Materials Science
Orbital angular momentum in crystalline solids: non-equilibrium theory
Sangeeta Sharma, Peter Elliott and Samuel Shallcross
We develop a general operator theory of orbital angular momentum (OAM) in periodic solids that applies equally to
equilibrium and coherently driven states. Employing the
crystal momentum representation of Adams and Blount, we construct a periodic-solid OAM operator exactly corresponding to the symmetrized $\hat{\mathbf r}\times\hat{\mathbf v}$ observable, and show that it separates into local circulation (LC) and itinerant circulation (IC) sectors that are Hermitian and $U(1)$, and more generally $U(N)$, gauge covariant. In
equilibrium, the LC and IC sectors reduce to the established modern theory of orbital magnetism; out of
equilibrium, the same operator structure exposes new contributions generated by
laser-written momentum-space textures and interband coherence. Applied to gapped
graphene, we uncover a rich phenomenology of ultrafast non-
equilibrium orbital angular momentum, including a quasi-stationary light-induced OAM. This latter finding, underpinned by a close connection between band-
manifold phase texture and nonequilibrium OAM, points towards new possibilities for light control over OAM in solids as well as a potentially rich role for topological band structures in designed OAM materials.
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Materials Science
Stacking-Controlled Altermagnetism and Topological Magnons in Bilayer CrI$_3$
Lu Xiao, Meng-Han Zhang and Dao-Xin Yao
Stacked van der Waals magnets provide a tunable route to altermagnetism, a compensated
magnetic order characterized by momentum-dependent
spin splitting, and to the topological
magnetic excitations that such order can host. In bilayer CrI$_3$, first-principles calculations and linear
spin-wave theory reveal stacking-controlled altermagnetic order and associated
magnon band
topology. Combining band-representation analysis with calculations of the dynamic structure factor relevant to inelastic
neutron scattering, we further characterize the chirally split topological
magnons and quantify their energy corrections and lifetimes using a many-body Green's-function approach. The interlayer
magnetic ground state is highly sensitive to the stacking geometry, thereby controlling the
magnon band
topology and transport responses. We further show that
magnon–
magnon interactions renormalize the
magnon dispersion and dynamic structure factor, with a particular focus on
magnon decay. Using van der Waals bilayer CrI$_3$ as a representative platform, our results establish stacking engineering as a structural route for tuning altermagnetism and associated topological
magnon excitations, opening avenues toward stacking-controlled spintronic and magnonic devices.
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Materials Science
Phase-Field Modeling of Liquid Phases with Ordering
Yanzhou Ji and Chengyin Wu
A strong negative enthalpy of mixing can lead to ordering in
liquid solutions. To describe ordering in
liquids, different thermodynamic descriptions of
liquid phases have been developed in the CALPHAD approach, including the associate model and the modified quasichemical model. In this study, we show that the key to directly incorporate these
liquid phase models and their corresponding CALPHAD-type free energy formulations in phase-field simulations, is to identify the internal processes and to properly define the internal thermodynamic variables. The free energy of the
liquid phase can then be converted to a function of component compositions and internal variables, and the
evolution of the internal variables will be determined via Allen-Cahn-type relaxation equations. We demonstrate this modeling strategy using the formation of stoichiometric phases from
liquid phases in Na-Sb and Na-Sn systems. This study will pave the way for predicting microstructure
evolution in
liquids with ordering using phase-field simulations.
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Materials Science
Controlling Switching Evolution in Lead-Free Perovskite-Inspired Chalcogenide Memristors for Neuromorphic Computing
Emmanuel Joseph Shaji, Zhiyuan Li, Srikanth Doddapaneni et al. (+9 more)
Memristors have emerged as key building blocks of neuromorphic computing architectures due to their ability to integrate data storage and processing. While metal halide perovskites have recently shown significant promise owing to their mixed electronic-ionic conduction and low-cost solution processability, their reliance on toxic lead and limited stability presents critical challenges. Here, we report environmentally friendly, low-toxicity AgBiS2-based solution-processable memristors exhibiting an ultra-low SET voltage of ~0.08 V and a high ON/OFF ratio of >104. First-principles calculations identify Ag interstitials as the energetically most favourable native defect and reveal low migration barriers within the Ag sublattice, for both interstitials and vacancies, facilitating ionic transport in the AgBiS2
lattice. Through interface and thickness engineering, the resistive switching behaviour can be systematically tuned from abrupt digital to gradual analog modes. Notably, thicker switching layers promote the
evolution of stable conductive pathways through intermediate metastable states, revealing a controllable filament
evolution process. Electrochemical impedance spectroscopy reveals pronounced negative capacitance (inductive) behaviour at low bias voltages, arising from coupled electronic-ionic dynamics. Consistent with this behaviour, pulse measurements demonstrate gradual conductance modulation under pulse trains, emulating synaptic responses relevant for neuromorphic computing. Finally, post-operando structural analysis reveals substantial morphological
evolution of the switching layer driven by repeated filament formation and rupture. Linking structural dynamics to switching variability provides important design principles for achieving reliable and durable sustainable memristors.
