endcap 0 (ME0) detector. Large-scale production started in 2024. ME0 is a six-layer station designed to extend pseudo-rapidity coverage to |{\eta}| = 2.8 from the previous maximum of |{\eta}| = 2.4, enhancing sensitivity to forward physics processes. Each endcap will host 18 ME0 stacks, with each stack comprising six triple-layer gas electron multiplier (GEM) chambers. The system adds up to six additional hits per track, which significantly improves
muon identification, spatial resolution, and robust track reconstruction at the first trigger level. Chamber production and quality control across multiple international sites ensure scalability and timely delivery. The ME0 design incorporates lessons learned from earlier GEM deployments, with improvements in electronics robustness, grounding, and
segmentation to withstand high background rates and minimize damage from discharges. This contribution provides a comprehensive overview of the ME0 detector concept, assembly strategy, quality assurance procedures, current production status, and its pivotal role in strengthening CMS
muon reconstruction during HL-LHC operations.
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Chemical Physics
Optimally Tuned Multiconfigurational Short-Range DFT for Linear Response Properties
Michał Hapka, Katarzyna Pernal and Ewa Pastorczak
Multiconfigurational short-range density functional theory (MC-srDFT) rigorously combines ground state wavefunction theory with DFT. Unlike single-reference range-separated hybrid functionals, MC-srDFT has lacked theoretically grounded protocols for choosing the system-specific range-separation parameter. To address this problem, we introduce an optimal-tuning scheme based on enforcing the correct exponential decay of the electron density. We show that the range-separation parameter can be determined from the ionization potential given by the smallest-magnitude
eigenvalue of the Extended Koopmans'
Theorem matrix constructed for the model
Hamiltonian. We validate this approach for static and dynamic dipole polarizabilities of ground-state molecular systems using MC-srDFT within both full linear response and its extended random phase approximation (ERPA) variant. Optimal tuning substantially improves polarizabilities relative to the commonly used universal $\mu = 0.4\,\mathrm{bohr}^{-1}$ parameter.
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Applied Physics · Cross-listed from cs.LG
Reducing hyperparameter sensitivity in measurement-feedback based Ising machines
Toon Sevenants, Guy Van der Sande and Guy Verschaffelt
Analog Ising machines have been proposed as heuristic hardware solvers for combinatorial optimization problems, with the potential to outperform conventional approaches, provided that their hyperparameters are carefully tuned. Their temporal
evolution is often described using time-continuous dynamics. However, most experimental implementations rely on measurement-feedback architectures that operate in a time-discrete manner. We observe that in such setups, the range of effective hyperparameters is substantially smaller than in the envisioned time-continuous analog Ising machine. In this paper, we analyze this discrepancy and discuss its impact on the practical operation of Ising machines. Next, we propose and experimentally verify a method to reduce the sensitivity to hyperparameter selection of these measurement-feedback architectures.
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Plasma Physics
Electric current dynamics in the stellarator coil winding surface model
Wadim Gerner, Anouk Nicolopoulos-Salle and Diego Pereira Botelho
In stellarator design, the coil winding surfaces $\Sigma\subset\mathbb R^3$ support current distributions $j$ that shape the
magnetic field. This work provides a theoretical framework explaining the emergence of centre and saddle point regions, a key feature in coil optimisation. For coil winding surfaces with a toroidal shape, we prove a dichotomy principle: the current distribution has both centre and saddle point regions or is no-where vanishing. For coil winding surfaces that consist of piecewise cylinders, we show that if $j$ is oppositely oriented on the two boundary circles, centre and saddle points appear, and all but finitely many field lines of $j$ are periodic. When $j$ admits a harmonic potential, all field lines are closed poloidal orbits. These results offer insights into current patterns on winding surfaces, with implications for coil design strategies and their simplification.
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Applied Physics
Air-stable bright entangled photon-pair source from graphene-encapsulated van der Waals ferroelectric NbOI2
Mayank Joshi, Mengting Jiang, Yu Xing et al. (+6 more)
Van der Waals (vdW) ferroelectrics are emerging nonlinear photonic materials that combine large second-order susceptibility ḩi}(2) with
heterostructure compatibility, offering an attractive route toward miniaturized spontaneous parametric down-conversion (SPDC) sources. However, vdW SPDC sources operating under continuous irradiation in air remain limited in low brightness and poor operational stability, as oxygen and moisture exposure, together with pump-induced heating, lead to material degradation and permanent damage. Here we demonstrate an air-stable, bright SPDC source based on ferroelectric NbOI2 enabled by
graphene encapsulation.
