Accelerator Physics
Emittance tuning of the FCC-ee high energy booster ring
Quentin Bruant, B Dalena, A Chance et al. (+4 more)
The Future Circular
Collider (FCC), in its leptonic configuration has been chosen by CERN as main proposition for the next high-energy
collider. This project aims to achieve luminosities one to two orders of magnitude higher than ever. Feasibility studies have led to the definition of tolerances on magnet imperfections and correction strategies. This is crucial for ensuring the performance of one of the main elements of the acceleration chain, the High Energy Booster (HEB) ring. The efficiency and overall performance of these strategies greatly influence new magnet specifications and tolerances, affecting main optic functions. Horizontal and vertical orbit corrections use horizontal and vertical kickers, respectively. Skew quadrupoles address vertical dispersion and transverse coupling. Normal quadrupoles correct the horizontal and vertical phase advances. This study simulates the distribution of these four corrector types to minimize the
equilibrium emittance at the extraction energy of 45.6 GeV. The calculated strengths of these correctors and the associated misalignments are presented. The study also discusses the limitations and drawbacks of the proposed correction strategy.
Read the full abstract ▾
All authors Quentin Bruant, B Dalena, A Chance, V Gautard, F Bugiotti, Adnan Ghribi, R Tomas
Computational Physics
A Variance-Decomposition Formula for Direct and Adjoint Monte Carlo Particle Transport Problems
Paul Rovel, Coline Larmier, Davide Mancusi et al. (+1 more)
In fixed-source Monte Carlo particle-transport problems, obtaining an acceptable variance on the sought response is of paramount importance. Currently, the only rigorous tool to analyze the variance of such games is the framework of the moment equations, which is unfortunately unwieldy to use in practice. In this paper, we establish a formula that enables expressing the variance of a Monte Carlo simulation as a sum of 'variance contributions' collected throughout the underlying particletransport process. This formula applies to both direct and adjoint games, and provides a new tool to pinpoint the variance-inducing mechanisms in Monte Carlo simulations, understand common variance-reduction techniques, and even conceive new ones. We showcase the use of the variance-decomposition formula on several applications. In particular, we revisit zero-variance schemes and analyze existing variance-reduction techniques, underlining their strengths and weaknesses. A few relevant numerical examples substantiate our theoretical findings.
Read the full abstract ▾
All authors Paul Rovel, Coline Larmier, Davide Mancusi, Andrea Zoia
Geophysics
A limit theorem linking continuous plate models to the topology of the plate boundary network
Seung-Sep Kim
Plate tectonics is described in two registers: a discrete one, in which rigid plates tile the sphere and meet at trivalent junctions, and a continuous one, in which Bercovici and Wessel (1994) replace hard plate outlines by smooth shape functions of finite boundary half-width $\delta^*$. We prove that the two registers are connected by a limit
theorem. Generalizing the shape functions to a signed-geodesic-distance construction, we show that they converge almost everywhere, exponentially in $1/\delta^*$, to the plate indicator functions; that the sharp plate mosaic is a finite regular trivalent CW decomposition of the sphere under an explicit structural hypothesis; and that a component-counted weighted Euler characteristic $\chi_w^{\delta^*}(\tau)$, built from the super-level sets of the shape functions, equals $V-E+F=2$ for all $\delta^*$ below an explicit threshold $\delta_0(\tau)$, with its face, edge and vertex terms converging separately to the numbers of plates, boundary arcs and triple junctions. The
theorem is proved unconditionally on the signed-distance offset cover under positive-reach, separation and bounded-sector hypotheses, and transferred to the normalized partition-of-unity field for thresholds $\tau<1/3$ by a comparison lemma whose junction-ball step, a planar single-crossing property, is certified numerically. The residual $|\chi_w-2|$ defines a topological diffuseness index for diffuse plate boundaries. For the present-day PB2002 network the hypotheses are measured to be non-vacuous, with $\delta_0(0.15)\approx2.8$ km. The
theorem is the mathematical foundation of the topological audit of plate reconstructions presented in a companion paper (Kim, 2026, submitted to Geoscience Frontiers).
Read the full abstract ▾
Medical Physics · Cross-listed from cs.CV
Fiber-Resolved Microstructure Quantification from Multi-Shell Diffusion MRI using Detection Transformers
Sebastian Endt, Marcus Wirth, Johannes Reinhold Schlund et al. (+1 more)
Fiber orientation and compartmental microstructure are central to the characterization of white matter tissue in
diffusion MRI, yet existing methods either resolve fiber orientations without quantifying microstructure, or quantify microstructure while assuming a fixed number of compartments and a single fiber direction. Nonparametric approaches that recover both require tensor-valued
diffusion encoding and computationally expensive Monte-Carlo inversion of an ill-posed inverse Laplace transform. We propose to reframe this problem as an object detection-like task, adopting the Detection
Transformer (DETR) architecture to jointly predict mean diffusivity (MD), fractional anisotropy (FA), main fiber direction, and signal fraction for a variable number of compartments per voxel from standard multi-shell
diffusion MRI with linear encoding. Hungarian matching during training resolves permutation invariance across compartments. We introduce mean Average Precision as a reproducible
benchmark metric. Evaluated on synthetic test data with up to five compartments per voxel, our model achieves $R^2=0.95$ for MD, $R^2=0.88$ for FA, and a median angular error of 4.2{\deg}, with performance scaling naturally with compartmental signal fraction.
