Materials Science · Cross-listed from physics.comp-ph
Machine Learning Hamiltonians are Accurate Energy-Force Predictors
Seongsu Kim, Chanhui Lee, Yoonho Kim et al. (+7 more)
Recently, machine learning
Hamiltonian (MLH) models have gained traction as fast approximations of electronic structures such as orbitals and electron densities, while also enabling direct evaluation of energies and forces from their predictions. However, despite their physical grounding, existing
Hamiltonian models are evaluated mainly by reconstruction metrics, leaving it unclear how well they perform as energy-force predictors. We address this gap with a
benchmark that computes energies and forces directly from predicted
Hamiltonians. Within this framework, we propose QHFlow2, a state-of-the-art
Hamiltonian model with an SO(2)-equivariant backbone and a two-stage edge update. QHFlow2 achieves $40\%$ lower
Hamiltonian error than the previous best model with fewer parameters. Under direct evaluation on MD17/rMD17, it is the first
Hamiltonian model to reach NequIP-level force accuracy while achieving up to $20\times$ lower energy MAE. On QH9, QHFlow2 reduces energy error by up to $20\times$ compared to MACE. Finally, we demonstrate that QHFlow2 exhibits consistent scaling behavior with respect to model capacity and data, and that improvements in
Hamiltonian accuracy effectively translate into more accurate energy and force computations.
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All authors Seongsu Kim, Chanhui Lee, Yoonho Kim, Seongjun Yun, Honghui Kim, Nayoung Kim, Changyoung Park, Sehui Han, Sungbin Lim, Sungsoo Ahn
Materials Science
Atomically Precise Electron Beam Sculpting of Bilayer h-BN: The Role of Crystallographic Orientation and Milling Strategy
Ondrej Dyck, Andrew R. Lupini, Ivan Vlassiouk et al. (+3 more)
Achieving atomic precision in top-down manufacturing remains a fundamental challenge nanofabrication technology. Here, the focused electron beam of a scanning transmission electron microscope is used to demonstrate atomically precise sculpting of hexagonal boron nitride (h-BN)
bilayers, achieving nanoribbons as narrow as 6 Å with atomically smooth edges. The key to this precision lies in understanding how the underlying atomic structure, particularly in twisted
bilayer systems, influences the milling process. High-angle annular dark-field imaging combined with multislice simulations reveals distinct intensity signatures that allow identification of different stacking arrangements within moiré patterns. Mathematical analysis of moiré
lattices provides a predictive framework for determining optimal cutting directions, with cuts along armchair directions yielding superior edge quality compared to zigzag orientations. Surprisingly, a sequential milling approach, where a small electron beam subscan area is translated during the process, produces significantly better results than parallel milling of the entire target region. To understand these differences we implemented a
stochastic milling model that reveals that sequential milling minimizes unwanted exposure to surrounding material through beam tail effects. These findings establish a framework for achieving atomic precision in electron beam sculpting of two-dimensional materials and provide fundamental insights applicable to the broader challenge of top-down nanofabrication.
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All authors Ondrej Dyck, Andrew R. Lupini, Ivan Vlassiouk, Matthew Brahlek, Rob Moore, Stephen Jesse
Materials Science
Strain-Rate- and Line-Length-Dependent Screw Dislocation Glide Mechanisms in BCC Refractory Metals and Alloys
Subhendu Chakraborty and Liang Qi
Plastic flow in body-centered cubic (BCC) metals and dilute/concentrated
alloys is governed by the motion of <111> screw dislocations, whose glide is often impeded by cross-kinks (jogs). While existing strengthening models typically treat depinning as defect-assisted cutting or dislocation bowing, the combined strain-rate and dislocation-line-length dependence of cross-kink stability and effective obstacle spacing remains insufficiently resolved at the atomistic scale. Here, we combine conventional molecular dynamics and strain-boost hyperdynamics to investigate screw-dislocation glide in pure Nb and Mo, dilute Nb-Mo
alloys, and equiatomic NbMo at 300 K over strain rates from 10^3 to 10^7 s^-1 and dislocation line lengths from 15 to 50 nm. We first demonstrate that low-strain-rate simulations require sufficiently long dislocation lines to capture consistent cross-kink behavior and strength-determining pinning events. Using the 50~nm configurations, we show that cross-kinks form not only in concentrated
alloys but also in pure BCC metals, with their stability governed by the relative rates of kink nucleation and migration on primary and cross-slip planes, which differ between Nb- and Mo-rich systems due to distinct core structures and non-Schmid responses. At high strain rates, depinning proceeds predominantly via vacancy-interstitial cluster formation. In contrast, at low strain rates and long line lengths, alternative pathways emerge, including lateral cross-kink migration, three-dimensional forward–backward cross-slip, and prismatic loop formation. The effective obstacle spacing controlling the critical resolved shear stress therefore emerges from coupled thermodynamic roughening and kinetic
evolution. These findings highlight the intrinsically rate-, length-, and chemistry-dependent nature of screw-dislocation strengthening in BCC
alloys.
