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Physics Physics

physics Watch Physics

Monday 28 September

Space Physics · Cross-listed from astro-ph.SR

Numerical analysis of electron density and response time delay during solar flares in mid-latitudinal lower ionosphere

Sayak Chakraborty and Tamal Basak

Impacts of solar flare vary at different parts of the lower ionosphere depending on its proximity to the direct exposure of incoming solar radiation. The quantitative analysis of this phenomenon can be attributed to the solar zenith angle (chi(t)) profile over the ionosphere. We numerically solve the electron continuity equation to obtain the lower ionospheric electron density profile (Ne(t)). The electron production rate (q(t)) is governed by the (i) X-ray profile (phi(t)) of the flare, (ii) chi(t)-values during the flare occurrence, etc. For analyzing the X-ray profile during flares, we use the GOES-15 satellite observations. Since we are working on an electron continuity equation based simplified ionospheric model, we confined our analysis to the comparatively stable mid-latitude ionosphere only. We choose three flares each from C, M, and X-classes for Ne(t)-profile computation. We observe that temporal Ne(t)-profiles differ when computed for the lower ionosphere over different discrete latitudes. Further, we compute the spatial Ne(t)-profile across the mid-latitude region at the time when phi(t) = phi_max. Now we assume that these flares repeat themselves every day of a year (DoY) at the same time of a day and we compute Ne(t)-profiles for each day. We found a seasonal effect on the Ne(t)-profile due to solar flare. Further, we investigate the response time delay (Delta t) of the lower ionosphere, which is the time difference between incidence of X-ray and the respective change in Ne(t)-profiles during solar flares. Strong seasonal effects on Ne(t)-profile and Delta t are the unique results of this work.
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Applied Physics · Cross-listed from cond-mat.mes-hall

Observing Colossal and Tunable Near-field Thermal Radiation with A Highly Sensitive Annular Micro-thermocouple Junction

Ken Araki, Wei Han Won and Liping Wang

Near-field radiative heat transfer between two objects has been theoretically predicted and experimentally demonstrated to exceed far-field blackbody limit across nanoscale vacuum gap distance enabled by evanescent surface waves coupling, while significant enhancement by more than 100 times usually requires sub-50-nm vacuum gaps around room temperature. This is challenging for parallel-plate configuration with millimeter sample sizes due to intrinsic wafer bow and contaminant particles. Sphere-plate configuration with a microsphere attached to bimaterial cantilevers or micro-thermocouple tips has been used to experimentally demonstrate near-field radiative heat transfer down to 30-nm gaps, but it is much less developed because of the challenges in the sophisticated sensor fabrication, low sensitivity and weak signals. In this work, we overcome these challenges by an annular micro-thermocouple junction fabricated at the end of a glass fiber with straightforward thin-film deposition to achieve high measurement accuracy with large Seebeck coefficient 25 uV/K and thermal resistance 8.7e6 K/W. With a silica microsphere attached underneath the micro-thermocouple junction, we report experimental observation of colossal near-field radiation heat transfer over blackbody limit down to 10-nm gap up to 2600 times with quartz and 900 times with doped silicon. Upon phase transition of VO2 thin film emitter, tunable near-field heat transfer up to 430-fold enhancement is experimentally demonstrated at 15-nm gap with 64% reduction. Experimental data agrees well with rigorous modeling based on fluctuational electrodynamics and Derjaguin approximation, and underlying mechanism is understood by energy transmission calculations. The results will advance the experimental study and fundamental understanding of energy transport at nanoscale gaps.
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Plasma Physics

Electrons with Anomalous Energy Generated in Vacuum Diodes with Different Pulse Duration

Vasily Kozhevnikov, Victor Tarasenko, Maxim Vorobyov et al. (+2 more)

This paper presents results from an experimental study in which anomalous energy electrons (AEE) appear in vacuum diodes under varying amplitude and pulse-duration conditions. AEE refer to high-energy discharge electrons with kinetic energy $T$ exceeding $eU$ values (where $e$ is the electron charge and $U$ is the amplitude gap voltage). Here, two experimental setups were used to generate electron beams with different pulse durations. The electron energy spectra were reconstructed from the beam attenuation curves using an AI-driven methodology for solving an ill-posed inverse problem for the Fredholm equation. This confirms that vacuum diodes generate electron beams with a significant AEE fraction (up to 25~%) only when operating with nanosecond-long voltage pulses. It has been confirmed that AEE in vacuum is generated using nanosecond- or sub-nanosecond voltage pulses. Experiments also indicate that photoelectric absorption of bremsstrahlung cannot produce significant AEE in vacuum discharges.
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All authors Vasily Kozhevnikov, Victor Tarasenko, Maxim Vorobyov, Evgenii Baksht, Andrey Kozyrev

Atomic Physics · Cross-listed from quant-ph

Observation of a topological edge state among localized bulk states in the anisotropic quantum Rabi model

Sungjoo Lim, Chanyang Im, Christopher G. Yale et al. (+7 more)

Topological phases are governed by discrete symmetries that protect boundary modes against local perturbations. When translational periodicity is absent, the bulk states also become localized, so that a topological edge state can no longer be distinguished from them by spatial localization alone. Here, we investigate the topological edge state (TES) and bulk eigenstates of the anisotropic quantum Rabi model (AQRM) in a trapped-ion quantum simulator. The AQRM hosts a topological phase in a one-dimensional synthetic lattice, whose translational symmetry is broken by the non-uniform couplings scaling with the site index. While both the TES and bulk states show localized distributions, we find that the TES exhibits well-defined chirality and near-complete spin–boson separability as signatures of the topological phase, in contrast to the bulk states. Phase-space tomography further reveals that the bosonic component of the TES is a squeezed vacuum state, with squeezing up to 6.45 dB. These results identify the TES through its intrinsic topological signatures and establish eigenstate-level characterization as a route to probing topological phenomena.
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All authors Sungjoo Lim, Chanyang Im, Christopher G. Yale, Brian K. McFarland, Edward C. Tortorici, Daniel S. Lobser, Melissa C. Revelle, Susan M. Clark, Mahn-Soo Choi, Junki Kim

