Chemical Physics
Multidimensional Design of Metal-Nitrogen-Carbon Electrocatalysts for Direct Propylene Epoxidation
Songbo Ye, Qingyuan Han, Jingwen Chi et al. (+6 more)
Propylene
oxide is a major industrial chemical whose production currently relies on hazardous chlorine- or peroxide-based oxidants. Direct electrochemical epoxidation using water as the oxygen source offers a sustainable alternative, but controlling oxygen-atom transfer against the competing oxygen
evolution reaction remains a fundamental challenge. Here, we show that propylene epoxidation selectivity cannot be described by oxygen binding energy alone, but is jointly governed by oxygen adsorption, the potential of zero charge, and applied potential. By combining theoretical calculations with pH-field-coupled microkinetic modeling across 41 metal-nitrogen-carbon single-atom catalysts, we first identified an optimal oxygen-binding window and Co as the most favorable metal center. We then found that peripheral substituents can tune the PZC while largely preserving the optimal oxygen adsorption energetics, thereby providing an independent design dimension to further optimize the already favorable Co active site. This sequential, multidimensional design strategy identified CoPc-NH2-CNT as the optimal catalyst, delivering a record PO Faradaic efficiency of 70-80 percent for direct propylene epoxidation in aqueous electrolyte under ambient conditions. These results establish interfacial electrostatics as an independently tunable design dimension for controlling selective oxygen-atom transfer in electrocatalysis.
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All authors Songbo Ye, Qingyuan Han, Jingwen Chi, Yuan Huang, Heng Liu, Di Zhang, Hitoshi Shiku, Li Wei, Hao Li
Optics
Full-field and Bloch-periodic-factor discretizations: Accuracy and phantom modes
Igor Tsukerman
One can distinguish four major formulations of the Bloch-mode problem in linear periodic media. First, there are two common complementary ways of slicing the Bloch variety (Bloch wave vectors paired with the corresponding frequencies): frequency vs. wave vector or, conversely, wave vector vs. frequency. This paper deals exclusively with the latter option, directly applicable to lossy media, evanescent waves, and complex band structures. A separate crossroads is the full field (FF) vs. the
lattice-periodic Bloch factor (PF) formulations. These are fully equivalent on the continuous level, but common discretization techniques may break this equivalence not just approximately but qualitatively. In the FF problem, the primary unknown
eigenvalue is the Bloch phase factor (not the wavenumber), which enters only through opposite-boundary coupling. The PF formulation, on the other hand, injects the Bloch wavenumber into the differential operator and leads to a volume quadratic pencil. Consequently, standard PF discretizations of the type considered here, in contrast with the corresponding FF discretizations, violate the reciprocal-
lattice (Brillouin-zone-shift) covariance – which, as the theory and numerical examples in the paper show, may lead to inaccurate or even nonphysical modes. From the mathematical perspective, the paper highlights the role of structure-preserving
algorithms. The practical importance of the results and recommendations stems from the wide adoption of PF formulations in optics, photonics, and beyond – such as the computation of propagating and evanescent Bloch modes in photonic and acoustic structures, topological photonics, effective-mass and topological
insulator band theories.
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Fluid Dynamics
Long-time behavior of optimal mixing in an advection-diffusion shell model
Jiajia Guo, Baole Wen, Christian Seis et al. (+1 more)
We investigate the long-time behavior of optimal mixing in an advection-
diffusion equation using a shell model framework. Our focus is on quantifying the decay of the scalar variance, measured by the negative Sobolev norm $H^{-1}$, under enstrophy-constrained stirring. We perform long-time computations using both local-in-time (maximizing the instantaneous mixing rate) and global-in-time (maximizing mixedness at a prescribed final time) optimization strategies. For mixing with
diffusion ($\kappa>0$), the numerical results show that the scalar length scale eventually becomes limited by a generalized Batchelor scale, in close agreement with theoretical predictions. In this regime, the $H^{-1}$ mix-norm decays exponentially in time with a decay rate that is independent of the diffusivity $\kappa$. Compared with the purely advective case ($\kappa = 0$),
diffusion significantly enhances the long-time mixing rate; moreover, increasing diffusivity further improves mixing efficiency by reducing the prefactor of the exponential decay. Guided by these numerical observations, we derive new conditional lower bounds on the $H^{-1}$ norm whose exponential decay rates are strictly independent of the diffusivity parameter $\kappa$, for all $\kappa > 0$. We further establish conditional upper bounds on the maximal rate of enhanced dissipation of the scalar variance, showing that the effective
diffusion time scale is at least of the order $|\log\kappa|$.
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All authors Jiajia Guo, Baole Wen, Christian Seis, Charles R. Doering
Chemical Physics
Is the TDDFRT representation of a molecular electronic transition unique?
Jérémy Morere and Thibaud Etienne
In this article a piece of the reference time-dependent density-functional response theory (TDDFRT) representation of molecular electronic transitions is studied through the one-body reduced difference density
matrix. A first derivation route for that object has been reported in the literature based on the substitution of TDDFRT-related objects into an exact functional expression. We show in the text that the functional providing the exact object is not unique, and we question whether substituting the same TDDFRT-related objects into the alternative functional expressions leads to acceptable candidates for the approximate TDDFRT one-body reduced difference density
matrix, which would question the unequivocality of the TDDFRT representation of molecular electronic transitions. We first directly address the problem using a novel diagrammatic
language, inspired by Dyck
languages, and specifically designed for dealing with this kind of issue. In the last part of the article we question whether it is possible or not to recast the problem in a more general framework - the equations-of-motion framework. Through all the text, the case of time-dependent Hartree-Fock is compared with that of TDDFRT to highlight what makes them so different when those questions are at stake.
