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Physics Condensed Matter

cond-mat Watch Condensed Matter

Tuesday 13 October · still arriving

Materials Science

Polarons in Lead-Halide Perovskites: What the Structural Evidence Does and Does Not Establish

Inhee Maeng, Young Mi Lee, Jinwoo Park et al. (+2 more)

Photogenerated carriers in lead-halide perovskites are widely believed to move as large polarons, whose accompanying lattice distortion is thought to screen the carrier from charged defects. Three recent findings call this picture into question. First, time-resolved structural probes now measure the photoinduced lattice distortion directly; yet calculations show that thermal motion of the lattice alone can raise the carrier effective mass-an increase previously taken as evidence for polarons-so that it remains unclear whether the lattice distortion around a carrier is really a polaron. Second, the carrier-lattice coupling strengthens as the material is thinned from 3D to 2D. In the two main families of 2D perovskites, Ruddlesden-Popper and Dion-Jacobson, the carrier couples to the lattice through different channels, and their contrasting behavior is set by which coupling channel dominates rather than by lattice stiffness alone. Third, the first direct test of defect tolerance does not point to polaron screening; its authors attribute defect tolerance to the soft, disordered lattice. New transport data show that polaron screening reduces scattering between carriers, but not that it protects carriers from defects. Polaron formation, even if established, therefore does not establish polarons as the cause of defect tolerance. Separating the two requires measuring the lattice distortion as a function of excitation density, over a range spanning the low densities of a solar cell under sunlight and the much higher densities used in most ultrafast experiments. On this basis, we propose experiments that can clarify how photogenerated carriers behave in lead-halide perovskites.
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All authors Inhee Maeng, Young Mi Lee, Jinwoo Park, Seung Jae Oh, Min-Cherl Jung

Materials Science

Learning from fracture: Teaching language models to design through physical experience

Bo Ni

How can language models learn to optimize physical designs through experience? We introduce FracLearn, an experience-abstraction-generalization framework evaluated through a deliberately unfamiliar, Tetris-inspired fracture game. The model first proposes architectures and examines simulated fracture responses to accumulate experience. Abstraction organizes these observations into conditional principles linking reinforcement placement, crack evolution and load transfer, with supporting evidence and failure conditions. Generalization challenges the model to combine these principles in new architectures and surpass the earlier design record. Campaigns demonstrate progressive refinement and recovery through hypothesis revision, yielding a best composite score 11.0% above the experience-stage record. Fracture analyses connect improved performance to crack deflection and temporary trapping as toughening mechanisms, alongside greater residual load capacity. Learning occurs through external knowledge and decisions, without model-weight updates. This approach offers a route to scientific agents that turn computational experiments into reusable physical knowledge for materials discovery and engineering design.
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Statistical Mechanics

Proof of the exact correlation-length exponent $\nu = \frac{\sqrt[3]{2}}{2}$ and algebraic relations for $\beta$ and $\gamma$ in the 3D Ising model

Aleksandr Liivand

Based on our previous results regarding critical exponent estimates for the 3D Ising model and scaling relations [1], we have proven that the exact value of the correlation-length exponent is $\frac{\sqrt[3]{2}}{2}$. This is in very close agreement with the most precise numerical estimates currently available. Using a group theoretical approach and some propositions of constructive geometry, we derived an exact construction of algebraic expressions for the critical exponents $\gamma$ and $\beta$. We proved that these expressions depend solely on the exact value of $\nu\,$.
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Quantum Gases

Skin phase transition of a skyrmion in a ferromagnetic superfluid

Wooyoung Yun, Se Kwon Kim and Hiromitsu Takeuchi

Compared with a scalar Bose-Einstein condensate (BEC), a spinor BEC supports richer phenomena because of its internal spin degrees of freedom, including topological solitons such as domain walls or skyrmions. Here, we theoretically reveal three distinct axisymmetric soliton structures formed as a circular domain wall in a spin-1 BEC, connected within fixed winding numbers. We refer to these structures as the broken-axisymmetry (BA)-skin, antiferromagnetic (AF)-skin and normal-core phases. Their occurrence is controlled by the two dimensionless parameters in the absence of linear Zeeman coefficient $p=0$: $q/\mu$, the ratio of the quadratic Zeeman coefficient to the chemical potential, and $c_s/c_n$, the ratio of spin interaction to the density interaction. The BA-skin and AF-skin phases exhibit local easy-plane magnetic order and local easy-plane spin-nematic order, respectively, while the normal-core phase is similar to a doubly quantized vortex analogous to that of a scalar BEC with a density depletion at the center. We obtain semi-analytical estimates of selected transition boundaries separating these phases by analyzing the Bogoliubov-de Gennes equation and also numerically confirm them by solving the Gross-Pitaevskii equation. We estimate the skyrmion radius using a hydrostatic approximation and compare the result with numerical solutions. We envision our results on skyrmions in a spin-1 BEC can trigger further investigation on novel soliton structures in a magnetic superfluid.
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Materials Science

Hydrogen-Bond Density Drives the Order-Disorder Phase Transition from Orientational to Vibrational in Polyalcohol Plastic Crystals

Ares Sanuy, Carlos Escorihuela-Sayalero, Pol Lloveras et al. (+5 more)

