Statistical Mechanics
Nonequilibrium Maxwell-Demon NEMD formalism for transport: I. Extrapolating shear viscosity to the hydrodynamic limit
Hesam Arabzadeh and Brad Lee Holian
The Maxwell-Demon nonequilibrium molecular dynamics method for measuring the shear viscosity divides the fluid simulation
cell into two regions of width w in the $x$-direction, with the particles allowed to move freely between the two sides. The Demon maintains equal and opposite regional average particle velocities in the $y$-direction by applying an acceleration needed to sustain the nonequilibrium steady state (NESS), which includes both total force balance and an instantaneous correction for
diffusion of particles across boundaries. We show that the driven velocity profile in x is a natural hydrodynamic response and not imposed by the constraint. For the present shear geometry, the measured NESS profile in Eulerian slabs is represented accurately by a piecewise
parabolic form, reminiscent of planar Poiseuille flow. The
parabolic profile gives an estimate of the kinematic viscosity from work done on the shearing fluid, as well as an
entropy production estimate, derived from heat removal by the Nosé-Hoover thermostat that keeps each regional average temperature constant. We show that the mechanical work supplied by the Demon equals the heat removed by the thermostat. Once NESS driving is removed, the
parabolic velocity profile relaxes exponentially in short order to sinusoidal, the natural transverse momentum-
diffusion eigenmode. Our Maxwell-Demon shear method provides a direct NEMD formalism for obtaining shear viscosity from momentum
diffusion, work, and
entropy balances. Our results in three dimensions for increasing system size $N$ (the number of particles) demonstrate that shear viscosity approaches an asymptote (the so-called hydrodynamic limit) as $N^{-1/3}$.
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Other Condensed Matter · Cross-listed from physics.med-ph
Proposal of a consistent value for the mean excitation energy of liquid water using advanced modeling, detailed simulations and precission proton Bragg curves
Pablo de Vera, Isabel Abril, Flávio Matias et al. (+3 more)
Protontherapy precision depends on accurately knowing the stopping power of
liquid water, as it is the most abundant component of soft body tissues. Difficulties when working with volatile
liquids have led to a long-standing debate regarding the values of the stopping power and the mean excitation energy $I$ of
liquid water. The equivalence between the stopping power of
liquid water and amorphous ice per unit mass density across all clinically relevant
proton energies opens the possibility to obtain a reliable estimation of the $I$-value of
liquid water. In this work, we employ two complementary theoretical methodologies to assess the
proton stopping power of water. The MELF-GOS (Mermin Energy Loss Function - Generalized Oscillator Strengths) approach, based on the perturbative dielectric formalism, serves as an accurate reference for sufficiently high energies, while the novel non-perturbative TDDFT-Penn (time-dependent density functional theory - Penn) method ensures accuracy for energies even below the stopping maximum. These stopping-power values were incorporated into the simulation code SEICS (Simulation of Energetic Ions and Clusters through Solids) to evaluate
proton Bragg curves from $1$ to $230$ MeV. Comparisons with available precise depth-dose measurements provide critical insights that help establish the consistent value $I = 79.4$ eV for
liquid water, providing robust physical grounds for improving range prediction for treatment planning.
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All authors Pablo de Vera, Isabel Abril, Flávio Matias, Julian M. B. Shorto, Hélio Yoriyaz, Rafael Garcia-Molina
Materials Science
What makes a useful molecular model of biochar? A community roadmap
Valentina Sierra-Jimenez, Jonathan P. Mathews, Luca Bellucci et al. (+22 more)
Biochars are disordered carbonaceous materials produced by biomass pyrolysis, with applications spanning soil amendment, water remediation, carbon storage, and functional materials. Although they share structural features with other disordered carbons such as coal, kerogen, and activated carbons, the questions posed to biochar models are distinct, and no single model can answer all of them equally well. Model usefulness must be defined relative to a specific question and validated against independent experimental observables. This community roadmap, arising from a CECAM workshop, critically maps current molecular approaches: experimentally guided top-down reconstruction, mimetic bottom-up simulation, and hybrid methods. We argue that first-generation models have been more successful than is often acknowledged, provided they are built at sufficient length scale and with explicit control over microporosity and bulk chemistry. Structural and
equilibrium interfacial properties are increasingly tractable with classical force fields, whereas dynamic and reactive behaviours require selective use of reactive methods within multiscale workflows. A parallel, largely unaddressed gap concerns the mineral and ash components of biochar, and the changes the material undergoes during ageing in soil. We identify seven open questions current models cannot yet answer reliably, and five community priorities: force field benchmarking, open model and data repositories, shared classification and metadata standards, ensemble validation, and training in reproducible practice. Across these, sustained interaction with experimentalists is essential to ground models in real observables and document where they fail. Progress will be accelerated by adapting transferable methods from coal, kerogen, and clay-organic matter frameworks rather than repeating trial-and-error development.
