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Physics Condensed Matter Materials Science

cond-mat.mtrl-sci Watch Materials Science

Tuesday 7 April

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

Introduction to Mechanics and Structures

Martina Scapin

This work provides a comprehensive overview of the fundamental concepts in continuum mechanics, focusing on the behaviour of materials under mechanical loads. It discusses the distinction between elastic and plastic, highlighting their atomic origins and macroscopic implications. Elastic behaviour is examined via Hooke's law and constitutive matrices, while plasticity is treated through yield surfaces, flow rules, and hardening laws, including isotropic and kinematic hardening. In addition, the theoretical foundations and design principles of pressure vessels and thin axisymmetric shells, focusing on their mechanical behaviour under internal or external pressure, is discussed. The analysis is based on shell theory, assuming thin walls and axisymmetric geometry, which simplifies the stress distribution into membrane stresses. The work also addresses buckling phenomena under external pressure, secondary stresses at geometric discontinuities, and design provisions from the EN 13445 standard.
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Materials Science · Cross-listed from cond-mat.mes-hall

Controlled dewetting and phase transition hysteresis of VO2 nanostructures

Peter Kepič, Petra Kalousková, Tomáš Šikola et al. (+1 more)

As artificial intelligence continues to grow, so does the need for more efficient ways to process data. Besides moving from electronic to photonic circuits, a promising approach is to integrate phase-change materials. Vanadium dioxide (VO$_2$) exhibits an ultrafast, near-room-temperature phase transition, characterized by hysteresis and large optical modulation – making it a promising candidate for short-term memories and for mimicking neural behavior in brain-like computing systems. While the hysteresis behavior of VO$_2$ has been well studied in thin films and nanostructures, practical control and device integration have been limited only to thin films. Here, we demonstrate control over the phase transitions of VO$_2$ nanocylinders via lithographic patterning, controlled crystallization, and controlled dewetting. Because nanostructures are easier to address and consume less power than films, the ability to fabricate them with tailored geometry and hysteresis properties directly on integrated platforms is a key step toward scalable, energy-efficient memory and neuromorphic photonic devices.
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All authors Peter Kepič, Petra Kalousková, Tomáš Šikola, Filip Ligmajer

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

ORION: Unifying Top-Down and Bottom-Up Chemical Space Sampling for a Universal Organic Force Field

Zherui Chen, Jiayu Zhang, Yuxuan Tian et al. (+7 more)

Empirical force fields remain the primary tool for large-scale molecular simulation, yet their limited flexibility and transferability often hinder predictive modeling in chemically complex condensed-phase systems. Here we present ORION, a universal machine-learning force field for C, H, O, N, S, and P systems developed within the Neuroevolution Potential (NEP) framework. To enhance transferability across diverse chemical environments, ORION was trained on a chemically rich dataset constructed through an integrated top-down and bottom-up strategy, enabling accurate descriptions of complex organic configurations, reactive intermediates, and weak intermolecular interactions. ORION achieves near-density-functional-theory accuracy while retaining the efficiency required for large-scale molecular dynamics simulations. On the test set, it predicts atomic forces with substantially higher accuracy than ReaxFF while running 215.5 times faster under identical hardware conditions, making simulations on the hundreds-of-nanoseconds timescale readily accessible. The model provides a balanced description of bond breaking and formation, aromatic growth, hydrogen bonding, van der Waals interactions, and {\pi}-stacking, demonstrating strong transferability across both reactive and nonreactive systems. These results establish ORION as a practical and general force field for predictive simulations in chemistry and materials science, and provide an effective route toward universal machine-learning force fields with both high accuracy and broad applicability.
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All authors Zherui Chen, Jiayu Zhang, Yuxuan Tian, Zhoulin Liu, Sining Dai, Yanghui Li, Cong Chen, Dingyuan Tang, Yajun Deng, Qingxia Liu

Materials Science

The effect of Nb and O on the martensitic transformation in the Ti-Nb-O alloys

Kristián Šalata, Dalibor Preisler, Josef Stráský et al. (+3 more)

This study examines the influence of niobium and oxygen on phase stability, crystal structure, and martensitic transformation pathways in Ti-Nb-O alloys. A series of Ti-(8-28)Nb-(0-3)O (at.%) alloys were prepared and solution-treated in the $\beta$-phase field. Microstructure and crystallography were characterized by X-ray diffraction, electron microscopy, and reciprocal-space mapping. A 2D-XRD orientation simulation approach was applied to distinguish all 12 crystallographically equivalent $\alpha"$ martensitic variants originating from a single prior $\beta$ grain, enabling detailed diffraction analysis. This method further allowed quantitative evaluation of the atomic shuffle parameter y, describing the $\beta\rightarrow\alpha"$ transformation. The results demonstrate that Nb primarily governs $\alpha"$ martensite evolution. Increasing Nb stabilizes the $\beta$ phase and shifts the $\alpha"$ structure toward higher symmetry, as reflected by systematic changes in lattice parameters and increasing shuffle parameter y, indicating suppression of transformation toward the hexagonal $\alpha'$ phase. Oxygen, in contrast, modifies transformation pathways. At lower Nb contents, it suppresses the $\omega$ phase formation and promotes $\beta\rightarrow\alpha"$ transformation, while at higher Nb levels it inhibits long-range martensitic transformation, resulting in retained $\beta$ or competing $\omega$ phase. These effects are attributed to local lattice distortions induced by interstitial oxygen.
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All authors Kristián Šalata, Dalibor Preisler, Josef Stráský, Jiří Kozlík, Lukáš Horák, Václav Holý

