Showing 1–50 of 1,003 results for author: Müller, D

Olaf: Bringing an Animated Character to Life in the Physical World

Authors: David Müller, Espen Knoop, Dario Mylonopoulos, Agon Serifi, Michael A. Hopkins, Ruben Grandia, Moritz Bächer

Abstract: Animated characters often move in non-physical ways and have proportions that are far from a typical walking robot. This provides an ideal platform for innovation in both mechanical design and stylized motion control. In this paper, we bring Olaf to life in the physical world, relying on reinforcement learning guided by animation references for control. To create the illusion of Olaf's feet moving… ▽ More Animated characters often move in non-physical ways and have proportions that are far from a typical walking robot. This provides an ideal platform for innovation in both mechanical design and stylized motion control. In this paper, we bring Olaf to life in the physical world, relying on reinforcement learning guided by animation references for control. To create the illusion of Olaf's feet moving along his body, we hide two asymmetric legs under a soft foam skirt. To fit actuators inside the character, we use spherical and planar linkages in the arms, mouth, and eyes. Because the walk cycle results in harsh contact sounds, we introduce additional rewards that noticeably reduce impact noise. The large head, driven by small actuators in the character's slim neck, creates a risk of overheating, amplified by the costume. To keep actuators from overheating, we feed temperature values as additional inputs to policies, introducing new rewards to keep them within bounds. We validate the efficacy of our modeling in simulation and on hardware, demonstrating an unmatched level of believability for a costumed robotic character. △ Less

Submitted 18 December, 2025; originally announced December 2025.

A discrete approach to Dirichlet L-functions via spectral models: special values and zeros

Authors: Anders Karlsson, Dylan Müller

Abstract: We study Dirichlet $L$-functions via discrete analogs $L_n$ arising from the spectral theory of cyclic graphs as $n\rightarrow \infty$. Using a refined Euler-Maclaurin asymptotic expansion due to Sidi, together with an independent polynomiality property of these finite spectral sums at integers, we obtain exact special-value formulas, even starting at $n=1$. This yields new expressions for certain… ▽ More We study Dirichlet $L$-functions via discrete analogs $L_n$ arising from the spectral theory of cyclic graphs as $n\rightarrow \infty$. Using a refined Euler-Maclaurin asymptotic expansion due to Sidi, together with an independent polynomiality property of these finite spectral sums at integers, we obtain exact special-value formulas, even starting at $n=1$. This yields new expressions for certain trigonometric sums of interest in physics, and recovers, by a different method, the striking formulas of Xie, Zhao, and Zhao. It also gives infinite families of recursion relations among special values of the Riemann zeta function and of Dirichlet $L$-functions. Concerning zeros, we prove that, for odd primitive characters, a natural asymptotic functional equation for the discrete functions $L_n$ is equivalent to the Generalized Riemann Hypothesis for the corresponding Dirichlet $L$-function. We also provide some remarks about the non-existence of possible real zeros. △ Less

Submitted 1 December, 2025; originally announced December 2025.

Comments: 24 pages

MSC Class: 11M (Primary) 11L03; 35K08; 39A12 (Secondary)

Effect of substrate miscut angle on critical thickness, structural and electronic properties of MBE-grown NbN films on c-plane sapphire

Authors: Anand Ithepalli, Saumya Vashishtha, Naomi Pieczulewski, Qiao Liu, Amit Rohan Rajapurohita, Matthew Barone, Darrell Schlom, David A. Muller, Huili Grace Xing, Debdeep Jena

Abstract: We report the structural and electronic properties of niobium nitride (NbN) thin films grown by molecular beam epitaxy on c-plane sapphire with miscut angles of $0.5^\text{o}$, $2^\text{o}$, $4^\text{o}$, and $10^\text{o}$ towards m-axis. X-ray diffraction (XRD) scans reveal that the full width at half maximum of the rocking curves around the 1 1 1 reflection of these NbN films decreases with incr… ▽ More We report the structural and electronic properties of niobium nitride (NbN) thin films grown by molecular beam epitaxy on c-plane sapphire with miscut angles of $0.5^\text{o}$, $2^\text{o}$, $4^\text{o}$, and $10^\text{o}$ towards m-axis. X-ray diffraction (XRD) scans reveal that the full width at half maximum of the rocking curves around the 1 1 1 reflection of these NbN films decreases with increasing miscut. Starting from 76 arcsecs on $0.5^\text{o}$ miscut, the FWHM reduces to almost 20 arcsecs on $10^\text{o}$ miscut sapphire indicating improved structural quality. Scanning transmission electron microscopy (STEM) images indicate that NbN on c-sapphire has around 10 nm critical thickness, irrespective of the substrate miscut, above which it turns columnar. The improved structural property is correlated with a marginal increment in superconducting transition temperature $T_\text{c}$ from 12.1 K for NbN on $0.5^\text{o}$ miscut sapphire to 12.5 K for NbN on $10^\text{o}$ miscut sapphire. △ Less

Submitted 17 November, 2025; originally announced November 2025.

Robot Crash Course: Learning Soft and Stylized Falling

Authors: Pascal Strauch, David Müller, Sammy Christen, Agon Serifi, Ruben Grandia, Espen Knoop, Moritz Bächer

Abstract: Despite recent advances in robust locomotion, bipedal robots operating in the real world remain at risk of falling. While most research focuses on preventing such events, we instead concentrate on the phenomenon of falling itself. Specifically, we aim to reduce physical damage to the robot while providing users with control over a robot's end pose. To this end, we propose a robot agnostic reward f… ▽ More Despite recent advances in robust locomotion, bipedal robots operating in the real world remain at risk of falling. While most research focuses on preventing such events, we instead concentrate on the phenomenon of falling itself. Specifically, we aim to reduce physical damage to the robot while providing users with control over a robot's end pose. To this end, we propose a robot agnostic reward function that balances the achievement of a desired end pose with impact minimization and the protection of critical robot parts during reinforcement learning. To make the policy robust to a broad range of initial falling conditions and to enable the specification of an arbitrary and unseen end pose at inference time, we introduce a simulation-based sampling strategy of initial and end poses. Through simulated and real-world experiments, our work demonstrates that even bipedal robots can perform controlled, soft falls. △ Less

Submitted 13 November, 2025; originally announced November 2025.

All-nitride superconducting qubits based on atomic layer deposition

Authors: Danqing Wang, Yufeng Wu, Naomi Pieczulewski, Prachi Garg, Manuel C. C. Pace, C. G. L. Bøttcher, Baishakhi Mazumder, David A. Muller, Hong X. Tang

Abstract: The development of large-scale quantum processors benefits from superconducting qubits that can operate at elevated temperatures and be fabricated with scalable, foundry-compatible processes. Atomic layer deposition (ALD) is increasingly being adopted as an industrial standard for thin-film growth, particularly in applications requiring precise control over layer thickness and composition. Here, w… ▽ More The development of large-scale quantum processors benefits from superconducting qubits that can operate at elevated temperatures and be fabricated with scalable, foundry-compatible processes. Atomic layer deposition (ALD) is increasingly being adopted as an industrial standard for thin-film growth, particularly in applications requiring precise control over layer thickness and composition. Here, we report superconducting qubits based on NbN/AlN/NbN trilayers deposited entirely by ALD. By varying the number of ALD cycles used to form the AlN barrier, we achieve Josephson tunneling through barriers of different thicknesses, with critical current density spanning seven orders of magnitude, demonstrating the uniformity and versatility of the process. Owing to the high critical temperature of NbN, transmon qubits based on these all-nitride trilayers exhibit microsecond-scale relaxation times, even at temperatures above 300 mK. These results establish ALD as a viable low-temperature deposition technique for superconducting quantum circuits and position all-nitride ALD qubits as a promising platform for operation at elevated temperatures. △ Less

Submitted 11 November, 2025; originally announced November 2025.

Comments: 4 figures in the main text

$L^p$-estimates for FIO-cone multipliers

Authors: Stefan Buschenhenke, Spyridon Dendrinos, Isroil A. Ikromov, Detlef Müller

Abstract: The classical cone multipliers are Fourier multiplier operators which localize to narrow $1/R$-neighborhoods of the truncated light cone in frequency space. By composing such convolution operators with suitable translation invariant Fourier integral operators (FIOs), we obtain what we call FIO-cone multipliers. We introduce and study classes of such FIO-cone multipliers on $\Bbb R^3$, in which the… ▽ More The classical cone multipliers are Fourier multiplier operators which localize to narrow $1/R$-neighborhoods of the truncated light cone in frequency space. By composing such convolution operators with suitable translation invariant Fourier integral operators (FIOs), we obtain what we call FIO-cone multipliers. We introduce and study classes of such FIO-cone multipliers on $\Bbb R^3$, in which the phase functions of the corresponding FIOs are adapted in a natural way to the geometry of the cone and may even admit singularities at the light cone. By building on methods developed by Guth, Wang and Zhang in their proof of the cone multiplier conjecture in $\Bbb R^3,$ we obtain $L^p$-estimates for FIO-cone multipliers in the range $4/3\le p\le 4$ which are stronger by the factor $R^{-|1/p-1/2|}$ than what a direct application of the method of Seeger, Sogge and Stein for estimating FIOs would give. An important application of our theory is to maximal averages along smooth analytic surfaces in $\Bbb R^3.$ It allows to confirm a conjecture on the the critical Lebesgue exponent for a prototypical surface from a small class of ``exceptional'' surfaces, for which this conjecture had remained open. △ Less

Submitted 7 November, 2025; originally announced November 2025.

MSC Class: 42B15; 35S30; 42B25

Disorder-broadened topological Hall phase and anomalous Hall scaling in FeGe

Authors: Chaman Gupta, Chris Matsumura, Hongbin Yang, Sarah Edwards, Rebeca M. Gurrola, Jiun-Haw Chu, Hanjong Paik, Yongqiang Wang, David A. Muller, Robert Streubel, Tzu-Ming Lu, Serena Eley

Abstract: Magnetic skyrmions are topologically protected spin textures that are promising candidates for low-power spintronic memory and logic devices. Realizing skyrmion-based devices requires an understanding of how structural disorder affects their stability and transport properties. This study uses Ne$^{+}$ ion irradiation at fluences from $10^{11}$ to $10^{14}$ ions-cm$^{-2}$ to systematically vary def… ▽ More Magnetic skyrmions are topologically protected spin textures that are promising candidates for low-power spintronic memory and logic devices. Realizing skyrmion-based devices requires an understanding of how structural disorder affects their stability and transport properties. This study uses Ne$^{+}$ ion irradiation at fluences from $10^{11}$ to $10^{14}$ ions-cm$^{-2}$ to systematically vary defect densities in 80 nm epitaxial FeGe films and quantify the resulting modifications to magnetic phase boundaries and electronic scattering. Temperature- and field-dependent Hall measurements reveal that increasing disorder progressively extends the topological Hall signal from a narrow window near 200K in pristine films down to 4K at the highest fluence, with peak amplitude more than doubling. Simultaneously, the anomalous Hall effect transitions from quadratic Berry curvature scaling to linear skew scattering behavior, with the skew coefficient increasing threefold. These results establish quantitative correlations between defect concentration, skyrmion phase space, and transport mechanisms in a chiral magnet. It demonstrates that ion-beam modification provides systematic control over both topological texture stability and electrical detectability. △ Less

Submitted 7 November, 2025; originally announced November 2025.