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All authors Valentina Sierra-Jimenez, Jonathan P. Mathews, Luca Bellucci, Edo Boek, Carla de Tomas, Manuel Garcia-Perez, Stef Ghysels, Paola Giudicianni, Corinna Maria Grottola, Kelly Anne Hawboldt, Robert L. Johnson, Fenna B.E. Kolff, Jean-Marc Leyssale, Diego Liberati, Francisco J. Martin-Martinez, Jacob W. Martin, Ondřej Mašek, Mohammad Mezbah Ul Hoque, Audrey Ngambia, Amaël Obliger, Frederik Ossler, Muhammad Riaz, John M. Tobin, Xiaolei Zhang, Valentina Erastova
Materials Science
Barium Hexaferrite Thin Films as a Scalable Magnetic-Insulator Platform for Proximity-Engineered Spintronics
Shyam Sundar Poriah, Sanjana D. S., Agrim Sharma et al. (+8 more)
Rare-earth iron garnets, such as yttrium iron garnet (YIG) and thulium iron garnet (TmIG), are the
benchmark magnetic insulators for spintronic and magnonic devices, but achieving usable perpendicular
magnetic anisotropy (PMA) in these materials typically relies on substrate strain- engineering, requiring careful
lattice-matching and specific growth conditions that constrain ma- terial accessibility. Here we establish sputter grown barium hexaferrite (BaFe12O19, BaM) as a
magnetic-
insulator alternative with strong intrinsic perpendicular anisotropy, requiring no strain engineering. X-ray diffraction, transmission electron microscopy and Raman spectroscopy confirm stoichiometric films with atomically smooth surfaces, while first-principles calculations corroborate a robust ferrimagnetic ground state. The films exhibit square out-of-plane hysteresis with a coercive field of nearly 0.1 T. Unlike rare-earth garnets, the perpendicular anisotropy in BaM is intrinsic to its magnetoplumbite
crystal structure, arising independent of highly ordered strain. Interfaced with Pt and with exfoliated BiSbTeSe2 (BSTS), BaM induces proximity induced anomalous
Hall trans- port, confirming efficient interfacial exchange coupling, while the BSTS/BaM
heterostructure shows an additional
Hall contribution suggestive of non-collinear interfacial
spin textures. These results position BaM thin films as a scalable
magnetic-
insulator platform for spintronic and topological
heterostructure devices beyond the constraints of garnet chemistry.
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All authors Shyam Sundar Poriah, Sanjana D. S., Agrim Sharma, Sreelakshmi M. Nair, Pankaj Bhardwaj, Laxmipriya Nanda, Aryaman Das, Jagadish Rajendran, R. S. Patel, Manish Jain, Dhavala Suri
Other Condensed Matter
Tracking Chirality during Molecular Motor Photoisomerization via Simulated Time-Resolved Circular Dichroism
Leonardo Biancorosso, Ali Hassanali, Mauro Stener et al. (+3 more)
Ultrafast spectroscopic techniques are widely used to investigate photoinduced processes, yet they remain largely blind to molecular chirality. Here we introduce a framework for simulating time-resolved electronic circular dichroism (TRCD) along an ensemble of nonadiabatic molecular dynamics, and apply it to the photoisomerization of a second-generation molecular motor[1]. While transient absorption captures the overall excited-state dynamics, it cannot distinguish the two photoproduct pathways. The TRCD response, by contrast, resolves the stereochemical branching: trajectories returning to the stable P isomer (right-handed helix) retain a distinct chiroptical band in the visible region, whereas those forming the M isomer (leftl-handed helix) become chiroptically dark as they twist through the conical intersection. This asymmetry constitutes a directly measurable signature of the stereochemical branching, offering a real-time probe of the formation of molecular chirality and concrete predictions for future TRCD experiments.
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All authors Leonardo Biancorosso, Ali Hassanali, Mauro Stener, Emanuele Coccia, Marta Monti, Gonzalo Díaz Mirón
Statistical Mechanics · Cross-listed from quant-ph
Transitions to super-radiance in ensembles of incoherently pumped emitters
Laszlo Rassaert and Tommaso Roscilde
Super-radiance is a striking phenomenon resulting from the collective interaction of emitters with light, and it is fundamentally related to the appearance of long-range correlations between the dipole moments of the emitters. As such, it represents a distinct phase of dissipative many-body systems compared to the case of independent emitters, similarly to how ferromagnetism stands out in
magnetic materials in contrast to paramagnetism. In this work we address the conditions under which the transition to super-radiance can occur in ensembles of emitters subject to dephasing and individual decay – a situation which is particularly relevant to the case of emitters in the solid state. The simplifying assumption of an ensemble of permutationally invariant emitters allows for the efficient solution of both the dissipative dynamics after a pulsed excitation, as well as of the steady state under incoherent pumping. This exact solution allows us to
benchmark a truncated cumulant expansion approach, which can give predictions for arbitrarily big system sizes. We show that super-radiance is fundamentally robust to sizable dephasing and non-radiative decay rates, both under a pulsed excitation, as well as under continuous pumping. This robustness is the result of the collective acceleration effect of super-radiant emission with respect to the individual coupling to a local environment. We establish the universal critical scaling laws at the transition between normal radiance and super-radiance; and we show that, in the super-radiant phase, significant finite-size crossovers can be observed before reaching the asymptotic scaling regime. Our results pave the way for future experiments to provide a quantitative characterization of the scaling properties of super-radiance, seen as a distinct non-
equilibrium many-body phase in ensembles of incoherently pumped emitters.