Materials Science · Cross-listed from cond-mat.mes-hall

Indication of Stochastic Photothermal Dynamics around a Topological Defect in a Chiral Magnet

Dongxue Han, Asuka Nakamura, Takahiro Shimojima et al. (+4 more)

Chiral magnets host topologically protected spin textures whose nonequilibrium dynamics are crucial in phase transitions and domain evolution, yet ultrafast defect-mediated processes remain poorly understood. Here, we investigate photothermally induced helical-to-paramagnetic phase transition in Co$_9$Zn$_9$Mn$_2$ using pump-probe Lorentz transmission electron microscopy (LTEM). Following the suppression of the magnetic stripe contrast induced by femtosecond pulsed laser, we observe a directional recovery process of magnetic order driven by the anisotropic thermal diffusion, toward the thick region that effectively acts as a heat sink. Remarkably, around a magnetic edge dislocation, the magnetic contrast recovery exhibits a pronounced delay accompanied by a transient blurring of LTEM contrast. These findings suggest that the recovery dynamics around the magnetic edge dislocation proceed through multiple relaxation paths that are selected stochastically. Our results indicate a possible enhancement of stochasticity around topological defects during the recovery dynamics of magnetic phase transitions.
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All authors Dongxue Han, Asuka Nakamura, Takahiro Shimojima, Kosuke Karube, Yasujiro Taguchi, Yoshinori Tokura, Kyoko Ishizaka

Materials Science

A Physics-Informed Chemical Rule for Topological Materials Discovery

Xinyu Xu, Arif Ullah and Ming Yang

Topological phases of matter$\unicode{x2013}$comprising both insulators and semimetals$\unicode{x2013}$offer great potential for quantum applications, but identifying new candidates remains challenging due to expensive first-principles simulations and labor-intensive experimental workflows. Here we introduce a physics-informed chemical rule that integrates compositional, orbital and crystallographic descriptors within an interpretable linear framework. By explicitly encoding electron filling, space-group symmetry and orbital-resolved chemical environments, our method overcomes a fundamental limitation of composition-only heuristics$\unicode{x2013}$their inability to distinguish polymorphs with identical stoichiometry but different crystal structures. Using only elemental characteristics, our approach reduces a material's topological propensity to a single, physically interpretable score, enabling rapid and high-throughput assessment. The model achieves superior predictive performance while maintaining physical transparency, and identifies candidate topological materials where conventional symmetry indicators fail. Consequently, our framework enables rapid and interpretable exploration of complex materials spaces, establishing a scalable paradigm for the intelligent discovery of next-generation topological and quantum materials.
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Materials Science

A coupled fully kinetic hydrogen transport and ductile phase-field fracture framework for modeling hydrogen embrittlement

Abdelrahman Hussein, Yann Charles, Jukka Kömi et al. (+1 more)

Modeling hydrogen embrittlement (HE) is a long-standing engineering challenge, which has experienced significant developments in recent years. Yet, there is a gap in modeling the effect of the kinetics of hydrogen segregation at dislocations and the resulting interaction between ductile tearing and hydrogen-induced brittle fracture. In this work, a comprehensive chemo-mechanical framework is developed by coupling the fully kinetic hydrogen transport model with the geometric phase-field fracture method. A novel driving force is proposed that utilizes a hyperbolic tangent function of stress triaxiality to ensure that plastic dissipation contributes to fracture only under tensile conditions, phenomenologically representing void-driven ductile damage. The model successfully predicts the hydrogen-dependent shift in damage initiation from the specimen core to the surface. More importantly, hydrogen segregation at dislocations was shown to be crucial for modeling the multiple surface cracking experimentally observed at the necking region. Furthermore, the framework captures the competition between loading rates and diffusion kinetics, resolving the transition from multiple circumferential surface cracking at high strain rates to center-initiated single crack at lower rates. Finally, the model reproduced the experimental J-resistance curves for compact tension specimens, showing the transition from ductile tearing to embrittled crack.
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All authors Abdelrahman Hussein, Yann Charles, Jukka Kömi, Vahid Javaheri

Materials Science · Cross-listed from cond-mat.mes-hall

Room Temperature Anisotropic Photoresponse in Low-Symmetry van der Waals Semiconductor CrPS$_4$

Cédric A. Cordero-Silis, Daniel Vaquero, Teresa López-Carrasco et al. (+2 more)