Instrumentation and Detectors

EXO-200 Public Data Release for AI/ML Applications

S. Al Kharusi, G. Anton, I. Badhrees et al. (+103 more)

We present a public release of a subset of calibration data from the EXO-200 experiment, a liquid xenon time projection chamber designed to search for neutrinoless double-beta decay in $^{136}$Xe. The released dataset consists of approximately one million events collected during $^{228}$Th calibration campaigns performed near the end of Phase-II operations between February 26 and March 2, 2018. For each event, the dataset includes raw detector waveforms from the U-wire, V-wire, and avalanche photodiode (APD) readout channels together with reconstructed event quantities including charge energy, light energy, rotated energy, and charge-cluster information. The dataset is distributed in HDF5 format and is intended to support data preservation, educational activities, and the development of modern machine-learning techniques for rare-event physics. This paper describes the detector, dataset contents, file structure, and public access mechanism.
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All authors S. Al Kharusi, G. Anton, I. Badhrees, P. S. Barbeau, V. Belov, T. Bhatta, M. Breidenbach, T. Brunner, G. F. Cao, W. R. Cen, C. Chambers, B. Cleveland, M. Coon, A. Craycraft, T. Daniels, L. Darroch, S. J. Daugherty, J. Davis, S. Delaquis, A. Der Mesrobian-Kabakian, R. DeVoe, A. Dolgolenko, M. J. Dolinski, J. Echevers, B. Eckert, W. Fairbank Jr., D. Fairbank, J. Farine, S. Feyzbakhsh, P. Fierlinger, Y. S. Fu, D. Fudenberg, P. Gautam, R. Gornea, G. Gratta, C. Hall, E. V. Hansen, J. Hoessl, P. Hufschmidt, M. Hughes, A. Iverson, A. Jamil, C. Jessiman, M. J. Jewell, A. Johnson, A. Karelin, L. J. Kaufman, T. Koffas, R. Krücken, A. Kuchenkov, K. S. Kumar, Y. Lan, A. Larson, B. G. Lenardo, D. S. Leonard, G. S. Li, S. Li, Z. Li, C. Licciardi, Y. H. Lin, R. MacLellan, T. McElroy, T. Michel, B. Mong, D. C. Moore, K. Murray, O. Njoya, O. Nusair, A. Odian, I. Ostrovskiy, H. Peltz Smalley, A. Perna, A. Piepke, A. Pocar, F. Retière, A. L. Robinson, P. C. Rowson, S. Schmidt, D. Sinclair, K. Skarpaas, A. K. Soma, V. Stekhanov, M. Tarka, S. Thibado, J. Todd, T. Tolba, T. I. Totev, R. Tsang, B. Veenstra, V. Veeraraghavan, P. Vogel, J.-L. Vuilleumier, M. Wagenpfeil, J. Watkins, M. Weber, L. J. Wen, U. Wichoski, G. Wrede, S. X. Wu, Q. Xia, H. Xu, D. R. Yahne, L. Yang, Y.-R. Yen, O. Ya. Zeldovich, T. Ziegler

Atmospheric and Oceanic Physics

Low latency global carbon budget reveals strong land sink recovery in 2025

Philippe Ciais, Piyu Ke, Xiangjun Tian et al. (+32 more)

The atmospheric CO2 growth rate fell sharply in 2025, from a record 3.76 $\pm$ 0.09 ppm yr-1 in 2024 to 2.06 $\pm$ 0.09 ppm yr-1 (NOAA marine boundary layer observations), below the 2015-2022 mean of 2.47 ppm yr-1, even as fossil CO2 emissions rose by 0.7% to 10.38 GtC yr-1. Here we present a low-latency global and regional carbon budget for 2025, combining three dynamic global vegetation models (DGVMs) and ocean model emulators with four atmospheric inversions constrained by OCO-2 satellite retrievals. The global net land sink reached 2.36 $\pm$ 0.16 GtC yr-1 in 2025 (DGVMs: 2.04 $\pm$ 0.24; inversions: 2.68 $\pm$ 0.20 GtC yr-1), strengthening by 2.81 $\pm$ 0.31 GtC yr-1 from 2024 and exceeding the 2015-2022 mean by 0.71 $\pm$ 0.13 GtC yr-1. Ocean uptake (3.11 $\pm$ 0.36 GtC yr-1) remained similar to 2024, making the land sink rebound the dominant driver of the slowdown in CO2 growth. Tropical lands shifted from net sources in 2024 to net sinks in 2025, with enhanced uptake across much of Africa and northern Eurasia, and land flux anomalies covaried with GRACE terrestrial water storage. Where the sink had weakened substantially in 2023-2024, about 80% of the area showed some recovery, with overall recovery of 87.3% (DGVMs) to 99.5% (inversions). Recovery exceeded 100% in the tropics but remained incomplete in the northern extratropics, indicating a strong but spatially uneven rebound of the land carbon sink.
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All authors Philippe Ciais, Piyu Ke, Xiangjun Tian, Stephen Sitch, Wei Li, Xiaomeng Du, Xiaofan Gui, Ben Poulter, Thomas Colligan, Auke M. van der Woude, Anne-Wil van den Berg, Wouter Peters, Zhu Liu, Zhu Deng, Zhe Jin, Yilong Wang, Junjie Liu, Sudhanshu Pandey, Chris O'Dell, Jiang Bian, John Miller, Xin Lan, Jefferson Goncalves De Souza, Michael O'Sullivan, Pierre Friedlingstein, Youngryel Ryu, Helin Zhang, Julien Alléon, Yi Xi, Daniel S. Goll, Lei Zhu, Guido R. van der Werf, Yitong Yao, Shilong Piao, Frédéric Chevallier

