Materials Science · Cross-listed from physics.geo-ph
Correlative Microstructural Analysis of a Weathered Nantan Meteorite Fragment
Graeme J. Francolini, Brendan V. Dyck, Paul Mack et al. (+1 more)
The weathering of iron-rich phases within meteorites is a multi-stage process that significantly alters the microstructure and chemical composition based both on the environmental condition at the location of landing and the exposure time since the fall. This work investigates the resulting phases of this process in a correlative and comparative manner using a naturally weathered Nantan meteorite fragment. Techniques including X-ray Photoelectron Spectroscopy (XPS), Energy Dispersive X-ray Spectroscopy (EDS), and X-ray Fluorescence Spectroscopy (XRF) were used for chemical and compositional determination and X-ray Diffraction (XRD) and Electron Backscatter Diffraction (EBSD) for phase determination and microstructural analysis. The combination of these techniques revealed the meteorite
matrix to be predominantly composed of magnetite, with goethite and maghemite present as secondary phases. The meteorite had two pairs of distinct regions, one separated by Ni content and the other by grain size and Cl distribution. The Ni content regions are believed to have formed from a former Widmanstätten pattern, with the high ($\geq$2.6 at%) and low ($\leq$0.9 at%) Ni regions from plessite and kamacite, respectively. The kamacite and taenite within the plessite are suggested to have weathered through distinct mechanisms. The grain size regions were separated by a visible boundary 100-200 $\mu$m wide where grains transitioned rapidly from fine to elongated grains and the Cl content changed from low and dispersed to concentrated spots. Lastly, a brecciated phase was found within the sample and was identified as a large cohenite grain which exhibited signs of weathering, including vein-like extensions of the
matrix composed of NiO and magnetite, and deposits of iron and nickel carbonates.
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All authors Graeme J. Francolini, Brendan V. Dyck, Paul Mack, Ben Britton
Materials Science · Cross-listed from physics.geo-ph
Iron spin crossover in ferropericlase and its effect on lower-mantle thermal conductivity
Alexander F. Goncharov, Irina Chuvashova, Eric Edmund et al. (+29 more)
Thermal conductivity of Earths lower mantle controls heat transfer across the core-mantle boundary (CMB) and strongly influences mantle convection. We report direct measurements of the thermal conductivity of single-
crystal ferropericlase (Mg$_{1-x}$Fe$_x$O, $x = 0.09$-0.13), the second most abundant lower-mantle mineral, using optical
laser flash and X-ray free-electron
laser heating in diamond-anvil
cells up to $\sim2200$~K and 130~GPa. These experiments provide the first conductivity data for ferropericlase at simultaneous lower-mantle pressures and temperatures. A marked reduction in conductivity between 60 and 100~GPa at $\sim1700$~K is consistent with the iron
spin crossover. Combined with our previous results for Fe- and Fe,Al-bearing bridgmanite, the data define a lower-mantle conductivity profile that increases with pressure to $\sim10$~W\,m$^{-1}$\,K$^{-1}$ near the CMB, constraining mantle heat flux, plume buoyancy, and long-term geodynamic
evolution.
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All authors Alexander F. Goncharov, Irina Chuvashova, Eric Edmund, JungFu Lin, Zena Younes, Nicolas Jaisle, Axel Phelipeau, Carmen Sanchez-Valle, Christoph Otzen, Clemens Prescher, Hanns-Peter Liermann, Nico Giordano, James McHardy, Karen Appel, Michal Andrzejewski, S. V. Rahul, Minxue Tang, Jolanta Sztuk-Dambietz, Thomas Michelat, Torsten Laurus, Malcolm McMahon, Mark Robertson, Rachel Husband, Efim Kolesnikov, Sebastien Merkel, Silvia Boccato, Carsten Baehtz, Guillaume Morard, Emma S. Bullock, Cornelius Strohm, Zuzana Konopkova, Ryan Stewart McWilliams
Materials Science · Cross-listed from cs.DB
Ontology-based knowledge graph infrastructure for interoperable atomistic simulation data
Abril Azocar Guzman, Sarath Menon, Tilmann Hickel et al. (+1 more)
The reuse of atomistic simulation data is often limited by heterogeneous formats, incomplete metadata, and a lack of standardized representations of workflows and provenance. Here we present an ontology-based infrastructure for representing and integrating atomistic simulation data as a knowledge graph. The approach combines domain ontologies with a software framework that enables data capture both from existing
datasets and directly from simulation workflows at the point of generation. Heterogeneous data from multiple sources are normalized into a common, ontology-aligned representation, enabling consistent querying and analysis across
datasets. We demonstrate these capabilities through the integration of grain boundary data, cross-
dataset analysis of material properties, and extraction of derived thermodynamic quantities from existing simulations. In addition, workflows are represented in a machine-readable form, enabling both forward provenance tracking and partial reconstruction of computational procedures. The resulting knowledge graph contains over 750,000 triples describing nearly 8,000 computational samples. This work provides a practical framework for improving the findability, interoperability, and reuse of atomistic simulation data.
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All authors Abril Azocar Guzman, Sarath Menon, Tilmann Hickel, Stefan Sandfeld
Materials Science
Metallic d-wave altermagnetism in WFeB: a platform for electrically switchable perpendicular spin-splitter response
Eranga H. Gamage, Zhen Zhang, Subhadip Pradhan et al. (+9 more)
We report the synthesis and
magnetic characterization of WFeB and identify it as a metallic d-wave altermagnet representative of a broader TiNiSi-type family.
