Statistical Mechanics · Cross-listed from physics.chem-ph
Structure-resolved free energy estimation of the 38-atom Lennard Jones cluster via population annealing
Akie Kowaguchi and Koji Hukushima
We systematically investigate the thermodynamic landscape of the 38-atom Lennard–Jones cluster LJ$_{38}$ using Population Annealing (PA), a method suited for systems with challenging double-funnel energy landscapes. By employing an adaptive temperature schedule, we demonstrate that thermodynamic observables, such as internal energy and heat capacity, converge robustly when the population size is sufficiently large. To gain deeper insights into the competing basins, we introduce an integrated framework that combines PA reweighting factors with structure-resolved analysis. Using quenched configurations characterized by potential energy and Steinhardt's bond-orientational order parameters, we identify three structural basins, FCC-like, icosahedral, and
liquid-like, via dimensionality reduction and clustering. This framework enables the direct computation of structure-resolved free energy differences from population fractions, providing a quantitative mapping of the thermodynamic competition between the funnels. The resulting structural crossovers are consistent with the heat-capacity peak, demonstrating PA as a promising and scalable framework for structure-resolved thermodynamics in complex molecular systems.
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Materials Science
Chemical Vapor Deposition of Epitaxial Chromium Nitride Thin Films
Lewis J. Adams, Sara Baserga, Laurent Souqui et al. (+3 more)
Chromium nitride (CrN) is a thermoelectric transition metal nitride whose properties are strongly influenced by stoichiometry, substrate choice, and defect chemistry. CrN is routinely synthesized by physical vapor deposition (PVD), its growth by chemical vapor deposition (CVD) has been limited by the lack of suitable chromium precursors capable of producing carbon-, oxygen-, and chlorine-free films. CVD of contamination-free Cr compounds is notoriously difficult, with carbon-free Cr compounds thought unattainable below 1000 C. Here, we report epitaxial, carbon- and chlorine-free CrN thin films synthesized by thermal CVD. Single-phase CrN films (~110 nm) were deposited on c-plane alpha-Al2O3 using in-situ generated chromium chlorides. Films exhibit n-type conduction with a Seebeck coefficient of -36 uV/K, comparable to PVD-grown CrN. These results present a routeto highly crystalline rock-salt CrN films for defect engineering, doping, and alloying with reduced implantation-related damage capabilities previously largely confined to PVD-based techniques.
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All authors Lewis J. Adams, Sara Baserga, Laurent Souqui, Enji Sadek, Linus von Fieandt, Per Eklund
Materials Science
Machine-learned interatomic potential for titanium carbide MXenes: Application to ion irradiation simulations
Jesper Byggmästar
A computationally efficient and accurate machine-learned (ML) interatomic potential is developed for bare Ti$_{n+1}$C$_n$ MXenes. With a diverse set of structures computed with density functional theory, the trained ML potential demonstrates good accuracy and robustness to a wide range of bond distances and environments, making it a useful tool for molecular dynamics simulations of MXenes subjected to mechanical load or irradiation. The ML potential is applied to simulations of light and heavy ion irradiation, gathering insight into the statistics and probabilities of sputtering, reflection, defect creation, and implantation into bare Ti$_{n+1}$C$_n$ MXene sheets. The results provide guidelines for defect engineering of MXenes through ion irradiation and implantation. Additionally, the ML potential development provides a landmark recipe for enabling machine-learning-driven atomistic simulations of other MXenes.
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Materials Science
Nine-element machine-learned interatomic potentials for multiphase refractory alloys
Jesper Byggmästar, Tiago Lopes, Zheyong Fan et al. (+1 more)
New refractory
alloys are being continuously designed and characterised for applications requiring good high-temperature mechanical properties and stability. Computational design from atomistic simulations is limited by interatomic potentials missing key elements, being too inaccurate, or computationally too slow for large-scale simulations. Here we present development of a refractory
alloy database and two computationally efficient and general-purpose machine-learned potentials (tabGAP and NEP). We also design a cross-sampling strategy for effective sampling of training data using predictions from two potentials with completely different underlying architecture. The potentials support arbitrary
alloy compositions of elements in groups four to six in the periodic table (Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W). The database is diverse yet multitargeted to enable simulations of refractory metals and
alloys across different pure-metal, solid-solution, intermetallic, and glassy phases. We demonstrate the usefulness of the potentials by reproducing known pressure-, temperature-, and solute-induced phase transitions, grain boundary segregation, and simulations of radiation damage in the WTaCrVHf metallic glass.
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All authors Jesper Byggmästar, Tiago Lopes, Zheyong Fan, Tapio Ala-Nissila
Mesoscale and Nanoscale Physics
Constructing Exceptional Knots and Links with Arbitrary Braiding Topology
Bin Jiang, Aolong Guo, Qilin Cai et al. (+1 more)
Exceptional
knots and links represent a remarkable class of non-Hermitian metals in which exceptional degeneracies form knotted or linked
manifolds in momentum space. Here, we report a universal construction framework for realizing exceptional
knots and links with arbitrary braiding
topology in 3D minimal two-band non-Hermitian systems. Our approach combines braid theory with semiholomorphic
polynomials to establish a direct correspondence between braid words and non-Hermitian Bloch
Hamiltonians. This framework enables the realization of a broad variety of exceptional configurations, including
torus knots, lemniscate
knots, nonfibred
knots,
hyperbolic knots, and multi-component links, within explicit tight-binding
Hamiltonians. Furthermore, we demonstrate controllable topological transitions in which exceptional
knots can be continuously untied through redistribution and reconnection of exceptional points, accompanied by transient exceptional chains and changes in spectral complex energy braiding. Our results establish a universal route toward programmable non-Hermitian
knot topology and provide a versatile platform for exploring knotted band degeneracies and their associated physical phenomena across photonic, acoustic, mechanical, and cold-atom systems.
