Optics
Increasing The Sensitivity of a Surface Plasmon Resonance Sensor using Ti3C2 Mxene
Rostyslav Terekhov, Zoya Eremenko and Sergii Kulish
Sensors based on the phenomenon of surface plasmon resonance have limited sensitivity, up to 123 degree/Refractive Index Unit, which restricts their applicability in detecting subtle changes gin biological media (around 10-5 RIU). To address this, a sensor dependent on the Kretschmann-Reather configuration was modified using Ti3C2 MXene material. The sensor was modeled using the finite element method (FEM), yielding the dependence of the wave reflection parameter on the angle of incidence. Founded on these results, the sensitivity of the sensor was calculated and demonstrated a 10% improvement compared to the conventional Kretschmann-Reather configuration. In addition, human biological real-world samples were modeled using refractive index data, and the error of the proposed approach was determined to be approximately 3%.
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Space Physics
Self-organization of local streamline structures and energy transfer rate in compressible plasma turbulence
Simone Benella, Virgilio Quattrociocchi, Emanuele Papini et al. (+4 more)
We examine how local streamline
topology and energy cascade rate self-organize in
plasma turbulence for both compressible and incompressible regimes. Using a fully-compressible
Hall-magnetohydrodynamic simulation, we quantify the subgrid-scale energy transfer and analyze its relationship to streamline structures by means of grandient tensor
geometric invariants of the velocity field. Our results highlight how streamline
topology is crucial for diagnosing
turbulence: for nearly-incompressible fluctuations the energy is primarily transferred to smaller scales through strain-dominated and stable-vortical structures, while is back-transferred towards larger scales through unstable-vortical structures. Compressible fluctuations, on the contrary, do not show a clear topological selection of the energy transfer since the overall direction of the local cascade rate is found to be determined by the sign of $-\nabla\cdot u$ (
plasma volumetric compression or expansion).
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All authors Simone Benella, Virgilio Quattrociocchi, Emanuele Papini, Andrea Verdini, Simone Landi, Maria Federica Marcucci, Giuseppe Consolini
Geophysics · Cross-listed from astro-ph.EP
Setting up the physical principles of resilience in a model of the Earth System
Orfeu Bertolami and Magnus Nyström
Resilience is a property of social, ecological, social-ecological and biophysical systems. It describes the capacity of a system to cope with, adapt to and innovate in response to a changing surrounding. Given the current
climate change crisis, ensuring conditions for a sustainable future for the habitability on the
planet is fundamentally dependent on Earth System (ES) resilience. It is thus particularly relevant to establish a model that captures and frames resilience of the ES, most particularly in physical terms that can be influenced by human policy\footnote{See page 4 for examples of strategies}. In this work we propose that resilience can serve as a theoretical foundation when unpacking and describing metastable states of
equilibrium and energy dissipation in any dynamic description of the variables that characterise the ES. Since the impact of the human activities can be suitably gauged by the planetary boundaries (PBs) and the
planet's temperature is the net result of the multiple PB variables, such as $\text{CO}_2$ concentration and radiative forcing, atmospheric aerosol loading, atmospheric ozone depletion, etc, then resilience features arise once conditions to avoid an ES runaway to a state where the average temperature is much higher than the current one. Our model shows that this runaway can be prevented by the presence of metastable states and dynamic friction built out of the interaction among the PB variables once suitable conditions are satisfied. In this work these conditions are specified. As humanity moves away from Holocene conditions, we argue that resilience features arising from metastable states might be crucial for the ES to follow sustainable trajectories in the Anthropocene that prevent it run into a much hotter potential
equilibrium state.
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Optics
Terahertz metasurface sensor with graphene microstrips for biosensing: modeling and application
K. S. Kuznetsova, V. A. Pashynska and Z. E. Eremenko
The study is devoted to development and optimization of a metasurface-based sensor with
graphene constituents for potential biosensing applications. A unit
cell of the proposed metasurface consists of a thin flexible dielectric substrate layer with a centrally positioned
graphene microstrip. As a result of numerical modeling of spectral properties of the metasurface by COMSOL Multiphysics software in terahertz range from 5 to 35 THz the absorption spectrum maxima (resonance modes) are revealed. The following stages of the study demonstrate that placement of a layer of tested
liquid sample (water or Bovine Serum Albumin (BSA) solution) on the metasurface causes a low frequency shift of the plasmonic resonance mode chosen for biosensing measurements. This frequency shift, along with the change in the amplitude of the absorption peak, are highly sensitive to the refractive index of the tested
liquid sample. The obtained results demonstrate the potential of the developed metasurface-based sensor with
graphene microstrips for application as a sensing structure to determine
proteins and other biomolecules in
liquid samples.