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Quantum Gases
Three-body contact for fermions. II. Non-degenerate limit
Xavier Leyronas and Félix Werner
A fundamental quantity characterizing
Fermi gases with zero-range interactions is the three-body contact $C_3$, which determines several observables including the number of nearby triplets of
fermions and the three-body loss rate in cold atom experiments, as shown in a companion article. Here, we compute $C_3$ to leading order in the non-degenerate limit for the homogeneous gas with negative or infinite scattering length $a$. At $a=\infty$, using a wavefunction approach, we obtain the analytical expression of $C_3$, which has a remarkably slow $1/T^{0.22728}$ dependence on the temperature $T$. In the Feynman diagram technique, which we use for $a<0$, the correct three-body short-distance correlations emerge only after a non-trivial cancellation between leading order two-body and three-body correlations, and the resulting power-law scaling comes from the large-wavevector tail of the 3-body T-
matrix, which we derive inspired by the analytical solution of the three-body problem at the
unitary limit.
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Soft Condensed Matter
Growth-Induced Transitions in Viscoelastic Matter
Valentin Slepukhin and Oskar Hallatschek
Growth is a fundamental process in living systems. Although the stress-deformation response of growing materials is often described as either purely elastic or purely viscous, many biological tissues, from biofilms to tumors, exhibit both elastic and viscous behavior. Here, we show that this viscoelastic response can crucially control the mechanics of proliferating matter when the growth rate becomes comparable to the rate of stress relaxation. Focusing first on the prototypical case of a growing elastic beam, we find that the dynamics are governed by a single dimensionless parameter, $g \tau$, where $g$ is the growth rate and $\tau$ is the viscoelastic relaxation time. While the limits $g \tau \to 0$ and $g \tau \to \infty$ recover purely viscous and purely elastic behavior, respectively, the intermediate regime is not merely a smooth crossover between them. Instead, qualitatively new dynamics emerge at $g \tau \sim 1$, including rapid transitions between metastable states that occur in neither limiting regime. We then develop a general, growth-compatible theoretical framework in which unconstrained growth is intrinsically stress-free, extending the analysis to other prototypical geometries and enabling simulations of more realistic growing biological materials. Within this framework, sharp mechanical transitions arise when stress generated by exponential growth accumulates faster than it can be dissipated by viscoelastic relaxation.
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Mesoscale and Nanoscale Physics
Field-Gated Programming of Nested Skyrmion Bags by Spatially Selective Skyrmion Writing
Shuhuang Lin and Shirong Lin
Nested skyrmion bags are composite
magnetic solitons whose internal occupation numbers provide an expanded state space for multilevel information encoding. However, deterministic and region-selective writing of individual skyrmions into nested bags remains challenging. Here, using micromagnetic simulations, we propose a field-gated nanocontact protocol for generating and programming double-layer nested skyrmion bags S(m,S(n)). The protocol exploits the opposite field responses of bag regions with antiparallel
magnetic backgrounds. Spatially segmented perpendicular
magnetic fields selectively expand either the outer region or the inner bag until it overlaps with a nanocontact. A
spin-polarized current pulse of the appropriate polarity then nucleates one skyrmion in the selected region, while subsequent field-driven contraction moves the written skyrmion away from the contact and restores the writing site for repeated operation. Starting from an empty nested bag S(0,S(0)), this procedure enables the controlled construction of S(m,S(0)), S(0,S(n)), and general S(m,S(n)) states. The simulated total topological charge changes in unit steps during sequential writing, confirming one-at-a-time and region-selective control. These results provide a possible route toward programmable nested topological states for multilevel spintronic information encoding.
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Materials Science
Claude-SpinDynamics: a cross-platform, dual-precision CPU/GPU micromagnetic simulator with native mumax3 script compatibility
Chun-Yeol You and Jaeyong Cho
Quantitative spintronics increasingly depends on a handful of GPU micromagnetic codes, all of which require NVIDIA hardware and single precision throughout, leaving researchers without such hardware unable to run even the standard validation problems. We report Claude-SpinDynamics (Claude-SD), a new open source micromagnetic simulator with a cross platform C++20 core (Windows and Linux) and a Python interface that closes this gap: a complete CPU build, validated by the same test suite as the GPU path, runs every unit test and uMAG standard problem with no accelerator at all, alongside GPU builds offering both single and double precision, a choice of two demagnetization FFT backends, and natively implemented
spin-orbit,
spin-transfer, and Zhang-Li torques, Dzyaloshinskii-Moriya interaction, and percell materials.Claude-SD natively interprets mumax3's .mx3 scripting
language, so existing community scripts run unmodified; under matched conditions the two codes agree
cell-by
cell to single-precision round-off, and, together with mumax+ and OOMMF, to within 2% on the uMAG dynamic-switching standard problem, with MuMax-CO agreeing to mumax3 to float32 round-off on the same problem. Benchmarked head-to-head against these three codes, Claude-SD's single-precision build is the fastest solver on small and two-dimensional problems and remains competitive at the largest grid sizes, while its double-precision and dual-FFT-backend paths are unmatched among GPU micromagnetic codes. A GPU replicabatching extension further advances an entire ensemble of finite-temperature trajectories in a single kernel launch per step, giving one to two orders of magnitude of throughput over a per-trial loop while reproducing single-trajectory results to numerical round-off. The complete source is openly licensed and distributed with runnable example notebooks and documentation for independent reproduction.
