Computational Physics · Cross-listed from astro-ph.IM
A Scalable Fast Multipole Method Poisson Solver for the RAMSES code: I. Unigrid Algorithm
Jun-Young Lee and Romain Teyssier
We present a scalable Poisson solver with $O(N)$ complexity based on the fast multipole method (FMM) implemented in RAMSES. Our FMM constructs a hierarchy of FMM grids on top of the pre-existing Cartesian grid which is used to compute the force for hydrodynamics or particle-mesh simulations. In contrast to the $O(N)$ multigrid solver (MG) - an iterative method that requires multiple V-cycles through a multi-resolution hierarchy of Cartesian grids - the FMM
algorithm performs just one upward pass through the same hierarchy, during which multipole expansions are accumulated and shifted, followed by a single downward pass, in which local expansions are propagated. Numerical tests indicate that FMM attains accuracy comparable to that of MG for smooth potentials and is particularly well-suited for problems with isolated boundary conditions, since it avoids the approximate Dirichlet boundary conditions required by MG schemes. Although in theory FMM requires around 30 times more floating-point operations than MG, its higher arithmetic intensity leads to comparable performance and better scalability relative to MG.
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Computational Physics · Cross-listed from quant-ph
Computational regimes in matrix-product-state-based quantum trajectory simulations
Aaron Sander, Simon Cichy, Martin Eigel et al. (+4 more)
Efficient simulation of open
quantum systems is central to modeling noisy
quantum hardware and many-body dynamics. In trajectory-based tensor network methods, cost is often associated with trajectory-level quantities such as
entanglement growth or bond dimension. However, the total cost of a fixed-accuracy simulation also depends on statistical sampling, and the interplay between per-trajectory complexity and sampling effort remains poorly understood. Here we introduce a cost-resolved framework for
matrix product state (MPS)-based
quantum trajectory simulations that decomposes total cost into
memory per trajectory, runtime per trajectory, and sampling effort. We show that physically equivalent
stochastic unravelings of the same Lindblad dynamics do not necessarily reduce total cost, but instead redistribute cost between trajectory complexity and statistical convergence. This trade-off is quantified by two dimensionless inflation factors: a bond dimension inflation $\alpha$ and a sampling inflation $\kappa$, which together determine the preferred unraveling under hardware-dependent
memory and parallelism constraints. We provide a practical protocol for extracting $(\alpha,\kappa)$ from modest pilot simulations and demonstrate it using
benchmarks across multiple noise channels. The resulting decision maps show that the computationally favorable unraveling can change with noise strength, time-step resolution, system size, and available parallelism. These results establish unraveling choice as a hardware-aware simulation design problem rather than an intrinsic optimization of trajectory
entanglement alone.
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All authors Aaron Sander, Simon Cichy, Martin Eigel, Jens Eisert, Maximilian Fröhlich, Tom Peham, Robert Wille
Atomic Physics · Cross-listed from cond-mat.quant-gas
Digital programming of spin correlations in a fermionic lattice quantum simulator
Yann Kiefer, Lars Fischer, Zijie Zhu et al. (+2 more)
Analog
quantum simulation provides a highly controlled platform to study diverse
quantum many-body phenomena. However, current methods for state initialisation are limited to thermal ensembles or uncorrelated product states. Here we present a hybrid approach that complements analog preparation with a digital
quantum-gate protocol. This approach enables the engineering of target states with specific, long-range
spin-correlations from the same initial resource state. By applying collisional gates to adiabatically prepared and filtered four-
fermion singlet chains, we program diverse
spin-correlation patterns, including that of a Heisenberg chain. We measure the
spin correlations using a sequence of
quantum gates followed by singlet-pair measurements. Our method paves the way to the targeted preparation of strongly correlated states of matter.
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All authors Yann Kiefer, Lars Fischer, Zijie Zhu, Konrad Viebahn, Tilman Esslinger
Applied Physics · Cross-listed from cond-mat.mtrl-sci
Electrostatic Charge Model for Dual-Layer Oxide Thin-Film Transistors
Måns J. Mattsson, John F. Wager and Matt W. Graham
A simple electrostatic two-equation model for dual-layer thin-film transistor (TFT) operation is developed. The model distributes electrostatic charge between the top and bottom
semiconductor layers, and the resulting transfer and mobility curves accurately simulate experimental dual-layer a-IGZO/a-IZO TFT operation. The model further provides an analytic expression that maps charge confinement in the high-mobility a-IZO bottom
semiconductor layer with the a-IGZO top-layer thickness and the conduction-band offset. By considering both a-IGZO/a-IZO layer charge partition and competing thickness-dependent oxygen vacancy trap density effects, the model suggests an optimal a-IGZO layer thickness of 9 to 12 nm. Importantly, this general electrostatic model extends to most dual-layer TFT systems and calculates how the top
semiconductor layer TFT turn-on voltage changes sharply with the conduction band offset and layer thickness.