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All authors Bin Jiang, Aolong Guo, Qilin Cai, Jian-Hua Jiang
Mesoscale and Nanoscale Physics · Cross-listed from physics.optics
Atomic-scale Stark-shift spectroscopy and microscopy of organic molecules
Xabier Arrieta, Sofia Canola, Ruben Esteban et al. (+2 more)
In conventional optical Stark-shift spectroscopy, molecules are exposed to spatially homogeneous static electric fields that shift the energies of their spectral lines. These shifts are attributed to the molecular electronic properties, such as variation of dipolar moment and polarizability of the molecule associated with photo(de)excitation. In realistic environments containing structural defects and nanoscale heterogeneities, however, molecules experience internal electric fields that vary strongly on the molecular scale, rendering the standard Stark selection rules inapplicable. Here we develop an extended theory of atomic-scale Stark shift, addressing such scenarios. Specifically, we present a detailed theoretical analysis of an experimentally relevant configuration where the atomically sharp tip of a light-assisted scanning tunneling microscope is used to controllably apply inhomogeneous electrostatic fields to representative molecular dyes spanning several molecular families. We decompose the total Stark shift into linear and quadratic contributions and show that they contain different information about the molecular properties. Concretely, spatial variations of the linear Stark shift as the tip scans across the molecule enable subnanometric mapping of the charge redistribution between ground and excited electronic states, with high sensitivity to molecular composition and chemical functionalization. The quadratic Stark contribution, in contrast, reflects changes in the conventional dipolar polarizability upon excitation. Together, these results establish nanoscale Stark-shift spectroscopy as a powerful tool for resolving excited-state charge dynamics in single molecules under realistic, strongly inhomogeneous electric fields.
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All authors Xabier Arrieta, Sofia Canola, Ruben Esteban, Javier Aizpurua, Tomáš Neuman
Strongly Correlated Electrons
Quantum oscillations and linear magnetoresistance in ultraclean CaVO$_3$ thin films
M. Müller, M. Espinosa, O. Chiatti et al. (+3 more)
Advances in epitaxy of transition metal
oxides with perovskite structure allow novel insights into transport mechanisms of strongly correlated electron systems, which are of interest for future transparent electronics. In this study, we investigate magnetotransport properties of thin epitaxial CaVO$_3$ films, grown coherently strained on LaAlO$_3$, and demonstrate for the ultraclean limit
quantum oscillations.
Fermi liquid behavior is detected in the temperature-dependent resistivity $\rho(T) \sim T^2$ up to 20 K, with effective mean free paths exceeding up to 20 times the film thickness (38 nm). Shubnikov-de Haas oscillations and a non-linear
Hall resistance reveal two electron-like (1 and 2) and one hole-like (h) carriers, reflecting the three-fold
Fermi surface of orthorhombic CaVO$_3$, with effective charge carrier densities and mobilities at 4.2 K of: (1) $n_1 \approx 9.3 \cdot 10^{21}{}cm^{-3}$ with low mobility $\mu_1 \approx 926{}cm^{2}V^{-1}s^{-1}$, (2) $n_2 \approx 7.2 \cdot 10^{19}{}cm^{-3}$ with high mobility $\mu_2 \approx 6600 {}cm^{2}V^{-1}s^{-1}$, and (h) $n_h \approx 2.2 \cdot 10^{18}{}cm^{-3}$ with $\mu_h \approx 1500{}cm^{2}V^{-1}s^{-1}$. A non-saturating linear magnetoresistance dominates at low temperatures, exceeding the value for single
crystals by 30$\%$. Our findings on epitaxial films demonstrate the delicate interplay of multiple carriers with correlations stemming from a non-spherical nested
Fermi surface of a perovskite structure with orthorhombic distortion.
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All authors M. Müller, M. Espinosa, O. Chiatti, T. Kuznetsova, R. Engel-Herbert, S. F. Fischer
Materials Science · Cross-listed from cs.AI
BeamPERL: Parameter-Efficient RL with Verifiable Rewards Specializes Compact LLMs for Structured Beam Mechanics Reasoning
Tarjei Paule Hage and Markus J. Buehler
Can reinforcement learning with hard, verifiable rewards teach a compact
language model to reason about physics, or does it primarily learn to pattern-match toward correct answers? We study this question by training a 1.5B-parameter
reasoning model on beam statics, a classic engineering problem, using parameter-efficient RLVR with binary correctness rewards from symbolic solvers, without teacher-generated
reasoning traces. The best BeamPERL checkpoint achieves a 66.7% improvement in Pass@1 over the base model. However, the learned competence is anisotropic: the model generalizes compositionally (more loads) but fails under topological shifts (moved supports) that require the same
equilibrium equations. Intermediate checkpoints yield the strongest
reasoning, while continued optimization degrades robustness while maintaining reward. These findings reveal a key limitation of outcome-level
alignment: reinforcement learning with exact physics rewards induces procedural solution templates rather than internalization of governing equations. The precision of the reward signal - even when analytically exact - does not by itself guarantee transferable physical
reasoning. Our results suggest that verifiable rewards may need to be paired with structured
reasoning scaffolding to move beyond template matching toward robust scientific
reasoning.