Comments: 20 pages, 5 figures

The Mathematisation of the World: Uncovering the Socio-Economic Tensions for Ethics in Mathematics Education

Authors: Dennis Müller

Abstract: The mathematisation of the socio-economic sphere, where mathematics actively constructs social reality, presents a challenge for studies on ethics in mathematics and its education. While existing scholarship on ethics in mathematics offers insights, it often remains philosophically driven and disconnected from other relevant disciplines. This paper addresses this gap by asking how debates on ethic… ▽ More The mathematisation of the socio-economic sphere, where mathematics actively constructs social reality, presents a challenge for studies on ethics in mathematics and its education. While existing scholarship on ethics in mathematics offers insights, it often remains philosophically driven and disconnected from other relevant disciplines. This paper addresses this gap by asking how debates on ethics in mathematics and its education can be connected with economic sociology, and what socio-economic tensions become visible through this connection. Drawing from concepts such as imagined futures, varieties of capitalism, and variegated capitalism, we synthesise a new perspective. This analysis reveals six interconnected tensions: a socio-economic valuation gap regarding ethics education; the multifaceted implementation of mathematics across different capitalist systems; its material opaqueness; a growing gap between economic power and social unaccountability; the enclosure of imagination limiting sustainable futures; and the erosion of multilateralism, which challenges critical pedagogy. The paper's contribution is a first step towards a structural socio-economic framework that links the limited literature on ethics in mathematics with these broader sociological perspectives. △ Less

Submitted 27 October, 2025; originally announced October 2025.

Comments: 50 pages, 7 figures, 1 table

MSC Class: 00A30; 01A80; 97A40; 97B99; 97C60; 97D20; 97E20; 97M10

Mind the Gap -- Imaging Buried Interfaces in Twisted Oxide Moirés

Authors: Harikrishnan KP, Xin Wei, Chia-Hao Lee, Dasol Yoon, Yonghun Lee, Kevin J. Crust, Yu-Tsun Shao, Ruijuan Xu, Jong-Hoon Kang, Ce Liang, Jiwoong Park, Harold Y. Hwang, David A. Muller

Abstract: The ability to tune electronic structure in twisted stacks of layered, two-dimensional (2D) materials has motivated the exploration of similar moiré physics with stacks of twisted oxide membranes. Due to the intrinsic three-dimensional (3D) nature of bonding in many oxides, achieving atomic-level coupling is significantly more challenging than in 2D van der Waals materials. Although clean interfac… ▽ More The ability to tune electronic structure in twisted stacks of layered, two-dimensional (2D) materials has motivated the exploration of similar moiré physics with stacks of twisted oxide membranes. Due to the intrinsic three-dimensional (3D) nature of bonding in many oxides, achieving atomic-level coupling is significantly more challenging than in 2D van der Waals materials. Although clean interfaces with atomic level proximity have been demonstrated in ceramic bicrystals using high-temperature and high-pressure processing to facilitate atomic diffusion that flattens rough interfaces, such conditions are not readily accessible when bonding oxide membranes. This study shows how topographic mismatch due to surface roughness of the membranes can restrict atomic-scale proximity at the interface to isolated patches even after obvious issues of contaminants and amorphous interlayers are eliminated. In hybrid interfaces between a chemically inert 2D material and an oxide membrane, the reduced ability of the 2D material to conform to the membrane's step-terrace topography also limits atomic-scale contact. In all these material systems, the interface morphology is best characterized using cross-sectional imaging and is necessary to corroborate investigations of interlayer coupling. When imaging the bicrystal in projection, conventional through-focal imaging is found to be relatively insensitive to the buried interface, whereas electron ptychography reliably resolves structural variations on the order of a nanometer. These findings highlight interface roughness as a key challenge for the field of oxide twistronics and emphasizes the need for reliable characterization methods. △ Less

Submitted 27 October, 2025; originally announced October 2025.

Comments: 27 pages, 6 figures, 13 supplementary figures

Subatomic Heroes

Authors: Anshika Bansal, Guido Bell, Aritra Biswas, Diptaparna Biswas, Anastasia Boushmelev, Carsten Busse, Markus Cristinziani, Carmen Diez Pardos, Qader Dorosti, Sebastian Edelmann, Thorsten Feldmann, Ivor Fleck, Jan Hahn, Dennis Horstmann, Tobias Huber, Jack Jenkins, Wolfgang Kilian, Danny Koschwitz, Nils Krengel, Martin Lang, Björn Lange, Alexander Lenz, Eleftheria Malami, Thomas Mannel, Ilija Milutin , et al. (22 additional authors not shown)

Abstract: Sharing the amazing achievements of the (particle) physics world with the general public is at the heart of the mission of the Subatomic Heroes, based at the University of Siegen, Germany. Originally this started out as an endeavor of theoretical particle physics, now we are steadily spreading out to cover and include more branches of physics and science. Our activities range from merging art with… ▽ More Sharing the amazing achievements of the (particle) physics world with the general public is at the heart of the mission of the Subatomic Heroes, based at the University of Siegen, Germany. Originally this started out as an endeavor of theoretical particle physics, now we are steadily spreading out to cover and include more branches of physics and science. Our activities range from merging art with public physics lectures via marvelous artistic performances at the local theater, over dedicated events for high-school students, to our Subatomic Heroes channel on Instagram and TikTok where you may also find out when and where our famous "hadronic ice-cream" will be served next! So follow us on https://www.instagram.com/subatomic_heroes and https://www.tiktok.com/@subatomic_heroes. △ Less

Submitted 24 October, 2025; originally announced October 2025.

Comments: 6 pages, 4 figures, Proceedings of The European Physical Society Conference on High Energy Physics (EPS-HEP2025), 7-11 July 2025, Marseille, France

Report number: SI-HEP-2025-23, P3H-25-082

The Ethical and Sustainable Concerns Triangle: A Framework for Navigating Discourses in Mathematics and Its Education

Authors: Dennis Müller, Maurice Chiodo, Michael Meyer

Abstract: The literature on ethics and sustainability in mathematics and its education is increasingly complex and fragmented, potentially leading to communication breakdowns between different scholarly traditions. To address this, the paper introduces the "Ethical and Sustainable Concerns Triangle," a framework that maps discourses based on their relative concern for three areas (represented as three verti… ▽ More The literature on ethics and sustainability in mathematics and its education is increasingly complex and fragmented, potentially leading to communication breakdowns between different scholarly traditions. To address this, the paper introduces the "Ethical and Sustainable Concerns Triangle," a framework that maps discourses based on their relative concern for three areas (represented as three vertices in the triangle): "Mathematics", "Community", and "Society/Planet". By integrating a systems theoretic perspective, we analyse discourses as dynamic systemic reactions to external irritations. Our analysis reveals that the field's fragmentation can be explained by the "location effect": the phenomenon whereby a discourse's position within the triangle shapes its perception and acceptance of other scholarship. By mapping key discourses and educator archetypes, the framework functions as a meta-heuristic tool. Ultimately, it serves not only to facilitate critical reflection but also as a call for the epistemic humility and dialogue needed to advance the field. △ Less

Submitted 13 October, 2025; originally announced October 2025.

Comments: 86 pages, 16 figures, 7 tables

MSC Class: 00A30; 01A80; 97A40; 97B99; 97C60; 97D20; 97E20; 97M10

Picoflares in the Quiet Solar Corona: Solar Orbiter Observations Halfway to the Sun

Authors: O. Podladchikova, A. Warmuth, L. Harra, L. Dolla, C. Verbeeck, M. Mierla, L. Rodriguez, S. Parenti, M. K. Georgoulis, S. J. Hofmeister, N. Engler, M. J. West, A. M. Veronig, P. Antolin, S. Purkhart, D. M. Long, É. Buchlin, M. Haberreiter, A. N. Zhukov, H. Safari, A. F. Battaglia, E. Soubrié, V. Büchel, S. Gissot, A. De Groof , et al. (11 additional authors not shown)

Abstract: X-ray observations of the Sun led Eugene Parker to propose nanoflares as the basic energy-release units that heat the solar corona. Decades later, Solar Orbiter's Extreme Ultraviolet Imager (HRIEUV), operating halfway between Earth and the Sun, revealed thousands of even smaller brightenings in the quiet corona - tiny "campfires" that are smaller and far more frequent than the fundamental nanoflar… ▽ More X-ray observations of the Sun led Eugene Parker to propose nanoflares as the basic energy-release units that heat the solar corona. Decades later, Solar Orbiter's Extreme Ultraviolet Imager (HRIEUV), operating halfway between Earth and the Sun, revealed thousands of even smaller brightenings in the quiet corona - tiny "campfires" that are smaller and far more frequent than the fundamental nanoflares observed from 1 AU. We analyze over 12,000 of these events, deriving their thermal energies using multiple geometric models to account for volume uncertainties. Although absolute values vary, all models yield consistent power-law energy distributions and ranges, confirming their flare-like behavior. These picoflares, spanning $10^{20}$--$10^{24}$ erg, were detected by the Solar Orbiter EUI Imager while the spacecraft was at 0.56 AU from the Sun. They occur up to sixty times more often than nanoflares seen from Earth orbit and supply about 1% of the quiet-Sun coronal heating power. This previously unseen energy source may be a missing component in the solar energy balance. Their discovery extends the flare energy spectrum to smaller scales, and future Solar Orbiter observations at 0.28 AU may reveal the most fundamental flare events that sustain the million-degree solar corona. △ Less

Submitted 29 November, 2025; v1 submitted 11 October, 2025; originally announced October 2025.