Read the full abstract ▾
All authors Sebastian Endt, Marcus Wirth, Johannes Reinhold Schlund, Marion Irene Menzel
Chemical Physics
Computing electron overlap integrals for Gausslet orbitals on cubic lattices
Xianrui Yin, Fereshteh Ghasempour and Christian B. Mendl
Gausslet orbitals on a cubic
lattice, introduced in [Steven R. White, J. Chem. Phys. 147, 244102 (2017)], represent a localized, smooth, and systematically refineable basis set featuring a "diagonal" approximability of the electron repulsion integral tensor. The present work develops efficient
algorithms for evaluating overlap integrals required for electronic-structure simulations using these Gausslets, specifically the kinetic, nuclear, and electron repulsion integrals (both with and without the diagonal approximation). Computational efficiency improvements rest on the exploitation of translation, permutation, and octahedral
symmetries, a reordering and precomputation of nested sums, as well as an early truncation of small coefficients. Our
algorithms reduce the number of electron repulsion integrals on a $5 \times 5 \times 5$ grid from $125^4 = 244140625$ to $138187$ due to
symmetries, and achieve a wall-clock runtime for evaluating the remaining integrals with a truncation tolerance of $10^{-5}$ in under 1 second on a laptop computer. We apply the developed methodology to compute the ground state of the hydrogen atom and molecule as a demonstration.
Read the full abstract ▾
Chemical Physics
A strategic roadmap for an atomistic machine-learning ecosystem
Jörg Behler, Michele Ceriotti, Cecilia Clementi et al. (+47 more)
Data-driven machine learning (ML) techniques have become an essential tool in many domains of science. Their application to atomistic simulations of matter is particularly widespread and impactful. This success is due largely to the existence of a well-developed and established physics-based modeling framework, ranging from first-principles electronic-structure calculations to molecular dynamics and statistical sampling, into which ML was integrated naturally to reshape long-standing trade-offs between accuracy, efficiency, and scale. Nevertheless, this integration raises both conceptual and practical challenges, from choosing between data-centric and physics-based modeling approaches to adapting established software stacks to modern hardware accelerators and ML libraries. As the field evolves rapidly, fueled in part by widespread enthusiasm but also by tangible impact, it seems appropriate to take a moment to consider the current state of the art and open challenges, and reflect on what can be done to better coordinate efforts across the community. With this goal in mind, several members of this community met in Lausanne in January 2026 at CECAM to discuss
algorithms, models, software and hardware infrastructure, and the most promising scientific applications that have become possible thanks to the use of artificial intelligence in atomic-scale simulations. This strategic roadmap paper summarizes the outcomes of these discussions, suggesting some long-term goals, and some concrete actions, to establish a healthy, sustainable and impactful atomistic ML ecosystem.
Read the full abstract ▾
All authors Jörg Behler, Michele Ceriotti, Cecilia Clementi, Gábor Csányi, Alin-Marin Elena, Aditi Krishnapriyan, Joseph W. Abbott, Fabio Affinito, Albert P. Bartók, Ilyes Batatia, Filippo Bigi, Florian N. Brünig, Yannick Calvino Alonso, Giuseppe Carleo, Aurélie Champagne, Stefan Chmiela, Marc L. Descoteaux, Ralf Drautz, Alexandra Farcas, Meng Gao, Rohit Goswami, Michael F. Herbst, Christian Holm, James R. Kermode, Alexander L. M. Knoll, Tobias Kreiman, Marcel F. Langer, Hoang-Thien Luu, Yury Lysogorskiy, Mihai-Cosmin Marinica, Rocco Meli, Klaus-Robert Müller, Frank Noé, Mohamadhosein Nosratjoo, Simon Olsson, Christoph Ortner, Aldo S. Pasos-Trejo, Anyang Peng, Eric Qu, Andrea Rizzi, Mariana Rossi, Bassem Sboui, Gregor N. C. Simm, Alexandre Tkatchenko, Jacopo Venturin, O. Anatole von Lilienfeld, William C. Witt, Brandon M. Wood, Tigany Zarrouk, Fabian Zills
Applied Physics · Cross-listed from cond-mat.mtrl-sci
Operando Charge Carrier Dynamics by Intensity-Modulated Photoluminescence: From Perovskite Thin Films to Solar Cells
Constantin Bach, Dong Won Kim, Yitian Du et al. (+2 more)
The charge-carrier lifetime in halide perovskites often varies by orders of magnitude with the injection level, complicating the analysis of conventional time-resolved photoluminescence and comparisons of reported lifetimes. We use intensity-modulated photoluminescence spectroscopy (IMPLS) as a frequency-domain alternative. Carried out under operating conditions, it yields the lifetime at a specific injection level, e.g., one-sun-equivalent illumination. On thin films, IMPLS and steady-state photoluminescence yield comparable lifetimes. For perovskite/transport-layer stacks, the IMPLS response becomes more complex. A dedicated transfer function disentangles charge transfer from interface recombination and predicts an upper limit on the charge-carrier lifetime in the solar
cell. In devices, electrical methods such as intensity-modulated photovoltage spectroscopy (IMVS) often fail to measure the lifetime due to capacitive effects. IMPLS shows not only the same two characteristic frequencies as IMVS, but also a third one, thereby resolving the lifetime itself.
Read the full abstract ▾
All authors Constantin Bach, Dong Won Kim, Yitian Du, Yana Vaynzof, Carsten Deibel
Chemical Physics
Autocata: Reaction-network-guided autonomous catalyst screening via capability-centric AI
Ruili Li, Rui Qi, Qingqing Mao et al. (+3 more)
Artificial intelligence (AI) has expanded the accessible space for catalyst discovery, yet executing complex screening campaigns still requires constant human intervention to bridge high-level reaction objectives with underlying computational pipelines. Here, we present Autocata, a knowledge-embedded autonomous
agent that scales catalytic expertise by encoding multistep reaction pathways into reusable,
language-invocable capabilities. Powered by a
Transformer pretrained on two million catalyst-adsorbate structures, the system incorporates a capability registry of 79 adsorbate-specialized generative models spanning complex reaction networks across carbon, nitrogen, oxygen, and hydrogen intermediates. Given a high-level natural-
language prompt, Autocata deconstructs reaction networks, queries and configures matching generative models, generates candidate surface configurations, enforces physical and
geometric validity filtering, and coordinates machine-learning potentials evaluations to rank promising materials. Crucially, the
agent adaptively reconfigures workflows when encountering capability boundaries or task bottlenecks. We demonstrate these capabilities across CH4 activation, nitrogen reduction reaction (N2RR), and CO2-to-methanol pathways. By shifting from static toolkits to an adaptive,
language-driven computational infrastructure, Autocata lowers the barrier for expert-level catalyst discovery and offers a scalable paradigm for self-driving catalytic research.