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Atomic Physics · Cross-listed from quant-ph
Analytical Theory of Higher-Order Collective Spin Interactions in Cavity Quantum Electrodynamics
Leilani Ainsworth, Chase Gomes, Joseph Prescott et al. (+5 more)
Cavity-mediated collective-
spin interactions are commonly described by a quadratic one-axis twisting
Hamiltonian. However, the underlying atom-light interaction naturally generates nonlinearities to arbitrary order. Here, we derive a closed-form analytical expression for the complete hierarchy of cavity-mediated collective-
spin interactions. We show that the nonlinear coefficients $\chi_k$ are governed by Chebyshev
polynomials, with $k$ the order of nonlinearity. This yields a universal scaling $\chi_k\propto\eta^k$ with the single-atom cooperativity $\eta$ and a description of their dependence on cavity detuning. The result provides a systematic framework for determining when higher-order nonlinearities become relevant and when the quadratic approximation breaks down. We identify experimentally relevant regimes in which higher-order terms substantially modify collective-
spin dynamics, accelerating the generation of
quantum correlations and
quantum Fisher information, and demonstrate that finite-order expansions can accurately reproduce the full cavity-mediated
evolution. Our results establish a general framework for understanding higher-order nonlinearities in cavity
quantum electrodynamics and their role in collective
entanglement and
quantum-enhanced sensing.
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All authors Leilani Ainsworth, Chase Gomes, Joseph Prescott, Kaley Wilcox, Jack Sullivan, Esteban Teran, Manav Bilakhia, Simone Colombo
Fluid Dynamics · Cross-listed from math.OC
Optimizing bounds for energy-constrained optimal cooling problems in two dimensions
Pedro Blöss Braga and Giovanni Fantuzzi
We study optimal control problems for incompressible fluids in two-dimensional domains with a cold boundary and internal heat sources and sinks. Given a kinetic energy budget, measured by the square of a nondimensional Péclet number $\mathrm{Pe}$, the goal is to maximize a cooling efficiency inversely proportional to the mean square gradient of the fluid's temperature. Using Lagrange duality, we formulate a well-posed dual problem whose solution yields an upper bound on the maximum cooling efficiency $\mathcal{E}(\mathrm{Pe})$. We then numerically approximate the dual problem using a convergent hierarchy of semidefinite programs obtained via discretization. We illustrate this computational approach on optimal cooling problems in a square and in an annulus, explaining also how problem
symmetries can be exploited to reduce computational complexity. Finally, we construct admissible points for the dual problem to obtain new analytical upper bounds on the optimal cooling efficiency $\mathcal{E}(\mathrm{Pe})$. Specifically, we prove that $\mathcal{E}(\mathrm{Pe}) \lesssim \mathrm{Pe}^{2}$ for arbitrary domains and heat distributions, and that $\mathcal{E}(\mathrm{Pe})\lesssim \mathrm{Pe}^{2}/ \ln^2\mathrm{Pe}$ for cooling flows in disks and annuli with heat source/sink distributions with a positive azimuthal average. These results generalize and improve known efficiency bounds for energy-constrained cooling flows in a disk.
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Accelerator Physics
On-axis top-up injection with multipole injection kicker in FCC-ee
Sen Yue, Yann Dutheil and Masamitsu Aiba
Top-up injection is required in FCC-ee to maintain high luminosity given the short beam lifetime. The present baseline relies on on-axis off-energy injection with a conventional orbit bump; in this work, an alternative on-axis scheme based on a multipole injection kicker (MIK) is investigated. The FCC-ee beam parameters place challenging demands on the MIK field distribution, which must combine a large field-free region for the circulating beam, a narrow transition region, and a sufficiently uniform field in the injection region across all operation modes. To relax these challenging requirements while limiting the residual kick on the circulating beam, a compensated MIK scheme is proposed, in which a compensation MIK (CMIK) is placed upstream of the main MIK, and the required optics and field-
symmetry conditions between the two devices are derived for different phase advances. The pulsed-conductor kicker is adopted as the preferred design, and its influence on the circulating beam and its injection performance are evaluated for the Z operation mode. In the Z mode, the near-zero phase-advance compensated scheme limits the single-pass emittance growth of the circulating beam to about $7\%$ horizontally and below $2\%$ vertically, while the injected beam requires transfer-line pre-compensation to mitigate the field gradient in the injection region. These results indicate that compensated MIK injection is a promising candidate for FCC-ee on-axis top-up injection.