Comments: 13 pages, 7 figures, submitted to Astronomy & Astrophysics (A&A)

MSC Class: 85A40; 62P35; 62M10 ACM Class: J.2; I.5.4; G.3

Ethical and sustainable mathematics is localised: why global paradigms fail and culturally-situated practices are essential

Authors: Dennis Müller, Maurice Chiodo

Abstract: This paper identifies several different interconnected challenges preventing the move towards more ethical and sustainable mathematics education: the entrenched belief in mathematical neutrality, the difficulty of simultaneously reforming mathematics and its pedagogy, the gap between academic theory and classroom practice, and the need for epistemic decolonisation. In this context, we look at both… ▽ More This paper identifies several different interconnected challenges preventing the move towards more ethical and sustainable mathematics education: the entrenched belief in mathematical neutrality, the difficulty of simultaneously reforming mathematics and its pedagogy, the gap between academic theory and classroom practice, and the need for epistemic decolonisation. In this context, we look at both bottom-up and top-down approaches, and argue that globalised frameworks such as the United Nations' Sustainable Development Goals are insufficient for this transformation, and that ethical and sustainable forms of mathematics ought not to be built using these as their (philosophical) foundation. These frameworks are often rooted in a Western-centric development paradigm that can perpetuate colonial hierarchies and fails to resolve inherent conflicts between economic growth and ecological integrity. As an alternative, this paper advocates for embracing localised, culturally-situated mathematical practices. Using the Ethics in Mathematics Project as a case study within a Western, Global North institution, this paper illustrates a critical-pragmatic, multi-level strategy for fostering ethical consciousness within a specific research community, and shows how this may be achieved in otherwise adversarial circumstances. △ Less

Submitted 7 October, 2025; originally announced October 2025.

Comments: 35 pages. Keywords: Sustainable Mathematics Education, Ethics in Mathematics, Praxis, Philosophy of Mathematics Education, Critical Mathematics Education, Sustainable Development Goals (SDGs)

MSC Class: 97D20; 97A40; 97B99; 00A30; 01A80

Advancing Automated Spatio-Semantic Analysis in Picture Description Using Language Models

Authors: Si-Ioi Ng, Pranav S. Ambadi, Kimberly D. Mueller, Julie Liss, Visar Berisha

Abstract: Current methods for automated assessment of cognitive-linguistic impairment via picture description often neglect the visual narrative path - the sequence and locations of elements a speaker described in the picture. Analyses of spatio-semantic features capture this path using content information units (CIUs), but manual tagging or dictionary-based mapping is labor-intensive. This study proposes a… ▽ More Current methods for automated assessment of cognitive-linguistic impairment via picture description often neglect the visual narrative path - the sequence and locations of elements a speaker described in the picture. Analyses of spatio-semantic features capture this path using content information units (CIUs), but manual tagging or dictionary-based mapping is labor-intensive. This study proposes a BERT-based pipeline, fine tuned with binary cross-entropy and pairwise ranking loss, for automated CIU extraction and ordering from the Cookie Theft picture description. Evaluated by 5-fold cross-validation, it achieves 93% median precision, 96% median recall in CIU detection, and 24% sequence error rates. The proposed method extracts features that exhibit strong Pearson correlations with ground truth, surpassing the dictionary-based baseline in external validation. These features also perform comparably to those derived from manual annotations in evaluating group differences via ANCOVA. The pipeline is shown to effectively characterize visual narrative paths for cognitive impairment assessment, with the implementation and models open-sourced to public. △ Less

Submitted 30 September, 2025; originally announced October 2025.

Beyond Softmax: A New Perspective on Gradient Bandits

Authors: Emerson Melo, David Müller

Abstract: We establish a link between a class of discrete choice models and the theory of online learning and multi-armed bandits. Our contributions are: (i) sublinear regret bounds for a broad algorithmic family, encompassing Exp3 as a special case; (ii) a new class of adversarial bandit algorithms derived from generalized nested logit models \citep{wen:2001}; and (iii) \textcolor{black}{we introduce a nov… ▽ More We establish a link between a class of discrete choice models and the theory of online learning and multi-armed bandits. Our contributions are: (i) sublinear regret bounds for a broad algorithmic family, encompassing Exp3 as a special case; (ii) a new class of adversarial bandit algorithms derived from generalized nested logit models \citep{wen:2001}; and (iii) \textcolor{black}{we introduce a novel class of generalized gradient bandit algorithms that extends beyond the widely used softmax formulation. By relaxing the restrictive independence assumptions inherent in softmax, our framework accommodates correlated learning dynamics across actions, thereby broadening the applicability of gradient bandit methods.} Overall, the proposed algorithms combine flexible model specification with computational efficiency via closed-form sampling probabilities. Numerical experiments in stochastic bandit settings demonstrate their practical effectiveness. △ Less

Submitted 4 October, 2025; originally announced October 2025.

Active-Learning Inspired $\textit{Ab Initio}$ Theory-Experiment Loop Approach for Management of Material Defects: Application to Superconducting Qubits

Authors: Sarvesh Chaudhari, Cristóbal Méndez, Rushil Choudhary, Tathagata Banerjee, Maciej W. Olszewski, Jadrien T. Paustian, Jaehong Choi, Zhaslan Baraissov, Raul Hernandez, David A. Muller, B. L. T. Plourde, Gregory D. Fuchs, Valla Fatemi, Tomás A. Arias

Abstract: Surface oxides are associated with two-level systems (TLSs) that degrade the performance of niobium-based superconducting quantum computing devices. To address this, we introduce a predictive framework for selecting metal capping layers that inhibit niobium oxide formation. Using DFT-calculated oxygen interstitial and vacancy energies as thermodynamic descriptors, we train a logistic regression mo… ▽ More Surface oxides are associated with two-level systems (TLSs) that degrade the performance of niobium-based superconducting quantum computing devices. To address this, we introduce a predictive framework for selecting metal capping layers that inhibit niobium oxide formation. Using DFT-calculated oxygen interstitial and vacancy energies as thermodynamic descriptors, we train a logistic regression model on a limited set of experimental outcomes to successfully predict the likelihood of oxide formation beneath different capping materials. This approach identifies Zr, Hf, and Ta as effective diffusion barriers. Our analysis further reveals that the oxide formation energy per oxygen atom serves as an excellent standalone descriptor for predicting barrier performance. By combining this new descriptor with lattice mismatch as a secondary criterion to promote structurally coherent interfaces, we identify Zr, Ta, and Sc as especially promising candidates. This closed-loop strategy integrates first-principles theory, machine learning, and limited experimental data to enable rational design of next-generation materials. △ Less

Submitted 9 October, 2025; v1 submitted 2 October, 2025; originally announced October 2025.

Comments: 7 pages, 6 figures (7 images)

Six-dimensional cosmological models with conformal extensions

Authors: Daniel Muller, Sergey G. Rubin, Ilya L. Shapiro, Alexey Toporensky

Abstract: We consider the background cosmological solutions in the $6D$ (six-dimensional) model with one time and five space coordinates. The theory of our interest has the action composed by the Einstein term, cosmological constant, and two conformal terms constructed from the third powers of the Weyl tensor. It is shown how the highest derivative terms in the equations of motion can be isolated that opens… ▽ More We consider the background cosmological solutions in the $6D$ (six-dimensional) model with one time and five space coordinates. The theory of our interest has the action composed by the Einstein term, cosmological constant, and two conformal terms constructed from the third powers of the Weyl tensor. It is shown how the highest derivative terms in the equations of motion can be isolated that opens the way for their numerical integration. There are flat anisotropic solutions which make one of the flat isotropic subspaces to be static. Depending on the value of bare cosmological constant, either two-dimensional or three-dimensional subspace can be static. In particular, there is a physically favorable solution with three ``large'' space coordinates and two extra inner dimensions stabilized. This solution is stable for a wide range of coupling constants, but this requires a special value of the bare cosmological constant. △ Less

Submitted 1 October, 2025; originally announced October 2025.

Using Aberrations to Improve Dose-Efficient Tilt-corrected 4D-STEM Imaging

Authors: Desheng Ma, David A Muller, Steven E Zeltmann

Abstract: Tilt-corrected imaging methods in four-dimensional scanning transmission electron microscopy (4D-STEM) have recently emerged as a new class of direct ptychography methods that are especially useful at low dose. The operation of tilt correction unfolds the contrast transfer functions (CTF) of the virtual bright-field images and retains coherence by correcting defocus-induced spatial shifts. By perf… ▽ More Tilt-corrected imaging methods in four-dimensional scanning transmission electron microscopy (4D-STEM) have recently emerged as a new class of direct ptychography methods that are especially useful at low dose. The operation of tilt correction unfolds the contrast transfer functions (CTF) of the virtual bright-field images and retains coherence by correcting defocus-induced spatial shifts. By performing summation or subtraction of the tilt-corrected images, the real or imaginary parts of the complex phase-contrast transfer functions are recovered, producing a tilt-corrected bright field image (tcBF) or a differential phase contrast image (tcDPC). However, the CTF can be strongly damped by the introduction of higher-order aberrations than defocus. In this paper, we show how aberration-corrected bright-field imaging (acBF), which combines tcBF and tcDPC, enables continuously-nonzero contrast transfer within the information limit, even in the presence of higher-order aberrations. At Scherzer defocus in a spherically-aberration-limited system, the resultant phase shift from the probe-forming lens acts as a phase plate, removing oscillations from the acBF CTF. We demonstrate acBF on both simulated and experimental data, showing it produces superior performance to tcBF or DPC methods alone, and discuss its limitations. △ Less

Submitted 1 October, 2025; originally announced October 2025.

Comments: 22 pages, 13 figures

Heavy flavor angular correlations as probes of the glasma

Authors: Dana Avramescu, Vincenzo Greco, Tuomas Lappi, Heikki Mäntysaari, David I. Müller

Abstract: We study the effect of the glasma fields, formed in the early stage of heavy-ion collisions, on the transport of $Q\bar{Q}$ pairs produced back-to-back. We find that for pairs with moderate initial transverse momentum $p_T$ evolving in glasma fields with sufficiently large saturation momentum $Q_s$, the azimuthal correlation $C({Δφ})$ is quickly affected. The decorrelation widths $σ_{Δφ}$ during t… ▽ More We study the effect of the glasma fields, formed in the early stage of heavy-ion collisions, on the transport of $Q\bar{Q}$ pairs produced back-to-back. We find that for pairs with moderate initial transverse momentum $p_T$ evolving in glasma fields with sufficiently large saturation momentum $Q_s$, the azimuthal correlation $C({Δφ})$ is quickly affected. The decorrelation widths $σ_{Δφ}$ during the glasma and Quark Gluon Plasma (QGP) phases are comparable. △ Less

Submitted 29 September, 2025; originally announced September 2025.