Neopentane-derived plastic crystals undergo order-disorder phase transitions accompanied by exceptionally large entropy changes, making them attractive solid-state materials for thermal energy storage and barocaloric cooling. Yet it remains unclear whether the microscopic nature of this entropy change is an intrinsic property of each compound or can be understood generally and tuned systematically through molecular design. Here, we use classical molecular dynamics simulations of neopentyl glycol (NPG), pentaglycerine (PG), and pentaerythritol (PE), validated against experimental transition temperatures, enthalpy changes, and phase diagrams, to show that the character of the phase-transition entropy undergoes a complete inversion as the hydrogen-bond connectivity of the molecule increases: orientational disordering dominates in NPG, whereas lattice vibrations dominate in PE and become remarkably robust against pressure. PG occupies a balanced intermediate regime in which the orientational, conformational, and vibrational channels contribute comparably. This inversion reflects two coupled microscopic mechanisms: a growing low-frequency gap between the phonon densities of states of the ordered and disordered phases as the hydrogen-bond network densifies, and an increase in molecular symmetry that progressively suppresses orientational disordering. Because the vibrational channel is substantially less sensitive to pressure than the orientational one, hydrogen-bond density emerges as a practical, chemically accessible design handle for tailoring not only the magnitude but also the pressure robustness of the caloric response in hydrogen-bonded plastic crystals, with direct implications for the search for new barocaloric and thermal-energy-storage materials.
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All authors Ares Sanuy, Carlos Escorihuela-Sayalero, Pol Lloveras, María del Barrio, Hassan Allouchi, Josep-Lluís Tamarit, Luis Carlos Pardo, Claudio Cazorla

Mesoscale and Nanoscale Physics

A Full-Spectrum Theory of Moiré Superlattices

Tong Liu, X. R. Wang and Jiansheng Wu

Conventional theoretical descriptions of moiré superlattices, epitomized by the Bistritzer–MacDonald continuum model, are fundamentally constrained by low-energy Dirac cone approximations and first-shell interlayer coupling truncations. While successful for small-angle twisted bilayer graphene near charge neutrality, these frameworks fail to provide universal analytical solutions for strained lattices, high-energy band behaviors, or arbitrary commensurate geometries. Here, we establish a unified, approximation-free analytical formalism for generalized moiré systems based on three pillars: (i) a rigorous two-dimensional-to-one-dimensional supercell cyclic mapping, (ii) exact block-circulant diagonalization of intralayer Hamiltonians, and (iii) quasi-degenerate Löwdin perturbation theory centered on arbitrary energy degeneracy points across the full spectrum. Unlike traditional schemes anchored to the Dirac point, our approach enables systematic perturbative expansions around any band crossing, unifying the characterization of low-energy flat bands with high-energy degeneracies. We demonstrate that this framework perfectly recovers the Bistritzer–MacDonald model in the low-energy limit while extending its reach to heterostrained systems and high-energy regimes. By incorporating high-order interlayer coupling shells, we reveal an intrinsic scale self-similarity in moiré lattices and predict the emergence of higher-order magic angles and tunable flat bands driven by secondary degeneracies. This work eliminates the long-standing lack of universal analytical tools for diverse moiré platforms, providing a full-spectrum theoretical foundation for band engineering and the exploration of exotic correlated and topological states in van der Waals heterostructures.
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Disordered Systems and Neural Networks · Cross-listed from math.PR

Sampling from the Sherrington-Kirkpatrick Model up to Criticality

Hang Du, Brice Huang, Sidhanth Mohanty et al. (+1 more)

We prove that the simulated annealing algorithm efficiently samples from the Sherrington-Kirkpatrick model at inverse temperature $\beta < 1$ to an arbitrary inverse polynomial accuracy in total variation.
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All authors Hang Du, Brice Huang, Sidhanth Mohanty, Amit Rajaraman

Superconductivity

Observation of stacked vortex structure with number controllable Majorana zero mode in Bi$_{2}$Te$_{3}$/FeTe heterojunction

Xinru Han, Zhaokun Luo, Quanxin Hu et al. (+16 more)

Creation and control of Majorana zero modes (MZMs) in solid devices is the key challenge in realizing fault tolerant topological quantum computation. Here we report spectroscopic evidence for number controllable MZMs in stoichiometric one quintuple layer Bi$_{2}$Te$_{3}$/FeTe heterostructures, together with the observation of stacked vortices in superconducting islands. Low temperature scanning tunnelling microscopy and spectroscopy reveal a long period moire superlattice arising from lattice mismatch, which induces nanoscale modulation of superconducting gaps. Magnetic vortices exhibit a stacked core structure with two layer derived bound state branches: a dispersive branch evolving into an X-shaped pattern away from the vortex center, and a quantum-limit branch characterized by a spatially unsplit MZMs together with discrete integer quantized Caroli-de Gennes-Matricon states. Notably, MZMs are observed in all measured vortices, and their number can be tuned by magnetic field and the size of Bi$_{2}$Te$_{3}$ islands. These results establish stoichiometric 1QL-Bi$_{2}$Te$_{3}$/FeTe as a patterned superconducting interface that enables isolated and few-vortex Majorana configurations, providing a controlled platform for fuse MZMs.
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All authors Xinru Han, Zhaokun Luo, Quanxin Hu, Xingjian Zhu, Xiaohong Pan, Dan Mu, Xinyu Liang, Zulin Xu, Qiheng Wang, Ryan L. Kong, Mingyuan Li, Zhengliang Zhang, Fazhi Yang, Mojun Pan, Xin Liu, Qiangqiang Gu, Baiqing Lv, Gan Wang, Hong Ding