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Materials Science
Dispersion and lifetimes of magnons in non-collinear magnets from time dependent density functional theory
David Eilmsteiner, Arthur Ernst and Paweł A. Buczek
We investigate the
spin dynamics of the non-collinear kagome triangular anti-ferromagnet Mn$_3$Rh using linear response time-dependent density functional theory. To this end, we present a novel first principles computational scheme for the evaluation of the dynamical susceptibility based on the non-collinear KKR Green's functions method and a symbolic computer
algebra.This approach allows us to address the Landau decay of
spin waves into non-collinear electron-hole Stoner pairs being inaccessible to adiabatic methods. Our calculations reveal three distinct Goldstone modes dispersing linearly in the long-wavelength regime giving rise to the three
magnon branches and we discuss their non-trivial spatial polarizations. The
spin-waves turn out to be defined in the whole Brillouin zone but their Landau damping becomes substantial away from the zone's center. Surprisingly,
magnons of comparable momenta and energies can feature, depending on their chirality, considerably different attenuation, in some cases of predominantly resonant character. We trace this effect to the interplay between the
magnon eigenvectors and the intrinsically
spin-polarized altermagnetic band structure and the resulting spectrum of non-collinear Stoner states.
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Materials Science
Machine learning assisted High-Throughput study of M$_4$X$_3$T$_x$ MXenes
Sakshi Goel and Arti Kashyap
In this work, we employ a machine-learning-assisted high-throughput density functional theory framework to systematically investigate the stability, electronic structure, and
magnetic ground states of 234 M$_4$X$_3$T$_x$ MXenes. The machine learning model predicts
lattice parameters with up to 94% accuracy using a relatively small training
dataset and significantly reduces structural optimization time in high-throughput calculations. Based on total energy and density-of-states analyses, we classify the
magnetic nature of MXenes across different transition- metal compositions and surface terminations. Ti-, Zr-, Hf-, Nb-, and Ta-based MXenes are found to be non-
magnetic metals for all functional groups considered, while Sc- and Y-based systems exhibit a range of behaviors including weak ferromagnetism and semiconducting character. V- and Fe-based MXenes are identified as
antiferromagnetic metals, whereas Cr- and Mn-based MXenes yield 16
ferromagnetic systems with
spin polarization ranging from 50% to 100%.
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Soft Condensed Matter
Study of flow of crystals and deformable particles in a channel and the effective segregation of soft and hard particles
Padmanabha Bose and Smarajit Karmakar
Soft matters whose constituents are deformable are ubiquitous in nature especially in biological systems-including
cells and their organelles-as well as in foams and emulsions. The capacity for deformation in these soft materials gives rise to a range of intriguing phenomena, such as glassy behavior without any size dispersity, cluster
crystal formation, and re-entrant melting. Deformability also plays a crucial role in facilitating essential biological processes, such as the flow of blood through veins and arteries. In this work, we investigate assemblies of two-dimensional (2D) polymeric, non-overlapping rings, which mimic deformable particulates in 2D using extensive molecular dynamics simulations. The rings are confined in a rectangular channel with hard walls perpendicular to the flow direction, mimicking natural flow conditions. We analyze the flow properties of these deformable particle assemblies at two different stiffness values. To further asses the impact of deformability, we examine the same monodisperse system at higher densities for the stiffer rings, where deformation is necessary and a fluid layer emerges at the channel edges. Finally, we explore a mixture of rings with two distinct stiffnesses and observe effective segregation of soft and hard particles at small channel widths.
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Statistical Mechanics · Cross-listed from quant-ph
Fermi-Dirac thermal measurements: A framework for quantum hypothesis testing and semidefinite optimization
Nana Liu and Mark M. Wilde
Quantum measurements are the means by which we recover messages encoded into
quantum states. They are at the forefront of
quantum hypothesis testing, wherein the goal is to perform an optimal measurement for arriving at a correct conclusion. Mathematically, a measurement operator is Hermitian with
eigenvalues in [0,1]. By noticing that this constraint on each
eigenvalue is the same as that imposed on
fermions by the Pauli exclusion principle, we interpret every eigenmode of a measurement operator as an independent effective fermionic mode. Under this perspective, various objective functions in
quantum hypothesis testing can be viewed as the total expected energy associated with these fermionic occupation numbers. By instead fixing a temperature and minimizing the total expected fermionic free energy, we find that optimal measurements for these modified objective functions are
Fermi-Dirac thermal measurements, wherein their
eigenvalues are specified by
Fermi-Dirac distributions. In the low-temperature limit, their performance closely approximates that of optimal measurements for
quantum hypothesis testing, and we show that their parameters can be learned by classical or hybrid
quantum-classical optimization
algorithms. This leads to a new
quantum machine-learning model, termed
Fermi-Dirac machines, consisting of parameterized
Fermi-Dirac thermal measurements-an alternative to
quantum Boltzmann machines based on thermal states. Beyond hypothesis testing, we show how general semidefinite optimization problems can be solved using this approach, leading to a novel paradigm for semidefinite optimization on
quantum computers, in which the goal is to implement thermal measurements rather than prepare thermal states. Finally, we propose
quantum algorithms for implementing
Fermi-Dirac thermal measurements, and we also propose second-order hybrid
quantum-classical optimization
algorithms.
