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Physics Physics

physics Watch Physics

Monday 27 April

Fluid Dynamics

Mass-Transfer Control With Microbubbles in Highly Turbulent Decaying Flows

Vivek Kumar, Prasoon Suchandra, Jason Rom et al. (+3 more)

We hypothesize that combining extreme turbulence with a minute reduction in surface tension $\sigma$ (surface tension of the liquid) using surfactant provides a simple and scalable route for controlling micron scale bubble size in gas–liquid systems. To test this, we generate high-intensity turbulence using a multiphase pump [turbulent intensity $\ge 40\%$; Taylor Reynolds number $Re_\lambda=\mathcal{O}(10^3)$; bulk Reynolds number $Re=\mathcal{O}(10^5)$] feeding a straight duct, which produces a decaying turbulent flow where, without additives, bubble coalescence dominates and causes monotonic downstream growth in the mean diameter $d_\mathrm{avg}$ of the bubbles. This growth is governed by the turbulent dissipation rate $\varepsilon$. High-speed imaging, back-lit shadowgraph and particle shadow velocimetry (PSV) quantify bubble statistics ($d_\mathrm{avg}$, and the bubble-size distribution) and turbulence metrics (turbulent kinetic energy $k$, turbulence intensity $\mathcal{I}$, and dissipation rate $\varepsilon$). We then introduce a minute amount ($\sim 0.01\%$ critical micelle concentration) of additive that produces a slight reduction in $\sigma$, used here only as an interfacial tuning knob because the same change in surface tension can be achieved with non surface active agents. This small decrease in $\sigma$ enhances breakup, slightly suppresses coalescence, and makes smaller bubbles more breakup prone, resulting in reduced $d_\mathrm{avg}$ and a narrower bubble-size distribution. Turbulence statistics remain unchanged within experimental uncertainty, indicating that the effect arises entirely from interface rather than hydrodynamic changes. Overall, combining extreme turbulence with a minute reduction in surface tension offers a low complexity and tunable lever for setting bubble-size distributions and intensifying mass transfer in industrial multiphase flows.
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All authors Vivek Kumar, Prasoon Suchandra, Jason Rom, Shivam Prajapati, Suhas Jain, Cyrus Aidun

Atomic Physics

Collective Strong Coupling of Thermal Atoms to Integrated Microring Resonators

Xiaoyu Cheng, Benyamin Shnirman, Alexandra Köpf et al. (+5 more)

Strong coupling between atomic ensembles and high-quality optical cavities enables collective and nonlinear phenomena that are central to cavity quantum electrodynamics (cQED). Although many experiments have been performed on this topic, most of them have focused on cold atoms. Here, we experimentally demonstrate collective strong coupling between thermal rubidium (Rb) vapor and high-quality silicon nitride microring resonators (MRRs) on an integrated photonic chip. We observe cavity mode splitting, with a measured collective coupling strength of $g_N/2\pi \approx 1\,\mathrm{GHz}$ and a collective cooperativity of $C_N\approx2$ at $110\,^\circ\mathrm{C}$, indicating coherent energy exchange between the atomic ensemble and the cavity mode despite rapid decoherence in the thermal vapor system. We infer an average of $20$ atoms participating in the collective interaction, yielding a single-atom cooperativity of $C_0=0.1$ and approaching the single-atom strong-coupling regime. Our results establish the integrated thermal vapor MRR platform as a robust, compact, and scalable system for studying collective and nonlinear phenomena in cQED.
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All authors Xiaoyu Cheng, Benyamin Shnirman, Alexandra Köpf, Guangcanlan Yang, Hong X. Tang, Hadiseh Alaeian, Tilman Pfau, Robert Löw

History and Philosophy of Physics · Cross-listed from astro-ph.SR

Analyses on Wassenius' Report for Total Solar Eclipse in 1733: Quantifications of the Solar Radius and the Earliest Reported Prominences

Hisashi Hayakawa, Mitsuru Sôma, Noortje Peek et al. (+3 more)

Total solar eclipses (TSEs) offer a unique opportunity to observe the solar atmosphere, detect limb phenomena, and accurately measure the solar radius. Following the TSE in 1733, Wassenius first reported the existence of prominences to the scientific community. Wassenius' original manuscript is held in the Royal Academy Archives of Sweden; this study translates his report and documents the associated source materials and local eclipse visibility. The solar radius (R_Sun) during the TSE in 1733 are 696250 +/- 170 km and 959.99 +/- 0.24" in the absolute and apparent scales, respectively. This result contrasts with the modern standard (helioseismic) R_Sun of 695780 +/- 160 km and 959.34 +/- 0.22"; however, it is consistent with the solar radius recorded in 1715. The observed prominences are located at +23.5 +/- 22.5{\deg}, +66.5 +/- 22.5{\deg}, and -68.5 +/- 22.5{\deg} in the heliographic latitude. The appearance of prominences at such high latitudes contrasts with the sunspot butterfly diagram for 1725-1750, confirming 1733 as a solar minimum. These high-latitude prominences can potentially be attributed to the so-called 'polar rush' prominences that appear a few years after a solar minimum. If they are categorised as 'polar rush' prominences, the solar minimum must be re-dated to before 1733 May. Furthermore, the latitudes of at least two of the prominences reported by Wassenius enable their classification as quiescent prominences, suggesting the presence of a polarity inversion line in the polar regions in early 1733.
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All authors Hisashi Hayakawa, Mitsuru Sôma, Noortje Peek, Jean-Pierre Rozelot, Stanislav Gunár, Alexei Pevtsov