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Computational Physics · Cross-listed from math.NA
Coupling-Aware Vanka Smoothing for Multigrid Preconditioning of the Implicit Immersed Boundary Equations
Cole Gruninger and Boyce E. Griffith
The immersed boundary (IB) method models fluid–structure interaction using the natural Lagrangian and Eulerian formulations of structural mechanics and fluid dynamics, respectively, but explicit time discretization of the IB force imposes a stiffness-dependent upper bound on the time-step size. Treating these forces implicitly removes this restriction, but the resulting coupled linear systems become increasingly difficult to solve as the structural stiffness increases. Algebraically eliminating the Lagrangian degrees of freedom yields a reduced Eulerian velocity–pressure IB system. Here we introduce a coupling-aware Vanka (CAV) smoothing strategy to enable effective multigrid preconditioning of this system. CAV patches are built as unions of standard pressure-centered Vanka patches, with the graph of the Eulerian elasticity
matrix determining which patches are combined. Under grid refinement, CAV patch sizes remain bounded, and the computational cost of each multigrid cycle thereby grows linearly with the number of Eulerian degrees of freedom. Tests using target-point, membrane, and beam force laws show that CAV-preconditioned FGMRES reduces the relative residual by ten orders of magnitude in $9--15$ iterations, with little growth under grid refinement. In a nonlinear
benchmark modeling flow past a flexible fiber, the average number of FGMRES iterations per Newton solve increases only from $8.6$ to $9.5$ as the Eulerian grid is refined from $32\times32$ to $256\times256$
cells. To our knowledge, CAV provides the first robust multigrid strategy for time-dependent implicit IB formulations.
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Computational Physics · Cross-listed from cond-mat.mtrl-sci
Efficient simulation of second-order phase transitions in quantum anharmonic materials
Andrea Baldanza and Lorenzo Monacelli
When a
crystal undergoes a second-order structural phase transition, such as in ferroelectrics, Peierls, and charge-density waves, the diverging fluctuations of the order parameter lead to the break- down of the standard
phonon quasiparticle picture. Simulating these highly anharmonic regimes is notoriously challenging, as methods such as molecular dynamics suffer from a critical slowdown near the transition point, while the harmonic approximation fails dramatically at saddle points of the energy landscape stabilized by
quantum or thermal ionic fluctuations. This work introduces a new approach, based on the variational free-energy principle, to predict critical long-range behavior and dynamical spectra in strongly anharmonic systems, even when
quantum ionic fluctuations dominate. The proposed framework builds upon the
stochastic self- consistent harmonic approximation but reduces its computational scaling with the number of atoms, N, from O(N^6) to O(N^2) and the
memory requirement from O(N^4) to O(N). We
benchmark the method on the prototypical lead-free metal-halide perovskite CsSnI3, a promising candidate for photovoltaic engineering, simulating its phase stability and Raman spectrum near the phase transition, where the breakdown of the quasiparticle picture becomes evident. We demonstrate the effectiveness of the method by computing the full free-energy Hessian and the critical temperature in a supercell with 1080 atoms. Such simulations would have required tens of thousands of years with the legacy approach; it is now feasible in a few hours on consumer hardware.
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Optics
A Reflective Metasurface for High-Efficiency Terahertz Cross-Polarization Conversion
Muhammad Fayyaz Kashif, Shobit Agarwal, Antonio Iodice et al. (+5 more)
We demonstrate a reflective terahertz (THz) polarization conversion metasurface based on a double-ring aluminum resonator on a quartz substrate. Full-wave simulations are used to optimize the unit-
cell geometry and investigate the polarization conversion mechanism through phase analysis, polarization ellipses, and surface-current distributions. The proposed metasurface exhibits a near-unity polarization conversion ratio (PCR) at resonance frequencies of 0.333 THz and 0.361 THz. Reflection-mode terahertz time-domain spectroscopy (THz-TDS) measurements show good agreement with the simulated results. The measured PCR exceeds 87% at both resonance frequencies, demonstrating efficient reflection-mode polarization conversion.
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All authors Muhammad Fayyaz Kashif, Shobit Agarwal, Antonio Iodice, Daniele Riccio, Junaid Yaseen, Zahra Mazaheri, Gian Paolo Papari, Antonello Andreone
Atmospheric and Oceanic Physics · Cross-listed from cs.CY
Meteorology-driven Causal Nowcasting of Fugitive Landfill Emissions from Measured Coupling Timescales
Timothy C. Pearce, David J. T. Smith, Alec Dobney et al. (+1 more)
Which meteorological processes control exposure to fugitive gases downwind of a source, and on what timescales, have largely been inferred from dispersion theory and partial field evidence. Here we show that the meteorological drivers of elevated hydrogen sulphide (H$_2$S) exposure at a long-monitored European landfill, and the timescales over which each acts, can be identified directly from monitoring data. Wind direction, wind speed and atmospheric pressure form the causal core, with the share of directed information carried by pressure increasing with aggregation scale. The recovered timescales are consistent with those expected from the underlying atmospheric processes. We use these driver timescales to initialise CAIRN (Causal-Anchored Inference for Receptor Nowcasting), a machine-learning nowcaster with fast and slow
memory components. Trained on past exceedances of WHO guideline levels, CAIRN nowcasts them from surface weather measurements and the calendar alone, without hand-engineered features. Combining four such nowcasters produces a site-level, tiered alert that agrees substantially with that generated by a direct sensor network and tracks an independent record of community odour reports. Meteorological variables can therefore serve as an inference-time proxy for exposure relative to WHO guideline levels, and they link atmospheric dynamics to community impact as an episode unfolds.
