Plasma Physics · Cross-listed from astro-ph.SR
Non-linear dynamo waves in magnetized shear flows: a candidate mechanism for the solar cycle
Uddipan Banik
Large-scale
magnetic reversals in
planets,
stars, and
accretion disks require a non-linear theory coupling the
magnetic field to its sustaining flow. We derive exact non-linear solutions to incompressible magnetohydrodynamics in background rotational shear, with large-scale velocity and
magnetic fields linear in position (affine form). They reveal a cyclic feedback between meridional circulation, shear,
magnetic tension, induction, and the Coriolis force. The steady-circulation reversal orbit, or dynamo wave, periodically reverses all
magnetic components and supports slower modulations. In the solar near-surface shear layer (NSSL), moderate shear permits stable 11-year reversals with poleward speeds of 8-12 m/s and poloidal and toroidal amplitudes of 4-11 G and 2.2-3.8 kG near 1 Mm depth, in reasonable agreement with regional observations. At stronger shear, an 88-year reversal exhibits a 203-207-year modulation, reproducing the Gleissberg and Suess-de Vries activity timescales but requiring weaker poloidal fields. Such reversals, however, remain observationally unestablished. At tachocline density, an 11-year reversal and the inferred return flow require toroidal fields of hundreds of kilogauss over a broad shear range, exceeding large-scale-field reconstructions but partly permitted by seismic upper limits. This favors a shallow NSSL origin for a local cycle, while leaving open the issue of a global dynamo. Toroidal dominance prolongs the cycle because shear acting on a weak poloidal field changes only a small fraction of the stronger toroidal field per rotation. Balancing the weak
magnetic torque against advection and the Coriolis force also requires slow circulation, which gradually turns the poloidal field and reverses toroidal induction. Efficient poloidal regeneration would shorten this delay. A long solar cycle therefore favors a primarily shear-driven dynamo.
Read the full abstract ▾
Fluid Dynamics
Comment on "Note on the start-up of Couette flow for viscoelastic fluids" [Phys. Fluids 35, 113108 (2023)]
Ivan C. Christov
I show that the initial conditions imposed by Balan [Phys. Fluids 35, 113108 (2023)] are incompatible with a nonzero retardation time, so that the problem solved is not impulsive start-up, but, as I prove exactly, the start-up of a plate ramped as $1 - \mathrm{e}^{-t/\lambda_2}$ on the retardation timescale $\lambda_2$ (a material parameter, not a setting in a rheometer). Values digitized from that paper's viscoelastic figures fall on the ramped-plate solution, not on Tanner's start-up solution. The large negative wall normal stress reported for the corotational model is a grid-dependent artifact due to the same error in the initial data. An open-source code repository provides annotated notebooks reproducing every figure and number in this Comment.
Read the full abstract ▾
Data Analysis and Statistics · Cross-listed from hep-ph
Optimizing Cell-Based Negative Weight Mitigation with Optimal Transport
Lauren Hay, Rishabh Jain, Matt LeBlanc et al. (+1 more)
As the accuracy of experimental results in high energy physics (HEP) increases, so does the demand for precision Monte Carlo (MC) simulation. Higher-accuracy event generation at next-to-leading order (NLO) and beyond brings with it negatively weighted events. These negatively weighted events reduce the statistical power of samples, increasing the number of events that need to be produced and straining already limited computational resources. We present a post-hoc reweighting scheme that employs
cell-based resampling using an IRC-safe metric to define the
cell radii. An optimal metric embeds kinematically similar events within the same
cells, minimizing bias when reweighted. This motivates our exploration of a Optimal Transport (OT) based distance metrics. We compare the performance of the reweighting
algorithm with different choices of metric, and explicitly demonstrate the performance on simulated Z+jets events produced at NLO accuracy.
Read the full abstract ▾
All authors Lauren Hay, Rishabh Jain, Matt LeBlanc, Jennifer Roloff
Physics and Society · Cross-listed from cs.CV
AlphaEarth distinguishes cities but compresses urban variation
Andrew Renninger
Cities differ in built form, land cover and development history, complicating comparison across places and time. Satellite foundation models map Earth's surface onto common numerical representations. Yet the tasks and targets used to shape them typically do not focus on cities: globally consistent labels for urban function do not exist, and many
datasets - especially land cover and land use classifications - collapse the built environment into few classes. Here we audit the representation, focusing on AlphaEarth but with broader applicability to other Earth
embeddings, by probing the geometry and geography of
embeddings for 1,000 urban areas in 162 countries. We find that cities occupy a shifted but overlapping region on the hypersphere, 62.7{\deg} from the global mean direction, and continent and
climate predict 24.3% of variation among the mean directions of urban centres in excluded countries. Inside cities, degrees of urbanisation carry 8.9% of the variation, and what they leave holds shared directions whose local orientation varies, not one universal axis of urbanisation. Retained variation is itself unequal: dispersion within urban centres is 14.1% greater per standard deviation of national development, even after adjusting for population, land area and continent. Further controls suggest cities in developing countries present less contrast in vegetation and texture, and dispersion follows that contrast: full adjustment for it leaves at most 6.4% of the gradient. Annually, a city's representation moves nearly eight times more than redrawing its own pixels explains, and contracts where the 2022 loss of Sentinel-1B removed a pass direction. AlphaEarth's representations therefore support comparison across regions, while the differences between its annual layers are not yet validated for comparison over time.
