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Physics Physics

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Wednesday 29 October

Accelerator Physics

Fast chaos indicator from auto-differentiation for dynamic aperture optimization

Ji Qiang, Jinyu Wan, Allen Qiang et al. (+1 more)

Automatic differentiation provides an efficient means of computing derivatives of complex functions with machine precision, thereby enabling differentiable simulation. In this work, we propose the use of the norm of the tangent map, obtained from differentiable tracking of particle trajectories, as a computationally efficient indicator of chaotic behavior in phase space. In many cases, a one-turn or few-turn tangent map is sufficient for this purpose, significantly reducing the computational cost associated with dynamic aperture optimization. As an illustrative application, the proposed indicator is employed in the dynamic aperture optimization of an ALS-U lattice design.
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All authors Ji Qiang, Jinyu Wan, Allen Qiang, Yue Hao

Data Analysis and Statistics

Response to Comment from Robert Cousins on Confidence intervals for the Poisson distribution

Frank C. Porter

Robert Cousins has posted a comment on my manuscript on “Confidence intervals for the Poisson distribution”. His key point is that one should not include in the likelihood non-physical parameter values, even for frequency statistics. This is my response, in which I contend that it can be useful to do so when discussing such descriptive statistics.
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Physics and Society

Bidirectional yet asymmetric causality between urban systems and traffic dynamics in 30 cities worldwide

Yatao Zhang, Ye Hong, Song Gao et al. (+1 more)

Understanding how urban systems and traffic dynamics co-evolve is crucial for advancing sustainable and resilient cities. However, their bidirectional causal relationships remain underexplored due to challenges of simultaneously inferring spatial heterogeneity, temporal variation, and feedback mechanisms. Here we present a spatio-temporal causality framework that bridges correlation and causation by integrating spatio-temporal weighted regression with spatio-temporal convergent cross-mapping. Characterizing cities through urban structure, form, and function, the framework uncovers bidirectional causal patterns between urban systems and traffic dynamics across 30 cities on six continents. Our findings reveal asymmetric bidirectional causality, with urban systems exerting stronger influences on traffic dynamics than the reverse in most cities. Urban form and function shape mobility more profoundly than structure, even though structure often exhibits higher correlations. This does not preclude the reversed causal direction, whereby long-established mobility patterns can also reshape the built environment over time. Finally, we identify three causal archetypes: tightly coupled, pattern-heterogeneous, and workday-attenuated, which support city-to-city learning and inform context-sensitive strategies in sustainable urban and transport planning.
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All authors Yatao Zhang, Ye Hong, Song Gao, Martin Raubal

Fluid Dynamics

Conditional neural field for spatial dimension reduction of turbulence data: a comparison study

Junyi Guo, Pan Du, Xiantao Fan et al. (+2 more)

We investigate conditional neural fields (CNFs), mesh-agnostic, coordinate-based decoders conditioned on a low-dimensional latent, for spatial dimensionality reduction of turbulent flows. CNFs are benchmarked against Proper Orthogonal Decomposition and a convolutional autoencoder within a unified encoding-decoding framework and a common evaluation protocol that explicitly separates in-range (interpolative) from out-of-range (strict extrapolative) testing beyond the training horizon, with identical preprocessing, metrics, and fixed splits across all baselines. We examine three conditioning mechanisms: (i) activation-only modulation (often termed FiLM), (ii) low-rank weight and bias modulation (termed FP), and (iii) last-layer inner-product coupling, and introduce a novel domain-decomposed CNF that localizes complexities. Across representative turbulence datasets (WMLES channel inflow, DNS channel inflow, and wall pressure fluctuations over turbulent boundary layers), CNF-FP achieves the lowest training and in-range testing errors, while CNF-FiLM generalizes best for out-of-range scenarios once moderate latent capacity is available. Domain decomposition significantly improves out-of-range accuracy, especially for the more demanding datasets. The study provides a rigorous, physics-aware basis for selecting conditioning, capacity, and domain decomposition when using CNFs for turbulence compression and reconstruction.
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All authors Junyi Guo, Pan Du, Xiantao Fan, Yahui Li, Jian-Xun Wang

Chemical Physics · Cross-listed from quant-ph

The Phase-Coupled Caldeira-Leggett Model: Non-Markovian Open Quantum Dynamics beyond Linear Dissipation

Ao-Xiang Chang, Yu Su, Zi-Fan Zhu et al. (+3 more)

