Strongly Correlated Electrons · Cross-listed from quant-ph
Low-Depth Initial-State Preparation for Ground-State Energy Estimation of Two-Dimensional Strongly Correlated Systems
Ryo Watanabe and Keisuke Fujii
Quantum algorithms for ground-state energy estimation require initial states with non-negligible fidelity to the ground state. Guided by classical simulations, we design shallow
circuits for the two-dimensional half-filled Hubbard model by first preparing an approximate Heisenberg ground state and then applying charge-fluctuation gates derived directly from the Schrieffer-Wolff (SW) generator. For the Heisenberg model, we demonstrate effective parameter transfer from a $4\times4$
lattice to
lattices up to $10\times10$ without further optimization. We obtain promising lower bounds on the ground-state fidelity using tensor-network simulations, variational Monte Carlo reference states, and estimates of the ground-state energy and singlet gap. Exact $4\times4$ calculations show that the SW gates substantially improve the Hubbard ground-state fidelity of the embedded Heisenberg state. The successful Heisenberg parameter transfer supports the use of these small-
lattice results to design shallow Hubbard
circuits on larger
lattices beyond the reach of classical simulations. For a $10\times10$
lattice, the estimated preparation cost is on the order of $10^5$ $T$ gates, including Clifford+$T$ synthesis, which is well within the megaquop regime. These results offer a route to low-cost initial-state preparation for ground-starongly correlated systems.
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Mesoscale and Nanoscale Physics
Effects of residual exchange coupling on simultaneously driven spin qubits
Heun Mo Yoo, Tanner M. Janda, Victor Yu et al. (+3 more)
Exchange coupling and microwave drives are widely used control mechanisms for
spin qubits. However, the influence of exchange coupling on microwave-driven
spin dynamics is not fully understood. We report simultaneous drive measurements of two
spin qubits as a function of exchange coupling and drive power. When the Rabi frequency exceeds exchange, we observe a beating pattern in the Rabi oscillations. In the opposite limit, two exchange-split patterns emerge in the Rabi chevrons. We find that these exchange-induced effects are suppressed when the difference in the Rabi frequencies exceeds the exchange coupling. A theoretical model is developed that reproduces the main features in our data.
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All authors Heun Mo Yoo, Tanner M. Janda, Victor Yu, Michael J. Gullans, Adam R. Mills, Jason R. Petta
Statistical Mechanics · Cross-listed from quant-ph
Pauli spectrum and nonstabilizerness of random fermionic Gaussian states
Xhek Turkeshi, Piotr Sierant and Poetri Sonya Tarabunga
We characterize the magic of random fermionic Gaussian states through their Pauli spectrum and their stabilizer
entropies. For the Haar ensemble of Majorana Gaussian states we derive closed finite-$N$ expressions for the average stabilizer purities and reconstruct the Pauli spectrum exactly, as a mixture of products of beta-distributed variables resolved by Majorana weight. The stabilizer
entropies, and their filtered versions, freeze at Rényi index $q_c=2$: for $q>2$ the magic density is $1/(q-1)$, controlled by rare weight-two Majorana strings. Resolving these contributions requires exponentially many characteristic samples, making direct sampling estimates of the stabilizer
entropies inefficient in this frozen regime. For Haar-random Gaussian states at fixed filling, we express the averaged stabilizer purities at positive integer $q$ as coefficient integrals over $\mathrm{SU}(2q)$, with dimension independent of $N$, and evaluate the $q=2$ case exactly. We establish that the frozen density describes typical states in the Majorana and half-filled number-conserving ensembles and relate these ensembles to the corresponding $\mathrm{SYK}_2$ ground states. Finally, we show that typical fermionic Gaussian states retain certified magic after discarding any fixed fraction $\kappa<\kappa_\star^{\rm G}\simeq0.7613$ of their modes in the thermodynamic limit, remaining magical even beyond the $2/3$ threshold for Haar-random states in the full
Hilbert space.
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Materials Science
Twin Domains in Van der Waals Quaternary Oxides
Dorothée S. Mader, Niels Brumby, Xiaosheng Yang et al. (+10 more)
Optical anisotropy is the basis for many intriguing phenomena in van der Waals materials, including
hyperbolic polaritons and extreme birefringence. Stacking and twisting van der Waals materials along the out-of-plane direction emerged as a powerful route to tailor this anisotropy, but designing lateral interfaces remains a challenge. Here, twin domains are reported in the van der Waals quaternary oxides MgTeMoO$_6$, MnTeMoO$_6$, ZnTeMoO$_6$, and CoTeMoO$_6$ - materials that possess strong in-plane optical anisotropy and second-order nonlinearity. The domains naturally form in their orthorhombic
crystal structure and extend over hundreds of micrometers. It is proposed that this stability is achieved by the domain wall acting as a diagonal mirror plane in the
crystal structure, parallel to the (1-10) or (110)
crystal planes, resulting in nearly opposite birefringence between domains. This hypothesis is experimentally confirmed by determining the angle between the
crystal axes of neighboring domains using polarization-resolved optical microscopy, infrared-visible sum-frequency generation microscopy, and transmission electron microscopy. The latter further allowed an estimate of the domain wall thickness. Overall, the observation of twin domains with orthogonal optical anisotropy opens new routes to use van der Waals quaternary oxides for birefringent waveguiding, polariton steering, and frequency conversion applications.