Materials Science

A Parameter-Free Reference Hamiltonian for Tight-Binding Vanadium Oxides

Jun-qiang Lu

Density-functional tight binding has yet to yield a transferable description of vanadium oxides; for the standard second-order construction and the tested cells, we demonstrate that two of its approximations deviate by electron volts and offset each other in the band-index centroid separation between the V 3d and O 2p bands: the neutral two-centre reference omits electron-volt environmental terms, whereas the Mulliken charge model assigns the compact V 3d shell less than half of the self-consistent potential shift observed in PBE. Instead, we build a tight-binding model where the reference Hamiltonian derives from superposed neutral atoms, the full PBE exchange-correlation potential and frozen-core orthogonality, without adjustable terms, and where the orbital-resolved charge response employs three screening factors obtained from PBE linear response and an unscreened Coulomb range. Relative to the primary PAW reference, the plane-wave Harris-Foulkes energy differences of VO2 are matched by the reference energy at fixed geometry and the stated numerical settings, with signed deviations of +2.6 meV/atom for M1 relative to rutile (an energy difference of 92 meV/atom) and +6.2 and -5.2 meV/atom under 3% volume compression and expansion; the charge response reproduces 14 of 22 fitted and 5 of 8 held-out matched PBE responses within one tolerance, and 28 of the 30 within twice the tolerance, and in rutile the fitted first-radial V 3d shift is -4.39 eV, 0.47 eV more negative than PBE's -3.92 eV, whereas the tested DFTB2 charge model gives -1.53 eV. The convexity of the spinless model at fixed geometry prevents it from spontaneously differentiating symmetry-equivalent sites under the symmetries respected by its implemented operators and sampling, so within this model such differentiation in the Magnéli phases must arise from electron-lattice coupling or an explicit spin or correlation term.
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Materials Science

A grand potential for molten-salt electrochemical phase diagrams from first principles

Lorenzo Bastonero, Lucio Colombi Ciacchi and Nicola Marzari

The clean-energy transition is driving unprecedented demand for metals, whose extraction is among the largest industrial sources of carbon emissions. Molten-salt electrolysis offers a path toward carbon-free production of aluminum, iron, titanium, and critical metals. However, corrosive operating conditions and complex, multicomponent feedstocks make experimental development costly and challenging. Here, we introduce a first-principles framework for constructing electrochemical phase diagrams of multicomponent systems to accelerate electrode and electrolyte design and guide the selection of compatible materials and operating conditions for electrochemical cells. We derive a grand potential that generates these diagrams at scale, and supply free energies using first-principles data obtained through a fine-tuned foundation machine-learned interatomic potential. For silicon in molten CaCl$_2$, the construction not only recovers the experimental diagram when using the first-principles framework on a limited set of structures, but also predicts further domains when extended to a larger set of crystal structures. Finally, we demonstrate the predictive power of the approach by screening ten alkali and alkaline-earth halide electrolytes for aluminum electrowinning. Three thermodynamic criteria identify NaF, where cryolite emerges as the stable aluminum-bearing phase, recovering the chemistry that Hall and Héroult established empirically in 1886. The framework provides a first-principles basis for selecting compatible electrodes, electrolytes, and operating conditions toward low-carbon metal production from complex source materials.
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Materials Science · Cross-listed from astro-ph.EP

Elastic Properties of Titan Tholins Revealed by Brillouin Spectroscopy

Mark A. Johnson, Jose R. Montes-Bojorquez, Adis Husic et al. (+7 more)

Titan tholins, complex organic solids synthesized by simulating photochemical processes in Titan's atmosphere, are widely regarded as analogs to organic surface materials on the Saturnian moon. Their mechanical properties are critical for understanding the evolution of Titan's geomorphology and crustal layering. Here, we report the first direct experimental determination of the complete set of aggregate elastic moduli of tholins using Brillouin spectroscopy. Measurements conducted on tholin powder samples yielded a bulk modulus of 11 (2) GPa and a shear modulus of 3.9 (5) GPa, from which we derive a Young's modulus of 11 (1) GPa and a Poisson's ratio of 0.34 (3). Thin-film samples synthesized under slightly different conditions yield similar results. Compared with other Titan-relevant organic compounds, tholins exhibit the highest elastic stiffness measured to date, exceeding even H2O ice in both bulk and shear moduli. These results support the interpretation that complex organics contribute significantly to the mechanical stability and transport behavior of Titan's equatorial dune sediments. Measurements following prolonged atmospheric exposure reveal persistent stiffening of tholins that remains after vacuum treatment, likely associated with minor chemical modification and physical reorganization, highlighting the importance of controlled sample handling for future laboratory investigations. This study also demonstrates the effectiveness of Brillouin spectroscopy as a non-destructive technique for characterizing planetary organic materials and provides new constraints relevant to future seismic and geophysical investigations by the Dragonfly mission.
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All authors Mark A. Johnson, Jose R. Montes-Bojorquez, Adis Husic, Xinting Yu, Wenhao Su, Jia-Sen Hu, Ryan Blase, Edward Patrick, Alan Whittington, Jin S. Zhang