We introduce the Phase-Coupled Caldeira-Leggett (PCL) model of quantum dissipation and develop an exact framework for its dynamics. Unlike the conventional Caldeira-Leggett model with linear system-bath coupling $H_{\mathrm{SB}}\propto\hat F$, the PCL model features an exponential interaction $H_{\mathrm{SB}}\propto e^{i\lambda \hat F}$, where $\hat F$ denotes the collective bath coordinate. This model unifies concepts from quantum Brownian motion and polaron physics, providing a general platform to study phase-mediated dissipation and decoherence beyond the linear-response regime. Despite its nonlinear system-bath coupling, the Gaussian nature of the environment allows a nonperturbative and non-Markovian treatment of PCL model within the algebra of dissipative quasiparticles. We obtain an exact closed-form equation of motion for the reduced density operator, and numerical simulations reveal distinctive dynamical behaviors that deviate markedly from those predicted by the conventional Caldeira-Leggett model.
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All authors Ao-Xiang Chang, Yu Su, Zi-Fan Zhu, Yao Wang, Rui-Xue Xu, YiJing Yan

Instrumentation and Detectors

Automated Supervised Identification of Thunderstorm Ground Enhancements (TGEs)

Davit Aslanyan

Thunderstorm Ground Enhancements (TGEs) are bursts of high-energy particle fluxes detected at Earth's surface, linked to the Relativistic Runaway Electron Avalanche (RREA) mechanism within thunderclouds. Accurate detection of TGEs is vital for advancing atmospheric physics and radiation safety, but event selection methods heavily rely on expert-defined thresholds. In this study, we use an automated supervised classification approach on a newly curated dataset of 2024 events from the Aragats Space Environment Center (ASEC). By combining a Tabular Prior-data Fitted Network (TabPFN) with SHAP-based interpretability, we attain 94.79% classification accuracy with 96% precision for TGEs. The analysis reveals data-driven thresholds for particle flux increases and environmental parameters that closely match the empirically established criteria used over the last 15 years. Our results demonstrate that modest but concurrent increases across multiple particle detectors, along with strong near-surface electric fields, are reliable indicators of TGEs. The framework we propose offers a scalable method for automated, interpretable TGE detection, with potential uses in real-time radiation hazard monitoring and multi-site atmospheric research.
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Fluid Dynamics

Shear-layer effects on the dynamics of unsteady premixed laminar counterflow flames

Jose G Rivera Lizarralde, Aditya Potnis and Abhishek Saha

The influence of flow non-uniformity and unsteadiness on premixed flames is of considerable interest due to its direct relevance to practical combustion systems. The steady counterflow flame has long served as a canonical configuration for investigating flame dynamics under controlled, spatially non-uniform conditions. A commonly studied variation, referred to as the unsteady counterflow, introduces a controlled temporal perturbation to the otherwise steady flow from the nozzles, thereby enabling the systematic examination of the coupled effects of unsteadiness and non-uniformity. Prior investigations have focused on flame dynamics along the line of symmetry, where the reduced dimensionality of the problem facilitates analysis. In the present study, we extend this perspective by experimentally examining flame behavior at off-center locations, where multi-dimensional effects of non-uniformity and unsteadiness are more pronounced. Results reveal markedly different dynamics away from the centerline, characterized by a dominant contribution from higher harmonic responses. Further analysis of the associated vortex dynamics in the shear layer demonstrates that the intensity of these vortical structures directly governs the strength of the observed higher harmonics, and thereby the altered flame behavior.
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Optics · Cross-listed from cond-mat.mes-hall

Single-shot laser-pulse-induced magnetization reversal in CoFeB/MgO-based magnetic tunnel junctions

Junta Igarashi, Sébastien Geiskopf, Takanobu Shinoda et al. (+9 more)

We demonstrate single-shot laser-pulse-induced magnetization reversal in rare-earth-free CoFeB/MgO magnetic tunnel junctions (MTJs), a material system widely adopted in spin-transfer torque magnetic random-access memory (STT-MRAM). By tuning the Ru capping layer thickness, we modify the laser energy absorption profile and observe magnetization reversal from the parallel (P) to antiparallel (AP) state, with switching observed for $t_\text{Ru} \geq 2.0\,$ nm. Furthermore, we detect magnetization reversal in a micro-scale MTJ device via the tunnel magnetoresistance (TMR) effect. Our findings suggest that ultrafast spin transport, dipolar interactions, or a combination of both may contribute to the switching process, although the precise mechanism remains to be clarified. This work represents a significant step toward integrating ultrafast optical control with MTJ technology.
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All authors Junta Igarashi, Sébastien Geiskopf, Takanobu Shinoda, Butsurin Jinnai, Yann Le Guen, Julius Hohlfeld, Shunsuke Fukami, Hideo Ohno, Jon Gorchon, Stéphane Mangin, Michel Hehn, Grégory Malinowski

Geophysics

Discovery of Late Triassic volcanic ash layers in the deep-water zone of the Nanpanjiang Basin (South China) and the possibility of Carnian Pluvial Episode correlation