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Statistical Mechanics
Mermin's dielectric function and the f-sum rule
Thomas Chuna, Jan Vorberger, Thomas Gawne et al. (+2 more)
Mermin's dielectric function [N.D. Mermin, Phys. Rev. B 1, 2362 (1970)] is widely assumed to satisfy the f-sum rule because he constrains his ansatz with the continuity equation. However, we identify a moment-closure problem in Mermin's use of the continuity equation. Further, we show that the Mermin's model can be derived without invoking continuity. We describe how other approaches such as the “completed Mermin” model of Chuna and Murillo [Phys. Rev. E 111, 035206 (2025)] remedy this closure issue. We then inspect the f-sum rule for both the original and completed Mermin models and find for the Mermin ansatz that collision frequencies scaling as $\omega$ must violate the f-sum rule, whereas constant, real, positive collision frequencies will satisfy it, with the caveat that, in practice, convergence with respect to the upper integration limit $\omega_{\max}$ is sufficiently slow that finite-domain numerical evaluations exhibit apparent violations, regardless of wavenumber $q$. We also find that collision frequencies with constant imaginary components cause f-sum rule violations. We conclude that if Mermin's model is fit to data via optimizing its collision frequency, then the f-sum rule is not inherently satisfied; constraints, though broad, are needed in order to assume the f-sum rule is satisfied. Further, if the f-sum rule is theoretically satisfied, but violations still appear, then these deviations ought to be included in the error estimates.
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All authors Thomas Chuna, Jan Vorberger, Thomas Gawne, Tobias Dornheim, Michael S. Murillo
Statistical Mechanics
Pushing-Induced Arrest Across Lattices and Dimensions
Itamar Shitrit, Ofek Lauber Bonomo and Shlomi Reuveni
Tracer-media interactions can give rise to transport phenomena beyond classical models; e.g., obstacle pushing can eliminate percolation. We demonstrate that the existing "snowplow" mechanism proposed to explain this effect fails in 3D. We show that confinement is governed by emergent trapping-rare "door-closing" events that occur with an approximately constant probability per step at low obstacle densities, thus yielding exponential survival. This allows prediction of the time-dependent mean-squared displacement from short-time estimates of the
diffusion constant and trapping probability, providing a minimal description of pushing-induced arrest across
lattices and dimensions.
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Mesoscale and Nanoscale Physics
Interfering trajectories in a ballistic Andreev cavity
Pankaj Mandal, Marcel Kaschper, Fernando Dominguez et al. (+9 more)
The conventional description of transport through the interface between a normal conductor and a
superconductor reduces the system to a one-dimensional problem treating Andreev reflection based on a zero-dimensional Sharvin type point-contact model, and effectively neglects all considerations of device geometry. While this has been successful in systems where conductance in the normal material is in the diffusive transport regime, such an over-simplification of the problem fails in other transport regimes. In particular, when transport is ballistic as in a typical
semiconductor-
superconductor hybrid structure, geometrical effects are inherently important, and a proper description must consider a one-dimension contact injecting into a two-dimensional ballistic cavity. We present the first study of this regime and explore the bias-voltage dependence of Andreev transport in a cavity-type device comprised of a high mobility HgTe
quantum well side-contacted by one superconducting and one normal contact, each creating a one-dimensional interface. The enhanced conductance from Andreev transport features two finite bias conductance peaks, observed at energies within the energy gap of the
superconductor. Interestingly, these two peaks respond differently to the application of a perpendicular-to-plane
magnetic field. Using a semi-classical model for the
quantum transport within the cavity, we are able to attribute each peak to a different class of ballistic trajectories. One class is dominated by normal reflection, and its interference condition is independent of
magnetic field, whereas the other one contains retro-reflected Andreev processes at the
superconductor interface. These create closed trajectories that are strongly suppressed by
magnetic field due to Aharonov-Bohm and Doppler shift effects.
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All authors Pankaj Mandal, Marcel Kaschper, Fernando Dominguez, Soumi Mondal, Lukas Lunczer, Dongyun Chen, Martin P. Stehno, Ewelina M. Hankiewicz, Björn Trauzettel, Teun M. Klapwijk, Charles Gould, Laurens W. Molenkamp
Mesoscale and Nanoscale Physics
Probing persistent spin textures through nonlinear magnetotransport
Neelanjan Chakraborti, Akash Dey, Snehasish Nandy et al. (+2 more)
Persistent
spin textures (PST) are special
spin configurations in
spin-orbit-coupled systems in which the
spin polarization acquires a
symmetry-enforced momentum-independent orientation, leading to exceptionally long
spin lifetimes and persistent
spin helices. Identifying direct experimental probes of PST, however, remains challenging because conventional
quantum-
geometric responses are strongly suppressed in this regime. Here, we show that PST systems isolate
spin-rotation
quantum geometry, which manifests through distinctive nonlinear magnetotransport responses. Using both a fine-tuned Rashba-Dresselhaus two-dimensional electron gas and a
symmetry-enforced cubic
spin-splitting model realizing PST, we demonstrate that PST suppresses conventional and Zeeman
quantum-
geometric contributions, leaving the
spin-rotation
quantum geometric tensor as the sole source of nonlinear
magnetic-current and
spin-magnetization responses. Remarkably, the nonvanishing response components exhibit identical direction-independent behavior as a function of chemical potential, providing a distinctive signature of PST. We further show that, in the Rashba-Dresselhaus two-dimensional electron gas at the PST point, these qualitative signatures remain robust even in the presence of a cubic Dresselhaus term that breaks the exact SU(2)
symmetry. Our results establish nonlinear magnetotransport as an experimentally accessible probe of PST and their underlying
spin-rotation
quantum geometry.