The crystalline and optical anisotropy of low-symmetry two-dimensional (2D) materials can enable strong dichroic responses, enhancing polarization contrast for photonic and optoelectronic devices. Here, we unveil pronounced optical and optoelectronic anisotropy in chromium thiophosphate CrPS$_4$ arising from the strong coupling between light polarization and its intrinsic crystal symmetry. Linearly polarized reflectivity and scanning photocurrent measurements in the 1.37-2.48 eV range reveal a robust dichroic response. The linear dichroism in reflection RLD reaches ~50, while in photocurrent PCLD it increases to ~60, with a sign reversal of the RLD between 1.6-1.8 eV, enabling strong narrow-band polarization contrast at room temperature. We attribute these anisotropic responses to the interaction between polarized light and Cr$^{3+}$ d-orbital T$_1$ and T$_2$ transitions. Spatially resolved photocurrent mapping further shows that the photocurrent is strongly dependent on the crystallographic axis: a 3-fold enhancement is obtained along the b-axis compared to the a-axis, yielding a clear 180{\deg} modulation of photoresponse across different contact orientations. Together, our findings establish CrPS$_4$ as a highly anisotropic 2D semiconductor with strong linear dichroism and polarization-sensitive photoresponse at room temperature. These characteristics highlight CrPS4 as a promising platform for narrow-band polarized photodetectors, anisotropic photo-transport, and future 2D spintronic and magneto-optical devices.
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All authors Cédric A. Cordero-Silis, Daniel Vaquero, Teresa López-Carrasco, Harshan Madeshwaran, Marcos H. D. Guimarães

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

Development of a 3D-CNN-based Prediction Model for Migration Barriers in Plasma-Wall Interactions

Seiki Saito, Keisuke Takeuchi, Hiroaki Nakamura et al. (+5 more)

Understanding the long-term transport of hydrogen isotopes in plasma-facing materials, such as tungsten, is critical for the steady-state operation of magnetic confinement fusion reactors. However, dynamically updating the transition parameters for kinetic Monte Carlo (kMC) simulations as the atomic structure evolves under continuous plasma irradiation remains a severe computational bottleneck. Conventionally, calculating these migration barriers requires the iterative and computationally expensive Nudged Elastic Band (NEB) method. To overcome this limitation, this article presents a highly efficient surrogate model for predicting migration barriers using a three-dimensional Convolutional Neural Network (3D-CNN), establishing the final component necessary to realize on-the-fly molecular dynamics (MD) and kMC hybrid simulations. The proposed deep learning model takes a two-channel volumetric input, the local three-dimensional potential energy distribution and the voxelized spatial coordinates of the initial and final trapping sites, to directly output the migration barrier as a scalar value. Trained on a comprehensive dataset of tungsten-hydrogen configurations evaluated using the Embedded Atom Method (EAM) potential, the model demonstrated robust predictive accuracy, achieving a Mean Absolute Error (MAE) of 0.124 eV and a high coefficient of determination of 0.890. Furthermore, utilizing GPU acceleration, the inference time is reduced to approximately 2.7 milliseconds per barrier, achieving a speed-up ratio of over 23,000 compared to conventional NEB calculations. This extraordinary acceleration effectively resolves the computational barrier of transition rate evaluations, paving the way for large-scale, dynamic modeling of plasma-wall interactions.
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All authors Seiki Saito, Keisuke Takeuchi, Hiroaki Nakamura, Yasuhiro Oda, Kazuo Hoshino, Yuki Homma, Shohei Yamoto, Yuki Uchida

Materials Science

Ab initio GW-BSE theory of optical activity in $\alpha$-quartz

Xiaoming Wang and Yanfa Yan

We present an ab initio many-body theory of optical activity in solids within the GW-BSE framework. Dielectric spatial dispersion is formulated in two complementary forms: exciton envelope modulation and sum-over-exciton-states expansion. Our application to $\alpha$-quartz reveals that the envelope-modulated formulation captures the low-frequency region, whereas the sum-over-exciton-states formulation is essential to reproduce the correct full frequency dependence. Comparisons with the independent-particle approximation and simple local-field corrections further highlight the decisive role of excitonic many-body effects in shaping the spectral dispersion of optical activity in solids.
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Materials Science

Rationalizing defect formation energies in metals and semiconductors with semilocal density functionals

Jorge Vega Bazantes, Timo Lebeda, Akilan Ramasamy et al. (+4 more)

The study of defects in materials is of utmost importance for technological applications and the design of new materials. In this work, we analyze the performance of density functional approximations on two prototypical sets of defective systems: monovacancies in eight fcc metals, and interstitials in the semiconductor Si-diamond. Specifically, we compute defect formation energies using the local density approximation, the Perdew-Burke-Ernzerhof generalized gradient approximation, the meta-generalized gradient approximations (meta-GGAs) strongly constrained and appropriately normed (SCAN), its regularized version (r2SCAN), the Lebeda-Aschebrock-Kummel (LAK) meta-GGA, and the Heyd-Scuseria-Ernzerhof screened hybrid functional. For metals, the local density approximation shows better performance compared to the other approximations, whereas for silicon, the meta-generalized gradient approximation Lebeda-Aschebrock-Kummel yields outstand- ing accuracy, surpassing the hybrid functional and approaching the results of more computationally demanding Quantum Monte Carlo methods. To rationalize the different performances, we study the semilocal ingredients rs, s and {\alpha} in both the pristine and defective structures. We identify critical regions that indicate the observed trends of the defect formation energies and pave the way for improving density functional approximations.
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All authors Jorge Vega Bazantes, Timo Lebeda, Akilan Ramasamy, Kanun Pokharel, Ruiqi Zhang, John Perdew, Jianwei Sun