Fluid Dynamics

Exact dispersion relation for linear surface waves on arbitrary vertical shear

Kjell S. Heinrich and Simen Å. Ellingsen

We derive the formal solution to the dispersion relation for linear surface waves on a horizontal mean current with arbitrary vertical dependence. The problem is cast in a Green's function framework for the Rayleigh equation, neglecting viscosity but making no further approximations about the mean velocity profile. The solution is the dispersion relation in the form of a single, implicit equation relating – and containing only – the velocity profile, wave frequency, and wavenumber. By isolating curvature effects in a path-ordered exponential, we obtain a solution that serves as a natural starting point for systematic approximations. We demonstrate that our solution reduces to the expression found by Shrira (1993, J. Fluid Mech. 252, 565–584) in the deep-water limit, yields known asymptotic approximations, and recovers known analytical solutions in special cases.
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Computational Physics · Cross-listed from quant-ph

Adaptive Tensor Network Sampling for Quantum Optimal Control

Zeki Zeybek, Rick Mukherjee and Peter Schmelcher

Quantum optimal control (QOC) provides a systematic framework for achieving high-fidelity operations in quantum systems and plays a central role in tasks such as gate synthesis, state transfer, and pulse design. Existing QOC methods broadly fall into two categories: gradient-based and gradient-free algorithms. The associated optimization landscape is often high-dimensional, non-convex, and populated by numerous local minima, making efficient gradient-free search strategies essential. To address this, we introduce a gradient-free matrix product state/tensor train (MPS/TT) sampling heuristic for discrete quantum optimal control. In our approach, the MPS defines a score function over the space of discrete control parameters, which in turn induces a sampling distribution over candidate control sequences. This distribution is iteratively refined through selection of better performing sequences and local tensor updates to bias the search toward high-performing sequences. We evaluate the method on a range of benchmark problems, including single-qubit state transfer, Bell-pair preparation, qutrit gate implementation, and open-system population transfer. Across these tasks, the method exhibits stable convergence behavior and competitive empirical performance relative to established gradient-free baselines. These results suggest that tensor network sampling offers a viable heuristic framework for discrete quantum control.
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Fluid Dynamics

Stable fluid-rigid body interaction algorithm using the direct-forcing immersed boundary method (DF-IBM)

E. Farah, A. Ouahsine, P. G. Verdin et al. (+1 more)

The direct-forcing immersed boundary method (DF-IBM) algorithm previously developed by the authors is extended by coupling the Navier-Stokes equations with the Newton-Euler equations for rigid body dynamics within the DF-IBM framework. This coupling broadens the applicability of the previous development, from stationary or prescribed motion to flow-induced (free) motion cases. To address fluid-rigid body interactions under a partitioned approach, an implicit coupling algorithm is developed to handle strongly coupled interface conditions. Stability and convergence issues, particularly stemming from critical solid-fluid density ratios and from the rigid body approximation of internal mass effects in rotational dynamics, are mitigated using a fixed relaxation technique for the rigid body kinematics to ensure numerical robustness. Additionally, the proposed algorithm leverages the previously developed DF-IBM formulation and the predictor-corrector strategy of the pressure implicit with splitting of operators (PISO) algorithm by omitting the momentum predictor step and the costly corrector loops from the implicit iterations. The method is validated against several benchmark cases, demonstrating robustness, stability, and efficiency in capturing complex fluid-rigid body interactions across a range of challenging scenarios.
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All authors E. Farah, A. Ouahsine, P. G. Verdin, B. Kaoui

Medical Physics

Weighted Directional Total Nuclear Variation for Joint Yttrium-90 PET/SPECT Reconstruction with CTAC-derived Guidance

S Porter, D Deidda, D R McGowan et al. (+3 more)

Quantitative post-treatment activity imaging is essential for personalised dosimetry after Yttrium-90 selective internal radiation therapy (SIRT). Yttrium-90 PET offers high spatial resolution but is extremely low-count, whereas bremsstrahlung SPECT has higher count statistics but is degraded by blur, scatter, and septal penetration. Since both modalities image the same microsphere distribution, joint synergistic reconstruction can exploit their physical coupling, with CT attenuation correction (CTAC) providing additional anatomical guidance. We propose weighted directional total nuclear variation (w-dTNV), a joint variational regulariser for coupled PET/SPECT reconstruction with CTAC-guided anisotropy. w-dTNV penalises the nuclear norm of a dual-modality Jacobian, promoting co-located, geometry-consistent edges without forcing intensity correlation. Directionality is derived from the CTAC attenuation map $\mu$ and applied to PET/SPECT gradients, allowing efficient per-voxel spectral computations. PET/SPECT scale disparity is mitigated using data-driven modality normalisation from preliminary reconstructions. We evaluate w-dTNV on a NEMA IEC phantom with 20 bootstrapped PET noise realisations and on 9 post-SIRT patients with 45 lesions, against dTV, w-TNV, and sequential hybrid kernel expectation maximisation (SHKEM). In the phantom, w-dTNV improves recovery coefficients over dTV and w-TNV, and improves recovery over SHKEM for the smallest spheres. In patients, w-dTNV gives higher tumour-to-background ratios than SHKEM at comparable background activity. These results suggest that CTAC-guided synergistic variational coupling improves lesion recovery and clinical lesion contrast, offering a practical route towards more stable post-SIRT Yttrium-90 activity estimates for personalised dosimetry.
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All authors S Porter, D Deidda, D R McGowan, J Anton-Rodriguez, S Arridge, K Thielemans