Read the full abstract ▾
All authors Ruili Li, Rui Qi, Qingqing Mao, Ritankar Das, Beien Zhu, Yi Gao
Computational Physics · Cross-listed from math.NA
Structure-preserving Fourier Neural Operators for Long-time Cahn-Hilliard Dynamics under Coarse Temporal Supervision
Yingli Li and Y.C. Zhou
Accurate long-time prediction of Cahn-Hilliard dynamics, particularly in the late-stage coarsening regime, is computationally demanding because it requires simulations over extended physical time. Various machine learning approaches have been developed to speed up the simulations through efficient surrogate evaluations. While the learned map can be accurate over a single learned interval, small prediction error will accumulate when the map is repeatedly applied in a long-time prediction, leading to large numerical errors. In this work, we identify the structure function for the coarsening dynamics of the Cahn-Hilliard equation and propose a two-stage structure-preserving
Fourier Neural Operator (FNO) learning framework. Stage~I introduces a bound-conforming FNO (bcFNO) with a soft, magnitude-dependent amplitude penalty. Stage~II retains the bound-conforming objective and supplements it with structure-function regularization, yielding a structure-preserving FNO (spFNO) that corrects late-stage coarsening statistics. Numerical experiments on the various spatial resolutions and reference-solver time steps suggest that the bcFNO stabilizes the long-time prediction by suppressing large amplitude excursions, without changing the low online cost of the FNO. Furthermore, the structure-function term consistently reduces discrepancies in the normalized structure function and provides further correction in associated late-stage coarsening quantities, including the $L^3(t)$ growth trend and the scaling collapse. These results indicate that our multi-stage implementation of physical principles in operator learning could be applied to other multiscale systems with statistical scaling, such as the functionalized Cahn–Hilliard equations and
turbulence, provided that the state constraints and statistical observables are adapted to the governing dynamics.
Read the full abstract ▾
Optics · Cross-listed from quant-ph
Fixed-Point Bifurcations and Path-Information Bounds in Nonlinear Exceptional-Point Sensing
Xue-Hao Yu and Cong-Feng Qiao
Nonlinear exceptional points (NEPs) were proposed to circumvent the noise–responsivity cancellation that limits linear exceptional-point sensors. At an NEP, the stationary frequency response can be singular while the
Hamiltonian Petermann factor remains finite, suggesting a genuine signal-to-noise ratio (SNR) enhancement. Yet recent model-specific analyses of nonlinear fluctuations have reached conflicting conclusions. We prove that any singular stationary frequency response requires a fixed-point bifurcation of the nonlinear dynamics. In local fluctuation regimes, the singularities in responsivity and long-time frequency uncertainty are both set by the critical relaxation rate, leading to exact cancellation in the SNR and precluding any divergent enhancement. Furthermore, we derive a path-information bound that applies across both local and nonlinear fluctuation regimes and places a finite ceiling on the long-time SNR enhancement. Notably, this upper bound is attainable in the local fluctuation regime, implying that frequency readout under such parameter settings is information-optimal.
Read the full abstract ▾
Computational Physics
The Ganglion Network Model: Evolving Trapped Phases in Porous Media
Yashar Mehmani
Partially miscible ganglia trapped within porous media, spanning one or multiple pores and evolving through diffusive mass transfer, are common in subsurface (e.g., CO$_2$ and H$_2$ storage) and manufacturing (e.g., fuel
cells) applications. We present the ganglion network model (GNM), a reduced-order method for simulating how a population of such ganglia evolves inside an arbitrary porous microstructure. GNM operates on a tree graph, the ganglion network, extracted from the pore-scale image of a given sample. Each point on the graph encodes a possible ganglion configuration in the void space, without any loss of
geometric or topological complexity. The
evolution of a population is modeled by representing each ganglion as a particle on the graph and tracking it according to a set of rules formulated herein. The rules capture capillary events such as pore invasion, retraction, snap-off, fragmentation, and merger. Unlike pore-network models, another graph-based modeling tool at the pore scale, GNM solves no system of equations and its cost scales with ganglion count, not domain size. We validate GNM against an image-based pore-network model in 2.5D and 3D domains with populations undergoing ripening, dissolution, and growth. We find good agreement in ganglion statistics, aggregate properties, and spatial configuration. We further argue that the ganglion network is the statistical space needed for extending kinetic theories of Ostwald ripening from single- to multi-pore ganglia, and provide an outline for how to do this. GNM opens the door to modeling other dynamics of trapped phases in porous media.
Read the full abstract ▾
Physics and Society
Social comparison shapes the evolution of cooperation in structured populations
Xiaojin Xiong, Qin Li, Jurgen Kurths et al. (+2 more)
Human cooperation unfolds in social environments where individuals influence each other through payoff-based learning and social comparison, the tendency to evaluate fitness relative to others. However, it is still unclear how social comparison and population structure jointly shape cooperation. Here, we incorporate social comparison theory into evolutionary dynamics on structured populations, letting fitness depend on individual and neighbour payoffs weighted by a comparison parameter. Under weak selection, we derive conditions favoring cooperation and find that the proposed comparison nonlinearly reshapes the critical benefit-to-cost ratio. Even one individual applying this protocol can affect the population, especially in heterogeneous networks. When comparison tendencies vary, the full distribution, not just the mean, determines evolutionary outcomes. Using a swarm-intelligence-based framework across typical and empirical networks, we identify cooperation-maximizing patterns: optimal states exhibit heterogeneous comparison tendencies, yet collectively align toward assimilative development. These results provide a basis for designing social incentives that harness comparison to promote collective cooperation in human groups.