Optics

Surface-Normal Electric Spin in Obliquely Excited Parallel-Slit SPP Fields

Naoki Ichiji

Handedness-dependent light-matter interactions require control of local electric-field rotation, but generating controlled surface-normal spin textures is difficult under oblique illumination. Here, we show that oblique s-polarized illumination of a periodic array of parallel slits produces spatially alternating surface-normal electric spin, with its period tunable by the incidence angle. We further show that the spin texture changes qualitatively between substrate- and air-side excitation depending on whether the SPP field interferes with the excitation field. Under substrate-side excitation, the spin distribution follows the conventional spin-flow relation, whereas under air-side excitation, interference between the SPP and excitation fields produces a distinct spatial periodicity and allows the spin extrema to coincide with the field-intensity maxima. A simple interference model reproduces these contrasting spin textures.
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Chemical Physics

Machine-learned Laplacian-level density functional from exact exchange-correlation potentials and energies

Arghadwip Paul, Bikash Kanungo, Sambit Das et al. (+1 more)

We present NNLap, a machine-learned Laplacian-level exchange-correlation (XC) functional that augments PBE with a neural-network correction depending on the electron density, its gradient, and its Laplacian. The model is trained on exact XC potentials and energies, obtained through inverse density-functional theory (DFT) calculations on configuration-interaction densities. Despite training on only a few systems – five atoms and three molecules – the model achieves remarkable accuracy on thermochemistry benchmarks, competing with the meta-GGA functionals SCAN and r2SCAN. It also attains accurate total energies, comparable to SCAN and better than r2SCAN and B3LYP. This shows that a Laplacian-level model, trained on exact XC potentials and energies, can reach the accuracy of meta-GGAs without their orbital dependence.
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All authors Arghadwip Paul, Bikash Kanungo, Sambit Das, Vikram Gavini

Optics

Monolithically integrated photonic neural network at driven-dissipative criticality

Yuming Zhang, Ruoran Wang, Jingcheng Li et al. (+4 more)

Artificial intelligence increasingly demands computing architectures capable of adaptive representation, temporal information processing, and robust operation under uncertainty. However, conventional photonic neural architectures typically rely on predefined computational operations and separate functional modules, limiting the ability of physical systems to exploit their intrinsic dynamics for intelligence. Here, we demonstrate a monolithically integrated active photonic neural network (APNN) that harnesses driven-dissipative criticality as a physical computational resource. By balancing optical excitation and dissipation, the system operates near a critical regime where weak input-dependent variations are amplified into separable representations while stable attractor dynamics are preserved. The APNN integrates an optoelectronic nonlinear array and a reconfigurable symmetric optical coupling matrix on a silicon photonic chip, enabling recurrent physical computation within a compact closed loop. We demonstrate nonlinear classification with 95.2% accuracy using only 100 training samples and 44 trainable parameters. And it remains robust at an ultralow signal-to-noise ratio of -30 dB. Beyond static recognition, the same driven-dissipative dynamics enable long-range temporal feature extraction from electrocardiogram signals and associative recovery of corrupted patterns through attractor-based relaxation. These results establish driven-dissipative criticality as a unified mechanism for representation enhancement, temporal information retention, and error-resilient computation, providing a pathway toward scalable photonic systems in which intelligence emerges from intrinsic physical dynamics.
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All authors Yuming Zhang, Ruoran Wang, Jingcheng Li, Hailong Zhou, Jianji Dong, Zhipei Sun, Xinliang Zhang

Chemical Physics

Basis Functions for Time-Dependent Kohn-Sham Inversion

Paul M. Zimmerman, Joshua Kammeraad, Duy-Khoi Dang et al. (+1 more)

Floquet theory provides insight into the inversion of time-dependent Kohn-Sham density functional theory. Specifically, mathematical derivations show that the fundamental frequencies of a time-dependent wavefunction solution are the leading-order harmonics for the TD-KS state. Numerical tests of the resulting ansatz in 1D and 3D for atomic and molecular cases demonstrate its utility. In particular, low $L_{2}$ errors in the time-dependent density and longitudinal current were found, even though currents were not an explicit optimization objective. In all, the proposed inversion ansatz provides exchange-correlation potentials from time-dependent wavefunctions, is highly interpretable, and may significantly help in the development of nonadiabatic density functionals.
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All authors Paul M. Zimmerman, Joshua Kammeraad, Duy-Khoi Dang, Harish S. Bhat