Applied Physics

Reflector-Free, Highly Confined Love-Like SAWs Enabled by a Phononic Metasurface for Real-Time Monitoring of Cell Dynamics

Jessica Monaldi, Francis Kosior, Julio A. Iglesias Martínez et al. (+4 more)

Surface acoustic wave (SAW) devices are widely used in sensing and biosensing but generally suffer from strong attenuation in liquid environments. Conventional approaches rely on reflectors to reduce these losses, yet these components remain difficult to optimize: limited device miniaturization, and increase fabrication complexity. Here, we introduce an innovative design strategy that integrates a phononic metasurface with tailored electromechanical properties of the substrate to generate a type of shear-horizontal (SH) surface resonance modes that exhibit strong lateral confinement and extremely low radiation into both the substrate bulk and the free surface, eliminating the need for reflectors. This approach enables highly tailorable surface acoustic resonances with distinctive enhanced dynamic strain-energy confinement leading to significantly higher quality factors than conventional SAW devices, particularly in water-loaded conditions. We show the fabrication and experimental validation of the proposed phononic metasurface-based SAW resonator and showcase its biosensing capabilities through real-time monitoring of cellular death.
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All authors Jessica Monaldi, Francis Kosior, Julio A. Iglesias Martínez, Laurent Badie, Halima Alem, Frédéric Sarry, Mourad Oudich

Classical Physics · Cross-listed from math.DG

The linear Elasticity complex: a natural formulation

Romain Lloria and Boris Kolev

We reformulate the Elasticity complex and Saint-Venant's compatibility condition using the generalized differential complex of Dubois-Violette-Henneaux. This is just a slight and natural modification of the de Rham complex to take account of the index symmetry of the tensors involved. An integrating formula to recover the displacement from the strain and similar to the Poincaré formula is provided. Finally, a Hodge star operator and a dual complex is introduced, which allows to recover stress potentials in dimensions 2 and 3.
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Medical Physics

Potential pof laser-driven VHEEs towards FLASH radiotherapy: Monte Carlo dosimetric study of single-field pencil beam scanning of a brain tumor

Leonida A. Gizzi, Damiano Del Sarto, Federico Avella et al. (+5 more)

Radiotherapy with Very High Energy Electron (VHEE) beams is being extensively investigated for the treatment of deep-seated tumours, even in view of novel protocols based on the so-called FLASH effect. Laser WakeField Acceleration (LWFA) provides a compact and affordable accelerator technology for VHEE electron beams, featuring ultra-high instantaneous dose rates and holding the promise to provide Ultra-High (average) Dose Rates (UHDRs) needed to activate the FLASH effect, with major efforts ongoing worldwide to fulfill this promise. Therapeutic doses are already at reach, using pencil beams produced via LWFA. These beams typically exhibit significant energy spread, and small transverse size. These features are rather different from those of other beams considered so far in radiotherapy studies. In view of a rapid clinical translation of LWFA-VHEE beams it is therefore of paramount importance to assess the role of these properties in the dose delivery to the patient. Here we present a study carried out via start-to-end (PIC and Monte Carlo) simulations, of the main dosimetric features of a realistic laser-driven VHEE pencil beam targeted on a brain tumor. The entire tumor coverage is achieved by a scanning procedure; the dose pattern resulting from tessellation, i.e. the overlapping of adjacent beamlets, and the role of energy spread are thoroughly discussed. Dose Volume Histograms are presented, and their quality is discussed. The impact of the FLASH effect is also considered, introducing a degree of healthy tissue sparing in the modelling. Finally, the foreseen technological path toward the achievement of FLASH dose rates with LWFA-VHEE beams is briefly outlined.
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All authors Leonida A. Gizzi, Damiano Del Sarto, Federico Avella, Gabriele Bandini, Simona Piccinini, Daniele Panetta, Davide Terzani, Luca Labate

Applied Physics

Vectorial Acoustic Multiplexed Holography

Yuan Tian, Hao Ge, Jiangpo Zheng et al. (+3 more)

Encoding more information into wave fields is a central goal in imaging, communication, and wave control. Optical holography benefits from polarization multiplexing, but acoustic holography remains largely limited to pressure-only encoding because sound in fluids lacks naturally independent vector channels. Here, we show that particle velocity can serve as a practical multiplexing degree of freedom despite the intrinsic pressure-velocity coupling governed by the acoustic Euler equation. We develop a physics-informed inverse-design approach that incorporates acoustic propagation and pressure-velocity coupling to create a binary metasurface for vector-field acoustic holographic multiplexing. Experiments demonstrate dual-channel multiplexing on the in-plane velocity components v_x and v_y, and further extend to three-channel multiplexing by incorporating pressure p, with high-fidelity reconstruction and low cross-talk. This approach adds a new information dimension without reducing spatial or spectral bandwidth and enables broader forms of wave-based information encoding and multiplexed wave control.
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All authors Yuan Tian, Hao Ge, Jiangpo Zheng, Xiujuan Zhang, Ming-Hui Lu, Yan-Feng Chen

Physics and Society

Leadership, Cooperation and Conflicts in Physics – Research Leaders' Perspective

Maike Reimer, Bianca Burkert, Theresa Goerg et al. (+1 more)