Read the full abstract ▾
All authors Xiaojin Xiong, Qin Li, Jurgen Kurths, Attila Szolnoki, Minyu Feng
Medical Physics
An Uncertainty-Guided Digital Twin Framework for Online Adaptive Proton Therapy in Head and Neck Cancer: A Feasibility Study
Yizhou Wu, Ryan J. Sanford, Huiqiao Xie et al. (+12 more)
Objective: Head and neck (HN)
proton therapy spans six to seven weeks of anatomical change, while offline replanning takes about a week. We present an uncertainty-guided digital twin (UGDT) framework that forecasts treatment-day anatomy before treatment and evaluate whether it generates online adaptive
proton therapy (APT) plans of clinical quality. Approach: A library of 302 longitudinal deformations from 88 previously treated HN patients was transported onto each new patient's treatment planning CT (TPCT) using two-step multi-atlas deformable image registration (DIR) built on a pretrained CT foundation model, generating about 284 predicted CTs (pdCTs) with contours per patient. Dispersion of propagated clinical target volume (CTV) contours defined a patient-specific robust margin. In ten patients, the quality assurance CT (QACT) triggering a replan represented treatment-day anatomy, and the physician-approved replan was the baseline. The pdCT most similar to the QACT (pdCT-H) and one from the lowest quartile (pdCT-L) were planned to within about 5% of baseline plan quality, forward-calculated on the QACT, and reoptimized to generate online APT plans. Main results: pdCT plans scored within -0.7% (pdCT-H) and -1.0% (pdCT-L) of baseline. Forward calculation on QACT reduced high-dose CTV D98% to 88.3% and 85.5%. After online reoptimization, D98% recovered to 98.3 +/- 0.3% and 98.2 +/- 0.3%, versus 98.5 +/- 0.4% at baseline. Spinal cord and brainstem doses remained below tolerance, and plan quality scores were within -1.1% (p = 0.19) and -1.7% (p = 0.01) of baseline. Significance: UGDT generated online APT plans comparable in quality to physician-approved offline replans using anatomy forecast before treatment, enabling a transition from reactive offline replanning toward anticipatory online adaptation.
Read the full abstract ▾
All authors Yizhou Wu, Ryan J. Sanford, Huiqiao Xie, Jie Ding, Shupeng Chen, Tung-Ho Wu, Ping-Hsiu Wu, Justin Roper, Jun Zhou, Minglei Kang, Bill Stokes, Sibo Tian, David S. Yu, Xiaofeng Yang, Chih-Wei Chang
Chemical Physics
An atom-based machine-learned dipole-moment model and application to conjugated systems
Yuansheng Zhao, Tamio Yamazaki, Ryohei Hosoya et al. (+2 more)
We introduce a method for predicting the dipole moments of molecular systems by systematically decomposing the total dipole moment into the sum of atomic contributions using the wavefunction from density-functional-theory calculations, and then use graph neutral networks to predict this effective atomic dipole moment from input atomic structure. It is demonstrated that the dipole moments and dielectric function can be accurately predicted even for complicated conjugated systems where the previous bond-based model [Phys. Rev. B 110, 165159] fails. The inference cost of our model scales linearly with the number of atoms and is about 3 times faster compared with Born-effective-charge-based schemes, but shows similar or better accuracy for dielectric function at terahertz range.
Read the full abstract ▾
All authors Yuansheng Zhao, Tamio Yamazaki, Ryohei Hosoya, Yu-ichiro Matsushita, Shinji Tsuneyuki
Optics
Frequency comb generation in active optomechanical cavities
Madhurendra Mishra and Adarsh Ganesan
We numerically investigate frequency-comb generation in an electrically pumped vertical-cavity surface-emitting
laser coupled to two mechanical modes of a suspended mirror. Displacement-dependent cavity loss and radiation-pressure feedback couple the mechanical motion to the carrier and
photon densities, producing harmonics and combination-frequency components in the
photon-density spectrum. Mechanical quality factors primarily control the intensity of comb lines, whereas resonance frequencies determine their spectral distribution. The initial mechanical amplitudes influence the threshold current and relative spectral intensities, with the simultaneous excitation of both modes yielding a broader set of observable components. These results establish the dependence of the calculated comb structure on mechanical dissipation, resonance frequencies, and initial conditions in an active optomechanical cavity.
Read the full abstract ▾
Optics
Surface quantum response of plasmonic heterodimers: revealing the hidden nonlocality
Ida Juliane Bundgaard, Christos Mystilidis, Frederik Wulff Christensen et al. (+7 more)
We probe the
quantum response of plasmonic heterodimers with extended (plane waves) and localized (point dipoles and electron beams) sources. Implementing the surface-response formalism based on Feibelman parameters, we explore nanosphere heterodimers where one particle is characterized by electron spill-in, while the other by spill-out. We identify situations where the two
quantum surface corrections that act in opposite spectral-shift directions can cancel out, producing mesoscopic spectra that do not differ from the predictions of the local-response approximation. This picture changes dramatically once localized sources are implemented, allowing one to selectively probe the
quantum response of the particles independently. Such probing occurs in terms of higher-order modes in the case of electron beams, or for the complete modal landscape in the case of point dipole sources. This description offers a new insight into the mechanisms governing light–matter interactions in extreme plasmonic architectures, and opens new perspectives for the understanding and tailoring of plasmonic–
quantum emitter architectures.