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All authors Dorothée S. Mader, Niels Brumby, Xiaosheng Yang, Nele Stetzuhn, Eduardo Ortega, Christian Carbogno, Katayoun Gharagozloo-Hubmann, Sebastian F. Maehrlein, Martin Wolf, Kirill Bolotin, Peining Li, Niclas S. Mueller, Alexander Paarmann
Mesoscale and Nanoscale Physics
A self-compliant selector-free memory with functional bismuth ferrite
Suman Roy, Subhabrata Das, Priyanka Sahu et al. (+5 more)
In-
memory computing requires selector-free operation, low variability, and stable multilevel switching, all of which remain challenging to achieve in a single system. Here, we demonstrate a robust selector-free
memory based on a multiferroic BiFeO3 that combines self-compliance with stable bipolar switching, high yield, long endurance, and extended retention. We observe that BiFeO3 memristive crossbar arrays become self-compliant through a naturally formed TiOx interfacial layer, enabling forming-free bipolar switching without external selector devices, addressing major bottlenecks that have limited the practical deployment of resistive
memories. The array delivers stable bipolar switching with a 97% device yield, endurance beyond 105 cycles, and data retention exceeding 105 s with low cycle-to-cycle and device-to-device variability. In addition, the devices exhibit 16 well-separated conductance states, enabling 4-bit multilevel operation via precise pulse-controlled programming for digit visualization in edge applications. Noise measurements reveal predominantly 1/f-type behavior with minimal random telegraph noise, indicating robust conductance stability across the programmable window. These results establish BiFeO3 as a practical materials platform for multibit, selector-free crossbar
memories offering high reliability, making it well-suited for scalable computing architectures based on functional materials.
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All authors Suman Roy, Subhabrata Das, Priyanka Sahu, Majid Ahmadi, Sameer Kumar Mallik, Kaushik Ghosh, Saroj Prasad Dash, Satyaprakash Sahoo
Materials Science
The Case Against Hall-Petch Hardening in High Entropy Carbide Ceramics
Ali Sarikhani, Ana C. Feltrin, Gregory E. Hilmas et al. (+5 more)
Grain size is often used with the Hall-Petch relationship to justify differences in hardness in ceramic materials. Herein, hardness does not vary systematically with grain-size for fully dense, single-phase (Cr,Mo,Ta,V,W)C1-{\delta} high
entropy carbide ceramics for grain sizes that varied by a factor of three. Fully dense, single-phase ceramics with grain sizes from 9.3+/-0.3 to 28.8+/-0.7 microns exhibited a pronounced indentation size effect, with Vickers hardness decreasing from ~28-30 GPa at 0.49 N to ~20-21 GPa at 9.81 N, and Berkovich nanohardness ranging from 26 to 30 GPa at 10 mN. However, at a given load, hardness remained within a narrow range across the grain-size series, and no consistent Hall-Petch dependence was resolved. The lack of grain-size dependence likely indicates that the deformation volume sampled by the indenter was not controlled by grain-boundary interactions; instead, hardness was governed primarily by indentation load and local response of the rock salt carbide
matrix.
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All authors Ali Sarikhani, Ana C. Feltrin, Gregory E. Hilmas, David W. Lipke, Douglas E. Wolfe, Stefano Curtarolo, Shen J. Dillon, William G. Fahrenholtz
Superconductivity
Wrinkles and Magnetic Flux Trapping in Graphite Nanoflakes: A Possible Source and Manifestation of Room-Temperature Superconductivity
Muhammad Saad, Sergey I. Nikitin, Dmitry A. Tayurskii et al. (+1 more)
The paper reports on a possible manifestation of local high-temperature superconductivity in graphite nanoflakes obtained from the bulk pyrolytic graphite by an extended grinding and subsequent annealing at 670 K in the air. Analysis of transmission electron microscopy (TEM) images shows that such treatment leads to a formation of high-density wrinkle-type defects at the basal-plane surface. The sample reveals the
magnetic flux trapping - one of the key signatures of superconductivity - that persists up to 390 K and above. At the same time, both the as-ground (unannealed) and annealed in the
vacuum samples manifest negligible flux trapping, and no wrinkles in their TEM images were found. The direct correlation between the appearance of high-density wrinkle arrays and the detection of the trapped
magnetic flux above the room temperature strongly suggests that wrinkled regions serve a source of local high-temperature superconductivity in graphite.
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All authors Muhammad Saad, Sergey I. Nikitin, Dmitry A. Tayurskii, Roman V. Yusupov
Mesoscale and Nanoscale Physics
Opto-Electrical Detection of Donor Bound Excitons in Silicon-on-Insulator Substrate
A. Kanniainen, A. S. Kumar, A. Sammak et al. (+2 more)
Spin of a donor bound electron in silicon has been shown to be a very coherent
qubit system but lacks a coherent optical interface. There does, however, exist a donor bound
exciton transition that can be excited optically but decays dominantly via an Auger recombination producing an electrical signal. This provides an opto-electrical pathway for
spin readout. Scaling this readout to single-
spin level will require interfacing the
spins with silicon photonics for efficient guiding of
photons, which in turn will require moving to silicon-on-
insulator (SOI) substrates. Here we demonstrate first ensemble opto-electronic measurements of donor bound
excitons in SOI material, including isotopically purified 28-Si device layers. The experiments show pronounced shifts in the resonance wavelengths and broadenings of the transition linewidths compared to the bulk experiments.