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Materials Science · Cross-listed from cond-mat.mes-hall
Electron viscosity and device-dependent variability in four-probe electrical transport in ultra-clean graphene field-effect transistors
Richa P. Madhogaria, Aniket Majumdar, Nishant Dahma et al. (+5 more)
Hydrodynamic electrons in high-mobility
graphene devices have demonstrated great potential in establishing an electronic analogue of relativistic
quantum fluid in solid-state systems. One of the key requirements for observing viscous electron flow in an electronic channel is a large momentum-relaxation path, a process primarily limited by electron-
impurity/
phonon scattering in
graphene. Over the past decade, multiple complex device geometries have been successfully employed to suppress momentum-relaxing scattering mechanisms; however, experimental observations have been found to be sensitive to the device fabrication process and architecture, raising questions about the signature of electron hydrodynamics itself. Here, we present a study on multiple ultra-clean
graphene field-effect transistors (FETs) in a simple, rectangular four-terminal device architecture. Using electrical transport measurements, we have characterised the pristine quality of the
graphene FETs and examined the variation of electrical resistance in the doped regime as a function of carrier density and temperature. Our results reveal strong device-dependent variability even in the most simple architecture that we attribute to competing momentum-conserving and momentum-relaxing scattering mechanisms, as well as coupling to contacts. Further, we have proposed a phenomenological method for analysing the results, which yields transport parameters in accordance with recent experiments. This simple experimental strategy and analysis can serve as an efficient tool for extracting the viscous electronic contribution in state-of-the-art high-mobility
graphene FETs.
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All authors Richa P. Madhogaria, Aniket Majumdar, Nishant Dahma, Pritam Pal, Rishabh Hangal, Kenji Watanabe, Takashi Taniguchi, Arindam Ghosh
Materials Science
A Kinetic Phase-Field Model of Diffusion Bonding: A Nonlocal Approach to Interface Coalescence
Maryam Khodadad, Noel Walkington, Suresh Kalyanam et al. (+2 more)
Conventional phase-field models often drive solid-solid interfaces to coalesce when in close proximity. This feature limits their use for processes like
diffusion bonding, where the interfaces might need to remain distinct under certain thermodynamic conditions. We develop a kinetic phase-field model to address this problem, using an
evolution equation based on a
geometric conservation law for interfaces, rather than the gradient descent
evolution that is typical in phase-field modeling. This formulation enables us to specify complex kinetic laws, and we use this to incorporate a physically-motivated
geometric criterion to control interface merging. This criterion, based on nonlocal higher-derivative
curvature invariants of the phase field, can be temperature-dependent, allows for a range of behaviors from complete coalescence to the preservation of distinct boundaries. Simulations show controlled bonding kinetics, demonstrating capabilities that are not available with existing methods for modeling interfaces that must remain distinct under given thermodynamic conditions.
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All authors Maryam Khodadad, Noel Walkington, Suresh Kalyanam, Matteo Pozzi, Kaushik Dayal
Materials Science
Understanding the kinetics of static recrystallization in Mg-Zn-Ca alloys using an integrated PRISMS simulation framework
David Montiel, Philip Staublin, Supriyo Chakraborty et al. (+6 more)
Recrystallization is a phenomenon in which a plastically deformed polycrystalline microstructure with a high dislocation density transforms into another that has low dislocation density. This
evolution is driven by the stored energy in dislocations, rather than grain growth driven by grain boundary energy alone. One difficulty in quantitative modeling of recrystallization is the uncertainty in material parameters, which can be addressed by integration of experimental data into simulations. In this work, we compare simulated static recrystallization dynamics of a Mg-3Zn-0.1Ca wt.%
alloy to experiments involving thermomechanical processing followed by measurements of the recrystallization fraction over time. The simulations are performed by combining PRISMS software for
crystal plasticity and phase-field models (PRISMS-Plasticity and PRISMS-PF, respectively) in an integrated computational materials engineering framework. At 20% strain and annealing at 350 {\deg}C, the model accurately describes recrystallization dynamics up to a mobility-dependent time scale factor. While the average grain boundary mobility and the fraction of plastic work converted into stored energy are not precisely known, by fitting simulations to experimental data, we show that the average grain boundary mobility can be determined if the fraction of plastic work converted to stored energy is known, or vice versa. For low annealing temperatures, we observe a discrepancy between the model and experiments in the late stages of recrystallization, where a slowdown in recrystallization kinetics occurs in the experiments. We discuss possible sources of this slowdown and propose additional physical mechanisms that need to be accounted for in the model to improve its predictions.