Neutron diffraction, Mössbauer spectroscopy, and magnetometry establish a collinear altermagnetic ordering confirmed by first-principles calculations. The electronic structure shows a nonrelativistic
spin splitting of approximately 100 meV, but it also supports a strong
spin-splitter transport response. This demonstrates that efficient
spin-current generation can occur even with such modest band splitting.
Symmetry analysis shows that selected film orientations permit deterministic switching of the Néel vector by current-induced staggered torques, enabling electrical control of a perpendicular
spin-splitter response. These results establish WFeB and related TiNiSi-type antiferromagnets as a platform for electrically switchable charge-to-
spin conversion driven by altermagnetic
symmetry.
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All authors Eranga H. Gamage, Zhen Zhang, Subhadip Pradhan, Ajay Kumar, David R. Ramgern, V. Ovidiu Garlea, Yaroslav Mudryk, Saeed Kamali, Douglas Warnberg, Kirill D. Belashchenko, Vladimir Antropov, Kirill Kovnir
Statistical Mechanics
Self-similar summation of virial expansions
V.I. Yukalov and E.P. Yukalova
Virial expansions are the series in powers of density assumed to be small. However, the equations of state require to consider finite densities for which virial expansions, as a rule, diverge. In order to extrapolate a virial expansion to the values, where this expansion diverges, one uses summation methods. The most often used method is the Padé summation, which has several deficiencies. First of all, Padé approximants are not uniquely defined, suggesting a large table of admissible variants. Second, often there appear spurious unphysical poles. On the contrary, in those cases where the existence of a pole is physically motivated, Padé approximants do not necessarily exhibit it. A new approach for the summation of virial expansions is suggested, based on self-similar approximation theory. The method is regular and uniquely defined. It allows for the determination of physically motivated poles. The accuracy of self-similar approximants is not worse than that of the best Padé approximants with fitting parameters or of Monte Carlo simulations. The self-similar summation is based solely on virial expansions, involving no fitting parameters. In some cases, self-similar summation allows for reconstructing the sought functions exactly. The approach is illustrated by summing virial expansions for hard-disk fluids, hard-sphere fluids, and systems with power-law potentials.
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Materials Science
Spatially modulated morphotropic phase boundaries in a compressively strained multiferroic thin film
Ting-Ran Liu, Xiangwei Guo, Sajid Husain et al. (+5 more)
The coexisting rhombohedral-like (R', MA) and tetragonal-like (T', MC) monoclinic phases in compressively strained bismuth ferrite thin films exhibit exceptional piezoelectric and
magnetic properties. While previous studies have largely focused on probing the morphotropic phase boundaries (MPBs) comprising ordered R'/T' twins, their self-organizing structures remain not fully explored. Here, we observed two types of interphase boundaries in a 60 nm-thick BiFeO3 film epitaxially grown on a LaAlO3 substrate by employing multi-modal diffraction-based electron microscopy. First, the flat MPBs form lines extending >1 mm, and repeat almost every ~20 um. Additionally, we uncover a new type of phase boundary with zig-zag regions of alternating R'/R' and T'/T' twin domains. Cross-sectional multislice electron ptychography confirms the atomic-scale polarization rotation across the MPB, with out-of-plane strain varying >15%. Plan-view electron backscatter diffraction reveals the
lattice disclination of ~1.5-degrees across the zig-zag interphase boundaries, while having >2.5 degrees within the MPB. Phase-field modeling suggests that the formation of zig-zag phase boundaries arises from balancing between Landau and elastic energies. We speculate that such well-ordered interphase boundaries are associated with mesoscale in-plane strain modulations, thus providing a way to engineer and harness their properties for potential device applications.
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All authors Ting-Ran Liu, Xiangwei Guo, Sajid Husain, Maya Ramesh, Pushpendra Gupta, Darrell Schlom, Ramamoorthy Ramesh, Yu-Tsun Shao
Statistical Mechanics
Dielectric response as a source of viscosity in polar liquids
David S. Dean and Haim Diamant
Transport coefficients and dielectric relaxation in
liquids are often treated as distinct manifestations of molecular dynamics. We show that, in polar
liquids, orientational dipolar fluctuations generate a substantial contribution to the shear viscosity that can be expressed in terms of dielectric response parameters. Using a Green-Kubo approach formulated in terms of dipolar body-force correlations, we derive an explicit relation linking the viscosity contribution to the static permittivity and the Debye relaxation time. With a single microscopic cutoff length fixed from one temperature, the theory predicts the temperature dependence of the viscosity for water and several alcohols using independently measured dielectric data. The results identify a general mechanism by which slow polarization dynamics generate an additional, and in strongly polar
liquids often dominant, contribution to the viscosity, providing a quantitative bridge between dielectric spectroscopy and rheology.