Liangjun Wu

This study presents new geochronological constraints for the Niluo Member within the slope-basin facies of the Late Triassic Nanpanjiang Basin, eastern Tethys. The basin underwent a significant marine-to-continental transition during this period. Previous biostratigraphic studies on platform facies were hindered by inconclusive conodont zonation, leaving the chronology of slope-basin deposits poorly resolved. To address this, we identified volcanic ash layers within the Niluo Member in the Wangmo area. Zircon U-Pb dating of these ashes yielded weighted mean ages of 229.9 Ma and 229.0 Ma, establishing a Carnian depositional age. This result is significantly younger than previous estimates and coincides with the CPE. The Niluo Member is interpreted as a period of slow, oxygen-deficient sedimentation, contrasting with the rapid turbidite deposition of the enclosing formations. This depositional hiatus likely facilitated the preservation of the datable ash layers. The Carnian age and unique lithology suggest the Niluo Member may record the CPE in the slope environment, potentially linked to increased terrigenous input that suppressed carbonate production. Concurrent conodont sampling was unsuccessful, validating the documented difficulties in applying biostratigraphy in these deep-water settings. These new radiometric ages provide a crucial anchor point for Late Triassic stratigraphy in South China, demonstrate the potential for preserving CPE records in slope facies, and offer a new basis for regional and global correlation.
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Optics

Commissioning of the TeraNet Optical Ground Station Network

Sascha Schediwy, Aliesha Aden, Benjamin Dix-Matthews et al. (+13 more)

TeraNet is a new three-node OGS network that has been established in Western Australia. The network is built to support a broad range of space missions operating between LEO and the Moon, using both conventional and advanced optical technologies developed at UWA. It is designed to be spacecraft and mission agnostic, able to be adapted for compatibility with spacecraft using a variety of communication and timing protocols. The TN network comprises three ground station nodes, each of which are equipped to support direct-detection, bidirectional optical communication with LEO spacecraft. In addition, each node is focused on the development of a unique advanced optics technology. Specifically: -TN-1 is a 70cm aperture OGS located on the campus of UWA. It uses ultra-sensitive optical detectors and specialised modulation formats to maximise the information recovered from spacecraft at lunar distances. -TN-2 is a 70cm aperture OGS at the Yarragadee Geodetic Observatory 300km North of Perth. It is equipped with a commercial-grade adaptive optics system for efficient single-mode fibre coupling. This enables high-speed coherent communications and ultra-precise coherent timing and positioning between ground and space. -TN-3 is a 43cm aperture mobile OGS node built onto the back of a utility vehicle for rapid, tactical deployment anywhere in the world. It is setup for quantum communication and quantum-assured time transfer and can establish satellite communication links within ten minutes of arriving on site, day or night. In this paper, we report on the results and outcomes of the TN commissioning campaign, including optical communication links with on-orbit spacecraft, the performance of the TN-2 adaptive optics system, the rapid deployment capability of TN-3, remote network operations of all three nodes, and interoperability tests with other OGSs across Australia and New Zealand.
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All authors Sascha Schediwy, Aliesha Aden, Benjamin Dix-Matthews, Alex Frost, Amrita Gill, David Gozzard, Mike Kriele, Andrew Lance, Nicolas Maron, Ayden McCann, Shawn McSorley, Lilani Toms-Hardman, Shane Walsh, Larissa Wiese, Graeme Wren, Randall Carman

Computational Physics

Six-Dimensional Movable Antenna Enabled Wideband THz Communications

Wencai Yan, Wanming Hao, Yajun Fan et al. (+3 more)

In this paper, we investigate a six-dimensional movable antenna (6DMA)-enabled wideband terahertz (THz) communication system with sub-connected hybrid beamforming architecture at the base station (BS). In particular, the three-dimensional (3D) position and 3D rotation of each 6DMA surface can be flexibly reconfigured to mitigate the beam squint effects instead of introducing costly true-time-delay devices. We first analyze the normalized array gain in the 6DMA-enabled wideband THz systems based on the beam squint effects. Then, we formulate a sum-rate maximization problem via jointly optimizing 3D positions, 3D rotations, and hybrid analog/digital beamforming. To solve the non-convex problem, an alternating optimization algorithm is developed that decomposes the original problem into three subproblems, which are solved alternately. Specifically, given the positions and rotations of 6DMA surfaces, we first reformulate the objective function and design a semidefinite relaxation-based alternating minimization scheme to obtain the hybrid analog/digital beamforming. Then, the positions and rotations of the 6DMA surfaces are further optimized through a feasible gradient descent procedure. The final solutions are obtained by repeating the above procedure until convergence. Numerical results demonstrate the superior performance of the proposed scheme compared with conventional fixed-position antenna architectures.
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All authors Wencai Yan, Wanming Hao, Yajun Fan, Yabo Guo, Qingqing Wu, Xingwang Li