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Mesoscale and Nanoscale Physics
Temperature-Dependent Spin-Orbit Torque Generation in Perpendicularly Magnetized Topological Insulator-Magnetic Multilayer Heterostructures
Soumyarup Hait, Benjamin A. Brereton, Ahmet Yagmur et al. (+3 more)
We report a comprehensive temperature-dependent investigation of
spin-orbit torque (SOT) generation in
heterostructures comprising a perpendicularly magnetized metallic multilayer grown on top of a topological
insulator (TI) epilayer. Temperature-dependent second-harmonic
Hall measurements reveal distinct trends in the magnitude of the
spin-orbit torque across the studied
heterostructures. Samples incorporating Bi2Se3 exhibit torques reaching approximately 12 mT/(10^12 A m^-2) at 15 K, around 5 times larger than those in a multilayer without the topological layer. The structure with a thin 2-nm Ta buffer for the multilayer shows the strongest enhancement and a pronounced increase at low temperatures, highlighting efficient
spin-current generation from the topological surface states. In contrast, the sample with a 10 nm-thick Ta spacer exhibits reduced torque efficiency, consistent with partial attenuation of
spin transmission through the buffer. Systems lacking Bi2Se3 but containing two heavy metals (Ta and Pt) yield significantly smaller torques, around 2.5 mT/(10^12 A m^-2), despite the presence of conventional
spin Hall sources. These observations underscore the dominant role of TI-derived
spin-momentum-locked currents in driving large damping-like torques and their sensitivity to interfacial structure and buffer-layer thickness.
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All authors Soumyarup Hait, Benjamin A. Brereton, Ahmet Yagmur, Satoshi Sasaki, Gavin Burnell, Christopher H. Marrows
Materials Science
Linking Electronic Bonding and Short-range Order to Strength in $\alpha$-Titanium Alloys: A First-Principles Study
Md Faiz Akhtar, Nilesh P. Gurao and Somnath Bhowmick
The development of accurate strength prediction models for titanium
alloys is critical for advanced materials design. This study systematically examines how the mechanical properties of $\alpha$-Ti are affected by substitutional (X = Al, V, Mo) and interstitial (Y = H, C, N, O) alloying elements, with a focus on electronic bonding. Using density functional theory (DFT), we uncover the short-range ordering (SRO) of substitutional atoms and quantify their influence on the electronic bonding and mechanical behavior. The primary novelty of this work lies in developing a predictive model for tensile strength that goes beyond traditional empirical approaches. To quantify the contributions of individual solutes to strengthening, we use physically grounded
quantum-chemical descriptors, such as the Integrated
Crystal Orbital Hamilton Population (ICOHP), which is a direct measure of bond strength derived from first-principles calculations. The resulting formula quantitatively predicts the tensile strength of a wide range of $\alpha$-Ti
alloys, demonstrating a significant advancement in the computational design of high-performance structural materials.
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Mesoscale and Nanoscale Physics · Cross-listed from physics.optics
Driven Time Crystal in Low-Symmetry ENZ Conductors
Mario G. Silveirinha
In recent years, epsilon-near-zero (ENZ) materials have attracted a great deal of
attention in nonlinear optics, as they combine field enhancement with strong, ultrafast nonlinearities. In particular, transparent conducting
oxides, such as ITO, have emerged as a promising class of materials and have been extensively exploited to achieve temporal optical responses varying on the femtosecond scale using an optical pump. Most of the solutions discussed so far in the literature rely on effective $\chi^{(3)}$ modulations, wherein the dominant material response is controlled by the envelope of the optical pump. Here, it is shown that low-
symmetry conductors can provide an interesting alternative to transparent conducting
oxides and a more natural implementation of time-crystalline behavior in optical systems with optical-cycle modulation. I demonstrate that ENZ confinement, combined with the strong anomalous-velocity nonlinearity of low-
symmetry conductors, enables a subwavelength nanoparticle to develop a time-crystalline response under optical pumping. For sufficiently strong pumping, this response can overcome dissipative losses and lead to parametric amplification. Furthermore, the pump can strongly tailor the scattering and extinction of a weak probe and, in extreme cases, render the extinction negative. In this regime, the driven nanoparticle effectively amplifies the probe beam.
