Accelerator Physics
Damping dynamics of the centroid oscillation of a relativistic laser pulse in a plasma channel
Yuhui Xia, Zhenan Wang, Ziyao Tang et al. (+10 more)
The centroid oscillation of an offset
laser pulse propagating in a preformed
plasma channel is investigated through theoretical analysis and three-dimensional particle-in-
cell simulations. For non-relativistic
laser pulses, the mode leakage of a finite channel and the temporal walk-off between the fundamental and high order modes of a finite-duration
laser induce a decay in the
laser centroid oscillation. An analytical model characterizing these decay mechanisms is derived and validated by simulations. For relativistic
laser pulses, the slice-based centroid oscillation frequency develops an axial chirp due to relativistic channel modification and
photon deceleration. This chirp leads to phase mixing across different axial slices of the pulse, resulting in a rapid damping of the overall centroid oscillation. Understanding this oscillation damping is crucial for mitigating electron beam pointing jitter and maintaining beam quality in high-energy, channel-guided
laser wakefield accelerators.
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All authors Yuhui Xia, Zhenan Wang, Ziyao Tang, Jianghao Hu, Xinyang Liu, Letian Liu, Laifu Man, Zhuo Pan, Di Wu, Jacob R. Pierce, Xueqing Yan, Chen Lin, Xinlu Xu
Fluid Dynamics
A frictional control mechanism of circumpolar transport in barotropic reentrant channel models
Takuro Matsuta, Atsushi Kubokawa, Humio Mitsudera et al. (+1 more)
Recent studies have reported that an increase in the bottom drag coefficient can enhance the volume transport of the Antarctic Circumpolar Current. Several mechanisms have been proposed to explain this frictional control, including the regulation of the geostrophic velocity by baroclinic instability and the influence of the form stress associated with standing meanders and wind-driven gyres. In this study, the role of momentum transport associated with Rossby wave radiations from disturbances is investigated as a potential frictional control mechanism. To highlight roles of the Rossby wave radiation, numerical experiments are conducted using barotropic reentrant channel models with topographic obstacles. In the high-drag regime, the circumpolar component is wind-driven, and the imbalance between the westerlies and topographic form stress sustains a net eastward transport. In contrast, in the low-drag regime, the eddy-driven westward circumpolar current is formed. In this case, the eastward flow at the center of the double gyre becomes unstable to barotropic instability. Analyses of the wave activity flux and momentum budget indicate that the Rossby wave transports westward momentum both northward and southward from the unstable region, which is responsible for the westward circumpolar current formation and maintenance. Although the direct application of the barotropic channel model to oceans requires caution, our findings imply that Rossby wave radiations from jets may play a role in the frictional control of the Antarctic Circumpolar Current.
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All authors Takuro Matsuta, Atsushi Kubokawa, Humio Mitsudera, Tomomichi Ogata
Optics
Inverse-designed release-free optomechanical crystal with high photon-phonon coupling
David Hambraeus, Paul Burger, Johan Kolvik et al. (+2 more)
Interactions between light and mechanics provide a powerful interface between optical and microwave-frequency signals, with applications spanning classical signal processing and
quantum technologies. High-performance optomechanical devices require both strong
photon-
phonon coupling and tolerance to parasitic
laser heating. Release-free optomechanical
crystals provide improved thermal anchoring compared to suspended nanobeams, but have so far exhibited weaker
vacuum optomechanical coupling rates, leaving a trade-off between coupling strength and thermal robustness. Here, we largely close this gap: we design and experimentally demonstrate a release-free silicon optomechanical
crystal with a record
vacuum optomechanical coupling rate of about $g_\text{OM} / (2 \pi) = 800$ kHz, comparable to suspended state-of-the-art devices. The resulting optomechanical scattering rate $\Gamma_\text{OM}/(2 \pi)= 1.1$ kHz is nearly twice that of previous release-free implementations. This performance is achieved by combining physics-guided human intuition with a multiphysics inverse-design
algorithm introduced here for resonant optomechanical structures. Beyond the specific device demonstrated, the inverse-design framework is applicable to co-optimizing optical and mechanical resonances and eigenmodes more broadly. These results strengthen release-free optomechanical
crystals as a platform for fast, low-noise classical and
quantum optomechanics.
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All authors David Hambraeus, Paul Burger, Johan Kolvik, Philippe Tassin, Raphaël Van Laer
Optics
One-dimensional polarization-hybrid photonic crystal molecules
Tiantong Li and Katia Gallo
Photonic molecules, i.e. artificial structures composed of coherently coupled optical cavities, are paradigmatic systems for investigating fundamental phenomena across photonics,
quantum optics and topological physics. In recent years, photonic integrated
circuits have emerged as a particularly powerful platform for their realization, exploiting also additional synthetic dimensions afforded by the degrees of freedom of light. To date, however, photonic molecule implementations have relied almost entirely on geometries defined by spatial coupling and
lattice symmetries rather than polarization. Here, we introduce a fundamentally new class of photonic molecules in which polarization is exploited as the primary dimension in the device response. By harnessing fundamental guided-mode couplings sustained by engineered Bragg gratings in photonic waveguides, we establish a new paradigm to access in 1D formats the coupled-resonator physics traditionally associated with higher-dimensional or free-space systems, demonstrating prototypical devices which can support Fano resonances or resonance-splitting for signals in the telecom band. Besides corroborating the theoretical predictions, experimental realizations in thin film lithium niobate open new prospects for the further exploration of novel reconfigurable topological, non-Hermitian and
quantum photonic
circuits, relying on the intrinsic nonlinear and electro-optic functionalities of this platform.