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All authors A. Kanniainen, A. S. Kumar, A. Sammak, G. Scappucci, J. T. Muhonen
Superconductivity
Thermal Hall effect in elemental niobium, a two-gap superconductor
Jing Zhang, Xiaokang Li, Kamran Behnia et al. (+1 more)
Niobium holds a pivotal place in superconductivity history: it is not only the elemental
superconductor with the highest critical temperature, but also a long-standing candidate for multiband pairing whose evidence has remained controversial. A two-gap scenario was proposed as early as 1959, yet experimental proof stayed elusive. Here, through thermal Hall effect measurements, we unambiguously demonstrate a crossover in transverse thermal transport below $T_{\rm c}$, where the dominant carrier switches from hole-like to electron-like upon cooling. This crossover is a clear hallmark of two distinct superconducting condensates. Fitting our data to a two-gap Bardeen-Rickayzen-Tewordt (BRT) model yields a second energy gap of approximately 0.22 $k_{\rm B}T_{\rm c}$, only 11% of the dominant gap. This small gap size accounts for the difficulty in resolving the two-gap structure in earlier experiments. Crucially, the temperature at which the electron-like contribution to longitudinal thermal conductivity begins to dominate coincides with the onset of the sign reversal in the thermal Hall coefficient, confirming consistency between the two methods, and providing crucial information on the assignment of the superconducting gaps to their respective hole- or electron-like Fermi surface sheets. These findings not only resolve a longstanding controversy, but also establish that multigap superconductivity is far more common than previously assumed, and demonstrate the thermal Hall effect as a powerful probe for resolving gap multiplicities in
superconductors.
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All authors Jing Zhang, Xiaokang Li, Kamran Behnia, Zengwei Zhu
Mesoscale and Nanoscale Physics · Cross-listed from physics.optics
Extracting dipole orientations from asymmetric plasmonic nanostructures towards machine-learning-assisted spectropolarimetry
P. Christian Simo, Michaela Zbytovska, Annika Mildner et al. (+4 more)
In this work, nanoparticles with various asymmetries are analyzed for their azimuthal orientations using polarimetric dark-field spectroscopy at different analyzing angles of a linear polarizer. This approach reveals their spectral behavior in terms of electric far-field dipole intensities when modeled with an analytical dipole model. By simultaneously fitting the spectra from a set of analyzer angles, the respective dipole orientations are extracted. In a statistical approach, all non-repeating permutations are further studied with a machine learning
algorithm. The resulting azimuthal distribution of dipole orientations coincides well with the
geometric orientations derived from simulations and electron microscope images. A histogram gradient boosting regressor evaluates the impact of the measurement setup on the simultaneously fitted sets, linking the weights of the analyzer angles to the asymmetry in the plasmonic systems. This comprehensive spectroscopic method improves the accuracy of dipole orientation measurements and enables modern machine learning models to interpret potentially complex features of nanostructures.
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All authors P. Christian Simo, Michaela Zbytovska, Annika Mildner, Lukas Lang, Melanie Sommer, Dieter P. Kern, Monika Fleischer
Soft Condensed Matter
Liquid water stability as network resilience
Nicolás A. Loubet and Gustavo A. Appignanesi
Liquid water is unusually stable, remaining
liquid across a wide range of temperatures and even when confined to spaces only a few molecules wide. Yet it eventually fails in both settings: boiling at the critical temperature or drying in narrow pores, phenomena normally studied separately. Water's hydrogen-bond network is thought to underlie this stability, but tracking bond survival has not resolved the puzzle. Extending a concept we recently introduced for wetting (J. Am. Chem. Soc. 148, 21572 (2026)), we show that these stability limits can be accurately derived from a previously overlooked network property: resilience, the capacity to locally repair a broken bond. Only interactions too weak to be repaired constitute genuine defects, a far smaller population than conventional definitions suggest. While intact bonds remain abundant up to failure and conventionally broken bonds already percolate well before it, instability coincides with the percolation of these genuine defects. Comparing local interaction energies with a single intrinsic scale (identified a priori) predicts the wetting-hydrophobicity crossover, nanoconfinement cavitation, and bulk criticality. These phenomena are thus unified through a common molecular framework whose energetic scale also matches a long-unexplained spectroscopic feature of water. A single, parameter-free molecular criterion rooted in network resilience suffices.
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Mesoscale and Nanoscale Physics
Percolation threshold in cluster-assembled nanogranular palladium films and related strategies for hydrogen detection
A. Debot, D. Balakrishnan, A.M. Philippe et al. (+1 more)
Electrical transport in cluster-assembled nanogranular palladium films was investigated in-situ during growth by Supersonic Cluster Beam Deposition. The percolation onset is observed at a thickness of 1.5 nm, while the percolation completion at 2.5 nm. Films at the percolation thickness (2 nm) and beyond the percolation end (5 nm) were subsequently exposed ex-situ to 0.25% hydrogen in air to investigate how hydrogen-induced structural changes in Pd clusters modulate electrical transport and can be exploited as mechanism for selective hydrogen detection. Implantation of palladium clusters in soft
polymer styrene-ethylene-butylene-styrene was also explored to promote clusters separation and enhance
quantum tunnelling phenomena within electrical transport. The observed electrical transport pattern features stable baselines in air, suggesting material stability, fast response time (T50% < 10 s), and the coexistence of reversible and irreversible components in conduction changes induced by hydrogen exposure. The irreversible component is tentatively attributed to hydrogen-promoted partial coalescence of Pd clusters. The behaviour of the
polymer implanted samples suggests an alternative gas detection mechanism where hydrogen-induced
polymer swelling dominates over palladium structural modifications. These results disclose novel possible transducing routes for miniaturized hydrogen detection and
multimodal sensing.