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All authors Timothy C. Pearce, David J. T. Smith, Alec Dobney, Alessia Freddo
Accelerator Physics
Particle tracking with physics-informed deep learning methods
Matthias Remta, Anja Beck, Shanthalakshmi Kilambi et al. (+2 more)
Simulating the motion of charged particles in electromagnetic fields is essential for designing and optimising particle accelerators. Conventional tools rely on
symplectic integration schemes, which provide high accuracy but are computationally expensive. As a consequence, optimisation in moderate to high-dimensional parameter spaces as well as simulations of tens of thousands to millions of particles can be computationally prohibitive. This contribution explores the possibilities of employing modern machine-learning based tools, in particular SympNet and DeepONet, to enable fast particle simulations. A major novelty is the modification of the conventional SympNet architecture to enable learning of parametric
Hamiltonian dynamics. The models are trained and tested on a toy setup of a circular accelerator comprising two different types of quadrupole magnets with varying field strengths. All models achieved faster inference than the
symplectic integrator, at the expanse of significantly reduced accuracy. The SympNet implementation achieved the lowest mean squared error. Additionally, a DeepONet was employed to predict the
evolution of particle densities, derived from the single-particle simulations.
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All authors Matthias Remta, Anja Beck, Shanthalakshmi Kilambi, Thomas Zhang, Francesco Velotti
Geophysics · Cross-listed from stat.ML
Extending Occam's inversion with lasso fusion, overcomplete dictionaries, and isotropic total variation regularisation
Anandaroop Ray
Occam's inversion is a robust
algorithm to perform nonlinear geophysical inversion. It provides the smoothest model within observation noise, thereby discouraging geological overinterpretation. While Occam originally penalised l2 model roughness, l1 can be used to provide models that are visually sharp. However, l1 regularised geophysical inversion has diverged from the larger body of statistics and imaging literature. For example, l1 regularisation with a difference operator (i.e., total variation) in multiple dimensions has two distinct forms, only one of which is invariant to edge orientation – a distinction often overlooked in geophysics. In one dimension this reduces to the fused lasso problem in statistics. Within the Occam framework, for l2 data norm and l1 model regularisation, we show that lasso fusion through either synthesis or analysis leads to the same 1D problem. We extend the framework to multiple dimensions and to operators such as wavelet transforms in a dictionary, unravelling the mathematics behind three commonly used solvers for l1 regularised problems. These are coordinate descent, iteratively reweighted least squares (IRLS), and split Bregman. We use them to solve a sequence of problems that are linear (1D regression, 2D deblurring) and nonlinear (1D airborne transient electromagnetics) including a field data example. We recommend coordinate descent for 1D problems, and IRLS over split Bregman for 2D problems, with IRLS requiring up to an order of magnitude fewer least squares solves. We hope this work will further encourage geophysicists to adopt l1 regularised inversion, by presenting its various forms as a familiar Occam's inversion.
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Instrumentation and Detectors
Variable-Path-Length FTIR of E. coli in Aqueous Media
Jonathan Matsuura, Andrew Huang, Jaehyeon Kim et al. (+3 more)
Transmission infrared spectroscopy has been widely used for chemical analysis of biological samples in aqueous environments. However, its scope of applications has been limited by the path length, which is either too large or fixed, posing challenges for analyzing highly absorbing or heterogeneous samples. In this work, a mid-infrared (mid-IR) optical fiber probe was used for
Fourier transform infrared (FTIR) micro-spectroscopy of aqueous Escherichia coli (E. coli) samples, providing continuous tuning of optical path length and sampling of near-surface and bulk regions. The mid-IR absorbance of the
protein signal at 1548 cm-1 increased linearly with path length, consistent with the Beer-Lambert law. Path-length dependent spectra were used to calculate the spatially heterogeneous absorption coefficient of E. coli suspensions in aqueous media. The results demonstrate the ability of our fiber-based technique to resolve signals originating from different depths into the biological solution.
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All authors Jonathan Matsuura, Andrew Huang, Jaehyeon Kim, Ching-Ping Chang, Kai Zhang, Yingjie Zhang
Medical Physics
MRI-guided cardiac radiotherapy using a 1.5 T MR-linac: surveying emerging patterns of care
O. Akdag, S. Mandija, J. Pomp et al. (+11 more)
Background: Cardiac tumours are rare and treated by
surgical resections, which are complex, invasive and carry procedural risks. MRI-guided radiotherapy (MRgRT) noninvasively facilitates conformal dose deliveries using soft-tissue imaging and adaptive radiotherapy techniques. Aim: To assess the application of MRgRT for cardiac tumours, we conducted a patterns-of-care analysis with users of the 1.5T MR-linac. Materials & methods: A survey was distributed to users of the 1.5T MR-linac that treated patients with cardiac tumours. The survey included 30 questions concerning the patient cohort, imaging, treatment planning/simulation, radiotherapy treatment and treatment outcome. Results: Users from six international institutes completed the survey reporting twelve cardiac MRgRT treatments between 2021-2024. The median age[range] of the patients was 59[16-81] years with 50% of the cases concerning the treatment of primary tumours. The prescribed dose ranged between 30-60 Gy, with 30 Gy being prescribed the most (59%). In 75% of the cases, the treatment plans were delivered in five fractions with 6-8 Gy per fraction. Daily images were acquired with T1- and T2-weighted MRI and respiratory motion monitoring was performed in 92% of the cases using cine imaging. A single case was treated with intrafraction motion management using a novel vendor-provided gating solution on the MR-linac. Treatment outcomes were reported for 50% of the cases. All, but one case, attained local control using MRgRT without serious adverse events (grade$\geq$3). Conclusion: This study provides real-world insights into the feasibility and early outcomes to aid the development of cardiac MRgRT treatment recommendations and protocol harmonization.