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Physics and Society
Thermodynamic phase transitions reveal the resilience structure of urban traffic congestion
Luis E. Olmos
Understanding how cities transition from free-flowing to congested traffic remains a central open problem in urban science. Here we show that city-scale congestion undergoes a reproducible nonlinear transition analogous to an order-disorder phase transition in statistical mechanics, in which aggregate mobility acts as a control parameter and jam extent as a collective order parameter. Crucially, this analogy is not merely formal: we derive and empirically identify an effective thermodynamic temperature with concrete physical meaning, quantifying infrastructural heterogeneity and how broadly a city explores congestion configurations as demand increases. Low-temperature cities are congestion-fragile: small mobility increases trigger sharp, system-wide jam transitions. This framework further reveals that the macroscopic fundamental diagram is an incomplete description of the traffic state: it emerges as a projection of a richer free-energy landscape governed by
entropy-capacity trade-offs. Validated across 46 cities in Latin America and the Caribbean and independently confirmed with loop-detector data from 8 cities on three continents, these results establish a physics-based foundation for comparing urban traffic resilience and anticipating congestion regime shifts under changing mobility demand.
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Fluid Dynamics
Tethering and depth of submergence affect the swimming performance of undulatory robots
Alexandros Anastasiadis, Auke J. Ijspeert and Karen Mulleners
Over the past few decades, biomimetic robotic experiments have significantly advanced our understanding of undulatory swimming. Compared to animal experiments, robotic experiments offer repeatability and controlled parameter variations, but the
robots operate under constraints that differ from those experienced by their natural counterparts. Freely swimming
robots often remain on the surface, whereas most undulatory fish, including eels, are typically fully submerged during locomotion. Studies focusing on submerged swimming commonly rely on tethered
robots to maintain depth control. This study examines the performance implications for free versus tethered swimming at the surface, and for tethered swimming at the surface to tethered fully submerged swimming, using the robotic undulatory swimmer 1-guilla. The
robot was tested in two configurations: free swimming in a pool and tethered swimming in a water channel at the surface and at varying depths down to three body heights. We varied kinematic input parameters and quantified performance in terms of swimming speed, cost of transport, and body kinematics. At the surface, tethered swimming achieves speeds comparable to free swimming but at a lower energetic cost. The reduction in cost of transport is attributed to the mechanical stabilisation imposed by tethering. Increasing submergence depth improved both the maximum speed and energy efficiency by more than 10% relative to the surface swimming performance. As the body kinematics remained unchanged when submerged, the performance deficit near the surface is attributed to increased wave drag. Overall, our findings provide explanations and insights into discrepancies in results obtained for tethered and free-swimming robotic studies, they highlight the hydrodynamic challenges of surface locomotion, and can help explain why natural undulatory swimmers predominantly favour submerged propulsion.
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Fluid Dynamics
Evolution of passive scalar mixing layers in stratified and unstratified homogeneous turbulence
Stephen M. de Bruyn Kops, Peter N. Blossey and James J. Riley
High-resolution large-eddy simulations of decaying stratified and unstratified homogeneous
turbulence are used to understand the mixing of passive scalars in stably stratified flows. Two passive scalar mixing layers, one in the vertical direction and the other in the transverse direction, are a model for a plume that is very large relative to the length scale of the velocity. In the transverse direction, the
evolution of the passive scalar is broadly similar in the stratified and unstratified cases, although it does spread slightly faster when stratified. Also, the intensity of the scalar fluctuations is higher in the stratified case, and the turbulent/non-turbulent interface is more intermittent. In the vertical direction, though, the stratified case has almost no mixing because the stratification prevents large-scale stirring. Initially, the stratified passive layer grows until its width is proportional to the vertical integral length of the horizontal velocity, which is itself constrained to maintain the vertical Froude number order one. After this early growth, there is little additional spreading of the passive scalar. Modelling of the stratified scalar flux in the transverse direction is done effectively with a one-constant model if the mean profile is known, and a two-constant model if the profile shape must be assumed. In the latter case, the model is good only if the scalar is in quasi-
equilibrium with the velocity field such that the length scale of the scalar can be scaled from the kinetic energy. In this study, the Prandtl number of the active and passive scalars is 0.7. It is anticipated that the reverse buoyancy flux resulting from higher Prandtl numbers will affect the passive scalar mixing.