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Instrumentation and Detectors
Novel High-Radiopurity Doped Amorphous Silicon Resistors for Low-Background Detectors
A. Anker, P.C. Rowson, K. Skarpaas et al. (+9 more)
We present the results of a study of lightly doped amorphous silicon used as a resistive medium for high-radiopurity resistors in
nuclear and particle physics research instrumentation. Prototypes are produced for a Time Projection Chamber design for the nEXO neutrinoless double-beta decay search experiment that meet requirements for ultra-high radiopurity, good mechanical, cryogenic and high voltage performance, as well as useful
vacuum ultraviolet (VUV) reflectivity. Further study is warranted to refine production methods and to confirm that the technology used here is useful for more general applications.
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All authors A. Anker, P.C. Rowson, K. Skarpaas, S. Tsitrin, I.J. Arnquist, L. Kenneth S. Horkley, L. Pagani, T.D. Schlieder, E. van Bruggen, P. Kachru, A. Pocar, N. Yazbek
Optics
Optimization of the metal-dielectric metasurface unit cell for sensitivity enhancement in determination of IgG concentration in solutions
K. S. Kuznetsova, V. A. Pashynska and Z. E. Eremenko
This study focuses on developing a metal-dielectric sensor structure with optimized unit
cell geometry for determination of
protein Immunoglobulin G (IgG) concentration in aqueous solutions. The research combines both experimental and theoretical investigations, utilizing the differential microwave dielectrometry method and numerical modeling with COMSOL software. Complex permittivity (CP) values dependence of IgG water solutions on the
protein concentration was experimentally obtained at the microwaves using original microwave dielectrometer setup. It was shown that increase of IgG concentration resulted in the CP values of the solutions studied decrease. The experimentally obtained CP data for the IgG water solutions were used as a basis for microwave metal-dielectric metasurface unit
cell numerical modeling. The metal-dielectric metasurface consisting of Teflon substrate and plane copper microresonators was combined with a standard 96-well microplate used in clinical laboratories. Optimization of the obtained metasurface unit
cell revealed that the size and position of the copper microresonators within the unit
cell significantly impact the sensor sensitivity for determining the IgG concentration in aqueous solutions. The metasurface with the unit
cell containing four copper microresonators provided the most sensitive platform for detecting variations in the IgG concentration in the sample. The frequency shift of the reflection coefficient was directly related to changes in the
protein concentration. The calibration graph was developed for effective determination of IgG concentrations in the aqueous solutions.
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Plasma Physics
Comment on Nuclear Fusion 66, 016012 (2026) and arXiv:2508.03561 by Richard Fitzpatrick, A Simple Model of Current Ramp-Up and Ramp-Down in Tokamaks
Allen H Boozer
The paper
Nuclear Fusion 66, 016012 (2026) by Richard Fitzpatrick is based on fundamental errors in the physics of the
evolution of the poloidal
magnetic flux in tokamaks. With the certainty of Faraday's Law, the slippage of the poloidal relative to the toroidal flux is given by the loop voltage. Some of the errors may be endemic to the tokamak community, such as the focusing on a quantity $\psi$ that has a subtle relation to the poloidal flux while essentially ignoring the toroidal flux. Other errors are due to an
entanglement of in principle separable concepts and unjustified assumptions. Although
Nuclear Fusion allowed the use “private communication" as a reference to bolster misquotes, the response must be in arXiv because
Nuclear Fusion will not allow any mention of “private communication" in a response. This comment also derives a new and simple expression for the loop voltage, which has the validity of the parallel component of Ohm's law.
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Applied Physics
Comparison of reverse current mechanisms in GaN Schottky diodes grown on sapphire versus ammonothermal GaN substrates
B. Orfao, M. Abou Daher, H. Bouillaud et al. (+5 more)
In this work, we analyse the reverse current mechanisms in GaN Schottky barrier diodes (SBDs) grown on sapphire and native GaN substrates. For the sapphire-substrate sample, two conduction mechanisms are identified: Poole-Frenkel emission (PFE) and trap-assisted tunneling (TAT), with corresponding trap energy levels of 0.9 eV and 0.3 eV, respectively. In contrast, only PFE is observed in the GaN-substrate sample, with a trap energy of 0.75 eV, suggesting that the presence of TAT is related to the higher dislocation density in structures grown on sapphire substrates. The leakage mechanisms and associated trap energies are extracted by comparing experimental current-voltage (I-V) characteristics with a model that includes thermionic emission and tunneling contributions for different temperatures, from 298K up to 443K.
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All authors B. Orfao, M. Abou Daher, H. Bouillaud, Y. Roelens, P. Prystawko, R. Kucharski, M. Bockowski, M. Zaknoune
Optics · Cross-listed from quant-ph
Below-threshold error reduction in single photons through photon distillation
F. H. B. Somhorst, J. Saied, N. Kannan et al. (+14 more)
Photonic
quantum computers use the bosonic statistics of
photons to construct, through
quantum interference, the large entangled states required for measurement-based
quantum computation. Therefore, any which-way information present in the
photons will degrade
quantum interference and introduce errors. While
quantum error correction can address such errors in principle, it is highly resource-intensive and operates with a low error threshold, requiring numerous high-quality optical components. We experimentally demonstrate scalable, optimal
photon distillation as a substantially more resource-efficient strategy to reduce indistinguishability errors in a way that is compatible with fault-tolerant operation.