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Materials Science
Stress Asymmetry in Hard Magnetic Soft Materials
H. Gökçen Güner, Francois Barthelat, John D. Clayton et al. (+3 more)
Hard
magnetic soft materials – soft
polymers embedded with hard
magnetic particles – are modeled using continuum magnetomechanical formulations in which the deformation and the magnetization field are the primary kinematic variables. A recent question in such formulations is whether the Cauchy stress is symmetric, which is directly related to frame invariance and angular momentum balance. This note discusses energetically equivalent formulations, related by a change of variables between referential and current descriptions of the magnetization, and shows that they generally yield different Cauchy stresses, including a change in their
symmetry. Specifically, the formulation based on a referential magnetization produces a symmetric Cauchy stress, while that based on a current magnetization generally yields an asymmetric Cauchy stress. We highlight that when the internal variable (magnetization field) is at the energy-minimizing
equilibrium configuration, the divergences of these stresses are the same, and both stresses are symmetric.
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All authors H. Gökçen Güner, Francois Barthelat, John D. Clayton, Carlos Mora-Corral, Noel Walkington, Kaushik Dayal
Materials Science
Propagation-mediated amplification of \{11\={2}0\}-biased inversion domain boundary alignment in polarity-mixed GaN lateral overgrowth
Harim Song, Donghoi Kim and Chinkyo Kim
GaN polarity inversion and the associated inversion domain boundaries (IDBs) are frequently observed during lateral overgrowth and are often discussed in terms of the small energetic spread among competing IDB structures predicted by first-principles calculations. In circular mask openings, \(\{11\bar{2}0\}\)-aligned IDBs have previously been explained by
geometric closure of a single-polarity hexagonal domain at the circular boundary. Here we examine an experimentally distinct regime in which opposite-polarity domains already coexist within the opening before the later development of long, straight IDB traces. In this mixed-polarity regime, the final trace orientation cannot be attributed solely to the macroscopic circular boundary. Nevertheless, plan-view SEM line-trace statistics show that IDB orientations remain biased toward the \(\{11\bar{2}0\}\) family. To quantify how this bias develops during propagation, we perform distance-resolved, length-weighted orientation analysis in concentric annular regions defined from the opening center. The resulting metrics show that \(\{11\bar{2}0\}\)-biased
alignment is progressively amplified with propagation distance, while the orientation distribution becomes narrower, indicating systematic sharpening of the preferred
alignment state. We further apply the same ring-resolved statistical operators to minimal two-domain propagation simulations in a circular opening and find that a propagation-mediated anisotropy reproduces the observed radial amplification under fixed circular geometry. Together, these results establish a quantitative phenomenology of \(\{11\bar{2}0\}\)-biased IDB
alignment in polarity-mixed GaN lateral overgrowth on patterned sapphire and indicate that, although mask-boundary-imposed selection may describe single-polarity closure cases, the present mixed-polarity regime is better explained by propagation-mediated amplification.
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Mesoscale and Nanoscale Physics
Directly visualizing the energy level structure of quantum dot molecules
Heun Mo Yoo, Tanner M. Janda, Connor Nasseraddin et al. (+1 more)
The orbital,
spin and valley degrees of freedom in silicon
quantum dots support many modes of
spin qubit operation. However, it is generally challenging to obtain information about the energy level spectrum over large ranges of parameter space. We demonstrate a form of spectroscopy that is capable of mapping the energy level structure of a double
quantum dot as a function of level detuning, interdot tunnel coupling, and
magnetic field. In the one electron regime, we directly observe the transition from the atom like energy levels of isolated
quantum dots to molecular like bonding and anti bonding states with increasing interdot tunnel coupling. We also resolve the Zeeman splitting of ground and excited valley states in a
magnetic field. In the two electron regime, we gain access to the detuning dependent singlet triplet splitting. Our work may be extended to a broader class of systems, such as strong
spin-orbit materials or proximitized
quantum dots, allowing the direct extraction of various energy gaps.
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All authors Heun Mo Yoo, Tanner M. Janda, Connor Nasseraddin, Jason R. Petta
Mesoscale and Nanoscale Physics
Field-unmasked quantum geometry in a symmetry-forbidden photocurrent
Bumseop Kim, Aaron M. Burger, Zhenbang Dai et al. (+6 more)
Frequency- and polarization-resolved photocurrents provide a sensitive probe of hidden
symmetry and band geometry in
quantum materials. Here we study a
chiral cubic sillenite whose global
crystal symmetry forbids a longitudinal odd-in-B magneto-photocurrent in the Voigt geometry. Nevertheless, we observe a pronounced longitudinal response across the visible range that is predominantly linear in
magnetic field, persists below the band gap, and exhibits strong helicity selectivity, with the circular channel exceeding the linear one and reversing sign upon switching light helicity. We resolve this apparent contradiction by identifying defect-enabled, field-selected
spin ordering as the mechanism that lowers the effective
magnetic symmetry without altering the global
crystal structure. First-principles calculations show that O vacancies generate in-gap bound states and localized
magnetic moments on neighboring Bi-O units, stabilized by strong SOC. Although
symmetry-related vacancy configurations remain energetically degenerate and preserve the macroscopic T
symmetry at zero field, an applied
magnetic field selects a time-reversal-broken sector of the defect ensemble and reduces the effective
magnetic symmetry to the subgroup that leaves B invariant, thereby lifting the longitudinal selection rule. Importantly, this field-selected
symmetry reduction does more than activate a nominally forbidden photocurrent: it unmasks latent
quantum-
geometric responses encoded in the electronic structure. Momentum-resolved calculations show that the dominant circular and linear magneto-photocurrent channels spatially correlate with Berry-
curvature-rich and
quantum-metric-rich regions of the Brillouin zone, respectively. Our results establish field-selected defect
symmetry lowering as a route to revealing hidden
quantum geometry and activating forbidden nonlinear photocurrents in
chiral quantum materials.