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Accelerator Physics
Intrabeam scattering studies with large-emittance-ratio ion beams in the Relativistic Heavy Ion Collider, and implications for the Electron-Ion Collider
Yun Luo, Brendan Lepore, Kevin Mernick et al. (+2 more)
The Electron-Ion
Collider (EIC), to be constructed at Brookhaven National Laboratory, will collide polarized, high-energy electron beams with
hadron beams, achieving peak luminosities of up to $1.0 \times 10^{34}$ cm$^{-2}$ s$^{-1}$. To reach such luminosity, the EIC will employ flat-beam collisions at the interaction point. The design transverse emittance ratio will be about 10:1 in the
Hadron Storage Ring (HSR). Thanks to
stochastic cooling and precise decoupling, we successfully generated and accelerated gold ion beams with a large emittance ratio of 11:1 in the Relativistic Heavy Ion
Collider (RHIC). In this article, we present results of intrabeam scattering (IBS) measurements and modeling for large-emittance-ratio gold ion beam, both with and without controlled betatron coupling. To model the IBS growth, we use the formulas developed by Lebedev and Nagaitsev.
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All authors Yun Luo, Brendan Lepore, Kevin Mernick, William Bergan, Sergei Nagaitsev
Computational Physics · Cross-listed from cs.LG
A fully GPU-based workflow for building physics emulators of hypersonic flows
Fabian Paischer, Dylan Rubini, Deniz A. Bezgin et al. (+6 more)
The ability to resolve complex physical phenomena with high fidelity and at low computational cost is central to addressing key challenges in modern engineering. A
prime example lies in hypersonic flows, where the precise prediction of the full flowfield
topology, in particular with respect to shock wave location and intensity, is critical. Yet supersonic and hypersonic flows continue to be a stumbling block for traditional reduced-order models and neural emulators that struggle to capture steep gradients in flow states with physical consistency in applications of industrial relevance. To that end, we introduce a fully GPU based workflow that integrates accelerated data generation with the training of neural emulators augmented by uncertainty quantification and physics-aware refinement. Our workflow is enabled by a differentiable high-fidelity solver (JAX-Fluids) which we employ for rapid
dataset creation and residual-based improvement of the neural emulator to enhance physical consistency. Building on this framework, we first present a suite of model architectures and analyze their scaling behavior to expose their strengths and shortcomings. We then show that residual-based refinement enables training on cases where only mesh and input parameters are available, substantially reducing residuals and improving physical consistency. Together, differentiable simulation and residual-based refinement yield physics emulators that remain reliable beyond their training distribution, a key requirement for deploying surrogates in real-world engineering design loops.
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All authors Fabian Paischer, Dylan Rubini, Deniz A. Bezgin, Aaron B. Buhendwa, David Hauser, Florian Sestak, Johannes Brandstetter, Sebastian Kaltenbach, Nikolaus A. Adams
Biological Physics · Cross-listed from cond-mat.soft
Controlling Defects and Probing Dynamics in Active Nematics with Deep Reinforcement Learning
Russ Islam, Kyogo Kawaguchi and Yuto Ashida
Topological defects govern much of the flow behavior and orientational order in active nematics, making their control relevant for active matter physics, smart materials, and microfluidics. Applied activity patterns can induce self-propulsion of active nematic defects, but general-purpose methods for exploiting this effect to control defects remain largely unexplored. Here we use deep reinforcement learning (RL) to perform minimum-time position control of +1/2 defects in hybrid
lattice Boltzmann simulations of active nematodynamics. Spatiotemporally patterned activity, implemented as a control field in the active stress, steers defects through microchannel geometries and reveals finite-time reachable regions of defect position space. Reachability is shaped by director anisotropy, homeotropic wall anchoring, and the allowed activity patterns: local patterns steer defects in free domains but fail in junctions, whereas global patterns open otherwise inaccessible channels. In constrained geometries, the original defect may be unable to reach some goals intact, but controlled pair creation enlarges the effective reachable set by transferring control to a newly created +1/2 defect. The trained RL controllers outperform static and rule-based baselines, and controllers trained only on simple junctions can be combined without fine-tuning into a meta-controller that successfully steers defects through a larger test maze. Free energy visualizations show that guided defects write persistent, history-dependent distortions into the director field that can later be partially erased by -1/2 defects. Thus, RL-based control uncovers how confinement, anchoring, actuation geometry, and defect creation determine reachable motion in active nematics, providing a framework for other control tasks in soft and active matter.
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Physics and Society
A model of local and global reciprocity
Mari Kawakatsu, Yohsuke Murase, Taylor A. Kessinger et al. (+1 more)
We often decide how to treat friends based on observations of their past behavior, whereas actions toward strangers are typically guided by their public reputations. These two kinds of information underlie two classical mechanisms for the
evolution of cooperation$\unicode{x2014}$direct and indirect reciprocity$\unicode{x2014}$which have largely been studied in isolation. They are not interchangeable: we can recall the past actions of only a small circle of close contacts, whereas for the far larger pool of strangers we must rely on public reputations. Here we develop a mathematical framework built on this distinction. Each individual engages in direct reciprocity in local games within a finite neighborhood of friends, whose actions they observe directly, and in indirect reciprocity in global games with a large population of strangers, known only by reputation. Separating local and global interactions allows us to address two questions. First, can cooperation persist under a cognitively simple norm of judgment? We show that combining direct and indirect reciprocity resolves the scoring dilemma: conditional cooperators resist invasion by both unconditional cooperators and unconditional defectors, where indirect reciprocity alone would fail. Second, how should one treat a friend whose past behavior conflicts with their public reputation? We find that the strategies that maximize cooperation are forgiving$\unicode{x2014}$overlooking whichever piece of information is unfavorable$\unicode{x2014}$and that these forgiving strategies can often remain robust to invasion. By distinguishing between local and global scales of interaction and integrating information across them, our framework offers a more cognitively realistic account of how reciprocity sustains cooperation.