Comments: 4 pages, 4 figures, proceedings for Quark Matter 2025

Melting point depression of charge density wave in 1T-TiSe$_2$ due to size effects

Authors: Saif Siddique, Mehrdad T. Kiani, Omri Lesser, Stephen D. Funni, Nishkarsh Agarwal, Maya Gates, Miti Shah, William Millsaps, Suk Hyun Sung, Noah Schnitzer, Lopa Bhatt, David A. Muller, Robert Hovden, Ismail El Baggari, Eun-Ah Kim, Judy J. Cha

Abstract: Classical nucleation theory predicts size-dependent nucleation and melting due to surface and confinement effects at the nanoscale. In correlated electronic states, observation of size-dependent nucleation and melting is rarely reported, likely due to the extremely small length scales necessary to observe such effects for electronic states. Here, using 1T-TiSe$_2$ nanoflakes as a prototypical two-… ▽ More Classical nucleation theory predicts size-dependent nucleation and melting due to surface and confinement effects at the nanoscale. In correlated electronic states, observation of size-dependent nucleation and melting is rarely reported, likely due to the extremely small length scales necessary to observe such effects for electronic states. Here, using 1T-TiSe$_2$ nanoflakes as a prototypical two-dimensional (2D) charge density wave (CDW) system, we perform in-situ cryogenic electron microscopy with temperature down to 20 K and observe size-dependent nucleation and melting of CDWs. Specifically, we observe a melting point depression of CDW for 1T-TiSe$_2$ flakes with lateral sizes less than 100 nm. By fitting experimental data to a Ginzburg-Landau model, we estimate a zero-temperature correlation length of 10--50 nm, which matches the reported CDW domain size for 1T-TiSe$_2$. As the flake size approaches the correlation length, the divergence of the CDW correlation length near the transition is cut off by the finite flake size, limiting long-range order and thereby lowering the transition temperature. For very small flakes whose size is close to the correlation length, we also observe absence of CDWs, as predicted by the model. We thus show that an electronic phase transition follows classical nucleation theory. △ Less

Submitted 20 September, 2025; originally announced September 2025.

Detective quantum efficiency based comparison of HRTEM and ptychography phase imaging

Authors: Felix Bennemann, Angus I. Kirkland, David A. Muller, Peter Nellist

Abstract: High-resolution transmission electron microscopy (HRTEM) is an important method for imaging beam sensitive materials often under cryo conditions. Electron ptychography in the scanning transmission electron microscope (STEM) has been shown to reconstruct low-noise phase data at a reduced fluence for such materials. This raises the question of whether ptychography or HRTEM provides a more fluence-ef… ▽ More High-resolution transmission electron microscopy (HRTEM) is an important method for imaging beam sensitive materials often under cryo conditions. Electron ptychography in the scanning transmission electron microscope (STEM) has been shown to reconstruct low-noise phase data at a reduced fluence for such materials. This raises the question of whether ptychography or HRTEM provides a more fluence-efficient imaging technique. Even though the transfer function is a common metric for evaluating the performance of an imaging method, it only describes the signal transfer with respect to spatial frequency, irrespective of the noise transfer. It can also not be well defined for methods, such as ptychography, that use an algorithm to form the final image. Here we apply the concept of detective quantum efficiency (DQE) to electron microscopy as a fluence independent and sample independent measure of technique performance. We find that, for a weak-phase object, ptychography can never reach the efficiency of a perfect Zernike phase imaging microscope but that ptychography is more robust to partial coherence. △ Less

Submitted 18 September, 2025; v1 submitted 15 September, 2025; originally announced September 2025.

Cepstral Strain Mapping for Small Pixel-Count Detectors

Authors: Harikrishnan KP, Dasol Yoon, Yu-Tsun Shao, Zhaslan Baraissov, Luigi Mele, Christoph Mitterbauer, Erik Kieft, Stefano Vespucci, David A. Muller

Abstract: With the decreasing sizes of integrated-circuit components, the semiconductor industry is in growing need of high-throughput strain mapping techniques that offer high precision and spatial resolution, with desired industry goals of 0.01-0.1% and 1 nm respectively. As the fundamental limitation on the measurement precision is set by the Poisson noise, pixel array detectors with high saturation curr… ▽ More With the decreasing sizes of integrated-circuit components, the semiconductor industry is in growing need of high-throughput strain mapping techniques that offer high precision and spatial resolution, with desired industry goals of 0.01-0.1% and 1 nm respectively. As the fundamental limitation on the measurement precision is set by the Poisson noise, pixel array detectors with high saturation current, high dynamic range and fast readout are ideally suited for this purpose. However, due to the limited pixel count on these detectors, they do not work well with traditional strain mapping algorithms that were optimized to work on datasets with a large pixel count. Here, we evaluate the cepstral transform that was designed to address this problem, with the precision determined by the convergence, collection angles and dose while remaining insensitive to the pixel count. We test the performance of our method on silicon wedges and Si-SiGe multilayers, and using datasets collected at different conditions, we show how the measured strain precision scales as a function of dose, aperture size and sample thickness. Using precession gives a further improvement in precision by about 1.5-2x, whereas energy filtering does not have a significant impact on the cepstral method for device-relevant sample thickness ranges. △ Less

Submitted 10 September, 2025; originally announced September 2025.

Comments: 34 Pages, 7 Figures, 5 Supplementary Figures, 2 Supplementary Texts

Classifying Mitotic Figures in the MIDOG25 Challenge with Deep Ensemble Learning and Rule Based Refinement

Authors: Sara Krauss, Ellena Spieß, Daniel Hieber, Frank Kramer, Johannes Schobel, Dominik Müller

Abstract: Mitotic figures (MFs) are relevant biomarkers in tumor grading. Differentiating atypical MFs (AMFs) from normal MFs (NMFs) remains difficult, as manual annotation is time-consuming and subjective. In this work an ensemble of ConvNeXtBase models was trained with AUCMEDI and extend with a rule-based refinement (RBR) module. On the MIDOG25 preliminary test set, the ensemble achieved a balanced accura… ▽ More Mitotic figures (MFs) are relevant biomarkers in tumor grading. Differentiating atypical MFs (AMFs) from normal MFs (NMFs) remains difficult, as manual annotation is time-consuming and subjective. In this work an ensemble of ConvNeXtBase models was trained with AUCMEDI and extend with a rule-based refinement (RBR) module. On the MIDOG25 preliminary test set, the ensemble achieved a balanced accuracy of 84.02%. While the RBR increased specificity, it reduced sensitivity and overall performance. The results show that deep ensembles perform well for AMF classification. RBR can increase specific metrics but requires further research. △ Less

Submitted 28 August, 2025; originally announced August 2025.

Comments: Submission as part of the MICCAI MIDOG25 challenge

The (3+1)D structure of the dilute Glasma

Authors: Andreas Ipp, Markus Leuthner, David I. Müller, Sören Schlichting, Kayran Schmidt, Pragya Singh

Abstract: We study the (3+1)D structure of the Glasma in the dilute approximation, which allows us to describe the longitudinal dynamics that arise from the three-dimensional nuclear structure. We employ a nuclear model with tunable longitudinal and transverse fluctuation scales that generalizes the McLerran-Venugopalan model. We discuss the longitudinal profiles of the energy-momentum tensor and the transv… ▽ More We study the (3+1)D structure of the Glasma in the dilute approximation, which allows us to describe the longitudinal dynamics that arise from the three-dimensional nuclear structure. We employ a nuclear model with tunable longitudinal and transverse fluctuation scales that generalizes the McLerran-Venugopalan model. We discuss the longitudinal profiles of the energy-momentum tensor and the transverse structure of the local rest frame energy density. △ Less

Submitted 28 August, 2025; originally announced August 2025.

Comments: 4 pages, 2 figures, parallel talk given at the Quark Matter 2025 conference

Physically-Based Inverse Rendering Framework for PET Image Reconstruction

Authors: Yixin Li, Soroush Shabani Sichani, Zipai Wang, Wanbin Tan, Baptiste Nicolet, Xiuyuan Wang, David A. Muller, Gloria C. Chiang, Wenzel Jakob, Amir H. Goldan

Abstract: Differentiable rendering has been widely adopted in computer graphics as a powerful approach to inverse problems, enabling efficient gradient-based optimization by differentiating the image formation process with respect to millions of scene parameters. Inspired by this paradigm, we propose a physically-based inverse rendering (IR) framework, the first ever platform for PET image reconstruction us… ▽ More Differentiable rendering has been widely adopted in computer graphics as a powerful approach to inverse problems, enabling efficient gradient-based optimization by differentiating the image formation process with respect to millions of scene parameters. Inspired by this paradigm, we propose a physically-based inverse rendering (IR) framework, the first ever platform for PET image reconstruction using Dr.Jit, for PET image reconstruction. Our method integrates Monte Carlo sampling with an analytical projector in the forward rendering process to accurately model photon transport and physical process in the PET system. The emission image is iteratively optimized using voxel-wise gradients obtained via automatic differentiation, eliminating the need for manually derived update equations. The proposed framework was evaluated using both phantom studies and clinical brain PET data acquired from a Siemens Biograph mCT scanner. Implementing the Maximum Likelihood Expectation Maximization (MLEM) algorithm across both the CASToR toolkit and our IR framework, the IR reconstruction achieved a higher signal-to-noise ratio (SNR) and improved image quality compared to CASToR reconstructions. In clinical evaluation compared with the Siemens Biograph mCT platform, the IR reconstruction yielded higher hippocampal standardized uptake value ratios (SUVR) and gray-to-white matter ratios (GWR), indicating enhanced tissue contrast and the potential for more accurate tau localization and Braak staging in Alzheimer's disease assessment. The proposed IR framework offers a physically interpretable and extensible platform for high-fidelity PET image reconstruction, demonstrating strong performance in both phantom and real-world scenarios. △ Less

Submitted 27 August, 2025; originally announced August 2025.

Comments: This work has been submitted to the IEEE for possible publication

Low barrier ZrO$_x$-based Josephson junctions

Authors: Jaehong Choi, Maciej Olszewski, Luojia Zhang, Zhaslan Baraissov, Tathagata Banerjee, Kushagra Aggarwal, Sarvesh Chaudhari, Tomás A. Arias, David A. Muller, Valla Fatemi, Gregory D. Fuchs

Abstract: The Josephson junction is a crucial element in superconducting devices, and niobium is a promising candidate for the superconducting material due to its large energy gap relative to aluminum. AlO$_x$ has long been regarded as the highest quality oxide tunnel barrier and is often used in niobium-based junctions. Here we propose ZrO$_x$ as an alternative tunnel barrier material for Nb electrodes. We… ▽ More The Josephson junction is a crucial element in superconducting devices, and niobium is a promising candidate for the superconducting material due to its large energy gap relative to aluminum. AlO$_x$ has long been regarded as the highest quality oxide tunnel barrier and is often used in niobium-based junctions. Here we propose ZrO$_x$ as an alternative tunnel barrier material for Nb electrodes. We theoretically estimate that zirconium oxide has excellent oxygen retention properties and experimentally verify that there is no significant oxygen diffusion leading to NbO$_x$ formation in the adjacent Nb electrode. We develop a top-down, subtractive fabrication process for Nb/Zr-ZrO$_x$/Nb Josephson junctions, which enables scalability and large-scale production of superconducting electronics. Using cross sectional scanning transmission electron microscopy, we experimentally find that depending on the Zr thickness, ZrO$_x$ tunnel barriers can be fully crystalline with chemically abrupt interfaces with niobium. Further analysis using electron energy loss spectroscopy reveals that ZrO$_x$ corresponds to tetragonal ZrO$_2$. Room temperature characterization of fabricated junctions using Simmons' model shows that ZrO$_2$ exhibits a low tunnel barrier height, which is promising in merged-element transmon applications. Low temperature transport measurements reveal sub-gap structure, while the low-voltage sub-gap resistance remains in the megaohm range. △ Less

Submitted 15 August, 2025; originally announced August 2025.