Optics

Stimulated Electro-optic Scattering

Violet Workman and Gaurav Bahl

Stimulated Brillouin Scattering (SBS) couples light to acoustic waves and underpins applications ranging from sensing and signal processing to quantum photonics. In solids, this interaction is generally attributed to photoelasticity and the motion of dielectric boundaries. Here we show that piezoelectricity opens an additional, previously unrecognized pathway that can substantially reshape Brillouin gain. This contribution arises from the combined action of linear electro-optic “Pockels” scattering and its reciprocal $\chi^{(2)}$-mediated optical drive, which couple optical fields to the electric fields that accompany mechanical motion in piezoelectrics. We formally incorporate these effects into the modern theory of SBS and apply it to monolithic lithium niobate waveguides. In addition to large enhancement and suppression of the Brillouin gain, we surprisingly find occurrences of large gain where conventional theory would otherwise have predicted zero Brillouin interaction. These instances are the first identification of purely electro-optic SBS, a process we term stimulated electro-optic scattering, as it only involves interactions between electromagnetic fields even though a mechanical excitation is present. Our results establish the electro-optic tensor as a key governing parameter for Brillouin interactions in piezoelectric media and provide a new degree of freedom for engineering photon-phonon coupling independent of photoelasticity and index contrast.
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Fluid Dynamics

On the Rayleigh-Taylor instability of a diffuse interface

Marildo Kola, Daniel Israel and Aaron Towne

We present a formulation for the linear stability of the miscible Rayleigh-Taylor instability, retaining viscosity, Fickian mass diffusion, and non-Boussinesq effects. Rather than adopting the diffusive corrections of Duff, Harlow, and Hirt (1962) or relying on the viscous eigenvalue problem of Chandrasekhar (1961), the present formulation is derived directly from the incompressible variable-density governing equations. Both approaches are recovered as limiting cases, extending Morgan, Likhachev, and Jacobs (2016). The governing equations admit a time-evolving self-similar diffusive base state in which an error-function density profile is accompanied by a nonzero vertical base velocity required by the non-solenoidal constraint. Linearization yields a reduced coupled formulation for the vertical velocity and density perturbations, suitable for both theoretical analysis and computation. Within the quasi-steady-state approximation, we show that the formulation reduces, in the infinite-Schmidt-number limit, to the classical viscous immiscible theory and recovers the Chandrasekhar eigenvalue problem. At finite Schmidt number, diffusion alters the growth-rate spectrum and its high-wavenumber structure. Beyond the spectral cutoff we establish the scaling $\omega_r \sim -\lambda_m k^2$, where $\lambda_m$ is determined by the competition between the locally stratified kinematic viscosity and the reference diffusive scale. By isolating the effect of diffusion, we show that its stabilizing influence weakens with increasing density stratification. Buoyancy production becomes bimodal and shifts toward the lighter-fluid side, while pressure production grows to rival buoyancy and acts as an energy source on the heavier side of stratification. Together, these results clarify when classical diffusive corrections remain applicable and when diffusion and non-Boussinesq coupling must be retained.
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Optics

Tensor-Engineered Van der Waals NbOCl2 Resonant Metasurface for Polarization-entangled Bell State Generation

Xin Zeng, Wenna Du, Yun-Kun Wu et al. (+20 more)

Polarization-entangled photon pairs are essential resources for quantum information technologies, yet realizing compact sources with intrinsically controllable entanglement remains challenging. Van der Waals (vdW) nonlinear materials such as NbOCl2 provide atomically thin platforms for quantum light generation, yet their native crystalline anisotropies as natural materials impose limitations on accessible quantum states. Here, we develop a resonant vdW nonlinear metasurface based on NbOCl2 that exploits its intrinsic optical anisotropy to engineer polarization-dependent nonlinear responses. The anisotropic optical dispersion enables selective manipulation of resonant modes, resulting in a three-order-of-magnitude enhancement of the nonlinear response along the c axis. By further tailoring these resonant modes, we redistribute the effective second-order nonlinear susceptibility tensor between orthogonal polarization channels, balancing the spontaneous parametric down-conversion pathways along the b and c axes. This enables polarization-entangled photon generation with a measured fidelity of up to 92%. Our work establishes metasurface-enabled nonlinear optical engineering as a strategy for enhancing and controlling quantum light generation in vdW materials, providing a pathway toward scalable quantum photonic platforms.
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All authors Xin Zeng, Wenna Du, Yun-Kun Wu, Yuyang Zhang, Zhiyong Zhang, Kwok Kwan Tang, Ruihuan Duan, Xiaotian Bao, Yiyang Gong, Yuexing Xia, Yutong Zhang, Jinhan Zhao, Yubo Tian, Yangguang Zhong, Chuanxiu Jiang, Yubin Wang, Jianhui Fu, Shuai Yue, Guankui Long, Qing Zhang, Zheng Liu, Xi-Feng Ren, Xinfeng Liu

Physics and Society · Cross-listed from cs.CG

Using Persistent Homology to Analyze Access to Heterogeneous-Quality Resources and Heterogeneous-Severity Nuisances

Sarah Tymochko, Gillian Grindstaff, Abigail Hickok et al. (+2 more)

We develop a framework to use multiparameter persistent homology (PH) to examine access to heterogeneous-quality resources and exposure to heterogeneous-severity nuisances in a geographic region. Persistent homology, which is a type of topological data analysis {(TDA)}, has been employed previously to examine resource coverage. Unlike prior approaches, which used one-parameter PH to study resource coverage and nuisance exposure, our method accounts for heterogeneous-quality resources. Our framework, which employs a computationally-efficient approximation of multiparameter PH, allows one to study access to any resource ({or} exposure of any nuisance) using any notion of quality (or severity). Using the city of Chicago as an example region, we employ our framework to detect clusters of poor access to public parks, overexposure to landfills, and both underexposure and overexposure to pubs and bars.
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All authors Sarah Tymochko, Gillian Grindstaff, Abigail Hickok, Jiajie Luo, Mason A. Porter