Conflict in research teams was a near-ubiquitous phenomenon, with the three top issues being lack of respect or overconfidence, non-collegial behaviour and authorship. Most frequently involved (and perceived as most helpful) were informal sources of support, such as colleagues at the same institution and private contacts. Official institutional bodies were less often involved and often not perceived as helpful. In the majority of conflicts, there was no serious harm done to the research leaders involved, and qualification goals of conflicting parties could be reached. More wide-spread however were damages to research productivity such as delays or unpublished results. About two third of conflicts involved at least one person in a qualification process, demonstrating how inextricably research is linked with qualification, and that conflicts often occur in the complex entanglement of collaborative knowledge production and certification of individual research performance. Satisfaction with conflict development and its final resolution was fairly evenly distributed over the spectrum from complete dissatisfaction to complete satisfaction. Most research leaders changed their leadership practices in response to conflict experiences, showing that conflicts can be an opportunity to learn and grow.
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All authors Maike Reimer, Bianca Burkert, Theresa Goerg, Isabell M. Welpe

Instrumentation and Detectors

Investigation of the in-pixel response of the Mupix11 monolithic pixel sensor using a microfocus X-ray beam at Diamond Light Source

A.S. Rotelli, H. Augustin, D. Bortoletto et al. (+9 more)

MuPix11 is a High-Voltage Monolithic Active Pixel Sensor (HV-MAPS) developed for the tracking system of the Mu3e experiment. The in-pixel photon response of a MuPix11 sensor thinned to 70 {\mu}m was measured using an 8 keV X-ray beam with a 3 {\mu}m spot size at the B16 beamline at Diamond Light Source, emulating the passage of a minimum ionising particle (MIP). At nominal operating voltage and threshold, high-resolution scans across the pixel matrix show the detector response to be uniform. In the absence of reverse bias (0 V), the relative sub-pixel response is location-dependent as a reduced detection rate is observed at pixel boundaries.
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All authors A.S. Rotelli, H. Augustin, D. Bortoletto, A. Brooks, M. Grimes, A.J.A. Knight, M.S. Köppel, A.E. McDougall, R. Plackett, D M S Sultan, L. Vigani, S. Wood

Optics

Quantitative Pulse Shape-Instability Analysis Using 2D-Runs FROG

Pedram Abdolghader, Rana Jafari, Abinash Das et al. (+3 more)

We present a method for quantifying pulse-shape instability in a train of pulses using multi-shot Second-Harmonic-Generation Frequency-Resolved Optical Gating (SHG FROG). All versions of multi-shot FROG have previously shown the ability to distinguish stable from unstable pulse trains, as systematic differences appear between measured and retrieved traces when instability is present. This has proved possible because the recently introduced Retrieved-Amplitude N-grid Algorithmic (RANA) approach provides highly reliable pulse retrieval, even for unstable pulse trains and in the presence of noise, thus eliminating the possibility that algorithm stagnation, which mimics the effects of pulse-shape instability, could be confused for it. In other words, RANAs excellent performance ensures that any non-random discrepancies between measured and retrieved FROG traces reflect physical pulse-shape instability, rather than algorithmic stagnation. To begin to quantify such instability, we now introduce an instability parameter, R. It involves the use of the well-known statistical Runs test, which tests for systematic error in fits to one-dimensional (1D) data. A runs test counts the runs consecutive points in the plot of the difference between the data and fit with the same sign evaluating the goodness of the fit while minimizing the effects of random error. However, because FROG traces are functions of two variables, we must extend the usual 1D runs test to two dimensions, that is, to enumerate 2D runs hills and valleys in the difference between measured and retrieved 2D FROG traces. Many small 2D runs indicate only random noise-like differences and hence a stable pulse train, whereas few large runs reflect additional systematic error and hence pulse-shape instability.
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All authors Pedram Abdolghader, Rana Jafari, Abinash Das, Bilol Banerjee, E. P. Duchrist Crews, Rick Trebino

Geophysics · Cross-listed from nlin.CD

Estimating the Resilience of Non-Stationary Systems

Taylor Smith, Andreas Morr, Christof Schötz et al. (+1 more)

A wide body of work has applied the concept of critical slowing down to estimate the stability of different Earth system components. Most of them – such as global vegetation – are inherently non-stationary, for example due to strong seasonal forcing, which complicates the estimation of their resilience to external perturbations. Here, we introduce a new method to account for non-stationarity in estimating resilience for diverse synthetic and real-world data sets via a regression-based formulation of the Langevin Equation. Our method does not require extensive data pre-processing, is robust to gaps in the data record, and does not require regular time sampling. We further show that our method can incorporate time-varying data uncertainties, recover uncertainty bounds in stability estimates, and can be natively extended to examine spatial systems. Our method is a drop-in replacement for widely-used autocorrelation-based resilience estimates, and can be widely applied across Earth system components.
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All authors Taylor Smith, Andreas Morr, Christof Schötz, Niklas Boers

Fluid Dynamics

Numerical Investigation of Elastically-Mounted tandem Cylinders using an ALE Runge-Kutta Discontinuous Galerkin method