Photon distillation is an intrinsically bosonic, coherent error-mitigation technique which exploits
quantum interference to project single
photons into purified internal states, thereby reducing indistinguishability errors at both a higher efficiency and higher threshold than
quantum error correction. We observe unconditional error reduction (i.e., below-threshold behaviour) consistent with theoretical predictions, even when accounting for noise introduced by the
distillation gate, thereby achieving actual net-gain error mitigation under conditions relevant for fault-tolerant
quantum computing. We anticipate
photon distillation will find uses in large-scale
quantum computers. We also expect this work to inspire the search for additional intrinsically bosonic error-reduction strategies, even for fault-tolerant architectures.
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All authors F. H. B. Somhorst, J. Saied, N. Kannan, B. Kassenberg, J. Marshall, M. de Goede, H. J. Snijders, P. Stremoukhov, A. Lukianenko, P. Venderbosch, T. B. Demille, A. Roos, N. Walk, J. Eisert, E. G. Rieffel, D. H. Smith, J. J. Renema
Atomic Physics
Electron Energy Loss Spectroscopy of oriented targets and magnetic transitions
Ioannis Iatrakis and Valerii Brudanin
Electron beam energies in Transmission Electron Microscopes (TEMs) reach the relativistic realm constituting
Quantum Electrodynamics (QED) the appropriate framework for the study of electron matter interaction in TEMs. We focus on the inelastic scattering of relativistic electrons from a generic oriented target. The inelastic differential cross section factorizes to the fast electron part which is calculated analytically, and the dynamic form factor of the target, which encodes the response of the medium to the interaction with the beam. The properties of the dynamic form factor of oriented targets are analyzed. We then derive the scattering cross section of electrons by
magnetic targets where
spin-flip transitions are induced. We comment on the kinematic regimes where the coefficient of the transverse
magnetic interaction is amplified compared to the coulomb
matrix element.
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Optics
Spin waves excited by hard x-ray transient gratings
Peter R. Miedaner, Alexei A. Maznev, Mykola Biednov et al. (+49 more)
Recent progress in ultrafast x-ray sources helped establish x-rays as an important tool for probing
lattice and
magnetic dynamics initiated by femtosecond optical pulses. Here, we explore the potential of ultrashort hard x-ray pulses for driving
magnetic dynamics. We use a transient grating technique in which a spatially periodic x-ray excitation pattern gives rise to material excitations at a well-defined wave vector, whose dynamics are monitored via diffraction of an optical probe pulse. The excitation of a ferrimagnetic gadolinium bismuth iron garnet film placed in an external tilted
magnetic field by x-rays at the Gd L3 edge results in both
magnetic and non-
magnetic transient gratings whose contributions to the diffracted signal are separated by polarization analysis. We observe the magnetization precession at both longitudinal acoustic and
spin wave frequencies. An analysis with the Landau-Lifshitz-Gilbert equation indicates that the magnetization precession is driven by strain resulting from thermal expansion induced by absorbed x-rays. The results establish x-ray transient gratings as a tool for driving coherent
phonons and
magnons, with the potential of accessing wave vectors across the entire Brillouin zone.
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All authors Peter R. Miedaner, Alexei A. Maznev, Mykola Biednov, Marwan Deb, Carles Serrat, Nadia Berndt, Pietro Carrara, Cristian Soncini, Marta Brioschi, Daniele Ronchetti, Andrei Benediktovitch, Danny Fainozzi, Nupur Khatu, Eugenio Ferrari, Joan Vila-Comamala, Peter Zalden, Wojciech Gawelda, Martin Knoll, Paul Frankenberger, Ludmila Leroy, Talgat Mamyrbayev, Grigory Smolentsev, Simon Gerber, Alessandro Gessini, Filippo Bencivenga, Riccardo Cucini, Giorgio Rossi, Riccardo Mincigrucci, Ettore Paltanin, Majed Chergui, Claudio Masciovecchio, Stefano Bonetti, Yohei Uemura, Xinchao Huang, Han Xu, Frederico Alves Lima, Fernando Ardana-Lamas, Anders Madsen, Renato Torre, Luis Bañares, Jakub Szlachetko, Wojciech Blachucki, Matias Bargheer, Thomas Feurer, Robin Y. Engel, Martin Beye, Christian David, Urs Staub, Andrea Cannizzo, Cristopher Milne, Keith A. Nelson, Cristian Svetina
Fluid Dynamics
Coupled Level-Set Lattice Boltzmann Method on Adaptive Cartesian Grids
Julian Vorspohl, Yuxing Peng, Matthias Meinke et al. (+2 more)
A novel coupled level-set
lattice Boltzmann method on adaptive Cartesian grids for simulating
liquid-gas multiphase flows is presented. The approach addresses the inherent challenges of accurately modeling multiphase systems characterized by sharp interfaces and large density ratios. By employing separate solution
algorithms for each fluid phase which are coupled through boundary conditions at the interface the method is more accurate and more efficient. The study highlights the advantages of using
lattice Boltzmann methods together with level-set techniques to track interfaces effectively while facilitating adaptive mesh refinement. Applications to various test cases, e.g., immiscible stratified flow and rising bubbles, demonstrate the method's capability to capture complex interfacial dynamics and validate its accuracy against literature data.