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All authors David Montiel, Philip Staublin, Supriyo Chakraborty, Tracy Berman, Chaitali Patil, Michael Pilipchuk, Veera Sundararaghavan, John Allison, Katsuyo Thornton
Materials Science · Cross-listed from cond-mat.str-el
Ab Initio Auxiliary-Field Quantum Monte Carlo in the Thermodynamic Limit
Jinghong Zhang, Meng-Fu Chen, Adam Rettig et al. (+5 more)
Ab initio auxiliary-field
quantum Monte Carlo (AFQMC) is a systematically improvable many-body method, but its application to extended solids has been severely limited by unfavorable computational scaling and
memory requirements that obstruct direct access to the thermodynamic and complete-basis-set limits. By combining tensor hypercontraction with $\mathbf{k}$-point
symmetry, we reduce the computational and
memory scaling of ab initio AFQMC for solids to $\mathcal O(N^3)$ and $\mathcal O(N^2)$, respectively, with an arbitrary basis, comparable to
diffusion Monte Carlo. This enables direct and simultaneous thermodynamic-limit and complete-basis-set AFQMC calculations across insulating, metallic, and strongly correlated solids, without
embedding, local approximations, empirical finite-size corrections, or composite schemes. Our results establish AFQMC as a general-purpose, systematically improvable alternative to
diffusion Monte Carlo and coupled-cluster methods for predictive ab initio simulations of solids, enabling accurate energies and
magnetic observables within a unified framework.
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All authors Jinghong Zhang, Meng-Fu Chen, Adam Rettig, Tong Jiang, Paul J. Robinson, Hieu Q. Dinh, Anton Z. Ni, Joonho Lee
Materials Science · Cross-listed from cond-mat.soft
Design Principles for Fluid Molecular Ferroelectrics
Calum J Gibb, Jordan Hobbs, William C Ogle et al. (+1 more)
Fluid molecular ferroelectrics are a new class of organic materials where ferroelectricity is found in conjunction with 3D fluidity whilst still retaining spontaneous polarization values comparable to their traditional solid state counterparts. One of the major challenges for soft condensed matter physics is predicting whether a fluid molecular material will form ferroelectric phase with nematic or smectic order. Through the synthesis of forty five systematically varied molecules, and by analogy to solid molecular ferroelectrics, is it shown that subtle hydrogen fluorine substitution allows for tuneable syn-parallel pairing motifs resulting in either specific pairings leading too geometrically constrained lamellar order or diversified pairings stabilising nematic ordering. Large-scale, fully atomistic molecular dynamics simulations reveal that smectic ferroelectricity emerges from discrete lateral pairing modes, whereas nematic phases arise from a multiplicity of equivalent polar configurations. Together, these findings establish experimentally validated design principles for fluid molecular ferroelectrics and provide a predictive framework for engineering functional polar fluids.
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All authors Calum J Gibb, Jordan Hobbs, William C Ogle, Richard J Mandle
Materials Science
Growth and crystallographic structure of TiTe$_2$ on Au(111): From sub-monolayer structures to single- and multi-layer films
Andreas Raabgrund, Tilman Kißlinger, Alexander Wegerich et al. (+2 more)
We investigated the initial growth of TiTe$_2$ on Au(111) from sub-monolayer to multi-layer coverage by scanning tunneling microscopy (STM), low-energy electron diffraction intensity analysis (LEED-IV), and density functional theory (DFT). In the submonolayer regime we find a stable and well-ordered $(5\times\sqrt{3})_{\mathrm{rect}}$ superstructure consisting of separated TiTe$_2$ molecules, whereby the Ti atoms substitute Au atoms of the first substrate layer as proven by LEED-IV. By adding further Ti and Te in a 1:2 ratio and proper annealing dealloying sets in and a homogeneous 1T-TiTe$_2$ monolayer film on an unreconstructed substrate is formed. The resulting moiré structure is close to a $(4 \times 4)$ superstructure w.r.t. Au(111) and has a slightly expanded in-plane
lattice parameter compared to the 1T-TiTe$_2$ bulk value. With further stoichiometric deposition, thicker 1T-TiTe$_2$ films grow. Surprisingly, a five layer thick film exhibits an even larger
lattice-parameter (1.5 % larger than the bulk value). All LEED-IV analyses are based on best-fit R-factors of $R \le 0.13$.
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All authors Andreas Raabgrund, Tilman Kißlinger, Alexander Wegerich, Lutz Hammer, M. Alexander Schneider
Materials Science
Universal Framework for Decomposing Ionic Transport into Interpretable Mechanisms
KyuJung Jun, Pablo A. Leon, Jurğis Ruža et al. (+2 more)
Understanding mechanisms of ion transport in bulk materials is central to designing next-generation ion conductors for energy storage devices, yet studies employing all-atom molecular dynamics (MD) have largely been limited to reporting overall transport coefficients without a quantitative, spatiotemporally resolved breakdown of how charge is carried. We present a computational framework that analyzes MD trajectories to quantitatively interpret macroscopic transport by decomposing it into additive contributions from physically motivated events. They are defined either through heuristically identified microscopic transitions, capturing events such as single-ion hops, multi-ion hops, and vehicular motion, or through transitions between chemically interpretable coordination macrostates. The construction guarantees that attributed contributions sum exactly to the Onsager transport coefficients estimated via the Green-Kubo/Einstein formalism, while scanning the sampling window exposes characteristic temporal scales at which distinct transport mechanisms emerge and dominate. Applied across three prototypical electrolytes-inorganic
crystals,
liquids, and
polymers-the framework quantitatively resolves long-standing debates (e.g., the role of concerted motion and exchange), identifies dominant mechanisms and rate-limiting steps, quantifies their frequencies and effectiveness, and extracts activation energies for distinct transport modes, thereby distilling design rules for fast conduction. This general and reproducible analysis tool turns MD trajectories into quantitative mechanism maps, enabling the ion-conductor community to adjudicate mechanistic hypotheses and accelerate discovery.