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Statistical Mechanics · Cross-listed from quant-ph
Equivalence of quantum resources under ergodic dynamics
Sreemayee Aditya, Xhek Turkeshi and Piotr Sierant
Quantum resource theories characterize distinct forms of nonclassicality in many-body
quantum states, raising the question of whether these resources evolve independently under generic ergodic dynamics. Considering diagnostics quadratic in the state, we show that the dynamics of different resource measures become mutually interdependent and are governed by a few common degrees of freedom. For Haar-random
circuits, the purity together with a single resource witness suffices to reconstruct the remaining resource measures, as we demonstrate for coherence, various asymmetries, and number
entropies. The same relations hold, to a good approximation, under chaotic Floquet and continuous-time
Hamiltonian dynamics, establishing that this dynamical resource equivalence extends beyond random
circuits. We further derive an exact coherence–imaginarity relation, remarkably accurate also beyond Haar-random
circuits. Our results reveal an emergent simplification of many-body dynamics, in which sufficiently strong scrambling reduces seemingly distinct
quantum resources to a few common dynamical degrees of freedom.
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Materials Science
Broadband phonon-velocity suppression and a finite anisotropic crossover in twisted bilayer SnSe
Peng Kang, Wei Yin, Da Wan et al. (+9 more)
Moiré superlattices reshape
lattice dynamics without altering chemical composition, yet how
crystal anisotropy modifies this control remains unclear. We combine density-functional-theory (DFT)-calibrated
lattice-dynamical calculations with angle-matched untwisted controls to study puckered
bilayer SnSe across seven commensurate twist angles ($3.18^\circ$–$8.77^\circ$). At 300 K, twisting suppresses the band-path heat-capacity-weighted mean-square group velocity to 2.6–8.4% of the control values; the suppression spans a broad frequency range rather than a few soft branches. The velocity response crosses over between $4.78^\circ$ and $3.82^\circ$ into a regime where the relaxed stacking textures and frequency-resolved velocity profiles become self-similar, with the normalized mean-square velocity ratio spanning only 11.1% of its mean across the three smallest angles—a finite anisotropic crossover, not a singular-angle condition. Direct DFT–MACE force-constant agreement ($r=0.996$), uniform $4\times4\times1$ stability scans, and acoustic-sum-rule and path-density tests support the trend. The
equilibrium trend is defined by six structures after excluding one relaxation-sensitive case. These results extend
phonon twistronics to low-
symmetry layered materials and identify
crystal anisotropy as a key determinant of finite-angle
phonon crossover behavior.
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All authors Peng Kang, Wei Yin, Da Wan, Shulin Bai, Sirui Fan, Qi Zou, Hongfeng Li, Xiao Xiang, Zhen Li, Yu Liu, Lei Zheng, Li-Dong Zhao
Statistical Mechanics
The $6-\epsilon$ Expansion for Long-Range Lee–Yang and Percolation Criticality
Zhiyi Li, Kun Chen, Zhijie Fan et al. (+1 more)
The crossover from long-range (LR) to short-range (SR) criticality in percolation has remained unsettled because previous renormalization-group (RG) analysis within the $\epsilon'=3\sigma-d$ expansion fixes the anomalous dimension at $\eta=2-\sigma$, whereas SR percolation has $\eta_{\rm SR}<0$ near $d=6$. Sak's matching condition then places the crossover above $\sigma=2$, outside the regime in which the LR interaction dominates. In spatial dimension $d=6-\epsilon$, we formulate a perturbative expansion for the LR $\phi^3$ field theory and perform a one-loop RG analysis throughout the perturbatively accessible nonclassical regime $0<\delta<\epsilon/3$, where $\delta = 2-\sigma$. We derive the one-loop corrections to the critical exponents $\eta$ and $\nu$, which acquire nontrivial dependence on $\epsilon$ and $\delta$. They reduce to their mean-field values at the LR upper critical line and continuously recover the SR $6-\epsilon$ results as $\sigma\to2$. These results support a crossover threshold $\sigma_*=2$ and remove the apparent discontinuity of $\eta$ between the LR and SR values within this framework. The same approach also yields the anomalous and edge exponents of the LR Lee–Yang universality class and $q$-state Potts universality classes with $q<2$.
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All authors Zhiyi Li, Kun Chen, Zhijie Fan, Youjin Deng
Materials Science
DINO4DSTEM: A self-supervised framework for structural discovery in 4D-STEM
Daniel Khaykelson, Lothar Houben and Boris Rybtchinski
Nanodiffraction using 4D-STEM has become a key technique for quantitative nanoscale structural mapping in materials research, yet interpreting its high-dimensional
datasets in structurally complex materials remains a major bottleneck. Existing analysis workflows typically rely on structural models, manual annotation, predefined classes, or sample-specific heuristics, limiting their ability to characterize heterogeneous complex materials. Here, we introduce DINO4DSTEM, a self-supervised machine learning framework that automatically discovers structurally meaningful states directly from raw diffraction data. Without structural models, manual labels, a predefined number of classes, or system-specific parameter tuning, the framework learns representations that organize diffraction patterns by their intrinsic structural similarities, transforming large collections of low-dose measurements into quantitative nanoscale structure maps. Across diverse
datasets, DINO4DSTEM consistently identifies the dominant structural degrees of freedom, providing
segmentation without human supervision. We applied the framework to the crystallization of indomethacin, a beam-sensitive, multidomain pharmaceutical system, revealing that crystallinity emerges from a partially ordered precursor and spans a continuous spectrum of structural order. The discovered nanoscale structural states are mapped to reveal the
evolution of order across the specimen quantitatively. By replacing task-specific analysis with general self-supervised representation learning, DINO4DSTEM provides a broadly applicable framework for quantitative nanoscale structural mapping in complex materials, enabling the discovery of emergent structural organization in heterogeneous, beam-sensitive systems.