Alexios Papadimitriou, Spyridon Zafeiris and George Papadakis

This work presents a high-order Arbitrary-Lagrangian-Eulerian (ALE) Discontinuous Galerkin framework for simulating multi-body Vortex-Induced Vibrations. The ALE formulation extends a Runge-Kutta Interior-Penalty nodal DG solver with minimal additional computational overhead, incorporating discrete enforcement of the Geometric Conservation Law (GCL) to ensure free-stream preservation and Radial Basis Function (RBF) mesh deformation to handle large structural displacements. The framework is applied to elastically-mounted tandem cylinder configurations: a two-cylinder arrangement with cross-flow oscillations at Re=200, and a three-cylinder arrangement with two degrees of freedom at Re=150. In the three-cylinder case, the trajectories exhibit highly irregular behavior driven by complex wake interference, including a periodic attract-and-release mechanism governing the trailing cylinder's stream-wise response. Results are verified against established benchmarks through Lissajous curves, Poincaré phase maps, power spectra, and vortex shedding mode classification. An hp-refinement comparison demonstrates that increasing the polynomial order is more effective and computationally efficient than mesh refinement for capturing multi-body wake dynamics, as the low numerical diffusion of the high-order method preserves vortical structures over long distances on relatively coarse meshes. These findings highlight the importance of high-order methods for CFD-FSI applications where wake interactions drive the structural response.
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Atmospheric and Oceanic Physics

Amplified Urban Climate Extremes from Global Warming-Urbanization Synergy: A Physics-Informed Intelligence Paradigm

Qiuxia Wu, Yaqiang Wang and Huabing Ke

The nonlinear synergy between global warming and urbanization is amplifying extreme climate risks in cities worldwide. While observations and simulations confirm these compounding effects, two fundamental bottlenecks impede predictive understanding: (1) fragmented, case-specific perspectives that hinder the discovery of universal mechanisms, and (2) a methodological divide between computationally prohibitive high-resolution models and AI-based tools that lack physical interpretability at urban scales. This article advocates for a paradigm shift toward the deep integration of physical principles with data intelligence. To this end, we propose a transformative "Classification-Mechanism-Inference" (CMI) framework. Classification involves establishing a global urban "climate-morphology-development" typology to enable systematic comparison beyond isolated case studies. Mechanism advocates for physics-informed machine learning (PIML) as the core engine to develop efficient, physics-constrained surrogate models for uncovering nonlinear interactions. Inference leverages these models for high-throughput, tailored risk projection to directly inform context-specific adaptation planning. The CMI framework aims to bridge the cognitive and methodological gaps, thereby advancing urban climate science from phenomenological description towards mechanistic, predictive, and decision-relevant science, which is crucial for building climate-resilient cities globally.
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Chemical Physics

Vib2Conf: AI-driven discrimination of molecular conformations from vibrational spectra

Xin-Yu Lu, De-Yi Lin, Tong Zhu et al. (+3 more)

Retrieving or generating two-dimensional molecular structures on the basis of vibrational spectra has been well demonstrated via deep learning models. However, deciphering three-dimensional molecular conformations is still challenging, primarily due to spectral ambiguities caused by conformational heterogeneity, which are difficult to resolve. To address this limitation, we propose Vib2Conf, a deep learning model directly discriminating 3D molecular conformations from vibrational spectra. We implement an attentional resampler to distill conformation-sensitive features from sparse spectral signals, and integrate Mixture-of-Experts (MoE) to partition the conformational space for precise geometric mapping. These modules enable Vib2Conf to achieve state-of-the-art top-1 recall exceeding 95% on traditional spectrum-structure benchmarks, including QM9S, VB-Mols, and QMe14S. More importantly, Vib2Conf can discriminate near-isomeric conformers with a top-1 recall of 82.06% on VB-Confs test set, where conformational isomers differ by a root-mean-square deviation (RMSD) of only ~1 Å. In general, Vib2Conf is a promising method for fine-grained spectrum-to-conformation analysis.
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All authors Xin-Yu Lu, De-Yi Lin, Tong Zhu, Bin Ren, Hao Ma, Guo-Kun Liu

Applied Physics

Complementary-polarity double-layer LiTaO3 resonators for symmetry-selective SH2 excitation with ultrahigh electromechanical coupling (kt^2 = 25.7%)

Hao Yan, Zhen-hui Qin, Zhi-Wen Wang et al. (+7 more)

We report a structurally simple double-layer lithium tantalate (LiTaO3) bulk acoustic resonator that enables symmetry-selective excitation of the second-order thickness-shear (SH2) mode with ultrahigh electromechanical coupling. Two 31 deg Y-oriented single-crystal LiTaO3 films are rotation-bonded with complementary polarization (+X/-X) and driven by a longitudinal electric field. Matching between the effective piezoelectric symmetry and the SH2 mode yields an effective electromechanical coupling coefficient of kt^2 = 25.7% at 5.24 MHz. To our knowledge, this is the highest kt^2 reported for a LiTaO3 resonator architecture to date. The measured response is dominated by the target SH2 mode, with only weak parasitic features in the operating band. The structure is also tunable: the resonance frequency and coupling can be adjusted through geometric parameters while maintaining stable modal behavior, indicating good process tolerance. Finite-element analysis further suggests straightforward frequency scaling beyond 5 GHz by reducing the film and electrode thickness while preserving approximately 25% kt^2. In addition, introducing a SiO2 compensation layer is predicted to improve the temperature coefficient of frequency to approximately -25 ppm/deg C. These results establish complementary-polarity double-layer LiTaO3 as a practical platform for high-coupling, spurious-suppressed acoustic resonators and provide a scalable route toward wideband ultrasonic resonators, filters, and related transducers.
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All authors Hao Yan, Zhen-hui Qin, Zhi-Wen Wang, Shu-Mao Wu, Chen-Bei Hao, Hua-Yang Chen, Sheng-Nan Liang, Ke Chen, Si-Yuan Yu, Yan-Feng Chen