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All authors KyuJung Jun, Pablo A. Leon, Jurğis Ruža, Juno Nam, Rafael Gómez-Bombarelli
Materials Science
Understanding the influence of yttrium on the dominant twinning mode and local mechanical field evolution in extruded Mg-Y alloys
Chaitali Patil, Qianying Shi, Abhishek Kumar et al. (+2 more)
Twinning is a primary deformation mechanism in Mg
alloys. This study focuses on tension twins during uniaxial compression of Mg-Y
alloys, with three key aspects: the orientation specificity of twin grains, the relative
evolution of CRSS with increasing Y content, and the local stress and strain
evolution at twin sites. Experimental characterization and
crystal plasticity modeling were performed. In Mg-7wt.%Y, TT2-{112-1} tension twins were observed in addition to the common TT1-{101-2} twins. Increasing Y suppressed TT1 formation while promoting TT2 activity. A previously unreported group of crystallographic orientations with a higher global Schmid factor for <c+a> slip was identified, which exhibited TT1 twinning with increasing compression strain. To elucidate Y effects on twin activity and local mechanical fields, both TT1 and TT2 tension twin modes were incorporated into PRISMS-Plasticity, an open-source, finite element-based
crystal plasticity solver. Four binary Mg-Y
alloys were modeled under compression, and statistical analysis was conducted to correlate initial orientations, stress-strain distributions, and twin activities as functions of Y concentration. The plasticity analysis revealed that increasing Y decreases the CRSS ratio of prismatic and pyramidal slip relative to TT1 twinning, while the slip-to-twin CRSS ratio for TT2 increases, thereby serving as a potential indicator of differential twin activity with Y addition in Mg
alloys. Additionally, despite their small volume fraction, TT2 twin sites were predicted higher local strain accumulation locally, relative to the representative volume element and TT1 twins, suggesting their potential influence on localized phenomena such as recrystallization or twin nucleation. These findings provide insight into local mechanical behavior in Mg
alloys and support
alloy design for advanced engineering applications.
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All authors Chaitali Patil, Qianying Shi, Abhishek Kumar, Veera Sundararaghavan, John Allison
Materials Science
Stoichiometry Dependent Properties of Cerium Hydride: An Active Learning Developed Interatomic Potential Study
Brenden W. Hamilton, Travis E. Jones, Timothy C. Germann et al. (+1 more)
Cerium hydride has a variety of interesting properties, including a known
lattice contraction and densification with increasing hydrogen content. However, precise stoichiometric control is not experimentally straightforward and ab initio approaches are not computationally feasible for many properties such as melting and low temperature
diffusion. Therefore, we develop a machine-learned interatomic potential for cerium hydride that is valid for H to Ce ratios from 2.0 to 3.0. A query-by-committee active learning approach is used to develop the training set. Leveraging classical molecular dynamics simulations, we assess a range of properties and provide fundamental mechanisms for the trends with stoichiometry. A majority of the properties follow the trend of
lattice contraction, being governed by the stronger
lattice binding induced by adding octahedral atoms.
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All authors Brenden W. Hamilton, Travis E. Jones, Timothy C. Germann, Benjamin T. Nebgen
Materials Science
Addressing Ill-conditioning in Density Functional Theory for Reliable Machine Learning
L. Arnstein, J. Wetherell, R. Lawrence et al. (+2 more)
In principle, machine learning (ML) can be used to obtain any electronic property of a many-body system from its electron density within density functional theory. However, some physical quantities are highly sensitive to small variations in the density. This 'ill-conditioning' limits the accuracy with which these quantities can be learned as density functionals from a fixed amount of data. We identify sources of ill-conditioning present in density functionals that belong to two ubiquitous classes: 1) Physical quantities that are globally gauge-dependent, meaning they change value if a constant shift is applied to the external potential – for example, the total energy; 2) Functionals of the N-electron density that have an implicit dependence on the (N+1)-electron density, such as the fundamental gap. We demonstrate that widely used ML models exhibit orders-of-magnitude greater error when applied to these ill-conditioned density functionals compared to other functionals that fall into neither class, even when the global gauge is fixed to prevent constant shifts. Owing to an absence of ill-conditioning in potential functionals, we find that providing the external potential as input to the ML model leads to significantly improved predictions of quantities in these two classes.