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All authors Mari Kawakatsu, Yohsuke Murase, Taylor A. Kessinger, Joshua B. Plotkin
Plasma Physics
Robust Control of ECH Deposition Profiles on DIII-D
H.J. Farre-Kaga, A. Rothstein, K. Yasoda et al. (+5 more)
Electron Cyclotron Heating (ECH) is a key actuator in DIII-D and future tokamaks that provides auxiliary heating, localized current drive for scenario development and MHD stability, and even
impurity pump-out. Due to its control flexibility and applications, a gyrotron optimization
algorithm was developed to multitask and fine-tune the deposition location and heating power of each gyrotron while providing robustness to hardware failure. The ECH Optimization (ECHO)
algorithm finds the optimal gyrotron mirror angle and power to achieve a target ECH radial deposition profile. This optimization is accomplished in real-time using a parallelized neural network surrogate of the TORBEAM code combined with a genetic optimizer. This has been deployed in a DIII-D experiment and has been validated with experimental ECE measurements and post-experiment offline ray tracing, showing the
algorithm's reliability despite gyrotron failures and significant changes to
plasma parameters.
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All authors H.J. Farre-Kaga, A. Rothstein, K. Yasoda, J. Lestz, N. Chen, S.K. Kim, A. Jalalvand, E. Kolemen
Chemical Physics
Spin-Polarized Oxygen Evolution in Chiral-Molecule-Modified Plasmonic Photoanodes
Priscila Vensaus, Milad Sabzehparvar, Fatemeh Kiani et al. (+2 more)
Photoelectrochemical oxygen
evolution is limited not only by multi-electron charge-transfer kinetics but also by the
spin constraints associated with forming triplet O2. Here, we used a hybrid photoanode architecture to demonstrate
spin-polarization of plasmonic hot holes through a
chiral molecular layer. TiO2 photoanodes were modified with achiral Au nanoparticles to introduce visible-light plasmonic absorption, functionalized with cysteine as a
chiral molecular interface, and coated with a NiFe-based oxygen-
evolution catalyst. Wavelength-resolved photo-scanning electrochemical microscopy was used to directly detect locally evolved O2 under operando illumination while simultaneously monitoring the photoanode current.
Chiral functionalization with homochiral L-cysteine enhanced both photocurrent and local O2
evolution relative to racemic DL-cysteine controls. The chirality-dependent enhancement was most pronounced under visible excitation overlapping the Au plasmon resonance, including a 130% photocurrent increase. These results provide evidence that
chiral molecular layers, often used for
chiral nanoparticle synthesis, can directly modulate plasmon-derived hot-carrier transfer through the
chiral induced
spin selectivity (CISS) effect. This work establishes a
chiral plasmonic photoelectrochemical platform for coupling hot-carrier generation to
spin-dependent water oxidation.
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All authors Priscila Vensaus, Milad Sabzehparvar, Fatemeh Kiani, Germán García Martínez, Giulia Tagliabue
Chemical Physics
Orbital-optimized density functional calculations of excited electronic states: Recent advances and perspectives
Lorenzo Restaino, Giulia Gamboni, Elli Selenius et al. (+1 more)
Orbital-optimized (OO) density functional calculations provide a time-independent, variational route to electronic excitations, alternative to presently widely used time-dependent density functional theory (TDDFT) approaches. As the orbitals are optimized in a state specific way, these methods can provide a balanced description of excited states with different character, thereby overcoming several limitations of practical implementations of TDDFT. Driven by recent developments in
algorithms for obtaining excited states as saddle points on the electronic energy surface, OO methods have attracted increasing interest, maturing into an active and rapidly expanding area of research. Here, the theoretical foundations of the approach are clarified and an overview of recent methodological developments in excited-state orbital optimization is provided. An overview of methods for treating open-shell singlet excited states and current approaches for computing transition properties and spectra is also provided. Finally, recent applications to molecular Rydberg, charge-transfer, and core excitations are reviewed, with the aim of assessing the present accuracy and range of applicability of OO density functional calculations with common exchange and correlation functionals.
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All authors Lorenzo Restaino, Giulia Gamboni, Elli Selenius, Gianluca Levi
Optics
Electric-Field Mapping of Optically Perturbed CdTe Radiation Detectors
Adriano Cola, Lorenzo Dominici and Antonio Valletta
In radiation detectors, the spatial distribution of the electric field plays a fundamental role in their operation. Access to this field distribution is of strategic importance, especially when investigating the perturbing effects induced by incident radiation. For example, one dangerous effect that prevents their proper operation is the accumulation of internal space charge. Here, we probe the two-dimensional electric field in a Schottky CdTe detector using the Pockels effect and report on its local perturbation after exposure to an optical beam at the anode electrode. Our electro-optical imaging setup, together with a custom processing routine, allows the extraction of the electric-field vector maps and their dynamics during a voltage bias-optical exposure sequence. The results are in agreement with numerical simulations, allowing us to confirm a two-level model based on a dominant deep level. Such a simple model is indeed able to fully account for both the temporal and spatial dynamics of the perturbed electric field. This approach thus allows a deeper understanding of the main mechanisms affecting the non-
equilibrium electric-field distribution in CdTe Schottky detectors, such as those leading to polarization. In the future, it could also be used to predict and improve the performance of planar or electrode-segmented detectors.