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Optics · Cross-listed from quant-ph
Path integral quantization of the electromagnetic field in nonlinear dielectric materials
Arman Kashef, Oscar Perearnau Herrero, Alexander Szameit et al. (+2 more)
We construct a
quantum theory of light in nonlinear dielectric media with dispersion and absorption. We employ a mesoscopic model for the light-matter interaction that include a fourth-order nonlinearity in the material response. Quantization is performed by constructing an effective action in a path-integral formalism by integrating out matter and bath degrees of freedom. We show how a nonlinear response function associated with Kerr nonlinearity is obtained through the model and, after full field quantization, we derive the Feynman rules from this theory.
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All authors Arman Kashef, Oscar Perearnau Herrero, Alexander Szameit, Marco Ornigotti, Stefan Scheel
Optics
Drop-on-demand printed negative dielectric anisotropy liquid crystal droplets for adaptive complex beam manipulation and assessment
Jinge Guo, Xuke Qiu, Runchen Zhang et al. (+12 more)
Adaptive manipulation of vectorial optical fields are important for optical metrology, imaging, and structured light related applications, yet existing approaches often rely on bulky or sequentially operated systems. Here we demonstrate an inkjet-printed negative dielectric anisotropy nematic
liquid crystal droplet platform that unifies adaptive complex beam generation and full vectorial optical field sensing within a single printed architecture. For complex beam generation, voltage-driven director reconfiguration in the droplets produces tunable birefringence and wavelength-dependent polarization textures, including skyrmionic like optical fields. For adaptive full vectorial optical field sensing, the same droplet array enables spectral and polarization
retrieval through wavelength-dependent intensity patterns and division-of-wavefront polarimetry, while also functioning as a microlens array for Shack Hartmann wavefront sensing to reconstruct phase. These results establish negative dielectric anisotropy
liquid crystal droplets as a scalable soft-matter photonic system for adaptive beam manipulation and multidimensional optical field characterization.
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All authors Jinge Guo, Xuke Qiu, Runchen Zhang, Qihao Han, Liangyu Deng, Yishun Lu, Zimo Zhao, Mengmeng Li, Waqas Kama, Junseok Ma, Yongge Ma, Steve J Elston, Alfonso A. Castrejón-Pita, Stephen M Morris, Chao He
Applied Physics
Data-driven Initial Gap Identification of Piecewise-linear Systems using Sparse Regression and Universal Approximation Theorem
Ryosuke Kanki and Akira Saito
This paper proposes a method for identifying an initial gap in piecewise-linear systems from data. Piecewise-linear systems appear in many engineered systems such as degraded mechanical systems and infrastructures, and are known to show strong nonlinearities. To analyze the behavior of such piecewise-linear systems, it is necessary to identify the initial gap, at which the system behavior switches. The proposed method identifies the initial gap by discovering the governing equations using sparse regression and calculating the gap based on the universal approximation
theorem. A key step to achieve this is to approximate a piecewise-linear function by a finite sum of piecewise-linear functions in sparse regression. The equivalent gap is then calculated from the coefficients of the multiple piecewise-linear functions and their respective switching points in the obtained equation. The proposed method is first applied to a numerical model to confirm its applicability to piecewise-linear systems. Experimental validation of the proposed method has then been conducted with a simple mass-spring-hopping system, where the method successfully identifies the initial gap in the system with high accuracy.
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Fluid Dynamics · Cross-listed from astro-ph.SR
Unifying Transport Models of Thermohaline Convection in Stars
Valentin A. Skoutnev
Thermohaline convection is a standard chemical mixing process in
stellar interiors, yet its mixing efficiency is not fully settled. Competing theories predict turbulent
diffusion coefficients, $D_\mu$, that can differ by orders of magnitude, leading to uncertainties in
stellar models and interpretations of observations. This paper explores a potential resolution to existing discrepancies. We first complete the linear stability theory and identify two types of unstable modes: slow growing modes at large length scales and fast growing modes at small length scales. We then reevaluate $D_\mu$ considering the full spectrum of unstable modes and find that it can self-consistently interpolate between previously proposed theoretical scalings across the instability parameter space. The question of thermohaline mixing efficiency in
stars may be settled by future simulations that quantify the scale-dependent contributions of fast and slow modes to $D_\mu$ and determine how the modes dominating the transport change across parameter space.
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Atmospheric and Oceanic Physics
Towards accurate extreme event likelihoods from diffusion model climate emulators
Peter Manshausen, Noah Brenowitz, Julius Berner et al. (+2 more)
ML
climate model emulators are useful for scenario planning and adaptation, allowing for cost-efficient experimentation. Recently, the
diffusion model
Climate in a Bottle (cBottle) has been proposed for generation of atmospheric states compatible with boundary conditions of
solar position and sea surface temperatures. Crucially, cBottle can be guided to generate extreme events such as Tropical Cyclones (TCs) over locations of interest.
Diffusion models such as cBottle work by approximating the probability density of the training data. Here, we show use cases of the probability density estimates of atmospheric states obtained from this
climate emulator. Most importantly, these estimates allow us to calculate likelihoods of extreme events under guidance. When guiding the model towards states including TCs, comparing the probability density under the guided and unguided model enables us to quantify how much more likely the guidance has made the TC. We show how these odds ratios allow us to importance-sample from the TC distribution, reducing the standard error of the probability estimate compared to simple Monte Carlo sampling. Furthermore, we discuss results and limitations of the application of model probability densities to extreme event attribution-like experiments. We present these early but encouraging results hoping they will spur more research into probabilistic information that can be gained from
diffusion models of the atmosphere.