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All authors Emmanuel Joseph Shaji, Zhiyuan Li, Srikanth Doddapaneni, Bhavya Rakheja, Vikrant Chaudhary, Avantika Suthar, Lingyun Zhu, Jingxin Ma, Hongbin Zhang, Monojit Bag, Gerardo Hernandez-Sosa, Ramesh Kumar
Materials Science · Cross-listed from cond-mat.supr-con
A Different Perspective on Superconductivity in Crystalline Graphene: Exploiting Energetics
Ke Wang, Shicong Song and K. Levin
A central mystery of crystalline
graphene is why superconductivity is so widespread yet often confined to strange slivers near boundaries between distinct isospin-ordered metals. In this paper, we apply a “two-parent” energetic framework which we show can explain this unusual form of superconductivity without specifying the details of the necessarily present pairing attraction. First-order transitions are crucial here: when two normal isospin-ordered states are degenerate in free energy, even a small net superconducting energy gain may stabilize an
equilibrium superconductor. We demonstrate how this is possible even though the small energy gain from pairing is reduced by the expense of reconstructing the normal metal, which is needed to achieve superconducting compatibility. The first order degeneracy also gives superconductivity a choice between two normal state parents, favoring the state with the largest net free energy gain.
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Materials Science
A latent-space extrapolation grade built into graph atomic cluster expansion foundation potentials
Yury Lysogorskiy, Anton Bochkarev and Ralf Drautz
Foundation machine-learning interatomic potentials cover broad configurational and chemical spaces, but their reliability can vary across the atomic environments encountered during a simulation. Here we introduce the calibrated Mahalanobis (CALM) extrapolation grade $\gamma$, a piecewise differentiable per-atom quantity integrated into GRACE foundation models and evaluated alongside energies and forces in a single model pass. We define $\gamma$ from nearest-cluster Mahalanobis distances in latent feature space, setting $\gamma=1$ from the training-distance distribution separately for each element and cluster. Controlled tests show that a normalized random projection of the invariant many-body basis detects structural and chemical extrapolation. On different foundation
datasets, OMat24 and SMAX, $\gamma$ correlates with atomic force errors and separates structures with different error distributions. The CALM grade adds percent-level computational cost, and its spatial gradient guides uncertainty-biased data collection toward configurations with larger absolute force errors.
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Materials Science
Training and memory in a randomly driven fractal gel
Chloe W. Lindeman, Joshua D. Clugston, Justin C. Goodrich et al. (+3 more)
A variety of disordered materials, including jammed particulate systems and crumpled paper, can be trained to exhibit
memory of a cyclic strain amplitude. Only recently, however, have questions emerged around the whether random driving can impart similar training. Here, we employ x-ray
photon correlation spectroscopy to study a fractal nanoparticle gel trained via either deterministic cyclic shear or random shear bounded by strain amplitudes $-\gamma_t$ and $+\gamma_t$. Both types of training lead to microstructural reversibility, with a slower and more irregular training for the random protocol. In both cases, the shear induces redistribution of internal stress in the gel with corresponding irreversible strain displacements whose magnitudes decrease during training. Finally, we show that
memory of the random driving can be read out and quantified using a standard protocol.
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All authors Chloe W. Lindeman, Joshua D. Clugston, Justin C. Goodrich, Mark Sutton, James L. Harden, Robert L. Leheny
Materials Science · Cross-listed from cond-mat.mes-hall
Atomic-Scale Imaging of Lattice Relaxation and Topological Flat Bands in Helical Trilayer Graphene
Shudan Jiang, Zonglin Li, Yu Gu et al. (+15 more)
Helical trilayer
graphene (HTG) has emerged as a highly tunable moire
quantum material that hosts strong electronic correlations and nontrivial band
topology. However, the atomic-scale
lattice structure and local electronic properties have remained largely unexplored. Here we present a comprehensive real-space study of HTG using a combination of scanning near-field optical microscopy and low-temperature scanning tunneling microscopy. We directly image supermoire
lattice relaxation, revealing large triangular domains separated by sharp domain walls, as well as stripe domains connected by smoothly varying boundaries. Atomic-scale spectroscopy uncovers flat bands with a honeycomb electronic texture and one-dimensional boundary states confined to domain walls. By systematically varying the twist angle, we identify a magic angle of approximately 1.9{\deg}, substantially larger than the 1.6 degree predicted by theory. Our results establish a direct microscopic link between
lattice relaxation and flat bands in HTG.
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All authors Shudan Jiang, Zonglin Li, Yu Gu, Liang Liu, Dandan Guan, Yaoyi Li, Hao Zheng, Canhua Liu, Kenji Watanabe, Takashi Taniguchi, Shengwei Jiang, Xiaoxue Liu, Zhiwen Shi, Guorui Chen, Jinfeng Jia, Tingxin Li, Can Li, Shiyong Wang
Materials Science
Cluster-based Structural Similarity for Dataset Visualization and Data Selection for Machine Learning Interatomic Potentials
Yuto Iwasaki and Miguel A. Caro
Machine learning interatomic potentials (MLIPs) are essential components for accelerating simulation-driven materials design. Data-efficient MLIP training relies on data-selection strategies that maximize structural diversity while limiting computationally expensive first-principles calculations. A key challenge in such strategies is evaluating structural similarity, which involves a trade-off between retaining information on individual atomic environments and reducing computational cost. Here, we propose similarity evaluation methods that achieve both representational fidelity and computational efficiency. Our method represents each structure using a small set of characteristic atomic environments identified by k-medoids clustering and computes pairwise similarity through optimal matching between these representatives or their distributions. Molecular
benchmarks demonstrate that our method is approximately 50 times faster than the baseline method while also more clearly distinguishing structures with different chemical compositions. Similarity-based data-selection
benchmarks demonstrate that our methods improve the data efficiency and stability of force prediction in MLIPs.