Plasma Physics

Observer-Based Model Predictive Control for Isoflux Regulation in the EXL-50U Spherical Tokamak

Chang Lu, Jiayi Zhi, Xian Guo et al. (+1 more)

Isoflux control in spherical tokamaks requires coordinated plasma-current and boundary regulation under coil-voltage and computational constraints. This study develops observer-based constrained model predictive control (MPC) for EXL-50U. Vacuum-vessel spatial coarsening reduces a 538-state linearized model to 40 states while retaining control-relevant responses. A Kalman observer provides full-state estimates, and MPC jointly optimizes current and boundary-flux tracking with explicit voltage bounds. Matrix precomputation, specialized solver code generation, and structured linear-system solution reduce online computation. Nonlinear free-boundary Grad–Shafranov evolutive simulations evaluate nominal operation, measurement noise, configuration mismatch, a one-step delay, and combined noise and delay against proportional–integral–derivative (PID) control and a linear–quadratic regulator (LQR). Nominal MPC root-mean-square errors are 1.126 kA for plasma current, 0.0164 m for last-closed-flux-surface geometry, and 2.463 mWb for the worst flux channel, all below both baselines. With noise, the boundary error is 0.0199 m over the common evaluation window, and regulation is sustained throughout the test. MPC completes a limiter-to-divertor transition without relinearization or retuning and maintains regulation under delay and combined disturbances. Across all five cases, mean MPC computation times are 0.413–0.441 ms, with all 99th percentiles below 1 ms. These results support joint current and isoflux regulation and millisecond-scale computational feasibility, providing a basis for future EXL-50U experiments.
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All authors Chang Lu, Jiayi Zhi, Xian Guo, Tianyuan Liu

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

Native implementation of the machine-learned Skala exchange-correlation functional in CP2K: Unified one-centre reconstruction for molecular and condensed-phase calculations

Johann Pototschnig, Franz Pöschel, Jürg Hutter et al. (+1 more)

We implement the machine-learned Skala exchange-correlation (XC) functional natively in CP2K using its Gaussian and plane-wave (GPW) and Gaussian and augmented-plane-wave (GAPW) methods. A joint one-centre reconstruction of density, density gradients, and kinetic-energy density before functional evaluation preserves mixed gradient terms and nonlocal couplings between smooth and atom-local contributions in all-electron and pseudopotential calculations. An XC-specific GAPW representation resolves rapidly varying local contributions on atom-centred grids, reducing plane-wave requirements for pseudopotential calculations. The implementation includes Brillouin-zone sampling and point-group symmetry reduction. With sufficiently flexible orbital bases, it retains the molecular benchmark accuracy of our earlier GauXC formulation. All-electron Skala-D3(BJ) calculations yield a mean absolute error (MAE) of 1.54 kJ mol$^{-1}$ against diffusion Monte Carlo for crystalline CO$_2$, NH$_3$, and urea, spanning dispersion, quadrupolar electrostatics, and hydrogen bonding. For all thirteen DMC-ICE13 phases, the MAEs are 1.16 kJ mol$^{-1}$ for absolute lattice energies and 0.82 kJ mol$^{-1}$ for the twelve relative energies to ice Ih. At fixed experimental geometries, all-electron/mixed-core calculations give a band-gap MAE of 0.43 eV on a 15-material set, substantially below commonly used semilocal functionals and comparable to widely used hybrid functionals. The LC10 benchmark nevertheless reveals systematically underestimated equilibrium lattice constants, indicating structural overbinding. This framework connects molecular Skala to condensed-phase electronic structure and enables future development using periodic many-body reference data.
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All authors Johann Pototschnig, Franz Pöschel, Jürg Hutter, Thomas D. Kühne

Optics

Multimode oscillation and its route to chaos in transverse-mode-coupled optomechanical resonators

Motoki Asano, Lalit Maharjan and Hiroshi Yamaguchi

Nonlinear oscillators exhibit rich collective phenomena including synchronization, bifurcation, and chaos. Experimentally accessing multimode nonlinear dynamics, however, remains challenging because low-order internal resonances readily induce mode locking and suppress stable multimode oscillation. Here, we demonstrate a transverse-mode-coupled optomechanical platform that combines engineered collective mechanical modes with optically controllable nonlinear interactions. The engineered transverse mechanical modes enable stable two- and three-mode self-sustained oscillations while suppressing trivial low-order internal resonances. By varying the laser frequency, we observe successive beat-frequency fractionalization ({\Delta}f/2 and {\Delta}f/3), followed by a broadband state exhibiting enhanced trajectory divergence consistent with deterministic chaos. These results establish transverse-mode-coupled optomechanical resonators as a scalable and tunable platform for exploring high-dimensional nonlinear dynamics in interacting mechanical networks.
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Sunday 27 September

Classical Physics · Cross-listed from cond-mat.mes-hall

Magnetic localization and spectral asymmetry of a charged particle at a screw dislocation

Claudio Furtado, Omar Mustafa, Hassan Hassanabadi et al. (+1 more)