Fluid Dynamics

Exploratory Study of Chaotic Behavior in Walking Droplets

Emily Dunn, Bavand Keshavarz and Earl Dowell

The interaction of 'walking droplets' and capillary waves in a weakly subcritical Faraday wave experiment has been studied as a hydrodynamic analog to Bohmian quantum mechanics (see "Hydrodynamic Quantum Analogs", J. Bush and A. Oza, Rep. Prog. Physics (2021)). We report here experimental results of walking droplets interacting with supercritical Faraday waves with dimensionless acceleration of approximately 7.7, where the onset of Faraday instability occurs at dimensionless acceleration 6.3, in flat bath topography. Our working fluid is silicone oil with a kinematic viscosity of 20 cst that is placed as a 4.2 mm horizontal liquid layer in an intermediate-aspect-ratio circular bath with a radius to Faraday wavelength ratio of 5.8. We also use different 3D-printed subsurfaces that act as slit structures with local oil depth of 0.7 mm. We confirm expected behavior for walking droplets in the supercritical Faraday regime, such as erratic trajectories, droplets clustering together due to capillary effects, and spontaneous drop creation. We note a special case of walking-droplet behavior when the bath only partially displays Faraday waves. We discuss the influence of the lateral boundaries and slits on droplet trajectory in this chaotic regime and compare the measured trajectories found here to those single and double slit experiments previously studied in the subcritical Faraday regime.
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Plasma Physics

Magnetic Field Line Chaos, Cantori, and Turnstiles in Toroidal Plasmas

Allen H Boozer

The mathematical concepts of chaos, cartori, and turnstiles underlie a number of areas of tokamak and stellarator physics. Nevertheless, these concepts have seldom explicitly appeared in publications on fusion plasmas. The absence of physical intuition about these concepts is responsible for misunderstandings and slows developments in a number of areas: magnetic reconnection, the most important electromagnetic correction to what are called electrostatic microinstabilities, non-resonant divertors in stellarators, disruptions and damage from runaway electrons in tokamaks. Physicists become interested in new mathematical concepts when they give insights into and solutions to practical problems. The importance of this review is not only in explaining chaos, cartori, and turnstiles as mathematical concepts but also in illustrating their significance through applications.
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Atmospheric and Oceanic Physics

Evaluating Extreme Precipitation Forecasts: A Threshold-Weighted, Spatial Verification Approach for Comparing an AI Weather Prediction Model Against a High-Resolution NWP Model

Nicholas Loveday and Tracy Hertneky

Recent advances in AI-based weather prediction have led to the development of artificial intelligence weather prediction (AIWP) models with competitive forecast skill compared to traditional NWP models, but with substantially reduced computational cost. There is a strong need for appropriate methods to evaluate their ability to predict extreme weather events, particularly when spatial coherence is important, and grid resolutions differ between models. We introduce a verification framework that combines spatial verification methods and weighted proper scoring rules. Specifically, the framework extends the High-Resolution Assessment (HiRA) approach with threshold-weighted scoring rules. It enables user-oriented evaluation consistent with how forecasts may be interpreted by operational meteorologists or used in simple post-processing systems. The method supports targeted evaluation of extreme events by allowing flexible weighting of the relative importance of different decision thresholds. We demonstrate this framework by evaluating 32 months of precipitation forecasts from an AIWP model and a high-resolution NWP model. Our results show that model rankings are sensitive to the choice of neighbourhood size. Increasing the neighbourhood size has a greater impact on scores evaluating extreme-event performance for the high-resolution NWP model than for the AIWP model. At near equivalent neighbourhood sizes, the empirical CDF of the high-resolution NWP model only outperformed the empirical CDF of the AIWP model in predicting extreme precipitation events at short lead times. We also demonstrate how this approach can be extended to evaluate discrimination ability in predicting heavy precipitation. We find that the high-resolution NWP model had superior discrimination ability at short lead times.
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Tuesday 28 October

Atomic Physics

Real time synchronisation of a free-running atomic clock time base with UTC using GNSS signals for application in experimental physics

Claire Dalmazzone, Mathieu Guigue, Boris Popov et al. (+2 more)

We present the results obtained by applying, in real-time, a correction method to precisely synchronize a time base generated from a free-running atomic clock with the Coordinated Universal Time (UTC). The method uses the Global Navigation Satellite System (GNSS) signals to have regular time comparisons between the atomic clock generated time base and the GPS Time, perform linear fits of the measurements and extrapolate a correction to apply to the free-running signal. In this work, we apply for the first time this method in real-time. Two atomic clocks were tested, a low-cost Rubidium clock and a more expensive magnetic Caesium clock. We demonstrate that we can obtain a residual difference between the clock time base and the French official realization of UTC (UTC(OP)) in the range of $\pm 15$ ns with no apparent residual drift.
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All authors Claire Dalmazzone, Mathieu Guigue, Boris Popov, Stefano Russo, Vincent Voisin

Atmospheric and Oceanic Physics · Cross-listed from math-ph

On a wave kinetic equation with resonance broadening in oceanography and atmospheric sciences