Comments: 32 pages in manuscript format

Journal ref: APL Mater. 13, 111103 (2025)

Electron Ptychography Images Hydrogen Atom Superlattices and 3D Inhomogeneities in Palladium Hydride Nanoparticles

Authors: Zixiao Shi, Qihao Li, Himani Mishra, Desheng Ma, Héctor D. Abruña, David A. Muller

Abstract: When hydrogen atoms occupy interstitial sites in metal lattices, they form metal hydrides (MHx), whose structural and electronic properties can differ significantly from the host metals. Owing to the small size of hydrogen atom and its unique interactions with the host metal, MHx is of broad interest in both fundamental science and technological applications. Determining where the hydrogen is loca… ▽ More When hydrogen atoms occupy interstitial sites in metal lattices, they form metal hydrides (MHx), whose structural and electronic properties can differ significantly from the host metals. Owing to the small size of hydrogen atom and its unique interactions with the host metal, MHx is of broad interest in both fundamental science and technological applications. Determining where the hydrogen is located within the MHx, and whether it orders on the partially occupied interstitial sites is crucial for predicting and understanding the resultant physical and electronic properties of the hydride. Directly imaging hydrogen within a host material remains a major challenge due to its weak interaction with X-rays and electrons in conventional imaging techniques. Here, we employ electron ptychography, a scanning transmission electron microscopy technique, to image the three-dimensional (3D) distribution of H atoms in Palladium hydrides (PdHx) nanocubes, one of the most studied and industrially relevant MHx materials. We observe an unexpected one-dimensional superlattice ordering of hydrogen within the PdHx nanocubes and 3D hydrogen clustering in localized regions within PdHx nanocubes, revealing spatial heterogeneity in metal hydride nanoparticles previously inaccessible by other methods. △ Less

Submitted 14 August, 2025; originally announced August 2025.

Comments: 5 figures, 19 SI figures

Information in 4D-STEM: Where it is, and How to Use it

Authors: Desheng Ma, Guanxing Li, David A Muller, Steven E Zeltmann

Abstract: Contrast transfer mechanisms for electron scattering have been extensively studied in transmission electron microscopy. Here we revisit H. Rose's generalized contrast formalism from scattering theory to understand where information is encoded in four-dimensional scanning transmission electron microscopy (4D-STEM) data, and consequently identify new imaging modes that can also serve as crude but fa… ▽ More Contrast transfer mechanisms for electron scattering have been extensively studied in transmission electron microscopy. Here we revisit H. Rose's generalized contrast formalism from scattering theory to understand where information is encoded in four-dimensional scanning transmission electron microscopy (4D-STEM) data, and consequently identify new imaging modes that can also serve as crude but fast approximations to ptychography. We show that tilt correction and summation of the symmetric and antisymmetric scattering components within the bright-field disk -- corresponding to tilt-corrected bright field (tcBF) and tilt-corrected differential phase contrast (tcDPC) respectively -- enables aberration-corrected, bright-field phase contrast imaging (acBF) that makes maximal use of the 4D-STEM information under the weak phase object approximation (WPOA). Beyond the WPOA, we identify the contrast transfer from the interference between inelastic/plural scattering electrons, which show up as quadratic terms, and show that under overfocus conditions, contrast can be further enhanced at selected frequencies, similar to phase-contrast TEM imaging. There is also usable information encoded in the dark field region which we demonstrate by constructing a tilt-corrected dark-field image (tcDF) that sums up the incoherent scattering components and holds promise for depth sectioning of strong scatterers. This framework generalizes phase contrast theory in conventional/scanning transmission electron microscopy to 4D-STEM and provides analytical models and insights into full-field iterative ptychography, which blindly exploits all above contrast mechanisms. △ Less

Submitted 25 October, 2025; v1 submitted 28 July, 2025; originally announced July 2025.

Comments: 45 pages, 10 figures; corrected typos; submitted to Ultramicroscopy

First coordinated observations between Solar Orbiter and the Daniel K. Inouye Solar Telescope

Authors: Krzysztof Barczynski, Miho Janvier, Chris J. Nelson, T. Schad, A. Tritschler, Louise Harra, Daniel Müller, Susanna Parenti, Gherardo Valori, Gianna Cauzzi, Yingjie Zhu

Abstract: Solar Orbiter and the Daniel K. Inouye Solar Telescope (DKIST) are two of the newest facilities available to the solar physics community. The first coordinated observations of the Sun by these two facilities occurred over the course of one week in October 2022. The returned data are open-access and will provide a valuable resource to researchers in the field. We provide an overview of the datase… ▽ More Solar Orbiter and the Daniel K. Inouye Solar Telescope (DKIST) are two of the newest facilities available to the solar physics community. The first coordinated observations of the Sun by these two facilities occurred over the course of one week in October 2022. The returned data are open-access and will provide a valuable resource to researchers in the field. We provide an overview of the datasets collected by Solar Orbiter and DKIST through this coordination and discuss their scientific potential. Our aim is to demonstrate how these unique high-resolution coordinated observations, as well as similar observations obtained through subsequent campaigns, can help tackle important science questions in the field. Between 18 and 24 October 2022, Solar Orbiter and DKIST observed a decayed active region simultaneously. During this period, Solar Orbiter's separation angle with Earth decreased from 77° to 51°, enabling stereoscopic observations with DKIST. From Solar Orbiter, observations are provided by the Extreme Ultraviolet Imager (EUI), Polarimetric and Helioseismic Imager (PHI), and the Spectral Imaging of the Coronal Environment (SPICE) instruments. Meanwhile, DKIST observed using the Cryogenic Near Infrared Spectropolarimeter (CryoNIRSP), the Visible Broadband Imager (VBI), and the Visible Spectropolarimeter (ViSP). The first coordinated Solar Orbiter and DKIST campaign was a success, collecting observations multiple times over the week. We focus on three specific topics as representative examples, namely, coronal loop physics, the formation and evolution of the small-scale active region brightenings, and coronal rain dynamics. These open-access observations, and others like them, should help the solar physics community tackle key questions in the field. Such stereoscopic coordinated observations open up a new era in the analysis of the solar atmosphere. △ Less

Submitted 25 July, 2025; originally announced July 2025.

Comments: 16 pages, 8 figures; accepted for publication in A&A;

Improving Multislice Electron Ptychography with a Generative Prior

Authors: Christian K. Belardi, Chia-Hao Lee, Yingheng Wang, Justin Lovelace, Kilian Q. Weinberger, David A. Muller, Carla P. Gomes

Abstract: Multislice electron ptychography (MEP) is an inverse imaging technique that computationally reconstructs the highest-resolution images of atomic crystal structures from diffraction patterns. Available algorithms often solve this inverse problem iteratively but are both time consuming and produce suboptimal solutions due to their ill-posed nature. We develop MEP-Diffusion, a diffusion model trained… ▽ More Multislice electron ptychography (MEP) is an inverse imaging technique that computationally reconstructs the highest-resolution images of atomic crystal structures from diffraction patterns. Available algorithms often solve this inverse problem iteratively but are both time consuming and produce suboptimal solutions due to their ill-posed nature. We develop MEP-Diffusion, a diffusion model trained on a large database of crystal structures specifically for MEP to augment existing iterative solvers. MEP-Diffusion is easily integrated as a generative prior into existing reconstruction methods via Diffusion Posterior Sampling (DPS). We find that this hybrid approach greatly enhances the quality of the reconstructed 3D volumes, achieving a 90.50% improvement in SSIM over existing methods. △ Less

Submitted 24 July, 2025; v1 submitted 23 July, 2025; originally announced July 2025.

Comments: 16 pages, 10 figures, 5 tables

3D Atomic-Scale Metrology of Strain Relaxation and Roughness in Gate-All-Around (GAA) Transistors via Electron Ptychography

Authors: Shake Karapetyan, Steven E. Zeltmann, Glen Wilk, Ta-Kun Chen, Vincent D. -H. Hou, David A. Muller

Abstract: To improve transistor density and electronic performance, next-generation semiconductor devices are adopting three-dimensional architectures and feature sizes down to the few-nm regime, which require atomic-scale metrology to identify and resolve performance-limiting fabrication challenges. X-ray methods deliver three-dimensional imaging of integrated circuits but lack the spatial resolution to ch… ▽ More To improve transistor density and electronic performance, next-generation semiconductor devices are adopting three-dimensional architectures and feature sizes down to the few-nm regime, which require atomic-scale metrology to identify and resolve performance-limiting fabrication challenges. X-ray methods deliver three-dimensional imaging of integrated circuits but lack the spatial resolution to characterize atomic-scale features, while conventional electron microscopy offers atomic-scale imaging but limited depth information. We demonstrate how multislice electron ptychography (MEP), a computational electron microscopy technique with sub-Ångström lateral and nanometer-scale depth resolution, enables 3D imaging of buried features in devices. By performing MEP on prototype gate-all-around transistors we uncover and quantify distortions and defects at the interface of the 3D gate oxide wrapped around the channel. We find that the silicon in the 5-nm-thick channel gradually relaxes away from the interfaces, leaving only 60% of the atoms in a bulk-like structure. Quantifying the interface roughness, which was not previously possible for such small 3D structures but strongly impacts carrier mobility, we find that the top and bottom interfaces show different atomic-scale roughness profiles, reflecting their different processing conditions. By measuring 3D interface roughness simultaneously with strain relaxation and atomic-scale defects, from a single MEP dataset, we provide direct experimental values of these performance-limiting parameters needed for modeling and early fabrication optimization. △ Less

Submitted 9 July, 2025; originally announced July 2025.