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All authors A. Debot, D. Balakrishnan, A.M. Philippe, E. Barborini
Materials Science
Beyond binary at submicron dimensions: crossed-ellipse MTJ free layers as multi-state cells for spintronic crossbars
Arup Ghosh, Ariel Zaig, Colin Ducarme et al. (+3 more)
Spintronic crossbars are promising hardware platforms for energy-efficient neuromorphic computing, but conventional
magnetic tunnel junctions (MTJs) are binary, limiting the information density and synaptic precision of each crosspoint. Here, MuMax3 micromagnetic simulations are used to investigate how size and aspect ratio control the switching and remanent-state landscape of crossed-ellipse MTJ free layers, with permalloy as the main model system and CoFeB checks for transferability. At fixed 8:1 aspect ratio, shrinking the device from 16 micron x 2 micron to 80 nm x 10 nm lowers the absolute switching current but raises the switching field from about 11 Oe to about 301 Oe and increases the SOT current density. At fixed major axis 1.6 micron, aspect-ratio tuning produces a low-field four-state regime, while lower aspect ratios stabilize additional remanent states with lower switching fields and current densities. A 1.6 micron x 0.8 micron device exhibits twelve accessible remanent plateaus in its angle-resolved planar Hall response, with the resolved state count depending on both aspect ratio and absolute size. Projected MTJ readout gives multiple electrical levels, while minimum-energy-path calculations show that the twelve configurations are not all thermally independent. Independent MuMax+ calculations reproduce the multistate
topology and reveal lower-barrier multistep escape pathways between nominally distant states. These results define a geometry-dependent design window for scalable multistate spintronic crossbar
cells.
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All authors Arup Ghosh, Ariel Zaig, Colin Ducarme, Thomas Coppée, Flavio Abreu Araujo, Lior Klein
Materials Science
Lukas Cvitkovich, Klaus Zollner and Jaroslav Fabian
Active control of
magnetic damping is a crucial capability for the development of low-power, tunable spintronic devices. In this work, we demonstrate that the intrinsic
magnetic damping of 2D materials is highly sensitive to the position of the Fermi level relative to
spin-orbit driven anti-crossings, or “
spin hot spots.” Starting from a minimal multi-band model evaluated via Kamberský's breathing Fermi surface theory, we elucidate the fundamental role that energy gaps and band
alignment play in governing
magnetic dissipation. We then translate these theoretical insights to van der Waals
heterostructures, proposing a concrete mechanism for strongly tunable
magnetic damping via electrostatic gating. This concept is validated from first principles via density functional theory calculations on a realistic Fe$_3$GeTe$_2$/
graphene heterostructure. Our calculations reveal that an out-of-plane electric field selectively shifts the relevant energy bands, enabling the modulation of the
heterostructure's
magnetic damping over several orders of magnitude. These findings establish a robust theoretical framework and a promising materials platform for electrically tunable magnetization dynamics.
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Mesoscale and Nanoscale Physics
Two Strategies to Measure Spin-Orbit-Torque Efficiency Acting on the Insulating Magnet Li$_{0.5}$Al$_{0.7}$Fe$_{1.8}$O$_4$
Orion Smedley, Thow Min Jerald Cham, Daisy O'Mahoney et al. (+10 more)
For insulating magnets, widely-used electrically-based techniques to measure
spin-orbit-torque efficiency can suffer from poor signal-to-noise ratios and unwanted artifacts, while optical measurements based on conventional magneto-optical Kerr effect (MOKE) imaging often lack sufficient sensitivity. Here we analyze two strategies for measuring the
spin-torque efficiency acting on an insulating magnet, focusing on the torque from Pt acting on thin films of the low-damping magnet lithium aluminum ferrite (LAFO). First, lateral + longitudinal
spin-torque ferromagnetic resonance (ST-FMR) allows separation of the
spin-torque signal from artifacts due to
spin-pumping and resonant heating. With this technique, we measure a
spin-torque efficiency $\xi^\text{ST-FMR}_\text{DL}\sim\ 0.07$, comparable to but a bit less than for Pt acting on metallic ferromagnets. Second, we attempted optical measurements of
spin-orbit-torque-induced
magnetic deflection using Sagnac interferometry. We find that Sagnac interferometry provides sufficient sensitivity to measure signals from 14.4 nm LAFO thin films, but the inferred torque efficiency is much weaker than the result from lateral + longitudinal ST-FMR. We speculate that there may be another contribution to the Sagnac signal other than current-induced
magnetic deflection, perhaps an out-of-plane component of itinerant electron
spins. This becomes relevant for insulating magnets in which the magneto-optic coupling coefficient is very weak.