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All authors L. Arnstein, J. Wetherell, R. Lawrence, P. J. Hasnip, M. J. P. Hodgson
Materials Science
The rise and fall of an oxide: growth and evolution of Bismuth Oxide on Au(111)
Alberto Turoldo, Marco Bianchi, Deborah Perco et al. (+4 more)
We report a comprehensive, multi-technique study of bismuth
oxide growth on Au(111) under different conditions such as temperature and exposure to molecular oxygen. By combining synchrotron-based X-ray photoelectron spectroscopy and diffraction with low-energy electron diffraction and scanning tunneling microscopy, we elucidate the structural
evolution of the system during controlled oxidation and subsequent annealing. We find that Bi deposition induces well-defined surface reconstructions, whereas oxidation triggers the formation of a complex sequence of bismuth
oxide domains. High-resolution spectroscopic and diffraction data enable us to pinpoint the structure of the
oxide consistent with the $(201)$ surface of $\beta$-Bi$_2$O$_3$. In addition, angle resolved photoemission experiments and work function measurements reveal substantial electronic modifications triggered by a complex interplay of segregation of Bi and Au. Notably, a work function of 3.4 eV is achieved for oxidation at 423 K. These results provide
benchmark structural and electronic insights into the Bi
oxide/Au(111) system and establish a framework for integrating Bi$_2$O$_3$ as a contact material in devices using two-dimensional
semiconductors, where it can enable low contact resistance.
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All authors Alberto Turoldo, Marco Bianchi, Deborah Perco, Andrea Berti, Philip Hofmann, Alessandro Baraldi, Silvano Lizzit
Materials Science · Cross-listed from cond-mat.mes-hall
Exciton-Selective Phonon Coupling in a Lead Halide Perovskite
Pradeepa H. L., Sagnik Chatterjee, Sayantan Patra et al. (+6 more)
Exciton-
phonon interactions govern the optical response of
semiconductors, yet disentangling multiple coupling channels in lead halide perovskites remains challenging. We investigate CsPbBr3 microcrystals using photoluminescence, Raman and reflectance spectroscopy at low temperature, revealing the simultaneous presence of high-energy and Rashba
excitons, each accompanied by distinct
phonon replica series. High-energy
exciton replicas are uniquely spaced by approximately 9 meV, whereas Rashba
exciton replicas exhibit a characteristic approximately 6 meV spacing, indicating the specificity of the
exciton-
phonon coupling. Unsupervised machine learning applied to a large low-temperature photoluminescence
dataset reveals these replica features are prevalent. With increasing temperature, replica features broaden and merge, evolving into a dominant longitudinal optical
phonon coupling regime at room temperature. This work establishes direct spectroscopic evidence for concurrent,
exciton-specific
phonon coupling within a single material, offering new pathways to engineer light-matter interactions for optoelectronic and
phonon-
photon-based
quantum device applications.
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All authors Pradeepa H. L., Sagnik Chatterjee, Sayantan Patra, Swapneswar Bisoi, Saqlain Mushtaq, Hardeep, Akshay Singh, Ashish Arora, Atikur Rahman
Materials Science
Thermal Decoherence and Population Transfer of MeV Channeling Electrons in Diamond
Tadas Paulauskas
Channeling radiation from MeV-regime electrons is governed by transitions between quantized transverse bound states, but experimental spectra are strongly modified by thermal diffuse scattering. To capture these open-system dynamics, a frozen-
phonon multislice framework is combined with bound-state projection analysis to construct depth-dependent reduced density
matrices in selected transverse
manifolds. Beyond reproducing experimental channeling-radiation transition energies, this approach separates thermal population transfer, intra-
manifold decoherence, and cross-
manifold coherence loss. Applied to 16.9 MeV axial electron channeling in $\langle100\rangle$ diamond, the results show approximately exponential population decay from the initially occupied states, accompanied by strongly channel-dependent feeding among low-lying
manifolds. Starting from a coherent superposition within the degenerate 2p
manifold,
stochastic symmetry breaking by thermal displacements drives the intra-
manifold purity toward the maximally mixed limit, indicating rapid phase scrambling. Under 1s initialization, population transferred into the 2p and 3d
manifolds remains internally close to maximally mixed, yet a weak residual 2p-3d cross-
manifold coherence persists. This framework goes beyond static mean-field thermal broadening and provides a microscopic basis for evaluating population dynamics and coherence lifetimes in strongly quantized channeling-radiation systems.
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Materials Science · Cross-listed from cond-mat.stat-mech
Quantum-classical correspondence for spins at finite temperatures: theory and applications
A. El Mendili and M. E. Zhitomirsky
We derive a rigorous
quantum-to-classical mapping for interacting
spin systems at finite temperatures. Specifically, the large-$S$ asymptotic form of the partition function is given by the partition function of a classical model for vectors of length $S_C=\sqrt{S(S+1)}$.
Quantum corrections to the asymptotic result form a series in powers of $1/[S(S+1)]$. We calculate the leading temperature-independent
quantum correction to the
magnetic anisotropy constant. The established mapping justifies the use of classical Monte Carlo simulations for realistic
magnetic Hamiltonians, including anisotropic and frustrated interactions. As an application, we compute the Curie and Néel temperatures for a range of
magnetic materials with known microscopic interaction constants. The obtained transition temperatures are in good agreement with measured values. Monte Carlo results for the
magnetic susceptibility of the collinear antiferromagnet MnF$_2$ above and below the transition are also compared with experimental data.