Materials Science

Chemical Short-Range Order Regulates Hydrogen Energetics and Hydrogen-Dislocation Interactions in CoNiV

Beihan Chen, Dalia Sayed Ahmed, Yang Yang et al. (+1 more)

Chemical short-range order (CSRO) has emerged as a critical structural feature in concentrated alloys, yet its coupling with hydrogen remains an active discussion. Here, we develop a machine-learning interatomic potential for the Co-Ni-V-H system and investigate how CSRO regulates hydrogen energetics and dislocation behavior in CoNiV, an alloy with reported strong resistance to hydrogen embrittlement. We identify strong V-centered ordering that suppresses V-V clustering and significantly reshapes the hydrogen solution landscape. Compared to a chemically random alloy, the ordered state exhibits higher average hydrogen solution energies and a reduced population of strongly binding sites, indicating lower bulk hydrogen uptake. At partial dislocations, hydrogen preferentially segregates to tensile core regions, acting as a shallow, reversible trap with a much weaker effect compared to chemical trapping states. These results demonstrate that local chemical order strongly regulates hydrogen-dislocation coupling and provide an atomistic understanding for tuning hydrogen-assisted deformation in concentrated CoNiV alloys.
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All authors Beihan Chen, Dalia Sayed Ahmed, Yang Yang, Miaomiao Jin

Materials Science

Historical Foundation and Practical Guideline for Ferroelectric Switching Kinetic Studies

Yi Liang, Pat Kezer and John T. Heron

Electrical measurements of ferroelectric switching kinetics are widely used to probe the dynamics of polarization reversal, yet the influence of the measurement circuit is often underappreciated. In this paper, we show that the interplay between ferroelectric capacitors and circuit elements produces distorted, time-dependent voltage waveforms across the device, particularly in the sub-ns regime. We examine how these circuit contributions affect polarization transients extracted from PUND measurements. The resulting distortions scale with supply voltage, capacitor dimensions, and lumped circuit elements, but are not accounted for in conventional experimental analyses or analytical model fitting. We then critically assess existing nucleation and growth models and show that neglecting the time-varying voltage profile can lead to unphysical interpretations of switching kinetics, most notably in the extracted growth dimensionality represented by the Avrami exponent. Finally, we outline guidelines for future studies, emphasizing the need for direct voltage monitoring and circuit-aware de-embedding, as well as modeling frameworks that incorporate voltage-dependent nucleation and growth rates based on intrinsic material parameters.
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Materials Science · Cross-listed from cond-mat.mes-hall

Orbital-driven emergent transport in altermagnets

Junyeong Choi and Kyoung-Whan Kim

Altermagnets have recently emerged as a promising platform for spintronics due to their unique magnetic symmetry. However, most studies have focused on spin degrees of freedom, leaving the dynamic role of orbital degrees of freedom largely unexplored. In this work, we extend the altermagnet Hamiltonian to include the orbital degree of freedom as a dynamical variable and derive the resulting emergent electromagnetic fields (EEMFs). This approach allows us to demonstrate emergent electric fields controllable via lattice anisotropy and the resulting orbital and magnetic multipole currents. Furthermore, we show that non-vanishing emergent electric fields can arise even in simplified spin and orbital textures, particularly in the presence of dynamic lattice distortion. This formalism is generalizable to high-order altermagnets beyond d-wave systems.
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Materials Science · Cross-listed from cond-mat.str-el

H-NESSi: The Hierarchical Non-Equilibrium Systems Simulation package

Thomas Blommel, Jeremija Kovačević, Jason Kaye et al. (+3 more)

We present H-NESSi (The Hierarchical Non-Equilibrium Systems Simulation package), an open-source software package for solving the Kadanoff-Baym equations (KBE) of nonequilibrium Green's function (NEGF) theory using hierarchical low-rank compression techniques. The simulation of strongly correlated quantum systems out of equilibrium is severely limited by the cubic scaling in propagation time and quadratic memory growth associated with conventional two-time formulations. H-NESSi overcomes these limitations by combining high-order time-stepping schemes with hierarchical off-diagonal low-rank (HODLR) representations of the retarded and lesser Green's functions, enabling controllable accuracy at substantially reduced computational cost and memory usage. Imaginary time quantities are efficiently represented using the discrete Lehmann representation (DLR), allowing compact and accurate treatment of thermal initial states. The implementation supports multiorbital systems, adaptive singular value truncation, and both shared-memory (OpenMP) and distributed-memory (MPI) parallelization strategies suitable for large-scale lattice calculations. The workflow closely mirrors established NEGF frameworks while introducing compression transparently into the propagation procedure. Benchmark applications to driven superconductors within dynamical mean-field theory and to the two-dimensional Hubbard model demonstrate favorable scaling compared to conventional implementations, with asymptotic time complexity significantly below the cubic scaling of uncompressed approaches. H-NESSi thus enables long-time and large-system nonequilibrium simulations of correlated quantum materials which were previously computationally prohibitive.
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All authors Thomas Blommel, Jeremija Kovačević, Jason Kaye, Emanuel Gull, Jakša Vučičević, Denis Golež