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All authors Julian Vorspohl, Yuxing Peng, Matthias Meinke, Dominik Krug, Wolfgang Schröder
Biological Physics
Flow-wave coupling synchronizes oscillations in growing active matter
Lara Koehler, Elissavet Sandaltzopoulou, Frank Jülicher et al. (+1 more)
Oscillatory biochemical signals and mechanical forces must coordinate robustly during development, yet the principles governing their mutual coupling remain poorly understood. In syncytial embryos and
cell-free extracts, mitotic waves propagate across millimeter scales while simultaneously generating cytoplasmic flows, suggesting a two-way interaction between chemical oscillators and mechanics. Here, we combine experiments in Xenopus Laevis cytoplasmic extracts with a minimal particle-based model to reveal a mechanochemical feedback that stabilizes phase wave propagation. In contrast to previous models of oscillatory active matter, an asymmetric size cycle, slow growth and rapid shrinkage, combined with size-dependent mechanical interactions generates a net particle displacement and flows aligned with the wave direction, which in turn drive a synchronization transition. Our results show that mechanical forces actively maintain the coherence of biochemical waves, providing a general mechanism for long-range order in oscillating active matter.
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All authors Lara Koehler, Elissavet Sandaltzopoulou, Frank Jülicher, Jan Brugués
Instrumentation and Detectors
Data acquisition from high-rate detectors at MAX IV
Paul Bell, Michele Cascella, Felix Engelmann et al. (+7 more)
At MAX IV pixelated area detectors are operated at high frame rates to take advantage of the X-ray beam properties available from the fourth generation synchrotron in scattering, diffraction and imaging applications. A variety of
photon counting and charge integrating detectors and sCMOS cameras have been integrated into a common data acquisition (DAQ) system in which data are streamed to a central Kubernetes cluster, mounting an IBM Storage Scale (GPFS) file system. The DAQ system provides live feedback from the detectors/cameras and extends to enable on-the-fly data processing. Control system integration via Tango provides a standardised single interface for controlling all DAQ components. Starting from an overview of the detector types in use, we describe the design and implementation of the MAX~IV detector-DAQ system and report a quantitative study of its performance in terms of data throughput and detector operating rates.
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All authors Paul Bell, Michele Cascella, Felix Engelmann, Thomas Eriksson, Aleko Lilius, Zdenek Matej, Jeremy Metz, Andrii Salnikov, Clemens Weninger, Meghdad Yazdi
Chemical Physics · Cross-listed from cond-mat.soft
Topological origin of peak splitting in the structure factor of liquid water
Zoé Faure Beaulieu, Volker L. Deringer and Fausto Martelli
The splitting of the principal peak in the structure factor of
liquid water is commonly interpreted as evidence of a competition between two distinct local environments. Here, we show that this peak splitting arises from medium-range topological features of the hydrogen-bond network. Using atomistic simulations, we systematically decompose the structure factor into contributions from hydrogen-bonded rings of different sizes. We find that 5-8-membered rings, which dominate the network
topology of
liquid water at low temperatures, can directly explain the experimentally observed bimodal scattering signal. Among these, 5-membered rings are particularly persistent, maintaining distinct structural signatures even above room temperature. Our findings establish a direct link between the network
topology of
liquid water and experimentally accessible diffraction features, clarifying the microscopic basis of water's behaviour and suggesting a broader conceptual framework for interpreting the anomalies in tetrahedral network
liquids and glasses.
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Computational Physics · Cross-listed from cs.LG
GlueNN: gluing patchwise analytic solutions with neural networks
Doyoung Kim, Donghee Lee, Hye-Sung Lee et al. (+2 more)
In the analysis of complex physical systems, the objective often extends beyond merely computing a numerical solution to capturing the precise crossover between different regimes and extracting parameters containing meaningful information. However, standard numerical solvers and conventional deep learning approaches, such as Physics-Informed Neural Networks (PINNs), typically operate as black boxes that output solution fields without disentangling the solution into its interpretable constituent parts. In this work, we propose GlueNN, a physics-informed learning framework that decomposes the global solution into interpretable, patchwise analytic components. Rather than approximating the solution directly, GlueNN promotes the integration constants of local asymptotic expansions to learnable, scale-dependent coefficient functions. By constraining these coefficients with the differential equation, the network effectively performs regime transition, smoothly interpolating between asymptotic limits without requiring ad hoc boundary matching. We demonstrate that this coefficient-centric approach reproduces accurate global solutions in various examples and thus directly extracts physical information that is not explicitly available through standard numerical integration.