Young Ho Kim, Yuri V. Lvov, Leslie M. Smith et al. (+1 more)

In this work, we study a three-wave kinetic equation with resonance broadening arising from the theory of stratified ocean flows. Unlike Gamba-Smith-Tran(On the wave turbulence theory for stratified flows in the ocean, Math. Models Methods Appl. Sci. 30 (2020), no.1, 105–137), we employ a different formulation of the resonance broadening, which makes the present model more suitable for ocean applications. We establish the global existence and uniqueness of strong solutions to the new resonance broadening kinetic equation.
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All authors Young Ho Kim, Yuri V. Lvov, Leslie M. Smith, Minh-Binh Tran

Atmospheric and Oceanic Physics

Reflecting on a Decade of Formalized Tornado Emergencies

Edward C. Wolff, James S. Goodnight, Leanne Blind-Doskocil et al. (+3 more)

In 1999 the NWS began using the phrase "tornado emergency" to denote tornado warnings for storms with the potential to cause rare, catastrophic damage. After years of informal usage, tornado emergencies were formally introduced to 46 weather forecasting offices in 2014 as part of the impact-based warning (IBW) program, with a nationwide rollout occurring over the following years. In concert with the new tiered warning approach, the Warning Decision Training Division (WDTD) also introduced suggested criteria for when forecasters should consider upgrading a tornado warning to a tornado emergency, which includes thresholds of rotational velocity (VROT) and significant tornado parameter (STP). Although significant research has studied both tornado forecasting and tornado warning dissemination in the decade since, relatively little work has examined the effectiveness of the tornado emergency specifically. Our analysis of all 89 IBW tornado emergencies issued from 2014-2023 found that forecasters do not appear to follow the suggested criteria for issuance in the majority of cases, with only a handful of tornado emergencies meeting both the VROT and STP thresholds. Regardless, 70% of tornado emergencies were issued for EF-3+ tornadoes, and tornado emergencies covered 55% of all EF-4 tornadoes as well as 41% of all tornadoes resulting in 3 or more fatalities. Based on these results, we propose several updates to the current NWS training materials for impact-based tornado warnings.
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All authors Edward C. Wolff, James S. Goodnight, Leanne Blind-Doskocil, Elijah M. Conklin, Evan T. Gustafson, Joseph E. Trujillo-Falcón

Optics

Scalar computational primitives with perturbative phase interferometry

Christopher R. Schwarze, Anthony D. Manni, David S. Simon et al. (+1 more)

We describe how weak phase modulations applied to classical coherent light in specially modified linear interferometers can be used to perform primitive computational tasks. Instead of encoding operations within a fixed unitary state, the operations are enacted by moving from one state to another. This harnesses the particular phase parametrization of an interferometer, allowing entirely linear optics to produce nonlinear operations such as division and powers. This is due to the nonlinear structure of the underlying phase parametrizations. The realized operations are approximate but can be made more accurate by decreasing the size of the input perturbations. For each operation, the inputs and outputs are changes in phase relative to a fixed bias point. The output phase is ultimately read out as a change in optical power.
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All authors Christopher R. Schwarze, Anthony D. Manni, David S. Simon, Alexander V. Sergienko

Optics

Ultra-stable lasers using hollow-core fibre

Zitong Feng, Giuseppe Marra, Irene Barbeito Edreira et al. (+3 more)

Ultra-stable lasers are fundamental to a growing range of applications, including optical frequency metrology, fundamental physics and quantum sensing. Their outstanding performance is achieved by stabilizing their frequency to Ultra-Low Expansion (ULE) optical cavities. However, the complexity of fabrication and assembly of these systems - even for compact designs - has been limiting their widespread deployment. While micro-resonators and optical fibre delay lines offer alternatives, their performance is significantly limited by thermally-induced frequency drift. Here we demonstrate, for the first time to the best of our knowledge, a laser stabilised to a Hollow Core Fibre (HCF) achieving comparable performance to ULE cavity-stabilised lasers. We achieve a frequency instability of 4.6x10-15 at 1 s and a frequency drift of 88 mHz/s, reducible to 3.7 mHz/s with thermal correction. Furthermore, over 3-year characterization confirms the HCF's predictable long-term behaviour. These results and the simplicity of the HCF-based system pave the way to a high-performance and scalable solution for ultra-stable laser sources.
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All authors Zitong Feng, Giuseppe Marra, Irene Barbeito Edreira, Hesham Sakr, Francesco Poletti, Radan Slavik

Computational Physics

Breaking the Timescale Barrier: Generative Discovery of Conformational Free-Energy Landscapes and Transition Pathways

Chenyu Tang, Mayank Prakash Pandey, Cheng Giuseppe Chen et al. (+3 more)