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All authors Orion Smedley, Thow Min Jerald Cham, Daisy O'Mahoney, Sanyum Channa, Xin Yu Zheng, Anna Janni, Lauren J. Riddiford, Yuhan Liang, Bozo Vareskic, Zbigniew Galazka, Yunqiu Kelly Luo, Yuri Suzuki, Daniel C. Ralph
Statistical Mechanics
Reconstruction of the superstatistical temperature distribution from single-particle kinetic energies
Sergio Davis
The framework of superstatistics can be used to describe non-Maxwellian single-particle velocity distributions as mixtures of Maxwellian distributions, where the weight is imposed by a probability density associated to the inverse temperature $\beta = 1/(k_B T)$. Among the typical model choices for $\beta$ are gamma, inverse gamma and lognormal distributions, sometimes called the universality classes of superstatistics. Given a set of observed velocities, however, there is no direct method to determine the underlying temperature distribution, as temperature itself is not a phase-space observable, and numerically inverting the Laplace transform is an unreliable process. In this work, we show that Jaynes' principle of maximum
entropy can be used to successfully reconstruct the inverse temperature distribution from data, by using the logarithmic moments of the kinetic energy as constraints. Although the kinetic energy distributions for the three universality classes are almost indistinguishable at the same mean and variance of $\beta$, the method correctly discriminates between them.
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Mesoscale and Nanoscale Physics
Optical activation of nonlinear Hall effect in topological insulators with warped Fermi surface
Mohammad Shafiei, Farhad Fazileh and Milorad V. Milošević
Topological
insulators (TIs) with hexagonally warped Fermi surface are natural platforms for the nonlinear Hall effect, as warping breaks inversion
symmetry while preserving time-reversal
symmetry (TRS). Here we show that this inversion breaking alone is insufficient: although warping generates a strongly anisotropic Berry
curvature, the preserved threefold rotational
symmetry forces the
equilibrium Berry
curvature dipole (BCD) to vanish identically. We demonstrate that linearly polarized light removes this
symmetry obstruction: in the off-resonant Floquet regime, it lowers the rotational
symmetry while preserving TRS, thereby generating a finite BCD whose magnitude, orientation, and sign are continuously tunable by the light intensity and polarization. For realistic Bi$_2$Te$_3$ parameters, we show that the induced BCD reaches $\sim$0.03~nm, yielding microampere-scale nonlinear Hall currents under experimentally accessible conditions. Our results therefore establish Floquet
symmetry engineering as a route to activating the
symmetry-forbidden nonlinear transport on TI surfaces without breaking the TRS.
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Superconductivity
Multigap superconductivity in Ising superconductors: The case of (LaSe)1.14(NbSe2)m misfit layer compounds
Alexandra Palacio-Morales, Tomas Samuely, Ludovica Zullo et al. (+14 more)
Strong
spin-orbit coupling and broken inversion
symmetry in transition metal dichalcogenides give rise to Ising superconductivity, a
spin-protected pairing state first identified in monolayer NbSe$_2$ through in-plane critical fields far exceeding the Pauli limit. More recently, Ising superconductivity has been proposed as a potential route to unconventional and even topological superconductivity in bulk misfit compounds. Here, we investigate the superconducting order parameter of layered misfit compounds composed of alternating transition metal dichalcogenide and rocksalt layers, which host extremely doped, electronically decoupled NbSe$_2$ sheets within a three-dimensional
crystal. Using directional scanning tunneling spectroscopy on the misfit
superconductors (LaSe)$_{1.14}$(NbSe$_2$) and (LaSe)$_{1.14}$(NbSe$_2$)$_2$, we uncover a strongly anisotropic multigap superconducting state: a fragile gap on the $\Gamma$-centered Fermi-surface pocket coexists with a robust, intrinsic gap on the K and K$'$ pockets. These features are in quantitative agreement with momentum-resolved gaps $\Delta(\mathbf{k})$ obtained from anisotropic Migdal-Eliashberg calculations. The marked fragility of the $\Gamma$-centered gap, combined with the strong sensitivity of the critical temperature to non-
magnetic disorder, points to pairing beyond conventional $s$-wave
symmetry, potentially involving a topological order parameter. These results establish NbSe$_2$-based misfit compounds as a tunable bulk platform for multigap, unconventional superconductivity, with Ising protection offering a promising route toward topological pairing.