Read the full abstract ▾
All authors Ida Juliane Bundgaard, Christos Mystilidis, Frederik Wulff Christensen, Lorenz Huber, P. Elli Stamatopoulou, Carsten Rockstuhl, P. A. D. Gonçalves, Ulrich Hohenester, N. Asger Mortensen, Christos Tserkezis
Optics
Experimental Demonstration of Chiral Bound States in the Continuum Enabled by Time-reversal Symmetry Breaking
Hao-Chang Mo, Wen-Jin Zhang, Ze-Yu Wu et al. (+3 more)
Bound states in the continuum (BICs) provide a powerful route to optical resonances with vanishing radiation loss and enhanced light-matter interactions. Of particular interest are chiral BICs, whose resonant states exhibit intrinsic circular polarization and
spin-selective radiation. Magneto-optical photonic
crystals have recently been predicted to host such states through time-reversal-
symmetry breaking. Here, we experimentally demonstrate chiral BICs induced by time-reversal
symmetry breaking in a magneto-optical photonic
crystal, where an out-of-plane
magnetic field lifts a degenerate BIC pair into two nondegenerate resonances with opposite circular polarizations. Numerical simulations reveal the associated chiral phase
vortices and strong
spin-selective radiation, while microwave measurements confirm the predicted behavior through angle-resolved transmission spectra and pronounced broadband circular dichroism. Our results establish an experimental platform for exploring chiral BIC physics in magneto-optical photonic structures, opening new opportunities for
spin-selective photonics.
Read the full abstract ▾
All authors Hao-Chang Mo, Wen-Jin Zhang, Ze-Yu Wu, Xin-Tao He, Xiao-Dong Chen, Jian-Wen Dong
Optics · Cross-listed from cond-mat.mes-hall
Optical observation of interlayer spin correlation
Akiyoshi Park, Pranshoo Upadhyay, Andrey Grankin et al. (+16 more)
Spin correlations govern numerous collective behaviors of
quantum materials, underpinning exotic phenomena such as unconventional superconductivity and topological magnetism. In layered materials, the interlayer
spin correlation is important because it characterizes the
magnetic ground state and determines
spin transport across the interface. Yet interlayer
spin correlations have remained hard to measure directly, leaving one of the most basic quantities of two-dimensional magnetism out of experimental reach. Here we provide the first direct optical probe of interlayer
spin correlations, using two-
magnon Raman scattering in the van der Waals antiferromagnet (AFM) CrSBr, in remarkable agreement with a microscopic
spin-wave model without any fitting parameter. Moreover, the two-
magnon channel switches on only in the AFM state and vanishes when a
magnetic field takes the
crystal to a ferromagnetic state. We furthermore establish an
exciton-mediated variant of the technique, where tuning the
laser near the
exciton resonance enhances the signal roughly tenfold due to the
exciton's large oscillator strength.
Magnon-pair Raman spectroscopy thus opens a direct optical window into interlayer
spin correlations in van der Waals magnets, extendable to twisted bilayers and proximity-coupled
heterostructures.
Read the full abstract ▾
All authors Akiyoshi Park, Pranshoo Upadhyay, Andrey Grankin, Emil Viñas Boström, Mahdi Ghafariasl, Hassan Alnatah, Masoud Mohammadi-Arzanagh, Gautam Nambiar, Beini Gao, Alireza Alvandi, Sakthi Rajmano Madhan Kumar, Ghadah Alshalan, Isaac Sherwood, Mahmoud Jalali Mehrabad, You Zhou, Arun Ramanathan, Xavier Roy, Angel Rubio, Mohammad Hafezi
General Physics
Is the acceleration scale of the radial acceleration relation tracking the cosmic expansion rate?
G. T. Alcock
The MUSE-DARK survey has reported a highly significant increase of the radial acceleration relation (RAR) scale $a_0$ with
redshift over $0.33<z<1.44$, parametrized linearly and described by its authors as phenomenological. We confront the same binned measurements with physically motivated scalings, using the continuous family $a_0(z)=A(1+z)^{\gamma}$ as the primary statistic. We find $\gamma=0.78\pm0.15$ (statistical, plotted uncertainties read as $1\sigma$): a
redshift-independent scale ($\gamma=0$) is excluded at $5.3\sigma$ by a nested test, and the matter-density scaling ($\gamma=3/2$) at $4.8\sigma$. Hubble tracking, $a_0(z)\propto H(z)$, with effective exponent $\gamma\simeq1.10$, is consistent with the measurement within $2.1\sigma$ and is the information-criterion-preferred one-parameter description; its amplitude, $A=(1.40\pm0.03)\times10^{-10}$ m s$^{-2}$, lies 17 per cent above the canonical SPARC value, within the latter's systematic-dominated uncertainty. The survey's remark that the
evolution is 'faster than $H(z)$' is shown to be anchor-dependent: relative to a floating amplitude the binned growth is, if anything, mildly shallower than $H(z)$. All exponent-based conclusions are invariant under
redshift-independent rescalings of $a_0$, and hence robust to common-mode stellar-mass systematics. At current precision, the long-noted coincidence $a_0\sim cH_0$ survives its first confrontation with direct kinematic measurements at intermediate
redshift.