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All authors Bumseop Kim, Aaron M. Burger, Zhenbang Dai, Sayed Ali Akbar Ghorashi, Adam Abirou, Md Al Helal, Vladmir M. Fridkin, Jonathan E. Spanier, Andrew M. Rappe
Mesoscale and Nanoscale Physics
Dielectric control of ultrafast carrier dynamics and transport in graphene
Hai I. Wang, Xiaoyu Jia, Anand Nivedan et al. (+4 more)
Understanding the ultrafast dynamics of photoexcited charges in
graphene is essential, as the microscopic mechanisms underlying these dynamics determine many of
graphene's optical, optothermal, and optoelectronic properties. These are crucial properties for many functionalities and devices enabled by
graphene, such as high-speed photodectors. Therefore, beyond scientific understanding, it is highly desirable to control ultrafast carrier dynamics for practical applications. Here, we establish this control by engineering the dielectric environment of
graphene, thereby regulating both heating and cooling dynamics without altering the
Fermi energy, optical power, or ambient temperature. By combining optical pump-terahertz probe experiments with theoretical calculations, we show that dielectric screening suppresses carrier-carrier interactions and slows the dynamics. In particular, reduced carrier-carrier scattering delays the formation of a quasi-
equilibrium hot electron distribution, thus slowing carrier heating. It also slows carrier cooling because re-thermalization after optical-
phonon emission depends on the same interactions. The enhanced screening further reduces the energy of electron-hole puddles, thereby increasing charge mobility and the Seebeck coefficient. This ability to externally control internal
graphene dynamics and transport properties enables the optimization of device performance, such as the sensitivity of photodetectors for data communication and wireless communication applications.
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All authors Hai I. Wang, Xiaoyu Jia, Anand Nivedan, Mischa Bonn, Aron W. Cummings, Alessandro Principi, Klaas-Jan Tielrooij
Statistical Mechanics · Cross-listed from math.DG
Gradient systems and asymmetric relaxations in view of Riemannian geometry
Alessandro Bravetti, Miguel Ángel García Ariza and José Roberto Romero-Arias
In dually flat
manifolds, there is a deep connection between gradient flows and pregeodesics. This was one of the many important contributions of Amari to information geometry. In this paper, we extend the study of this relationship to general Riemannian
manifolds. Our result does not impose conditions of flatness on the connection or
symmetry on its non-metricity tensor, thus broadening the
geometric setting beyond Hessian
manifolds. Within this framework, we provide a criterion for comparing relaxation along two different gradient descent curves of a function, formulated in terms of the non-metricity tensor of a connection for which the gradient curves are pregeodesics. We use it to study Gaussian chains, whose relaxation trajectories coincide with gradient descent curves in the space of Gaussian distributions.Thus, we recover a recent result that establishes a universal asymmetry: warming up is faster than cooling down. Our work illustrates how
geometric insights rooted in Amari's legacy offer new perspectives for optimization problems and
stochastic processes.
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Soft Condensed Matter
Microscopic Basis for Recovery Rheology and the Nonequilibrium Structure,Yielding, and Flow of Dense Particle Suspensions
Anoop Mutneja and Kenneth S. Schweizer
The recent introduction of recovery rheology has provided qualitatively new physical insights into the yielding and flow of soft matter systems across diverse mechanically driven nonequilibrium protocols by separating the deformation strain into recoverable and unrecoverable components. A striking finding is that the fluid-like response associated with the gradually increasing unrecoverable strain ultimately leads to the continuous yielding transition from a solid to a
liquid. We build on the force and particle level Elastically Collective Nonlinear Langevin Equation theory of activated dynamics within a nonequilibrium microrheological framework to formulate a general statistical mechanical foundation of step-rate start-up shear response that relates recovery rheology to microscopic structure, relaxation, and elasticity. Quantitative applications to metastable hard and soft sphere colloidal suspensions reveal testable new predictions and interconnections between macroscopic and microscopic properties: (i) the steady state recoverable strain is directly related to the steady-state shear thinning; (ii) the transient stress overshoot amplitude varies non-monotonically with packing fraction and is quantitatively linked to the steady-state recoverable strain; (iii) the acquired unrecoverable strain dictates the stress overshoot strain; (iv) the predicted enormous reduction of the structural relaxation time under deformation is inversely related to the unrecoverable strain-rate.