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All authors Alexandra Palacio-Morales, Tomas Samuely, Ludovica Zullo, Raphaël T. Leriche, Pavol Szabó, Shunsuke Sasaki, Cesare Tresca, Hugo Le Du, Christophe Brun, François Debontridder, Giovanni Marini, Marek Kuzmiak, Jozef Kačmarčík, Laurent Cario, Matteo Calandra, Tristan Cren, Peter Samuely
Statistical Mechanics
Sharp Bounds on the Mean Efficiency of a Fluctuating Machine
Badr Farih
The efficiency of a machine at the scale of thermal fluctuations is random, and the conventional ratio $-W/Q_h$ has no moment of any order: its input heat fluctuates through zero. We work instead with the exergetic ratio $\eta=W/(W+T_0S)$, which lies in $[0,1]$ pointwise for non-negative dissipation, and ask what the energy budget alone determines about its mean. With $\alpha=T_0\langle S\rangle/W$ and $\sigma^2$ the relative variance of the dissipation, $1/(1+\alpha)<\langle\eta\rangle\le\sigma^2/(1+\sigma^2)+1/\{(1+\sigma^2)[1+\alpha(1+\sigma^2)]\}$, both ends sharp, with no distributional assumption. The floor is Jensen's inequality: fluctuating dissipation raises the mean efficiency above its deterministic value, and the mean alone gives nothing more. The ceiling is attained by an intermittently reversible law, dissipating nothing in a fraction $\sigma^2/(1+\sigma^2)$ of realisations, a prediction testable on trajectories. A third moment lifts the floor. Fixed delivered work is not required: when it too fluctuates, the bounds hold with the moments taken on $T_0S/W$, and the thermodynamic uncertainty relation on the work current converts the ceiling into a precision-efficiency frontier, whose zero-variance member is the known bound on a motor's ratio-of-means efficiency, shown here to be unsafe for the mean of the fluctuating ratio. Inside the interval lies the maximum-
entropy benchmark $\alpha^{-1}e^{1/\alpha}E_1(1/\alpha)$. Finally the bounds are worked out for a motor with futile cycles, observed until a fixed number of steps is delivered. There the dissipation cannot fall below the reversible cost of that work, and this floor $b$ sharpens the ceiling to $q/(1+\alpha b)+(1-q)/(1+\alpha c)$, $q=\sigma^2/[\sigma^2+(1-b)^2]$, $c=1+\sigma^2/(1-b)$, removing 40-67 per cent of the width. It is saturated when slips are rare: the extremal law is an operating regime, not an idealisation.
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Soft Condensed Matter
Analysis of Granular Flow in a Wedge-Shaped Hopper
Afroz Momin and Devang Khakhar
We study the velocity and stress distribution for a planar granular flow in a wedge-shaped hopper using Discrete Element Method simulations. Periodic boundary conditions are used in the direction normal to the plane of the flow to simulate a layer system. A parametric study is carried out varying system parameters (orifice width, wedge angle, wall friction and hopper height) and particle properties (particle diameter and particle friction). Scaling relations are obtained for the velocity and stress ratios. The data indicate that the stresses are not dependent on the shear rate, as in the Mohr-Coulomb rheology, however, the friction coefficient varies spatially, and the coaxiality condition is violated in some regions of the flow. A theory based on the scaling relations is presented, which gives good predictions for all the cases studied. Based on the theory, an expression for the mass flow rate, incorporating the effect of system parameters and particle properties is derived, which gives a close match with the computed mass flow rates for all the cases.
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Materials Science
Effect of Near-surface Thermal Spikes on Radiation Hardness of Gallium Oxide
Tomás Fernández Bouvier, Umutcan Bektas, Alexander Azarov et al. (+10 more)
Gallium oxide (Ga$_{2}$O$_{3}$) stands out as an extraordinary high radiation-tolerant
semiconductor, because its
lattice displacements induce polymorph transitions, holding material crystalline instead of leading to amorphization. Meanwhile, under extremely severe irradiation conditions many
crystals become amorphous, often starting from surfaces where the translation
symmetry breaks. Here, we show that the surface amorphization may prevail over the crystallisation in Ga$_{2}$O$_{3}$, however only if the mass and energy of irradiated ions produce sufficiently dense heat spikes in the immediate vicinity of the free surface. Applying machine-learned molecular dynamics simulations together with experimental broad-beam and focused ion-beam irradiations, we conclude that the presence of the free surface enables asymmetric displacements of Ga and O atoms, leading to a local non-stoichiometry. Consequently, when the affected cascade volume is sufficiently large, this compositional imbalance suppresses recrystallization and promotes amorphization. As such, our results are ready to use for tailoring irradiation conditions to either prevent or induce surface amorphization, depending on the requirements of the intended applications in Ga$_{2}$O$_{3}$ or other compound
semiconductors.
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All authors Tomás Fernández Bouvier, Umutcan Bektas, Alexander Azarov, Ru He, Nico Klingner, René Hübner, Paul Chekhonin, Aleksi Leino, Kai Nordlund, Javier García Fernández, Andrej Kuznetsov, Gregor Hlawacek, Flyura Djurabekova
Statistical Mechanics
Shannon Entropy as an Order Parameter for the Two-Dimensional Confined Coulomb Systems: Exact Balance Law, Topological Charge Sum Rule, and Boundary Saturation
Georgiy K. Lavrov and Eduard G. Nikonov
We establish a complete information-theoretic framework for the known minimum-energy configurations of the two-dimensional Thomson problem on a hard-wall disk, based on the Shannon
entropy of the Voronoi topological charge distribution. Two exact results are proven and hold for all studied systems of size $N=12$-$10^5$. First, an
entropy balance law decomposes the total
entropy into bulk, boundary, and mixing contributions without approximation. Second, a topological charge sum rule fixes the total charge at twice the number of boundary particles plus six; together with the observed separation of bulk and boundary coordination numbers, it yields exact relations between the populations of three- and four-coordinated boundary particles and the bulk charge. We further prove that the boundary
entropy is bounded by the binary maximum and that saturation of this bound is equivalent to the bulk charge per boundary particle approaching minus one half, a regime that the framework itself restricts to boundaries of at least twelve particles. All identities are verified numerically to machine precision, locating bulk nucleation at $56$ particles, the maximum of global disorder at $146$, and the onset of boundary saturation beyond about $1000$. The framework carries over to closed spherical geometry, where the total charge is fixed at twelve.