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Materials Science · Cross-listed from cond-mat.mes-hall
Thermal magnon transport in FM/AFM bilayers
Moumita Kundu and Ulrich Nowak
Progress in information processing relies on spintronics, where
magnetic states serve as efficient carriers for data storage and transfer. In this work, we theoretically study
magnon propagation in a bilayer composed of a ferro-magnet and an antiferromagnet. For this purpose, we probe the
spin Seebeck effect by introducing a spatially varying temperature profile. This generates a local
magnon excitation and a continuous
magnon flux from hot to cold regions which we quantify through the resulting non-
equilibrium magnon accumulation. Based on the chirality of these modes, we identify specific constraints for
magnon modes traveling either from the ferromagnet into the antiferromagnet or vice versa. A key finding is the observation of a thermally triggered
spin current in the antiferromagnet, a phenomenon typically absent in bulk antiferromagnets that obey time-reversal
symmetry. These results provide important insights into the design of
heterostructures for magnonic chirality-selective
spin transport.
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Materials Science
Engineering Grain Boundary Commensurability for Ferroelectric Stabilization in Hafnia-Based Films
Chuqiao Shi, Xinyan Li, Xing He et al. (+11 more)
Grain boundaries fundamentally dictate the macroscopic properties of polycrystalline materials by breaking long-range
symmetry. In ferroelectrics, these structural discontinuities are conventionally considered as detrimental features that induce depolarization fields and accumulate defects, thereby suppressing or pinning local polarization. Here, we demonstrate that the commensurability of grain boundaries inherently governs the polar-orthorhombic phase stability in polycrystalline Hf0.5Zr0.5O2 (HZO) thin films. Using depth-resolved multislice electron ptychography, we map the three-dimensional (3D) phase distribution across diverse boundaries with atomic resolution, revealing that highly commensurate grain boundaries effectively suppress the nonpolar-tetragonal phase by compensating for
lattice mismatch while mitigating
geometric frustration. Monte Carlo simulations uncover the atomistic and energetic origins of polar-phase stabilization, showing that commensurate grain boundaries favor the polar-orthorhombic phase more strongly than other grain boundaries. Guided by this principle, we epitaxially engineer HZO thin films to promote the preferential formation of commensurate grain boundaries, resulting in an approximately 60% enhancement in the remanent polarization of ferroelectric devices. These findings provide direct 3D experimental and theoretical evidence that establishes grain boundary commensurability as a critical degree of freedom for designing functional interfaces, guiding the construction of novel ceramics and polycrystalline ferroelectrics.
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All authors Chuqiao Shi, Xinyan Li, Xing He, Kaiji Zhao, Jesse Schimpf, Akash Surampalli, Adan Mireles, Sergio Puebla, Yi Jiang, Ramamoorthy Ramesh, Xiaofeng Qian, Lane W. Martin, Andrew M. Rappe, Yimo Han
Materials Science
Phonon-mediated closing of topological Floquet gaps in graphene: Non-phenomenological analysis
Royi Ledermann, Rave Hanoch Saadon, Adam Herling et al. (+1 more)
Floquet band engineering has been intensively studied for its potential to control material properties via
laser driving. In particular, Floquet topological states have been measured on the surface of Bi$_2$Se$_3$. Nonetheless, the original prediction of Floquet topological bands in
graphene remains unobserved, with works only measuring non-topological Floquet-Bloch states or indirect features. Here, we theoretically explore
graphene irradiated by circular
lasers with non-phenomenological electron-
phonon (e-ph) coupling. We show that e-ph interactions substantially broaden Floquet bands due to
graphene's large zero-point motion, as expected from phenomenological treatments. However, we further find that larger displacement $\Gamma$ optical
phonons reduce Floquet gaps by about half, even in absence of instrumentation broadening, and can also induce additional features like flat bands. These
phonons "enter the gap" and blur photoemission signatures. Including non-$\Gamma$
phonons closes the gap and greatly reduces visibility. When these modes couple to reasonably expected instrumentation broadening, the effect is significantly exacerbated. Our results shed light on the missing Floquet topological gaps and also lead to possible mitigation strategies: (i) Pre-pumping coherent
phonons to counteract blurring, or (ii), transition to a Dirac system with more favorable
phonon statistics. Moreover, our real-space analysis proposes that Floquet topological physics is alive in localized regions, such that properties of interest like transport should be accessible. We expect this work to impact experimental analysis and lead to set-ups where
graphene Floquet
topology might finally be directly observed.
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All authors Royi Ledermann, Rave Hanoch Saadon, Adam Herling, Ofer Neufeld
Materials Science
A transferable full-band Monte Carlo framework for complex alloy avalanche photodiodes
Shafat Shahnewaz, Hannaneh Karimi, Joe C. Campbell et al. (+1 more)
We present a physics-based multiscale full-band Monte Carlo framework for modeling avalanche multiplication and excess noise in complex
alloy avalanche photodiodes (APDs), demonstrated on Al$_{0.7}$InAsSb as a representative quaternary system. The framework links atomistic material structure to device-level avalanche statistics: an environment-dependent $sp^3d^5s^\ast$ tight-binding calculation resolves the full conduction- and valence-band structure of the random- or digital-
alloy configuration – including $\Gamma$, X, and L valley ordering, non-parabolicity, anisotropy, and
spin-orbit-induced valence-band splitting – and supplies the band-structure inputs for
stochastic high-field transport. A central element of the framework is a physics-derived treatment of
alloy-disorder scattering, in which the quaternary disorder potential is constructed from atomic valence differences, covalent radii, and Thomas–Fermi screening through a composition-weighted decomposition into binary contributions, complemented by composition-interpolated polar-optical, acoustic, intervalley-
phonon, impurity, and impact-ionization models. Because every material-dependent input is generated from the atomic composition and configuration by the same well-defined procedure, the framework transfers without structural modification to arbitrary zinc-blende ternary and quaternary
alloys. Applied to a \SI{1}{\micro\meter} Al$_{0.7}$InAsSb p-i-n APD, the framework reproduces the measured gain and excess-noise characteristics with only the impact-ionization softness parameters calibrated. The approach provides a documented, reproducible route for analyzing and designing complex
alloy APDs in which band structure, disorder, and scattering physics jointly determine gain and ionization statistics.