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All authors O. Akdag, S. Mandija, J. Pomp, M.P.W. Intven, X. Chen, J. Yang, S. Oh, M. Trombetta, H. Tan, J. De Leon, M. Jameson, E. Efe, C. Önal, M.F. Fast
Chemical Physics
Poly(1,4-anthraquinone) as an Organic Cathode Material: Simulation of Observable Bonding Properties to Li, Na, Mg, and Ca
Laura Femmer, Lukas Köbbing, Juliane Heitkämper et al. (+9 more)
Poly(1,4-anthraquinone) (P14AQ) has emerged as a promising cathode material, offering high capacity and good cycling stability, yet the atomic-scale mechanisms governing metal-ion binding and electrochemical behavior remain poorly understood. To address this, we investigate the binding mechanisms of Li, Na, Mg, and Ca to P14AQ using
quantum mechanical methods, particularly DFT and DFTB. A key challenge lies in the material's structural complexity: multiple conformers of P14AQ are energetically similar but kinetically isolated due to significant energy barriers. To account for this, we develop an automated method to generate all unique P14AQ conformers for a periodic
polymer chain without rotational duplicates through an orientation labeling scheme. For each conformer, we systematically place a metal atom adjacent to every oxygen site, enabling a complete exploration of binding configurations. We observe two structural motifs: a single metal-oxygen bond and coordination to two opposite oxygen atoms. While Li and Na exhibit continuous energy distributions, Mg and Ca show an energy gap between the two motifs, with a strong preference for the two-oxygen binding configuration. Galvanostatic measurements support these findings by showing lower gravimetric capacities for Ca and Mg than for Li and Na. For Na, the different numbers of metal-oxygen bonds are reflected in the two voltage plateaus observed experimentally. Overall, the combined computational and experimental results explain the higher capacity of monovalent ions in P14AQ: divalent ions cannot bind efficiently to a single oxygen site due to unfavorable energetics, and the conformational distribution of the
polymer chain prevents optimal coordination.
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All authors Laura Femmer, Lukas Köbbing, Juliane Heitkämper, Sibylle Riedel, Devran Cay, Florin Adler, Birgit Esser, Alexander J. C. Kuehne, Zhirong Zhao-Karger, Piotr de Silva, Juan Maria García-Lastra, Birger Horstmann
Chemical Physics · Cross-listed from cond-mat.mtrl-sci
Electrostatic Phenomenology Benchmarks for Machine-Learned Interatomic Potentials in Electrochemistry: Beyond the Energy-Force Metric
Barbara Sumić, Ria Vasdev, Sudheesh Kumar Ethirajan et al. (+8 more)
Accurate treatment of long-range interactions in machine learning interatomic potentials (MLIPs) is essential for electrochemical simulations. However, aggregate energy and force errors alone are insufficient to establish an MLIP's physical accuracy since they do not detect qualitative inconsistencies in the model such as the prediction of image-charge attraction, dielectric screening, or charge transfer. We introduce a
benchmark suite EPhEct (Electrostatic Phenomena for Electrochemistry) of focused test cases designed to evaluate MLIPs on electrochemically relevant physical phenomena. The tests probe for image-charge attraction at a metal electrode, the splitting between longitudinal and transverse optical
phonons as a probe of ionic and electronic screening, the dipole moment of interfacial water, and
Fermi-level pinning during ion discharge. These tests establish a qualitative diagnostic routine complementary to aggregate energy-force metrics.
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All authors Barbara Sumić, Ria Vasdev, Sudheesh Kumar Ethirajan, Jing Yang, Clotilde S. Cucinotta, Richard G. Hennig, Karsten Reuter, Stefan Ringe, Mira Todorova, Christoph Freysoldt, Jörg Neugebauer
Plasma Physics
Physics of Circular Polarized Ion-Scale Waves in Hybrid Simulations of Alfvénic Fluctuations
Hai Yang Harry Qian, Trevor A. Bowen, Carlos A. Gonzalez et al. (+5 more)
Ion cyclotron waves (ICW) and fast magnetosonic/whistler waves (FMW) are fundamental electromagnetic modes at ion kinetic scales, yet their generation mechanisms and roles in
plasma evolution remain poorly understood. We analyze a 2.5D hybrid simulation of broadband Alfvénic fluctuations, where the
proton velocity distribution is modeled as a sum of two bi-Maxwellian components: a thermal core and a drifting beam. Using wavelet-based wave identification, bi-Maxwellian VDF fitting, and the PLUME linear dispersion solver, we find that ICW behave as linear modes. Growth is intermittent, occurring when core temperature anisotropy builds up, and is driven mainly by the core (the beam contributes negligibly). Poynting flux analysis shows that ICW are predominantly forward-propagating, with a net energy flux ratio of $+1$ across all frequencies, consistent with the initial condition. FMW present a stark contrast: PLUME solutions often yield very small (near-zero) linear growth/damping rates. The species decomposition breaks down when $|\gamma/\omega_r| \gtrsim 0.368$, indicating that linear theory predicts these waves to be strongly damped and not describable by linear eigenmodes. Nevertheless, FMW are clearly observed in the wavelet helicity spectrogram, indicating that they are generated by nonlinear processes (e.g., parametric decay or phase steepening) and persist despite linear damping. The net energy flux ratio for FMW is close to $+1$ at low frequencies but decreases at higher frequencies, yet never reaches zero (net energy flow remains forward). These results demonstrate that ICW are linear, core-driven waves that transfer energy to the
plasma, while FMW are heavily damped, nonlinearly generated waves.