Materials Science

Electric Edelstein Effect in Two Dimensions

Xiaoran Lv, Tianqi Zhang, Chaoxi Cui et al. (+6 more)

We propose the electric Edelstein effect (EEE) in two-dimensional (2D) systems, where crossed out-of-plane and in-plane electric fields generate a spin polarization. The gate field converts the Edelstein response from a material-fixed spin polarization into a gate-tunable one. A symmetry analysis over all magnetic layer point groups shows that the EEE is allowed in a wide range of magnetic and nonmagnetic systems, including ferromagnets, antiferromagnets, and altermagnets. The EEE plays two distinct roles: it generates spin polarization where the linear Edelstein effect (EE) is symmetry-forbidden, as in centrosymmetric materials; and it adds a new gate-tunable component, generally non-collinear with the conventional one, where the EE is already present. The two fields then act as independent knobs and raise the control dimension of the spin polarization from one to two. Our microscopic theory traces the EEE to band-geometric quantities, which are strongly enhanced near small band gaps. First-principles calculations predict pronounced effects in the centrosymmetric ferromagnet MnSe2, where the linear EE is symmetry-forbidden. In the nonmagnet SnSe where the EE and EEE coexist, the two knobs cover an extended region of the spin sphere. Our findings establish the EEE as a symmetry-guided, gate-tunable route to electrical spin manipulation and open avenues for multistate spintronics in 2D quantum materials.
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All authors Xiaoran Lv, Tianqi Zhang, Chaoxi Cui, Yaling Li, Yi Shen, Weikang Wu, Ziming Zhu, Yugui Yao, Zhi-Ming Yu

Monday 12 October

Mesoscale and Nanoscale Physics · Cross-listed from physics.app-ph

Supercell-enabled inverse design of boundary-state dispersion for group-velocity control in pseudomagnetic phononic crystals

Hua-Yang Chen, Nan-Xin Yu, Zhen-Hui Qin et al. (+2 more)

Topological boundary states offer a platform for guided-wave propagation within bulk bandgaps. Their propagation characteristics depend not only on the spectral extent of the states but also on their dispersion within the operating bandwidth. A nearly linear dispersion provides more uniform group velocity across frequency, whereas a reduced slope enables slow-wave propagation. Despite advances in inverse design of prescribed dispersion relations and topological dispersion tuning, application-oriented characteristics of boundary-state dispersion have rarely been direct inverse-design objectives. Here, we develop a supercell-enabled, surrogate-assisted framework for the inverse design of boundary-state dispersion in a phononic crystal with pseudomagnetic fields. Area-preserving anisotropic deformation shifts the local Dirac points, and independent variation of 21 rows within the supercell provides distributed control over the boundary-state branch. A constraint-preserving surrogate reconstructs the dispersion from an initial frequency and positive frequency increments, with first- and second-difference losses introduced to capture local slope and curvature. Coupled with a genetic algorithm, the framework targets two prescribed-bandwidth design tasks: highly linear dispersion and slow-wave dispersion. For the former, finite-element verification yields relative group-velocity variations of 0.142% to 1.183% over prescribed bandwidths of 4% to 6%. For slow-wave designs, mean group velocities of 190 to 385 m/s are obtained over bandwidths of 0.2% to 0.8% while satisfying a 5% group-velocity variation limit. These results establish boundary-state dispersion as a direct inverse-design target beyond conventional bandwidth optimization, opening a flexible route to group-velocity control through application-oriented dispersion engineering over prescribed relative bandwidths.
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All authors Hua-Yang Chen, Nan-Xin Yu, Zhen-Hui Qin, Yan-Feng Chen, Si-Yuan Yu

Mesoscale and Nanoscale Physics

Energy levels of screened charged impurity in gapped graphene under magnetic field

Andrii Iurov and V.P. Gusynin

In this paper, we present a comprehensive theoretical study of the electronic states in gapped graphene in the presence of a charge impurity and a magnetic field. We focus on the impurity-induced modification of the Landau levels under the influence of an attractive or repulsive Coulomb potential. Since analytical solutions of the Dirac equation are not available in this case, the study is carried out using the Wentzel-Kramers-Brillouin (WKB) approach. Special attention is paid to the screening of the Coulomb potential in the presence of metallic gates situated at some distances from graphene sheet, as is commonly the case in experiments. Using both analytical and numerical methods, we determine the energy spectrum as a function of the magnetic field strength and impurity charge for screened and unscreened Coulomb potentials. Our results reveal a significant dependence of the energy levels for sample-to-gate distances of less than 50 nm.
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Mesoscale and Nanoscale Physics · Cross-listed from gr-qc

Mathematical framework for simulation of non-perturbative cavity optomechanics with plate-like dielectrics

Andrew Laeuger and Nancy Aggarwal

Cavity optomechanical models typically treat a sensor either as a perturbative polarizable object in a pre-existing optical field, or solve the field at a fixed equilibrium and linearize in a fixed optical-mode basis. Neither self-consistently determines the spatially resolved cavity field, the optical force, the mechanical equilibrium, and the back-action when a finite-thickness dielectric plate intercepts the entire optical mode and restructures the intracavity field. In this companion paper to [1], we develop that treatment by extending the vacuum two-photon formalism to finite-thickness planar dielectrics, incorporating their Fresnel reflection, internal propagation, optical forces, and mechanical back-action directly into the field equations. We derive the carrier and sideband transfer functions needed to describe the coupled optical-mechanical system, including optomechanical cooling and the response to external perturbations. The result is a systematic starting point for calculating detector response and noise budgets for cavity experiments with intracavity dielectrics, including gravitational wave detectors deploying levitated sensors.
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Mesoscale and Nanoscale Physics · Cross-listed from quant-ph