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All authors Doyoung Kim, Donghee Lee, Hye-Sung Lee, Jiheon Lee, Jaeok Yi
Plasma Physics
Dynamics of ion temperature gradient modes in burning plasma conditions in the presence of energetic particles
Roman Ivanov, Alessandro Biancalani, Alberto Bottino et al. (+4 more)
The interaction between energetic particles (EPs) and ion temperature gradient (ITG) modes is studied using the global particle in
cell ORB5 code. In this work, we extend previous studies to a broader range of EP temperatures, including the burning
plasma regime and to wider variety of EP distribution functions. Two main stabilization mechanisms are found to be effective in ITG stabilization confirming previous studies: direct dispersion relation modification (DDRM) effective only at intermediate EP temperatures and dilution effect (DE) which is independent of EP temperature and becomes dominant in burning
plasma regime ($T_f > 50T_i$). The study is further extended to slowing-down EP distributions which in contrast exhibit no DDRM-related stabilization. The findings are further validated in an ITER pre-fusion operation scenario and additionally compared with electromagnetic effects. In this scenario EP stabilization is found to be weaker than $\beta$-stabilization. Overall, these results provide better understanding of EP-ITG interactions over a wider range of EP parameters relevant to burning
plasma regime which is important for predicting
turbulence and confinement in future devices such as ITER.
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All authors Roman Ivanov, Alessandro Biancalani, Alberto Bottino, Didier Gossard, Thomas Hayward-Schneider, Alexey Mishchenko, Ruoyuan Wu
Optics · Cross-listed from quant-ph
Breaking the Exponential: Decoherence-Driven Power-Law Spontaneous Emission in Waveguide Quantum Electrodynamics
Stefano Longhi
We investigate the spontaneous emission of a two-level system coupled to a photonic waveguide, showing that dynamical dephasing in the
photon modes profoundly alters the decay law. In the absence of dephasing, the emitter displays conventional exponential decay followed by a long-time power-law tail – observable only at extremely low survival probabilities. Strikingly, when dephasing is introduced, a robust power-law decay emerges already at short times, driven by
photon diffusion in the dynamically disordered environment rather than spectral edge effects. These results reveal a novel, decoherence-induced mechanism for non-exponential spontaneous emission in waveguide QED platforms.
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Accelerator Physics
Non Destructive Testing
Gonzalo Arnau Izquierdo
The contribution introduces the principles, methods, and applications of non-destructive testing in the context of particle accelerators and related technologies is presented. Both surface inspection methods (visual testing, penetrant testing,
magnetic particle testing, eddy current testing) and volumetric methods (radiographic testing, ultrasonic testing) are presented, with examples drawn from CERN projects. Emphasis is placed on the capabilities, limitations, and practical considerations of each technique, highlighting their role in ensuring quality and safety during fabrication, procurement, and in-service inspections. The contribution also addresses standards, codes, and personnel qualification schemes, underlining their regulatory impact in fields such as pressure equipment and industrial piping. Finally, the importance of early integration of NDT into project planning is stressed, not only for compliance but also as a proactive quality and efficiency measure.
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Fluid Dynamics
Combined effects of evaporation, sedimentation and solute crystallization on the dynamics of aerosol size distributions on multiple length and time scales
Sina Zendehroud, Ole Kleinjung, Philip Loche et al. (+5 more)
We investigate three aspects of aerosol-mediated air-borne viral infection mechanisms on different length and time scales. First, we address the
evolution of the size distribution of a non-interacting ensemble of droplets that are subject to evaporation and sedimentation using a sharp droplet-air interface model. From the exact solution of the
evolution equation we derive the viral load in the air and show that it depends sensitively on the relative humidity. Secondly, from Molecular Dynamics simulations we extract the molecular reflection coefficient of single water molecules from the air-water interface. This parameter determines the water condensation and evaporation rate at a
liquid droplet surface and therefore the evaporation rate of aqueous droplets. We find the reflection of water to be negligible at room temperature but to rise significantly at elevated temperatures and for grazing incidence angles. Thirdly, we derive a thermodynamically consistent three-dimensional diffuse-interface model for solute-containing droplets that is formulated as a three-phase Cahn-Hilliard/Allen-Cahn system. By numerically solving the coupled system of equations, we explore representative scenarios that show that this model reproduces and generalizes features of the sharp-interface model. These interconnected studies on the dynamics of aerosol droplet evaporation are relevant in order to quantitatively assess the airborne infection risk under varying environmental conditions.