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Soft Condensed Matter
Retained-spin micropolar hydrodynamics from the Boltzmann–Curtiss equation
Satori Tsuzuki
We derive a retained-
spin micropolar hydrodynamic closure from the Boltzmann–Curtiss equation using a generalized Chapman–Enskog construction in which the local mean
spin is retained as a quasi-slow variable. Starting from the one-particle kinetic balance identities and the corresponding exact coarse-grained finite-size balances for mass, linear momentum, and intrinsic angular momentum, we keep the collisional-transfer contribution to the antisymmetric stress explicit in the
spin balance, decompose the first-order source into irreducible scalar, axial, and symmetric-traceless sectors, and show explicitly how the standard micropolar constitutive structure with coefficients $(\eta,\xi,\eta_r,\alpha,\beta,\gamma)$ emerges. This decomposition makes clear that the one-particle kinetic stress contributes only to the symmetric stress, whereas the rotational viscosity belongs to a collisional-transfer channel. For perfectly rough elastic hard spheres, we further obtain explicit dilute-gas estimates for the rotational viscosity $\eta_r$ from homogeneous
spin relaxation and for the transverse
spin-
diffusion combination $\beta+\gamma$ from a transport-relaxation calculation. Targeted event-driven molecular-dynamics simulations are used as a posteriori checks: expanded homogeneous-
spin density and roughness sweeps support the predicted $n^2$ and $K/(K+1)$ trends for $\eta_r$, while finite-$k$ transverse runs provide a qualitative diagnostic of the retained-
spin response. The result is a self-contained derivation and coefficient-level estimate of retained-
spin micropolar hydrodynamics that clarifies which parts of the closure are exact balance-law statements, which are first-order generalized Chapman–Enskog results, and which remain controlled rough-sphere estimates.
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Other Condensed Matter · Cross-listed from quant-ph
Quantum memory on a nanophotonic silicon chip
Stephan Rinner, Jonas Schmitt, Kilian Sandholzer et al. (+1 more)
Integrated photonic
circuits offer great promise for
quantum technologies. However, due to the rapid propagation of light, many envisioned applications require efficient on-chip
quantum memories with a programmable delay, compact footprint, and high fidelity. Implementing this based on standard
semiconductor processing technology is an outstanding challenge. Here, we realize such
memories using erbium-doped silicon waveguides, fabricated as part of a multi-wafer project by a nanophotonic foundry. We demonstrate light storage with a $44.2(9)\ \text{MHz}$ bandwidth and a programmable delay exceeding $1\ \mu \text{s}$ in a device with a footprint of only $1.5\times 10^{-2}\ \text{mm}^2$, outperforming on-chip delay lines by many orders of magnitude. The phase of the read-out light field is preserved with a visibility of $91.3(30)\ \%$. The efficiency of $1.89(28)\times 10^{-8}$ can be improved in future devices through resonator enhancement and higher dopant concentrations. With this, the demonstrated approach will pave the way towards applications in photonic
quantum computing based on scalable silicon processing technology.
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All authors Stephan Rinner, Jonas Schmitt, Kilian Sandholzer, Andreas Reiserer
Other Condensed Matter · Cross-listed from quant-ph
Scattering phase shift in quantum mechanics on quantum computers: non-Hermitian systems and imaginary-time simulations
Peng Guo, Paul LeVan, Frank X. Lee et al. (+1 more)
To overcome the fast oscillatory behavior of correlation functions for extracting scattering phase shift in real-time
quantum simulations encountered in Ref.\cite{Guo:2026qkx}, we propose and test two solutions in the present work. One is to simulate Hermitian systems in imaginary time, the other is to simulate non-Hermitian systems in real time. We demonstrate that both approaches lead to the problem of non-
unitary quantum evolution which can be solved by combining two
quantum algorithms: block encoding and Hadamard test. The combined
quantum algorithm does not require mid-
circuit measurements or adjustment of the input parameters of the
Hamiltonian, and can be easily implemented on
quantum computers. Numerical tests on
quantum simulators show that both approaches agree with exact solutions for a sufficiently long time before the signal is lost in statistical fluctuations. The results bode well for using non-Hermitian and imaginary-time simulations to circumvent oscillations inherent in real-time simulation of other
quantum systems. In particular, the non-Hermitian approach shows a decisive advantage over the imaginary-time one on the number of required ancillary
qubits, and hence is more practical to scale up.
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All authors Peng Guo, Paul LeVan, Frank X. Lee, Yong Zhao
Mesoscale and Nanoscale Physics
A footprint of zero-point entropy in higher-temperature magnetic thermodynamics
Sergey Syzranov and Arthur P. Ramirez
Identifying extensively degenerate zero-temperature states is key in characterizing
spin-
liquid-candidate materials and
spin ices. In experiments, finding zero-point
entropy (ZPE) is often attempted by measuring the
entropy released by a material when cooled down from very high to very low temperatures. Such investigations are often unreliable and lead to controversial results because accessible temperatures may be insufficient to accurately capture essential low- and high-temperature features of
magnetic materials. The purpose of this paper is to point out a simple, easily accessible signature of nonzero ZPE: the Maxwell's relation $\left(\partial S/\partial H\right)_T = \left(\partial M/\partial T\right)_H$ can appear violated if a vanishing ZPE is assumed incorrectly. This relation can further be used for estimating the ZPE. In many materials below characteristic temperatures, the criterion of non-vanishing ZPE has a particularly simple form: $\left(\frac{\partial C}{\partial H}\right)_T\left(\frac{\partial M}{\partial T}\right)_H<0$. We discuss these effects and the ZPE signature in the
benchmark test case of the well-studied
spin ice $Dy_2Ti_2O_7$.