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Materials Science
When Is Structural Lubricity Load Independent? The Role of Contact Geometry and Elastic Compliance
Hongyu Gao
Using molecular dynamics simulations of an incommensurate Au(111)/graphite interface, we investigate the conditions under which structural lubricity produces load-independent friction. We show that strict load independence occurs only in laterally infinite, area-filling contacts, where dissipation is governed by
phonon-mediated viscous coupling and the shear stress scales linearly with sliding velocity. Finite contacts with explicit boundary terminations exhibit substantially higher friction yet remain load independent up to a critical load. Load dependence arises only when elastic out-of-plane deformation near the contact line exceeds a critical amplitude, activating additional dissipation channels. These results demonstrate that contact geometry and local elastic compliance, rather than normal load itself, determine the onset and breakdown of load-independent structural lubricity.
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Materials Science · Cross-listed from cond-mat.mes-hall
Confinement Epitaxy of Large-Area Two-Dimensional Sn at the Graphene/SiC Interface
Zamin Mamiyev, Niclas Tilgner, Narmina O. Balayeva et al. (+3 more)
Confinement epitaxy beneath
graphene stabilizes exotic material phases by restricting vertical growth and altering lateral
diffusion, conditions unattainable on bare substrates. However, achieving long-range interfacial order while maintaining high-quality
graphene remains a significant challenge. Here, we demonstrate the synthesis of large-area quasi-free-standing monolayer
graphene (QFMLG) via the intercalation of a two-dimensional (2D) Sn. While the triangular Sn(1x1) interface exhibits a robust metallic band structure, the decoupled QFMLG maintains charge neutrality, confirmed by photoemission spectroscopy. Using high-resolution Raman spectroscopy and microscopy, we distinguish between direct intercalation and
diffusion-driven expansion, identifying the latter as the critical pathway to superior QFMLG crystalline quality. Temperature-dependent analysis reveals dynamical structural coupling between the decoupled QFMLG and the Sn interface, providing a novel degree of freedom for strain engineering. Beyond uncovering the
diffusion-driven mechanism, this work establishes metal intercalation as an effective strategy for tailoring durable
graphene-metal
heterostructures with tunable properties for next-generation
quantum materials platforms.
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All authors Zamin Mamiyev, Niclas Tilgner, Narmina O. Balayeva, Dietrich R.T. Zahn, Thomas Seyller, Christoph Tegenkamp
Materials Science
Hidden universality in dislocation-loops mediated three-dimensional crystal melting
Alessio Zaccone and Konrad Samwer
Understanding why and how crystalline solids melt remains a central problem in condensed-matter physics. Dislocation loops are fundamental topological excitations that control the thermodynamic stability of
crystals, yet their role in setting universal aspects of melting has remained unclear. Here we show, within dislocation-mediated melting theory, that the free-energy condition for loop proliferation leads to a universal ratio between the energy of a minimal dislocation loop and the thermal energy at melting. For minimal dislocation loops that begin to proliferate at the onset of melting, this ratio takes the purely
geometric value $\mathcal{E}_* = E_{\rm loop}/(k_B T_m) \approx 25.1$, independent of elastic
moduli and chemistry-dependent details. This result provides a microscopic explanation for recent empirical findings by Lunkenheimer et al., who identified a closely related universal energy scale $\approx 24.6$ from viscosity data. The same framework also rationalizes the empirical $2/3$ rule relating the glass-transition and melting temperatures.
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Materials Science
AI-Driven Structure Refinement of X-ray Diffraction
Bin Cao, Qian Zhang, Zhenjie Feng et al. (+4 more)
Artificial intelligence can rapidly propose candidate phases and structures from X-ray diffraction (XRD), but these hypotheses often fail in downstream refinement because peak intensities cannot be stably assigned under severe overlap and diffraction consistency is enforced only weakly. Here we introduce the whole-pattern expectation–maximization (WPEM)
algorithm, a physics-constrained whole-pattern decomposition and refinement workflow that turns Bragg's law into an explicit constraint within a batch expectation–maximization framework. WPEM models the full profile as a probabilistic mixture density and iteratively infers component-resolved intensities while keeping peak centres Bragg-consistent, producing a continuous, physically admissible intensity representation that remains stable in heavily overlapped regions and in the presence of mixed radiation or multiple phases. We
benchmark WPEM on standard reference patterns (PbSO$_4$ and Tb$_2$BaCoO$_5$), where it yields lower $R_p/R_{wp}$ than widely used packages (FullProf and TOPAS) under matched refinement conditions. We further demonstrate generality across realistic experimental scenarios, including phase-resolved decomposition in multiphase materials, quantitative recovery of mixture compositions, separation of crystalline peaks from amorphous backgrounds in semicrystalline systems, high-throughput operando
lattice tracking, automated refinement of compositionally disordered solid solutions, and quantitative phase-resolved analysis of complex archaeological samples from synchrotron powder XRD. By providing Bragg-consistent, uncertainty-aware intensity partitioning as a refinement-ready interface, WPEM closes the gap between AI-generated hypotheses and diffraction-admissible structure refinement on challenging XRD data.