Monday 6 April

Materials Science

Stability and superstructural ordering of alkali-triel-pnictide clathrates A$_8$T$_{27}$Pn$_{19}$

Frank Cerasoli, Xiaochen Jin, Genevieve Amobi et al. (+2 more)

Clathrates are a class of inclusion compounds that offer various useful and surprising phenomena, including superconductivity, thermoelectricity, and the potential for high-density ion storage. Stability conditions within the Alkali-Triel-Pnictide A$_8$T$_{27}$Pn$_{19}$ family of unconventional clathrates are investigated with high-throughput density functional theory calculations, establishing trends in formation energy, structural and electronic properties. Electronic structure calculations and first-principles molecular dynamics simulations show that the ionization potential of guest alkaline atoms strongly influences the stability of electron-exact clathrates and affects their rattler behavior. Targeted reactive synthesis from elemental precursors is attempted, resulting in two novel ternary compounds. However, the targeted clathrate phases are not obtained. Further analysis reveals that the stability of ATPn clathrate compounds containing heavy elements, such as bismuth, depends strongly on spin-orbit effects, which are often neglected in high-throughput studies that compute formation energies. Finally, chemically induced superstructural ordering is described in relation to Wyckoff sites in the prototypical type-I clathrate unit cell.
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All authors Frank Cerasoli, Xiaochen Jin, Genevieve Amobi, Kirill Kovnir, Davide Donadio

Materials Science · Cross-listed from cond-mat.soft

Ion-Containing Bottlebrush Elastomers as Pressure-Sensitive Electroadhesives

Hao Dong, Intanon Lapkriengkri, Nadia Chapple et al. (+7 more)

This study presents a materials-design framework for low-voltage pressure-sensitive electroadhesives based on ion-containing bottlebrush polymers that combine the on-demand reversibility of traditional electroadhesives with the tunable conformability typical of pressure-sensitive adhesives (PSAs). Two complementary bottlebrush polymers bearing pendant flexible side chains and independently tunable anionic or cationic groups were designed to form soft and tough elastomers after crosslinking. When the two oppositely charged bottlebrush networks were brought into contact, a smooth, continuous interface formed, which is locally charge neutral due to the presence of mobile counterions. At low voltages (less than 2 V), mobile ions migrate toward the electrodes, creating an interfacial heterojunction and significant electrostatic attraction that enhances adhesion, yielding an on/off ratio of up to more than 4.5. The low-voltage operation and PSA-like mechanics of bottlebrush electroadhesives, even at charge density as low as 18 C/g, create opportunities in applications such as soft robots, haptic devices, and biomedical devices.
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All authors Hao Dong, Intanon Lapkriengkri, Nadia Chapple, Hyunki Yeo, Alexandra Zele, Hiba Wakidi, Thuc-Quyen Nguyen, Michael L. Chabinyc, Christopher M. Bates, Megan T. Valentine

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

Information Entropy is a General-Purpose Collective Variable for Enhanced Sampling

Xiangrui Li and Daniel Schwalbe-Koda

Enhanced sampling methods typically require predefined collective variables (CVs) that presuppose knowledge of reaction coordinates, restricting the discovery of unanticipated transition mechanisms or intermediates. Here, we show that a local measure of information entropy in atomistic systems is a general-purpose CV for rare event sampling across molecular and condensed-phase systems. The method biases simulations toward entropy-changing configurations following a well-tempered metadynamics approach, thus balancing novelty and thermodynamic accessibility. Blind exploration of potential energy surfaces enables unsupervised discovery of metastable basins and reaction pathways, including competing transition channels inaccessible to conventional order parameters. We demonstrate the generality of the method across five systems spanning conformational sampling, homogeneous nucleation, glass formation, and solid-state phase transformations.
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Materials Science · Cross-listed from physics.optics

Deep-Subwavelength and Broadband Quarter-Wave Retardation in Ultrathin Hyperbolic MoOCl2

Georgy Ermolaev, Adilet Toksumakov, Valeria Maslova et al. (+10 more)

The miniaturization of polarization-controlling optical components is one of the central pursuits in nanophotonics. While traditional anisotropic materials require large propagation lengths to achieve the desired phase shifts, metasurfaces mitigate this size constraint but often introduce narrow operational bandwidths and high fabrication complexities. To bridge this gap, we introduce MoOCl2 as a promising material for ultracompact and broadband phase retardation. Building on its giant optical anisotropy, we experimentally demonstrate MoOCl2 quarter-wave plates with thicknesses of 77 nm and 98 nm. These flakes exhibit achromatic quarter-wave retardation across broad visible (445 - 525 nm) and near-infrared (730 - 945 nm) spectral windows, surpassing the fundamental thickness and bandwidth limitations of both conventional optical materials and artificial nanostructures. Moreover, MoOCl2 waveplates demonstrate up to lambda/4500 retardance tolerance at central wavelengths. As a result, this study establishes MoOCl2 as a building block for ultracompact polarization optics.
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All authors Georgy Ermolaev, Adilet Toksumakov, Valeria Maslova, Aleksandr Slavich, Anton Minnekhanov, Gleb Tselikov, Nikolay Pak, Andrey Vyshnevyy, Aljoscha Söll, Zdeněk Sofer, Aleksey Arsenin, Kostya S. Novoselov, Valentyn Volkov

