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Physics Physics

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Thursday 1 October

Optics

Digitally enhanced Multi-wavelength Stabilization using a Passive Fiber Frequency Reference

Hilma Karlsson, Vaishali Adya, Robert L. Ward et al. (+1 more)

High-stability frequency stabilization between lasers is essential for applications such as precision metrology, frequency dissemination, and quantum communications. We demonstrate the use of code-based multiplexing of a passive fiber-based interferometer for frequency stabilization and transfer between lasers separated by 85 GHz in frequency using a single shared passive reference. Out-of-loop characterization places an upper bound of sub-kHz/$\sqrt{\text{Hz}}$ on the transferred frequency noise at Fourier frequencies above 40 mHz, with a differential fractional stability of $7\times10^{-13}$ at 1 second. We also demonstrate continuous laser tunability and characterize the residual length noise coupling. Finally, we provide a framework for scaling the architecture to higher laser counts, offering a pathway to scalable, frequency agile stabilization of multiple light sources without the overhead of an optical frequency comb.
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All authors Hilma Karlsson, Vaishali Adya, Robert L. Ward, Chathura P. Bandutunga

Fluid Dynamics

Flow separation and turbulence production over trapezoidal protrusions

Shahar Zuckerman, Jibu Tom Jose, Ori Levi et al. (+1 more)

The influence of rear-face inclination on turbulence around a surface-mounted protrusion depends on the separated flow approaching its trailing edge. This dependence is examined experimentally using stereoscopic particle image velocimetry over twelve trapezoidal protrusions with upper-surface length-to-height ratios $L/h=1,2,3,4$ and rear-face angles of $30^\circ$, $45^\circ$, and $90^\circ$. Measurements were conducted at a height-based Reynolds number of approximately $5\times10^4$, with an incoming boundary-layer thickness comparable to the protrusion height. For $L/h=1$ and 2, a connected reverse-flow region extends over the upper surface into the wake. For $L/h=3$ and 4, upper-surface reattachment separates the two regions. Length governs this change in topology and the variation in turbulence levels at common spatial locations, whereas rear-face inclination has its strongest influence close to the surface. From $L/h=2$ onward, shallower faces have higher near-face turbulent kinetic energy, while vertical faces have lower energy but a larger spanwise fraction. A decomposition of the in-plane deviatoric production separates the effects of Reynolds-stress and mean-strain magnitudes from those of their relative principal-axis orientation. At the inclined faces, this contribution changes from negative to positive between $L/h=1$ and 2, before upper-surface reattachment occurs. Its subsequent increase with length is driven mainly by increasing tensor magnitudes, with the largest contribution at $L/h=4$ and $30^\circ$, where strong production remains close to the rear face. These results show that local turbulence production depends jointly on the flow state approaching the trailing edge and the position of the energetic shear layer relative to the rear surface.
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All authors Shahar Zuckerman, Jibu Tom Jose, Ori Levi, Omri Ram

Fluid Dynamics

A conservative micro-continuum-cellular automaton method for multispecies biofilm dynamics in complex flows

Soyoung Kim and Yinuo Noah Yao

We develop a conservative micro-continuum-cellular automaton method for simulating multispecies biofilm dynamics in complex flows. The proposed method couples the Darcy-Brinkman-Stokes equations, reactive transport, suspended bacteria, and biofilm dynamics with a two-stage cellular automaton algorithm for biofilm redistribution and interface evolution. While treating biofilms as evolving porous media, we ensure conservative redistribution of multispecies biomass across partially occupied cells. Donor and recipient cell volumes are explicitly accounted for to conserve biomass and preserve species composition on non-uniform meshes. The proposed method is assessed against diffusion-dominated benchmark cases, including single-species fingering and multispecies stratification, and is further evaluated through a mesh-convergence study for flow and growth over a rectangular bump. The framework is then applied to counter-diffusional biofilms in a membrane-aerated biofilm reactor as a canonical example. The results demonstrate that the framework captures the expected biofilm morphology and stratification in systems involving coupled flow, substrate transport, and biofilm dynamics. The proposed method provides a flexible computational approach for simulating multispecies biofilm dynamics in complex flows and geometries.
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Chemical Physics · Cross-listed from quant-ph

Molecular Dynamics with Nuclear Effects on Quantum Computers

Lukas Haßfurth, Juliane Heitkämper, Elias Walter et al. (+1 more)

Nuclear quantum effects are critical for describing proton transfer and hydrogen bonding, but their incorporation into quantum chemistry calculations is often computationally prohibitive on classical hardware. A promising alternative are quantum computers due to their linear scaling in the space requirements with system size. We introduce a novel hybrid quantum-classical algorithm for ab-initio molecular dynamics that incorporates nuclear quantum effects via the nuclear-electronic orbital method. The proposed algorithm evaluates ground state energies and forces on the quantum computer using a variational quantum eigensolver, while the molecular geometries are updated classically. We validate our approach through simulations of $\text{H}_2$, $\text{H}_2\text{O}$ and the Zundel ion $\text{H}_5\text{O}_2^+$, comparing the simulated vibrational spectra with experimental data. Upon inclusion of nuclear quantum effects, the proton shuttling movement in the Zundel ion becomes effectively barrierless, and errors in the simulated frequencies improve significantly. Employing compact hardware-efficient ansatz circuits we achieve results comparable to the more accurate UCCSD ansatzes, which hints towards the feasibility of executing our algorithm on near-term quantum devices.
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All authors Lukas Haßfurth, Juliane Heitkämper, Elias Walter, Birger Horstmann