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Strongly Correlated Electrons
Crystals Caught Doping: Metallic Wigner Crystals in Rhombohedral Graphene
Junkai Dong, Tomohiro Soejima, Daniel E. Parker et al. (+1 more)
Nearly a century after Wigner's initial proposal, electron
crystals are now a topic of intense experimental and theoretical interest. However, most proposed crystalline phases are commensurate and therefore become insulating in the presence of even weak pinning. In this work we discuss when a commensurate Wigner
crystal will spontaneously self dope and develop itinerant carriers, giving rise to an incommensurate and thus metallic Wigner
crystal (MWC). We develop a general criterion for the instability of the commensurate
crystal which involves the competition between the charge gap at commensurability and a “packing bias” whose sign selects whether electron or hole doping is preferred. We then apply these insights to rhombohedral multilayer
graphene, where calculations for commensurate
crystals reveal instabilities towards self-doping. Carrying out self-consistent Hartree-Fock over the landscape of incommensurate
crystals reveals the phase diagram, where a broad MWC phase appears directly adjacent to an insulating Wigner
crystal phase. Recent observations of an island of reversed
Hall conductance near a putative Wigner
crystal phase in rhombohedral
graphene are naturally explained by our theory.
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All authors Junkai Dong, Tomohiro Soejima, Daniel E. Parker, Ashvin Vishwanath
Mesoscale and Nanoscale Physics
Evidence of Metallic Wigner Crystal in Rhombohedral Graphene
Tonghang Han, Jackson P. Butler, Shenyong Ye et al. (+15 more)
When the Coulomb interaction dominates over kinetic energy, electrons can crystallize into a Wigner
crystal (WC). This paradigmatic correlated electronic phase has been realized in two-dimensional electron gases with
parabolic band dispersion and completely flat Landau levels under high
magnetic fields. Beyond these conventional contexts of electron crystallization, more exotic electron
crystals have been postulated but remain unexplored. For example, a metallic Wigner
crystal (mWC), in which itinerant carriers coexist with a pinned electron
lattice, has been proposed theoretically but considered difficult to realize. Non-
parabolic electron bands and
quantum geometry may facilitate mWC and other novel topological electron
crystals. Here we report transport evidence for WC and mWC in rhombohedral tetra-, penta-, and hexalayer
graphene in the charge density range 0.3-0.5x10^12 cm^-2. By flattening the conduction band with a gate-controlled displacement field D, we observe an insulating state at nonzero charge density that shows nonlinear, hysteretic current-voltage relations, signatures of a pinned WC, that are absent from the lower-density
insulator. Further increasing D reveals transport dominated by hole-like carriers with density up to only 15% of the nominal electron density, consistent with mWC. This mWC state is closely tied to the WC state, as both collapse simultaneously with increasing temperature or bias voltage. The mWC state shows
quantum Hall onset near 0.4 T and disobeys the Streda relation, indicating compressible charge exchange between itinerant holes and the transport-inert WC background. Our results establish rhombohedral
graphene as a platform for exploring novel electron
crystals, as well as possible nontrivial
topology, and new collective modes.
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All authors Tonghang Han, Jackson P. Butler, Shenyong Ye, Zhenqi Hua, Surajit Dutta, Zach Hadjri, Zhenghan Wu, Jixiang Yang, Junseok Seo, Phatthanon Pattanakanvijit, Emily Aitken, Kenji Watanabe, Takashi Taniguchi, Peng Xiong, Eli Zeldov, Zhengguang Lu, Raymond Ashoori, Long Ju
Disordered Systems and Neural Networks · Cross-listed from physics.optics
Observation of Floquet erratic non-Hermitian skin effect in photonic mesh lattice
Yeyang Sun, Shu Yang and Yi Yang
In ordered, translationally invariant non-Hermitian systems, the skin effect is understood as a boundary phenomenon: nonreciprocal hopping drives an extensive accumulation of eigenstates towards the edges, whereas the periodic-boundary spectrum remains Bloch extended. Here we experimentally reveal the opposite limit – a disorder-enabled, boundary-independent, and intrinsically bulk form of skin localization – the recently predicted erratic non-Hermitian skin effect (ENHSE), realized in a driven photonic platform. Using a time-multiplexed photonic mesh
lattice with programmable gain, loss, and phase modulation, we engineer spatially fluctuating imaginary gauge fields and realize a Floquet non-Hermitian
lattice whose global reciprocity can be tuned independently of strong local nonreciprocity. We observe a disorder-driven non-Hermitian topological transition between two oppositely directed disordered skin phases through a critical point of global reciprocity. At this transition, boundary skin accumulation disappears, yet the wave dynamics self-organizes into bulk-localized patterns without any interface, providing direct evidence of ENHSE. The measured localization profiles agree with simulations and exhibit the defining feature that distinct eigenstates share a common bulk-localized envelope determined by the disordered imaginary gauge fields. By further introducing controllable on-site disorder, we reveal the competition between ENHSE and Anderson localization, and show how increasing scattering progressively suppresses erratic skin dynamics. Our results help establish ENHSE as a unique disorder-induced non-Hermitian phenomenon and open a route to engineering localization, transport, and
topology beyond conventional Bloch and boundary-based paradigms.