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Disordered Systems and Neural Networks
Localization and Transport in a Non-Hermitian Hexagonal Harper Model
Akshey Rajoriya, Dibyajyoti Sahu, Suhas Gangadharaiah et al. (+1 more)
We investigate a one-dimensional non-Hermitian hexagonal Harper model with quasiperiodically modulated hopping amplitudes. In the Hermitian limit, the model exhibits metallic, insulating, and multifractal phases characterized by distinct eigenstate properties. Upon introducing non- Hermiticity, the phase diagram is qualitatively altered, with an expansion of metallic regions and strong boundary sensitivity arising from the non-Hermitian skin effect. By analyzing wave-packet dynamics, we uncover qualitatively distinct transport signatures in metallic, multifractal, and in- sulating regimes. In the metallic region, nonreciprocal hopping induces finite sliding, resulting in ballistic center-of-mass motion that is absent in the Hermitian model, while wave-packet spreading is simultaneously suppressed and exhibits diffusive scaling. Interestingly, the multifractal regime emerges as a distinct dynamical phase supporting both enhanced spreading and finite sliding, both primarily of superdiffusive nature, in contrast to metallic regions where sliding (spreading) shows ballistic (diffusive) scaling. These features are markedly different from their Hermitian counterpart. On the other hand, in the insulating region, both the spreading and sliding are strongly suppressed. We reconfirm these intriguing transport characteristics by investigating the distinct growth profile of single-particle
entanglement entropy where the effect of spreading of the wave-packet is clearly manifested. These results demonstrate that quasiperiodicity in hopping amplitudes, combined with non-Hermiticity, establishes the multifractal regime as a key mediator of transport.
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All authors Akshey Rajoriya, Dibyajyoti Sahu, Suhas Gangadharaiah, Tanay Nag
Soft Condensed Matter
Skimming transition in flexible granular sweeping
Y. Ochi and H. Katsuragi
A flexible body placed in a steady flow bends to reduce drag. This self-streamlining is a hallmark of fluid-structure interaction (FSI). Granular-structure interaction is equally ubiquitous in nature. However, it remains poorly understood. Thus, we investigate inertial granular-structure interaction (IGSI). Specifically, ejection induced by a flexible plate sweeping a granular bed is experimentally examined. We find that a faster sweep results in less ejection, particularly for a flexible plate. To understand the underlying physics of this behavior, a dimensionless number Sk is introduced as the ratio of the plate elastic timescale to the sweep timescale. At $\mathrm{Sk} \lesssim 1$, the plate deflection follows the self-streamlining law of FSI and induces substantial ejection. At $\mathrm{Sk} \gtrsim 1$, on the other hand, the plate skims the bed and the mass of ejected grains decreases sharply. Sk organizes IGSI as the granular counterpart of FSI.
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Soft Condensed Matter
Time-resolved sedimentation of dense potato-starch suspensions measured by optical coherence tomography
T. Saiki and H. Katsuragi
We demonstrate optical coherence
tomography (OCT) as a measurement technique for dense, optically opaque suspensions. Conventional optical methods cannot access the interior of such suspensions. OCT resolves individual potato-starch particles (${\sim}20~\mathrm{\mu m}$) as distinct scatterers, even though the suspension appears opaque to the eye. By tracking the vertical centroid position of the particle-laden layer $\langle Z \rangle(t)$ and the supernatant boundary $Z_\mathrm{sup}(t)$ in the same OCT image sequence, we obtain the instantaneous settling velocity $V(t)$ and the time-evolving effective volume fraction $\phi_\mathrm{eff}(t)$ simultaneously and continuously in time. To our knowledge, this is the first measurement to combine settling velocity and particle concentration into a single continuous trajectory within one sedimentation run. Conventional batch measurements yield only one velocity value per run. We applied this method to dense potato-starch suspensions, varying the initial volume fraction $\phi_0$ from 0.30 to 0.50 and the solvent density $\rho_\mathrm{L}$ from 1.0 to $1.3{\times}10^3~\mathrm{kg~m^{-3}}$ using aqueous sodium polytungstate solutions. The normalized velocity $V/V_\mathrm{Stokes}$ plotted against $\phi_\mathrm{eff}$ collapses onto a common trend consistent with both the Krieger–Dougherty model and the Richardson–Zaki law over $\phi_\mathrm{eff} \simeq 0.30$–$0.52$, confirming that the method captures physically reasonable hindered-settling behavior. These results establish OCT as a viable tool for probing internal dynamics in dense suspensions that were previously inaccessible to optical measurement.