Optics

Measuring Geometric Phase based on Indefinite Causal Order in a Sagnac Interferometer

Lucas Marques Fagundes, Finlay Campbell, Richard Aguiar Maduro et al. (+4 more)

Sagnac interferometers are important tools for precision measurements. Cancellation of common-path noise ensures a high degree of stability against external perturbations, while their sensitivity to rotations provides the basis of laser gyroscopes. Here we highlight another, less explored feature of Sagnac interferometers: the fact that optical elements are traversed in reverse order for clockwise and counterclockwise propagating light components. This provides the opportunity to explore a classical realization of indefinite causal order, where the order of events within a sequence is not fixed. We explore this concept to identify the geometric phase associated with two non-commuting polarization operations with a single measurement. This idea may have applications for the rapid determination of polarization manipulations or optical activity within chiral media, but foremost it provides a geometric illustration of indefinite causal order.
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All authors Lucas Marques Fagundes, Finlay Campbell, Richard Aguiar Maduro, Renné Medeiros de Araújo, Stephen M. Barnett, Sarah Croke, Sonja Franke-Arnold

Plasma Physics

A Nonlinear Two-Sheath Circuit Model for Low-Pressure Symmetric and Asymmetric Capacitively Coupled Radio-Frequency Plasmas

Katharina Noesges, Tim Bolles, Máté Vass et al. (+2 more)

We develop a nonlinear self-consistent two-sheath circuit model for low-pressure capacitively coupled plasmas that applies to geometrically symmetric as well as asymmetric discharges. The quasineutral plasma bulk is represented by an inductive-resistive element and coupled to stationary particle and electron-energy balances. Both boundary sheaths are treated dynamically using a lncosh sheath charge-voltage model with bounded differential elastance, based on a Riccati closure for the differential sheath width. The model recovers the quadratic depletion-sheath relation in the small-charge limit, while the characteristic sheath scales are determined from the RF-averaged sheath voltages using a collisionless Child-Langmuir/Bohm closure. The resulting four-variable RF subsystem contains the two sheath charges, the blocking-capacitor voltage, and the discharge current. In the symmetric monofrequent limit, the two sheath nonlinearities compensate strongly and the dc self-bias vanishes, whereas geometrical or electrical asymmetry breaks this compensation and enhances harmonic generation and plasma-series-resonance oscillations.
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All authors Katharina Noesges, Tim Bolles, Máté Vass, Ihor Korolov, Thomas Mussenbrock

Optics

Inherent Turbulence Immunity of Vector Vortex Beams in Free Space Quantum Key Distribution

Behnam Talari and Rouhollah Karimzadeh

Orbital angular momentum (OAM) multiplexing provides an infinite-dimensional discrete Hilbert space ideally suited for high-capacity free-space quantum key distribution (QKD). Nevertheless, pure scalar spatial modes carrying topological charge ($|\ell| \ge 1$) undergo severe decoherence when transmitted through terrestrial atmospheric turbulence. Turbulent refractive-index eddies split high-order vortex singularities, induce catastrophic intermodal crosstalk across adjacent topological channels, and rapidly drive the quantum bit error rate (QBER) well above the unconditional 11% security threshold associated with individual cloning attacks. Here, we demonstrate that hybrid polarization-OAM entangled states, known as vector vortex beams (VVBs), provide intrinsic, hardware-free immunity against turbulent perturbations. Because the optical anisotropy of terrestrial air is exceedingly small ($\Delta n < 10^{-9}$), refractive-index fluctuations couple symmetrically to orthogonal circular polarization modes as an identical common-mode scalar phase screen that cancels in the relative polarization-phase degree of freedom. By numerically propagating modal fields through modified power-spectral phase screens over turbulence strengths ranging from $D/r_0 = 0$ to $3.0$, we show that the VVB encoding protocol suppresses the asymptotic QBER from 42.0% to 4.8%, yielding an error-suppression factor of approximately 11.6 without requiring active adaptive optics or deformable mirrors.
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Data Analysis and Statistics · Cross-listed from cs.CL

No Model Required: Text Entropy Rate Filtering Mitigates Iterative Fine-Tuning Collapse

Lewis Mitchell

Iterative fine-tuning on synthetic data causes model collapse: output diversity narrows as rare patterns are progressively lost, a signature most visible as phrase-level repetition. Existing mitigations either require model log-probabilities, an external oracle, or continued access to real human data. Here we develop a new approach grounded in mathematical information theory: the non-parametric Kontoyiannis entropy rate estimator $h_k$, computed entirely from raw text via match-length statistics, with no model of any kind. We show that this is in fact a superior training-data filter on text-diversity metrics in a fully-synthetic, single-lineage fine-tuning setting. In a six-generation QLoRA collapse experiment on Llama-3.1-8B, logprob-based filtering (the most established model-access-requiring baseline) provides no significant text-diversity benefit on any metric ($p > 0.23$), whereas $h_k$-filtering yields $+42\%$ unique trigrams, $+30\%$ vocabulary, and $-19\%$ repetition (all $p < 0.001$). We validate $h_k$ as a cross-domain entropy proxy ($\beta = 0.924$, $R^2 = 0.746$) and collapse detector ($\rho = +0.454$, $p < 0.0001$) across 4~domains, 2~temperatures, 2~generator–scorer model pairs, and 1{,}520 generated documents. Our results demonstrate that information theoretic approaches to collapse mitigation are efficient, and suggest new approaches for maintaining multi-agent diversity.
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Plasma Physics