Materials Science

Many-body description of two-dimensional van der Waals ferroelectric $\alpha-$In$_2$Se$_3$

Denzel Ayala, Dimitar Pashov, Tong Zhou et al. (+3 more)

Two-dimensional (2D) van der Waals ferroelectrics are recognized for enabling many applications, from memory and logic to neuromorphic computing, as well as transforming other materials to control electronic phase transitions and topological states. While these materials are typically weakly correlated and expected to have their ground-state properties well described with the commonly used density functional theory, by focusing on bilayers and trilayers of In$_2$Se$_3$ we show that this approach may not be reliable. The underlying electronic structure strongly depends on the polarization structure of the multilayer system and is surprisingly challenging to accurately calculate, requiring a high-fidelity many-body theory of the quasiparticle self-consistent GW approximation. We develop this underlying description by extending the capabilities of Green function implementation within the open-source Questaal package. We show that even a sophisticated hybrid functional approach may fail to predict a nonvanishing gap in a bilayer In$_2$Se$_3$ and yields charge density, polarization, and band offsets that strongly deviate from the many-body picture. We discuss the implications of these computational advances for future opportunities in 2D ferroelectrics.
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All authors Denzel Ayala, Dimitar Pashov, Tong Zhou, Kirill Belashchenko, Mark van Schilfgaarde, Igor Žutić

Materials Science

Understanding insulating ferromagnetism in LaCoO3 films under tensile strain

Ali Barooni, Murod Mirzhalilov, Mohit Randeria et al. (+2 more)

LaCoO3 thin films grown under epitaxial tensile strain exhibit a robust ferromagnetic insulating state that is absent in the bulk. Despite many studies, both experimental and computational, the microscopic origin of this phenomenon is not well understood. In this work, density functional theory calculations are used to systematically investigate the magnetic ground state of stoichiometric LaCoO3 under epitaxial strain equivalent to that imposed by a SrTiO3 substrate. The results identify a ferromagnetic insulating ground state characterized by a unique ordered array of high-spin (HS) and low-spin (LS) Co3+ ions. The spin state ordering is best described as 2 x 2 columns that consist of alternating HS and LS Co3+ ions, separated by planes of LS Co3+ ions. This leads to HS-LS-LS repeating sequence of Co3+ ions in both pseudocubic [100] and [010] directions. Analysis of the electronic structure confirms the presence of an insulating gap. Evaluation of the superexchange interactions reveal ferromagnetic interactions between HS Co3+ ions via 90 degree paths, and antiferromagnetic interactions via 180 degree paths, both of which are facilitated by empty sigma* (eg) orbitals on the diamagnetic LS Co3+ ions. The strength and number of 90 degree ferromagnetic interactions are sufficient to overcome the competing 180 degree antiferromagnetic interactions stabilizing a ferromagnetic insulating state.
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All authors Ali Barooni, Murod Mirzhalilov, Mohit Randeria, Patrick M. Woodward, Maryam Ghazisaeidi

Materials Science

Zr Concentration-Dependent Sub-Lattice Phase-Field Model of Hf1-xZrxO2: Analysis of Phase Composition and Polarization Switching

Tae Ryong Kim and Sumeet K. Gupta

We develop a sub-lattice phase-field model of Hf1-xZrxO2 incorporating zirconium (Zr) concentration (x)-dependence. Our framework expands the time-dependent Ginzburg-Landau (TDGL) equation to the sub-lattice level and incorporates x-dependent interaction parameters and gradient coefficients. Our experimentally calibrated model captures the evolution of charge-voltage (Q-V) characteristics for x ranging from 0.5 to 1.0. The sub-lattice formulation explains the thermodynamic preference and kinetic transition barriers of competing orthorhombic phase (o-phase) and tetragonal phase (t-phase), while the phase-field framework enables spatially resolved analysis of polarization (P) and electric-field (E-field) profiles, allowing multi-domain (MD) polarization and mixed-phase states to emerge naturally. Our model reproduces the experimentally observed ferroelectric (FE)-to-anti-ferroelectric (AFE) transition as x increases from 0.5 to 1.0. At low Zr concentration (x = 0.5-0.6), the o-phase dominates, yielding distinct FE behavior. At high concentration (x = 0.9-1.0), the t-phase is stabilized, leading to AFE transitions. A key finding of our work is the unique behavior at intermediate Zr concentrations (x = 0.7-0.8). Here, the o- and t-phase energies are comparable, making the system strongly influenced by local variations in the electric field (E-field), which arise from stray fields near the domain walls. This non-uniform field distribution results in a mixed-phase composition and spatially staggered polarization reversal, which manifests as a more gradual Q-V evolution compared to other values of x. By linking energy landscapes to spatial field effects, the model provides insights into the FE-to-AFE crossover in Hf1-xZrxO2.
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Materials Science · Cross-listed from physics.app-ph