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Strongly Correlated Electrons
Quantum Many-Body Scars, Magnon-Pair Condensation, and Hilbert Space Fragmentation in an Anisotropic Heisenberg Model
J. Y. Liu-Sun and Z. Song
We investigate a
spin-$1/2$ anisotropic Heisenberg model on a
lattice consisting of two identical bipartite sublattices. A family of exact eigenstates generated by the restricted spectrum generating
algebra (RSGA) constitutes
quantum many-body scar states, characterized by subextensive
entanglement entropy and supporting. These scar states are
magnon-pair condensates exhibiting off-diagonal long-range order (ODLRO). At the resonance point of the inter-sublattice interaction, the model exactly maps onto a mixed
spin-$1$ and
spin-$0$ XY model on a bipartite
lattice, which decomposes into independent sub-
Hamiltonians labeled by all possible
spin configurations. Each
spin-$0$ particle is dynamically isolated from its neighbors and acts as a kinetic constraint, giving rise to emergent
Hilbert space fragmentation (HSF). Our work establishes an exactly solvable platform in which
quantum many-body scars,
magnon-pair condensation exhibiting off-diagonal long-range order, and
Hilbert space fragmentation naturally coexist.
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Materials Science
Network Topology of Hafnia-Based Amorphous Optical Coatings by Grazing-Incidence X-ray Total Scattering and Atomic Modeling
K. Prasai, B. LaBell, K. H. Lee et al. (+4 more)
Amorphous hafnia-based films are promising optical-coating materials for cryogenic GW detectors, but their performance depends on how doping and annealing modify the atomic network. We combine grazing-incidence X-ray total scattering measurements with experimentally constrained atomic modeling to study the as-deposited HfO$_2$ and 27% SiO$_2$-doped HfO$_2$ films annealed at 150$^\circ$C and 400$^\circ$C. Pure amorphous HfO$_2$ is a dense, high-coordination network of Hf-centered polyhedra with substantial edge- and face-sharing connectivity. Incorporating SiO$_2$ introduces stable SiO$_4$ tetrahedra, lowers the Hf and O coordination, and replaces highly connected Hf-rich oxygen environments with mixed Si–O–Hf bridges. This produces a chemically mixed network rather than isolated SiO$_2$-rich regions, and shifts the cation
topology toward corner-sharing connectivity. Annealing to 400$^\circ$C produces only modest additional structural relaxation. These results provide an atomic-scale description of how SiO$_2$ modifies the
topology of amorphous HfO$_2$-based coatings and suggest structural descriptors relevant to understanding their mechanical-loss behavior.
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All authors K. Prasai, B. LaBell, K. H. Lee, A. Mehta, B. Shyam, M. M. Fejer, R. Bassiri
Soft Condensed Matter
Shear effects in active models of normal and cancer cells
Souvik Sadhukhan, Rajsekhar Das, Lin Zhao et al. (+2 more)
Mechanical properties of biological tissues, driven by passive and active forces, play a vital role in several processes ranging from development to cancer metastasis. However, the dynamical responses of
cells in tissues, subject to mechanical deformations such as shear and the associated rheological properties, are not well characterized. Here, we use three-dimensional
agent-based models for normal and cancer tissues to investigate their responses to simple shear as a function of
cell stiffness and
stochastic active forces. In the normal epithelium, with uniform strength of active force, the yield stress as a function of shear rate follows the Herschel-Bulkley form over a range of
cell volume fraction. Strikingly, the shear rate dependence and the elasticity-dependent changes in the yield stress fall on master curves upon suitable scaling. To model cancer-like behavior, a certain fraction ($N_p$) of
cells was chosen to have enhanced activity and decreased stiffness. As $N_p$ increases, the extent of collective
cell movement decreases, transitioning from affine (collective) to non-affine (individualistic) movement, a finding that is in accord with imaging experiments. Simulations of a model of a stiff solid tumor, with radius $R_s$ embedded in normal tissue, show that as $R_s$ increases, the yield stress increases. Interestingly, the
cells migrate collectively as $R_s$ increases. A Gaussian Mixture Model (GMM) and a mean field theory quantitatively account for the simulation as well as experimental results on cancerous, non-cancerous, and a mixture of these two types. The combined theoretical and experimental study establishes that heterogeneity in stiffness and activity determines non-affine movements in normal and cancer tissues.