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All authors Shafat Shahnewaz, Hannaneh Karimi, Joe C. Campbell, Avik W. Ghosh
Materials Science
Second-order Real Nodal Lines in Nodal Surface Semimetals
Yunye Wang, Ruopu Zhao, Tianqi Zhang et al. (+1 more)
Real nodal lines (RNLs) featuring real Chern numbers and second-order boundary modes have attracted widespread
attention. In all previously reported realizations, an RNL is linked by another nodal line, and whether an RNL can be linked by other types of band degeneracies has remained open. Here, we propose a second-order real nodal-line semimetal in which a pair of RNLs is linked by a nodal surface. We show that this state can be realized in spinless systems with both $PT$ and nonsymmorphic $S_{2z}T$
symmetries, where the $S_{2z}T$-enforced nodal surface prevents the pair of RNLs from annihilation. Each nodal line carries a nontrivial real Chern number $\nu_R=1$, giving rise to topological hinge Fermi arcs located at a pair of $PT$-related hinges. Guided by this construction, we identify the interpenetrated
graphene network (IGN) as a promising material realization. First-principles calculations confirm that a pair of nodal lines traversing the Brillouin zone are linked by a nodal surface and each nodal line carries double nontrivial $Z_2$ charges. The bulk-boundary correspondence of IGN manifests as a pair of hinge Fermi arcs together with drumhead surface states. Our work establishes nodal surfaces as a new linking partner for real nodal lines and provides a roadmap for exploring higher-order real
topology in carbon-based and other light-element systems.
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All authors Yunye Wang, Ruopu Zhao, Tianqi Zhang, Weikang Wu
Materials Science · Cross-listed from physics.comp-ph
MyTm: An Automated Melting Temperature Calculation Toolkit
Y. S. Huang, H. X. Song, Y. Sun et al. (+7 more)
Melting temperature calculation is one of the important topics in computational materials science. In high-throughput in silico screening and artificial intelligence assisted design of materials, it usually requires a rapid and autonomous assessment of the melting temperature of the target. Unfortunately, molecular dynamics (MD) simulations of the melting point require many cumbersome and manual operations, making large-scale calculation of the melting point challenging. In this work, we introduce MyTm, a toolkit that employs MD to automatically determine the melting point. The method is fully modularized, and by combining these modules, the program enables fully automated melting calculations by using commonly adopted approaches, including the direct-heating method, the void method, the modified void method, the solid-
liquid coexistence method, and the Z method. Moreover, a machine learning (ML) method is proposed and employed to recognize and classify the solid like and
liquid like atoms, which effectively resolve the low accuracy issue in conventional classification approaches, thus making the automated high throughput pipeline of melting-point calculation possible. The robustness and efficacy of MyTm have been demonstrated by several well studied systems.
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All authors Y. S. Huang, H. X. Song, Y. Sun, J. L. Li, Y. L. Xu, F. C. Wu, Y. C. Gan, Y. F. Wang, H. Wang, Hua Y. Geng
Materials Science
An electromechanically coupled multiphase-field model with generalized kinetic relations for ferroelectrics
Hsu-Cheng Cheng and Dennis M. Kochmann
Classical phase-field models typically use the Allen–Cahn
evolution law to model interface motion as a consequence of energy-gradient descent. This implies a linear kinetic relation between the velocity of an interface (e.g., a domain wall in a ferroelectric) and its driving force. When nonlinear kinetic relations are to be modeled, as commonly found in ferroelectric ceramics, an alternative
evolution law and hence an alternative model is needed. Here, we propose an electromechanically (stress-driven) coupled multiphase-field framework with a general kinetic formulation, which integrates a prescribed kinetic relation directly into the
evolution law. We show that this model correctly evolves domain walls with the assigned nonlinear kinetics, e.g., of mixed exponential-power law type, following the so-called Merz–Stadler law. The multiphase formulation further enables distinct kinetic relations and interfacial energies to be assigned to different order-parameter pairs, which reflects the distinguishable properties of the different types of ferroelectric domain walls. The proposed framework is broadly applicable for simulating kinetic relations in materials with applications beyond ferroelectrics.
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Materials Science · Cross-listed from quant-ph
Boron vacancies in bulk h-BN created by high-energy He+ irradiation
P.G. Vilyuzhanina, S.V. Bolshedvorskii, R. Isayev et al. (+11 more)
While color centers in diamond and other three-dimensional
crystals are nowadays key elements of several types of sensors, color centers in two-dimensional materials are rapidly developing and promise high-tech applications. One of the interesting centers in 2D materials is the negatively charged boron vacancy in hexagonal boron nitride (h-BN), which has already shown some potential for magnetometry, but the reliable creation of boron vacancies remains challenging. Here, we demonstrate the fabrication of negatively charged boron vacancy color centers in bulk h-BN via implantation of helium ions at ~1 MeV, as confirmed by characteristic photoluminescence and optically detected
magnetic resonance. The resonance has a width of 180 MHz and exhibits the expected Zeeman shift of the resonance lines. The depth of the color center along the c-axis of the h-BN
crystal was measured and compared with predictions from Stopping and Range of Ions in Matter modeling. The experimental value and the modeled prediction are consistent within the reported uncertainties, as their 1{\sigma} intervals overlap, although the experiment shows a greater depth and a wider distribution.