Temperature Dependent Characteristics of Quasi-vertical AlN Schottky Diodes on Bulk AlN Substrate

Md Abdul Hamid, Nabasindhu Das, Advait Gilankar et al. (+3 more)

We report on the fabrication and temperature-dependent characterization of MOCVD-grown quasi-vertical AlN Schottky barrier diodes (SBDs) on bulk AlN substrates. The SBDs exhibited high current densities exceeding 2 kA/cm2 at 10 V, with a turn-on voltage of ~3.0 V (at 1 A/cm^2) and an on/off ratio >10^9 at room temperature. Stable rectifying operation was maintained up to 300 C (the highest measured temperature), with a pronounced increase in current density at elevated temperatures due to thermally activated carrier transport, accompanied by an increase in extracted Schottky barrier height and a reduction in ideality factor. Capacitance voltage measurements showed strong temperature dependence due to the deep donor nature of Si in AlN, resulting in an increase in the net donor concentration (ND-NA) from ~5x10^17 cm-3 at 300 K to ~1x10^18 cm-3 at 373 K. Temperature-dependent reverse-bias characteristics were consistent with Poole-Frenkel emission as the dominant leakage mechanism, with an estimated trap energy of ~0.34 eV. Characterization using transmission electron microscopy and energy-dispersive X-ray spectroscopy revealed a ~5 nm AlNxOy interfacial layer at the metal/semiconductor junction, which likely influences both forward and reverse transport. These results provide insight into carrier transport, leakage mechanisms, interface chemistry, and high-temperature characteristics, and guidance for the future development of high-performance AlN power devices.
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All authors Md Abdul Hamid, Nabasindhu Das, Advait Gilankar, Brad Lenzen, David J. Smith, Nidhin Kurian Kalarickal

Materials Science

Experimental measurements and modeling of characteristic time scales in single iron particle ignition

Liulin Cen, Yong Qian, XiaoCheng Mi et al. (+1 more)

Recyclable metal fuels such as iron are promising carbon-free energy carriers for heat and power. In such systems, particle ignition characteristics strongly affect combustion efficiency and combustor stability, making them critical for burner and reactor design. However, predictive ignition modelling remains limited by the lack of time-resolved data for single-particle solid-phase oxidation and phase transitions. In this work, digital in-line holography combined with ultra-high-speed single-color pyrometry is used to resolve characteristic solid-phase oxidation times of spherical micron-sized iron particles burning in well-defined hot oxidizing environments. Three temperature plateaus are identified, corresponding to FeO melting, the {\gamma}-Fe to {\delta}-Fe transition, and Fe melting, from which pre-melting oxidation times and melting durations are extracted. An ignition model based on solid-phase iron oxidation kinetics following a parabolic rate law, coupled with external-oxygen-transport-limited description, is used to simulate these characteristic times. The model accurately captures the FeO-scale pre-melting oxidation time, which is nearly independent of oxygen concentration, while the FeO, {\gamma}-Fe to {\delta}-Fe, and Fe melting stages show strong oxygen-concentration dependence consistent with external-oxygen-transport-limited reaction rates. These measurements and simulations provide the first diameter-resolved dataset for FeO and Fe melting processes and show that this modelling framework can quantitatively predict characteristic times for single iron particles in metal-fuel applications.
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All authors Liulin Cen, Yong Qian, XiaoCheng Mi, Xingcai Lu

Materials Science

Multiferroicity in the Presence of Exchange Bias: The Case of Spinel CoMn2O4

P. Kumar, P. Das, B. K. Kuanr et al. (+1 more)

Ferrimagnetic spinel materials of formula AB2X4, where A and B are transition metals and X is oxygen or sulphur, hold promise for the realization of multiferroic characteristics. In this work, we report synthesis of spinel CoMn2O4 and explore its magnetic, dielectric, and ferroelectric aspects and their correlations. Polycrystalline CoMn2O4 was synthesized by using the conventional solid-state method. The X-ray diffraction (XRD) and Raman spectroscopy confirmed the phase purity of the synthesized compound. The crystal structure was identified with tetragonal symmetry (I41/amd space group). DC magnetization measurements indicate two magnetic transitions: one at temperature T1 ~ 186 K, followed by another Yafet-Kittel (YK) ferrimagnetic transition at T2 ~ 86 K. A frequency independent anomaly in the temperature dependent dielectric permittivity is observed near the low magnetic ordering temperature (T2). This reflects the possibility of the correlation between lattice dynamics and spin ordering in spinel CoMn2O4. A substantial exchange bias was also observed below T2 ~ 86 K. The change in dielectric permittivity in the presence of applied magnetic field follows the square of the magnetization dependence, which is consistent with Ginzburg-Landau theory. However, the detailed pyroelectric current measurements reveal the absence of intrinsic ferroelectric order.
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All authors P. Kumar, P. Das, B. K. Kuanr, S. Patnaik