Quantitative schlieren imaging of a laser-ionized plasma channel in atomic vapor using symbolic regression

Gabor Demeter

The plasma channel of the AWAKE plasma wakefield acceleration experiment is created by laser ionization of rubidium vapor in a 10~m long vapor source. Verifying the properties of the channel — the radius of its fully ionized core and the width of its boundary sheath — that are vital for ensuring good-quality wakefields is important as is validating numerical models of ionizing pulse propagation. Near-resonant schlieren imaging can detect the plasma channel in the low-density vapor, but there is no direct inversion to obtain channel parameters from the images. We use symbolic regression to find compact analytical formulas that connect measurable properties of the schlieren signal to plasma profile parameters. Using databases of simulated signals generated with three different plasma edge-profile families, we obtain formulas with one and two fitting constants to predict the equivalent channel radius with an accuracy below twice the camera pixel size and formulas to predict the sheath width. Re-evaluating previous measurements with these formulas, we confirm a theoretically predicted power-law relationship between the channel radius and the energy of the ionizing laser pulse. The formulas can be adapted to new experimental conditions by refitting the constants on a modest set of new samples, providing a lightweight, interpretable alternative to evaluation with deep neural networks.
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Optics

Engineering photonic crystal slab modes for strong exciton-photon coupling and polariton dispersion control

Natalia Salakhova, Andrey Demenev, Vladimir Kulakovskii et al. (+1 more)

We numerically study exciton-photon coupling in a hybrid structure composed of a period-doubled Si3N4 photonic crystal slab and an hBN-encapsulated MoSe2 monolayer. Period doubling folds quasi-guided modes into the light cone and produces spectrally separated photonic branches whose radiative character is controlled by the symmetry of the doubled unit cell, resulting in one bright and one bound state in the continuum (BIC)-like mode at normal incidence. We then use the hBN thickness as an independent tuning parameter for the vertical field profile. Increasing the hBN thickness redistributes the resonant field toward the MoSe2 monolayer and enhances the Rabi splitting from about 3-4 meV to 8.5 meV, corresponding to an increase in the coupling strength from approximately 1.5-1.7 meV to 4.3-4.4 meV. Comparison between bright and BIC-like configurations shows that the quality factor primarily determines radiative visibility and linewidth, whereas the coupling strength is mainly governed by the field amplitude at the monolayer. We further show that the hBN layer controls the hybridization of folded TE-like modes at finite momentum, leading to an additional photonic gap whose size can be tuned by the hBN thickness and closes at an hBN thickness of approximately 113 nm. In the hybrid structure, we observe finite-momentum exciton-polariton states that, depending on the hBN thickness, form either through coupling to an isolated photonic branch or through coupling to two nearly degenerate photonic modes. These results identify period-doubled photonic crystal slabs as a flexible platform for controlling radiative coupling, exciton-photon overlap, and polariton dispersion in hybrid structures based on transition-metal dichalcogenide monolayers.
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All authors Natalia Salakhova, Andrey Demenev, Vladimir Kulakovskii, Nikolay Gippius

Physics Education

Determining the degree of randomness in multiple-choice question distractors

JPW Diener, BL Frick and J Kriek

Multiple-choice questions are a staple of educational assessment due to their efficiency, but their diagnostic ability is inherently limited. Traditional psychometric techniques infer misconceptions by fitting models to predetermined data patterns, making them ineffective for analysing small, novel, or non-standardised samples where such prior trends are absent. To address this, we introduce a new distractor analysis method based on Information Theory, which treats the randomness in incorrect answers not as noise but as a measurable signal in student answer patterns. By examining the entropy of distractor choices alongside overall accuracy, our approach offers a sample-sized-independent framework for classifying performance. Cross-referencing low-scoring item-level response data against independently documented misconceptions on a standard mechanics inventory shows that, for items identified as most concentrated (lowest degree of randomness), a majority of incorrect responses select the exact response coded to that item's named, interview-validated misconception. Furthermore, items independently flagged elsewhere as diagnostically unreliable are exactly the items the measure identifies as having the highest degree of randomness. The pedagogically actionable cases are those with low degree of randomness in a class's incorrect answers, signalling a shared, re-teachable misconception, whereas a high degree of randomness chiefly serves as the null case against which that concentration is judged. Our analysis is intended as a fast, single-administration triage step: a practical, theory-driven complement to established distractor-analysis methods that helps instructors flag which items warrant closer, more resource-intensive investigation.
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Plasma Physics

Electromagnetic drift-kinetic particle-in-cell model with energy and charge conservation for studying finite-$\beta$ plasmas