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All authors Hai Yang Harry Qian, Trevor A. Bowen, Carlos A. Gonzalez, Nikos Sioulas, Alfred Mallet, Kristopher G. Klein, Daniel Verscharen, Stuart D. Bale
Computational Physics · Cross-listed from cond-mat.mtrl-sci
Accurate and efficient calculation of atomic forces in solids with non-self-consistent hybrid functionals
Damian Contant and Maria Hellgren
Hybrid functionals are routinely employed self-consistently within the generalized Kohn-Sham framework. The evaluation of the nonlocal Fock exchange operator makes hybrid functional calculations computationally expensive, in particular with plane-wave basis sets. Here, we investigate the advantages of non-self-consistent hybrid functional calculations, focusing on the evaluation of atomic forces. The analytical force terms that arise due to non-self-consistency are computed using density functional perturbation theory (DFPT), as implemented within the
Quantum ESPRESSO distribution. A non-self-consistent hybrid force calculation thus consists of self-consistent DFPT calculations with a local or semi-local functional and a single evaluation of the Fock exchange operator. The overall computational cost is, thereby, reduced in general, especially for solids that require a dense Brillouin-zone sampling. Moreover, results for structural parameters are barely affected by self-consistency, and vibrational frequencies are typically agreeing within 0.5%, thus making non-self-consistent calculations an interesting alternative.
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Medical Physics
Birth of the Coil: another Milestone towards a fully reproducible low-field MRI scanner for head-imaging
Umberto Zanovello, Julia Pfitzer, Ariane Ernst et al. (+5 more)
Low-field
magnetic resonance imaging (MRI) provides an accessible, portable, and low-cost alternative to high-field scanners, expanding diagnostic imaging to point-of-care settings. However, widespread adoption is fundamentally hindered by a severely reduced signal-to-noise ratio (SNR). At low frequencies, radiofrequency (RF) coil conductor losses - rather than tissue sample losses - predominantly govern the system's total noise, making meticulous RF coil optimization critical to recovering image quality. This work presents an open-source, optimized solenoid head coil tailored for the 50 mT open-source scanner (OSII ONE v2.1). The paper validates production reproducibility across three independent international institutions and introduce an open-source connector with integrated digital circuitry for coil identification and DC or logic signals. Comprehensive benchtop measurements, Electromagnetic Interference (EMI) coupling analysis, Specific Absorption Rate (SAR) safety simulations, and phantom and human volunteer imaging confirm the design's efficacy, safety, and reproducibility. The results of the paper, when combined with the material provided in the open-source dedicated repositories, set the basis for a fully reliable and reproducible component for the open-source OSII ONE MRI scanner. In addition, the same optimization strategy and design material can be exploited for designing other RF coils for imaging of other body parts.
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All authors Umberto Zanovello, Julia Pfitzer, Ariane Ernst, Joshua Harper, Jack Hayhoe, Tri Nguyen, Lukas Winter, Nicola De Zanche
Plasma Physics
Enhancement of axial magnetic field generation during relativistic self-channeling of laser radiation propagating along thin films
Vitalia A. Kuleshova and Artem V. Korzhimanov
Within the framework of a stationary model of relativistic self-focusing, it is shown that the addition of a thin layer of denser
plasma on the propagation axis of a circularly polarized beam increases the axial
magnetic field generated by it via the inverse Faraday effect by more than a factor of 10 compared to the case of a homogeneous
plasma. The magnitude of the axial field in this case can exceed the
magnetic field of the
laser wave, which makes it possible, at radiation intensities $\sim 10^{26}$~W/cm$^2$ potentially achievable in the near future, to approach the level of teragauss quasi-stationary fields.
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Optics
Sharp Coherence Crossover and Multimode Phase Control in Coupled Exciton-Polariton Condensates
R. V. Cherbunin, A. D. Liubomirov, M. A. Chukeev et al. (+2 more)
We investigate the formation of mutual coherence and deterministic phase control in a multimode
exciton-polariton system realized in two optically induced traps with tunable intertrap coupling. As the coupling channel between the traps is progressively opened, the mutual first-order coherence rises sharply from nearly zero to values close to unity, revealing a threshold-like crossover from weakly correlated to highly coherent dynamics. We demonstrate deterministic multimode phase control with a short spatially localized nonresonant pulse, which allows the phase relations between selected polariton condensate modes to be set while keeping their amplitudes nearly unchanged. The induced phase transformation is read out directly from time-resolved interferograms and realized for different sets of modes in both intermediate- and high-coherence regimes. Within a reduced four-mode description, the control pulse implements an effective phase operator on the coherent multimode field with a fidelity exceeding 0.995. The same approach can be extended to larger networks of coupled traps, offering a route to optical control over phase relations in increasingly complex multimode polariton states.