Read the full abstract ▾
Computational Physics · Cross-listed from gr-qc
Physics-Informed Neural Networks for Static Black-Hole Exterior Metrics: Charge and Cosmological-Constant Sweeps
Huan Jin, Fei Wu and Fei Xue
We apply physics-informed neural networks (PINNs) to recover the time-time component of static, spherically symmetric black-hole exterior metrics from a reduced ordinary differential equation (ODE). Inspired by recent work on solving Einstein field equations with deep learning~\cite{Li2023}, we encode the
vacuum/charged exterior through a residual loss and an asymptotic boundary constraint—not by
embedding analytic metric terms such as $2M/r$ directly into the network output. Unlike the distributed PINN (DPINN) strategy of Ref.~\cite{Li2023}, which partitions the radial domain into subdomains with separate networks, we employ a unified fully connected network over the entire interval $[r_{\min},r_{\max}]$, avoiding spurious jumps at subdomain interfaces. Moreover, whereas Ref.~\cite{Li2023} restricts training to $r\in(10,300)$—well outside the steep $1/r$ and $1/r^2$
curvature of the inner exterior—we begin at $r_{\min}=10^{-2}M$, spanning three decades in radius and covering the strongly varying region that DPINNs sidestep by domain truncation. Holding the mass fixed at $M=1$, we sweep electric charge $Q\in\{0,0.5,1.0,1.1\}$ at $\Lambda=0$ and sweep $\Lambda\in\{0,0.1,-0.1\}$ at $Q=0.5$. All configurations achieve relative $\mathcal{L}_2$ errors below $6\%$ against the analytic reference. For the representative case $Q=0.5$, $\Lambda=0.1$, three independent trainings with fixed seeds yield relative errors of $3.89\%$, $1.08\%$, and $2.76\%$ (mean $2.58\%$, standard deviation $1.41\%$), demonstrating robustness of the mesh-free approach without labeled field data.
Read the full abstract ▾
Optics
Efficient simulation of millimeter-scale complex-modulated integrated Bragg gratings via hierarchical locally periodic eigenmode expansion
Rui Cheng, Jia Meng, Ping Yu et al. (+2 more)
We propose a structure-aware, hierarchical locally periodic eigenmode expansion (HLP-EME) framework for efficiently simulating millimeter-scale integrated Bragg gratings (IBGs) with complex modulation on silicon-on-
insulator platforms. HLP-EME discretizes continuously varying grating parameter profiles into piecewise-constant blocks and exploits the resulting local periodicity of the physical grating structure by reusing the S-
matrix of a representative period within each block. This strategy reduces full-device simulation times for millimeter-scale IBGs to a few minutes, providing a speedup exceeding three orders of magnitude over conventional 3D-FDTD simulations. The method accommodates diverse IBG configurations, including intra-mode gratings, mode-converting multimode gratings and grating-assisted contra-directional couplers. Experimental results validate the predicted reflection spectra of several complex-modulated IBGs and the reflection phase response of one representative design. The framework is further extended to curved waveguides and successfully captures
curvature-induced spectral distortions in millimeter-long, Gaussian-apodized spiral IBGs. Combining full-vectorial modeling with high computational efficiency, HLP-EME provides a powerful tool for designing and optimizing long, complex IBG-based photonic devices.
Read the full abstract ▾
All authors Rui Cheng, Jia Meng, Ping Yu, Jihao Wang, Zikun Xie
Data Analysis and Statistics
Simulation-Based Inference and Unbinned Asimov Construction with Hybrid Neural Density Estimation
Rafael Coelho Lopes de Sa and Jay Sandesara
High-dimensional, unbinned neural simulation-based inference often relies on neural ratio estimation, which uses expressive supervised models to estimate density ratios, but a learned ratio by itself provides neither an explicit normalized density nor a generative model. Flow-based surrogate models instead enable tractable density evaluation and efficient sampling, but residual density-estimation errors can limit precision for complex implicit distributions. We propose hybrid neural density estimation, which uses a flow to define a parameter-independent reference distribution, and classifiers to estimate target-to-reference density ratios. Multiplying a learned ratio by the reference density gives an evaluable target density surrogate. The ratio also provides importance weights for integration and resampling. We show how this representation defines an exact Asimov
dataset, whose maximum likelihood fit returns the generating parameters. We also show how the tractable reference supplies renewable samples for pseudo-experiments and how it enables methods to reduce the Monte Carlo variance of expected test statistic calculations. We demonstrate the construction in a toy statistical model motivated by high-energy physics measurements, but for which the exact densities are known analytically.
Read the full abstract ▾
Accelerator Physics · Cross-listed from hep-ex
Climate, Science, and the FCC: Think Clearly, Choose Wisely, Act Swiftly
Patrick Janot and Christophe Grojean
Could a
collider become the symbol of everything that must be abandoned to save the
climate? In the summer of 2026, this question ran through discussions within the Particles and Fields Division of the French Physical Society. An unacceptable carbon footprint, two EPR nuclear reactors, five hundred deaths, billions to be reallocated and, to top it all, unclear scientific objectives: the objections to CERN's Future Circular
Collider (FCC) appeared to form an overwhelming indictment. This essay examines each charge in turn. Grounded in a clear and compelling scientific
vision, it confronts emotionally charged imagery with orders of magnitude, distinguishes a project's footprint from its full balance sheet, considers the costs of postponing or abandoning it, and explores ways of reducing its impacts-including the still uncertain prospect of natural hydrogen. The
climate alone does not decide the future of the FCC; it requires us to judge the available choices by their real consequences. Developed for the public debate organised by France's National Commission for Public Debate (CNDP) and for the Swiss consultation, this essay is intended for the broadest possible readership, well beyond particle physics and the scientific community. It frames the FCC debate as a choice about the kind of society we want to build: should we organise society around renunciation and ever-shrinking horizons, or transform how we produce and choose a future worth wanting?