Imaging the topological sign of a Dirac mass with spin-polarized tunneling

Payman Kazemikhah, Ruolun Zhang, Byunghoon Kim et al. (+5 more)

A Dirac mass is characterized not only by its magnitude but also by its sign, which influences the Berry curvature, Hall response, and the propagation direction of chiral boundary modes. However, local spectroscopy of magnetic topological surfaces has so far accessed only the gap magnitude $|M|$. Here we show how the topological sign of $M$ can be determined locally. We first establish the sign information that spin-resolved tunneling can access and the reference required to obtain an absolute Hall sign. Although spin–orbital entanglement generally obscures the relation between measured spin and Dirac pseudospin, crystalline rotation symmetry alleviates this ambiguity at the band edge. On a $C_3$ axis, an axial tip couples to a fully spin-polarized state whose tunneling asymmetry directly reveals the sign of the occupied-band Berry curvature, independent of orbital composition. We further show that the zeroth Landau level provides an independent mass-sign measurement with an unpolarized tip, while opposite domains and their chiral domain-wall modes provide self-consistency checks. These results pave the way for imaging the locations where the topological character changes within a material, rather than merely identifying where a spectral gap appears.
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All authors Payman Kazemikhah, Ruolun Zhang, Byunghoon Kim, Xiangyu Luo, Zachary Ducorsky, Yu-Peng Zhu, Md. Shafayat Hossain, M. Zahid Hasan

Materials Science

Crystal growth, magnetic, and transport properties of the candidate altermagnet Cr$_{1/4}$NbS$_{2}$

Nashra Pistawala, Martin R. Lees, Geetha Balakrishnan et al. (+1 more)

Altermagnetism is a rapidly emerging paradigm in condensed matter magnetism, yet the number of experimentally confirmed altermagnetic candidates remains remarkably limited. Cr$_{1/4}$NbS$_2$ has long been theoretically proposed as a promising altermagnet, but there are no reports on experimental investigation of single crystals. Here, we report the successful growth of high-quality single crystals of Cr$_{1/4}$NbS$_2$ and present its structural, thermal, magnetic, and transport properties. Single-crystal X-ray diffraction provides detailed crystallographic information and confirms the hexagonal crystal symmetry ($P6_3/mmc$), one of the criteria for altermagnetic behavior. Resistivity and heat capacity measurements unambiguously identify an antiferromagnetic Néel temperature of $T_N \sim 141$ K. Magnetotransport measurements reveal non-trivial features in the magnetoresistance and Hall response, indicative of quantum diffusive transport phenomena. Cr$_{1/4}$NbS$_2$, synthesized here for the first time in single-crystalline form, therefore serves as an experimental platform for probing the rich spectrum of phenomena predicted for altermagnetic materials.
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All authors Nashra Pistawala, Martin R. Lees, Geetha Balakrishnan, Daniel A. Mayoh

Superconductivity · Cross-listed from hep-th

Type I-II/II-I vortices from dimension-six self-interaction

A. Alonso-Izquierdo, A. González-Parra, K. Slawinska et al. (+2 more)

We show that adding a dimension-six self-interaction to the Abelian-Higgs or Ginzburg Landau model results in a remarkable and rich structure of multi-vortex solutions. Besides the standard type I (attractive) and type II (repulsive) regimes, separated by the Bogomol'nyi-Prasad-Sommerfeld limit (BPS), we identify two new possibilities: type I-II vortices, which attract at short distances and repel at long distances; and type II-I vortices, with the opposite behaviour. Specifically, within the type II-I regime, multi-vortex solutions form chunks of the Abrikosov lattice or clusters. Conversely, in the I-II regime, axially symmetric multi-vortices are stable; nevertheless, depending on the model parameters and topological charge (winding number), they can be either the ground state or not.
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All authors A. Alonso-Izquierdo, A. González-Parra, K. Slawinska, A. Wereszczynski, T. Winyard

Materials Science

High-Resolution X-ray Spectro-Microscopy of Au Microcrystallites

Chaitali Sow, Martin Seyrich, Johannes Hagemann et al. (+4 more)

Understanding the functionalities of heterogeneous catalysts requires high-resolution nanoscale chemical imaging tools. X-ray spectro-ptychography offers this capability by enabling X-ray absorption near edge structure (XANES) based amplitude and phase contrasts imaging with high spatial resolution. However, such demonstrations at energies above 10 keV, such as the Au L-edges, remain limited due to weak X-ray absorption cross-sections and correspondingly, low signal levels. Here, multimodal spectro-microscopy has been performed on Au microcrystallites that host both conventional face-centered cubic (fcc) and unconventional non-cubic lattices, particularly important for catalysis. Amplitude and phase contrasts across the $\mathrm{L}_3$ edge are resolved on thin Au crystallites (150 nm to 1.71 $\mu\mathrm{m}$) with 45 nm spatial resolution. The resulting XANES spectra exhibit excellent agreement with standard transmission and fluorescence contrasts, while providing complementary chemical insights with improved spatial resolution. Quantitative retrieval of anomalous phase variations ~12 keV further demonstrates the suitability of this method for high-resolution chemical imaging across a wide range of functional materials.
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All authors Chaitali Sow, Martin Seyrich, Johannes Hagemann, Giridhar U. Kulkarni, Thomas F. Keller, Christian G. Schroer, Andreas Schropp