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All authors Neelanjan Chakraborti, Akash Dey, Snehasish Nandy, Sudeep Kumar Ghosh, Kush Saha
Disordered Systems and Neural Networks · Cross-listed from cs.LG
Specialization of softmax attention heads: insights from the high-dimensional single-location model
M. Sagitova, O. Duranthon and L. Zdeborová
Multi-head
attention enables
transformer models to represent multiple
attention patterns simultaneously. Empirically, head specialization emerges in distinct stages during training, while many heads remain redundant and learn similar representations. We propose a theoretical model capturing this phenomenon, based on the multi-index and single-location regression frameworks. In the first part, we analyze the training dynamics of multi-head softmax
attention under SGD, revealing an initial unspecialized phase followed by a multi-stage specialization phase in which different heads sequentially align with latent signal directions. In the second part, we study the impact of
attention activation functions on performance. We introduce the Bayes-softmax
attention, which achieves optimal prediction performance in this setting.
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Strongly Correlated Electrons
Dynamics of Charge-Density-Wave puddles in 2$H$-NbSe$_2$
Shreya Kumbhakar, Marina Esposito, Anjan Kumar N M et al. (+15 more)
Electronic phases in
quantum materials are often governed by nanoscale inhomogeneity, where local order develops within spatially confined regions or puddles. A prominent example is an incommensurate charge-density-wave (I-CDW) that comprises locally commensurate domains. In 2$H$-NbSe$_2$, such an I-CDW state persists alongside
lattice anharmonicity and superconductivity, raising fundamental questions about the dynamical stabilization of CDW order in puddles. Here, we probe the puddle-dynamics in 2$H$-NbSe$_2$. Raman scattering reveals a strong Fano-coupling between the interlayer shear vibration and the CDW amplitude mode. Time-resolved reflectivity measurement shows a low-frequency ~0.15 THz coherent overdamped oscillation onsetting within the CDW regime at ~17 K, pointing towards a so far unexplored transition. This we identify as a Fano-coupled
phonon-CDW hybrid emerging from the collective dynamics of CDW puddles. We establish a framework for capturing intertwinned glassy dynamics with an overdamped oscillatory model. Such dynamics in 2H-NbSe2 highlight how
lattice pinning and electronic correlations in layered materials affect the CDW order, which is crucial for the design of novel Van der Waals devices.
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All authors Shreya Kumbhakar, Marina Esposito, Anjan Kumar N M, Liwen Feng, Tommaso Confalone, Vasilisa Gerega, Flavia Lo Sardo, Rafiqul Alam, Davide Masarotti, Francesco Tafuri, Thomas Böhm, Mahmoud Abdel-Hafiez, Sushmita Chandra, Claudia Felser, Kornelius Nielsch, Nicola Poccia, Stefan Kaiser, Golam Haider
Mesoscale and Nanoscale Physics
Trainable Neuromorphic Spintronic Hardware Via Analog Finite-Difference Gradient Methods
Catarina Pereira, Alex Jenkins, Eleonora Raimondo et al. (+8 more)
Spintronic nano-
neurons offer a promising route towards energy-efficient, high-performance hardware neural networks thanks to their inherent low-input nonlinear dynamics. However, training such networks remains a major bottleneck as it depends on oversimplified models of device behaviour and is highly sensitive to device variability. Here, we introduce a hardware architecture that overcomes these limitations by enabling on-device generation of gradients. First, we introduce theoretically and demonstrate experimentally that
magnetic tunnel junctions can generate tunable and complex nonlinear responses. Building on this, we implement an analogue finite-difference approach to enable on-chip training in spintronic neural networks with one and two hidden layers. We experimentally implemented device in the loop backpropagation in a
magnetic tunnel junction based neural network, achieving a classification accuracy of 93.3% despite pronounced device variability. During training, the gradients generated by the proposed analog
neurons closely match the values derived numerically, without incurring computational overhead. Via physical simulations, we also demonstrate that this approach can be scaled up to support training in deep architectures. Our results pave the way for reliable, trainable and fully analogue spintronic neural networks, opening up new possibilities for next-generation, energy-efficient artificial intelligence hardware.