Computational Physics · Cross-listed from cs.LG

SolarTformer: A Transformer Based Deep Learning Approach for Short Term Solar Power Forecasting

Ankan Basu, Jyotiraditya Roy, Aditya Datta et al. (+2 more)

Accurate forecasting of solar power output is essential for efficient integration of renewable energy into the grid. In this study, an attention-based deep learning model, inspired by transformer architecture, is used for short-term solar power forecasting. Our proposed model, "SolarTformer", is designed to predict solar power output from meteorological data. Unlike traditional models, SolarTformer leverages self-attention mechanisms to effectively capture temporal dependencies and spatial variability in solar irradiance. In addition, the proposed methodology includes feeding power station-specific metadata into the model, which helps to generalize between power stations located at different locations and with different panel configurations and in different seasons. Our experiments demonstrate that SolarTformer significantly outperforms previous models on the same data set. In particular, the model exhibits strong performance on both clear and cloudy days, indicating high robustness and generalizability. These findings highlight the potential of attention-based architectures in enhancing the accuracy of solar forecasting, contributing to a more reliable management of renewable energy.
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All authors Ankan Basu, Jyotiraditya Roy, Aditya Datta, Prayas Sanyal, Sumanta Banerjee

Accelerator Physics

Resonant RF Wakefield Coupling for Radiation-Reaction Control of 3D Betatron Dynamics in Hybrid Laser Plasma Accelerators

A. A. Molavi Choobini and M. Shahmansouri

Hybrid laser plasma radiofrequency (RF) acceleration architectures signify a promising advancement in addressing the stability challenges associated with traditional laser wakefield accelerators. A thorough theoretical and numerical analysis of the three-dimensional dynamics of ultra-relativistic electron bunches in these hybrid systems is presented, clearly explaining how transverse beam stability, betatron oscillation polarisation, and radiative cooling work. By combining analytical models of spatiotemporal plasma wakefield modulation and phase dependent RF-driven oscillations with fully self-consistent 3D particle in cell (PIC) simulations, incorporating classical radiation reaction (RR) via the Landau Lifshitz model (with quantum parameter to account for synchrotron like losses during betatron oscillations. The findings indicate that the external RF fields operate as a tunable lattice, allowing for exact adjustment of amplitude, frequency, and carrier-envelope phase, which facilitates deterministic regulation of transverse focussing gradients and betatron amplitudes. A regime of resonant alignment between RF fields and natural betatron frequencies is established; this resonance enhances controlled transverse excursions while concurrently diminishing parasitic oscillations via increased radiative damping, resulting in substantial emittance reduction and the alleviation of synchrotron-like energy losses. Also, the detailed stability maps and 3D force landscapes show that the gamma factor growth rates change over time depending on the interaction between longitudinal field gradients and initial injection conditions. The paper's results give a clear picture of the nonlinear, resonant, and damping events that happen in hybrid accelerators. They also make it possible to get ultra stable, high-quality electron beams with the right polarisation states.
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Physics and Society

Functional Dismantling of Network Relaxation through Slow-Branch Susceptibility

Kaiming Luo and Huiying Zhou

Robustness of relaxation on asymmetric networks is not determined by connectivity alone, because the slow collective mode can be complex and may change its spectral identity under adaptive damage. We introduce a slow-branch susceptibility framework for functional dismantling of network relaxation. Starting from the projected relaxation dynamics, we show that the relevant robustness observable is the real part of the selected nonzero Laplacian branch, which controls the long-time decay of the nonstationary sector. Node deletion is then treated as a dimension-changing compression of the operator, leading to a modal susceptibility (MS) score that estimates the first-order reduction of the branch-tracked relaxation rate from the biorthogonal support of the slow mode. In the reciprocal limit, the same construction reduces to the weighted Fiedler sector, placing directed and weighted-undirected networks within a common spectral-response formulation. Tests on synthetic and real-world networks show that MS identifies vulnerability patterns that differ from standard centrality-based attacks and edge-level spectral proxies. These results resolve a modal-selection ambiguity in non-Hermitian robustness analysis and provide a spectral basis for functional dismantling in asymmetric networks.
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Chemical Physics

A Machine-Learned Symbolic Committor for a Chemical Reaction: Retinal Isomerization

Kai Töpfer, Gianmarco Lazzeri, Vittoria Ossanna et al. (+4 more)

The thermal cis-trans isomerization around the C$_{13}$=C$_{14}$ double bond of retinal is a prototypical high-barrier reaction whose mechanism hinges on subtle out-of-plane bending motions. We apply Artificial Intelligence for Molecular Mechanism Discovery (AIMMD) to N-retinylidene-lysine in vacuum, learning the committor from unbiased molecular dynamics trajectories generated by two-way shooting. Parametrizing the logit of the committor, rather than the committor itself, allows the neural network to resolve the reaction coordinate across the full transition region, not only at the isocommittor surface $p_B(\mathbf{x}) = 0.5$. Holdback input randomization identifies four proper dihedrals around the reactive bond as the informative coordinates, while the improper dihedrals at C$_{13}$ and C$_{14}$ prove unsuitable because reactant, transition, and product states share the same values. Symbolic regression then distills the network into compact analytical expressions and shows that a nonlinear coupling of all four dihedrals is required to reproduce the S-shaped, stepwise pathway seen in the transition path ensemble. This S-shape is absent from the minimum-free-energy path: it arises from the non-equilibrium dynamics of the short ($\sim 0.13$ ps) transition events combined with the mass asymmetry between heavy-atom and hydrogen-bearing dihedrals. An interpretable, machine-learned committor thus exposes dynamical features of the mechanism to which the free-energy surface is blind. The workflow requires no prior assumptions about the reaction coordinate and extends naturally to other isomerizations and to chemical reactions more broadly.
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All authors Kai Töpfer, Gianmarco Lazzeri, Vittoria Ossanna, Florian Renner, Gianluca Lattanzi, Roberto Covino, Bettina G. Keller