We study a charged particle moving outside a spiral (screw) dislocation in a uniform magnetic field. Although the geometry is locally Euclidean, its global screw identification couples the conserved axial and azimuthal quantities through the combination \(\mathcal{J}=\ell-\beta k\) After eliminating the cyclic coordinates, the radial motion is governed by an inverse-square term determined by $\mathcal{J}$ and a harmonic confining term generated by the magnetic field. The corresponding reduced quantum problem is exactly solvable for $q\mathcal{B}_0\neq0$, yielding an equally spaced spectrum with an orientation-dependent energy shift. The radial probability density depends on $\mathcal{J}$, and opposite values of the screw-modified quantum number have identical spatial profiles but different energies for a fixed magnetic-field orientation. The radial size scales as \(\sqrt{\langle r^2\rangle_n}\propto |q\mathcal{B}_0|^{-1/2}\), with the magnetic length setting the localization scale. At $q\mathcal{B}_0=0$, the confining term vanishes, and the problem reduces to a scale-free inverse-square system. Thus, the screw parameter enters the dynamics through the global coupling of the conserved axial and azimuthal quantities, whereas the magnetic field provides radial confinement and its orientation determines the spectral asymmetry.
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All authors Claudio Furtado, Omar Mustafa, Hassan Hassanabadi, Abdullah Guvendi

General Physics

Gravity, Entropy, and the Second Law: A Pedagogical Perspective

Jorge Pinochet and Giorgio Sonnino

An interesting question for discussion in university physics courses is whether gravity can conflict with the second law of thermodynamics. In this work, we address this question by comparing the behavior of an ideal gas in two conceptually different regimes: one in which gravitational effects can be neglected and another in which self-gravity becomes important. In the first case, free expansion provides the familiar example in which an increase in the volume accessible to the gas leads to an increase in entropy. Self-gravitating systems behave differently: gravitational contraction can reduce the volume occupied by matter while releasing energy and increasing the entropy of the surroundings. This illustrates why the usual intuition that increasing entropy necessarily corresponds to increasing spatial dispersion cannot be applied directly to self-gravitating systems. Using elementary thermodynamic arguments and a minimal amount of mathematics, we discuss this apparent tension and show how entropy must be considered for the complete system, including emitted radiation. The discussion is finally extended to black holes and Hawking radiation, where the appropriate framework is the generalized second law of thermodynamics. The analysis is intended to provide a pedagogical bridge between elementary thermodynamics and some of the distinctive thermodynamic properties of gravitating systems.
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Data Analysis and Statistics · Cross-listed from cs.IT

Statistical Dark Matter: Synergy is everywhere but is hard to capture

Alberto Liardi, Fernando E. Rosas, Daniele Marinazzo et al. (+3 more)

The information-theoretic construct of synergy refers to the statistical structure that is contained in three or more variables, but not in any subset of them. Here we leverage recent advances in multivariate information theory to show that synergy is far more prevalent in complex systems than previously thought. In particular, we show that many-body systems tend to become strongly dominated by synergy as the number of subcomponents grows. At the same time, our results also reveal that commonly used tools for statistical modelling severely underestimate synergistic structures. These findings imply that synergy constitutes a prevalent, yet often invisible, informational component of complex systems. With a certain poetic licence, we interpret these results as suggesting that synergy is the dark matter of statistics - interdependencies that we know exist, but standard instruments fail to detect.
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All authors Alberto Liardi, Fernando E. Rosas, Daniele Marinazzo, Thomas F. Varley, Michael Gastpar, Pedro A.M. Mediano

Atmospheric and Oceanic Physics

BridgeCast: Bridging Ocean Wave Forecasts to Reanalysis via Flow Matching with Exogenous Variables

Siyu Gan, Dongsheng Luo, Kunxiaojia Yuan et al. (+2 more)

Ocean wave forecasting is essential for maritime safety, offshore operations, and coastal resilience, yet remains challenging due to systematic biases in physics-based models. Physical models, while widely used, rely on approximations and parameterizations that limit their accuracy under complex ocean-atmosphere conditions. To enhance ocean wave forecasting, we propose BridgeCast, within a physics-AI hybrid framework for bias correction. BridgeCast is a probabilistic model based on conditional flow matching (CFM) that learns to transform physical model forecasts into reanalysis-like fields. It treats physical forecasts as corrupted observations and employs a continuous-time generative process to bridge their distribution toward that of reanalysis data. BridgeCast is parameterized by a Transformer-based architecture that enables spatiotemporal modeling, incorporation of exogenous atmospheric variables, and flexible inference via both ordinary and stochastic differential equation formulations. Extensive experiments on real-world datasets demonstrate that BridgeCast consistently outperforms state-of-the-art baselines across regions and forecast lead times.
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All authors Siyu Gan, Dongsheng Luo, Kunxiaojia Yuan, Dongjin Song, Jingchao Ni

Atomic Physics · Cross-listed from cond-mat.quant-gas

Spectral Crossings Diagnose Multiply Quantized Vortex Splitting

Jongheon Baek, Hiroyuki Tajima and Gihwan Nam

A charge-$r$ coupling vortex can remain as one fused order-$r$ zero or split into $r$ nearby unit-charge zeros after weak symmetry breaking. The two configurations have the same total winding and the same number of dark states in the lowest-Landau-level projection, so neither the imposed orbital angular momentum nor the total winding number distinguishes their local geometry. We study this distinction in a two-component Haldane-sphere model coupled by a Raman field. For monopole flux $N$, the detuning $\tau_{\rm c}$ at which a defect branch crosses zero energy scales as $|\tau_{\rm c}|\sim N^{-(r+1)}$ for a fused zero and $N^{-2}$ for resolved unit zeros. A Feshbach reduction establishes these asymptotic laws for the full multiband Hamiltonian, and numerical calculations in a monopole-harmonic basis confirm the fused-vortex scaling for $r=1,2,3$. For a partially split cluster of diameter $d$, the scaling crosses over when $d$ is comparable to the magnetic length $\ell_B$, allowing $d$ to be inferred spectroscopically even below the spatial resolution of direct imaging.
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Space Physics

An autonomous approach to model daytime behavior of sub-ionospheric VLF signals over short and medium propagation paths in India