Comments: 43 pages, 14 figures

Unconventional superlattice ordering in intercalated transition metal dichalcogenide V$_{1/3}$NbS$_2$

Authors: Shannon S. Fender, Noah Schnitzer, Wuzhang Fang, Lopa Bhatt, Dingbin Huang, Amani Malik, Oscar Gonzalez, Veronika Sunko, Lilia S. Xie, David A. Muller, Joseph Orenstein, Yuan Ping, Berit H. Goodge, D. Kwabena Bediako

Abstract: The interplay between symmetry and topology in magnetic materials makes it possible to engineer exotic phases and technologically useful properties. A key requirement for these pursuits is achieving control over local crystallographic and magnetic structure, usually through sample morphology (such as synthesis of bulk crystals versus thin-films) and application of magnetic or electric fields. Here… ▽ More The interplay between symmetry and topology in magnetic materials makes it possible to engineer exotic phases and technologically useful properties. A key requirement for these pursuits is achieving control over local crystallographic and magnetic structure, usually through sample morphology (such as synthesis of bulk crystals versus thin-films) and application of magnetic or electric fields. Here we show that V$_{1/3}$NbS$_2$ can be crystallized in two ordered superlattices, distinguished by the periodicity of out-of-plane magnetic intercalants. Whereas one of these structures is metallic and displays the hallmarks of altermagnetism, the other superlattice, which has not been isolated before in this family of intercalation compounds, is a semimetallic noncollinear antiferromagnet that may enable access to topologically nontrivial properties. This observation of an unconventional superlattice structure establishes a powerful route for tailoring the tremendous array of magnetic and electronic behaviors hosted in related materials. △ Less

Submitted 27 June, 2025; originally announced June 2025.

Comments: 7 pages, 4 figures

28 Years of Sun-as-a-star Extreme Ultraviolet Light Curves from SOHO EIT

Authors: Emily Sandford, Frédéric Auchère, Annelies Mortier, Laura A. Hayes, Daniel Müller

Abstract: The Solar and Heliospheric Observatory (SOHO) Extreme-ultraviolet Imaging Telescope (EIT) has been taking images of the Solar disk and corona in four narrow EUV bandpasses (171Å, 195Å, 284Å, and 304Å) at a minimum cadence of once per day since early 1996. The time series of fully-calibrated EIT images now spans approximately 28 years, from early 1996 to early 2024, covering solar cycles 23, 24, an… ▽ More The Solar and Heliospheric Observatory (SOHO) Extreme-ultraviolet Imaging Telescope (EIT) has been taking images of the Solar disk and corona in four narrow EUV bandpasses (171Å, 195Å, 284Å, and 304Å) at a minimum cadence of once per day since early 1996. The time series of fully-calibrated EIT images now spans approximately 28 years, from early 1996 to early 2024, covering solar cycles 23, 24, and the beginning of cycle 25. We convert this extensive EIT image archive into a time series of `Sun-as-a-star' light curves in EIT's four bandpasses, providing a long-term record of solar EUV variability. These Sun-as-a-star light curves, available for download from https://doi.org/10.5281/zenodo.15474179, trace the Sun as if it were a distant point source, viewed from a fixed perspective. We find that our EUV light curves trace the $\sim$ 11-year solar activity cycle and the $\sim$ 27-day rotation period much better than comparable optical observations. In particular, we can accurately recover the solar rotation period from our 284Ålight curve for 26 out of 28 calendar years of EIT observations (93% of the time), compared to only 3 out of 29 calendar years (10% of the time) of the VIRGO total solar irradiance time series, which is dominated by optical light. Our EIT light curves, in conjunction with Sun-as-a-star light curves at optical wavelengths, will be valuable to those interested in inferring the EUV/UV character of stars with long optical light curves but no intensive UV observations, as well as to those interested in long-term records of solar and space weather. △ Less

Submitted 23 June, 2025; originally announced June 2025.

Comments: 19 pages, 14 figures. Accepted for publication in A&A. Abstract abridged

Journal ref: A&A 699, A353 (2025)

XHEMTs on Ultrawide Bandgap Single-Crystal AlN Substrates

Authors: Eungkyun Kim, Yu-Hsin Chen, Naomi Pieczulewski, Jimy Encomendero, David Anthony Muller, Debdeep Jena, Huili Grace Xing

Abstract: AlN has the largest bandgap in the wurtzite III-nitride semiconductor family, making it an ideal barrier for a thin GaN channel to achieve strong carrier confinement in field-effect transistors, analogous to silicon-on-insulator technology. Unlike SiO$_2$/Si/SiO$_2$, AlN/GaN/AlN can be grown fully epitaxially, enabling high carrier mobilities suitable for high-frequency applications. However, deve… ▽ More AlN has the largest bandgap in the wurtzite III-nitride semiconductor family, making it an ideal barrier for a thin GaN channel to achieve strong carrier confinement in field-effect transistors, analogous to silicon-on-insulator technology. Unlike SiO$_2$/Si/SiO$_2$, AlN/GaN/AlN can be grown fully epitaxially, enabling high carrier mobilities suitable for high-frequency applications. However, developing these heterostructures and related devices has been hindered by challenges in strain management, polarization effects, defect control and charge trapping. Here, the AlN single-crystal high electron mobility transistor (XHEMT) is introduced, a new nitride transistor technology designed to address these issues. The XHEMT structure features a pseudomorphic GaN channel sandwiched between AlN layers, grown on single-crystal AlN substrates. First-generation XHEMTs demonstrate RF performance on par with the state-of-the-art GaN HEMTs, achieving 5.92 W/mm output power and 65% peak power-added efficiency at 10 GHz under 17 V drain bias. These devices overcome several limitations present in conventional GaN HEMTs, which are grown on lattice-mismatched foreign substrates that introduce undesirable dislocations and exacerbated thermal resistance. With the recent availability of 100-mm AlN substrates and AlN's high thermal conductivity (340 W/m$\cdot$K), XHEMTs show strong potential for next-generation RF electronics. △ Less

Submitted 19 June, 2025; originally announced June 2025.

Relative-phase dependence of dynamically assisted electron-positron pair creation in the superposition of strong oscillating electric-field pulses

Authors: J. Braß, D. M. Müller, S. Villalba-Chávez, K. Krajewska, C. Müller

Abstract: Production of electron-positron pairs in the superposition of oscillating electric-field pulses with largely different frequencies is studied, focussing on the impact of relative phases between the pulses. Various field configurations are considered: superpositions of either two or three pulses of equal duration as well as combinations of a long low-frequency and a short high-frequency pulse. We s… ▽ More Production of electron-positron pairs in the superposition of oscillating electric-field pulses with largely different frequencies is studied, focussing on the impact of relative phases between the pulses. Various field configurations are considered: superpositions of either two or three pulses of equal duration as well as combinations of a long low-frequency and a short high-frequency pulse. We show that the relative phase of superimposed high-frequency modes can exert a sizeable effect on the total numbers of produced pairs, enhancing them by about 10-30% for the considered field parameters. △ Less

Submitted 30 May, 2025; originally announced May 2025.

Comments: 9 pages, 7 figures

AMOR: Adaptive Character Control through Multi-Objective Reinforcement Learning

Authors: Lucas N. Alegre, Agon Serifi, Ruben Grandia, David Müller, Espen Knoop, Moritz Bächer

Abstract: Reinforcement learning (RL) has significantly advanced the control of physics-based and robotic characters that track kinematic reference motion. However, methods typically rely on a weighted sum of conflicting reward functions, requiring extensive tuning to achieve a desired behavior. Due to the computational cost of RL, this iterative process is a tedious, time-intensive task. Furthermore, for r… ▽ More Reinforcement learning (RL) has significantly advanced the control of physics-based and robotic characters that track kinematic reference motion. However, methods typically rely on a weighted sum of conflicting reward functions, requiring extensive tuning to achieve a desired behavior. Due to the computational cost of RL, this iterative process is a tedious, time-intensive task. Furthermore, for robotics applications, the weights need to be chosen such that the policy performs well in the real world, despite inevitable sim-to-real gaps. To address these challenges, we propose a multi-objective reinforcement learning framework that trains a single policy conditioned on a set of weights, spanning the Pareto front of reward trade-offs. Within this framework, weights can be selected and tuned after training, significantly speeding up iteration time. We demonstrate how this improved workflow can be used to perform highly dynamic motions with a robot character. Moreover, we explore how weight-conditioned policies can be leveraged in hierarchical settings, using a high-level policy to dynamically select weights according to the current task. We show that the multi-objective policy encodes a diverse spectrum of behaviors, facilitating efficient adaptation to novel tasks. △ Less

Submitted 29 May, 2025; originally announced May 2025.

Comments: SIGGRAPH 2025

The Problem of Algorithmic Collisions: Mitigating Unforeseen Risks in a Connected World

Authors: Maurice Chiodo, Dennis Müller

Abstract: The increasing deployment of Artificial Intelligence (AI) and other autonomous algorithmic systems presents the world with new systemic risks. While focus often lies on the function of individual algorithms, a critical and underestimated danger arises from their interactions, particularly when algorithmic systems operate without awareness of each other, or when those deploying them are unaware of… ▽ More The increasing deployment of Artificial Intelligence (AI) and other autonomous algorithmic systems presents the world with new systemic risks. While focus often lies on the function of individual algorithms, a critical and underestimated danger arises from their interactions, particularly when algorithmic systems operate without awareness of each other, or when those deploying them are unaware of the full algorithmic ecosystem deployment is occurring in. These interactions can lead to unforeseen, rapidly escalating negative outcomes - from market crashes and energy supply disruptions to potential physical accidents and erosion of public trust - often exceeding the human capacity for effective monitoring and the legal capacities for proper intervention. Current governance frameworks are inadequate as they lack visibility into this complex ecosystem of interactions. This paper outlines the nature of this challenge and proposes some initial policy suggestions centered on increasing transparency and accountability through phased system registration, a licensing framework for deployment, and enhanced monitoring capabilities. △ Less

Submitted 26 May, 2025; originally announced May 2025.