Read the full abstract ▾
Space Physics
Revisiting magnetotelluric theory: layered induction and geoelectric fields
Dennies Bor
We review the magnetotelluric (MT) theory of a horizontally layered Earth under plane wave forcing and its use in calculating geoelectric fields. Starting from Maxwell's equations, we derive the
diffusion approximation and the impedance recursion by matching electric and
magnetic fields at layer boundaries. The equivalent transmission line connects the surface impedance to attenuation with depth, apparent resistivity, phase, and the electric response to a
magnetic time series. Three evaluations accompany the derivation, each against its own reference. First, under the storm of 10 and 11 May 2024, electric fields calculated from layered models fitted to measured impedance tensors differ from those calculated from the tensors by a median vector error of 46% across 1614 EarthScope MT sites with sufficient period coverage. Reducing each tensor to its antisymmetric projection alone gives a median of 46%, while the layered fit and numerical processing give medians of 1.9% and 1.3%. Second, the
magnetic field reconstructed at each of 20 withheld observatories from the remaining network has a median vector error of 48%. In a common electric field comparison, the
magnetic reconstruction gives the larger electric vector error in 96% of 32,280 observatory-tensor pairs, with medians of 93% against 46% for the layered representation. Third, when conductivity inferred at MT sites is interpolated by kriging into withheld regions, the median relative error of the predicted impedance is 57.3%, and 11 of 149 withheld sites have an apparent resistivity discrepancy of a factor of 100 or more at one or more periods. The vector comparison retains differences in the amplitude and direction of the electric field that are relevant to calculating induced voltages along transmission line routes.
Read the full abstract ▾
Atomic Physics · Cross-listed from quant-ph
Rayleigh-Wood Threshold Controls Nonlinear Photon Correlations in Atomic Arrays
Zai-Cheng Xiao and Li-Ping Yang
Two-dimensional atomic arrays provide versatile free-space
quantum optical interfaces by coupling
photons to collective
lattice modes. Here we show that the Rayleigh–Wood anomaly provides a sharp control mechanism for both linear and nonlinear
photon scattering from a single atomic layer. As the
lattice spacing crosses the diffraction threshold, newly opened radiative channels rapidly broaden the dominant collective modes, converting specular reflection into diffuse off-axis scattering. In transmission, momentum-space Fano interference between the incident and collectively scattered fields produces single-
photon intensity zeros that strongly reshape the momentum-space correlation pattern. An integrated nonlinear contrast reveals a pronounced channel asymmetry: upon opening the Rayleigh channels, the connected two-
photon contribution drops by several orders of magnitude relative to the factorized background in transmission, while remaining close to unity in reflection. These results establish radiative diffraction thresholds as a control principle for
quantum nonlinear optics in single-layer atomic arrays.
Read the full abstract ▾
Instrumentation and Detectors
Kilopixel Performance of the Kinetic Inductance Detectors for the Terahertz Intensity Mapper
Justin S. Bracks, Reinier M. J. Janssen, Steve Hailey-Dunsheath et al. (+32 more)
We characterize a flight-grade array of lumped-element kinetic inductance detectors (LEKIDs) developed for the long-wavelength module of the Terahertz Intensity Mapper (TIM). From an 864-pixel science array, we select 490 well-isolated resonators spanning the focal plane and readout band and measure their thermal response, optical responsivity, and noise using a ZCU111-based multitone readout system intended for flight. The thermal and optical response of the detector population is consistent with previous single-pixel measurements and can be described by Mattis-Bardeen theory under the influence of a change in Cooper pair or quasi-particle density. Under increasing blackbody loading, the measured noise transitions from a thermal generation-recombination-dominated floor to
photon-noise-limited scaling, with the majority of detectors achieving
photon-noise-limited operation by approximately 400 fW incident power, with a median detector noise-limited NEP of $NEP_{det} = 1.1\times10^{-17} W \sqrt{Hz}$. The measured
photon-noise scaling implies a median optical efficiency of approximately 0.67, indicating additional unknown loss sources between the detectors and blackbody radiator, tentatively attributed to losses in the waveguide. These results demonstrate that the TIM LEKID architecture retains the required sensitivity when scaled to kilopixel-class arrays; the principal remaining challenges are array-level tone optimization, resonator tracking, and identification of frequency-domain collisions.
Read the full abstract ▾
All authors Justin S. Bracks, Reinier M. J. Janssen, Steve Hailey-Dunsheath, Talia Saeid, Bruce Bumble, Logan Foote, Elijah Kane, Lun-Jun Liu, Charles M. Bradford, Shubh Agrawal, James E. Aguirre, Hrushi Athreya, Brockton S. Brendal, Karia Dibert, Peter Dow, Jeffrey P. Filippini, Jianyang Fu, Karolina G. Garcia, Christopher E. Groppi, Bradley Johnson, Dylan Joralmon, Wooseok Kang, Garrett K. Keating, Ryan P. Keenan, Ian N. Lowe, Alex Manduca, Aashrita Mangu, Daniel P. Marrone, Philip D. Mauskopf, Evan C. Mayer, Sydnee ODonnell, Simon Tartakovsky, Mathilde Van Cuyck, Joaquin D. Vieira, Jessica A. Zebrowski
Atmospheric and Oceanic Physics
Long-term Variations of Cyclone-Influenced Exchange Flows in the Coastal Arctic in 2000-2025
Chunyan Li
Summertime synoptic weather regulates Arctic land-ocean exchange through meteorological tides during the ice-free period, when the predominantly easterly regime of the Beaufort High is punctuated by episodic Arctic cyclones, changing coastal winds to westerlies. We analyze wind-driven exchange flows in a northern Alaska lagoon using a 26-year reconstruction (2000-2025) validated by new in situ observations and FVCOM simulations. Phase-space EOF analysis of ERA5 data links deep cyclonic anomalies to 89% of Extreme Eastward Throughflow (EET) events (24 of 27). Coastal easterly occurrence frequencies show a consistent decrease (up to 38.3% at > 10 m/s) while low-to-moderate westerly occurrence frequencies increase by up to 30.9% but decrease by 40.6% at extreme westerly wind speeds (>10 m/s). Offshore ERA5 wind occurrence frequencies increased, with extreme offshore events (> 10 m/s) increasing by 82.4%. Local ASOS observations reveal a near-monotonic ~88% reduction in atmospheric forcing variability (East-West Wind Excursion variability drops from 2.98X10^4 to 0.35X10^4 km). The In-Out Flow Excursion variability declines steadily by 70% from 1.40X10^3 to <0.40X10^3 km, while the positive (outward) flow ratio variability contracts steadily from 7.4% to below 2.2%. Extreme Westward Throughflow (EWT) events were infrequent, but were concentrated in the most recent period, with five of six events occurring in 2022-2024 and one in 2013. These results indicate that the summertime coastal regime remains predominantly easterly, but exhibits progressively lower variability, while the wind distribution shifts toward more frequent low-to-moderate westerlies. This
evolution of weather-forcing state may impact flushing and biogeochemical connectivity of Arctic lagoon ecosystems.