O. P. Morozov, V. A. Kurshakov and I. V. Timofeev

The paper proposes a generalization of the fully implicit energy- and charge-conserving electromagnetic particle-in-cell method to the case where the lightest type of plasma particle (electrons) is described in the drift-kinetic approximation. This allows us to remove the very strict time step limitation of this method requiring to resolve the gyrorotation of electrons. Since the drift-kinetic model is only applicable to light particles whose contribution to plasma polarization is small, this model can be further simplified by neglecting the electron polarization drift and including in Maxwell's equations, in addition to the current of gyrocenters, only the magnetization current. In order for such a hybrid model to retain conservative properties in finite-difference form, a method of self-consistent interpolation of $\nabla B$ in the mirror force from grid to particle and the magnetization vector from particle to grid is proposed. Unlike existing drift-kinetic models, this model is not limited to considering small perturbations near a given equilibrium, and therefore allows one to study the formation of plasma equilibria in regimes with a finite ratio of plasma to magnetic field pressure. Testing of the parallel code implemented in C++ using the PETSc library confirmed the fulfillment of the finite-difference laws of energy and charge conservation, as well as the ability of the model (in the drift-kinetic version for all types of particles) to correctly reproduce the diamagnetic effect and longitudinal ion-acoustic wave.
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Optics · Cross-listed from cond-mat.mes-hall

Sector-Resolved Winding Selection Rules for Structured-Light-Driven dc Currents

Tomohiro Tamaya and Kenichi L. Ishikawa

Structured light can generate electronic dc currents with azimuthal winding, yet the rules governing their winding order $m$ remain unclear. Here we identify sector-resolved winding selection rules using graphene as a clean two-dimensional platform. By decomposing the current response into local and gradient sectors at the current-operator level, we show that the winding order is determined not by the optical orbital angular momentum $\ell$ alone, but by the angular structure of the corresponding current operators together with projection onto the azimuthal direction. For scalar Laguerre-Gaussian beams, linear polarization yields $m=|\ell\pm1|$ in the local sector and $m=|\ell|,|\ell\pm2|$ in the gradient sector, whereas circular polarization with helicity $\sigma=\pm1$ selects $m=|\ell-\sigma|$ in the local sector and $m=|\ell+2\sigma|$ in the gradient sector. Numerical time-evolution calculations verify these rules and further show that helicity can select an $m=0$ branch, producing an azimuthally uniform circulating current whose radial profile determines the axial magnetic field $B_z(z)$. Our results provide a sector-resolved organizing principle for classifying and controlling structured-light-driven dc currents with tailored winding structures.
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Chemical Physics

Deciphering the internal conversion and triplet formation in thymine via time-resolved multi-center X-ray photoelectron spectroscopy

Xiaojun Wang, Woojin Park, Dennis Mayer et al. (+12 more)

The photostability of DNA nucleobases relies on ultrafast nonradiative relaxation following ultraviolet excitation. In thymine, the competition between internal conversion (IC) and intersystem crossing (ISC) determines the balance between ultrafast energy dissipation and potentially long-lived photochemistry. However, the precise molecular mechanisms linking early singlet-state electronic and structural evolution to the formation of long-lived triplet dark states remain unresolved. Here, we investigate the gas-phase photodynamics of thymine using time-resolved multi-center X-ray photoelectron spectroscopy (tr-mc-XPS) supported by state-of-the-art ab initio quantum calculations. By simultaneously probing multiple carbon 1s core levels, we track site-specific electronic and nuclear dynamics. We discover that the long-lived dark state is not initially a pure triplet; rather, it exists as a mixture of singlet and triplet states for the first 240 ps before transitioning fully to the $^3\pi\pi^*$ state. On the femtosecond timescale, we resolve atom-specific vibrational coherence at $\sim$730 cm$^{-1}$ (a ring-breathing mode with a period of 46 fs) that survives the initial $^1\pi\pi^*$ to $^1n\pi^*$ IC. Furthermore, we identify a critical dual role for the methyl group (C9H$_3$). During singlet relaxation, the methyl group acts as an inertia spectator, and its mass hinders direct internal conversion to the ground state. Upon $^3\pi\pi^*$ state formation, however, state-dependent hyperconjugation couples the methyl group to the pyrimidine ring, turning it into a highly sensitive electronic reporter of ISC. These findings establish tr-mc-XPS as a powerful approach for disentangling the complex, multi-timescale photodynamics governing nucleobase stability.
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All authors Xiaojun Wang, Woojin Park, Dennis Mayer, Fabiano Lever, Giovanni Cirmi, Michele Di Fraia, Benjamin Erk, Marion Kuhlmann, Zane Phelps, Oksana Plekan, Federico Pressacco, Sebastian Schulz, Cheol Ho Choi, Miquel Huix-Rotllant, Markus Gühr

Accelerator Physics

The JO&Minna Framework: A Method to Interpret Depth Profiles of Activation

Edil Mustafin

A novel analytical framework, designated as the JO&Minna method, is presented for interpreting the depth profiles of residual activation induced by heavy-ion beams in thick structural materials. By combining the limiting fragmentation and factorization principles, the JO-factor formalism rigorously links the spatial distribution of projectile-like fragments to the range derivatives and electronic stopping powers. Furthermore, the Minna procedure establishes a mathematical link via profile differentiation and convolution analysis to reconstruct the original energy spectrum and energy spread of the fragments from the measured activity profiles. The methodology is benchmarked using experimental data for $^{46}\text{Sc}$ production from $^{238}\text{U}$ irradiation in aluminum, demonstrating a reliable and straightforward approach for extracting fragment production kinematics and energy distributions in accelerator materials research.
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Accelerator Physics