Comments: 27 pages. This is an early concept paper, and we plan to add further content to it over time. Please get in touch if you want to be part of its further development. Keywords: algorithmic collision, AI agents, algorithmic ecosystem, flash crash, multiagent systems

ACM Class: I.2.11; K.4.1; K.4.2; K.5.2

WeedNet: A Foundation Model-Based Global-to-Local AI Approach for Real-Time Weed Species Identification and Classification

Authors: Yanben Shen, Timilehin T. Ayanlade, Venkata Naresh Boddepalli, Mojdeh Saadati, Ashlyn Rairdin, Zi K. Deng, Muhammad Arbab Arshad, Aditya Balu, Daren Mueller, Asheesh K Singh, Wesley Everman, Nirav Merchant, Baskar Ganapathysubramanian, Meaghan Anderson, Soumik Sarkar, Arti Singh

Abstract: Early identification of weeds is essential for effective management and control, and there is growing interest in automating the process using computer vision techniques coupled with AI methods. However, challenges associated with training AI-based weed identification models, such as limited expert-verified data and complexity and variability in morphological features, have hindered progress. To a… ▽ More Early identification of weeds is essential for effective management and control, and there is growing interest in automating the process using computer vision techniques coupled with AI methods. However, challenges associated with training AI-based weed identification models, such as limited expert-verified data and complexity and variability in morphological features, have hindered progress. To address these issues, we present WeedNet, the first global-scale weed identification model capable of recognizing an extensive set of weed species, including noxious and invasive plant species. WeedNet is an end-to-end real-time weed identification pipeline and uses self-supervised learning, fine-tuning, and enhanced trustworthiness strategies. WeedNet achieved 91.02% accuracy across 1,593 weed species, with 41% species achieving 100% accuracy. Using a fine-tuning strategy and a Global-to-Local approach, the local Iowa WeedNet model achieved an overall accuracy of 97.38% for 85 Iowa weeds, most classes exceeded a 90% mean accuracy per class. Testing across intra-species dissimilarity (developmental stages) and inter-species similarity (look-alike species) suggests that diversity in the images collected, spanning all the growth stages and distinguishable plant characteristics, is crucial in driving model performance. The generalizability and adaptability of the Global WeedNet model enable it to function as a foundational model, with the Global-to-Local strategy allowing fine-tuning for region-specific weed communities. Additional validation of drone- and ground-rover-based images highlights the potential of WeedNet for integration into robotic platforms. Furthermore, integration with AI for conversational use provides intelligent agricultural and ecological conservation consulting tools for farmers, agronomists, researchers, land managers, and government agencies across diverse landscapes. △ Less

Submitted 24 May, 2025; originally announced May 2025.

Formalising Human-in-the-Loop: Computational Reductions, Failure Modes, and Legal-Moral Responsibility

Authors: Maurice Chiodo, Dennis Müller, Paul Siewert, Jean-Luc Wetherall, Zoya Yasmine, John Burden

Abstract: We use the notion of oracle machines and reductions from computability theory to formalise different Human-in-the-loop (HITL) setups for AI systems, distinguishing between trivial human monitoring (i.e., total functions), single endpoint human action (i.e., many-one reductions), and highly involved human-AI interaction (i.e., Turing reductions). We then proceed to show that the legal status and sa… ▽ More We use the notion of oracle machines and reductions from computability theory to formalise different Human-in-the-loop (HITL) setups for AI systems, distinguishing between trivial human monitoring (i.e., total functions), single endpoint human action (i.e., many-one reductions), and highly involved human-AI interaction (i.e., Turing reductions). We then proceed to show that the legal status and safety of different setups vary greatly. We present a taxonomy to categorise HITL failure modes, highlighting the practical limitations of HITL setups. We then identify omissions in UK and EU legal frameworks, which focus on HITL setups that may not always achieve the desired ethical, legal, and sociotechnical outcomes. We suggest areas where the law should recognise the effectiveness of different HITL setups and assign responsibility in these contexts, avoiding human "scapegoating". Our work shows an unavoidable trade-off between attribution of legal responsibility, and technical explainability. Overall, we show how HITL setups involve many technical design decisions, and can be prone to failures out of the humans' control. Our formalisation and taxonomy opens up a new analytic perspective on the challenges in creating HITL setups, helping inform AI developers and lawmakers on designing HITL setups to better achieve their desired outcomes. △ Less

Submitted 25 September, 2025; v1 submitted 15 May, 2025; originally announced May 2025.

Comments: 31 pages. Keywords: Human-in-the-loop, Automated decision making system, Human oversight in sociotechnical systems, Oracle machine, AI safety, Trustworthy AI

ACM Class: F.1; H.1.2; I.2.0; K.4.1

PtyRAD: A High-performance and Flexible Ptychographic Reconstruction Framework with Automatic Differentiation

Authors: Chia-Hao Lee, Steven E. Zeltmann, Dasol Yoon, Desheng Ma, David A. Muller

Abstract: Electron ptychography has recently achieved unprecedented resolution, offering valuable insights across diverse material systems, including in three dimensions. However, high-quality ptychographic reconstruction is computationally expensive and time consuming, requiring a significant amount of manually tuning even for experts. Additionally, essential tools for ptychographic analysis are often scat… ▽ More Electron ptychography has recently achieved unprecedented resolution, offering valuable insights across diverse material systems, including in three dimensions. However, high-quality ptychographic reconstruction is computationally expensive and time consuming, requiring a significant amount of manually tuning even for experts. Additionally, essential tools for ptychographic analysis are often scattered across multiple software packages, with some advanced features available only in costly commercial software like MATLAB. To address these challenges, we introduce PtyRAD, an open-source software framework offers a comprehensive, flexible, and computationally efficient solution for electron ptychography. PtyRAD provides seamless optimization of multiple parameters--such as sample thickness, local tilts, probe positions, and mixed probe and object modes--using gradient-based methods with automatic differentiation (AD). By utilizing PyTorch's highly optimized tensor operations, PtyRAD achieves up to a 17x speedup in reconstruction time compared to existing packages without compromising image quality. In addition, we propose a real-space depth regularization, which avoids wrap-around artifacts and can be useful for twisted two-dimensional (2D) material datasets and vertical heterostructures. Moreover, PtyRAD integrates a Bayesian optimization workflow that streamlines hyperparameter selection. We hope the open-source nature of PtyRAD will foster reproducibility and community-driven development for future advances in ptychographic imaging. △ Less

Submitted 10 July, 2025; v1 submitted 12 May, 2025; originally announced May 2025.

Comments: 17 pages, 6 figures

Journal ref: Microscopy and Microanalysis, Volume 31, Issue 4, August 2025, ozaf070

Solar Orbiter's 2024 Major Flare Campaigns: An Overview

Authors: Daniel F. Ryan, Laura A. Hayes, Hannah Collier, Graham S. Kerr, Andrew R. Inglis, David Williams, Andrew P. Walsh, Miho Janvier, Daniel Müller, David Berghmans, Cis Verbeeck, Emil Kraaikamp, Peter R. Young, Therese A. Kucera, Säm Krucker, Muriel Z. Stiefel, Daniele Calchetti, Katharine K. Reeves, Sabrina Savage, Vanessa Polito

Abstract: Solar Orbiter conducted a series of flare-optimised observing campaigns in 2024 utilising the Major Flare Solar Orbiter Observing Plan (SOOP). Dedicated observations were performed during two distinct perihelia intervals in March/April and October, during which over 22 flares were observed, ranging from B- to M-class. These campaigns leveraged high-resolution and high-cadence observations from the… ▽ More Solar Orbiter conducted a series of flare-optimised observing campaigns in 2024 utilising the Major Flare Solar Orbiter Observing Plan (SOOP). Dedicated observations were performed during two distinct perihelia intervals in March/April and October, during which over 22 flares were observed, ranging from B- to M-class. These campaigns leveraged high-resolution and high-cadence observations from the mission's remote-sensing suite, including the High-Resolution EUV Imager (EUI/HRI_EUV), the Spectrometer/Telescope for Imaging X-rays (STIX), the Spectral Imaging of the Coronal Environment (SPICE) spectrometer, and the High Resolution Telescope of the Polarimetric and Helioseismic Imager (PHI/HRT), as well as coordinated ground-based and Earth-orbiting observations. EUI/HRI_EUV operating in short-exposure modes, provided two-second-cadence, non-saturated EUV images, revealing structures and dynamics on scales not previously observed. Simultaneously, STIX captured hard X-ray imaging and spectroscopy of accelerated electrons, while SPICE acquired EUV slit spectroscopy to probe chromospheric and coronal responses. Together, these observations offer an unprecedented view of magnetic reconnection, energy release, particle acceleration, and plasma heating across a broad range of temperatures and spatial scales. These campaigns have generated a rich dataset that will be the subject of numerous future studies addressing Solar Orbiter's top-level science goal: "How do solar eruptions produce energetic particle radiation that fills the heliosphere?". This paper presents the scientific motivations, operational planning, and observational strategies behind the 2024 flare campaigns, along with initial insights into the observed flares. We also discuss lessons learned for optimizing future Solar Orbiter Major Flare campaigns and provide a resource for researchers aiming to utilize these unique observations. △ Less

Submitted 12 May, 2025; originally announced May 2025.

Initial conditions for Starobinsky inflation in general quadratic gravity

Authors: Daniel Muller, Alexey Toporensky

Abstract: We consider initial conditions leading to Starobinsky inflation in the general quadratic gravity, where the action of the theory contains one more curvature square invariant in addition to $R^2$. We have chosen corresponding coefficients in a way so that the inflationary solution keeps to be stable. Our numerical results show that despite the configuration of initial conditions in the $(H,R)$ plan… ▽ More We consider initial conditions leading to Starobinsky inflation in the general quadratic gravity, where the action of the theory contains one more curvature square invariant in addition to $R^2$. We have chosen corresponding coefficients in a way so that the inflationary solution keeps to be stable. Our numerical results show that despite the configuration of initial conditions in the $(H,R)$ plane, leading to Starobinsky inflation can change considerably from the $R^2$ theory, realization of inflation does not need a fine-tuning of the initial conditions. △ Less

Submitted 7 May, 2025; originally announced May 2025.

Towards a Critical Pragmatic Philosophy of Sustainable Mathematics Education

Authors: Dennis Müller

Abstract: This paper proposes critical pragmatism as a philosophy of sustainable mathematics education to bridge the gap between critical theory and the existing patchwork implementations. Combining existential sustainability as a holistic concept with pragmatic frameworks from the ethics in mathematics education literature creates a foundation enabling critical reflection and pragmatic implementation. We o… ▽ More This paper proposes critical pragmatism as a philosophy of sustainable mathematics education to bridge the gap between critical theory and the existing patchwork implementations. Combining existential sustainability as a holistic concept with pragmatic frameworks from the ethics in mathematics education literature creates a foundation enabling critical reflection and pragmatic implementation. We outline how their synthesis naturally leads to a three-stage implementation strategy: cultivating an ethical classroom culture, engaging with ethnomathematics, and tackling complex sustainability problems. Our critical pragmatic approach attempts to build a new philosophical perspective to equip teachers and students with the mathematical competencies, critical perspectives, and ethical grounding necessary to navigate and contribute to a sustainable future and to provide new analytic pathways. △ Less

Submitted 23 April, 2025; originally announced April 2025.