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Optics
Temporal glide symmetry enforces a parity sideband selection rule in scalar bulk media
Miguel Camacho
Symmetry is a powerful way to control coupling between photonic mode families. In spatially periodic structures, glide
symmetry can protect band contacts and suppress stop bands. Here we show a different role for temporal glide, a spatiotemporal counterpart combining reflection with a half-period time translation. In a scalar time-modulated trilayer waveguide, temporal glide imposes an exact selection rule linking frequency conversion to transverse-mode
symmetry: the parity content of every Floquet eigenstate alternates with sideband index, up to a state-dependent sign. In scattering, this means that a mode of definite parity can emit only into the opposite transverse parity at odd sidebands and into the same parity at even sidebands. We verify the rule directly in bulk Floquet eigenstates and in finite-section time-domain simulations. An incident odd waveguide mode is converted into an even frequency sideband, while all
symmetry-forbidden output channels at the analysed sidebands are suppressed to numerically negligible values. Rather than acting as a temporal copy of spatial-glide band sticking, temporal glide provides a distinct
symmetry principle for converting electromagnetic energy between selected modes and frequencies.
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Fluid Dynamics
Effects of mean flow skew on turbulent shear layers. Part II. Experimental investigation
Dipendra Gupta, Vedant Kumar, Johan Larsson et al. (+1 more)
Planar turbulent mixing layers, formed by the interactions of two parallel streams with different velocities, have been studied far more than three dimensional (3D) turbulent mixing layers, in which the incoming streams are skewed, and not parallel. Yet many practical shear flows are 3D. Here, we develop and validate an experimental methodology to generate and characterize skewed turbulent mixing layers and to quantify how mean-flow skew modifies mixing layer dynamics. We introduce skew with a spanwise deflection of the mean flow using turning vanes mounted near the trailing edge of a splitter plate, and we use cross-wire anemometry to investigate the downstream
evolution of the flow. Relative to the planar configuration, the skewed mixing layer exhibits systematic reductions in both mean and turbulent quantities, with deviations reaching approximately 40%. Despite these quantitative differences, the fundamental characteristics of the mixing layer remain largely unchanged. Mean-velocity profiles collapse under similarity scaling, shear-layer thicknesses retain approximately linear downstream growth, and Reynolds-stress profiles preserve their characteristic near-Gaussian form. Townsend's structure parameter, which quantifies the efficiency of turbulent momentum transport, remains approximately invariant between the planar and skewed configurations, in contrast to skewed turbulent boundary layers, wherein comparable mean flow skewing reduces the parameter by approximately 30%. These results indicate that mean flow skew modifies turbulent mixing layers quantitatively while exerting only a secondary influence on their underlying dynamics. This study establishes a controlled experimental framework and empirical
benchmark for future investigations of three-dimensional free-shear
turbulence.
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All authors Dipendra Gupta, Vedant Kumar, Johan Larsson, Gregory P. Bewley
Chemical Physics · Cross-listed from cond-mat.soft
Exploratory digital alchemy for colloidal crystal discovery
Shih-Kuang Lee, Sun-Ting Tsai and Sharon C. Glotzer
Digital Alchemy (DA), introduced by Van Anders et al., is a statistical mechanics-based generalized thermodynamic ensemble method that employs computer simulations to optimize colloidal particle design. This approach applies the principles of statistical mechanics to predict and tailor particle attributes that lead to desired self-assembled structures or material properties. However, as an inverse design method, its main limitation is that the target structure must be known a priori. Therefore, the optimal design from DA does not guarantee the targeted structure is the most or the only stable one. This highlights the importance of forward design with an exploratory scheme for optimizing novel colloid designs, which becomes more suitable in such cases. In this paper, we introduce Exploratory Digital Alchemy (EDA), an enhanced forward design scheme that begins by releasing the constraint of the target
crystal from DA, followed by an exploration-oriented bias that has been extensively used in enhanced sampling methods such as metadynamics (MetaD). We demonstrate the utility of EDA through examples involving particles interacting via a two-dimensional Lennard-Jones Gauss potential (LJGP) and a three-dimensional oscillating pair potential (OPP). We applied EDA to study the free energy landscapes given different potential parameters of LJGP at different temperatures. With the exploratory scheme, we've also successfully identified a wide range of OPP potential parameters that stabilize metastable Frank-Kasper phases. Our approach fuses the standard DA framework with metadynamics, which could potentially be useful for studying alchemical reactions in a generalized ensemble.