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Materials Science
From bending to stretching driven peeling of heterogeneous adhesives
Laurent Ponson
We study theoretically the peeling behavior of adhesives. Adopting a fracture mechanics approach, we derive the equation of motion of the adhesion front propagating at the interface between the adhesive and the substrate from which the peel strength is inferred. The originality of our approach lies in the description of the interplay during peeling between the stretching and the bending modes of deformation of the adhesive that is described as a F{\”o}ppl-Von Karman's thin film. Considering first a straight adhesion front, we retrieve the most salient feature of homogeneous adhesives, namely a peeling angle dependent peel strength driven by bending at large angles and by stretching at low angles. We also derive the shape of the adhesive that can be described using a single bending length scale derived from our model. We then investigate the impact of adhesion heterogeneities. We evidence that the deformations of the adhesion front are governed by a non-local interface elasticity the strength of which decreases with the peeling angle. This phenomenon reflects the transition between a stretching dominated peeling at low angle to a bending driven peeling at large angles that is captured in our model. This transition impacts the stability of adhesive fronts that relaxe more slowly from perturbations and gives rise to a stronger toughening effect in presence of a disorder distribution of adhesion energy at low peeling angles. Overall, this study sheds light on the central role played the elastic deformations of adhesives on their peeling behavior. The proposed framework unfolds the complex interplay between the deformation of adhesives and the peeling driving force that may be leveraged to engineer heterogeneous adhesives with enhanced properties. It also provides rich insights on the mechanisms underlying the emergence of non-local elasticity in interface problems.
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Soft Condensed Matter
Geometry-induced flocking and topological sound on a defect-free curved surface
Devendra Saini, Atanu Bhatta and Pritha Dolai
We study an ordered polar active flock on a
torus and show that topological sound persists on a compact curved surface without topological defects or physical boundaries. Using the covariant Toner Tu theory, we derive an effective nonHermitian Dirac operator whose
curvature-induced mass changes sign across the outer and inner equators, producing two Jackiw Rebbi domain walls. These support co-propagating but distinct chiral edge excitations: a density mode localized on the positively curved outer equator and a Goldstone mode localized on the negatively curved inner equator. The bulk bands possess opposite half-integer Chern numbers whose jumps across the domain walls are determined by the sign of the Gaussian
curvature. We further show that the localised modes are protected by a one-dimensional Callias index
theorem, while the sum of the local indices obeys the Poincare Hopf
theorem on the compact surface. Our results establish that
curvature alone, independent of defects and boundaries, is sufficient to generate and protect topological sound in active matter, providing a unified connection between non-Hermitian
topology, differential geometry, and hydrodynamic theory of collective motion.
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Soft Condensed Matter
Active pistons extract work by periodic compression alone
Paul Bernard, Euijoon Kwon, Benjamin Buhl et al. (+2 more)
Active matter is liable to invent protocols that evade the constraints of
equilibrium thermodynamics. We put forward active pistons that extract work by periodic compression alone without changing any bulk property of the system. Such pistons necessarily couple the perturbation imposed by an external operator with some degrees of freedom internal to active components. We illustrate this design principle with elastic networks composed of self-aligning motile particles. For slow protocols, self-
alignment always overwhelms mechanical friction when the internal activity exceeds a specific threshold controlled by fluctuations. We identify the key response coefficient that helps delineate regimes of work extraction, and reveal that the corresponding phase diagram follows a master curve with re-entrance in terms of noise amplitude. Overall, our active pistons embody a novel design principle with broad implications for building innovative engines far from
equilibrium.
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All authors Paul Bernard, Euijoon Kwon, Benjamin Buhl, Étienne Fodor, Olivier Dauchot
Mesoscale and Nanoscale Physics
Unified contact-free formulation of linear-response transport theory
Ziqian Wang and Ji Feng
Thermal Hall transport is conventionally formulated as a current–current Kubo response supplemented by an energy-magnetization correction obtained through an auxiliary pseudogravitational
magnetic field. Motivated by this structure, we develop a unified contact-free moment formulation of linear response to scalar sources. For channels whose source contributions vanish in the dc limit, the transport coefficient is the low-frequency residue of a retarded moment–moment correlator, with equal-time endpoint contributions incorporated before the dc limit is taken. The formulation uses only scalar sources, requires no separate magnetization subtraction, is independent of the local-current gauge, and is invariant under admissible redistributions of bond or interaction energy. It organizes particle, grand-energy, and specified
spin-density channels within a common source–moment response
matrix. Quadratic Landau–Lifshitz
spin waves illustrate the method: the secular growth of polarization-moment correlations isolates the finite Hall residue and yields the
magnon thermal Hall and
spin Nernst coefficients from the same paraunitary band geometry. The formulation provides a unified operator framework that simplifies the organization of particle, thermal, and
spin transport and can streamline calculations by eliminating separate contact and magnetization corrections.
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Materials Science
Optical properties of single-crystal As2Se3 from first principles
Juan J. Meléndez
Crystalline As2Se3 is a promising layered chalcogenide for mid-infrared photonics and optoelectronic applications, yet its fundamental electronic and optical properties remain debated. This paper combines density functional theory with G0W0 quasiparticle corrections and Bethe-Salpeter equation (BSE) calculations to deliver a robust description of its band structure and optical response. The G0W0 results reveal an indirect band gap of 2.31 eV with nearly degenerate direct transitions at Gamma, while the BSE spectra exhibit strong polarization-dependent anisotropy and pronounced excitonic effects. Several bound
excitons are identified near the absorption onset, clarifying the role of electron-hole interactions and resolving previous controversies regarding the nature of the optical gap and the contribution of lone-pair states. This work provides a unified and quantitatively accurate picture of As2Se3, reinforcing its potential for next-generation infrared and anisotropic photonic devices.