Read the full abstract ▾
Optics
Tunable hyperbolic metamaterials for brightening single-photon emission
Manobina Karmakar, Pavel Klok, Filip Ligmajer et al. (+1 more)
Hyperbolic metamaterials (HMMs) are widely used to enhance spontaneous emission through their exceptionally large photonic density of states, yet large Purcell factors (>100) rarely translate into brighter emission, exposing both a practical limitation of HMMs as brightening platforms and a flaw in the Purcell factor itself as a figure of merit. Here we systematically evaluate spontaneous emission enhancement and
photon outcoupling in metal-dielectric HMMs (TiO$_2$/Ag and phase-change Sb$_2$S$_3$/Ag) using effective medium theory, transfer
matrix calculations, and full-wave FDTD simulations. We show that planar HMMs with Purcell factors exceeding 100 have external
quantum efficiencies below 0.01, yielding an actual radiative Purcell factor (PF$_{rad}$) of only 1-2. Nanopatterning the HMM into a photonic
crystal efficiently couples trapped high-k modes into free space, raising PF$_{rad}$ by an order of magnitude. Incorporating the phase-change material Sb$_2$S$_3$ further enables dynamic tuning of the
hyperbolic spectral window and active control over emission brightness: an optimized nanostructured design reaches PF$_{rad}$ ~ 10-15 in the near-infrared, with switching contrast exceeding a factor of 15 upon phase transition. These results establish the radiative Purcell factor as a more reliable figure of merit than the conventional Purcell factor for engineering
quantum emitters, and provide a practical route toward actively tunable, bright single-
photon sources based on
hyperbolic metamaterials.
Read the full abstract ▾
All authors Manobina Karmakar, Pavel Klok, Filip Ligmajer, Leonardo de S. Menezes
Medical Physics
Cone-beam artifact reduction in Gamma Knife CBCT images using a line-arc-line scan trajectory
Alexandra Alain-Beaudoin, Håkan Nordström, Luc Beaulieu et al. (+1 more)
Objective. Gamma Knife cone-beam computed
tomography (CBCT) images suffer from distinct cone-beam artifacts for some patients, due to the conical X-ray beam which is oriented to intersect the detector perpendicularly at its inferior edge. The use of an exotic scan trajectory which provides more complete sampling across the field of view than the current arc can reduce cone-beam artifacts. In this study, the feasibility of a line-arc-line scan trajectory for the Gamma Knife CBCT is assessed through a proof of concept. Approach. Using a customized research Gamma Knife, a Catphan 503 and an anthropomorphic head phantom are imaged for different exotic scan trajectories, consisting of combinations of arcs and lines. The CBCT images for each trajectory are then reconstructed with an iterative
algorithm. Main results. A line-arc-line trajectory provides the largest image quality improvement among the investigated trajectories, with no noticeable cone-beam artifacts in the CBCT images: the axial interfaces between modules of the Catphan are well defined, and the superior edge of the phantom on the axial axis is sharper by 92%, compared with the current arc trajectory. For the head CBCT images, the proposed trajectory removes most of the cone-beam artifact on the superior side of the skull, characteristic of the current Gamma Knife CBCT images. The artifact reduction also leads to a visual improvement in terms of uniformity in both phantoms. Significance. A line-arc-line CBCT scan trajectory would be feasible on the Gamma Knife with limited changes to the current configuration, and could produce images with improved image quality.
Read the full abstract ▾
All authors Alexandra Alain-Beaudoin, Håkan Nordström, Luc Beaulieu, Joakim da Silva
Computational Physics
CTrex: A Research-Oriented Framework for Kernel- and Projection-Level Algorithm Development in CT Reconstruction
Karel Desplenter, Wannes Goethals, Thomas De Schryver et al. (+3 more)
Micro-computed
tomography (micro-CT) is increasingly applied to imaging scenarios in which object motion, deformation, truncated fields of view, continuous rotation, or unconventional scanning protocols violate assumptions underlying classical reconstruction pipelines. Existing reconstruction frameworks efficiently support standard geometries and workflows but offer limited control over how projection operators are discretized and evaluated. This restricts methodological exploration when accuracy depends on ray sampling, interpolation, or operator coupling. We introduce CTrex, a research-oriented, GPU-accelerated iterative reconstruction framework that exposes the projection-correction-backprojection pipeline down to the kernel level. Rather than treating these operators as fixed black boxes, CTrex represents them as editable computational building blocks within a unified iterative structure. This allows non-ideal acquisition effects to be incorporated directly into the numerical operators rather than treated as external corrections. CTrex combines GPU-level accessibility with an extensible geometry formulation based on homogeneous-coordinate view
matrices, allowing rigid and affine transformations, detector misalignments, and time-varying geometries to be expressed consistently. It supports circular, helical, offset, and conveyor-belt trajectories while keeping GPU kernels agnostic to the trajectory definition. Applications in motion- and deformation-aware reconstruction, event-based 4D imaging, cylindrical-coordinate reconstruction, and extended-field-of-view CT illustrate how operator-level adaptations enable strategies difficult to realize in conventional frameworks. CTrex provides a flexible platform for CT reconstruction research under realistic and unconventional imaging conditions.