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All authors Catarina Pereira, Alex Jenkins, Eleonora Raimondo, Mario Carpentieri, Ensieh Iranmehr, Luana Benetti, Subhajit Roy, Ricardo Ferreira, Joao Ventura, Giovanni Finocchio, Davi Rodrigues
Materials Science
Hydrostatic Pressure Driven Band Gap Tuning and Self-Trapped Exciton Formation in (4FPEA)$_2$SnBr$_{4}$ Halide Perovskite
Rafał Bartoszewicz, Jakub Ziembicki, Ewelina Zdanowicz et al. (+5 more)
Two-dimensional tin halide perovskites provide a highly tunable platform for
exciton phonon coupling and local
lattice distortions, enabled by their intrinsically soft
lattice. We report a combined temperature and pressure dependent photoluminescence study of the layered perovskite (4FPEA)$_{2}$SnBr$_{4}$. At room temperature, its optical response is dominated by near band edge (NBE)
excitons, which
redshift linearly under hydrostatic pressure up to $\sim$3 GPa, indicating a rigid band edge behavior without phase transitions. Cooling reveals a broad, strongly Stokes shifted self-trapped
exciton (STE) emission, evidencing a crossover from delocalized to self localized excitonic states. Strikingly, while NBE emission
redshifts under pressure, STE emission exhibits an anomalous blueshift, reflecting pressure induced modification of the
exciton phonon energy landscape. In contrast, the iodide analogue (4FPEA)$_{2}$SnI$_{4}$ shows no STE emission under identical conditions, highlighting the critical role of
lattice rigidity and dielectric screening in stabilizing self-trapped
excitons.
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All authors Rafał Bartoszewicz, Jakub Ziembicki, Ewelina Zdanowicz, Artur P. Herman, Jesús Sánchez-Diaz, Samrat Das Adhikari, Iván Mora-Seró, Robert Kudrawiec
Mesoscale and Nanoscale Physics
Demonstration of Robust Chiral Edge Transport in Field-Induced Chern Insulator MnBi2Te4 Devices with Engineered Geometric Defects
Pinyuan Wang, Jun Ge, Jiawei Luo et al. (+5 more)
Chiral edge states in Chern
insulators are theoretically predicted to propagate unidirectionally along the sample boundary with inherent robustness against local perturbations, which manifests as the immunity to
impurity-induced backscattering, a key factor for the development of robust, high-performance
quantum devices. However, the direct experimental verification of the robustness of
chiral edge states remains scarce. Here, we experimentally validate the robustness of the
chiral edge states in MnBi2Te4 devices by introducing engineered
geometric defects via atomic force microscopy (AFM) nanomachining. Specifically, under a moderate perpendicular
magnetic field, the MnBi2Te4 devices exhibit the Chern
insulator state, characterized by a quantized
Hall plateau and simultaneously vanishing longitudinal resistance. To verify the robustness of this topological state, we modify the device geometry by creating a structural cut that reaches the substrate using AFM nanomachining. Remarkably, the quantization behavior survives this drastic modification. The robust nature of the
chiral edge transport is further confirmed by two-terminal, three-terminal, and nonlocal measurements, demonstrating that the edge currents can survive the engineered cut. Our results provide strong evidence for the robustness of
chiral edge states against
geometric disruption and establish AFM nanomachining as a promising technique for topological
quantum devices engineering.
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All authors Pinyuan Wang, Jun Ge, Jiawei Luo, Xiaoqi Liu, Jiayi Yang, Fucong Fei, Fengqi Song, Jian Wang
Other Condensed Matter
Tuning competing electronic phases in monolayer VSe$_2$ via interface hybridization
Ishita Pushkarna, Árpád Pásztor, Greta Lupi et al. (+2 more)
Competing electronic phases in two-dimensional transition metal dichalcogenides constitute a fertile platform for uncovering emergent ground states and elucidating the control parameters that govern the correlated electron phases. Among these materials, vanadium diselenide is particularly compelling: while the bulk hosts a well-established charge density wave (CDW), monolayers exhibit markedly different electronic behavior. Here, we identify three distinct electronic regimes in mechanically exfoliated VSe$_2$ flakes on Au(111) substrates, where interfacial hybridization, charge transfer, and strain act as primary tuning parameters of electronic order. Monolayers strongly coupled to gold show complete suppression of the CDW, accompanied by the emergence of moiré modulations. In contrast,
bilayers preserve the in-plane $4a \times 4a$ CDW characteristic of the bulk limit. Strained, electronically decoupled monolayers formed in suspended membrane and bubble regions stabilize a $\sqrt{3}a\times\sqrt{7}a$ CDW phase, underscoring the reversible role of substrate interaction and hybridization.
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All authors Ishita Pushkarna, Árpád Pásztor, Greta Lupi, Adolfo O. Fumega, Christoph Renner
Materials Science
Remote Plasma Polymers of Iron (II) Phthalocyanine in Polyacrylonitrile-Derived Carbon Electrospun Fibers as Electrode for Supercapacitors
Jose M. Obrero, Jorge PV Tafoya, Michael Thielke et al. (+6 more)
Remote
plasma-assisted vapour deposition under nitrogen (RPAVD-N2) is introduced as a single-step, solvent-free, room-temperature strategy to integrate iron(II) phthalocyanine (FePc) into carbon nanofiber (CNF) scaffolds for high-performance pseudocapacitive electrodes. In this process, CNFs are activated by low-energy N2 remote
plasma and subsequently exposed to sublimated FePc, which undergoes controlled
plasma polymerisation to form conformal, nitrogen-rich FePc-derived coatings while preserving Fe-N coordination. By tuning the
plasma power, the degree of crosslinking, defect generation and molecular fragmentation is precisely controlled. Structural and spectroscopic analyses reveal progressive incorporation of amine, nitrile and oxygenated functionalities while maintaining the Fe-N coordination environment, with 30 W power providing the optimal balance between structural integrity and defect density.