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Fluid Dynamics · Cross-listed from cond-mat.soft
Limits of constant-parameter constitutive models for hydrogels under inertial cavitation
Tianyi Chu, Joseph Beckett, Zhiren Zhu et al. (+2 more)
Mechanical characterization of soft materials at high strain rates is challenging due to their high compliance, nonlinear viscoelastic behavior, and potentially history-dependent responses. Inertial microcavitation rheometry (IMR) addresses this challenge by coupling
laser-induced cavitation (LIC) experiments with numerical simulations of bubble dynamics models to infer constitutive models and material parameters. Both IMR and its variants infer parameters that depend on the chosen fitting window, which suggests that a constant-parameter constitutive model is insufficient to describe the full cavitation event. We use this window dependence to identify when the constant-parameter assumption fails, rather than to report a single effective parameter set. The constitutive parameters are estimated over moving, overlapping windows using a modified iterative ensemble Kalman smoother with multiple data assimilation (MIEnKS-MDA). Within the neo-Hookean Kelvin–Voigt (NHKV) constitutive model, we obtain time-resolved estimates of the constitutive response in polyacrylamide (PAAm) hydrogels with different crosslinker concentrations. The inferred shear modulus and viscosity generally decrease and then plateau during cavitation, while exhibiting relatively weak temperature sensitivity. For gelatin gels, by contrast, the inferred property
evolution shows a pronounced temperature dependence, with distinct trends at low and high temperatures. Moreover, both the apparent shear modulus and viscosity exhibit significant variations during the first two bubble collapses. These results show that time-resolved parameter estimation within the prescribed NHKV constitutive structure can diagnose where the constant-parameter model assumption falls short during cavitation, thereby guiding the development of improved physics-based models of complex bubble–material interactions.
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All authors Tianyi Chu, Joseph Beckett, Zhiren Zhu, Jonathan B. Estrada, Spencer H. Bryngelson
Physics and Society · Cross-listed from cs.CY
The Tone of Awareness: Topic, Sentiment, and Toxicity Maps During Mental Health Month on TikTok
Henrique Ferraz de Arruda, Andreia Sofia Teixeira, Pranay Gundala Reddy et al. (+3 more)
Despite raising concerns about the mental health effects associated with the usage of TikTok, little is known about how related content is framed by creators and received by audiences. We collect the content of 28,341 TikTok
videos and 80,130 comments from Mental Health Awareness Month (May) in 2023 and 2024 via the TikTok Research API, and study how the tone of awareness varies across topics and years. We characterize "tone" as the emotional and interpersonal framing of mental health discourse, operationalized through sentiment and toxicity measures. We extract topics from
video text using BERTopic and log-odds keywords, then quantify topic-conditioned sentiment (XLM-T) and toxicity (Detoxify) separately for
video transcriptions and comments. Sentiment captures the affective valence of content, while toxicity reflects the presence of harmful or abusive
language. We find a stable set of recurring themes across years, spanning clinical conditions, emotional disclosure, self-care, and campaign-oriented content, with engagement highly skewed toward a small subset of topics. All sentiment and toxicity analyses are computed separately for
video content and comments, allowing us to distinguish between content production and audience reception. Sentiment in
videos is often negative for emotionally charged topics, while comments tend to shift toward more mixed or positive polarity, especially for suicide prevention. Toxicity is low in median overall, but exhibits longer-tailed outliers in comments than in
videos that are more pronounced in comments and concentrated in specific topics (e.g., "Duet", "Suicide Prevention", and "Psychisch"). Overall, our results provide a topic-level decomposition of mental health discourse on TikTok during awareness-month campaigns.
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All authors Henrique Ferraz de Arruda, Andreia Sofia Teixeira, Pranay Gundala Reddy, Anindya Mondal, Kleber Andrade Oliveira, Filipi Nascimento Silva
Optics
Programmable Synthetic Motion at a Time-Varying Interface
A. C Harwood, D. Cielecki, T. V. Raziman et al. (+3 more)
Space-time metamaterials that exhibit synthetic motion promise arbitrary control over the momentum, frequency and energy of scattered light, but realising the required space-time modulation in a programmable way remains a challenge. Here we program synthetic motion using a single spatial light modulator in a 4f geometry which imprints a continuously tunable pulse-front tilt onto a high-intensity pump pulse, inducing reflectivity modulations at a sub-wavelength indium tin
oxide thin film with synthetic velocities spanning the sub- and superluminal regimes. The angle-resolved spectrum of a scattered probe pulse reveals space-time diffraction patterns whose gradient and bandwidth vary continuously with synthetic velocity, in excellent agreement with theory. Splitting the shaped pump into two independently controlled pulses yields space-time double-slit diffraction with tunable fringe separation and frequency-momentum gradient. This programmable platform opens a path towards non-linear and periodic space-time trajectories for tabletop analogue studies of relativistic phenomena and space-time metasurfaces.
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All authors A. C Harwood, D. Cielecki, T. V. Raziman, S. A. Maier, S. Vezzoli, R. Sapienza
Data Analysis and Statistics · Cross-listed from cond-mat.mtrl-sci
Symmetry-electronic fingerprints reveal competing magnetic phases in two-dimensional materials
Addis Fuhr, Zachary R. Fox, David Parker et al. (+1 more)
Two-dimensional magnets offer compelling platforms for spintronics and
quantum technologies, yet predicting their
magnetic ground states, moments, and anisotropy remains challenging. This limitation primarily arises because existing machine-learning representations encode chemical environments without capturing the
symmetry or exchange physics that govern magnetism. In this work, we introduce the
symmetry-electronic fingerprint (SEF), a physically interpretable representation that encodes crystallographic
symmetry operations, Wyckoff-site geometry, together with site-resolved electronic structure. Combined with ensemble learning with random forests, the SEF accurately classifies
magnetic ordering while regressing moments alongside anisotropy energies while simultaneously resolving the distinct regimes of itinerant Stoner ferromagnetism from localized superexchange. What sets the SEF-trained models apart is that regions of elevated model uncertainty are not a failure but a diagnostic, identifying materials where these mechanisms compete. First-principles calculations on Co- and Ni-based halides and
oxides confirm that these regions correspond to genuine near-degenerate FM and AFM phases with
magnetic frustration, suppressed anisotropy, and emergent non-collinear ordering. By encoding
symmetry together with exchange physics directly into the representation unlike conventional descriptors, the SEF transforms model uncertainty into a compass pointing toward two-dimensional materials where small perturbations drive transitions between collinear, frustrated, or non-collinear
magnetic phases.