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All authors P.G. Vilyuzhanina, S.V. Bolshedvorskii, R. Isayev, A.S. Doroshkevich, A.A. Tatarinova, V.V. Soshenko, I.S. Cojocaru, A.M. Kozodaev, S.M. Drofa, A. Chernyavskiy, N.I. Salangin, A.N. Smolyaninov, A.Y. Kuntsevich, A.V. Akimov
Materials Science
First-Principles Study of I$_2$ and CH$_3$I Adsorption on Transition Metal Decorated 2D-Material substrates : Insights from Electronic Structure and Reaction Kinetics
M. Meena and P. Anees
Radioactive iodine species, particularly I$_2$ and CH$_3$I, pose significant environmental and technological hazards owing to their high
volatility, chemical stability, and relatively weak interaction with traditional substrate and sorption materials. In this work, we proposed a series of transition-metal (TM) (Fe, Ni, Cu, Zn) decorated boron-doped
graphene (BDG) and 2D-MoTe2 substrates for efficient adsorptive capture and mitigation of such volatile Iodine species. Using systematic first-principles density functional theory (DFT) calculations, we elucidate the microscopic origin of the enhanced adsorption by analyzing the changes in the electronic structure upon adsorption. More importantly, we analyzed the thermodynamic and kinetic feasibility of adsorption on these newly designed substrates using Climbing-Image Nudged Elastic band (CI-NEB) calculations and found that TM decoration serves as an effective catalytic center, thereby making the reaction thermodynamically and kinetically feasible. Conversely, the reaction becomes thermodynamically and kinetically unfavorable on pristine substrates in the absence of a TM atom as a catalytic center. This work deepens our understanding of the electronic origin of the enhanced adsorption and reaction kinetics, and the predictions made will be useful for experimental realization.
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Materials Science
Realization of ZnSe-based Field-Effect Transistors operating at Cryogenic Temperatures as a Platform for Future Spin-Qubit Applications
Christine Falter, Yurii Kutovyi, Nils von den Driesch et al. (+12 more)
The wide-bandgap compound
semiconductor ZnSe is a promising host material for the realization of electron
spin-
qubits. Its non-degenerate conduction band and the potential for isotopic nuclear
spin purification promise long
spin coherence times. Key requirements for such devices include reliable electrostatic control of electrons in ZnSe and low-resistance ohmic contacts that remain functional at cryogenic temperatures. In this work, we utilize a novel Shadow Wall technique for molecular-beam epitaxy combined with in-situ deposition of Al ohmic contacts to realize normally-off ZnSe-based field-effect transistors. The devices exhibit linear output characteristics and effective gate control of the channel from room temperature down to 5 K, confirming low-resistance ohmic contacts to the undoped ZnSe channel. The drain current can be modulated by several orders of magnitude through electrostatic gating, with threshold voltages of approximately 3 V and field-effect mobilities exceeding 100 cm2/Vs over the investigated temperature range. Self-consistent Schrödinger-Poisson and drift-
diffusion simulations reproduce the measured transfer characteristics and provide insight into the role of interface electrostatics in determining the channel formation and threshold voltage. These results demonstrate the potential of gated ZnSe
heterostructures for future
spin-
qubit applications.
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All authors Christine Falter, Yurii Kutovyi, Nils von den Driesch, Denny Dütz, Nataliya Demarina, Prateek Kaul, Anjana Rajan, Jiayuan Zhang, Thomas J. Smart, Leqi Zhou, Lidia Kibkalo, András Kovács, Qing-Tai Zhao, Lars R. Schreiber, Alexander Pawlis
Materials Science
Surface-Dependent Phonon Dynamics in 9-Armchair Graphene Nanoribbon Arrays
Ángel Labordet Álvarez, Gabriela Borin Barin, Michel Calame et al. (+1 more)
Atomically precise
graphene nanoribbons (GNRs) have electronic properties tunable through their width and edge structure, making them promising building blocks for nanoscale electronics and optoelectronics. Their integration into devices also requires understanding how the supporting surface and neighbouring ribbons affect their vibrations. Here, we use temperature-dependent Raman spectroscopy to compare five configurations of 9-armchair GNR arrays differing in substrate,
alignment and coverage. Transferring the same unaligned film from Au to a Raman-optimised Al$_2$O$_3$-coated support reduces the measured $D$- and $G$-mode
redshift rates by about 80%. This reduction is consistent with a substantial thermoelastic contribution. The fitted strain and anharmonic contributions depend on the assumed thermal expansion and strain transfer. Compared with the low-coverage array, the dense aligned Au array has higher extrapolated reference frequencies ($\omega_0$). On first heating, its $G$-mode linewidth decreases and then increases, forming an unusual intermediate-temperature minimum. The $D$-mode linewidth shows a similar, less pronounced variation. These findings establish substrate choice and array arrangement as routes to engineer nanoribbon vibrations, extending control beyond the atomic structure defined during synthesis.