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All authors Bin Cao, Qian Zhang, Zhenjie Feng, Taolue Zhang, Jiaqiang Huang, Lu-Tao Weng, Tong-Yi Zhang
Materials Science · Cross-listed from quant-ph
Why the Casimir Force for Magnetic Metals Computed by the Lifshitz Theory Using the Drude Model Disagrees with the Measurement Data
G. L. Klimchitskaya, C. C. Korikov and V. M. Mostepanenko
We consider the Casimir force in configurations with
magnetic metal plates and analyze the reasons why the predictions of the Lifshitz theory using the dielectric permittivity of the Drude model are inconsistent with the measurement data. For this purpose, the contributions of the electromagnetic waves with the transverse
magnetic and transverse electric polarizations to the Casimir force are computed using the Lifshitz theory expressed in terms of the pure imaginary Matsubara frequencies. Furthermore, the fractions of the evanescent and propagating waves in these contributions are found using an equivalent formulation of the Lifshitz theory along the real frequency axis. All computations are performed for Au-Ni and Ni-Ni plates using the Drude model and the experimentally consistent
plasma model over the separation region from 0.5 to 6~mum, where the total force value is determined by conduction electrons. It is shown that the transverse
magnetic contribution to the Casimir force does not depend on the used model of the dielectric permittivity, so that the total difference between the predictions of the Lifshitz theory using the Drude model and the measurement data is determined by the transverse electric contribution. In doing so, as opposed to the case of nonmagnetic metals, both fractions of the evanescent and propagating waves in this contribution depend on the model of the dielectric permittivity used in computations, whereas the
magnetic properties of the plate metal influence the Casimir force solely through the fraction of propagating waves in the transverse electric contribution. The issue of a more adequate theoretical description of the electromagnetic response of
magnetic metals is discussed.
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Materials Science · Cross-listed from cond-mat.str-el
Stripe antiferromagnetism in van der Waals metal HoTe3 decoupled from charge density wave order
Weiyi Yun, Ryota Nakano, Ryo Misawa et al. (+10 more)
The $R\mathrm{Te}_3$ ($R = \text{rare earth}$) family of layered van der Waals (vdW) compounds hosts coexisting
magnetic and charge density wave (CDW) orders, yet the interplay between these degrees of freedom remains little explored. Combining polarized and unpolarized
neutron diffraction on single-
crystal $\mathrm{HoTe}_3$, we identify two distinct
antiferromagnetic (AFM) phases, both exhibiting a collinear $\uparrow\uparrow\downarrow\downarrow$ motif within individual vdW layers. The two phases are distinguished by the vdW stacking of
magnetic layers:
ferromagnetic (FM) stacking in the higher-temperature AFM-II phase, here termed “vertical-stripe”, and AFM stacking in the AFM-I ground state, here termed “tilted-stripe”; the two phases have propagation vectors $\boldsymbol{q}_{\mathrm{m2}} = (0.48, 0, 0)$ and $\boldsymbol{q}_{\mathrm{m1}} = (0.5, 0.5, 0)$, respectively. In contrast to the CDW-driven exotic magnetism in $\mathrm{DyTe}_3$, $\mathrm{TbTe}_3$, and $\mathrm{GdTe}_3$, we find no evidence for coupling between magnetism and CDW in $\mathrm{HoTe}_3$. The relative
alignment between AFM and CDW propagation vectors, as well as single-ion anisotropy, are likely essential for generating coupled
spin/charge orders in layered vdW systems.
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All authors Weiyi Yun, Ryota Nakano, Ryo Misawa, Rinsuke Yamada, Shun Akatsuka, Yoshichika Onuki, Priya Ranjan Baral, Hiraku Saitoh, Ryoji Kiyanagi, Takashi Ohhara, Taro Nakajima, Taka-hisa Arima, Max Hirschberger
Materials Science
Unveiling and quantifying the topology-dependent pre-melting of nanoparticles
Marthe Bideault, Arnaud Allera, Ryoji Asahi et al. (+2 more)
The melting of metallic nanoparticles is governed by surface premelting, a phenomenon traditionally modeled as the isotropic growth of a uniform
liquid shell. Challenging this classical view, we report facet-dependent premelting in hexagonal close-packed Co nanoparticles, arising from the structural heterogeneity of their surface. In molecular dynamics simulations (587 to 13047 atoms), the onset of surface mobility is observed as low as 20% of the bulk melting point, driven by the early disordering of stepped $\{01\bar{1}1\}$ facets. These facets consistently melt nearly 150 K below flat $\{0001\}$ facets, regardless of particle size. We show that both surface and facet melting temperatures scale with nanoparticle size through the Gibbs-Thomson effect, and determine a size-dependent critical
liquid layer thickness that triggers complete melting of the nanoparticle, which saturates near three atomic layers. Our results confirm recent experimental observations of surface premelting and extend the framework to anisotropic particles with facet-orientation-dependent behavior.