Molecular transitions – such as protein folding, allostery, and membrane transport – are central to biology yet remain notoriously difficult to simulate. Their intrinsic rarity pushes them beyond reach of standard molecular dynamics, while enhanced-sampling methods are costly and often depend on arbitrary variables that bias outcomes. We introduce Gen-COMPAS, a generative committor-guided path sampling framework that reconstructs transition pathways without predefined variables and at a fraction of the cost. Gen-COMPAS couples a generative diffusion model, which produces physically realistic intermediates, with committor-based filtering to pinpoint transition states. Short unbiased simulations from these intermediates rapidly yield full transition-path ensembles that converge within nanoseconds, where conventional methods require orders of magnitude more sampling. Applied to systems from a miniprotein to a ribose-binding protein to a mitochondrial carrier, Gen-COMPAS retrieves committors, transition states, and free-energy landscapes efficiently, uniting machine learning and molecular dynamics for broad mechanistic and practical insight.
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All authors Chenyu Tang, Mayank Prakash Pandey, Cheng Giuseppe Chen, Alberto Megías, François Dehez, Christophe Chipot

Chemical Physics

FSIM: A Pedagogical and Extensible HPC Framework for the Hartree-Fock Method

Mario Hernández Vera

Efficient computation of molecular integrals and Hartree-Fock energy remains a central topic in quantum-chemistry algorithm development. Although many sophisticated open-source packages are available, understanding their implementations from first principles can be difficult for students and developers alike. In this work, we present a concise overview and an extensible pedagogical framework that implements the Hartree-Fock method and the McMurchie-Davidson scheme for molecular integral evaluation. The implementation follows an object-oriented design in C++, emphasizing clarity and modularity. We also discuss strategies for parallel execution, including distributed computing with MPI and shared-memory parallelization with OpenMP. Beyond presenting a working reference, this work establishes a learning platform for further exploration, including suggested mini-projects for algorithmic optimization and HPC scalability. The accompanying open-source library, FSIM, described in this work, serves as a compact resource for teaching and research in computational chemistry and high-performance computing.
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Applied Physics · Cross-listed from cond-mat.mes-hall

Magneto-optical spectroscopy based on pump-probe strobe light

Shihao Zhou, Yujie Zhu, Chunli Tang et al. (+12 more)

We demonstrate a pump-probe strobe light spectroscopy for sensitive detection of magneto-optical dynamics in the context of hybrid magnonics. The technique uses a combinatorial microwave-optical pump-probe scheme, leveraging both the high-energy resolution of microwaves and the high-efficiency detection using optical photons. In contrast to conventional stroboscopy using a continuous-wave light, we apply microwave and optical pulses with varying pulse widths, and demonstrate magnetooptical detection of magnetization dynamics in Y3Fe5O12 films. The detected magneto-optical signals strongly depend on the characteristics of both the microwave and the optical pulses as well as their relative time delays. We show that good magneto-optical sensitivity and coherent stroboscopic character are maintained even at a microwave pump pulse of 1.5 ns and an optical probe pulse of 80 ps, under a 7 megahertz clock rate, corresponding to a pump-probe footprint of ~1% in one detection cycle. Our results show that time-dependent strobe light measurement of magnetization dynamics can be achieved in the gigahertz frequency range under a pump-probe detection scheme.
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All authors Shihao Zhou, Yujie Zhu, Chunli Tang, Rui Sun, Junming Wu, Yuzan Xiong, Ingrid E. Russell, Yi Li, Dali Sun, Frank Tsui, Binbin Yang, Valentine Novosad, Jia-Mian Hu, Wencan Jin, Wei Zhang

Fluid Dynamics · Cross-listed from cond-mat.soft

Flow-Induced Phase Separation for Active Brownian Particles in Four-Roll-Mill Flow

Soni D. Prajapati, Kusum Seervi, Akshay Bhatnagar et al. (+1 more)

We investigate the collective dynamics of active Brownian particles (ABPs) subjected to a steady two-dimensional four-roll-mill flow using numerical simulations. By varying the packing fraction ($\phi$), we uncover a novel flow-induced phase separation (FIPS) that emerges beyond a critical density ($\phi \geq 0.6$). The mean-square displacement (MSD) exhibits an intermediate bump between ballistic and diffusive regimes, indicating transient trapping and flow-guided clustering. The effective diffusivity decreases quadratically with $\phi$, while the drift velocity remains nearly constant, demonstrating that large-scale transport is primarily dictated by the background flow. Number fluctuations show a crossover from normal to giant scaling, signaling the onset of long-range density inhomogeneities in the FIPS regime. Our findings provide new insights into the coupling between activity, crowding, and flow, offering a unified framework for understanding phase behavior in driven active matter systems.
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All authors Soni D. Prajapati, Kusum Seervi, Akshay Bhatnagar, Anupam Gupta