Read the full abstract ▾
All authors Karel Desplenter, Wannes Goethals, Thomas De Schryver, Marjolein Heyndrickx, Jan Aelterman, Matthieu N. Boone
Plasma Physics
Singularities of the cold plasma theory: Modeling challenges for ICRF operation in low-density edge plasma
Wouter Tierens, Chris Klepper, Raymond Diab et al. (+1 more)
Sustained ICRF operation in a fusion power plant may require low edge densities to mitigate
plasma-wall interactions, a regime which was recently achieved in WEST with very little impurity sputtering. Cold
plasma theory, however, predicts singular radiofrequency electric fields in this regime, both at the lower hybrid resonance and along the resonance cones, raising the question of whether standard collisional cold
plasma models suffice to describe low-density edge ICRF at all. Collisions in principle remove these singularities, replacing them with finite but sharply peaked fields. We derive these peak length scales analytically and confirm them with a 2D finite-element simulation using exponential mesh refinement, achieving micrometer resolution where needed. We conclude that edge collisions in cold
plasma do not remove the need to resolve length scales ordinarily associated with hot-
plasma and Bernstein-wave physics.
Read the full abstract ▾
All authors Wouter Tierens, Chris Klepper, Raymond Diab, Guillaume Urbanczyk
Biological Physics · Cross-listed from q-bio.PE
Consistent determination of stability regimes in natural ecological communities from abundance time series
Alberto Megías, Vicente J. Ontiveros, David Alonso et al. (+1 more)
The stability of an ecological community is conventionally defined through species interactions, which quantify how species affect one another. Interaction strengths are notoriously difficult to measure in species-rich assemblages. Long-term monitoring of species-rich communities, however, provides species abundance time series, increasingly available across habitats and taxa but not yet connected to the stability properties of the communities they describe. Here we develop a statistical framework that infers stability regimes in large ecological communities directly from abundance time series. We study
stochastic Generalized Lotka–Volterra dynamics with random interactions and environmental fluctuations using dynamical mean-field theory, reducing the multispecies system to an effective
stochastic process for a representative species. The theory predicts three stability regimes (stable coexistence, intermittent dynamics close to extinction, and unbounded growth) separated by analytical boundaries, and shows that environmental stochasticity systematically destabilizes coexistence by promoting intermittent low-abundance dynamics. The resulting steady-state species abundance distribution is a Gamma law that ties the dynamical phases to the variability-based stability metrics used in empirical studies. Recasting the effective dynamics as a multivariate regression model, we infer interaction statistics, environmental variability and the characteristic timescale of the dynamics from community data, without reconstructing the interaction network. Applied to natural communities spanning a broad range of habitats and taxa, the method resolves contrasting stability regimes and yields quantitative estimates of species extinction risk.
Read the full abstract ▾
All authors Alberto Megías, Vicente J. Ontiveros, David Alonso, José A. Capitán
Computational Physics · Cross-listed from math.NA
High-dimensional extreme eigenvalue problems: low-rank tensor parametrization and optimization
Pengfei Hao, Renfeng Peng, Yang Kuang et al. (+1 more)
High-dimensional extreme
eigenvalue problems often arise from molecular vibrational models, electronic structure calculations, and
quantum mechanics. Directly solving these problems suffers from the curse of dimensionality. Instead of tackling the problem in high-dimensional ambient space, we reformulate the problem through low-rank tensor formats, which can significantly reduce the computational cost and storage. Specifically, we consider the Rayleigh–Ritz problem on bounded-rank tensors in the tensor train format, which enables a more flexible choice of rank parameters. Moreover, we consider a smooth parametrization for bounded-rank tensors by introducing slack variables, leading to a smooth
manifold structure. The original Rayleigh–Ritz problem is therefore transferred to an optimization problem on the
manifold. We develop the Riemannian geometry and propose optimization methods for solving extreme
eigenvalue problems. In practice, the Kronecker-product structure in
Hamiltonian and PDE operators is employed to simplify the computation. Numerical experiments on the harmonic oscillator, the Laplace operator, the Schrödinger equation, the layered cluster problem, and the Bose–Einstein model demonstrate that the proposed method achieves accuracy comparable to full-space eigensolvers with reduced computation time and avoids storing full vectors. The results also show numerical rank reduction during iteration and robustness when the rank parameter is over-estimated. In addition, the fixed-point iterations illustrate that the proposed methods are able to serve as an inner eigensolver for solving nonlinear
eigenvalue problems.
Read the full abstract ▾
All authors Pengfei Hao, Renfeng Peng, Yang Kuang, Bin Gao
Medical Physics
Melanin- and linewidth-corrected pulse-oximetry
Giles Blaney, Jodee Frias, Ravi Durbha et al. (+2 more)
Current pulse oximeters tend to overestimate arterial saturation in darkly pigmented patients, increasing occult hypoxemia rates. One proposed mechanism is spectral coloring: melanin contributes to reshaping the detected spectrum of a broad-linewidth source, making the wavelength-dependent parameters behind calibration skin-tone-dependent. Diffuse optics theory allows for a melanin- and linewidth-corrected calibration equation, which to our knowledge has never been tested in vivo. In this work, we fit the empirical linear equation, the diffuse optics theoretical equation, and its corrected form to the public OpenOximetry Repository—considering 98 patients with paired arterial blood-draws, raw photoplethysmograms, and skin reflectance spectra. Accuracy is practically indistinguishable between the three approaches, yet the saturation error's dependence on melanin falls from +2.8 to +1.2 to +0.1 percentage points per unit melanin volume fraction—consistent with the spectral-coloring theory. None of these dependencies on melanin are statistically resolved here—a cohort roughly ten times larger is needed—so the correction is backed by theory and its expected direction. All three calibration approaches carry two degrees of freedom: the theoretical equation drops into existing calibration procedures, and its melanin-aware form needs a per-patient skin-tone measurement. This work demonstrates how the theoretical pulse oximetry model can be applied in calibration.