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Materials Science · Cross-listed from cs.LG
Perspective: Towards sustainable exploration of chemical spaces with machine learning
Leonardo Medrano Sandonas, David Balcells, Anton Bochkarev et al. (+20 more)
Artificial intelligence is transforming molecular and materials science, but its growing computational and data demands raise critical sustainability challenges. In this Perspective, we examine resource considerations across the AI-driven discovery pipeline–from
quantum-mechanical (QM) data generation and model training to automated, self-driving research workflows–building on discussions from the “SusML workshop: Towards sustainable exploration of chemical spaces with machine learning” held in Dresden, Germany. In this context, the availability of large
quantum datasets has enabled rigorous benchmarking and rapid methodological progress, while also incurring substantial energy and infrastructure costs. We highlight emerging strategies to enhance efficiency, including general-purpose machine learning (ML) models, multi-fidelity approaches, model
distillation, and active learning. Moreover, incorporating physics-based constraints within hierarchical workflows, where fast ML surrogates are applied broadly and high-accuracy QM methods are used selectively, can further optimize resource use without compromising reliability. Equally important is bridging the gap between idealized computational predictions and real-world conditions by accounting for synthesizability and multi-objective design criteria, which is essential for practical impact. Finally, we argue that sustainable progress will rely on open data and models, reusable workflows, and domain-specific AI systems that maximize scientific value per unit of computation, enabling efficient and responsible discovery of technological materials and therapeutics.
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All authors Leonardo Medrano Sandonas, David Balcells, Anton Bochkarev, Jacqueline M. Cole, Volker L. Deringer, Werner Dobrautz, Adrian Ehrenhofer, Thorben Frank, Pascal Friederich, Rico Friedrich, Janine George, Luca Ghiringhelli, Alejandra Hinostroza Caldas, Veronika Juraskova, Hannes Kneiding, Yury Lysogorskiy, Johannes T. Margraf, Hanna Türk, Anatole von Lilienfeld, Milica Todorović, Alexandre Tkatchenko, Mariana Rossi, Gianaurelio Cuniberti
Statistical Mechanics · Cross-listed from cs.LG
Temporal Memory for Resource-Constrained Agents: Continual Learning via Stochastic Compress-Add-Smooth
Michael Chertkov
An
agent that operates sequentially must incorporate new experience without forgetting old experience, under a fixed
memory budget. We propose a framework in which
memory is not a parameter vector but a
stochastic process: a Bridge
Diffusion on a replay interval $[0,1]$, whose terminal marginal encodes the present and whose intermediate marginals encode the past. New experience is incorporated via a three-step Compress–Add–Smooth (CAS) recursion. We test the framework on the class of models with marginal probability densities modeled via Gaussian mixtures of fixed number of components~$K$ in $d$ dimensions; temporal complexity is controlled by a fixed number~$L$ of piecewise-linear protocol segments whose nodes store Gaussian-mixture states. The entire recursion costs $O(LKd^2)$ flops per day – no backpropagation, no stored data, no neural networks – making it viable for controller-light hardware. Forgetting in this framework arises not from parameter interference but from lossy temporal compression: the re-approximation of a finer protocol by a coarser one under a fixed segment budget. We find that the retention half-life scales linearly as $a_{1/2}\approx c\,L$ with a constant $c>1$ that depends on the dynamics but not on the mixture complexity~$K$, the dimension~$d$, or the geometry of the target family. The constant~$c$ admits an information-theoretic interpretation analogous to the Shannon channel capacity. The
stochastic process underlying the bridge provides temporally coherent “movie” replay – compressed narratives of the
agent's history, demonstrated visually on an MNIST latent-space illustration. The framework provides a fully analytical “Ising model” of continual learning in which the mechanism, rate, and form of forgetting can be studied with mathematical precision.
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Superconductivity · Cross-listed from hep-ph
Role of the equivalence principle in gauge and axial symmetries of Yukawa coupling, and the strong CP problem
Konstantin V. Grigorishin
It is demonstrated the fundamental role of the equivalence principle in
gravity for the Yukawa coupling between scalar and
fermion fields. The Kibble-Zurek mechanism for formation of topological defects as vortexes and monopoles breaks down in system with a global gauge
symmetry only. At the same time, the different vacuums can occur, which are separated be domain walls. The equivalence principle makes the strong violation of CP invariance impossible. Thus the
axion hypothesis becomes redundant.
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Disordered Systems and Neural Networks · Cross-listed from physics.soc-ph
Urban scaling and the dynamics of individual cities
Ulysse Marquis and Marc Barthelemy
Cities are often compared through scaling laws, usually expressed as power-law relations between population size and aggregate urban quantities related to infrastructure, socioeconomic activity, or environmental impacts. These laws are influential because their exponent is often interpreted as describing what happens when a city grows, with implications for urban theory, planning, and policy. Here, we show that this interpretation is generally misleading. An exponent measured by comparing many cities at one date does not, in general, describe the trajectory of any individual city. Instead, it reflects a statistical pattern produced by cities with different histories, constraints, institutions, and growth paths. Apparent sublinear or superlinear scaling can therefore arise even when individual cities follow simpler dynamics, as we show for the area–population relation. Cross-sectional scaling laws can reveal system-level regularities, but should not be used alone to infer growth mechanisms or guide policy for a given city.