Atmospheric and Oceanic Physics · Cross-listed from astro-ph.EP

The Fate of Hydrogen and Helium: From Planetary Embryos to Earth- and Neptune-like Worlds

Akash Gupta, Haiyang Luo, Jie Deng et al. (+1 more)

Hydrogen, helium, silicates, and iron are key building blocks of rocky and gas-rich planets, yet their chemical interactions remain poorly constrained. Using first-principles molecular dynamics and thermodynamic integration, we quantify hydrogen and helium partitioning between molten silicate mantles and metallic cores for Earth-to-Neptune-mass planets. Hydrogen becomes strongly siderophilic above $\sim$25 GPa but weakens beyond $\sim$200 GPa, whereas helium remains lithophilic yet increasingly soluble in metal with pressure. Incorporating these trends into coupled structure-chemistry models suggests that majority of hydrogen and helium reside in planetary interiors, not atmospheres, with abundances strongly depending on planet mass. Such volatile exchange may influence the redox states of secondary atmospheres, longevity of primordial envelopes, predicted CHNOPS abundances, and emergence of helium-enriched atmospheres, while He 1083 nm and H Lyman-$\alpha$ lines provide potential probes of atmosphere-interior exchange. These findings link atomic-scale interactions to planetary-scale observables, providing new constraints on the origins of Earth-to-Neptune-sized worlds.
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All authors Akash Gupta, Haiyang Luo, Jie Deng, Adam Burrows

Computational Physics · Cross-listed from cond-mat.mtrl-sci

Stabilisation of hBN/SiC Heterostructures with Vacancies and Transition-Metal Atoms

Arsalan Hashemi, Nima Ghafari Cherati, Sadegh Ghaderzadeh et al. (+3 more)

When two-dimensional atomic layers of different materials are brought into close proximity to form van der Waals (vdW) heterostructures, interactions between adjacent layers significantly influence their physicochemical properties. These effects seem particularly pronounced when the interface exhibits local order and near-perfect structural alignment, leading to the emergence of Moiré patterns. Using quantum mechanical density functional theory calculations, we propose a prototypical bilayer heterostructure composed of hexagonal boron nitride (hBN) and silicon carbide (SiC), characterized by a lattice mismatch of 18.77% between their primitive unit cells. We find that the removal of boron atoms from specific lattice sites can convert the interlayer interaction from weak vdW coupling to robust localized silicon-nitrogen covalent bonding. Motivated by this, we study the binding of transition-metal adatoms and formulate design guidelines to enhance surface reactivity, thereby enabling the controlled isolation of single-metal atoms. Our machine-learning-assisted molecular dynamics simulations confirm both dynamical stability and metal anchoring feasibility at finite temperatures. Our results suggest the hBN/SiC heterostructure as a versatile platform for atomically precise transition-metal functionalization, having potential for next-generation catalytic energy-conversion technologies.
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All authors Arsalan Hashemi, Nima Ghafari Cherati, Sadegh Ghaderzadeh, Yanzhou Wang, Mahdi Ghorbani-Asl, Tapio Ala-Nissila

Computational Physics · Cross-listed from cond-mat.soft

Energy-Conserving Contact Dynamics of Nonspherical Rigid-Body Particles

Haoyuan Shi, Christopher J. Mundy, Gregory K. Schenter et al. (+1 more)

Understanding the contact dynamics of nonspherical particles beyond the microscale is crucial for accurately modeling colloidal and granular systems, where shape anisotropy dictates structural organization and transport properties. In this paper, we introduce an energy-conserving contact dynamics framework for arbitrary convex rigid-body particles, integrating vertex-boundary interactions in 2D with vertex-surface and edge-edge detection in 3D. This formulation enables continuous force evaluation and strictly prevents particle overlap while conserving total energy during translational and rotational motion. Simulations of polygonal and polyhedral particles confirm the framework's stability and demonstrate its capability to capture packing behavior, anisotropic diffusion, and equations of state. The framework establishes a robust and extensible foundation for investigating the nonequilibrium dynamics of complex nonspherical particle systems, with potential applications in colloidal self-assembly, granular flow, and hydrodynamics.
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All authors Haoyuan Shi, Christopher J. Mundy, Gregory K. Schenter, Jaehun Chun

Optics

Gouy Phase-Related Effects in the Free-Space Optical Modulation of Free Electrons

Zhexin Zhao, Yiqi Fang, Mevlana Yunus Uludağ et al. (+1 more)