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All authors R. V. Cherbunin, A. D. Liubomirov, M. A. Chukeev, E. S. Sedov, A. V. Kavokin
Chemical Physics
Reaction-Transformation-Aware Flow Matching for Generalizable Transition State Generation
Kaipeng Zeng, Wenxi Zhai, Shengrui Xu et al. (+5 more)
Transition-state (TS) structures define the energetic barriers and mechanistic pathways of elementary chemical reactions, yet their identification remains computationally demanding because conventional saddle-point searches require expensive
quantum-mechanical calculations. Recent machine-learning approaches have accelerated TS generation by predicting structures from reaction endpoint information, but they primarily learn
geometric correspondence between endpoints and TSs, leaving the structural transformations underlying elementary reactions implicitly represented. To address this limitation, we introduce TransTS, a reaction-transformation-aware framework for generalizable TS generation from atom-mapped reactant-product pairs. TransTS explicitly learns atom-level structural transformations between reaction endpoints and integrates them with a unified atom-aligned
geometric representation of reactants, TSs and products, enabling reaction-aware equivariant generation of TS geometries. TransTS is designed to provide reliable TS initial guesses for subsequent
quantum-chemical refinement, where generated structures are evaluated not only by
geometric similarity but also by their ability to converge to validated saddle points and recover the intended reaction pathways. Across IID and zero-shot OOD
benchmarks, TransTS demonstrates improved TS initialization quality, with particularly strong generalization to unseen reaction distributions. On the challenging GDB-10-rxn and GDB-17-rxn OOD
benchmarks, TransTS generates TS candidates that more frequently converge to validated saddle points and recover the intended elementary reactions after refinement than existing approaches under the same training regime. Scaling reaction coverage and model capacity further improves both
geometric fidelity and refinement outcomes.
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All authors Kaipeng Zeng, Wenxi Zhai, Shengrui Xu, Jie Zhao, Bowen Li, Shiyue Wang, Junchi Yan, Tong Zhu
Chemical Physics
Linear-response time-dependent density-functional theory with local range separation: Core and valence resonances of the neon atom
Jari van Gog, Felipe Zapata and Julien Toulouse
We investigate range-separated hybrids (RSHs) and locally range-separated hybrids (LRSHs) for linear-response time-dependent density-functional theory (TDDFT) Sternheimer calculations of the photoionization spectrum of the Ne atom. This system constitutes a stringent test for approximate exchange-correlation treatments because it exhibits both valence and core resonances with very different energy scales. Building on previous work employing a simple one-parameter local range-separation function, we assess here a more flexible two-parameter range-separation function designed to improve the high-density limit. We compare photoionization spectra and resonance parameters obtained with RSHs and LRSHs. We find that the LRSH approach with the two-parameter range-separation function provides an overall satisfactory description of the photoionization spectrum, including energies for both valence and core resonances. The lifetimes of the 2s $\rightarrow$ np valence resonances are also reasonably reproduced since their decay does not involve double excitations. In contrast, the lifetimes of the 1s $\rightarrow$ np core resonances are overestimated by orders of magnitude because their Auger decay channels involve double excitations that are absent in adiabatic, single-determinant TDDFT. Obtaining more accurate resonance widths within linear-response range-separated TDDFT would require using multideterminant schemes and/or adding a frequency-dependent response kernel.
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Chemical Physics
Different Environments in Quantum Statistical Mechanics: To $\beta$ or not to $(k_{\mathrm{B}} T)^{-1}$
Ellen T. Ekstrøm, Jacob Pedersen and Ida-Marie Høyvik
We piece together textbook material to re-derive the parameter $\beta$ entering reduced density operators from
quantum statistical mechanics. By re-deriving $\beta$, we show that the content of $\beta$ depends on the particular application of statistical mechanics. We focus on two different applications, namely, electronic-structure theory (new) and thermodynamics (standard). Specifically, we show that $\beta$ becomes proportional to the inverse
Fermi energy, when the electronic states of a system interact with the infinitely many valence electrons in a metal. On the other hand, when the environment is a heat bath, $\beta$ takes the well-known form of inverse temperature. To highlight the importance of using the correct form of $\beta$, we explore statistical descriptions of a potassium atom adsorbed on a gold surface, where the valence electrons of gold represent the environment of potassium. By treating $\beta$ as the inverse
Fermi energy, we are able to qualitatively reproduce the fractional charging of potassium. In contrast, if we interpret $\beta$ as the inverse temperature, extremely high and unphysical temperatures are required to reproduce the same qualitative picture. Thereby, we illustrate the fallacy of not separating the mathematical framework of
quantum statistical mechanics from its pervasive thermodynamic application.
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Data Analysis and Statistics · Cross-listed from nucl-th
Bayesian inference of event-by-event collision geometry from charged-particle multiplicity in heavy-ion collisions
Yige Huang, Fu-Peng Li, Hanwen Feng et al. (+1 more)
We propose the Inference-driven Participant Determination (IPD) method, a
Bayesian framework for inferring event-by-event posterior distributions of the number of participants ($N_{\text{part}}$) and binary collisions ($N_{\text{coll}}$) from final-state charged-particle multiplicities in relativistic heavy-ion collisions. The joint distribution of $(N_{\text{part}}, N_{\text{coll}})$ obtained from the Monte-Carlo Glauber model is used as the prior, while negative binomial distributions calibrated to charged-particle multiplicity fluctuations define the likelihood. This approach replaces conventional hard-cut centrality classification with a probabilistic assignment based on $N_{\text{part}}$, making the multiplicity–geometry smearing explicit and reducing the impact of volume fluctuations on downstream observables. A closure test using an UrQMD-MCG hybrid model at $\sqrt{s_{NN}} = 19.6$~GeV shows that the method yields well-calibrated posterior distributions with negligible bias and improves the reconstruction of net-
proton cumulants relative to conventional multiplicity-based centrality selection.