Read the full abstract ▾
All authors Giles Blaney, Jodee Frias, Ravi Durbha, Sergio Fantini, Valencia Koomson
Fluid Dynamics
Magneto-coalescence phenomena in colliding ferrofluid droplets
Neeladri Sekhar Bera, Abhishek Kumar Jaiswal, Devranjan Samanta et al. (+1 more)
We explore the collision hydrodynamics of a ferrofluid droplet falling freely onto a sessile droplet of the same
liquid, in the presence of a horizontal
magnetic field; a configuration that couples droplet-on-droplet coalescence with concomitant field-governed wetting and spreading. Using high-speed imaging, we track the events through crown formation, radial spreading, and rim detachment (under specific conditions), across three ferrofluid concentrations, two substrates of different wettability (glass and PET), and a range of impact velocities and
magnetic field strengths. The maximum crown height is noted to scale as $H_{c,\max}/D_t\sim Fr^{0.5}$ ($Fr:$ Froude number); well below the ballistic upper bound of $H_{c,\max}/D_t\sim Fr$. At zero-field, the maximum spreading collapses onto the boundary-layer scaling $\beta_{0,\max}\sim (We_0/Oh)^{1/6}$ ($We_0$: Weber number, $Oh:$ Ohnesorge number) when expressed in terms of the merged impact velocity, and coalesced-drop size . With the field applied, a bulk-dissipation energy balance predicts $\beta_{\max}\sim Z^{1/5}$, where $Z$ combines the
magnetic-driving, and inertial-capillary-viscous terms, but the observations instead follow a markedly weaker $\sim Z^{1/11}$, a deficit traced to enhanced dissipation from the magnetoviscous effects, and manifested via an effective Ohnesorge number. Finally, rim detachment occurs beyond a field- and height-dependent threshold, described by a size-independent criterion $Fr^2Bo\approx 3550$ ($Bo:$ Bond number), above which the rim may fragment into daughter droplets. These scalings provide predictive tools for magnetically assisted printing, droplet-on-demand systems, and coating processes, where repeated droplet collisions occur on the residual
liquid droplet or layer.
Read the full abstract ▾
All authors Neeladri Sekhar Bera, Abhishek Kumar Jaiswal, Devranjan Samanta, Purbarun Dhar
Chemical Physics · Cross-listed from cond-mat.soft
Large integration time-step in molecular dynamics simulation artificially enhances the strength of hydrophobic interaction
Dilipkumar N. Asthagiri
Molecular dynamics simulations are used to compute the potential of mean force (PMF) between two united-atom methane molecules at several temperatures and integration time-steps. At a fixed time-step, the contact minimum of the PMF deepens with increasing temperature, as expected for hydrophobicity driven association. Compared with a time-step of 0.5 fs, one that preserves equipartition, larger time-steps alter the PMF and make the contact minimum more favorable. Thus even relative free energy values are sensitive to time-steps that break equipartition. Using quasichemical theory, we partition the free energy of association into hydrophobic and hydrophilic contributions. The hydrophilic contribution opposes association and is insensitive to the time-step. Thus the artificial enhancement of association at larger time-steps comes entirely from the hydrophobic contribution. We explain this behavior through the temperature dependence of the internal pressure of the
liquid. The same analysis also accounts for earlier observations of the
liquid's p-V behavior under conditions that break equipartition.
Read the full abstract ▾
Chemical Physics · Cross-listed from cond-mat.mtrl-sci
Ultrafast Electron Microscopy: A Quantitative Platform for Nonequilibrium Materials Research
David J. Flannigan and Swarit Ahmed Shadman
Macroscopic materials function is determined not merely by
equilibrium structure, but by how carriers,
phonons, fields, defects, interfaces, and collective order dynamically evolve after perturbation. Ultrafast electron microscopy (UEM) uniquely bridges this gap, coupling femtosecond-to-nanosecond timing with real-space, reciprocal-space, and energy-resolved contrast. Here, we review how these integrated capabilities now quantitatively map energy flow and conversion in electronic materials, decode electronic-structural coupling in
quantum and correlated systems, and isolate the localized propagation of carriers, polaritons, strain, and
phonons across optoelectronic and nanomechanical architectures. We emphasize the predictive design rules now emerging from direct metrology of carrier-
lattice coupling, momentum-resolved
phonon thermalization, defect-controlled phase transformations, and authentic operando switching. We critically assess UEM's ongoing maturation from a qualitative, proof-of-concept experiment into a rigorous, quantitative materials platform. This paradigm shift is actively driven by integrated
multimodal detection, advanced energy-resolved spectroscopy, high-repetition-rate sources, multidimensional (4D and 5D) acquisition, and physically constrained forward modeling. Ultimately, the central opportunity lies in establishing a complete, causal link from initial excitation to localized energy flow, transient structure, and macroscopic device function. By prioritizing quantitative reproducibility and robust mechanistic interpretation, UEM is positioned to engineer materials in functional states that
equilibrium thermodynamics simply cannot describe.