Read the full abstract ▾
Chemical Physics
A Visual Understanding of Circular Dichroism Spectroscopy: The Case of Interacting Chromophores
Braden M. Weight, Aaron Forde and Sergei Tretiak
Exciton coupling shapes the photophysical and chiroptical properties of molecular aggregates, but a microscopic, real-space understanding of how intermolecular interactions generate circular dichroism (CD) remains incomplete. Here, we computationally investigate the microscopic origins of CD in coupled molecular dimers to separate intermolecular (topological) from intramolecular (intrinsic) chirality. A combination of time-dependent density functional theory and transition chiral tensor (TCT) analysis provides a quantitative, visually intuitive framework showing how electronic coupling strength, intermolecular separation, and relative angular orientation govern chiroptical response. We find that pairs of individually achiral chromophores exhibit robust, topological chiroptical signals which give rise to the conventional Cotton effect, a standard interpretation of derivative features in circular dichroism spectra. We next establish that intrinsically chiral chromophores, on the other hand, disrupt the Cotton effect due to the competition between intrinsic chirality and topological chirality. This work establishes a foundation for interpreting
exciton-coupled CD spectra in molecular aggregates, supramolecular assemblies, and nanostructures using the TCT approach, which offers an attractive tool for analyzing chiroptical responses in complex multi-chromophore systems relevant to emerging
quantum technologies.
Read the full abstract ▾
Computational Physics · Cross-listed from cond-mat.str-el
Sign problem and criticality in world-line quantum Monte Carlo methods
Rubem Mondaini and Richard T. Scalettar
Discussions of the sign problem usually focus on its role as the primary limitation of the applicability of
quantum Monte Carlo methods for reliably solving
quantum many-body models. Its behavior as a function of spatial
lattice size, doping, temperature, and interaction strengths has been carefully characterized within the determinant
quantum Monte Carlo
algorithm. Here, we consider the much less well-explored question of the temperature- and size-dependence of the sign problem in the world-line
quantum Monte Carlo method. We review how, in this
algorithm, even sampling the partition function of free
fermions yields a sign problem, which we connect to non-analytic behavior in the corresponding reference model, hard-core
bosons on a
lattice. The latter exhibits a finite-temperature Kosterlitz-Thouless phase transition in 2D, and we show how this non-analytic behavior is imprinted on the average sign of the weights for the corresponding free
fermion 2D tight-binding model. Our results thus show how phase-transition information can remain encoded in the sign problem even after direct sign sampling becomes impractical.
Read the full abstract ▾
Fluid Dynamics
Separated flows over swept wings across transitional Reynolds numbers
Laura Victoria Rolandi, Jonathan Quang Tran and Kunihiko Taira
We explore the effect of wing sweep on the interaction between leading-edge separation and tip
vortex for a finite wing with sweep angles $\Lambda=0^\circ$ to $45^\circ$, semi-aspect-ratio $sAR=2$, angle of attack $14^\circ$, and chord-based Reynolds numbers $Re=600$, $1000$, $2500$, $5000$, and $10000$. Through this parametric study, we seek to bridge the knowledge gap between existing studies on separated flows over swept finite wings at low Reynolds numbers ($Re\approx 10^2$) and the behavior of turbulent flows at higher Reynolds numbers ($Re\approx10^4$). The flow structure significantly varies over this range of parameters, altering the lift characteristics. Specifically, increasing $\Lambda$ at $Re\leq5000$ reduces the lift, while increasing $\Lambda$ at $Re>5000$ enhances the lift. The flow modifications that lead to this effect with increasing Reynolds number are analyzed in terms of the streamwise vorticity, revealing three key effects: (i) a change in the sign of streamwise vorticity with increasing sweep angle, due to the higher spanwise velocity component, (ii) the progressive weakening and eventual disappearance of the tip
vortex, when increasing the wing sweep, is accompanied by the emergence of a dominant inboard vortical structure near the root, and (iii) at higher Reynolds numbers and sweep angles, this inboard vortical structure merges with the main wake, intensifying the shear-layer roll-up and giving rise to a leading-edge
vortex with an extended reattached region over the wing. These findings reveal how the flow characteristics vary with Reynolds number, transitioning from regimes where swept wings offer limited benefit to regimes where they significantly enhance the aerodynamic performance. This study contributes to a deeper understanding of swept-wing wake dynamics, which is crucial for modern air vehicle design.
Read the full abstract ▾
Optics
Metaphotonics for High-Harmonic Generation
Pavel Tonkaev, Anton Rudenko, Ziwen Wang et al. (+2 more)
We summarise the recent advances on the generation of high-order harmonics in optical metaphotonic structures such as isolated resonators and metasurfaces. In such subwavelength-patterned structures, extreme nonlinear effects are expected from the interaction between ultrafast
laser pulses and structured planar surfaces which support various resonances. High-harmonic generation (HHG) is considered as an effective tool for realising extreme ultraviolet light sources and attosecond pulses, and it was observed previously in gases,
liquids, and solids. Resonant metaphotonics can offer a novel sub-wavelength platform for the HHG effects, and it can provide new strategies for the design of efficient integrated light sources. We start our discussion from a brief overview of HHG in gases,
liquids, and unstructured solids, and then move to summarising the recent experimental observations of HHG in individual resonant nanoparticles and resonant dielectric metasurfaces. We focus on different types of resonances (plasmonic vs. Mie resonances vs. bound states in the continuum), and also present the observation of non-integer power dependencies of the generated harmonics driven by strong resonances. We also mention current unsolved problems and identify new promising research directions involving HHG in metasurfaces.