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All authors Peter Manshausen, Noah Brenowitz, Julius Berner, Karthik Kashinath, Mike Pritchard
Optics
Neural-network reconstruction of THz transmission spectra using electrically tunable AlGaN/GaN plasmonic-crystal analyzer
A. Witkowska, M. Dub, P. Sai et al. (+4 more)
We demonstrate machine learning (ML) based reconstruction of terahertz transmission spectra using an electrically tunable grating-gate AlGaN/GaN plasmonic-
crystal analyzer. The analyzer encodes the transmission spectrum into a voltage-dependent intensity, which is then inverted by an ML
algorithm. A feedforward neural network trained on a synthetic
dataset is validated experimentally on four samples in standard
Fourier Transform Infrared (FTIR) mode and in direct (fixed-mirror) acquisition mode. The network achieves a mean square error (MSE) of the reconstruction of 0.015 in FTIR mode and 0.038 in direct mode, correctly identifying six out of seven ground-truth resonances in each mode. Against a first-difference Tikhonov regularization baseline, the mean reconstruction error is reduced 3.6 times in FTIR mode and 1.55 times in direct mode, with fewer spurious peaks and lower peak-position errors. Voltage-tunable plasmonic filtering combined with neural-network inversion establishes an interferometer-free architecture for THz spectral reconstruction.
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All authors A. Witkowska, M. Dub, P. Sai, P. Tiwari, M. Sakowicz, J. A. Majewski, W. Knap
Optics
Attosecond-Stable Two-Dimensional Spectroscopy by a Sagnac-Based Modulating System and a sub-4-fs Continuum Source
Wei-Chung Feng, Bo-Han Chen, Chih-Hsuan Lu et al. (+3 more)
We present a two-dimensional electronic spectroscopy (2DES) platform driven by a novel Coherent Loop-based Integrated Modulating and Beamsplitting System (CLIMBS). Coupled with an octave-spanning multiple-plate continuum (MPC) source, CLIMBS enables broadband, phase-coherent measurements with attosecond-level time delay precision. Its Sagnac-inspired, nearly common-path geometry provides exceptional long-term phase stability without active feedback, eliminating beam walk-off and preserving beam pointing during delay scans. Delay calibration using spectrally resolved interferometric fringes yielded a wedge angle in excellent agreement with the designed geometry, confirming precise, linear coherence time control. The MPC technique generates broadband excitation pulses spanning 550–980 nm and temporally compressed to 3.7 fs. This bright, few-cycle source enables simultaneous interrogation of widely separated electronic and vibronic transitions, with high temporal and spectral resolution, allowing 2DES to capture vibronic cross peaks, energy-transfer pathways, and undistorted ground-state bleaching (GB), stimulated emission (SE), and excited-state absorption (ESA) features across a broad spectral window. System performance was benchmarked on chlorophyll-a in methanol, where the excitation bandwidth fully covers the $Q_x$ and $Q_y$ bands, ensuring distortion-free spectra. The nearly collinear configuration of CLIMBS eliminates beam walk-off during delay scanning, supports ultrabroadband few-cycle 2DES enabled by the high-brightness MPC source, and maintains attosecond-level phase stability, providing a simple and robust platform for high-fidelity multidimensional spectroscopy.
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All authors Wei-Chung Feng, Bo-Han Chen, Chih-Hsuan Lu, Howe-Siang Tan, Shang-Da Yang, Kai Chen
Physics and Society
Rumor Propagation and Supervision during Confrontation: An Importance-Driven SIRQS Network Model
Wenjie Zhang, Juan Wu, Minyu Feng et al. (+2 more)
The societal impact of rumor spreading is becoming increasingly severe; yet, current research remains relatively one-sided, typically focusing on either rumor propagation or rumor control while neglecting the confrontational and dynamically evolving relationship between them. To address this gap, we propose a novel confrontation framework for rumor modeling. We extend the classical Susceptible-Infected-Recovered-Susceptible (SIRS) model into an Ignorant-Spreader-Stifler-Vigilant-Ignorant (SIRQS) framework by introducing a vigilant state and a confrontation mechanism, thereby capturing subtle differences in individual states during rumor propagation and in their confrontational behavior toward supervisors. At the same time, supervisors patrol the network through random walks guided by node propagation importance, enabling targeted monitoring of rumor spreaders and individuals with a high risk of spreading rumors. Using a microscopic Markov chain approach, we further characterize heterogeneous node behavior and individual differences, and couple the propagation and supervision processes to model node-state transition patterns. We conduct simulations on networks with three different sizes, various
topologies, and a real-world network. The results show that the supervision subject, the preference effects associated with the number of supervisors, and the confrontation mechanism are key factors in supervision, and largely determine the effectiveness of rumor propagation control in the simulations, reflecting the substantial influence of these three mechanisms in real-world spreading scenarios. Finally, through multiple evaluation indicators, we provide references for determining the optimal number of supervisors.