Read the full abstract ▾
Optics
Angular-Channel Capacity and Nonlocal Wavefront Engineering for Phased-Array–Lens Systems
Mohammad Soltani and George V. Eleftheriades
Wide-angle beam steering with high directivity is fundamentally limited by diffraction, restricting the scan performance of phased-array systems. Here, we establish the bounds governing beam steering with high directivity in phased-array–lens systems and show that nonlocal dielectric metalenses can operate close to these limits. Unlike conventional metalenses that suppress mutual coupling, our approach exploits nonlocal electromagnetic interactions to laterally redistribute energy and expand the spatial width of phase coherence across the aperture. This enables efficient wavefront engineering for uniformly excited planar phased arrays using conventional linear progressive phasing. We further reveal the roles of diffraction and evanescent waves in determining the maximum coherent-aperture expansion and directivity enhancement. The resulting angular-channel bounds identify two distinct operating regimes for passive linear phased-array–lens systems: scan-resolution enhancement and directivity enhancement with preserved scan range. Guided by these limits, we derive geometry-independent design equations and develop an adjoint-based inverse-design framework for passive linear metalenses, enabling planar and cylindrical implementations that operate close to the fundamental limits. We experimentally validate a compact cylindrical metalens that preserves the $50^\circ$ scan range and scan resolution of a phased array with a $3.5\lambda_0$-wide aperture while providing a $2.5$–$3.5\,\mathrm{dB}$ increase in peak directivity. These results establish nonlocal dielectric metalenses as a practical platform for diffraction-limited wavefront engineering in phased arrays.
Read the full abstract ▾
Plasma Physics · Cross-listed from cond-mat.mtrl-sci
From Processing to Functionality: Engineering Accessible Material States in Cu-Embedded SiO$_x$ Memristive Devices
Tobias Gergs, Rouven Lamprecht, Sahitya Yarragolla et al. (+5 more)
Resistive switching in oxide-based devices is widely governed by
stochastic defect processes, yet a predictive link between fabrication conditions and functional behavior remains elusive. Here, we establish a multiscale framework connecting
plasma-defined deposition conditions to macroscopic device functionality in sputtered SiO$_x$/Cu/SiO$_x$-based systems. By combining large-scale statistical analysis of more than 50,000 experimentally characterized devices with physics-based
plasma and atomistic simulations, we show that device behavior does not emerge from deterministic process-to-performance mappings, but from a probabilistic cascade spanning defect formation, defect-state
evolution, and functional-regime emergence. Data-driven clustering reveals a continuous functional state space composed of operational switching types, while inverse modeling identifies the reconstructed oxygen-vacancy density as an effective latent descriptor capturing the combined influence of structural disorder and defect
topology. This latent descriptor is strongly coupled to both Cu redistribution and electrical response, linking otherwise hidden material properties to observable device characteristics. Furthermore, macroscopic switching behavior is argued to arise from ensemble integration across spatially heterogeneous subdomains, providing a physical explanation for the pronounced variability of large-area devices. These findings shift the perspective from deterministic defect engineering toward probabilistic defect-state design and establish a physically grounded framework for understanding and controlling functional variability in such oxide-based systems, such as memristive or resistive-switching devices.
Read the full abstract ▾
All authors Tobias Gergs, Rouven Lamprecht, Sahitya Yarragolla, Ole Gronenberg, Luca Vialetto, Hermann Kohlstedt, Thomas Mussenbrock, Jan Trieschmann
Optics
Ultrabroadband Optical Pulses with Quadratic Spectral Phase Part I: Temporal Form
George A. Hine
A time-domain representation for ultrabroadband optical pulses that includes chirp, stretching, and absolute phase is presented. This is accomplished by holomorphic
Fourier transform of a class of spectra that include spectral phase up to second order, resulting in derivatives of the Faddeeva function $w(\zeta)$. Relating the real part of this pulse to physical quantities, its shape is investigated. The derivative order $\eta$ is shown to dictate relative spectral bandwidth. The absolute phase determines the mixing of more localized (Gaussian decay) and less localized (algebraic decay) components of $w(\zeta)$. Second order spectral phase accounts for linear dispersion and is shown to cause a sheering of the Wigner-Ville distribution of the pulse, akin to the linear chirp that would be seen in the quasi-monochromatic case. Along with $\eta$ it determines the pulse length and number of oscillations. A correspondence with the slowly varying envelope approximation is derived for large order $\eta$, although the algebraic decay persists.
Read the full abstract ▾
Optics
Inverse design of large-scale freeform meta-optics by breaking the memory wall of full-wave simulation
Hyoseok Park and Yeonsang Park
Gradient-based design of large optical surfaces hits a
memory wall: a stored-history time-domain adjoint needs $24N_t$ bytes per
cell for $N_t$ steps, and every field update is bound by
memory bandwidth. Here we present a validated framework that removes this wall and makes globally optimized freeform meta-optics practical. Its finite-difference time-domain adjoint reconstructs interior fields backwards from two recorded planes, so volume
memory no longer grows with $N_t$, and fused kernels raise
memory-bandwidth use on an H200 GPU from 6.4% to a remarkable 56% of peak. Overlapping tiles, coupled by angular-spectrum propagation and distributed over GPUs, keep tile
memory independent of the aperture. Within hours on one GPU node, we optimize, against one global objective, every density variable of single-layer metasurfaces 100 to 200 $\mu$m across, and design a nine-wavelength lens, a color hologram and a polarization-switched hologram. Confirmed by an independent solver within 1.6%, they substantially outperform meta-atom designs, reaching 1.64, 1.65 and 1.24 times their objectives.