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All authors Addis Fuhr, Zachary R. Fox, David Parker, Ayana Ghosh
Applied Physics · Cross-listed from cond-mat.mes-hall
Micron-sized magnonic 3-port rectilinear circulator
Kevin S. Weber, Loic Temdie, Vincent Castel et al. (+6 more)
The development of miniaturized non-reciprocal microwave technologies compatible with integrated
circuit architectures remains a critical challenge for modern information technology. Here, we present the first experimental characterization of a micron-sized prototypical
magnon circulator. Taking advantage of the
chiral excitation of
spin-waves via nanowire gratings, we propose an original design of a circulator involving three channels of rectilinear and unidirectional
spin-wave beams. We demonstrate via a full 3-port
spin-wave spectroscopy a genuine
spin-wave circulation between the three ports. The narrow frequency band of operation can be tuned over a broad range of frequencies ($2$-$8$ GHz) with both an external field of up to $100$ mT, and the dimensions of the grating specifying the wavevectors. This proposed scheme opens up possibilities for new architectures of integrated and miniaturized non-reciprocal microwave devices.
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All authors Kevin S. Weber, Loic Temdie, Vincent Castel, Timmy Reimann, Morris Lindner, Carsten Dubs, Yves Henry, Matthieu Bailleul, Vincent Vlaminck
Physics Education · Cross-listed from gr-qc
Beyond the Metric: Geometrical Measurability as a Constraint on Quantum Gravity
Matteo Tuveri
This paper develops an epistemological constraint on
quantum gravity grounded in the empirical meaning of general relativity. The central claim is that a complete recovery of general relativity requires an effective metric, a continuum limit, or Einstein-like dynamics together with the physical conditions under which relational geometrical quantities can be objectively determined. These conditions concern the dynamical stability of measuring devices and reference systems, causal accessibility among physical systems, record formation, and invariance under admissible descriptions. In classical general relativity, they are usually implicit in the use of clocks, rods, light signals, freely falling bodies, detectors, and gauge-invariant observables. In
quantum gravity, however, they become non-trivial because spacetime geometry may be emergent, effective, thermodynamic, relational, or frame-dependent. This claim is developed through four cases: Rindler horizons and the Unruh effect, black-hole thermodynamics and Jacobson's equation-of-state derivation, gravitational-wave detection, and Weyl and conformal
gravity. The latter is discussed as a critical limiting case in which conformal invariance raises a sharp question about whether scale-dependent measurements of space and time can be physically fixed. Implications for
quantum gravity are also discussed using emergent
gravity and
quantum reference frames as examples. The perspective developed in the study suggests a general epistemological constraint on
quantum gravity: any viable approach must recover the physical possibility of objective geometrical measurement together with geometry itself.
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Accelerator Physics · Cross-listed from cond-mat.supr-con
Current patterns and loss contributions in CORT cables carrying AC current
Benoît Vanderheyden, Julien Dular, Louis Denis et al. (+4 more)
Conductor-on-round-tube (CORT) cables are a potential solution for carrying AC power in a small cross-section. Due to the geometry of the cable and the helical arrangement of the coated conductors (CC), the current follows a non-trivial pattern inside each CC. For instance, for the case of a single-layer cable, the current flow is mostly axial along the outer face of the CCs and mostly azimuthal along their inner face. Such a current distribution, known as the Garber current pattern, affects the transport AC losses. In numerical models, commonly adopted simplifications are either based on straight conductors or infinitely thin CCs. Such approaches neglect the Garber current pattern and thus misrepresent both the detailed current flow within the CC and the resulting 3D distribution of the fields. In this work, the detailed 3D current distribution in the CCs is investigated in a one-layer CORT cable, as a function of the cable geometrical parameters such as the conductor thickness, the pitch angle, and the gap between adjacent CCs. In particular, the impact of the Garber current pattern is studied on the two largest contributions to the AC losses, namely the surface losses (associated with the penetration of the component of the
magnetic field parallel to the wide faces of the superconducting layer) and the edge losses (associated with the penetration of the perpendicular component of the
magnetic field occurring in the vicinity of the gaps between the CCs). The detailed distribution of the currents in the CCs is examined and its relationship with the different AC loss mechanisms is established. This study is carried out by means of an effective 2D model that uses a system of coordinates conforming with the helical structure of the cable.