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All authors Wenjie Zhang, Juan Wu, Minyu Feng, Qin Li, Matjaž Perc
Fluid Dynamics · Cross-listed from astro-ph.GA
What is the Strouhal number of turbulence driven by supernovae?
James R. Beattie, Isabelle Connor and Enrico Ramirez-Ruiz
The Strouhal number, ${\rm{St}}=t_{\rm cor}/t_{\rm out}$, measures the temporal coherence of turbulent driving relative to the outer-scale eddy turnover time. In
turbulence-box models one commonly sets ${\rm{St}}=1$, although recent work by \citet{Grete2025_density_distribution} and \citet{Scannapieco2025_density_distribution} has shown that
turbulence statistics, especially the mass-density distribution in compressively driven
turbulence, are sensitive to this choice. In this Letter, we compute ${\rm{St}}$ directly from the measured two-time correlation tensor and outer-scale eddy time in stratified multiphase ISM simulations of Milky Way-like and starburst disks. We find isotropic median values ${\rm{St}}=0.26^{+0.30}_{-0.16}$ for the Milky Way-like model and ${\rm{St}}=0.25^{+0.11}_{-0.12}$ for the starburst model. These values are consistent with the picture that
supernova remnants (SNRs) drive
turbulence locally near $R_{\rm cool}$, where the unstable contact discontinuity in the expanding SNR sets comparable forcing and eddy times, ${\rm{St}}(R_{\rm cool})\approx 1$. The reconstructed scale-dependent curves reach ${\rm{St}}=1$ at a nearly universal outer-scale fraction, $\ell_\ast/\ell_{\rm out}\approx0.12\text{--}0.13$ ($\ell_\ast\approx25\text{--}32\,\rm{pc}$), so the standard ${\rm{St}}=1$ prescription is not an outer-scale model of SN-driven ISM
turbulence, but a local-scale approximation tied to injection near the cooling radius of the SNR.
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Applied Physics
Effect of Adding Wave Diffractors Within Reverberation Chambers on the Frequency Spacing of Adjacent Resonant Modes
François Sarrazin and Guillaume Andrieu
This paper takes advantage of a recent method able to extract the characteristics of resonant modes in a metallic enclosure such as a reverberation chamber (RC). The aim here is to analyze, the effect of inserting curvilinear objects within a parallelepiped RC on the chamber performances, particularly from the point of view of the frequency spacing of adjacent resonant modes. Two configurations are compared: one is a parallelepiped RC with added curvilinear diffracting objects, and the other is the same chamber without diffractors but with added absorbers to compensate the decrease of the quality factor. The obtained results exhibit differences that fall within the measurement uncertainties.
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Plasma Physics
Assessing the role of ITER ECE oblique view in resolving non-thermal emissions
Saeid Houshmandyar and William L. Rowan
Systematic discrepancies between electron temperature (Te) measurements from radially viewing electron cyclotron emission (ECE) and Thomson scattering (TS) diagnostics have been observed in multiple tokamaks and are widely attributed to non-Maxwellian features in the electron velocity distribution function (EVDF). As the International Thermonuclear Experimental Reactor (ITER) is expected to operate at much higher core temperatures than present devices, accurate Te measurements from ECE become increasingly critical, particularly in the presence of non-Maxwellian EVDFs. This work presents ECE spectra simulations performed to assess the diagnostic capability of the ITER oblique view under ITER conditions where non-Maxwellian distributions are present. The results show that above a critical oblique angle, Doppler broadening becomes the dominant effect, masking fine-scale spectral signatures of non-thermal distortions across a wide range of conditions. Furthermore, at higher EC harmonics in either polarization, the spectra remain largely unaffected by non-thermal emissions, enabling reliable reconstruction of Te-profiles. These findings demonstrate that the ITER oblique ECE view retains sufficient sensitivity to non-thermal electrons while providing robust and accurate Te-profile measurements under reactor-relevant conditions. The results presented here also have direct implications for ITER ECE system operation and channel selection, particularly in the presence of non-Maxwellian electron populations.
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Chemical Physics
Discovering Reaction Mechanisms with Transition Path Sampling-Based Active Learning of Machine-Learned Potentials
Ashique Lal, Rik S. Breebaart, Peter G. Bolhuis et al. (+1 more)
Machine-learned interatomic potentials (MLPs) provide near density functional theory (DFT) accuracy at reduced computational cost, but their reliability depends on representative training data and often deteriorates in transition-state regions governing rare events. We introduce an active-learning framework in which Transition Path Sampling (TPS) serves as a targeted data-generation engine for constructing MLPs accurate in barrier regions. TPS generates ensembles of unbiased reactive trajectories, and a committee-based uncertainty estimate identifies configurations for selective DFT labeling and retraining. Iterating this cycle systematically refines the potential energy surface in dynamically relevant regions, without the need of prior knowledge of the mechanism. Applied to electrochemical CO$_2$ reduction to CO on copper in explicit water, the approach removes nonphysical artifacts present in early models, achieves near-DFT energy and force accuracy, and enables stable long-time sampling of reactive pathways. Extended TPS simulations reveal multiple dynamically accessible protonation mechanisms. This work establishes TPS as an efficient and principled active-learning strategy for reactive molecular simulations at electrochemical interfaces.