Fluid Dynamics

A Particle Multi-Relaxation Bhatnagar-Gross-Krook Method for Rarefied Monatomic Gas Mixtures

Inchan Kim, Joonbeom Kim, Woonghwi Park et al. (+1 more)

Kinetic models based on the Bhatnagar-Gross-Krook (BGK) framework provide an efficient alternative to the Boltzmann equation for rarefied gas flows; however, existing formulations for gas mixtures remain limited in representing pair-dependent relaxation processes and recovering correct Navier-Stokes-Fourier (NSF) transport behavior. A particle-based unified BGK (UBGK) model for monatomic gas mixtures is developed by extending the single-species UBGK framework to a multi-relaxation formulation. The model preserves the pairwise interaction structure of the mixture Boltzmann equation, enabling independent species-pair relaxations for an arbitrary number of species. The relaxation properties of the mixture UBGK model are determined by matching the production terms to those of the Boltzmann equation, ensuring correct NSF-level transport behavior. The model is implemented within the particle framework and validated against DSMC using four benchmark cases: homogeneous relaxation, Poiseuille flow, Couette flow, and hypersonic flow around a cylinder. The results demonstrate that the mixture UBGK model captures species-specific non-equilibrium effects, including species-dependent differences in velocity and temperature, across a range of mole fractions and Knudsen numbers in good agreement with DSMC. Furthermore, cost and accuracy analyses show that the mixture UBGK model becomes more efficient than DSMC at sufficiently large time step sizes, but its first-order accuracy suggests further improvement through higher-order schemes.
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All authors Inchan Kim, Joonbeom Kim, Woonghwi Park, Eunji Jun

Fluid Dynamics

Multilevel radial basis function surrogates for noise-robust DSMC-CFD coupling

Arshad Kamal, Arun K. Chinnappan, James R. Kermode et al. (+1 more)

Hybrid methods for simulating rarefied gas flows reduce computational cost by coupling a particle-based model, typically the direct simulation Monte Carlo (DSMC) method, to a continuum-based solver, i.e. a computational fluid dynamics (CFD) code. However, widespread adoption of these methods is hindered by numerical instabilities caused by statistical noise and difficulties in applying them to complex geometries. To be effective, a hybrid framework must be robust to noise, reliable in not introducing errors to the flow physics, automated, and flexible enough for complex spatial domains. Previous iterations of the micro-macro-surrogate-sparse (MMS-Sparse) method successfully addressed the first three requirements using Bayesian surrogate models to provide smooth, constitutive corrections to the CFD. However, they relied on global basis functions, limiting their applicability to relatively simple geometries. In this work, we address the fourth requirement - flexibility - by introducing a set of multilevel radial basis functions (RBFs) to represent the smooth corrections within the MMS-Sparse framework. Unlike global polynomials, multilevel RBFs can resolve broad and fine flow details locally, potentially allowing the method to be applied to more complex geometrical systems than previously considered. We couple this approach with a finite-volume CFD solver (OpenFOAM) and validate it using the rarefied lid-driven cavity flow problem. This serves as a rigorous test case for spatially two-dimensional coupling. Our results demonstrate that this enhanced MMS-Sparse method produces estimates in good agreement with benchmarks while retaining the noise-robust and automated benefits of the Bayesian approach.
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All authors Arshad Kamal, Arun K. Chinnappan, James R. Kermode, Duncan A. Lockerby

Optics

Compact fiber-based compression of a 1 W, 76 MHz Yb laser to 15 fs for broadband ultrafast applications

Marco Polastri, Erika Benedetti, Eva Arianna Aurelia Pogna et al. (+1 more)

We demonstrate a compact scheme for generating sub-20-fs pulses from a commercial ytterbium femtosecond laser delivering 80 fs pulses at 76 MHz repetition rate with 1 W average power. Spectral broadening is achieved in a photonic crystal fiber (PCF), followed by dispersion compensation using broadband chirped mirrors. By systematically varying the fiber length and coupled power, we investigate the interplay between nonlinear spectral broadening and higher-order dispersion. While the spectral bandwidth increases monotonically with fiber length, the achievable pulse duration exhibits a clear minimum due to accumulation of uncompensated higher-order phase, primarily third-order dispersion. An optimal fiber length of 80 mm yields nearly transform-limited 15.4 fs pulses. Shorter fibers provide insufficient broadening, whereas longer fibers, despite offering larger bandwidth, compromise the pulse temporal quality. Stable sub-20-fs operation is demonstrated at average powers exceeding 600 mW, and noise measurements indicate that the system performance is limited by the Yb seed laser. These results identify an optimal nonlinear interaction regime in PCF-based compression and establish a practical design rule linking spectral broadening to higher-order phase for compact ultrafast Yb laser sources.
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All authors Marco Polastri, Erika Benedetti, Eva Arianna Aurelia Pogna, Nicola Coluccelli