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All authors Sina Zendehroud, Ole Kleinjung, Philip Loche, Lydéric Bocquet, Roland R. Netz, Erica Ipocoana, Dirk Peschka, Marita Thomas
Plasma Physics
Toward Quantitative Electric-Field Measurements of Inception Clouds in Nanosecond Discharges Using E-FISH Assisted by Machine Learning
Mhedine Alicherif, Edwin Sugeng, Zhijan Yang et al. (+2 more)
This study investigates the spatio-temporal
evolution of the electric field during the early stages of a nanosecond positive corona discharge in atmospheric-pressure air by combining time-resolved E-FISH measurements, machine-learning-assisted field inversion (based on a recently developed operator-learning model), and iCCD optical emission imaging. The objective is to quantitatively characterize the electric field in the vicinity of the high-voltage electrode during inception and the transition toward streamer formation. By averaging over a large number of discharge events and operating in a regime where the discharge remains statistically axisymmetric, the proposed approach enables reconstruction of the electric-field profiles with nanosecond resolution. The results show a rapid increase of the field during the first nanoseconds, followed by the formation of a shell-like structure exhibiting the highest reduced fields prior to destabilization. The reconstructed reduced electric-field magnitude reaches peak values in the range of approximately 230-270 Td, with an estimated uncertainty of about 20-30% associated with calibration and profile-shape effects. These values correspond to the regime where electron-impact excitation and photoionization processes become highly efficient, consistent with the observed transition from a stable inception cloud to streamer destabilization. After the onset of streamer branching, increasing asymmetry limits the applicability of the inversion, and the reconstructed fields represent averaged contributions rather than the local field at individual streamer heads. The methodology thus identifies the conditions under which quantitative E-field mapping is reliable and establishes a framework for extending electric-field diagnostics to the inception phase of nanosecond atmospheric discharges.
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All authors Mhedine Alicherif, Edwin Sugeng, Zhijan Yang, Deanna A. Lacoste, Tat Loon Chng
Physics and Society
When legs and bodies synchronize: Two-level collective dynamics in dense crowds
Thomas Chatagnon, Mohcine Chraibi, Julien Pettré et al. (+2 more)
Ultra-dense crowds, in which physical contact between people cannot be avoided, pose major safety concerns. Nevertheless, the underlying dynamics driving their collective behaviours remain poorly understood. Existing dense crowd models, mostly two-dimensional and contact-based, overlook biomechanical mechanisms that govern individual balance motion. In this study, we introduce a minimal two-level pedestrian model that couples upper body and legs dynamics, allowing us to capture transitions between balanced and unbalanced states at the individual scale. Whereas previous models fail to achieve it, this coupling gives rise to emergent collective behaviours observed empirically, such as self-organized waves and large-scale rotational motion within the crowd. The model bridges basic individual biomechanical concepts and macroscopic flow dynamics, offering a new framework for modelling and understanding collective motions in ultra-dense crowds.
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All authors Thomas Chatagnon, Mohcine Chraibi, Julien Pettré, Armin Seyfried, Antoine Tordeux
Chemical Physics
A Non Linear Spectral Graph Neural Network Simulator for More Stable and Accurate Rollouts
Salman N. Salman, Sergey A. Shteingolts, Ron Levie et al. (+1 more)
Molecular dynamics (MD) simulations are a central tool in science and engineering enabling the study of dynamical behavior and the link between microscopic structure and macroscopic function. Their high computational cost, however, has motivated extensive efforts to develop accelerated alternatives. A promising approach is the use of machine-learning-based simulators that allow for substantially larger time steps than conventional MD. Among these, graph neural network (GNN)-based methods have been found to be especially attractive given that they naturally encode the inductive bias of interacting particle systems, however current architectures remain limited in accuracy and stability. In particular, standard message-passing schemes struggle to efficiently propagate long-range information. Here, we investigate whether spectral-GNN simulators can overcome these limitations by explicitly representing a simulated system in a global eigenmode basis, and therefore better capture long-range and collective behavior. Focusing on disordered elastic networks, model systems for complex materials and biological structures such as
proteins, we compare spatial, linear spectral, and nonlinear spectral GNN architectures. We find that while spectral representations alone are not sufficient, nonlinear spectral models substantially outperform alternatives. By learning both the time-dependent dynamics and the mixing of eigenmodes, these models more accurately capture the system's slow, global modes, which dominate macroscopic behavior. This leads to a marked reduction in systematic particle-position error and significantly improved prediction of global physical properties.