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All authors Ashique Lal, Rik S. Breebaart, Peter G. Bolhuis, Evert Jan Meijer
Atomic Physics
Theoretical Calculation of Electron Transfer Between Calcium Ground-State Atoms and Rydberg Atoms
Alisée Bouillon and Matthieu Génévriez
We calculated the electronic interaction associated with the exchange of an electron between an atom of calcium excited to a Rydberg state ($n\sim 10-15$) and another, neighbouring calcium atom in its ground state. In this range the Rydberg states have an energy that is comparable to the electron affinity of Ca, enabling resonant or near resonant charge transfer at large internuclear separations (200-700 $a_0$). We calculated the interaction strength while systematically and critically assessing the approximations made, and found it to be large, ranging from $10^{-5}$ $E_h$ (70 GHz) to $10^{-8}$ $E_h$. Charge transfer is thus expected to be efficient and to significantly affect the molecular dynamics at a range of internuclear distances where ultralong range Rydberg molecules also exist.
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Optics · Cross-listed from cond-mat.mtrl-sci
Deterministic positioning of circular Bragg gratings using atomic force lithography for high-performance quantum dot light sources
Sai Abhishikth Dhurjati, Moritz Langer, Yared G. Zena et al. (+6 more)
Semiconductor quantum dots (QDs) grown by molecular beam epitaxy are excellent
quantum emitters, but their random spatial distribution hinders deterministic coupling to optical microcavities. We demonstrate a room-temperature atomic force microscopy (AFM)-assisted nano-oxidation lithography technique enabling QD positioning with a radial displacement of $51(28)$ nm. Free-standing asymmetric circular Bragg gratings incorporating AFM-positioned GaAs QDs exhibit a $245$-fold photoluminescence enhancement and fine-structure splitting (FSS) comparable to bulk QDs. Polarization-resolved spectroscopy and finite-difference time-domain simulations show robust emission for displacements up to $50$ nm (Stokes parameter $\lvert S \rvert < 0.05$ ). The devices display stable FSS and polarization imbalance below $5 \, \%$ , confirming precise, reproducible
alignment and potential for high fidelity devices. This scalable approach enables deterministic integration of high-performance QDs with photonic cavities, advancing practical
quantum light sources for
quantum information technologies.
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All authors Sai Abhishikth Dhurjati, Moritz Langer, Yared G. Zena, Ahmad Rahimi, Liesa Raith, Martin Bauer, Frank H. P. Fitzek, Riccardo Bassoli, Caspar Hopfmann
Fluid Dynamics
Turbophoresis of inertial particles in inhomogeneous turbulence generated by oscillating grids
E. Elmakies, O. Shildkrot, N. Kleeorin et al. (+2 more)
Turbophoresis in inhomogeneous turbulent flows leads to the formation of large-scale nonuniform particle number density distributions of inertial particles. This effect is associated with an effective drift velocity directed toward regions of lower
turbulence intensity. It depends on the Stokes and Reynolds numbers, as well as on the gradient of the
turbulence intensity. In the present study, turbophoretic transport is experimentally investigated in air turbulent flows generated by one-grid and two-grid oscillating systems. The flow velocity field is measured using Particle Image Velocimetry, and the particle spatial distribution is obtained by applying image processing techniques. To isolate the effect of particle accumulation due to turbophoresis from that associated with mean fluid flow, the measured particle number density of inertial particles is normalized by the corresponding distribution obtained for noninertial tracer particles under identical flow conditions. The measurements show preferential accumulation of inertial particles in regions of minimum mean-square turbulent velocity, consistent with the expected behavior of turbophoretic transport.
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All authors E. Elmakies, O. Shildkrot, N. Kleeorin, A. Levy, I. Rogachevskii
Fluid Dynamics
Pressure-equilibrium-preserving and fully conservative discretization of compressible flow equations for real and thermally perfect gases
Gennaro Coppola, Alessandro Aiello and Carlo De Michele
Numerical simulations of compressible real-fluid flows are notoriously plagued by spurious pressure oscillations arising in regions of abrupt flow variations. As a possible remedy, several numerical formulations enforce the pressure
equilibrium condition for the compressible Euler equations, typically at the cost of spoiling the correct conservation of total energy or by overspecifying the thermodynamical variables. This study proposes for the first time a numerical discretization procedure which is able to discretely preserve the full conservation of the linear invariants (mass, momentum and total energy) and to exactly enforce the pressure
equilibrium condition. The method also preserves the conservation of kinetic energy by convection, and is based on the specification of nonlinear numerical fluxes for mass and internal energy which depend on the details of the equation of state. Both thermally perfect and real gases with an arbitrary equation of state are considered, and a simplified approximate pressure
equilibrium preserving formulation with excellent performances is also proposed. The effectiveness of the novel formulations is assessed through a series of numerical simulations in supercritical and transcritical conditions with some of the most popular cubic equations of state.