Sayak Chakraborty, Tamal Basak, Sourav Palit et al. (+1 more)

We present an autonomous model to simulate the daytime variation of sub-ionospheric Very Low Frequency (VLF) signal amplitude, beginning with the computation of the D-region electron density by numerically solving the electron continuity equation (ECE). From the resulting altitude-dependent electron density profile (Ne), we extract Wait's ionospheric parameters (h' and beta) using a log-linear fitting method. The study focuses on two VLF propagation paths (one short and one medium in length) in India, originating from the VTX (18.2 kHz) transmitter. The model employs the Long Wave Propagation Capability (LWPC) framework to reproduce the daytime VLF signal amplitude profile. It accurately captures the daytime variations observed at the Bengaluru (BAN) station, where the ground-wave component is dominant, as well as at Khukurdaha, WB (KHK). A quantitative comparison between the simulated (Asim) and observed (Aobs) amplitudes shows good agreement, supporting the physical consistency and potential predictive capability of the proposed approach.
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All authors Sayak Chakraborty, Tamal Basak, Sourav Palit, Sandip K. Chakrabarti

Fluid Dynamics

Wave-Particle Decomposition for Kinetic Equations II: Full Boltzmann equation

Chang Liu and Kun Xu

The wave-particle decomposition of kinetic relaxation equations is extended to the full Boltzmann equation. An identity transformation writes the collision term as a relaxation toward the local Maxwellian plus a conservative remainder. Along characteristics, the equilibrium integral over a local kinetic horizon defines the wave, and its exact complement, the particle, carries the remainder. The moments of the wave and particle equations are extended Navier-Stokes equations, and the total conservation law and the particle equation form the wave-particle multiscale equations, a closed system, exact for every horizon, that spans a continuous spectrum from the Boltzmann equation to Navier-Stokes hydrodynamics, parametrized by the horizon-to-relaxation ratio. A coupled implicit wave-particle iteration solves this system, updating the conservative variables with the particle fixed and the particle with the predicted wave and an endpoint traction from the macroscopic prediction. Under discrete compatibility assumptions the iteration preserves the discrete Boltzmann solution as its fixed point, and its near-continuum convergence factor is bounded by a particle transport share decaying exponentially with this ratio. Tests on a normal shock, Couette and cavity flows, and hypersonic cylinder and Apollo flows agree with the conventional iterative scheme in rarefied and transition regimes and with Navier-Stokes solutions near the continuum limit. There the wave-particle iteration needs about three to more than four orders of magnitude fewer iterations than this scheme or, where it did not converge, its Shakhov version, and the Boltzmann solution costs one to four times the wall time of a Navier-Stokes solver on the same mesh.
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Fluid Dynamics · Cross-listed from cs.LG

Learning in the Transverse Subspace: A Minimal Representation for Divergence-Free Operator Learning

Yifei Sun

Divergence-free vector fields are fundamental state variables in incompressible flows and many PDE systems. Redundant parameterizations, including Neural Conservation Law (NCL) potentials, map multiple auxiliary representations to the same physical field. Our experiments show that this redundancy can reduce static representation-fitting error by enlarging the set of equivalent solutions, but the resulting many-to-one mapping does not provide a unique state for operator learning. We introduce a minimal representation that encodes a real \(D\)-component divergence-free vector field on a \(D\)-dimensional domain as a real \((D-1)\)-component field on the same domain. Exploiting the transverse structure imposed by incompressibility in Fourier space, we use a Householder orthogonal transformation to construct the reduced coordinates directly. For periodic and closed impermeable fields, the transform is invertible, isometric, and angle-preserving. For open nonperiodic flows, Fourier extension constructs a compatible periodic field, and a minimum-energy rule selects a unique reduced representation. Neural operators then learn temporal evolution entirely in this reduced space. At inference, the predicted \((D-1)\)-component field is decoded directly into a physical divergence-free \(D\)-component field, without predicting an ambient field or applying post-hoc projection. Experiments on static fitting and temporal prediction reveal a task-dependent trade-off: redundancy facilitates static optimization, whereas unique invertible coordinates provide a well-defined state for temporal dynamics. By removing unconstrained longitudinal or null directions from the learned state space, the proposed formulation achieves lower prediction error and greater robustness while preserving divergence freedom by construction.
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Optics

Tapered Fibre Fabry-Pérot Cavities for the Generation of Near-Visible, Phase-locked, and Broadband Kerr Frequency Combs

Matthew Macnaughtan, Yiqing Xu, Miro Erkintalo et al. (+2 more)

We demonstrate the generation of near-visible, broadband Kerr frequency combs using a tapered fibre Fabry-Pérot resonator. By incorporating micron-diameter tapered silica fibre into a pulsed-driven Fabry-Pérot configuration, we exploit geometric dispersion engineering to overcome the strong normal dispersion intrinsic to silica (and nearly all materials) at short wavelengths, achieving net anomalous dispersion in the near-visible regime. The strong mode confinement of the tapered fibre simultaneously enhances the Kerr nonlinearity by over an order of magnitude relative to standard fibre, substantially reducing the threshold drive power for nonlinear structure formation. Through precise control of the cavity fabrication, we experimentally demonstrate and numerically confirm a rich suite of coherent, phase-locked nonlinear structures, including conventional anomalous dispersion cavity solitons, zero dispersion solitons with spectral bandwidths exceeding 16 THz between 750 nm and 800 nm, as well as switching wave combs and broadband Raman solitons, the latter also reported here for the first time in the near-visible regime. Our results establish the tapered fibre Fabry-Pérot resonator as a versatile platform for near-visible and potentially visible coherent frequency comb generation.
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All authors Matthew Macnaughtan, Yiqing Xu, Miro Erkintalo, Stéphane Coen, Stuart G. Murdoch