High-Field Brightness Limits of Alkali-Antimonide Photocathodes

Peng-Wei Huang, Zhiyuan Wang, Xinlong Cao et al. (+6 more)

Pushing the brightness limit of electron sources requires simultaneously minimizing the intrinsic emittance and maximizing the accelerating field. Alkali-antimonide photocathodes exhibit excellent properties at low fields, yet their high-field photoemission physics remains poorly understood owing to acute vacuum sensitivity. Here, we demonstrate robust photoemission from alkali-antimonide photocathodes in a radio-frequency gun at peak fields exceeding 100 MV/m, with quantum efficiency maintained above 1% for more than two weeks, enabling the first systematic measurements of their high-field brightness limits. The measured field dependence of the intrinsic emittance provides direct insight into photoemission physics at high fields. Near-threshold photoemission further reduces the intrinsic emittance, increasing the attainable brightness, and reveals constraints imposed by surface roughness. This work opens the high-field regime for advanced photocathodes, extending the brightness limits of electron sources.
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All authors Peng-Wei Huang, Zhiyuan Wang, Xinlong Cao, Zhuoxuan Liu, Lianmin Zheng, Yingchao Du, Wenhui Huang, Chuanxiang Tang, Renkai Li

Optics

Stochastic switching in quadrupole-trap-based levitodynamic systems

Vadim Rybin, Semyon Rudyi and Dmitrii Shcherbinin

We have theoretically investigated the stochastic switching dynamics of a charged microparticle in a quadrupole trap near the principal parametric resonance at atmospheric pressure. We show that this behavior arises from a balance of thermal noise-activated escape and dissipative return between linear and nonlinear dynamical regimes of the particle motion. The switching follows an effective Gibbs distribution with an effective potential barrier between the regimes and effective system temperature, which are both experimentally accessible. Because the effective barrier depends on particle mass and charge, and since thermal noise reveals rather than obscures the transition, this exponential dependence could provide the foundation to precision noise-aware detection schemes. The system further offers a highly adjustable platform for studying activation dynamics in non-equilibrium parametric systems.
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Chemical Physics

SupraTITO: Transferable Generative Molecular Dynamics for Supramolecular Systems

Weilong Chen, Nuno Costa and Julija Zavadlav

Peptide sequence governs both the structures formed through supramolecular assembly and the dynamics by which they emerge, but predicting either requires resolving slow collective processes among many interacting molecules. Molecular dynamics (MD) provides microscopic insight into these processes, yet the long timescales of assembly and the vast peptide sequence space make systematic exploration computationally demanding. We introduce SupraTITO, a transferable generative molecular dynamics (GenMD) framework for supramolecular systems, demonstrated through peptide self-assembly. SupraTITO learns transferable implicit transfer operators (TITO) conditioned on peptide sequence, molecular topology, and periodic geometry, allowing configurations to be propagated over physical intervals much longer than an MD integration step. On a comprehensive dipeptide benchmark, SupraTITO generalizes to held-out sequences and reproduces sequence-dependent structures and dynamics while maintaining molecular integrity over long rollouts. Compared with direct ensemble prediction trained on the same trajectory data, SupraTITO more accurately reproduces assembly structures while also resolving their temporal evolution. The learned dynamics generalize across peptide concentrations, including dilute conditions not represented during training. These results extend transferable GenMD to collective dynamics in periodic supramolecular systems and provide a foundation for modeling related processes beyond peptide assembly.
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Optics

Inverted Leakage Radiation Microscopy for measurement of nonreciprocity induced by a plasmonic metasurface with false chirality

Ahmed Lafeef Ettapuram Naduvilepurayil, Sahil Sahoo and Yuri Gorodetski

A system has a nonreciprocity when the light propagating through it in opposite directions encounters different optical adventures. This effect lies in the origin of various applications, including signal processing, noise reduction, unidirectional propagation, and sensing. Here, we experimentally demonstrate the detection of nonreciprocity in plasmonic metasurfaces and a method to determine its direction. We use the Inverted Leakage Radiation Microscopy setup to show that the non- reciprocity is linked to a false chirality of the medium. The setup behaves as an equivalent of the classical Sagnac loop providing an interference pattern at the image plane. The non-reciprocity is then detected by examining the topological order of the resulting image. Our system represents a compact and versatile method to detect false chirality in plasmonics and suggests a variety of potential areas of implementation in nanophotonics, chemistry and applied physics.
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Applied Physics · Cross-listed from cond-mat.mtrl-sci