Materials Science

Collision Cascade Morphology and Defect Nucleation in Pure Tungsten and an Equimolar Refractory W-Ta-Cr-V Alloy

Mark Fedorov, Francisco Javier Dominguez-Gutierrez, Damian Kalita et al. (+1 more)

Collision cascades and defect-formation mechanisms in pure W and the WTaCrV refractory high-entropy alloy were investigated using molecular dynamics simulations over primary knock-on atom (PKA) energies of 0.1-100 keV. Machine-learning tabulated Gaussian approximation potentials (tabGAP), complemented by short-range Ziegler-Biersack-Littmark (ZBL) interactions, were employed. Analysis of the heat spike lifetime, affected region, atomic relocations and surviving defects reveals an importance of subcascade formation and presence of light elements in the alloy. Cascade splitting into subcascades contributes to the distribution of the PKA energy over a larger region in the material, nucleating less dislocations and smaller voids - latter being present only in W and absent in WTaCrV. Since cascade splitting is more frequently observed in WTaCrV, the alloy shows better resistance to primary irradiation damage compared to pure W. Apart from cascade splitting, analysis of interstitials shows that Cr and to lesser extent V, being lighter and smaller elements, accommodate the damage in form of point defects, contributing to the mitigation of formation of the dislocation loops.
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All authors Mark Fedorov, Francisco Javier Dominguez-Gutierrez, Damian Kalita, Łukasz Kurpaska

Soft Condensed Matter

Meso-scale modeling of cohesive cavitation in polymer networks

Saleh Assadi, Zachary T. White, Fay Salmon et al. (+6 more)

Cavitation influences the failure of elastomers and soft adhesives, but connecting its microscopic initiation to subsequent cavity growth remains challenging. Here, we develop a discrete mesoscale framework that combines finitely extensible, breakable strands with local van der Waals interactions to examine how cohesion and network heterogeneity transform distributed dilation into localized damage. In the simulations, local cohesive instability produces a network-scale precursor whose subsequent expansion is coupled to strand stretching and rupture. Cohesive strength and intrinsic defect size govern the critical hydrostatic tension, while cohesion also controls the number and spatial distribution of cavities. Under confined loading, decreasing the layer thickness promotes multiple cavities and distributed strand rupture, extending the load-bearing response and increasing the critical work density of rupture. An energetic interpretation based on the thickness dependence of the crack-driving force connects these observations to competition between additional cavity nucleation and propagation of existing damage. The results link local cohesive instability to the collective evolution of cavities and rupture in disordered polymer networks.
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All authors Saleh Assadi, Zachary T. White, Fay Salmon, Nitin Hansoge, Zhuonan Liu, Victor Crespo-Cuevas, Jason Clapper, Nikolaos Bouklas, Franck J. Vernerey

Materials Science

Can Universal Machine-Learning Interatomic Potentials Capture Small Polarons? A Geometry-Based Benchmark in Transition-Metal Oxides

Vladislav Korostelev, Pjotrs Žguns and Konstantin Klyukin

Small polarons, formed when excess electrons become localized through lattice distortions, play a critical role in charge transport, catalysis, and degradation processes in transition-metal (TM) oxides. Whether universal machine-learning interatomic potentials (uMLIPs), which lack explicit electronic degrees of freedom, can describe them remains unclear. Here, we investigate H-induced electron localization across a range of TM oxides using a geometric descriptor based on TM-O bond-length expansion (Z-score) to identify polaronic distortions. We find that uMLIPs exhibit varying degrees of electron localization, from delocalized states to metastable small polarons, with their behavior strongly dependent on the training data and treatment of electronic correlations. In particular, the absence of Hubbard U corrections for specific TM cations in the training data leads to electron delocalization, in contrast to DFT+U predictions. Although some uMLIPs reproduce polaron hopping barriers during coupled proton-polaron transfer, all of them fail to stabilize spatially separated polaron-donor configurations, instead favoring localization near the H dopant.
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Disordered Systems and Neural Networks · Cross-listed from cs.LG

On the Statistical Mechanics of Attention

Jerry Yao-Chieh Hu, Yu-An Chen, Maojiang Su et al. (+1 more)

We revisit the statistical mechanics of Transformer attention via modern Hopfield models. We present a free-energy formalism to characterize parameter regimes and critical thresholds for robust attention inference. The resulting free-energy landscape incorporates randomness in query states and attention weights. Attention then emerges as the equation of motion of the corresponding statistical-mechanical system (i.e., modern Hopfield model) through the standard free-energy minimization principle. This allows us to identify first-order phase transitions in attention between glassy and robust-inference phases. We further extend to attentions with stochastic queries and keys. Together, our results provide a rigorous statistical-mechanics framework for characterizing phase structure and inference reliability across a family of Transformer attention mechanisms.
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All authors Jerry Yao-Chieh Hu, Yu-An Chen, Maojiang Su, Han Liu