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All authors Benoît Vanderheyden, Julien Dular, Louis Denis, Steffen Elschner, Christophe Geuzaine, Mariusz Wozniak, Francesco Grilli
Fluid Dynamics
Ultimate regime in Rayleigh-Darcy Convection
Garima Varshney, Anikesh Pal and Narsimha Reddy Rapaka
DNS of Rayleigh-Darcy convection in a 3D porous domain is performed at Ra $\in [10^3, 10^6]$ to investigate heat-transfer scaling, thermal boundary-layer dynamics, and flow-structure
evolution in the unexplored ultimate regime. The Nu exhibits an approximately linear dependence on Ra throughout the investigated range. However, a distinct change in slope is observed at $Ra \approx 4\times10^5$, indicating the onset of the ultimate regime. For $Ra \leq 2.5\times10^5$, our scaling is 6.25% lower than that reported by \cite{hewitt2014high}, while for $Ra \geq 4\times10^5$ our results are within 1.24% of the extrapolated ultimate-regime prediction of \cite{pirozzoli2021towards}. Analysis of thermal structure reveals formation of near-wall protoplumes that merge into large-scale columnar megaplumes. With increasing Ra, the size of the protoplumes decreases, whereas the numbers increase, thus enhancing boundary-layer convection and heat transport. The thermal boundary-layer thickness scales as ~ Ra^{-1} and ~ Nu^{-1}, corroborating the persistence of linear heat-transfer scaling in the ultimate regime. The thermal dissipation is found to be increasingly shifting from the boundary layer to the bulk with increasing $Ra$, further indicating that the finer protoplumes efficiently transport heat from walls to bulk. The flow structures are quantified using the dominant length scale using the mean wavenumber ($\overline{k}$). It exhibits linear variation with $Ra$ for near-wall structures, with a higher slope in the ultimate regime, signifying finer protoplumes. At the mid-plane, a weaker scaling suggests that the megaplumes also become finer with increasing $Ra$ in the ultimate regime, thus leading to efficient heat transport in the bulk.
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Chemical Physics · Cross-listed from cond-mat.stat-mech
Population dynamics of surface-mediated autocatalytic processes
Denis S. Grebenkov and Yilin Ye
We investigate the population dynamics of surface-mediated autocatalytic processes, in which particles diffuse in a complex environment towards surface regions where they can be either killed or replicated. These opposite mechanisms compete with each other and lead to a sophisticated
stochastic evolution of the population size. We provide a systematic analysis of the generating function of the population size. We also deduce its distribution, mean, variance and higher-order moments. For this purpose, we employ several equivalent descriptions of these quantities in terms of nonlinear integral equations and partial differential equations with nonlinear boundary conditions. We inspect the long-time behavior of the population dynamics in three regimes when the mean population size vanishes, reaches a steady-state level, or grows exponentially. A numerical solution of the underlying integral equations and independent Monte Carlo simulations support our theoretical predictions.
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Medical Physics · Cross-listed from eess.SY
Interaction Dynamics MPC for Knee Rehabilitation Exoskeletons: A Closed-Loop SEA Outer-Loop Study
Yongyan Cao and Jinshan Tang
Safe rehabilitation is an interaction-dynamics problem: the controller must regulate a prescribed motion while absorbing involuntary spasm, voluntary effort, actuator compliance, and model mismatch as disturbances. This paper instantiates the predictive interaction-dynamics framework of the base pHRI formulation on a SEA knee joint. SEA feedforward reduces the
gravity-compensated knee to the same scalar double integrator as the base framework, while a dynamic-residual measurement from spring deflection supplies an interaction-disturbance observation. A steady-state target converts the estimated disturbance into a cancelling input, and a finite-horizon quadratic program regulates deviations from that target under range-of-motion, torque, and velocity constraints. The evaluation matches stiffness and damping across controllers so gains cannot be attributed to higher impedance. Under a motion-opposing $15\unit{Nm}$ step, classical impedance and MPC without estimation produce about $500\unit{mrad}$ steady-state error, whereas Kalman-augmented interaction MPC reduces this to $1.17\unit{mrad}$ at 100~Hz and $0.70\unit{mrad}$ at 500~Hz; the 500~Hz peak is $7.27\unit{mrad}$. In 30 randomized trials, the 95th-percentile peak is $21.57\unit{mrad}$. Bounded Assist-as-Needed scheduling, a corrective-channel energy tank, constrained OSQP stress cases, direct MuJoCo execution, and a posture-clamped MyoSuite knee slice are implemented. The framework holds on a single-mass, closed-inner-loop SEA approximation; an explicit two-mass plant with a finite-bandwidth, pole-placed inner torque loop (Section~VIII) confirms this for nominal tracking but shows delivered torque can overshoot the commanded bound by 21.7% near saturation. Scope excludes clinical intent recognition, full-system passivity, safety certification, hardware trials, and multi-joint validation.
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Optics · Cross-listed from cond-mat.mes-hall
Phonon polariton confinement in isotopically pure MOVPE-grown BN triangles
Maximilian Scharpey, Oskar Schröer, Daniel Wigger et al. (+5 more)
Phonon polaritons, quasiparticles formed by the resonant hybridization of light and
lattice vibrations, exhibit unique properties like the possibility of
hyperbolic dispersions. In this context, exfoliated hexagonal boron nitride (hBN) has emerged as a promising material for
phonon polariton-based research. However, to advance toward practical applications, it is essential to demonstrate efficient
phonon-polariton propagation and confinement in large-area epitaxial BN. To address this topic, we use metalorganic vapor phase epitaxy (MOVPE)-grown BN and investigate the
phonon polariton properties using scattering-type scanning near-field optical microscopy (s-SNOM) and nanoscale
Fourier transform infrared spectroscopy (nano-FTIR). We report remarkably long
phonon polariton propagation lengths, indicating the high crystalline quality of the BN layer. By using epitaxially grown isotopically pure triangular islands, we further demonstrate efficient
phonon-polariton confinement with mode patterns tunable by the incident light wavelength. Our results pave the way for implementing all-epitaxial, microscale, high-quality polariton resonators for nanophotonics and
quantum optics.