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All authors Souvik Sadhukhan, Rajsekhar Das, Lin Zhao, Wolfgang Losert, D. Thirumalai
Materials Science
Discovering Physically Interpretable Mathematical Expression for Predicting CO2 Adsorption in Metal-Organic Frameworks via Machine Learning-Symbolic Regression
Yimin Shao, Shengluo Ma, Shenghong Ju et al. (+2 more)
This work presents a machine learning-symbolic regression (ML-SR) strategy to develop a physically interpretable formula for predicting low pressure CO2 adsorption capacity in hypothetical metal-organic frameworks (hMOFs). Four ML models were trained on a small
dataset of 1,000 samples, and five key descriptors-largest cavity diameter, pore limiting diameter, void fraction, gravimetric surface area, and number of hydrogen atoms-were identified through SHAP and feature importance analyses. Symbolic regression was then employed to derive a concise adsorption formula, Q=aA, where a represents an adsorption baseline (mmol/g) and A is a dimensionless adsorption number incorporating four structural descriptors. We interpret A as the ratio between an adsorption binding force and a
diffusion driving force, revealing how pore
topology and surface chemistry jointly influence adsorption. Validation against a comprehensive
dataset of 137,652 hMOFs demonstrates that this formula achieves over 70% prediction accuracy for 62,448 structures, confirming strong applicability within defined structural and operational ranges. Unlike conventional black box ML models, the proposed physics-guided expression enables efficient prediction and provides clearer insight into adsorption mechanisms.
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All authors Yimin Shao, Shengluo Ma, Shenghong Ju, Yijun Shi, Wei Li
Mesoscale and Nanoscale Physics
Symmetry-Tunable Skyrmions and Merons in Magnetic Nanodisks via Spatially Engineered Anisotropy
X. D. Wang, J. F. Oliveira da Silva, Z. H. Tao et al. (+3 more)
We demonstrate that spatially engineered
magnetic anisotropy can stabilize skyrmion and meron
spin textures in
magnetic nanodisks even in the absence of Dzyaloshinskii-Moriya interaction (DMI). Using a constrained analytical model and micromagnetic simulations, we show that competing perpendicular and in-plane anisotropies can generate non-collinear topological textures in non-
chiral magnetic systems. We further show that DMI and dipolar interactions lift the helicity degeneracy and select preferred
chiral configurations; micromagnetic simulations were used to identify physically stable states. These results establish anisotropy-patterned nanodisks as a platform for studying DMI-free topological
spin textures and their controllable
magnetic response. We also show that arrays of anisotropy-engineered skyrmions can control
spin-wave transmission by manipulating their vorticity arrangement, pointing to reconfigurable magnonic elements based on non-
chiral topological textures.
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All authors X. D. Wang, J. F. Oliveira da Silva, Z. H. Tao, H. M. Dong, K. Chang, M. V. Milošević
Materials Science
Probing three-dimensional structures of complex colloidal quantum dots at the single-atomic level
Qikai Wu, Meng Pei, Jiancheng Zhang et al. (+6 more)
Colloidal
quantum dots (QDs) are promising optoelectronic materials due to their size-tunable properties, yet their three-dimensional (3D)
quantum confinement makes electronic states highly sensitive to structural and chemical heterogeneity, which critically impacts their optoelectronic performance. Accurately resolving the 3D atomic structure with sub-angstrom precision is thus essential for rational design. Here, we applied atomic electron
tomography (AET) to determine, for the first time, the 3D atomic structure of complex core/shell QDs, resolving over 14,000 atoms per particle. Our reconstructions reveal surface morphology, eccentric cores, and nearly atomically abrupt heterovalent interfaces and identify anisotropic shell growth directed by twin boundaries. Utilizing an AET-derived atomic structure, we performed large-scale
quantum mechanical calculations to uncover an orientation-dependent strain accommodation mechanism where the heterogeneous strain is compensated at interfaces and twin boundaries. Furthermore, our results reveal strain-induced localized states near the band edge, which contribute to the key features of the experimental ensemble absorption spectrum. This work sets a new
benchmark for atomic-level characterization, establishing a powerful framework for the rational design of next-generation nanomaterials.
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All authors Qikai Wu, Meng Pei, Jiancheng Zhang, Wei Xu, Tianding Xu, Colin Ophus, Zaiping Zeng, Botao Ji, Yao Yang
Superconductivity
Anharmonicity and Nonadiabaticity in Hydride Superconductors
Shashi B. Mishra, Francesco Belli, Eva Zurek et al. (+1 more)
We study superconductivity in representative hydrides using anharmonic
phonons, electron-
phonon vertex corrections, and full-bandwidth Eliashberg theory. The high-pressure binary hydrides H3S, YH6, and YH9 must be treated with both anharmonic and nonadiabatic corrections, whereas the ambient-pressure PdH/PdD/PdT series is strongly anharmonic but remains adiabatic, reproducing the inverse isotope effect without sizable vertex contributions. LaBeH8 exhibits weak anharmonicity, while vertex corrections reduce the critical temperature (Tc) by approximately 4 K, leaving the predicted Tc above experiment. To identify when treatments beyond harmonic, adiabatic Migdal-Eliashberg theory are required, we introduce the anharmonic renormalization $A_{\lambda}$ and the vertex ratio $R_{V}$ as material-specific diagnostics.
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All authors Shashi B. Mishra, Francesco Belli, Eva Zurek, Elena R. Margine