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All authors Yige Huang, Fu-Peng Li, Hanwen Feng, Nu Xu
Classical Physics
Huygens' principle and field equivalence relations for cylindrical enclosing surfaces
Andrey Osipov and Sergei Tretyakov
Frequency-domain Huygens' principle and equivalence relations for an infinitely long cylindrical surface enclosing the field sources or scatterers are addressed. Assuming the harmonic dependence of the fields along the cylinder axis, we derive three equivalent versions of line-integral representations for the fields in free space at any point outside the enclosing cylinder. In one of the forms, similarly to the Helmholtz-Kirchhoff scalar diffraction theory, the integrals contain a longitudinal field component and its normal derivative. Another presented form contains longitudinal and normal field components, similarly to the three-dimensional boundary integral representations by Stratton and Chu. This form eliminates the need to calculate the field derivatives. Furthermore, we present a two-dimensional version of the three-dimensional Schelkunoff-Franz representation, which contains only tangential components of the fields, complies with the field equivalence
theorem and can be regarded as a rigorous formulation of Huygens' principle for a cylindrical enclosing surface. All three forms of the line-equivalence relation are exact and describe the fields through equivalent field distributions on a line enclosing the cross section of the source region. A physical interpretation of Huygens' principle in terms of conical waves emanated by virtual linear sources on the cylindrical surface enclosing the true sources is given. Specialization of the relations to the intermediate and far-field zones are presented. The derived expressions are applicable for studies of scattering and radiation in a very general class of structures that are infinite and periodic along one direction.
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Optics
Transient Chirp Dynamics in Terahertz Quantum Cascade Lasers
Xianglong Bi, Xuhong Ma, Wenjian Wan et al. (+9 more)
Laser frequency chirp is a ubiquitous dynamical process in
semiconductor lasers, vital for frequency-modulated photonic systems. In the mid-infrared (MIR) and terahertz (THz) ranges,
quantum cascade
lasers (QCLs) are ideal sources with high power, narrow linewidth and compact size. While chirp dynamics in MIR QCLs have been studied, the transient chirp behavior of THz QCLs–particularly the thermal chirp on microsecond to millisecond timescales–remains largely unexplored. Here, we experimentally investigate transient thermal chirp dynamics in single-mode THz QCLs via an on-chip heterodyne scheme. Twin monolithically integrated single-mode QCLs are used: one pulsed QCL as the device under test, and one continuous-wave (CW) QCL serving as both local oscillator (LO) and ultrafast THz detector. The frequency chirp is mapped to the
radio-frequency (RF) domain by heterodyne down-conversion. By varying current and temperature, we observe three distinct chirp features: unidirectional down-chirp, V-shaped chirp, and unidirectional up-chirp. A two-node thermal model reproduces the dynamics with good agreement with experiments. Chirp dynamics in the multi-mode regime are also identified, showing the potential for sensitive dynamic spectral characterization. These findings deepen the understanding of THz QCL thermal chirp mechanisms and support applications in THz frequency combs, frequency-modulated continuous-wave (FMCW) radar, and high-speed coherent communications.
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All authors Xianglong Bi, Xuhong Ma, Wenjian Wan, Binbin Liu, Guibin Liu, Ziping Li, Yanming Lu, Zhiwei Qin, Yunxiang Zhu, Ziyu Guo, J. C. Cao, Hua Li
Optics
Multi-quantum-channel mediated tunable single-photon skyrmions from metasurfaces
Yan Wang, Zhenyu Guo, Minggui Liang et al. (+3 more)
Quantum optical skyrmions, as topologically robust
quantum information carriers, hold transformative potential for resilient high-dimensional
quantum information networks. However, their practical exploitation was still restricted to a single
quantum channel, which precludes the multiplexing essential for practical high-capacity
quantum networks. Here, we utilize a metasurface to achieve multi-channel
quantum state distribution of the polarization-entangled
photon pairs, inducing a two-
photon bunching effect in both the
spin and spatial dimensions with compact flat optics. At the spatial bunching port, controlled manipulation of the
spin-orbit interaction enables the generation of a tunable single-
photon skyrmion pair. In contrast to any prior skyrmion generation, the single-
photon skyrmions are mediated and topologically controlled by
quantum measurement in multiple channels. Concurrently, during the amplitude and phase modulation process, both the skyrmion localization and the texture helicity can be precisely customized. The proposed tunable single-
photon skyromions offer multidimensional controllability and topological stability provide a viable path toward noise-resilient high-dimensional
quantum information processing.
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All authors Yan Wang, Zhenyu Guo, Minggui Liang, Shuangchun Wen, Yijie Shen, Hailu Luo
Physics Education
PhysTrackX: Open-Source Software for Kinematics Tracking for the Physics Laboratory
Isam Abdullah Balghari and Muhammad Sabieh Anwar
Video analysis software is a staple of the modern undergraduate physics laboratory, enabling students to extract quantitative kinematic data from real-world phenomena. In this paper, we introduce PhysTrackX, a modern, open-source
video analysis framework designed to minimize cognitive load and maximize pedagogical value. Built with a streamlined graphical interface, PhysTrackX features a comprehensive set of functionalities including
video trimming, spatial calibration, and coordinate system definition with robust, machine learning based tracking. Furthermore, it introduces an integrated optical character recognition (OCR) module for synchronizing kinematic motion with external digital sensor displays, an interactive geometry plugin for in-situ spatial measurements, and a data management tool. In this article, we introduce the software and its functionality through some case studies.
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