Optics

Bound state in the continuum induced room-temperature superfluorescence

Haijun Tang, Hamdi Barkaoui, Can Huang et al. (+6 more)

Superfluorescence is a collective emission from several quantum emitters that initially have random phases and are then synchronized through vacuum field interactions. Despite its fascinating prospects in quantum information processing, optical computing and advanced photonic devices, a key challenge in harnessing superfluorescence is alleviating its reliance on cryogenic conditions. Recently, room-temperature superfluorescence has been successfully achieved using upconverted nanoparticles and quasi two-dimensional lead halide perovskites. These approaches, however, are restricted to a few specific material designs and unsuitable for wide promotion. Here, we report a universal strategy to elevate the operating temperature of superfluorescence. We reveal that the symmetry-protected optical bound state in the continuum (BIC) can break the size limitation of superfluorescence ({\lambda}^3) and correlate distant but similar emitters without violating the selection rules, significantly accelerating synchronization process and promoting the possibility of room-temperature superfluorescence. This effect has been experimentally verified using a series of BIC metasurfaces made of different lead halide perovskites. Key features such as the quadratic increase in transient peak intensity and the reduction in pulse width and build-up time at the BIC wavelength confirm the realization of room-temperature superfluorescence that is absent in the pristine material. A theoretical model is also built to explain the experimental observations. This research demonstrates that the operating temperatures of coherent macroscopic states can be effectively improved by artificial field, paving a critical step towards constructing building blocks for optical and quantum applications.
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All authors Haijun Tang, Hamdi Barkaoui, Can Huang, Xiong Jiang, Yixuan Zeng, Shumin Xiao, Shaohua Yu, Jiecai Han, Qinghai Song

Optics

Three-core fiber Fabry-Perot resonator for dual-frequency comb generation

Thomas Bunel, Antonio Cutrona, Debanuj Chatterjee et al. (+12 more)

Fiber Fabry-Perot resonators have proven their ability to generate broad and stable optical frequency combs, and are ideal devices for fiber systems as they are high-Q, compact, and easily integrated with FC/PC connectors. Here, we present an advanced fiber Fabry-Perot resonator designed for multi-frequency comb generation and spatial multiplexing. The resonator is fabricated using a three-core optical fiber and is able to generate two mutually coherent frequency combs while being locked to a driving laser. Multiplexing of the combs is achieved with a fan-in/fan-out system, enabling a fully fiber-based experimental setup. The generated combs, induced by cavity solitons, feature a 1.27 GHz repetition rate and a bandwidth above 40 nm. A slight difference in the group index of each core leads to a 112 kHz repetition rate offset between the combs, enabling dual-comb spectroscopy proof-of-concept measurement of a 0.1 nm absorption band.
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All authors Thomas Bunel, Antonio Cutrona, Debanuj Chatterjee, Damien Labat, Vincent Andrieux, Geraud Bouwmans, Andy Cassez, Antonin Moreau, Julien Lumeau, Manal Arbati, Alexis Bougaud, Benjamin Wetzel, Alessia Pasquazi, Matteo Conforti, Arnaud Mussot

Applied Physics

Evidence of Micron-Scale Ion Damage in (010), (110), and (011) ${\beta}-Ga_2O_3$ Epitaxial Layers

Carl Peterson, Chinmoy Nath Saha, Yizheng Liu et al. (+2 more)

We report on the experimental observation of up to 11.5 ${\mu}m$ deep charge depletion in (010), (110), and (011) ${\beta}-Ga_2O_3$ epitaxial layers due to ion damage from sputtering and inductively coupled plasma (ICP) etching processes whereas charge depletion in (001) ${\beta}-Ga_2O_3$ epitaxial layers was minimal. The orientation-dependent reduction in CV-measured charge density was first observed in $NiO_x$ reactively sputtered heterojunction p-n diodes (HJDs). When compared to reference low-damage Schottky barrier diodes (SBDs), the sputtered HJDs showed a $9.4{\times}$ increase in the specific on resistance $(R_{on,sp})$ and 85% reduction in net donor concentration $(N_D - N_A)$ at zero bias for sputter-damaged HJDs on (010) epitaxial layers whereas HJDs on (001) remained unchanged. Similarly, sputtered SiO2 caused a reduction of $N_D - N_A$ 11.5 ${\mu}m$ deep into the (010) material. Next, SBDs were fabricated on ${\beta}-Ga_2O_3$ surfaces previously etched via a BCl3 based ICP process and compared to SBDs on un-etched surfaces. The (010) SBDs on etched surfaces exhibited a $7.7{\times}$ increase in $R_{on,sp}$ and a 91% reduction in $N_D - N_A$ at zero bias where the (001) etched diodes exhibited little change. Additionally, (110) and (011) diodes fabricated on ICP damaged surfaces also saw a ~82% reduction in $N_D - N_A$ at zero bias, indicating (110) and (011) are also susceptible to ion damage. Damage in the (010), (110), and (011) diodes is potentially caused by energetic ions that travel into the open channels present along the [010] direction and create compensating point defects which could potentially diffuse further.
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All authors Carl Peterson, Chinmoy Nath Saha, Yizheng Liu, James S. Speck, Sriram Krishnamoorthy