Soft Condensed Matter

Machine-learned topological transport of magnetic colloids

Sebastian Wohlrab, Aneena Rinu Perayil, Piotr Kuświk et al. (+6 more)

By modulating the orientation of a uniform magnetic field it is possible to transport magnetic microparticles above magnetic patterns. In periodic patterns, driving control loops that wind around special orientations, which are dictated by the symmetry of the pattern, induce transport by one unit cell. The transport is topologically protected with trajectories controlled by the winding numbers of the control loop around the special orientations. In non-periodic patterns, the special orientations are position dependent, precluding a global characterization of the transport using a small set of winding numbers. We use here a reinforcement learning agent trained with Brownian dynamics simulation data that uses the local environment of the magnetic pattern to decide an action over the control loop. The agent autonomously discovers topological transport protocols directly from the particle response without prior characterization of the magnetic potential landscape. An agent trained solely on a complex non-periodic pattern directly generalizes to completely unseen periodic, quasicrystalline, and disordered substrates. Moreover, the agent dynamically adjusts the control loop to transport particles across lattice defects and to counterbalance external drift forces that would cause static topological loops to fail. By adding a pathfinding algorithm to the model, we additionally achieve simultaneous, independent transport of multiple identical colloids along different on demand trajectories. We demonstrate that the control loops can be directly transferred to an experimental setup to successfully drive magnetic microparticles across a non-uniform magnetic thin film.
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All authors Sebastian Wohlrab, Aneena Rinu Perayil, Piotr Kuświk, Maciej Urbaniak, Feliks Stobiecki, Arne J. Vereijken, Arno Ehresmann, Thomas M. Fischer, Daniel de las Heras

Materials Science · Cross-listed from physics.app-ph

Progress in Cavitated Front Gridline Ag Paste for PERC Solar Cell Architecture

Donald Intal, Sandra Huneycutt, Abasifreke Ebong et al. (+2 more)

This paper reports on the progress made with the cavitated silver paste on passivated emitter rear contact cells (PERC) to reduce the series resistance. Thus, electrical and microscopy characterizations are used to examine these resistive losses. Fifteen 252 cm2 PERC cells were fabricated for this assessment. Under standard test conditions, these cells achieved a mean conversion efficiency of 21.87% and a maximum of 22.07%. The mean measured fill factor was 79.98%, compared with a pseudo-fill factor of 83.60%, giving a mean deficit of 3.62 percentage points. Fill factor decreased with increasing area-normalized series resistance, which ranged from 0.53 to 0.83 ohm-cm2, while open-circuit voltage and short-circuit current remained unvaried. These electrical trends support resistive current extraction as the principal contributor to fill-factor variation. Electroluminescence imaging revealed localized emission nonuniformity. Cross-sectional scanning transmission electron microscopy showed variation in the glass interfacial layer, while scanning electron microscopy identified micrometer and sub micrometer silver particles. Conductive atomic force microscopy after sequential removal of the bulk metal and glass revealed spatially heterogeneous current response. These observations are consistent with nonuniform electrical access at the contact, where restricted conducting pathways may concentrate on current and increase local voltage drops. The combined characterization identifies interfacial heterogeneity and the connectivity of conducting pathways as priorities for understanding and reducing extraction losses in cavitated silver metallization.
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All authors Donald Intal, Sandra Huneycutt, Abasifreke Ebong, Dana Hankey, Marshall Tibbetts

Strongly Correlated Electrons · Cross-listed from quant-ph

Krylov Complexity and Quantum Many-body Scars in Non-Hermitian Kinetically Constrained Models

Arkaprava Sil, Rudraksh Kaushik, Souvik Kundu et al. (+2 more)

Thermalization in closed quantum systems and its breakdown are central to understanding the emergence of statistical behavior in isolated many-body systems. Kinetic constraints can hinder thermalization by restricting the allowed dynamics and giving rise to non-ergodic behavior. While various aspects of ergodicity breaking in kinetically constrained models have been extensively studied in Hermitian systems, their behavior in non-Hermitian kinetically constrained models remains less explored. In particular, the inequivalence of left and right eigenstate ensembles in non-Hermitian systems raises the question of whether they exhibit qualitatively different dynamical and eigenstate properties. In this work, we study non-Hermitian generalizations of the paradigmatic PXP and PPXPP models, focusing on their Krylov complexity, eigenstate structure, and dynamics. By exploiting the Arnoldi's algorithm, we find distinct behavior of Krylov complexity in the left and right eigenstate ensembles as non-Hermiticity is increased: in the right ensemble, it exhibits persistent oscillations with an increasing period, while in the left ensemble, it gradually saturates. The characteristic arch-shaped profile of the Arnoldi coefficients is also modified differently in the two ensembles, with a Hermitian-like arch re-emerging in the left ensemble at large non Hermiticity. Additionally, in the non-Hermitian PXP model, the right ensemble retains the characteristic scar structure, while the left ensemble develops additional non-scar eigenstates with anomalously low entanglement and large overlap with the vacuum state. These findings reveal that non-Hermiticity can affect Krylov dynamics and eigenstate structure differently in the left and right ensembles, demonstrating the need to treat them separately when characterizing non-Hermitian kinetically constrained systems.
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All authors Arkaprava Sil, Rudraksh Kaushik, Souvik Kundu, Prasant Mallik, Sudipto Singha Roy

Materials Science

Evidence of an anisotropic Griffiths phase in a two-dimensional van der Waals magnet