Read the full abstract ▾
All authors Pavel Tonkaev, Anton Rudenko, Ziwen Wang, Tran Trung Luu, Yuri Kivshar
Chemical Physics
Nuclear quantum effects enhance diffusion in supercooled ammonia through accelerated pyramidal inversion
Minwoo Kim, Hido Woo, Taeyong Park et al. (+2 more)
In
liquid ammonia, an imbalance between available hydrogen-bond donor and acceptor sites limits water-like tetrahedral connectivity, leaving sparse and short-lived associations within a densely packed
liquid. We ask whether
quantum-sensitive local rearrangements can nevertheless contribute appreciably to
diffusion in this weakly connected
liquid. We compare classical and thermostatted ring
polymer molecular dynamics from 170 to 250 K using an r$^2$SCAN-trained machine learning force field. Ammonia can change its molecular geometry as the nitrogen atom passes through the plane of the three hydrogen atoms, a motion known as pyramidal inversion. Nuclear
quantum effects increase the inversion rate and reduce its apparent activation energy. Inversion events are accompanied by transient reductions in hydrogen-bond coordination and local density within the first solvation shell. Although nuclear
quantum effects lower density and viscosity throughout the studied range,
diffusion remains only weakly affected in the warmer
liquid. The
quantum enhancement of
diffusion emerges below approximately 210 K and reaches 18% at 170 K, tracking the enhancement of cage escape. These observations support an inversion-assisted contribution to
diffusion, in which a spatially localized,
quantum-sensitive internal motion couples to cage relaxation without a persistent tetrahedral network.
Read the full abstract ▾
All authors Minwoo Kim, Hido Woo, Taeyong Park, Ji Woong Yu, Won Bo Lee
Optics · Cross-listed from quant-ph
Polarisation-resolved identification of spontaneous four-wave mixing processes in a multimode fused tapered fibre coupler
Jefferson Flórez, Chams Baker, Benjamin J. Sussman et al. (+3 more)
Integrated
quantum photonics benefits from
photon-pair sources that generate
photons directly in waveguides, where they can be efficiently collected, routed, and manipulated. Here, we investigate spontaneous four-wave mixing (SFWM) in a fused tapered-fibre microcoupler formed from two single-mode fibres, and report four contributions. First, we observe SFWM
photon pairs in this multimode device: its elliptical central region supports three spatial mode profiles, each with two polarisations, giving six guided modes, and pumping it near 800 nm yields two
photon pairs at 648/1048 nm and 660/1021 nm. Second, because several allowed SFWM processes produce similar wavelengths and wavelength alone does not identify their origin, we distinguish the processes by combining phase-matching calculations and selection rules with two independent polarisation measurements: the dependence of the coincidence rate on pump polarisation and polarisation
tomography of the generated
photons. Third, within the present geometrical model, we find that the 660/1021 nm pair is generated by co-polarised pump
photons in a process occurring entirely within a single higher-order mode, whereas the 648/1048 nm pair is generated by orthogonally polarised pump
photons in an intermodal process coupling a fundamental and a higher-order mode; the signal
photons from the two processes have similar output polarisations, while the idler
photons are nearly orthogonal. Fourth, extending the analysis beyond the measured operating point, we theoretically identify pairs of simultaneous SFWM processes driven by a common pump that could generate either polarisation
entanglement or composite spatial-polarisation
entanglement, depending on the pump wavelength. More generally, this work provides a practical strategy for identifying intermodal SFWM processes in multimode waveguides when spectral information alone is insufficient.
Read the full abstract ▾
All authors Jefferson Flórez, Chams Baker, Benjamin J. Sussman, Xiaoyi Bao, Jeff S. Lundeen, Lambert Giner
Applied Physics · Cross-listed from math.AP
Magnetic Stabilization of the Greenhill Instability and Hysteretic Switching in Slender Rods
Vivekanand Dabade, Likhit Ganedi, Mohd Tahseen et al. (+1 more)
In a bulk ferromagnet,
magnetic energy is six orders of magnitude weaker than elastic energy. Slenderness removes this disparity. In a thin ferromagnetic rod, the two energies scale differently with aspect ratio, and an ordinary nickel wire deforms nonlinearly even under modest remote fields. We develop a three-dimensional variational theory of a ferromagnetic Kirchhoff rod under
gravity and
magnetic loading and study it through bifurcation analysis, computation, and experiment. A uniform field can stabilize a vertical rod against buckling under its own weight, the classical Greenhill instability. We give sharp criteria for clamped and pinned supports in terms of an Airy-type principal
eigenvalue, and give a fine description of the character of the bifurcation landscape, which has qualitatively different features than its non-
magnetic counterpart. When subject to the field generated by a permanent magnet, we identify a rich bifurcation landscape as the magnet is rotated quasistatically along a circle of radius slightly larger than the rest length of the rod. We prove that instability occurs only through a planar mode and that each snap is a nondegenerate saddle-node. We also locate the two folds that produce hysteresis. Experiments on $50\,\mu$m nickel wires confirm the Greenhill threshold, restabilization at ~$30$ mT for both clamped and pinned supports, as well as the salient features of the hysteresis loop such as switching angles, width, and geometry of folds accurately. Our analysis and experiments offer principled design rules for field-stabilized filaments and remotely switched bistable actuators. Our analysis also offers a well-conditioned numerical method is valid near the switching events, where the direct numerical minimization is strongly ill-conditioned.
Read the full abstract ▾
All authors Vivekanand Dabade, Likhit Ganedi, Mohd Tahseen, Raghavendra Venkatraman