Benchmarking average atom potentials derived from atomic cluster expansions

Deepak Somani, Lorenzo Piersante and Anirudh Raju Natarajan

Average atom (A-atom) potentials provide a mean-field description of a chemically disordered alloy and are used to predict the properties of solid solutions without short-range order. Such potentials are usually averaged from an existing interatomic potential and are therefore only as accurate as the parent model. Accurate interatomic potentials are themselves difficult to parameterize and can require large training datasets. Here we benchmark a recently developed formalism that computes an exact A-atom potential directly from a linear atomic cluster expansion (ACE). We first fit a linear ACE to Fe-W data generated with an embedded atom method (EAM) potential. The resulting A-atom potential reproduces the properties of the disordered phase computed from an explicit random supercell and from a conventional A-atom potential averaged from the same EAM potential. We then fit a linear ACE to energies and forces computed from electronic structure calculations for about 1500 small Mo-Nb structures with an average of 7 atoms per structure. The A-atom potential derived from this ACE reproduces the DFT elastic constants, lattice parameter, mixing enthalpy, and Bain path of special quasirandom structures. Of the three chemical site bases, only the occupation basis also reproduces the DFT surface energies of the alloy. For Mo-Nb, the A-atom potential also predicts that the ideal solution entropy outweighs the destabilizing vibrational contribution to the finite-temperature free energy of the disordered phase. These benchmarks show that the properties of disordered alloys can be recovered from small training datasets when the chemical site basis is chosen carefully.
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Applied Physics · Cross-listed from cond-mat.mes-hall

Inertial Dynamics of a Skymeron

Mona Bhukta, Duc Minh Tran, Kilian Leutner et al. (+19 more)

Topological spin textures in antiferromagnets inherit the compensated magnetic order of the host material, resulting for skyrmions in the suppression of the skyrmion Hall effect and enabling ultrafast dynamics that make them attractive for low-power spintronic devices. In intrinsic antiferromagnets, the two sublattices are identical, but synthetic antiferromagnets offer additional control mechanisms by enabling independent tuning of the properties of two ferromagnetic layers. Analogous to a ferrimagnet, the total magnetic moment can be adjusted by compensating the two-layer moments. Here, we demonstrate that tuning the effective magnetic anisotropy difference between the two layers drives a spin-flop transition where one layer reorients and thus stabilizes an orthogonal configuration with one layer oriented along the out-of-plane direction and the other in-plane. In this transition, an antiferromagnetic skyrmion undergoes a homotopic reconfiguration into a complex spin texture comprising a skyrmion coupled to an in-plane bimeron. Element-specific X-ray microscopy resolves this texture layer by layer: one hosts an out-of-plane skyrmion coupled to an in-plane bimeron in the other. We refer to this previously unexplored three-dimensional spin texture as a skymeron. By using time-resolved pump-probe X-ray microscopy, we discover unique polarity ,dependent dynamics during current pulses: a short post-pulse inertia-like continuation of the motion, and a slower return towards the pinned initial state. Micromagnetic simulations reveal that the inertia-like propagation originates from the finite-time relaxation of reorientation of the skymeron. Our results establish layer-selective anisotropy engineering as a route to uncharted composite spin textures with internal dynamical degrees of freedom, not possible in conventional antiferromagnets with identical sublattices.
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All authors Mona Bhukta, Duc Minh Tran, Kilian Leutner, Takaaki Dohi, Nikolai S. Kiselev, Filipp N. Rybakov, Olle Eriksson, Fabian Kammerbauer, Sebastian Wintz, Markus Weigand, Hendrik Ohldag, Maria-Andromachi Syskaki, Robin Tietgen, Edoardo Mangini, Sabrina Kerber, Thibaud Denneulin, Joseph Vimal Vas, Rafal E. Dunin-Borkowski, Bastian Pfau, Oleg A. Tretiakov, Robert Frömter, Mathias Kläui

Medical Physics

Quiet, rapid 3D multiparametric mapping using magnetization-prepared zero echo time MRI

Alireza Samadifardheris, Shishuai Wang, Ana Beatriz Solana et al. (+10 more)

Purpose: To introduce and evaluate MuPa-ZTE, a quiet, rapid 3D framework combining native and magnetization-prepared zero echo time (ZTE) acquisitions for multiparametric mapping. Methods: MuPa-ZTE combines steady-state native ZTE with transient-state magnetization-prepared ZTE. Two implementations were evaluated: T2T1-ZTE for apparent proton density, T1, and T2 mapping, and T1-ZTE for apparent proton density and T1 mapping. Both were assessed in an ISMRM/NIST system phantom and two healthy volunteers; T2T1-ZTE was also demonstrated in a patient with brain metastases. The 10-minute phantom and 4.5-minute in vivo acquisitions were retrospectively truncated to 3, 2, and 1 minute and reconstructed with and without deep learning-based denoising. Evaluations included phantom agreement, precision, short-term repeatability, apparent SNR, edge sharpness, and consistency with full-duration in vivo maps. Results: T1 estimates remained close to nominal phantom values across implementations, durations, and reconstructions. T2 accuracy was maintained down to 2 minutes over the brain-relevant range, with limited sensitivity to longer T2 values. Denoising generally reduced variability and improved short-term repeatability. In vivo, 1.1-mm isotropic whole-brain maps were obtained in 4.5 minutes; denoising increased apparent SNR while preserving edge sharpness and yielded promising image quality after retrospective truncation to 2 minutes. Conclusion: MuPa-ZTE enables quiet, isotropic 3D multiparametric mapping within 4.5 minutes, supporting robust T1 mapping and T2 mapping over a brain-relevant range, with promising acceleration toward 2 minutes using deep learning-based denoising.
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All authors Alireza Samadifardheris, Shishuai Wang, Ana Beatriz Solana, Jose de Arcos, Noemi Sgambelluri, Emil Langensee Ljungberg, Sagar Mandava, Mika Vogel, Steven C. R. Williams, Stefan Klein, Juan Antonio Hernandez-Tamames, Dirk H. J. Poot, Florian Wiesinger