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Applied Physics · Cross-listed from cond-mat.soft
Adhesion-controlled sliding and the Stribeck curve in hydrophobic soft contacts
Ruibin Xu, Charlotte Spies, Michele Scaraggi et al. (+1 more)
We present an experimental and theoretical study of dry and glycerol-lubricated sliding for polymethyl methacrylate (PMMA) cylinders with different surface roughness sliding on polydimethylsiloxane (PDMS) rubber. This system represents a hydrophobic soft contact, where adhesion may persist even in the presence of the lubricant and thereby modify both the real contact area and the sliding response. Dry-friction measurements, combined with contact-area calculations that include adhesion, provide a baseline for the lubricated study. For the two sandblasted surfaces, the measured Stribeck curves are described reasonably well by a mean-field mixed-lubrication theory with a fitted velocity-independent effective interfacial shear stress. In contrast, the smooth surface exhibits qualitatively different behavior. We attribute this to an adhesion-controlled sliding mode involving macroscopic Schallamach-wave-like instabilities at low sliding speeds, which are progressively suppressed as the sliding speed increases and forced wetting reduces direct solid-solid contact. The results show that, for soft hydrophobic contacts, the Stribeck curve cannot always be understood from classical fluid flow and load sharing alone. For sufficiently smooth and adhesive surfaces, adhesion changes not only the real contact area but also the sliding mode itself.
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All authors Ruibin Xu, Charlotte Spies, Michele Scaraggi, B.N.J. Persson
Plasma Physics
A programmable stellarator-tokamak hybrid for million-scale magnetic-configuration discovery
Guodong Yu, Xianyi Nie, Gwanggeun Seo et al. (+10 more)
Tokamaks and stellarators are the leading
magnetic-confinement concepts for fusion, but they rely on complementary design principles. Tokamaks use simple axisymmetric coils and
plasma current, whereas stellarators use externally generated three-dimensional fields for steady-state operation. Here, we propose a programmable stellarator–tokamak hybrid that uses a fixed set of simple planar coils to access a broad
magnetic-configuration space. The device adds 288 dipole-field coils to a tokamak-like coil set, with only six independent coil geometries required by
symmetry. By programming coil currents, the same hardware generates more than 1.66 million optimized stellarator configurations spanning quasi-axisymmetry, quasi-helical
symmetry, and quasi-isodynamicity, as well as tokamak-relevant three-dimensional perturbations. Representative configurations exhibit nested
magnetic surfaces, low neoclassical transport, and favorable energetic-particle confinement. This approach enables rapid
magnetic-configuration discovery without hardware redesign.
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All authors Guodong Yu, Xianyi Nie, Gwanggeun Seo, Daxing Huang, Hengqian Liu, Junhao Liu, Jaebeom Cho, Hyun-Su Kim, Jinlin Xie, Ge Zhuang, Fazhu Ding, Jong-Kyu Park, Caoxiang Zhu
Physics and Society
Long-Range Correlation in Code Commit Dynamics as a Novel Indicator of Software Product Stability: A Detrended Fluctuation Analysis Study
Goran Mitevski
This work proposes the fractal scaling exponent alpha, estimated via Detrended Fluctuation Analysis (DFA) on the unaggregated time series of lines of code added per commit event in a software repository, as a novel process-level indicator of software product stability. The proposal rests on the hypothesis that stable software products arise from development processes characterised by long-range temporal correlations in commit behaviour: each code addition is shaped not only by the immediately preceding commits but by patterns extending weeks or months into the past and anticipating work to be done in the future. This hypothesis is tested on two non-overlapping 712-day time series of lines of code added per commit event, drawn from a closed-source software organisation and labeled as stable and unstable by the lead engineer on the basis of crash-analytics data. Applied to these series, DFA yields alpha = 0.70 (n_min = 16) for the stable period and alpha = 0.57 for the unstable period, with all estimates substantially above the shuffled-surrogate baseline (alpha ~= 0.50 +/- 0.01). Results are robust to three parameterisations (n_min in {4, 16, 48}) and validated against 1,000 surrogate time series per condition. Remarkably, the unstable period generated 3.2 times more commit events than the stable period, yet exhibited lower long-range
memory, demonstrating that commit volume alone does not predict stability, and that the temporal organisation of development activity is the key variable. This result can be situated in the broader literature on fractality in human creative production, discuss methodological limitations, and outline a research programme for deploying alpha as a continuous code-health indicator in version-control pipelines.
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Optics
Pseudovorticity of 2+1D optical solitons
Ludovica Falsi, Giuseppe Agostino, Alberto Villois et al. (+6 more)
In the hydrodynamic representation of a
quantum fluid or optical field, vorticity vanishes wherever the phase is well defined, and is instead localized at phase singularities, or
quantum vortices. Pseudovorticity, by contrast, characterizes local rotational structures, even in regions without singularities or net orbital angular momentum. We study both experimentally and numerically pseudovorticity in photorefractive
solitons and show that a detailed phase and amplitude analysis unveils a complex rotational flow dynamic: bright 2+1D
solitons are found to carry a pseudovorticity dipole, while quadrupoles emerge in
soliton fusion. The phenomenon, also explained using geometrical considerations, suggests a general picture according to which stable high-dimensional
solitons naturally carry a hierarchy of pseudovorticity multipoles, encoded in the local perturbed phase and amplitude.