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All authors Ángel Labordet Álvarez, Gabriela Borin Barin, Michel Calame, Mirjana Dimitrievska
Materials Science · Cross-listed from cond-mat.str-el
Tunability of the structural and magnetic transition in kagome material: PrIr$_3$B$_2$
Gayathri V, Irshad K A, Sathishkumar M et al. (+2 more)
We report the temperature and pressure tunability of an unusual structural transformation associated with a two-step metal-
insulator-metal (MIM) transition in the kagome
lattice compound PrIr$_3$B$_2$ using synchrotron X-ray powder diffraction. At ambient conditions of temperature and pressure, the monoclinic ($C2/m$) and the hexagonal ($P6/mmm$) phases coexist as twinned structure in the
crystal. As the temperature (pressure) is decreased (increased), PrIr$_3$B$_2$ converts fully to monoclinic structure at $T =$ 280 K (at ambient pressure) and $P =$ 1.2 GPa (at room temperature). Temperature dependence of the monoclinic structure at ambient pressure presents complex
evolution of
lattice parameters with weak but clearly discernable anomalies at \SI{\sim 250} {\K} and \SI{\sim 110} {\K}, which are correlated with the second MIM transition and the linear to nonlinear temperature-dependent resistivity crossover, respectively. These anomalies are likely due to some charge order state causing a partially gapped Fermi surface. The
magnetic phase diagram of PrIr$_3$B$_2$ is also investigated from anisotropic measurements. At 10 K, a superzone gap opens near the antiferromagnetic transition, which does not close even in the polarized state. From the tunability of the
crystal structure and
magnetic and electronic ground state, promising electronic orders are indicated in this kagome metallic magnet.
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All authors Gayathri V, Irshad K A, Sathishkumar M, Boby Joseph, S. Manni
Materials Science
Nature of the 175-180 cm$^{-1}$ Raman Feature in Altermagnetic $\alpha$-MnTe
Pankaj Bhardwaj, Rajib Sarkar, Naresh Shyaga et al. (+6 more)
The 175 cm$^{-1}$ Raman mode in $\alpha$-MnTe has long been assigned to the material's only
symmetry-allowed
phonon ($E_{2g}$), yet first-principles calculations consistently place this mode below 100 cm$^{-1}$. Recent works have offered three competing explanations for the discrepancy: a weak
symmetry-lowering ($D_{6h}\rightarrow D_{3h}$)
phonon, an electronic plasmon arising from intrinsic hole self-doping, or an extrinsic MnTe$_2$ impurity signature. Here we resolve this controversy using stoichiometry-controlled molecular beam epitaxy (MBE). We deliberately vary the fraction of pyrite-type MnTe$_2$ secondary phase relative to the $\alpha$-MnTe
matrix. Across this series, the 175 cm$^{-1}$ mode tracks the presence of MnTe$_2$ essentially one-to-one – present wherever MnTe$_2$ is detectable, absent in stoichiometric, single-phase $\alpha$-MnTe at nominal
LASER powers. When
LASER power is ramped up, an additional mode emerges in MnTe as well positioned close to $\approx$ 175 cm$^{-1}$; our detailed temperatures dependendent Raman analysis identifies the
LASER power scorched region close to a distorted MnTe phase, confirmed via transmission electron microscopy. This direct, growth-controlled correlation identifies the mode as an extrinsic MnTe$_2$ impurity signature or as a distorted MnTe feature, rather than an intrinsic
phonon or plasmon feature, providing a practical diagnostic for phase purity in MBE-grown $\alpha$-MnTe and clarifying which Raman features can be reliably attributed to the intrinsic altermagnetic phase.
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All authors Pankaj Bhardwaj, Rajib Sarkar, Naresh Shyaga, Subhransu Kumar Negi, Kartick Biswas, Susmita Jana, Pavan Nukala, B. R. K. Nanda, Dhavala Suri
Materials Science
Three-Dimensional Topology from Stacking-Controlled Umklapp Scattering in Large-Angle Twisted Graphite
Juncheng Li, Cong Chen and Wang Yao
Large-angle twisted
graphene lies beyond the local-stacking description of conventional moiré systems: inequivalent rotation centers define distinct commensurate interfaces whose low-energy interlayer hybridization is governed by intervalley Umklapp tunneling. For three-dimensional twisted graphite assembled from interfaces with different crystalline
symmetries, a
symmetry-constrained effective model reveals that competition between nonchiral and chiral tunneling produces one-ring and two-ring nodal-line phases and a $C_3$-protected higher-order topological
insulator in the presence of sublattice (chiral)
symmetry. The nodal rings carry integer winding numbers, allowing oppositely wound rings to annihilate into the gapped phase. We further examine how sublattice-
symmetry breaking modifies these phases. Density functional theory (DFT) and atomistic calculations for $21.8^\circ$ twisted graphite identify the
equilibrium structure as a higher-order topological
insulator, while compression drives it into a topological Weyl semimetal phase. These findings establish the stacking sequence of
symmetry-inequivalent interfaces as a means of engineering band
topology in three-dimensional twisted structures.
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Materials Science · Cross-listed from physics.chem-ph
A strategic roadmap for an atomistic machine-learning ecosystem
Jörg Behler, Michele Ceriotti, Cecilia Clementi et al. (+47 more)
Data-driven machine learning (ML) techniques have become an essential tool in many domains of science. Their application to atomistic simulations of matter is particularly widespread and impactful. This success is due largely to the existence of a well-developed and established physics-based modeling framework, ranging from first-principles electronic-structure calculations to molecular dynamics and statistical sampling, into which ML was integrated naturally to reshape long-standing trade-offs between accuracy, efficiency, and scale. Nevertheless, this integration raises both conceptual and practical challenges, from choosing between data-centric and physics-based modeling approaches to adapting established software stacks to modern hardware accelerators and ML libraries. As the field evolves rapidly, fueled in part by widespread enthusiasm but also by tangible impact, it seems appropriate to take a moment to consider the current state of the art and open challenges, and reflect on what can be done to better coordinate efforts across the community. With this goal in mind, several members of this community met in Lausanne in January 2026 at CECAM to discuss
algorithms, models, software and hardware infrastructure, and the most promising scientific applications that have become possible thanks to the use of artificial intelligence in atomic-scale simulations. This strategic roadmap paper summarizes the outcomes of these discussions, suggesting some long-term goals, and some concrete actions, to establish a healthy, sustainable and impactful atomistic ML ecosystem.