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Materials Science
NLSTEM: Non-local denoising for enhanced 4D-STEM pattern indexing
Yichen Yang, Olivier Pierron, Josh Kacher et al. (+1 more)
4D-STEM-based orientation and phase mapping has enabled rapid microstructure quantification that can be directly combined with standard TEM- and STEM-based imaging modes. Typically, orientation mapping is coupled with beam precession (i.e. precession electron diffraction) to achieve high indexing rates, adding to the cost and often decreasing the spatial resolution of the approach. This paper introduces a new post processing approach modeled after the non-local pattern averaging and reindexing
algorithm developed for the electron backscatter diffraction community, wherein post-collection, patterns are averaged using a distance similarity parameter. Results from Ni and Au thin films show that indexing rates can be significantly improved using this post-processing technique due to improved signal-to-noise ratios in the diffraction patterns. Interestingly, the highest indexing rates are achieved in samples heavily damaged via ion irradiation, suggesting that averaging over curved
lattices further improves indexing rates.
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All authors Yichen Yang, Olivier Pierron, Josh Kacher, David Rowenhorst
Disordered Systems and Neural Networks
Pattern Expansion of Spin Glasses
Mutian Shen, Zohar Nussinov and Yang-Yu Liu
We introduce a systematic method for expanding general
spin-glass
Hamiltonians in terms of Mattis interactions, providing a novel perspective for understanding the fundamental differences between short-range Edwards-Anderson (EA) and mean-field Sherrington-Kirkpatrick (SK)
spin glasses. By iteratively extracting patterns from the coupling
matrix, we expand the original
spin-glass system into a Hopfield-like model (a series of Mattis interactions) plus a residual system. Our analysis reveals profound distinctions between EA and SK models: while EA models in two and three dimensions break into isolated subconnected sections after expansion, the SK model exhibits remarkable self-similar behavior, with the residual system preserving the mean-field structure and Gaussian statistics throughout the expansion process. This self-similarity manifests in exponential decay of residual
matrix norms and expansion coefficients, reflecting the inherent mean-field nature of the SK model. Furthermore, we demonstrate that pattern expansion can identify ultra-low energy excitations in EA models, revealing excitations with energies that decrease rapidly with expansion step. Through connected component analysis, we quantify the size-energy relationship of these independent excitation clusters, opening new avenues for understanding the low-energy landscape of
spin glasses and providing insights into the nature of metastable states.
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Mesoscale and Nanoscale Physics · Cross-listed from physics.app-ph
Enhanced nanocomposite susceptibility by field-alignment of superparamagnetic particles
Mathias Zambach, Miriam Varón, Thomas Veile et al. (+7 more)
Nanocomposites comprised of insulated
magnetic single-domain particles are promising candidates for high-frequency, eddy current free, soft
magnetic materials, but tend to suffer from low
magnetic susceptibility ($<20$). Particle
alignment has been proposed to increase nanocomposite susceptibility and reduce
magnetic losses but experimental verification has been lacking. Here,
magnetic nanocomposites containing 3-57 vol% field-aligned 11$\pm$3 nm maghemite particles in a poly-vinyl
matrix were investigated for potential use as high-frequency inductor core materials. The particles were aligned by a homogenous static
alignment field during nanocomposite drying, fixating the particle orientation. Particle aggregation was disproved by small-angle scattering. The dependence of the
alignment field strength and particle concentration on the nanocomposite's susceptibility and hysteresis losses were investigated from DC up to 922 kHz by vibrating sample magnetometry, AC-susceptibility and high-frequency hysteresis measurements. Nanocomposite susceptibility increased super-linearly with particle fraction due to weak particle interactions.
Alignment of the particles increased the nanocomposite susceptibility from 21 to 50 for samples with a particle content of 57 vol%. Hence, the synergy between particle
alignment and interaction allows for a higher than expected susceptibility of nanocomposites. The results show that magnetically aligning particles in a nanocomposite reduces
magnetic losses when using well-dispersed single-domain superparamagnetic nanoparticles. Measured nanocomposite susceptibility could be modelled by a combination of directional dependent Debye-models including mean-field interaction effects and partial particle
alignment. Measured susceptibility of 50 is among the highest obtained for nanocomposites, making it a relevant candidate for applications in power electronics.
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All authors Mathias Zambach, Miriam Varón, Thomas Veile, Matti Knaapila, László Almásy, Tomás S. Plivelic, Christer Johansson, Ziwei Ouyang, Marco Beleggia, Cathrine Frandsen
Mesoscale and Nanoscale Physics
Negative Electronic Friction and Non-Markovianity in Nonequilibrium Quantum Systems
Riley J. Preston, Samuel L. Rudge, Daniel S. Kosov et al. (+1 more)
We address the connection between negative electronic friction and non-Markovian effects in the nonadiabatic vibrational dynamics of molecules interacting with metal surfaces under nonequilibrium conditions. We show that a generic nonequilibrium mechanism leading to negative Markovian electronic friction, where molecular vibrations couple directly to inelastic electronic transitions, also introduces significant non-Markovian contributions to the electronic friction. To demonstrate these ideas, we investigate nonequilibrium charge transport through a molecular nanojunction containing a vibrationally coupled donor-acceptor model, where negative electronic friction reflects driving of the vibrational mode beyond standard Joule heating. By comparison to numerically exact, fully
quantum hierarchical equations of motion simulations, we verify that these non-Markovian effects have a significant impact on the nonequilibrium dynamics and even on the stability of the resulting Langevin equation.
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All authors Riley J. Preston, Samuel L. Rudge, Daniel S. Kosov, Michael Thoss