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All authors Marthe Bideault, Arnaud Allera, Ryoji Asahi, Jérôme Creuze, Erich Wimmer
Materials Science · Cross-listed from physics.optics
Breaking the Moss rule
Søren Raza, Kristian Sommer Thygesen and Gururaj Naik
Photonic devices depend critically on the dielectric materials from which they are made, with higher refractive indices and lower absorption losses enabling new functionalities and higher performance. However, these two material properties are intrinsically linked through the empirical Moss rule, which states that the refractive index of a dielectric decreases as its band gap energy increases. Materials that surpass this rule, termed super-Mossian dielectrics, combine large refractive indices with wide optical transparency and are therefore ideal candidates for advanced photonic applications. This Review surveys the expanding landscape of high-index dielectric and
semiconductor materials, with a particular focus on those that surpass the Moss rule. We discuss how electronic band structures with a large joint density of states near the band edge give rise to super-Mossian behavior and how first-principles computational screening can accelerate their discovery. Finally, we establish how the refractive index sets the performance limits of nanoresonators, waveguides, and metasurfaces, highlighting super-Mossian dielectrics as a promising route toward the next performance leap in photonic technologies.
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Materials Science
Negative Strain-Rate Sensitivity in Metallic Glasses Driven by Rejuvenation-Relaxation Competition: Kinetic Monte Carlo Simulations and a Minimal Effective Model
Tomoaki Niiyama, Akio Ishii, Takahiro Hatano et al. (+2 more)
When strain-rate sensitivity (SRS) is negative in metallic glasses, the material becomes weaker as the deformation rate increases, leading to accelerated plastic deformation and, eventually, catastrophic fracture. In this study, we elucidate the mechanism underlying the negative SRS using micromechanics-based kinetic Monte Carlo simulations that couple heterogeneous randomized shear transformation zone (STZ) models for metallic glasses. The model accounted for both the thermomechanical structural rejuvenation and relaxation of the energy barrier for thermal activation of STZs, incorporating a Kohlrausch-Williams-Watts (KWW)-type relaxation function. The present simulations systematically reproduce the dependence of flow stresses on strain rate, temperature, and the form of the relaxation function. The SRS tends to decrease at high strain rates and low temperatures in the simulations, and negative SRS appears when a compressed-exponential relaxation function is employed. Shear localization also appears; however, the conditions under which the observed localization emerges do not fully coincide with those leading to the negative SRS, leaving the dominant factor unclear. To clarify the dominant factor, we introduce a simplified theoretical model that reproduces flow stresses consistent with the simulation results. An analytical expression derived from the theoretical model reveals that negative SRS originates primarily from the temporal
evolution of the activation barrier. Specifically, negative SRS arises when the timescale of external loading exceeds that of STZ relaxation.
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All authors Tomoaki Niiyama, Akio Ishii, Takahiro Hatano, Tomotsugu Shimokawa, Shigenobu Ogata
Materials Science
Dislocation-ledge coupling governs semicoherent precipitate growth
Jin-Yu Zhang, Juan Du, Lin Yang et al. (+3 more)
Many crystalline materials acquire their properties as one
crystal phase grows through another, but a long-standing defect-kinetic puzzle remains: how dense interfacial dislocation networks between two
crystals advance, reorganize, and accommodate strain without relying on ordinary glide. Here, using in situ transmission electron microscopy, O-
lattice analysis, and three-dimensional phase-field
crystal simulations, we identify how semicoherent interfaces overcome this constraint during lath precipitate growth. In situ observations of austenite precipitates in duplex stainless steel reveal nanometer-high growth ledges propagating laterally along migrating habit planes. O-
lattice analysis shows how
lattice misfit prescribes a closed network across habit planes, side facets, and end faces. Simulations resolve the hidden three-dimensional dynamics: the network undergoes
diffusion-assisted, non-conservative motion, producing steady end-face advance and ledge-mediated broad-facet migration while accommodating transformation strain. These results reveal a general defect-kinetic route linking point-defect transport, dislocation-network motion, interface migration, and morphology
evolution.
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All authors Jin-Yu Zhang, Juan Du, Lin Yang, Frédéric Mompiou, Shigenobu Ogata, Wen-Zheng Zhang
Materials Science
Generative Inverse Estimation of 3D Atomic Coordination from Near-Edge Spectra via Equivariant Diffusion Models
Ren Okubo, Yu Fujikata, Izumi Takahara et al. (+1 more)
Extracting 3D atomic coordinates from spectroscopic data is a longstanding inverse problem. We present an equivariant
diffusion model that generates site-specific 3D structures directly from near-edge spectra (ELNES/XANES). Trained on Si-O
crystals, the model achieves radial accuracy comparable to Extended X-ray Absorption Fine Structure (EXAFS) (RMSD ~0.06 Å) but with superior coordination number precision (errors < 4.3% vs. EXAFS ~20%). Crucially, it reconstructs full 3D geometries including bond angles, overcoming the limitations of 1D radial distribution analysis. The model demonstrates robust out-of-distribution generalization, accurately predicting local structures in amorphous systems despite being trained exclusively on crystalline
lattices. Application to experimental O K-edge spectra from {\alpha}-quartz validates practical applicability. This generative approach outperforms template matching and establishes automated, quantitative 3D structure determination from spectroscopic data.
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All authors Ren Okubo, Yu Fujikata, Izumi Takahara, Teruyasu Mizoguchi