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All authors Jörg Behler, Michele Ceriotti, Cecilia Clementi, Gábor Csányi, Alin-Marin Elena, Aditi Krishnapriyan, Joseph W. Abbott, Fabio Affinito, Albert P. Bartók, Ilyes Batatia, Filippo Bigi, Florian N. Brünig, Yannick Calvino Alonso, Giuseppe Carleo, Aurélie Champagne, Stefan Chmiela, Marc L. Descoteaux, Ralf Drautz, Alexandra Farcas, Meng Gao, Rohit Goswami, Michael F. Herbst, Christian Holm, James R. Kermode, Alexander L. M. Knoll, Tobias Kreiman, Marcel F. Langer, Hoang-Thien Luu, Yury Lysogorskiy, Mihai-Cosmin Marinica, Rocco Meli, Klaus-Robert Müller, Frank Noé, Mohamadhosein Nosratjoo, Simon Olsson, Christoph Ortner, Aldo S. Pasos-Trejo, Anyang Peng, Eric Qu, Andrea Rizzi, Mariana Rossi, Bassem Sboui, Gregor N. C. Simm, Alexandre Tkatchenko, Jacopo Venturin, O. Anatole von Lilienfeld, William C. Witt, Brandon M. Wood, Tigany Zarrouk, Fabian Zills
Materials Science
Operando Charge Carrier Dynamics by Intensity-Modulated Photoluminescence: From Perovskite Thin Films to Solar Cells
Constantin Bach, Dong Won Kim, Yitian Du et al. (+2 more)
The charge-carrier lifetime in halide perovskites often varies by orders of magnitude with the injection level, complicating the analysis of conventional time-resolved photoluminescence and comparisons of reported lifetimes. We use intensity-modulated photoluminescence spectroscopy (IMPLS) as a frequency-domain alternative. Carried out under operating conditions, it yields the lifetime at a specific injection level, e.g., one-sun-equivalent illumination. On thin films, IMPLS and steady-state photoluminescence yield comparable lifetimes. For perovskite/transport-layer stacks, the IMPLS response becomes more complex. A dedicated transfer function disentangles charge transfer from interface recombination and predicts an upper limit on the charge-carrier lifetime in the solar
cell. In devices, electrical methods such as intensity-modulated photovoltage spectroscopy (IMVS) often fail to measure the lifetime due to capacitive effects. IMPLS shows not only the same two characteristic frequencies as IMVS, but also a third one, thereby resolving the lifetime itself.
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All authors Constantin Bach, Dong Won Kim, Yitian Du, Yana Vaynzof, Carsten Deibel
Materials Science · Cross-listed from physics.chem-ph
An atom-based machine-learned dipole-moment model and application to conjugated systems
Yuansheng Zhao, Tamio Yamazaki, Ryohei Hosoya et al. (+2 more)
We introduce a method for predicting the dipole moments of molecular systems by systematically decomposing the total dipole moment into the sum of atomic contributions using the wavefunction from density-functional-theory calculations, and then use graph neutral networks to predict this effective atomic dipole moment from input atomic structure. It is demonstrated that the dipole moments and dielectric function can be accurately predicted even for complicated conjugated systems where the previous bond-based model [Phys. Rev. B 110, 165159] fails. The inference cost of our model scales linearly with the number of atoms and is about 3 times faster compared with Born-effective-charge-based schemes, but shows similar or better accuracy for dielectric function at terahertz range.
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All authors Yuansheng Zhao, Tamio Yamazaki, Ryohei Hosoya, Yu-ichiro Matsushita, Shinji Tsuneyuki
Materials Science
First-Principles Study of Two-Dimensional CeN: Interplay between Structural Stability, Electronic Structure, and Magnetism
Jose Mario Galicia Hernandez, Jose Jorge Rios Ramirez, Gregorio Hernandez Cocoletzi et al. (+1 more)
In this work, the structural stability, and the electronic and
magnetic properties of a novel CeN monolayer derived from the (111) surface of bulk rock salt CeN are investigated using first-principles calculations. Dimensional reduction produces substantial changes in the local atomic environment, decreasing the Ce-N coordination from sixfold in the bulk to threefold in the monolayer and shortening the Ce-N bond length, while preserving an essentially planar hexagonal structure. Moreover, the electronic and
magnetic properties are significantly modified, which may be useful for practical applications. Both ferromagnetic (FM) and antiferromagnetic (AFM) configurations were investigated and found to be strongly bound, with the FM state slightly lower in energy. The
magnetic moments are predominantly localized on the Ce atoms and mainly originated from the Ce 4f states. The electronic structure also exhibits a pronounced dependence on
magnetic ordering, highlighting the connection between localized 4f states and the
spin-dependent electronic properties of the system. In addition, the calculated elastic properties confirm the mechanical stability of the monolayer, while
phonon calculations support its dynamical stability. Ab initio molecular dynamics simulations further indicate that the two-dimensional structure preserves its integrity under finite-temperature conditions. Overall, these results demonstrate that dimensional reduction significantly modifies the structural, electronic, and
magnetic behavior of CeN without compromising the stability of the resulting two-dimensional
lattice. The combination of structural robustness, localized Ce 4f magnetism, and
spin-dependent electronic properties make the CeN monolayer an interesting system for further exploration in two-dimensional magnetism and
spin-dependent applications.