Graphene provides robust environmental protection and can effectively supress pump induced degradation by enhancing heat dissipation. We report a record
photon-pair generation absolute rate of 258 Hz and a normalized brightness of 19,900 Hz/(mW.mm). Leveraging this stabilized platform, we further generate polarization entangled
photon pairs with 94% fidelity with respect to the maximally entangled Bell states from
graphene-encapsulated 90{\deg} twisted
bilayer NbOI2. Our results establish a practical and air-stable vdW ferroelectric SPDC platform that overcomes key limitations of existing vdW
quantum light sources and provides a viable pathway toward scalable, integrated entangled
photon sources for on chip
quantum photonics.
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All authors Mayank Joshi, Mengting Jiang, Yu Xing, Yuerui Lu, Jie Zhao, Ping Koy Lam, Syed M Assad, Xuezhi Ma, Young-Wook Cho
Computational Physics · Cross-listed from astro-ph.IM
Modified-gradient methods for exact divergence-free in meshless magnetohydrodynamics
Xiongbiao Tu, Qiao Wang, Liang Gao et al. (+1 more)
We present a novel gradient regularization to completely eliminate the
magnetic divergence error in meshless magnetohydrodynamics (MHD), which offers a high spatial resolution and conservative advantage, due to its Lagrangian nature. Comparing with the counterpart of constrained-gradient (CG) technique, we reform $\nabla \cdot \mathbf{B}=0$ by an implicit projection method to modify the
magnetic-field gradients. The accuracy of modified-gradient (MG) method is verified and it achieves exact divergence-free results with round-off precision, by using tests of shock tube, 2D and 3D
vortex, magneto-rotational instability, and especially, advection experiment, compared with CG method and the GIZMO code. It leads to noticeable improvement in pattern, amplitude and numerical dissipation of divergence error of
magnetic field.
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All authors Xiongbiao Tu, Qiao Wang, Liang Gao, Yifa Tang
Physics and Society
To trace or not to trace: analytical insights from network-based contact-tracing models
Giulia de Meijere, Andrea Pugliese, Gerardo Iñiguez et al. (+2 more)
Contact tracing is one of the most important control measures deployed during epidemics. Relying on the identification of contacts of known infected individuals, it necessitates a network perspective. Although pairwise models have been used extensively to study contact tracing, their analysis typically depends on a decoupling assumption-most commonly that contact tracing operates on a much faster timescale than disease transmission. Furthermore, contact tracing models often assume that all infected individuals become contact tracing-triggering, which is unrealistic given partial compliance to treatment. We relax both of these restrictive assumptions and provide a full analytical characterisation of the epidemic threshold in the pairwise mean-field model. Our analysis uses a fast-variables approach that captures the rapid early stabilisation of key network quantities. Inspired by mechanisms from social adoption dynamics, we introduce triplewise contact tracing in which an infected individual can be traced not only through direct contact with a single tracing-triggering neighbor (pairwise tracing), but also indirectly when connected to two tracing-triggering nodes simultaneously. For pure pairwise and pure triplewise contact tracing, we derive analytical expressions for critical contact tracing levels and demonstrate that when many infected individuals bypass treatment, the epidemic can become uncontrollable. When both contact tracing mechanisms operate together, we map out their combined contribution and relative impact on epidemic control. This unified framework yields rigorous and tractable threshold conditions for contact tracing dynamics on networks, extending the applicability of pairwise models beyond the fast-tracing regime and providing new insight into the interplay between disease progression, partial treatment compliance, and higher-order tracing processes.
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All authors Giulia de Meijere, Andrea Pugliese, Gerardo Iñiguez, Péter L. Simon, István Z. Kiss
Accelerator Physics
First Experimental Characterization of Plasma Parameters and Carbon Decontamination Rates in a Microwave Resonator Used in Particle Accelerators
Camille Cheney, Gabriel Abi-abboud, Stéphane Béchu et al. (+7 more)
In-situ
plasma processing of superconducting
radio frequency (SRF) cavities is a performance recovery technique used to mitigate the field emission limiting phenomenon. It has been proved very effective at major particle accelerator facilities such as SNS, CEBAF, FRIB, FNAL and C-ADS. This technique is based on the ignition of a noble-gas/oxygen
plasma inside the cavity over several hours to remove hydrocarbon-based contamination, responsible for the parasitic field emission degradation observed after several years of operation. Despite a large experimental R&D effort from the community,
plasma parameters and cleaning rates under various experimental conditions have never been directly evaluated. In this study,
plasma parameters were measured using a Langmuir probe and cleaning rates thanks to a quartz
crystal microbalance (QCM) coated with an amorphous carbon film to simulate a carbon-based contamination. In this article, the main results from a large parameter space are discussed along with guidelines for improving the
plasma processing effectiveness in SRF cavities. The encountered technical challenges are also discussed, as the SRF cavity is by design not intended to be a
plasma reactor.