Modulating the free-electron wave function with light brings new opportunities to create attosecond electron pulse trains, to probe the quantum coherence of systems with significantly improved spatial resolution, and to generate classical and non-classical states of light with wide tunability. It is therefore crucial to efficiently generate free-electron wave functions that are suitable for these applications. In this study, we theoretically investigate an efficient free-space optical modulation of free electrons with two counter-propagating Gaussian beams. We find that the Gaussian beams' Gouy phase not only plays a crucial role in the interaction, but also enables straight-forward generation of valuable free-electron states, including comb-shape spectra with similar amplitudes, and states with high degree of coherence. We also discuss the feasibility of demonstrating these Gouy phase-related effects with chirped femto-second laser pulses. Our study establishes a theoretical foundation and physical intuition about the role of the Gouy phase. It can provide guidance to efficiently shape the free-electron wave function for a wide range of quantum applications.
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All authors Zhexin Zhao, Yiqi Fang, Mevlana Yunus Uludağ, Peter Hommelhoff

Fluid Dynamics · Cross-listed from cond-mat.soft

Solute dispersion boosts the phoretic removal of colloids from dead-end pores

Yiran Li, Mobin Alipour and Amir Pahlavan

Predicting and controlling the transport of colloids in porous media is essential for a broad range of applications, from drug delivery to contaminant remediation. Chemical gradients are ubiquitous in these environments, arising from reactions, precipitation/dissolution, or salinity contrasts, and can drive particle motion via diffusiophoresis. Yet our current understanding mostly comes from idealized settings with sharply imposed solute gradients, whereas in porous media, flow disorder enhances solute dispersion, and leads to diffuse solute fronts. This raises a central question: does front dispersion suppress diffusiophoretic migration of colloids in dead-end pores, rendering the effect negligible at larger scales? We address this question using an idealized one-dimensional dead-end geometry. We derive an analytical model for the spatiotemporal evolution of colloids subjected to slowly varying solute fronts and validate it with numerical simulations and microfluidic experiments. Counterintuitively, we find that diffuseness of solute front enhances removal from dead-end pores: although smoothing reduces instantaneous gradient magnitude, it extends the temporal extent of phoretic forcing, yielding a larger cumulative drift and higher clearance efficiency than sharp fronts. Our results highlight that solute dispersion does not weaken the phoretic migration of colloids from dead-end pores, pointing to the potential relevance of diffusiophoresis at larger scales, with implications for filtration, remediation, and targeted delivery in porous media.
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Computational Physics · Cross-listed from cond-mat.soft

Machine Learning the Entropy to Estimate Free Energy Differences without Sampling Transitions

Yamin Ben-Shimon, Barak Hirshberg and Yohai Bar-Sinai

Thermodynamic phase transitions, a central concept in physics and chemistry, are typically controlled by an interplay of enthalpic and entropic contributions. In most cases, the estimation of the enthalpy in simulations is straightforward but evaluating the entropy is notoriously hard. As a result, it is common to induce transitions between the metastable states and estimate their relative occupancies, from which the free energy difference can be inferred. However, for systems with large free energy barriers, sampling these transitions is a significant computational challenge. Dedicated enhanced sampling algorithms require significant prior knowledge of the slow modes governing the transition, which is typically unavailable. We present an alternative approach, which only uses short simulations of each phase separately. We achieve this by employing a recently developed deep learning model for estimating the entropy and hence the free energy of each metastable state. We benchmark our approach calculating the free energies of crystalline and liquid metals. Our method features state-of-the-art precision in estimating the melting transition temperature in Na and Al without requiring any prior information or simulation of the transition pathway itself.
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Fluid Dynamics

Body-Free Simulation of Three-Dimensional Turbulent Cylinder Wakes

Zhicheng Wang, Theo Käufer, Khemraj Shukla et al. (+2 more)

We present a body-free simulation framework for three-dimensional turbulent cylinder wakes, in which the upstream cylinder is not explicitly resolved. Instead, the incompressible Navier–Stokes equations are solved in a simplified rectangular domain, and the inflow is prescribed using velocity profiles extracted from experimental measurements or pre-computed direct numerical simulations (DNS). We show that, for the Reynolds numbers considered here, prescribing low-dimensional inflow information at a single downstream location is sufficient to reconstruct the principal wake dynamics, including three-dimensionality, coherent vortex shedding, Reynolds-stress distributions, and spectral content, for $Re=500$, $5{,}000$, and $11{,}000$. Comparisons with full-body DNS and particle image velocimetry (PIV) measurements show good agreement in mean velocity profiles, Reynolds stresses, and dominant shedding frequencies. A local stability analysis further shows that successful wake reconstruction is obtained when the imposed inflow is selected from the absolutely unstable near-wake region. The results support existing theory that the essential dynamics of bluff-body wakes are governed primarily by the instability of the near-wake profile rather than by the explicit presence of the body itself. The role of the onset of three-dimensionality is elucidated for first time. Relative to full DNS with the cylinder present, the proposed body-free simulation offers a substantial reduction in computational cost, providing a physically interpretable and computationally efficient route for wake reconstruction, reduced-complexity simulation, and flow-control studies.
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All authors Zhicheng Wang, Theo Käufer, Khemraj Shukla, Michael Triantafyllou, George Em Karniadakis