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All authors Jose Mario Galicia Hernandez, Jose Jorge Rios Ramirez, Gregorio Hernandez Cocoletzi, Juan Francisco Rivas Silva
Materials Science
Defect Dynamics and Isomer Quenching in $^{229}$Th:CaF$_2$ Crystals
Ming Guan, Michael Bartokos, Kjeld Beeks et al. (+16 more)
Thorium-229 doped calcium fluoride ($^{229}$Th:CaF$_2$) is a leading candidate for a solid-state nuclear clock, owing to the unusually low energy of the $^{229}$Th isomer transition at 8.3557\,eV and the very high banggap of CaF$_2$ around 12\,eV. Here we report a temperature-dependent study of $^{229}$Th:CaF$_2$ luminescence, covering radioluminescence, photoluminescence, thermoluminescence, and afterglow. Using synchrotron x-ray excitation together with a cryo-
vacuum system, we resolve the thermal quenching of the self-trapped
exciton (STE) scintillation into its triplet and singlet components, identify eleven thermoluminescence glow peaks together with their associated emission bands, and separate the temperature-dependent components that make up the long-lived afterglow. The trap depths obtained in this way are closely correlated with the temperature dependence of x-ray-induced isomer quenching, including the enhanced isomer yield near \SI{-60}{\celsius} and the minimum near \SI{-80}{\celsius}, supporting the carrier-trapping picture in which trapped carriers are unavailable to quench the
nucleus. Taken together, these results connect the optical and nuclear observables of $^{229}$Th:CaF$_2$ through a unified picture of electron defects and Th-related traps, while providing a comprehensive characterization of the material relevant to solid-state nuclear clock operation.
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All authors Ming Guan, Michael Bartokos, Kjeld Beeks, Takahiro Hiraki, Shinji Kitao, Takahiko Masuda, Nobumoto Nagasawa, Koichi Okai, Ryoichiro Ogake, Martin Pimon, Noboru Sasao, Fabian Schaden, Thorsten Schumm, Makoto Seto, Kenji Tamasaku, Sayuri Takatori, Yoshitaka Yoda, Akihiro Yoshimi, Koji Yoshimura
Materials Science · Cross-listed from cond-mat.mes-hall
Optical observation of interlayer spin correlation
Akiyoshi Park, Pranshoo Upadhyay, Andrey Grankin et al. (+16 more)
Spin correlations govern numerous collective behaviors of
quantum materials, underpinning exotic phenomena such as unconventional superconductivity and topological magnetism. In layered materials, the interlayer
spin correlation is important because it characterizes the
magnetic ground state and determines
spin transport across the interface. Yet interlayer
spin correlations have remained hard to measure directly, leaving one of the most basic quantities of two-dimensional magnetism out of experimental reach. Here we provide the first direct optical probe of interlayer
spin correlations, using two-
magnon Raman scattering in the van der Waals antiferromagnet (AFM) CrSBr, in remarkable agreement with a microscopic
spin-wave model without any fitting parameter. Moreover, the two-
magnon channel switches on only in the AFM state and vanishes when a
magnetic field takes the
crystal to a ferromagnetic state. We furthermore establish an
exciton-mediated variant of the technique, where tuning the
laser near the
exciton resonance enhances the signal roughly tenfold due to the
exciton's large oscillator strength.
Magnon-pair Raman spectroscopy thus opens a direct optical window into interlayer
spin correlations in van der Waals magnets, extendable to twisted bilayers and proximity-coupled
heterostructures.
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All authors Akiyoshi Park, Pranshoo Upadhyay, Andrey Grankin, Emil Viñas Boström, Mahdi Ghafariasl, Hassan Alnatah, Masoud Mohammadi-Arzanagh, Gautam Nambiar, Beini Gao, Alireza Alvandi, Sakthi Rajmano Madhan Kumar, Ghadah Alshalan, Isaac Sherwood, Mahmoud Jalali Mehrabad, You Zhou, Arun Ramanathan, Xavier Roy, Angel Rubio, Mohammad Hafezi
Materials Science
Polymorph-Tunable Spin Texture and Excitonic Structures in Monolayer WSi2P4
Xianbo Chenwei, Yu Zhou, Ke Wu et al. (+6 more)
Two-dimensional
semiconductors that simultaneously possess a direct band gap and strong
spin-orbit coupling (SOC) are highly attractive for
quantum optoelectronics. Using first-principles GW plus Bethe-Salpeter equation (GW-BSE) calculations, we show that monolayer WSi2P4-an experimentally accessible member of the MSi2X4 family-hosts a direct K-valley gap together with strong SOC. Its three competing polymorphs (alpha, beta and gamma) are all direct-gap
semiconductors and are kinetically locked behind 1.3 eV migration barriers. The sigma_h mirror plane of the D3h of alpha and gamma phases enforces a persistent
spin texture across the Brillouin zone, weakly modulated near gamma by interband SOC mixing, whereas the polar beta phase (C3v) exhibits Rashba
spin splitting. SOC splits the doubly-degenerate lowest bright
exciton into dark states and splits the original absorption peak into two peaks (A and B), whose relative brightness is governed by the K-valley conduction-band splitting. alpha-WSi2P4 displays the brightest Peak A because a tiny band crossing of opposite-
spin branches can open a
spin-allowed radiative channel. These results establish the MSi2P4 family as a phase-tunable platform for the cooperative engineering of
spin texture, band splitting, and excitonic brightness within a single material system.
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All authors Xianbo Chenwei, Yu Zhou, Ke Wu, Gaofeng Xu, Ruixue Li, Yuan Li, Yabei Wu, Shaowen Xu, Fanhao Jia