Applied Physics

Determination of van der Waals thin film thickness via characteristic X-ray photon yield

Guoshun Luo, Shichao Zhao, Xinyu Liu et al. (+7 more)

Characteristic X ray generation is essential for producing X ray sources and for investigating light matter interactions. Here, we demonstrate that characteristic X ray generation can also be employed to determine the thickness of van der Waals thin films in a non destructive manner. Specifically, we establish a sixth order polynomial model that relates film thickness to the characteristic X ray yield. The model can both predict the X ray yield of graphite films at arbitrary thicknesses and infer the film thickness from a corresponding characteristic X ray yield. We validate the model by comparing its predicted X ray yields for graphite films with those obtained from Monte Carlo simulations. Additionally, we experimentally validated the reliability and feasibility of the proposed method for extracting graphite film thickness via X ray photon yield characterization. Our results provide a non destructive approach for determining the thickness of van der Waals thin films.
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All authors Guoshun Luo, Shichao Zhao, Xinyu Liu, Suguo Chen, Yue Wang, Ping Zhang, Zhigang Lu, Shaomeng Wang, Sunchao Huang, Yubin Gong

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

Inclined surfaces bias bacterial swarming expansion and edge organization

Pengxi Gu, Zixiang Lin, Hui Ren et al. (+5 more)

Bacterial swarming is the collective expansion of dense populations of motile cells across hydrated soft surfaces, yet gravity is usually treated as a fixed background condition in standard horizontal assays. We used surface inclination to examine how agar-surface orientation relative to gravity affects the expansion of Enterobacter sp. SM3 swarms. Colony expansion depended non-monotonically on inclination, with the strongest spreading at intermediate inclinations and weaker expansion under shallow or near-inverted conditions. Paired up-facing acute and down-facing obtuse configurations with the same tilt magnitude showed different expansion responses, suggesting that swarm expansion depended on both tilt magnitude and which side of the agar surface faced upward relative to gravity. Inclination also produced downhill-biased colony morphologies. PIV analysis showed generally lower mean activity under inclination than in the flat references, with a non-monotonic angular response and inward increases in speed and spatial velocity-correlation length. The downhill tip showed slower motion, shorter spatial correlations and smaller vortex cores than the upper region. To interpret the macroscopic trends, we extended a two-phase thin-film model by introducing an effective gravity coupling projected normal and tangential to the agar surface. Simulations suggest that the surface-tangential component mainly drives lateral redistribution, whereas the surface-normal component modifies film pressure and thereby pressure-dependent water replenishment, helping distinguish up-facing from down-facing configurations.
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All authors Pengxi Gu, Zixiang Lin, Hui Ren, Jin Zhu, Anqi Li, Omar Said, Weijie Chen, Zijie Qu

Optics

A Mid-Infrared Absorber Inspired by the Wing-Scale Photonic Structure of Troides magellanus

Amandine Marchand, Kevin Delmote, Olivier Deparis et al. (+1 more)

Growing environmental and energy-related challenges have intensified the search for technologies capable of improving energy recovery and resource efficiency. Bioinspired photonic structures offer promising opportunities for the development of efficient infrared absorbers by exploiting optical mechanisms found in natural systems. In this work, we present a bioinspired approach for the design and optimization of a broadband mid-infrared absorber based on the highly absorbing wing scales of the Troides magellanus Magellan birdwing butterfly. The characteristic geometry of the biological structure is rescaled and adapted to shift its optical response from the visible to the infrared spectral range. Numerical simulations are used to investigate the influence of the main geometrical parameters on the spectral absorptance and to identify an optimized configuration. The optimized structure achieves an absorption enhancement of 391% compared with a flat steel reference. The improved optical response is attributed to the optimized high-aspect-ratio geometry, which enhances multiple scattering and light trapping, thereby increasing the effective light-matter interaction area. These results demonstrate the potential of bioinspired geometric scaling as an effective strategy for engineering efficient infrared absorbers and tailoring their spectral response for thermal radiation management and infrared photonic applications.
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All authors Amandine Marchand, Kevin Delmote, Olivier Deparis, Sébastien R. Mouchet

Instrumentation and Detectors

State of the Art in Direct Dark Matter Detectors: Technologies, Performance, and Future Directions