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All authors Salman N. Salman, Sergey A. Shteingolts, Ron Levie, Dan Mendels
Atomic Physics · Cross-listed from cond-mat.quant-gas
Vorticity-Crystalline Order Coupling in Supersolids: Excitations and Re-entrant Phases
Malte Schubert, Koushik Mukherjee, Philipp Stürmer et al. (+1 more)
Rotation is a natural tool in ultracold gases to break time-reversal
symmetry, yet its impact on the collective excitations of supersolids remains largely unexplored. We show theoretically that tuning the rotation frequency, rather than the interparticle interactions, can trigger the superfluid-to-supersolid transition in Bose-Einstein condensates (dBECs). Computing excitation spectra in the presence of
vortices and persistent currents, we uncover a
vortex-driven de-softening mechanism whereby quantized vorticity elevates the gapless Goldstone mode to a finite-energy roton, restoring superfluidity. This effect results in re-entrant supersolid phases as a function of rotation frequency, revealing a fundamental coupling between topological defects and crystalline order.
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All authors Malte Schubert, Koushik Mukherjee, Philipp Stürmer, Stephanie Reimann
Atmospheric and Oceanic Physics
Non-stationary time series attribution for heatwaves over Europe
Pascal Meurer, Sebastian Buschow, Svenja Szemkus et al. (+1 more)
The increasing occurrence of extreme weather events since the beginning of the 21st century has led to the development of new methods to attribute extreme events to anthropogenic
climate change. The way in which the extreme event is defined has a major influence on the attribution result. A frequently overlooked aspect concerns the temporal dependence of extremes. This study presents an approach for attributing complete time series during extreme events to anthropogenic forcing. The approach is based on a non-stationary Markov process using bivariate extreme value theory to model the temporal dependence of the time series. We calculate the likelihood ratio of an observational time series from ERA5 given the distributions as estimated from CMIP6 simulations with historical natural-only and natural and anthropogenic forcing scenarios. The spatial fields are condensed by the extremal pattern index (EPI) as a compact description of spatial extremes. In addition, the study examines the extent to which attribution statements about the occurrence of extreme heat events change when the effect of the mean warming is eliminated. The resulting attribution statement provides very strong evidence for the scenario with anthropogenic drivers over Europe, especially since the beginning of the 21st century. For central and southern Europe, the influence of anthropogenic greenhouse gas emissions on heatwaves could already have been proven in the 1960s using today's knowledge. There is no reliable signal apart from a general shift in the temperature distribution, neither in terms of the temporal dependence of extreme heat days nor in terms of the shape of the extreme value distribution.
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All authors Pascal Meurer, Sebastian Buschow, Svenja Szemkus, Petra Friederichs
Atmospheric and Oceanic Physics
The Tara Polaris scientific vision: Advancing our understanding of the central Arctic Ocean to better address life in the Earth System
Marcel Babin, Lee Karp-Boss, Chris Bowler et al. (+19 more)
The Arctic Ocean is currently experiencing, at the forefront of global concerns, the pressures of
climate change and global pollution. To boost our ability to understand the state of this ecosystem, its
evolution in this context and its resilience, the Tara Ocean Foundation has built the Tara Polar Station (TPS), intended to become a permanent observatory of the central Arctic Ocean. The objective of this initiative is threefold: to deepen our knowledge of the foundations of life in an ice-covered polar ocean, to better understand the dynamics of the coupled ocean-ice-atmosphere system and the role of living organisms, and to identify long-term trends in the main characteristics of the Central Arctic Ocean ecosystem resulting from global change. In this article, we describe the
vision that guided the development of the Tara Polaris scientific programme, and more specifically the first of ten transpolar drifts that will be undertaken over the next 20 years aboard the TPS (Tara Polaris I, II, III, etc.). The research activities of the Tara Polaris I expedition will be grouped under four specific but interrelated themes: biosphere-atmosphere interactions, epi- and mesopelagic life in an ice-covered ocean, life in sea ice, and pollution. In addition, a theme that cuts across all environmental compartments and disciplines, and is implemented on all Tara Polaris expeditions, is the establishment of an observatory that will monitor the main sentinels of this ecosystem. This umbrella article introduces these different themes, which are then described in more detail in four other articles in this Special Feature, in addition to an article describing the technical characteristics of the TPS.
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All authors Marcel Babin, Lee Karp-Boss, Chris Bowler, Mathieu Ardyna, J. Michel Flores, Maxime Geoffroy, Jean-Francois Ghiglione, Kathy S. Law, Marcel Nicolaus, Benjamin Rabe, Julia Schmale, Silvia G. Acinas, Jody W. Deming, Pierre E. Galand, Thomas Linkowski, Clementine Moulin, Eric Pelletier, Igor Polyakov, Jean-Christophe Raut, Soren Rysgaard, Martin Vancoppenolle, Romain Troublé
Atomic Physics
Identifying Mechanism of Energy-Resolved Attoclock
Jia'nan Wu, Shiqi Shen, Jiayin Che et al. (+4 more)
We study above-threshold ionization (ATI) of atoms in strong elliptical
laser fields numerically and analytically. Recent
benchmark experiments for H showed that the attoclock offset angle related to each ATI ring increases remarkably with energy and this characteristic phenomenon can be attributed to the
laser-induced nonadiabatic initial velocity and position of the electron at the tunnel exit [PRL127, 273201 (2021)]. However, the specific mechanism of how the nonadiabatic effects influence this angle remains unclear. Here, by using a strong-field model that analytically and quantitatively decouples complex nonadiabatic effects and Coulomb effects, the detailed mechanism can be clearly identified. We show that due to nonadiabatic effects, the angles associated with lower (higher) energy rings are dominated by the main (minor) axis of the
laser ellipse, jumping from $0^o$ to $90^o$. These field-related rigid effects are softened by Coulomb-induced exit velocity closely related to system
symmetry, resulting in a significant but smooth increase in angle with energy.