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All authors Ludovica Falsi, Giuseppe Agostino, Alberto Villois, Francesco Coppini, Paolo M. Santini, Miguel Onorato, Aharon J. Agranat, Stefano Trillo, Eugenio DelRe
Computational Physics · Cross-listed from cond-mat.mtrl-sci
Gauge-Field-Mediated Symmetry Breaking of Matters Under Electromagnetic Fields and Its Impact on Spin Dynamics
Uiseok Jeong, Esmaeil Taghizadeh Sisakht, Angel Rubio et al. (+3 more)
When a condensed-matter system is subjected to external electromagnetic fields, the gauge-invariant formulation of physical operators must explicitly incorporate the gauge-field contribution. However, in the context of
spin-orbit coupling (SOC), this gauge-field term is often regarded as negligible or merely additive compared to the canonical SOC, which is typically localized near atomic cores. Here, we demonstrate that the
symmetry breaking and consequent
spin dynamics are governed by the gauge-field term, without which the
spins remain
symmetry-constrained. We perform real-time time-dependent density functional theory calculations to investigate
spin-orbit dynamics, focusing on representative cases with mirror, glide, and screw-rotational
symmetry. We demonstrate that when the gauge-field term in the time-dependent
Hamiltonian perturbs the
symmetry of the canonical term, a dynamical
spin state gradually develops during the time
evolution, beyond the
symmetry-frozen states. We suggest that, for nonequilibrium
spin-orbit dynamics, the gauge-invariant formulation of SOC is not only formally required but also quantitatively essential, even for a weak external field.
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All authors Uiseok Jeong, Esmaeil Taghizadeh Sisakht, Angel Rubio, Carsten A. Ullrich, Kyoung-Whan Kim, Noejung Park
Fluid Dynamics
Interface pinch-off in the presence of a soluble surfactant
M. Rubio, S. Rodríguez-Aparicio, J. M. Montanero et al. (+1 more)
We study numerically and experimentally the breakup of a pendant droplet loaded with a soluble surfactant. We consider the limit in which surfactant sorption is limited only by
diffusion. Surfactant transfer toward the interface is enhanced by convection. As a consequence,
diffusion does not constitute a significant barrier over most of the breakup, and surfactant sorption maintains the surface tension practically constant across the interface.
Diffusion hinders the surfactant sorption only very close to the interface pinch-off. The droplet shape in the
diffusion-limited model deviates significantly from that in the insoluble case over most of the breakup. In the insoluble case, the droplet shape is affected by surfactant depletion, which leads to a local increase in surface tension and Marangoni stress. The dynamics of a millimeter-sized droplet loaded with Surfynol 465 agree remarkably well with predictions from the
diffusion-limited model, without any parameter fitting, down to pinching times of the order of $10-20$ $\mu$s. Sodium dodecyl sulfate (SDS) produces essentially the same effects as those for Surfynol 465. Therefore, both Surfynol 465 and SDS maintain a practically constant surface tension throughout most of the droplet breakup. Slow-kinetics surfactants, such as Triton X-100, differ significantly from Surfynol 465 and SDS. The most evident effect of the surfactant adsorption energy barrier is the shortening of the filament that bridges the upper meniscus and the detached lower drop. Comparing the filament length to that of a clean interface with the same surface tension allows one to evaluate the rate of surfactant adsorption.
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All authors M. Rubio, S. Rodríguez-Aparicio, J. M. Montanero, M. A. Herrada
Fluid Dynamics
Harmonic Balance Unified Gas-Kinetic Scheme for Multiscale Periodic Flows
Yuze Zhu, Hangkong Wu, Yufeng Wei et al. (+1 more)
A time-domain harmonic balance unified gas-kinetic scheme (HB-UGKS) is developed for simulating periodic non-
equilibrium flows across all Knudsen regimes. By applying a time-spectral operator, the unsteady periodic problem is reformulated into a block-coupled, quasi-steady system. To preserve the intrinsic multiscale transportcollision flux coupling of the UGKS, a source-separated formulation incorporates the harmonic balance coupling strictly at the
cell-residual level. This allows the periodic limit cycle to be resolved directly via pseudo-time marching with local time-stepping, advancing all temporal collocation points simultaneously and completely bypassing physical startup transients. The framework is validated against three complementary
benchmarks: two small-amplitude sheardriven oscillatory flows and a thermally driven cavity under finite-amplitude excitation. The scheme accurately captures intricate non-
equilibrium kinetic phenomena, including aspectratio anti-resonance scaling, dynamic shear traction, nonlinear waveform distortions, and acoustic streaming. Across all cases, the HB-UGKS preserves the fidelity of standard timeaccurate simulations, while achieving order-of-magnitude speedups in high-frequency regimes where long transient timescales dominate.
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All authors Yuze Zhu, Hangkong Wu, Yufeng Wei, Kun Xu