Muad Ghaith and Khaled Obaideen

Identifying the particle nature of dark matter remains one of the most significant challenges in modern physics. Direct detection experiments aim to observe rare scattering events between dark matter particles and terrestrial targets, a task that demands extreme background suppression and sensitivity to minute energy depositions. This review critically assesses the current experimental landscape, organizing detector technologies by the fundamental physical trade-offs that define their scientific reach. We contrast the multi-tonne scalability of noble-liquid Time Projection Chambers (TPCs), which currently define the sensitivity frontier for high-mass Weakly Interacting Massive Particles (WIMPs), with the precision of cryogenic semiconductors and Charge-Coupled Device (CCD)- based sensors, which dominate the search for low-mass and sub-GeV candidates. Special emphasis is placed on the role of advanced reconstruction pipelines and machine learning (ML) as integral components of detector performance. Finally, we discuss the strategic roadmap for the next decade as experiments approach the neutrino fog, where coherent elastic neutrino-nucleus scattering (CE{\nu}NS) produces an increasingly significant and ultimately irreducible background that can mimic DM-induced nuclear recoils. We argue that future progress will rely not on a single technology but on a complementary global program combining increased target mass, ultra-low energy thresholds, improved background discrimination, and distinct observables - such as directionality and temporal signatures - to maintain robust discovery capability in the presence of neutrino-induced backgrounds.
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Applied Physics

Correlation Imaging via Hybrid Entanglement between Microwave Photons and Surface Acoustic Wave Phonons

Yu-Yuan Chen, Ling-An Wu and Yu-xi Liu

Establishing effective spatial correlations between distinct quantum fields is essential for multi-physics quantum sensing.~We here propose a correlation imaging approach based on the hybrid entanglement between microwave photons and microwave surface acoustic wave phonons generated via a superconducting quantum circuit.~At matched microwave frequencies, the five order-of-magnitude wavelength disparity between photons and phonons leads to a unique quantum entanglement resource, which allows the optimal field for probing an object to be distinct from that used for readout.~This resource enables two imaging modalities with giant spatial scaling: a $10^{-8}-10^{-6}$-fold demagnification of macroscopic object profiles onto microscopic phononic chips, and conversely, a $10^{4}-10^{6}$-fold magnification of microscopic object profiles into macroscopic photonic readout devices. By interfacing free-space long-wavelength microwaves with on-chip short-wavelength surface acoustic waves, our correlation imaging approach provides a general framework for cross-scale quantum sensing.
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Instrumentation and Detectors · Cross-listed from astro-ph.SR

Towards solar many-line inversions

Johannes Hölken

The Sun's atmosphere is significantly influenced by events that often measure only tens of kilometres in the horizontal direction. They happen in both quiet and active regions and are determined by magneto-hydro-dynamic processes. Their origins often lie in the lowest layers of the solar atmosphere. To improve the understanding of chromospheric and coronal heating, the solar dynamo and its activity cycle, or to improve the prediction of space weather events we must therefore enhance our understanding of such small-scale events. In this work I first describe the process of calibrating spectropolarimetric observations with very high spatial resolution and put together the building blocks for the mathematical proof the MOMFBD image reconstruction algorithm. In the second part I describe the FISS-SP prototype which is designed to fully exploit the resolution power of a 1.6m telescope. This instrument is built to provide suitable data for image reconstruction of spectropolarimetric data. The resulting data can resolve small-scale features down to the diffraction limit. The simultaneous interpretation of a few lines is still the norm for spatially highly resolved solar spectra. Theoretical works have shown that the simultaneous interpretation of multiple solar lines can greatly improve the robustness against noise. Using observations from FISS-SP I advance the theoretical idea into a practical technique. With this I can retrieve a coherent model of the local solar atmosphere, resolving structures down to a horizontal size of about 49~km. Finally, I use those new techniques to interpret an observation of a solar pore. In this last part I describe and analyse small-scale structures, which could not be spectropolarimetrically resolved before. Typical pore-boundary features - namely, dark striations with bright grains moving into the pore - are identified as convective features.
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Optics

Vector solitons in nonlinear lattices with competing SPM and XPM terms

Liangwei Zeng, Boris A. Malomed, Dumitru Mihalache et al. (+1 more)

We demonstrate that two-component photonic nonlinear lattices with opposite signs of the out-of-phase spatially modulated SPM and XPM coefficients can support two types of optical vector solitons (VSs), the fundamental and mixed ones. The fundamental VSs are composed of two Gaussian-like components, while the mixed ones combine Gaussian-like and dipole components. The structures and stability of both types of the VSs are investigated. The VSs are stable, except for narrow intervals at small values of the propagation constant.
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All authors Liangwei Zeng, Boris A. Malomed, Dumitru Mihalache, Xing Zhu

Optics · Cross-listed from cond-mat.mtrl-sci

Additive solid immersion lenses for enhanced collection efficiency of shallow NV centers by pulsed laser deposition and structurization of high-k amorphous oxides

Michael S. Bar, Nicole Raatz, Max Kneiss et al. (+3 more)

The negatively charged nitrogen-vacancy ($\mathrm{NV}^-$) defect in diamond can be used for quantum information processing via the readout of the radiative decay of the excited state into the ground state. The large refractive index mismatch of diamond and air causes low photon extraction efficiency due to total internal reflection losses. Here, we investigate the photon extraction numerically and report additive solid immersion lenses (SILs) consisting of the high-k dielectric yttria-stabilized-zirconia (YSZ) for surface modification and enhanced light extraction of shallow implanted nitrogen-vacancy centers. We demonstrate an average increase of 63% in saturation intensity using a scalable bottum up room temperature pulsed laser deposition and photolithography structurization process. The average necessary laser intensity for saturation is reduced by 33% by the SILs.
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All authors Michael S. Bar, Nicole Raatz, Max Kneiss, Andreas Koenig, Marius Grundmann, Holger von Wenckstern