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All authors Camille Cheney, Gabriel Abi-abboud, Stéphane Béchu, Alexandre Bès, Laurent Bonny, Thibaut Gerardin, Bruno Mercier, Eric Mistretta, Jonathan Yemane, David Longuevergne
Optics
Second-order supporting quadric method for designing freeform refracting surfaces generating prescribed irradiance distributions
Albert A. Mingazov, Dmitry A. Bykov, Evgeni A. Bezus et al. (+1 more)
We consider the inverse problem of calculating a refracting surface that generates a prescribed irradiance distribution in the far field for a collimated incident beam. This problem can be formulated as a mass transportation problem (MTP) with a quadratic cost function. To solve this problem, we propose a version of the supporting quadric method (SQM), in which the calculation of the quadric parameters is reduced to the problem of minimizing a convex function. We obtain simple analytical expressions for the second derivatives of this function, making it possible to calculate the quadric parameters using second-order optimization methods. This allows us to refer to the proposed method as the second-order SQM. We demonstrate high efficiency of this approach by designing several optical surfaces that generate complex irradiance distributions. We also consider the application of the second-order SQM to nonimaging optics problems described by MTPs with a non-quadratic cost function.
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All authors Albert A. Mingazov, Dmitry A. Bykov, Evgeni A. Bezus, Leonid L. Doskolovich
Optics
Radiative-channel valley topological laser
Seonyeong Kim, Markus Scherrer, Jakub Dranczewski et al. (+3 more)
Active topological photonic systems enable robust light control and new pathways for
semiconductor lasing. However, their intrinsically non-Hermitian nature, combining gain, radiation leakage, and material loss, makes the underlying physics more complex, and prior studies have mostly focused on gain competition while the influence of loss channels is less examined. Here, we experimentally demonstrate radiative-channel-driven topological lasing in a valley photonic
crystal consisting of isolated InP nanorods on an
insulator, achieving room-temperature single-mode operation within an about 4 lambda scale cavity. Loss-included simulations show that material absorption and radiative leakage can be exploited to establish the lasing pathway. Local off-edge pumping provides spatial evidence of topological edge-guided lasing. Berry-
curvature calculations, reflecting unit-
cell symmetry breaking, verify the valley-
Hall interface in a triangular
lattice. Geometry and temperature tuning identify a narrow spectral window where the above-light-line edge branch matches the gain-loss balance and remains decoupled from bulk bands. These results clarify how the loss landscape governs topological lasing and establish a radiative-edge design framework for active topological photonics. The substrate-supported nanorod architecture additionally offers a scalable route for on-chip implementation.
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All authors Seonyeong Kim, Markus Scherrer, Jakub Dranczewski, Heinz Schmid, Kirsten Moselund, Chang-Won Lee
Applied Physics
Dielectric Barrier Corona Discharge Anomaly by Ionic Wind under Unipolar Voltage Excitation
Gan Fu
An anomalous back discharge movement phenomenon is induced by a set of dielectric barrier corona discharges (DBCD) at unipolar half-sine voltage waveforms, where the back discharge has a time delay that relates to the applied voltage level. An ionic wind model is employed to analyze the physical behavior. Theoretical explanation and quantitative analysis are presented in this study based on abundant experimental results of 5 typical insulating materials and a FEP insulating cable. A numerical model is derived, which indicates that the back discharge can be activated under a relatively low potential voltage level in this study. The results highlight that the back discharge movement phenomenon behaves distinctly under half-sine voltage with negative polarity, yielding a significantly different partial discharge (PD) pattern with positive polarity. Besides, PD amplitude dependent on dielectric thickness is demonstrated by plotting in phase resolved partial discharge (PRPD) pattern. Furthermore, comparative experiments are conducted with respect to the variation of air gap length and dielectric geometry, manifesting different influences on PD amplitude.
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