Plasma processing enhances the capacitance by nearly one order of magnitude compared to sublimated FePc films, underscoring the critical role of
plasma-induced molecular integration. The FePc30W@CNFs electrode delivers 80.9 F/g at 0.25 A/g (areal capacitance 0.92 mF/cm2 at 2.9 mA/cm2), achieves 7.42 Wh/kg at 225 W/kg, and retains 86.5% of its initial capacitance after 6000 cycles. These results demonstrate that remote
plasma polymerisation enables robust, high-rate and durable phthalocyanine-based electrodes, establishing RPAVD as a scalable platform for next-generation energy-storage materials.
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All authors Jose M. Obrero, Jorge PV Tafoya, Michael Thielke, G.P. Moreno-Martínez, Lidia Contreras-Bernal, Jose Ferreira de Sousa Jr, Juan Ramón Sánchez-Valencia, Angel Barranco, Ana B. Jorge Sobrido
Statistical Mechanics · Cross-listed from quant-ph
Many-Body Structural Effects in Periodically Driven Quantum Batteries
Rohit Kumar Shukla and Cheng Shang
While
quantum batteries have been widely studied under static driving, their performance under periodic driving in many-body systems has received only limited
attention. In this Letter, we uncover structural principles showing that many-body structure fundamentally determines the charging performance of a collective
spin-1/2
quantum battery driven by a periodic Ising charger. In particular, interaction range, boundary conditions, system size, and integrability – capturing graph connectivity, geometry, even-odd effects, and many-body dynamics – emerge as critical factors for enhancing stored energy and charging power. First, we analyze how connectivity scaling and boundary geometry shape battery performance. We show that long-range interacting chargers exhibit superextensive energy storage, approaching the fundamental upper bound over broad ranges of driving periods and system sizes. In contrast, nearest-neighbor chargers achieve optimal charging only under finely tuned commensurability conditions. Moreover, we find that open boundary conditions (OBC) enhance robustness compared to periodic boundary conditions (PBC). Second, we examine the role of integrability under periodic driving. We demonstrate that nonintegrability enhances energy storage by suppressing conserved quantities and promoting ergodic Floquet dynamics, thereby enabling efficient population of the many-body spectrum. Through systematic structural optimization across multiple parameters, we identify long-range nonintegrability as a central resource for fast, scalable, and robust charging of collective
quantum batteries. Our results clarify how structural features of many-body systems, together with periodic driving, can be harnessed to achieve efficient collective charging dynamics.
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Statistical Mechanics
Ising Models of Cooperativity in Muscle Contraction
Elaheh Saadat, Matthieu Caruel, Stefano Gherardini et al. (+6 more)
Regulation of contraction in striated muscle is controlled by a dual mechanism involving both thin filaments containing actin and thick filaments containing myosin. The thin filament is activated by calcium ions binding to troponin, leading to tropomyosin azimuthal displacement which allows the activation of a regulatory unit (composed of one troponin, one tropomyosin and seven actin monomers) that exposes the actin sites for interaction with the myosin motors. Motor attachment to actin contributes to spreading activation within and beyond a regulatory unit along the thin filament through a cooperative mechanism. We introduce a one-dimensional Ising model to elucidate the mechanism of cooperativity in thin filament activation in relation to the force generated by the attached myosin motor. The model characterizes thin filament activation and cooperativity using only two parameters: one related to calcium concentration and the other to the force exerted by the attached myosin motor, which is modulated by temperature. At any force, the model is able to determine the extent of actin-myosin interactions on a correlation length ranging from two to seven actin monomers in addition to the seven actin monomers of the regulatory unit. Our theoretical predictions are successfully tested on experimental data, and our tests also include the condition of hindered filament activation by the use of the specific drug Omecamtiv Mecarbil (OM). According to our model, the effect of OM results in an anti-cooperativity mechanism accounting for the experimental data.
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All authors Elaheh Saadat, Matthieu Caruel, Stefano Gherardini, Ilaria Morotti, Matteo Marcello, Marco Caremani, Marco Linari, Ivan Latella, Stefano Ruffo
Mesoscale and Nanoscale Physics · Cross-listed from quant-ph
Fractional topology and multi-period re-quantization in open quantum systems
Xi Wu, Xiang Zhang and Fuxiang Li
We study fractional topological numbers in open
quantum systems described by the Gorin–Kossakowski–Sudarsha–Lindblad master equation. Under
symmetry conditions ensuring quantization, we show that single-valued physical states in momentum space give rise to integer winding numbers that remain integer during time
evolution. Fractional values arise when this condition is effectively relaxed, such that the
topology is evaluated over a restricted sector or exhibits an effective multi-branch structure. In these cases, the winding number is not quantized over the fundamental Brillouin zone and can depend continuously on system parameters, with discontinuities at purity-gap closings. However, when extended over multiple momentum periods, the winding recovers integer quantization. These effects are illustrated in a Su–Schrieffer–Heeger chain with gain and loss and can be probed in long-range hopping photonic
lattices with fractional fillings via Bloch state
tomography. Our results provide a unified framework for understanding fractional
topology in open
quantum systems.