Read the full abstract ▾
Applied Physics
Topological-Insulator Heterophase Gate Stacks for Transistor Electrostatics
Minuk Song, Jiwan Kim, MD Gius Uddin et al. (+25 more)
Conventional gate-stack scaling reduces dielectric thickness and increases permittivity while largely treating the position and electronic character of the gate-side screening boundary as fixed. Here we show that this boundary can be engineered by converting the surface of the topological
insulator Bi\textsubscript{2}Se\textsubscript{3} into insulating high-$\kappa$ BiF\textsubscript{3}. Position-resolved calculations reveal a gap-opened immediate amorphous-BiF\textsubscript{3}/crystalline-Bi\textsubscript{2}Se\textsubscript{3} interface and a reconstructed gap-closed Bi\textsubscript{2}Se\textsubscript{3}-derived state in the adjacent subinterface layer, accompanied by a localized interfacial dipole. Capacitor measurements independently resolve a finite series response consistent with electronic compressibility at this boundary; this response lowers the nominal stack capacitance. Despite this capacitance penalty, MoS\textsubscript{2} transistors with closely matched BiF\textsubscript{3} thicknesses and a common BiF\textsubscript{3}/MoS\textsubscript{2} channel-side material interface exhibit near-thermionic-limit switching, negligible hysteresis and substantially weaker subthreshold drain-bias dependence than BiF\textsubscript{3}-only controls. These results identify the position and electronic character of the gate-side screening boundary as additional design variables for transistor electrostatics beyond nominal dielectric capacitance.
Read the full abstract ▾
All authors Minuk Song, Jiwan Kim, MD Gius Uddin, Wonseok Kim, Jihun Park, Han Uk Lee, Dong Won Jeon, Dohyung Lee, Lide Yao, Jouko Lahtinen, Gyunghyun Jang, Soohyun Min, Yonas Tsegaye Megra, Xiaoqi Cui, Seungwoo Choi, Yunyun Dai, Sang Hoon Chae, Keun Su Kim, Dong-Ho Kang, Hyeon-Jin Shin, Wooseok Song, Seth Ariel Tongay, Chul-Ho Lee, Manish Chhowalla, Sung Beom Cho, Zhipei Sun, Kibog Park, Hoon Hahn Yoon
Instrumentation and Detectors
Impact of the position effects in the energy calibration of the BULLKID detector
Matteo Folcarelli, Matteo Cappelli, Giorgio Del Castello et al. (+6 more)
BULLKID is a cryogenic solid-state detector based on an array of silicon dice equipped with Kinetic Inductance Detectors (KIDs) for low-mass dark matter searches and coherent elastic
neutrino-
nucleus scattering. Energy calibration can be performed using optical LED
photon bursts, which are absorbed within the first few hundred nanometers of the substrate. An energy calibration using a $^{241}$Am gamma-ray source revealed a $<10\%$ discrepancy with the LED calibration. A recent study by Matava and Williams suggested that
phonon statistics and
phonon collection varying with the interaction position could explain such a discrepancy. This work evaluates the impact of these effects on the calibration of the BULLKID detector, demonstrating they both have a negligible effect. We clarify the results obtained during the $^{241}$Am calibration regarding the origin of the observed discrepancy.
Read the full abstract ▾
All authors Matteo Folcarelli, Matteo Cappelli, Giorgio Del Castello, Camilla Bonomo, Angelo Cruciani, Daniele Delicato, Leonardo Pesce, Davide Quaranta, Marco Vignati
Applied Physics
Integration of p-type Cr2O3 on Ultra-Wide Bandgap AlGaN PolFETs with 2.5 kV Breakdown Voltage
Jonathan Pratt, Yizheng Liu, Ijaas Mohamed et al. (+8 more)
We demonstrate UWBG AlGaN polarization-graded field-effect transistors (PolFETs) incorporating room-temperature sputtered p-type oxide Cr2O3 as a P-N heterojunction gate, enabling superior field management for lateral device breakdown compared to conventional Schottky gates. Devices using reverse-graded n-AlGaN contact layers achieved a record-low Rc of 0.56 {\Omega}-mm. The Cr2O3 gate exhibits a turn-on voltage VON > 3.5 V and a positive threshold voltage shift of +1.63 V relative to Schottky gates, displaying enhanced channel depletion from the P-N junction. Fabricated devices show high Imax (590 mA/mm) and ION/IOFF of 3x10^7. Devices exhibited state-of-the-art VBR > 2.5 kV with 11.2 m{\Omega}-cm^2 specific on-resistance (LGD = 9.55 um, average breakdown field > 2.5 MV/cm), while shorter gate-drain devices showed high breakdown fields up to 5.3 MV/cm with 0.28 m{\Omega}-cm^2 specific on-resistance. These results showcase sputtered p-Cr2O3 as a viable, low thermal budget P-N junction gate technology for high-Al-composition AlGaN transistors in RF and power electronics.