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Disordered Systems and Neural Networks
Statistical levels and spatial modes of Fock-space heterogeneity in many-body localization crossovers
Yu-Jing Liu and Chen Cheng
Near many-body localization crossovers, local
memory fluctuates strongly among eigenstates and ensemble realizations, but the observed heterogeneity combines contributions from different statistical levels. We develop a statistical framework based on configuration-space distance distributions that uses a variance decomposition to separate fluctuations within eigenstates, between eigenstates of one sample, and between samples. Applying this framework to random-cosine and quasiperiodic-cosine Ising ensembles with the same one-site field marginal, we find that, at the exact-diagonalization sizes studied, the clearest difference occurs among disorder realizations or quasiperiodic phase samples, whereas within-eigenstate and within-sample eigenstate-to- eigenstate contributions remain broadly comparable. Spatial covariances show that random outer fluctuations have a much stronger uniform component, whereas quasiperiodic phase fluctuations are organized more strongly at finite wave number and partly cancel in the spatial average controlling the distance center. We find that, near the crossover in the random ensemble, the ensemble-averaged distance center is particularly sensitive to changes in the nominal field strength. Combined with sample-to-sample differences in the realized field amplitude, this mean response accounts for much of the sample-to-sample variation in the distance center. Analysis of half-chain
entanglement further shows that its sample-to-sample fluctuations likewise reflect the combined effects of amplitude variations and its own mean response. An application to a fixed-magnetization
spin chain demonstrates the framework in a constrained configuration space. Resolving both statistical level and spatial mode therefore provides a more complete picture of sample- dependent many-body
memory and its configuration-space probability geometry.
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Strongly Correlated Electrons
Engineering Competing Fractional Topological States in a BHZ Superlattice
Ke-Chen Liu and Chen Cheng
Scalar superlattices offer a route from simple topological bands to fractionalized states, while multicomponent systems raise the additional question of which fractional order is energetically selected. We show that a scalar superlattice applied to the Qi-Wu-Zhang (QWZ) Chern-
insulator model produces an isolated C = +1 miniband with strongly improved Berry-
curvature and
quantum metric uniformity. Exact diagonalization of the projected interaction identifies a 1/3 fractional Chern
insulator in this miniband. Restoring the time-reversed partner to form the Bernevig-Hughes-Zhang (BHZ) superlattice yields, at total filling 2/3, a balanced fractional
quantum spin Hall (FQSH)-like state and a fully polarized 2/3 fractional Chern
insulator. Their competition separates two aspects of fractional-state control: the finite-momentum intercomponent coupling governs the stability of the balanced FQSH-like state, whereas a uniform pseudospin anisotropy shifts the relative energies of conserved sectors and can switch the global ground state between the balanced and polarized states. These results establish a minimal
lattice setting that connects topological-miniband reconstruction, fractional-state formation, and fractional-state selection, illustrating how distinct levels of control can be combined to navigate competing fractional topological states.
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Materials Science
Benchmarking Machine-Learning Interatomic Potentials for Dynamical Stability in Inorganic Semiconductor Nanocrystals: A CdSe Case Study
M. Usman, M. Suleymanova, Z. U. Abideen et al. (+2 more)
Machine-learning interatomic potentials (MLIPs) enable nanosecond-scale atomistic simulations of inorganic
semiconductor nanocrystals, but low errors on held-out configurations do not necessarily guarantee stable molecular dynamics. We
benchmark five graph-neural-network MLIPs, SchNet, PaiNN, NequIP, Allegro and MACE, for dynamical stability in a chloride-passivated cadmium selenide nanocluster containing 149 atoms. The models were trained under harmonized conditions on 1,000 configurations and evaluated against 2,000 held-out configurations generated using density functional theory, considering force accuracy, computational efficiency, uncertainty and structural stability during 1 ns simulations at 300 K. Allegro produced the lowest validation force mean absolute errors, ranging from 12 to 14 meV per angstrom, whereas SchNet produced the largest, ranging from 84 to 110 meV per angstrom. This ranking did not predict dynamical robustness: NequIP remained stable for 1 ns without additional training data, whereas MACE became stable only after an ensemble-based active-learning procedure added 34 uncertainty-selected configurations. PaiNN and Allegro remained unstable after the addition of 100 and 95 configurations, respectively, and SchNet also failed to achieve stable dynamics within the tested augmentation budget. Under the
benchmark hardware conditions, the machine-learning potentials required 6-66 ms per molecular-dynamics step, compared with approximately 20 s for density functional theory. These results show that held-out prediction errors alone are insufficient for selecting interatomic potentials for finite, surface-dominated nanostructures. Reliable deployment requires long-timescale dynamical testing combined with uncertainty-guided refinement, while the effectiveness and data efficiency of active learning remain strongly architecture dependent.
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All authors M. Usman, M. Suleymanova, Z. U. Abideen, M. Fernández-Pendás, I. Infante