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Strongly Correlated Electrons
Dualities and Topological Classification of the $S=1$ Pyrochlore Spin Ice
Sena Watanabe, Yukitoshi Motome and Haruki Watanabe
We derive dual descriptions of the trivial paramagnetic, $\mathbb{Z}_2$-deconfined, and $\mathbb{Z}_2$-confined U(1) Coulomb phases of the $S=1$ pyrochlore
spin ice. In the monopole-free limit, the system can be effectively mapped onto 3D $XY$ and Ising loop-gas models depending on the
spin anisotropy, which provides theoretical estimates for the phase boundaries, while a macroscopic flux vector classifies the topological sectors via
geometric parity rules. Thermal unit monopoles permit endpoints of both directed
spin loops and unoriented vacancy loops. The resulting relevant
symmetry-breaking fields in the dual $XY$ and Ising descriptions predict rounded crossovers near the monopole-free critical points. Monte Carlo simulations support this monopole-induced rounding.
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Materials Science · Cross-listed from physics.optics
Influence of Inter-Pulse Delay and Geometric Constraints on Damage and Optical Characteristics in thin Metal Targets Irradiated by Double Ultrashort Laser Pulses
George D. Tsibidis
Femtosecond pulsed
laser systems constitute powerful tools for the high-precision structuring of materials at micro/nano-scale resolutions. A critical parameter influencing the efficacy of ultrafast
laser-material interactions is the
laser-induced damage threshold (LIDT), which is defined as the minimum
laser fluence required to induce irreversible modification to the material surface. While extensive studies have addressed single-pulse damage mechanisms, the response of thin metallic films to double-pulse femtosecond irradiation, particularly when the film thickness is of the order of the optical penetration depth, remains, generally, unexplored. In this work, we present a rigorous theoretical investigation into the spatiotemporal
evolution of energy deposition, thermalization processes and optical parameter changes under double-pulse excitation conditions. The analysis considers key parameters including the inter-pulse delay and the film thickness to evaluate their influence on the LIDT for a range of technologically relevant metals: Au, Ag, Cu, Al, Ni, Ti, Cr, Pt, W, Mo and Stainless Steel (100Cr6). ). A comparative analysis highlights the potential of controlled double-pulse irradiation schemes to manipulate energy coupling efficiency, improve the spatial selectivity of
laser-induced modifications and compile a comprehensive LIDT database for commonly used industrial materials. The approach is aimed to provide a robust foundation for the design and optimization of advanced
laser micromachining and nanofabrication protocols across a broad spectrum of metallic systems.
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Statistical Mechanics · Cross-listed from quant-ph
O-Sensing: Operator Sensing for Interaction Geometry and Symmetries
Meng Ye-Ming and Shi Zhe-Yu
We ask whether the
Hamiltonian, interaction geometry, and
symmetries of a
quantum many-body system can be inferred from a few low-lying eigenstates without knowing which sites interact with each other. Directly solving the
eigenvalue equations imposes constraints that yield a highly degenerate subspace of candidate operators, where the local
Hamiltonian is hidden among an extensive family of conserved quantities, obscuring the interaction geometry. Here we introduce O-Sensing, a protocol designed to extract the
Hamiltonian and
symmetries directly from these states. Specifically, O-Sensing employs parsimony-driven optimization to extract a maximally sparse operator basis from the degenerate subspace. The
Hamiltonian is then selected from this basis by maximizing spectral
entropy (effectively minimizing degeneracy) within the sampled subspace. We validate O-Sensing on Heisenberg models on connected Erdős–Rényi graphs, where it reconstructs the interaction geometry and uncovers additional long-range conserved operators. We establish a learnability phase diagram across graph densities, featuring a pronounced “confusion” regime where parsimony favors a dual description on the complement graph. These results show that sparsity optimization can reconstruct interaction geometry as an emergent output, enabling simultaneous recovery of the
Hamiltonian and its
symmetries from low-energy eigenstates.
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Soft Condensed Matter
Topological defects in buckled colloidal monolayers
Aaron L. Galper, Henrik N. Barck, Conor M. Floyd et al. (+5 more)
When colloidal particles are vertically confined to a gap of between 1.3-1.6 particle diameters, they pack into buckled
crystals of particles in either "up" or "down" states. Neighboring particles tend to occupy opposite states, analogous to the behavior of
antiferromagnetic spins. The particles sit on a nearly-triangular
lattice, and the
spins of trios of adjacent particles are geometrically frustrated. Two levels of translational order exist in this system: that of the underlying triangular
lattice in the horizontal plane, and that of the emergent frustrated
spin lattice in the vertical dimension. We study the topological defects of both levels of translational order, and we find that both types of defects play a role in
crystal grain boundary structure and
spin domain coarsening. We classify the
spin defects and outline the basic rules for their motion, and we observe interactions between dislocations and
spin defects. Finally, we map the phase space of
spin coarsening in the buckled monolayer, characterizing which types of defects drive the dynamics. Understanding defect formation, motion, and interaction in the buckled monolayer is the first step in predicting the material properties and aging of this geometrically frustrated, self-assembled system.
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All authors Aaron L. Galper, Henrik N. Barck, Conor M. Floyd, Elliot A. Snyder, Charlie J. Schofield, Sorin A. P. Jayaweera, Ian G. McGuire, Sharon J. Gerbode