Read the full abstract ▾
All authors Jonathan Pratt, Yizheng Liu, Ijaas Mohamed, Seungheon Shin, Akilesh Srikanth, Md Saklain Morshed, Brianna A. Klein, Andrew Armstrong, Andrew A. Allerman, Sriram Krishnamoorthy, Siddharth Rajan
Medical Physics · Cross-listed from q-bio.PE
Mathematical Modelling of Within-Host HIV Dynamics with Cytotoxic Immune Response and Antiretroviral Therapy
Maxim Polyakov and Yulia Chernikova
We develop and analyse a mechanistic ODE model of within-host HIV dynamics that includes uninfected CD4$^+$ T
cells, latently and productively infected
cells, free virions, and cytotoxic immune effectors. Antiretroviral therapy is described by two time-dependent efficacy functions that separately reduce new infections and virion production. We show that solutions remain non-negative and bounded, identify the infection-free and endemic
equilibria, and derive the basic reproduction number together with the local stability condition for the infection-free state. We also obtain treatment-dependent suppression thresholds and an analytical estimate of when this threshold is crossed as treatment efficacy declines. Numerical simulations consider untreated infection, ART initiation, periodic variation in efficacy, and progressive loss of treatment effect. Without therapy, the model approaches a state of persistent infection. ART reduces viral load and promotes recovery of the CD4$^+$ T-
cell population, while the latent reservoir persists under the parameter set considered. Periodic changes in treatment efficacy produce sustained forced oscillations. These results connect the analytical threshold conditions with the treatment-dependent dynamics of the model and provide a basis for future calibration, sensitivity analysis, uncertainty quantification, and extensions informed by clinical data.
Read the full abstract ▾
Computational Physics · Cross-listed from nucl-th
Rapid Uncertainty Quantification on a Latent Field using Fisher Information
Karl Daningburg, A. E. Lovell, Arvind T. Mohan et al. (+1 more)
Many inverse problems in physics infer an unobserved field from measurements connected to it through a governing equation. We combine Fisher information with a differentiable solver to quantify uncertainty in both the inferred field and its predicted observables, using a local Gaussian approximation without sampling the full parameter posterior. Applied to nuclear optical potentials, the method distinguishes features constrained by scattering measurements from those whose uncertainty remains set by the prior. Our flexible model improves agreement with held-out cross sections and achieves nearly nominal coverage with narrower predictive intervals than an established
Bayesian optical model.
Read the full abstract ▾
All authors Karl Daningburg, A. E. Lovell, Arvind T. Mohan, R. O'Shaughnessy
Fluid Dynamics
Thermal behaviour of settling heavy inertial particles in turbulent natural convection
Hamid Reza Zandi Pour, Guido Boffetta, Stefano Musacchio et al. (+1 more)
We investigate the dynamical and thermal behaviour of settling heavy inertial particles in bulk turbulent thermal convection. Direct numerical simulations are performed in a homogeneous Rayleigh-Benard convection setup, where a uniform mean temperature gradient drives statistically homogeneous convective
turbulence seeded with passive inertial particles. We quantify the effects of dynamic and thermal inertia on particle statistics. In particular, we investigate how the fluctuations of particle velocity and temperature can be traced back to the correlations between particle and fluid variables, and how particle temperature statistics is affected by the interplay of settling and preferential concentration.
Read the full abstract ▾
All authors Hamid Reza Zandi Pour, Guido Boffetta, Stefano Musacchio, Filippo De Lillo
Instrumentation and Detectors · Cross-listed from cs.RO
High-Voltage Optocoupler Amplifier for Electrostatic Actuators
George C. Jurgiel, Alex S. Miller and Jeffrey H. Lang
Many electrostatic actuators require multi-kilovolt drive voltages at sub-milliamp currents, a task poorly suited for conventional switching devices. As an alternative, we demonstrate a high-voltage amplifier using optocouplers as active elements. The amplifier produces a 20-kV peak-to-peak output with up to 500 Hz bandwidth while maintaining a minimal component count. By using optocouplers as linear devices in feedback, lower harmonic distortion and higher bandwidth are achieved than offered by equivalent PWM amplifiers. This design improves the viability of electrostatic actuators by providing a simpler method to achieve useful drive waveforms.
Read the full abstract ▾
Fluid Dynamics · Cross-listed from cond-mat.soft
Extracting hydrogel properties by watching hydrogel particles moving through solid ice
Yanxia Feng, Se-Hyeong Jung, Camillo Sirvinski et al. (+4 more)
Strikingly, when hydrogel particles are embedded in ice in a temperature gradient, they move through the solid ice towards warmer temperatures, while swelling as they warm up. This motion comes from a flow of unfrozen water through the hydrogel mesh, driven by a process known as `cryosuction'. Here, we show how one can use this behavior to measure – with extremely high resolution – a range of different hydrogel transport properties, and how these change as a hydrogel deswells. These properties include permeability, compressibility, and poroelastic diffusivity: all of which are challenging to measure, but widely important for phenomena involving swelling, dehydration, transpiration and filtration. We demonstrate the measurement technique using poly(ethylene glycol) diacrylate (PEGDA) hydrogels. The technique uses picoliter-scale hydrogel volumes, and yields measurements of hydrogel properties that are consistent with existing literature data. The high resolution of our measurements also allows us to test commonly-used, classical predictions for hydrogel properties (derived assuming an idealized, homogeneous
polymer network in the hydrogel). We show that these classical predictions do not work well, highlighting the need for new models that can accurately describe real hydrogels.
Read the full abstract ▾
All authors Yanxia Feng, Se-Hyeong Jung, Camillo Sirvinski, Dan Balkanyi, Federico Paratore, Lucio Isa, Robert W. Style