Anupam K. Singh, Dhananjay Nair, Holger Meyerheim et al. (+2 more)

Over the last decade, the emerging field of two-dimensional van der Waals magnetism has focused mainly on long-range-ordered magnetic phases. Disordered magnetic phases, however, remain largely unexplored. Here, we report a highly anisotropic Griffiths phase driven by self-intercalation in the van der Waals magnet Fe5GeTe2. We find evidence of short-range-ordered ferromagnetic clusters embedded in a paramagnetic matrix, which dominate the magnetic response between the Curie temperature and a higher characteristic Griffiths temperature. An anomalous power-law divergence in the magnetic susceptibility, universal Lee Yang scaling, and distinct entropy anomalies provide further evidence for this phase. Strikingly, the non-analytic Griffiths response emerges exclusively under in-plane magnetic fields, whereas out-of-plane fields abruptly alter this behavior. This directional dependence indicates that the local magnetic interactions within nanoscale clusters are closely coupled to the layered lattice geometry. Our findings establish a regime of dimensionally constrained, disorder-driven magnetism and identify self-intercalated van der Waals magnets as a platform for investigating the interplay between magnetic disorder, anisotropy, and reduced dimensionality.
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All authors Anupam K. Singh, Dhananjay Nair, Holger Meyerheim, Stuart S. P. Parkin, Banabir Pal

Strongly Correlated Electrons

Quantum skyrmion molecule: two quantum skyrmions bound by coherent magnon exchange

Ricardo L. Silva

Quantum skyrmions in frustrated spin-$1/2$ magnets are bound states of a few flipped spins in a ferromagnetic background. Using exact diagonalization and density-matrix renormalization group calculations for the $J_1$–$J_2$ triangular-lattice model with easy-axis anisotropy $K$, we ask what happens when two such objects are brought together. The binding energy per flipped spin of a single skyrmion has a minimum at $\Nf^{*}=7$ for $K=0.02$–$0.04$, and skyrmions smaller than $\Nf^{*}$ do not exist as pairs: at any distance they coalesce into a single larger object, and pinning wells cannot prevent it. Two skyrmions of natural size, by contrast, form a bound molecule without pinning or external field, with binding energy $U\simeq-0.007J_1$ at $K=0.03$–$0.04$ and an equilibrium separation of $5.5$ lattice spacings that does not depend on $K$ or on the size of the simulation domain; at $K=0.02$, where the single skyrmion is itself marginally bound, the pair is unbound within our resolution. The two constituents keep their identity ($7+7$ flipped spins) and are coupled by a coherent exchange of magnons, visible as number fluctuations between the two halves of the domain in excess of those of two independent skyrmions and as in-plane spin correlations between the two skyrmion walls that reach $0.16$ across the gap. These correlations follow the vortex pattern of the two textures and lock their relative helicity, the quantum counterpart of the helicity-dependent interaction of classical skyrmions. We discuss the molecule as a self-assembled two-qubit element for skyrmion helicity qubits in a single layer.
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Strongly Correlated Electrons

Phase-Coherent Spin and Lattice Modulations in a Y-type Hexaferrite

Oskar Stepančić, Annika Stellhorn, Emma Campillo et al. (+7 more)

Spin-driven multiferroics owe their electric polarization to lattice distortions that follow the magnetic order, yet in incommensurate magnets symmetry normally forbids such a distortion at the fundamental magnetic wavevector, and the distortions themselves are rarely observed. Here we use half-polarized neutron scattering to show that the Y-type hexaferrite Ba$_{0.4}$Sr$_{1.6}$Mg$_2$Fe$_{12}$O$_{22}$ hosts, in its saturated high-field state, a magnetic modulation and a longitudinal lattice modulation at one and the same wavevector, locked in phase. The signature is a large nuclear-magnetic interference asymmetry, $r$ ~ 0.8, of the same sign at the two conjugate satellites and persisting to 275 K. The zone contrast and the field dependence of the satellite indicate an internal, field-induced longitudinal distortion. The modulated state coexists with the transverse-conical magnetoelectric phase and outlives it, revealing a fundamental-wavevector spin-lattice coupling in a magnetic field regime previously regarded as a simple ferrimagnet.
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All authors Oskar Stepančić, Annika Stellhorn, Emma Campillo, Kun Zhai, Niklas Ekström, A. Corani, D. Alba Venero, R. Cubitt, E. Blackburn, L. Shen

Mesoscale and Nanoscale Physics

Turning a superconducting diode into a Feynman ratchet to realize a nanoscale heat engine

Konstantinos Kontogeorgiou, İnanç Adagideli, Géraldine Haack et al. (+1 more)

Feynman's ratchet illustrates how work can be extracted from thermal fluctuations in the presence of two heat baths at different temperatures. While some experimental realizations of the ratchet effect have been studied, most lack miniaturization, which is important for some applications. Here, we propose a physical implementation of Feynman's ratchet based on Josephson junctions exhibiting the superconducting diode effect. This setup functions as a heat engine that generates work solely from a temperature gradient via thermal fluctuations. We analyze its output power and efficiency, and, through the Seebeck coefficient, identify the conditions under which the device operates in reverse, functioning instead as a Peltier cooler.
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All authors Konstantinos Kontogeorgiou, İnanç Adagideli, Géraldine Haack, Fabian Hassler