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All authors Jia'nan Wu, Shiqi Shen, Jiayin Che, Shang Wang, Weiyan Li, Guoguo Xin, Yanjun Chen
Chemical Physics
Characterization of Strongly Hyperfine-split Protons by DNP
Gian-Marco Camenisch, Nino Wili, Gunnar Jeschke et al. (+1 more)
Dynamic
nuclear polarization experiments use microwave irradiation to transfer the larger electron polarization to
nuclear spins of interest, and thus enhance the NMR transitions above thermal
equilibrium. How the polarization transfer from the electron
spin to the
nuclear spins in such experiments proceeds and which
nuclear spins close to an unpaired electron get polarized and contribute through
spin diffusion to the observable bulk
nuclear magnetization is not fully understood. We address these questions by combining reverse DNP and band-selective inversion pulses on
nuclear spins. We report the electron-detected NMR spectrum of
proton spins involved in the direct DNP process in Ox063 trityl samples with protonated and deuterated solvents and variable radical concentrations. We also determine the
spin-
diffusion barrier surrounding trityl and find that
proton spin diffusion is quenched for hyperfine coulings exceding ~250 kHz. This corresponds to a radius of the
spin diffusion barrier in the range from 5.4 to 6.8 Å. Burning a hole into the NMR spectrum of
proton spins involved in the direct DNP step reveals an electron-electron
spin diffusion process imprinted on the
proton spectrum. We explain this
diffusion process using a three-
spin system consisting of two electron
spins and one
proton and quantify the electron
spin diffusion rate constant.
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All authors Gian-Marco Camenisch, Nino Wili, Gunnar Jeschke, Matthias Ernst
Optics
Phonon Trapping Lateral Field Excited Suspended Bulk Acoustic Wave Resonators (XBARs)
Elnaz Shokati, Robert Thomas and Krishna C. Balram
Film bulk acoustic wave resonators (FBARs) underpin modern wireless communication by enabling compact, high-performance RF filters in modern smartphones. Traditionally, these FBAR devices work with quasi-plane waves of sound where the transverse extent of the acoustic field $\gg$ the acoustic wavelength ($\lambda_a$). On the other hand, strong modal confinement is needed for achieving the interaction strengths necessary for building efficient microwave to optical signal transducers (MW-OT) around an FBAR opto-mechanical cavity platform. While MW-OTs have traditionally been engineered around sub-{\mu}m scale optomechanical cavities, bulk acoustic wave approaches have inherent advantages in
phonon injection efficiency, optical power handling and manufacturability. A key limitation of the FBAR geometry is that the acoustic field is confined under the metal electrode which makes it challenging to engineer the small mode-volume, high quality factor optical cavities which are critical for achieving high transduction efficiency. Here, we consider lateral field excited suspended overtone bulk acoustic wave resonators (XBARs) as an alternative bulk wave platform for MW-OT, which overcome this limitation, and outline the requirements needed for building efficient MW-OT around this geometry. As a first step towards viability, we fabricate a small mode-volume
phonon trapping acoustic microresonator by shaping the piezoelectric layer into a spherical lens and show an improvement in modal confinement and quality factor ($\approx$ 4$\times$).
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Physics and Society
Adaptive behaviors neutralize bistable explosive transitions in higher-order contagion
Marco Mancastroppa, Márton Karsai and Alain Barrat
During contagion phenomena, individuals perceiving a risk of infection commonly adapt their behavior and reduce their exposure. The effects of such adaptive mechanisms have been studied for processes in which pairwise interactions drive contagion. However, contagion and the perception of infection risk can also involve ("higher-order") group interactions, leading potentially to new phenomenology. How adaptive behavior resulting from risk perception affects higher-order processes remains an open question. Here, we consider the impact of several risk-based adaptive behaviors on pairwise and higher-order contagion processes, using numerical simulations and an analytical mean-field approach. For pairwise contagion, adaptive mechanisms based on local (pairwise or group-based) risk perception impact only the endemic state, without affecting the epidemic phase transition. For higher-order contagion processes, instead, the adaptivity defuses the impact of non-linear group interactions: this reduces or even completely suppresses the parameter range in which bistability is possible, effectively transforming a higher-order contagion process into a pairwise one.
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