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All authors Maximilian Scharpey, Oskar Schröer, Daniel Wigger, Lena Miler, Jakub Iwański, Andrzej Wysmołek, Johannes Binder, Iris Niehues
Medical Physics
A beam–membrane biomechanical vocal fold model incorporating posturing and glottal conformation
Mohamed A. Serry, Matías Zañartu and Sean D. Peterson
The posture of the vocal folds produced by laryngeal muscle activation plays a central role in determining the dynamics of voice production. Abnormal vocal fold configurations are frequently associated with inefficient phonation and a variety of voice disorders. Although diverse glottal closure patterns have been observed clinically, the biomechanical mechanisms governing their dynamic behavior and resulting phonatory characteristics remain incompletely understood. Moreover, existing numerical models that incorporate the effects of the intrinsic musculature on posturing and glottal conformation are computationally expensive, which limits their suitability for large-scale parametric investigations. In this work, we introduce a computationally inexpensive vocal fold (VF) model wherein the body and cover VF layers are treated as a composite beam and a coupled membrane, respectively. Intrinsic laryngeal muscle activation, in addition to positioning the arytenoid cartilages and cricothyroid joint, introduces moments at the boundaries of the structure that influence glottal conformation. The model produces phonatory characteristics that are qualitatively consistent with those reported in high-fidelity finite-element models and clinical studies, thereby supporting its predictive capability while offering substantial computational advantage. The proposed framework provides biomechanical insights into the influence of incomplete glottal closure on phonation dynamics and may serve as a computationally tractable tool for investigating mechanisms underlying certain voice disorders.
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Instrumentation and Detectors
Tracking and timing measurements on irradiated TI-LGADs
A. Gomez-Carrera, L. Diehl, J. Duarte-Campderros et al. (+10 more)
Trench-isolated (TI) LGADs, developed at FBK, are pixelated LGAD implementations where pads are separated by physical trenches etched within the silicon substrate and filled with a dielectric material. Developed as a solution to the LGAD fill factor problem, this technology provides a path towards 4D tracking with reduced inefficiencies in the interpad regions. Through a dedicated 120 GeV SPS pion test beam campaign, the time resolution, efficiency, and inter-pad distance of carbon infused irradiated TI-LGADs is presented. Fluences up to 2.5$~$x$~$$10^{15}$$~$n$_{\text{eq}}$$~$cm$^{-2}$ are evaluated, for single trench implementations with varied trench width at a temperature of -25 $^o$C. The results show an interpad distance degradation with the irradiation of the detector and an optimal time resolution between 35 ps and 45 ps for all studied devices.
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All authors A. Gomez-Carrera, L. Diehl, J. Duarte-Campderros, M. Fernandez, V. Gkougkousis, B. Hiti, G. Kramberger, A. Macchiolo, Y. Padniuk, M. Puklavec, C. Torres Muñoz, I. Velkovska, I. Vila
Plasma Physics · Cross-listed from math.AP
A symmetry theorem for localizable steady solutions of the 3D Euler equations
Daniel Peralta-Salas and Radu Slobodeanu
A steady Euler flow is localizable if the pressure function is constant along its stream lines. This property was used by Gavrilov to construct the first smooth compactly supported steady states of 3D Euler. We prove that any analytic localizable 3D Euler flow in a bounded domain $\Omega$ is axisymmetric and $\Omega$ is a rotationally symmetric domain whose transverse section is a disk or an annulus with convex boundary curves. To the best of our knowledge, this is the first
symmetry theorem for 3D steady Euler flows. In the context of MHD
equilibria, this result shows that Grad's
conjecture holds true for
magnetic fields satisfying the isodynamic condition, a property introduced by Palumbo in the 1960's to minimize the effect of particle drifts in
plasma confinement devices.
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Optics
Real-Time Visualization of the Spatiotemporal Dynamics of 3D Solitons
XinGe Liu, Chaoyang Geng, Yunhan Yu et al. (+3 more)
Three-dimensional (3D) optical
solitons bear far stronger relevance to multi-dimensional nonlinear dynamics prevalent in complex physical, chemical and biological systems than conventional 1D
solitons, and they support far richer and more intricate phenomena arising from their spatiotemporal degrees of freedom. However, real-time recording of 3D
soliton evolution with simultaneous spatiotemporal resolution remains a critical challenging. Here, we demonstrate long-term real-time visualization of 3D
soliton dynamics with spatiotemporal resolution using high-speed photodetectors combined with joint space- and time-division multiplexing. We visually capture complex transient behaviors of 3D
solitons in a multimode fiber
laser and, by integrating our method with the time-stretch technique, simultaneously record pulse-resolved beam and spectral evolutions. We observe that during the birth of 3D
solitons, the highly multimode beam stabilizes for a substantial interval prior to spectral broadening, indicating that a large number of transverse modes have already locked before longitudinal mode proliferation. These findings highlight the critical importance of real-time spatiotemporal visualization for advancing ultrafast multimode
laser design and delivering new insights into high-dimensional nonlinear dynamics.
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All authors XinGe Liu, Chaoyang Geng, Yunhan Yu, Lixia Xi, Xiaoguang Zhang, Xiaosheng Xiao