Comments: 22 pages

MSC Class: 97D20; 97A40; 97B99; 00A30; 01A80

Machine-learned RG-improved gauge actions and classically perfect gradient flows

Authors: Kieran Holland, Andreas Ipp, David I. Müller, Urs Wenger

Abstract: Extracting continuum properties of quantum field theories from discretized spacetime is challenging due to lattice artifacts. Renormalization-group (RG)-improved lattice actions can preserve continuum properties, but are in general difficult to parameterize. Machine learning (ML) with gauge-equivariant convolutional neural networks provides a way to efficiently describe such actions. We test a mac… ▽ More Extracting continuum properties of quantum field theories from discretized spacetime is challenging due to lattice artifacts. Renormalization-group (RG)-improved lattice actions can preserve continuum properties, but are in general difficult to parameterize. Machine learning (ML) with gauge-equivariant convolutional neural networks provides a way to efficiently describe such actions. We test a machine-learned RG-improved lattice gauge action, the classically perfect fixed-point (FP) action, for four-dimensional SU(3) gauge theory through Monte Carlo simulations. We establish that the gradient flow of the FP action is free of tree-level discretization effects to all orders in the lattice spacing, making it classically perfect. This allows us to test the quality of improvement of the FP action, without introducing additional artifacts. We find that discretization effects in gradient-flow observables are highly suppressed and less than 1% up to lattice spacings of 0.14 fm, allowing continuum physics to be extracted from coarse lattices. The quality of improvement achieved motivates the use of the FP action in future gauge theory studies. The advantages of ML-based parameterizations also highlight the possibility of realizing quantum perfect actions in lattice gauge theory. △ Less

Submitted 22 April, 2025; originally announced April 2025.

Comments: 12 pages, 4 tables, 6 figures

Autonomous Human-Robot Interaction via Operator Imitation

Authors: Sammy Christen, David Müller, Agon Serifi, Ruben Grandia, Georg Wiedebach, Michael A. Hopkins, Espen Knoop, Moritz Bächer

Abstract: Teleoperated robotic characters can perform expressive interactions with humans, relying on the operators' experience and social intuition. In this work, we propose to create autonomous interactive robots, by training a model to imitate operator data. Our model is trained on a dataset of human-robot interactions, where an expert operator is asked to vary the interactions and mood of the robot, whi… ▽ More Teleoperated robotic characters can perform expressive interactions with humans, relying on the operators' experience and social intuition. In this work, we propose to create autonomous interactive robots, by training a model to imitate operator data. Our model is trained on a dataset of human-robot interactions, where an expert operator is asked to vary the interactions and mood of the robot, while the operator commands as well as the pose of the human and robot are recorded. Our approach learns to predict continuous operator commands through a diffusion process and discrete commands through a classifier, all unified within a single transformer architecture. We evaluate the resulting model in simulation and with a user study on the real system. We show that our method enables simple autonomous human-robot interactions that are comparable to the expert-operator baseline, and that users can recognize the different robot moods as generated by our model. Finally, we demonstrate a zero-shot transfer of our model onto a different robotic platform with the same operator interface. △ Less

Submitted 3 April, 2025; originally announced April 2025.

The impact of glasma on heavy quark spectra and correlations

Authors: Dana Avramescu, Vincenzo Greco, Tuomas Lappi, Heikki Mäntysaari, David. I. Müller

Abstract: We investigate the effect of the glasma classical color fields, produced in the very early stage of heavy-ion collisions, on the transport of heavy quarks. The glasma fields evolve according to the classical Yang-Mills equations, while the dynamics of heavy quarks is described by Wong's equations. We numerically solve these equations and compute the transport coefficient $κ$, which is anisotropic… ▽ More We investigate the effect of the glasma classical color fields, produced in the very early stage of heavy-ion collisions, on the transport of heavy quarks. The glasma fields evolve according to the classical Yang-Mills equations, while the dynamics of heavy quarks is described by Wong's equations. We numerically solve these equations and compute the transport coefficient $κ$, which is anisotropic and initially very large. Further, we extract observables sensitive to the initial glasma stage. The heavy quark nuclear modification factor $R_{AA}$ is affected by the glasma but the effect is moderate compared to the nPDF contribution. Our main finding is that the glasma has a large impact on the azimuthal correlation between $Q\overline{Q}$ pairs, initially produced back-to-back. △ Less

Submitted 1 April, 2025; originally announced April 2025.

Comments: 6 pages, 6 figures, proceedings for the 12th International Conference on Hard and Electromagnetic Probes of High-Energy Nuclear Collisions (Hard Probes 2024)

The Ethical Turn in Mathematics Education

Authors: Dennis Müller

Abstract: This article analyzes the emerging ethical turn in mathematics education, arguing that it is a nuanced extension of the sociopolitical turn. While sociopolitical studies of mathematics have highlighted systemic issues and group concerns (e.g., equity, diversity, exclusion), the newer scholarship on ethics in mathematics presents a sharpened focus on the individual responsibility of learners, teach… ▽ More This article analyzes the emerging ethical turn in mathematics education, arguing that it is a nuanced extension of the sociopolitical turn. While sociopolitical studies of mathematics have highlighted systemic issues and group concerns (e.g., equity, diversity, exclusion), the newer scholarship on ethics in mathematics presents a sharpened focus on the individual responsibility of learners, teachers, and mathematicians by explicitly engaging with philosophical ethics. We analyze key themes of the discourse, including the tension between "doing good" and "preventing harm," and present various philosophical foundations from which scholars have engaged with ethics: Levinas, non-Western perspectives, and pragmatism. We show that the ethical turn holds significant implications for training teachers, including self-reflection, responsibility towards the Other, historical and philosophical awareness, the role of mathematics in society, individual flexibility, cultural sensitivity, and courage to navigate the complex reality of today's mathematics classrooms. The article is designed to also serve as an introduction to ethics in mathematics education. △ Less

Submitted 30 March, 2025; originally announced March 2025.

Comments: 80 pages, 4 figures

MSC Class: 97D20; 97A40; 97B99; 00A30; 01A80

Uncertainty Distillation: Teaching Language Models to Express Semantic Confidence

Authors: Sophia Hager, David Mueller, Kevin Duh, Nicholas Andrews

Abstract: As large language models (LLMs) are increasingly used for factual question-answering, it becomes more important for LLMs to have the capability to communicate the likelihood that their answer is correct. For these verbalized expressions of uncertainty to be meaningful, they should reflect the error rates at the expressed level of confidence. However, when prompted to express confidence, the error… ▽ More As large language models (LLMs) are increasingly used for factual question-answering, it becomes more important for LLMs to have the capability to communicate the likelihood that their answer is correct. For these verbalized expressions of uncertainty to be meaningful, they should reflect the error rates at the expressed level of confidence. However, when prompted to express confidence, the error rates of current LLMs are inconsistent with their communicated confidences, highlighting the need for uncertainty quantification methods. Many prior methods calculate lexical uncertainty, estimating a model's confidence in the specific string it generated. In some cases, however, it may be more useful to estimate semantic uncertainty, or the model's confidence in the answer regardless of how it is verbalized. We propose a simple procedure, uncertainty distillation, to teach an LLM to verbalize calibrated semantic confidences. Using held-out data to map initial uncertainty estimates to meaningful probabilities, we create examples annotated with verbalized probabilities for supervised fine-tuning. We find that our method yields verbalized confidences that correlate well with observed error rates, even when compared to strong baselines, some of which are more than twenty times slower at inference time. Additionally, we demonstrate that our method can be applied to black-box models that allow API-based fine-tuning, resulting in estimates of uncertainty that are both more effective and more efficient than any of our baselines. △ Less

Submitted 12 December, 2025; v1 submitted 18 March, 2025; originally announced March 2025.

Microscopic mechanisms of flexoelectricity in oxide membranes

Authors: Harikrishnan KP, Varun Harbola, Jaehong Choi, Kevin J. Crust, Yu-Tsun Shao, Chia-Hao Lee, Dasol Yoon, Yonghun Lee, Gregory D. Fuchs, Cyrus E. Dreyer, Harold Y. Hwang, David A. Muller

Abstract: Modern electromechanical actuators and sensors rely on the piezoelectric effect that linearly couples strain and electric polarization. However, this effect is restricted to materials that lack inversion symmetry. In contrast, the flexoelectric effect couples strain gradients to electric polarization, and is a universal property in insulating materials of arbitrary symmetry. Flexoelectricity becom… ▽ More Modern electromechanical actuators and sensors rely on the piezoelectric effect that linearly couples strain and electric polarization. However, this effect is restricted to materials that lack inversion symmetry. In contrast, the flexoelectric effect couples strain gradients to electric polarization, and is a universal property in insulating materials of arbitrary symmetry. Flexoelectricity becomes prominent at the nanoscale from the inverse scaling of strain gradients with material dimensions. Here, we measure the strain-gradient-induced structural distortions in strontium titanate using multislice electron ptychography. This technique enables reliable picometer-scale measurements of the dominant oxygen-titanium distortions, correcting for artifacts that limited conventional imaging methods. This enables us to directly measure the sign of the net ionic contribution to the flexoelectric polarization. Guided by the experimental measurements, first-principles calculations show how the sign and magnitude of the bulk contribution to the flexoelectric coefficient in strontium titanate can be switched by tuning the strain state. Hybridization between the optical soft phonon and acoustic phonon modes drives this transition, yielding a large response and a polarity switch across the resonance. This strain-dependence might explain the sign discrepancy and orders of magnitude variation in the values of previously reported flexoelectric coefficients for strontium titanate. As the strain state of curved membranes can be tuned, our approach also suggests an approach to engineer nanoscale flexoelectric polarization using strain as a control parameter. △ Less

Submitted 17 March, 2025; originally announced March 2025.

Comments: 51 pages, 4 figures, 13 Supplementary Figures

A magnetic avalanche as the central engine powering a solar flare

Authors: L. P. Chitta, D. I. Pontin, E. R. Priest, D. Berghmans, E. Kraaikamp, L. Rodriguez, C. Verbeeck, A. N. Zhukov, S. Krucker, R. Aznar Cuadrado, D. Calchetti, J. Hirzberger, H. Peter, U. Schühle, S. K. Solanki, L. Teriaca, A. S. Giunta, F. Auchère, L. Harra, D. Müller

Abstract: Solar flares are the most powerful, magnetically-driven, explosions in the heliosphere. The nature of magnetic energy release in the solar corona that heats the plasma and accelerates particles in a flare, however, remains poorly understood. Here, we report high-resolution coronal observations of a flare (SOL2024-09-30T23:47) by the Solar Orbiter mission that reveal initially weaker but rapid reco… ▽ More Solar flares are the most powerful, magnetically-driven, explosions in the heliosphere. The nature of magnetic energy release in the solar corona that heats the plasma and accelerates particles in a flare, however, remains poorly understood. Here, we report high-resolution coronal observations of a flare (SOL2024-09-30T23:47) by the Solar Orbiter mission that reveal initially weaker but rapid reconnection events, on timescales of at most a few seconds, leading to a more prominent activity of similar nature that explosively cause a flare. Signatures of this process are further imprinted on the widespread raining plasma blobs with short lifetimes, giving rise to the characteristic ribbon-like emission pattern associated with the flare. Our novel observations unveil the central engine of a flare and emphasize the crucial role of an avalanche-like magnetic energy release mechanism at work. △ Less

Submitted 15 March, 2025; originally announced March 2025.

Comments: Online animations available from the corresponding author