Fluid Dynamics · Cross-listed from nlin.CD

Score-based stochastic reduced-order models of barotropic quasi-geostrophic turbulence

Ludovico Theo Giorgini

In many turbulent flows, the quantities of interest are coarse-grained variables, either because resolving the fine scales is computationally prohibitive or because only coarse observations are available; in both cases, data-driven reduced-order models that evolve only these variables are required. Even when the governing equations are known, their projection onto the coarse variables is not closed, and the central difficulty lies in representing the feedback of the unresolved scales so that the reduced model reproduces the statistical and dynamical properties of the projected high-resolution system. We address this problem with a score-based framework, constructing a reduced-order model of quasi-geostrophic turbulence whose stationary distribution coincides by construction with the distribution learned from data and whose dynamics are calibrated to reproduce the finite-time correlations of the energies in individual wavenumber shells. The resulting model reproduces the stationary statistics of the resolved flow with high accuracy, and its calibration is computationally inexpensive because it requires no forward integration of the model.
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Physics and Society

Learning to cooperate in a changing world: How caring about the future promotes cooperation across scales

Yuxin Geng, Xingru Chen, Xin Wang et al. (+4 more)

In social dilemmas, individuals need to forgo short-term temptations to achieve synergistic collective outcomes through cooperation. Previous work has examined mechanisms through which cooperation can evolve, including direct reciprocity, indirect reciprocity, environmental stochasticity, network reciprocity, and demographic stochasticity. These mechanisms have largely been studied under natural selection or social learning, where strategies with higher payoffs are more likely to spread. It is equally important to study cooperation under self-learning, where individuals adapt through their own experiences. Here, we focus on multi-agent reinforcement learning and derive analytical conditions under which these mechanisms stabilize cooperation under learning dynamics across scales. We find that reinforcement learning can steer self-interested individuals toward cooperation when they value the future over short-term temptation. Our work provides a unified approach to identifying the determinants of learning to cooperate in a changing world, thereby paving the way for the advancement of cooperative artificial intelligence.
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All authors Yuxin Geng, Xingru Chen, Xin Wang, Hongwei Zheng, Longzhao Liu, Shaoting Tang, Feng Fu

Optics · Cross-listed from cond-mat.mes-hall

Topological chiral edge modes in the continuum of a trivial bulk

Li Ge

Chiral edge modes in topological states of matter are routinely found inside band gaps, which may be deformed and shifted as a function of the lattice momentum. Here we show that they can exist inside a bulk band instead, i.e., in the continuum. These chiral edge modes in the continuum (CEMICs) are induced by introducing an imaginary magnetic flux to an otherwise topologically trivial two-dimensional system, realized by imposing asymmetric couplings along the edge. While the stronger couplings provide a preferred circulation direction, we find that the edge localization of CEMICs does not originate from the non-Hermitian skin effect, which is typically associated with systems featuring asymmetric couplings and the open boundary condition. Instead, this edge localization is due to the energy exchange between the system and the environment at the asymmetric coupling junctions, which leads to a parity-time (PT) transition in the Brillouin zone. Intriguingly, the onset of CEMICs is independent of the system size in the macroscopic limit, which is given by the golden ratio between the asymmetric couplings and their geometric average.
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Plasma Physics

A packet-level digital hardware twin for commissioning megahertz diagnostic edge AI and plasma control system integration in tokamaks

Semin Joung, Abhilasha Dave, Luca Scomparin et al. (+6 more)

High-bandwidth plasma diagnostics increasingly provide inputs to machine learning and signal-processing algorithms intended for real-time tokamak control, but the complete path from diagnostic sampling to control-system handoff is difficult to commission because of their sampling rates. We develop a packet-level digital hardware twin for the megahertz diagnostic edge-AI architecture. The simulator represents a 64-channel, 1 MHz beam emission spectroscopy (BES) diagnostic embedded in a 96-channel dual-carrier acquisition system, two 48-channel streams with SPAD0 sample counters, 10 GbE Hardware UDP transport, packet loss and network jitter, FPGA parsing and dual-carrier alignment, causal preprocessing and edge inference, a compact Ethernet result packet, receiver-side shared state, and a 1 kHz PCS-like control cycle. Binary UDP payloads and PCAP files are generated rather than emulating transport only at the array level. With a baseline of 20 samples per packet, each carrier generates 50,000 packets s$^{-1}$ and 100 MB s$^{-1}$ of user payload. A 110 ms reference run produces 11,000 HUDP packets; an intentionally dropped 20-sample packet is detected by the sample-counter continuity logic and invalidates the two overlapping 128-sample inference windows without silent interpolation. For valid windows, the configured engineering latency model gives a median last-input-to-shared-memory latency of 91.6 us and a 99th percentile of 108.1 us. A separate operating-system loopback test sends binary FPGA-result datagrams through a UDP receiver into POSIX shared memory and preserves packet sequence and CRC for 20/20 packets. Interactive GUI interfaces expose timing, packetization, network faults, inference thresholds, and control-state inspection. The framework provides a reproducible environment for testing diagnostic-to-accelerator interfaces and fail-safe behavior for deployment on fusion devices.
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All authors Semin Joung, Abhilasha Dave, Luca Scomparin, Filipp Khabanov, Zheng Yan, Benedikt Geiger, George McKee, Ryan N. Coffee, David R. Smith