Materials Science · Cross-listed from cond-mat.str-el

Effects of Spin Fluctuation and Disorder on Topological States of Quasi 2D Ferromagnet Fe1/5CrTe2

M. Lamba, P. Saha, K. Yadav et al. (+2 more)

We present a thorough magnetization and magneto-transport study of the diluted Fe-intercalated CrTe2 family member, Fe1/5CrTe2, a van der Waals ferromagnet. Fe1/5CrTe2 shows an elevated Curie transition temperature of 182 K in comparison to the Fe1/3CrTe2 composition, indicating the sensitive role of Fe concentration in modulating magnetic exchange interactions within the CrTe2 framework. The saturated magnetization exhibits a quadratic dependence with temperature, indicating the presence of long-wavelength spin fluctuations. Analysis of the temperature dependent resistivity reveals a dominant T3/2 contribution over the typical T2 behavior, signaling substantial coupling between conduction electrons and localized spins. The magnetoresistance shows a linear and non-saturating negative field dependency throughout a wide temperature range below TC, which is compatible with the increasing suppression of spin-disorder dispersion related to ferromagnetic spin fluctuations. A thorough analysis of the anomalous Hall effect (AHE) shows that extrinsic skew-scattering contribution, which is associated to Fe-related disorder, dominates the anomalous Hall response. The systematic separation of intrinsic and extrinsic components reveals that, over a wide temperature range, the intrinsic anomalous Hall conductivity scales linearly with the saturation magnetization, despite the substantial extrinsic dominant background. The linear behavior of intrinsic anomalous Hall conductivity with magnetization is in line with a long wavelength spin-fluctuation framework, where thermal spin disorder lowers net magnetization without significantly altering the underlying electronic structure. These findings reveal Fe1/5CrTe2 as a newly investigated van der Waals ferromagnet where spin fluctuations and disorder coexist with a well-defined intrinsic Berry-curvature contribution to the Hall response.
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All authors M. Lamba, P. Saha, K. Yadav, N. Kamboj, S. Patnaik

Materials Science

Engineering 2D high-temperature ferromagnets with large in-plane anisotropy via alkali-metal decoration in a tetragonal CoSe monolayer

Yiran Peng, Yanfeng Ge, Yong Liu et al. (+1 more)

Two-dimensional (2D) ferromagnetic materials with high Curie temperature ($T_{\rm c}$) and large magnetic anisotropy energy (MAE) are critical for nanoscale spintronics but remain rare. We propose, via first-principles calculations, that adsorbing alkali atoms ($A$ = Li, Na, K, Rb, Cs) onto a tetragonal CoSe monolayer transforms it into a series of stable 2D ferromagnetic metals, $A$CoSe, with an in-plane easy axis. Notably, LiCoSe is a half-metal. These functionalized monolayers exhibit dramatically enhanced ferromagnetism compared to the pristine layer, with $T_{\rm c}$ > 300 K and MAE > 800 $\mu$eV/Co. The coupled alkali atoms amplify the local magnetic moment of Co ions, reinforce ferromagnetic Ruderman-Kittel-Kasuya-Yosida (RKKY) and superexchange couplings, and concurrently weaken the direct antiferromagnetic exchange between Co ions. Furthermore, tensile strain can further elevate the MAE (via band shifting) and increase $T_{c}$ (by strengthening the nearest-neighbor exchange $J_1$). Among them, NaCoSe exhibits the highest MAE and excellent strain-modulated $T_{c}$, rendering it the most promising candidate material. Our results establish alkali-metal decoration as an effective strategy for realizing 2D ferromagnets with high $T_{\rm c}$ and large MAE in tetragonal lattices.
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All authors Yiran Peng, Yanfeng Ge, Yong Liu, Wenhui Wan

Materials Science · Cross-listed from cond-mat.supr-con

Two-Channel Allen-Dynes Framework for Superconducting Critical Temperatures: Blind Predictions Across Five Orders of Magnitude and a Quantum-Metric No-Go Result

Jian Zhou

We present a two-channel extension of the Allen-Dynes framework that unifies phonon-mediated and spin-fluctuation-mediated pairing channels for predicting superconducting critical temperatures. Channel 1 employs the standard Allen-Dynes formula with material-specific electron-phonon coupling; Channel 2 incorporates a spin-fluctuation coupling parameter extracted from inelastic neutron scattering data. Blind predictions for 19 materials spanning conventional superconductors, MgB2, iron pnictides, iron chalcogenides, heavy fermions, cuprates, and hydrides achieve R-squared = 0.96 across five orders of magnitude in Tc (0.4-250 K) without free parameters. We further demonstrate a quantum-metric no-go result: the Peotta-Torma geometric superfluid weight, while essential for flat-band systems, cannot serve as a universal predictor of Tc because it correlates with band-structure topology rather than pairing strength. The framework identifies the spin-fluctuation channel as the dominant contributor to Tc enhancement in unconventional superconductors, providing quantitative design rules for materials with Tc above 100 K.
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Materials Science

Transforming Discarded Thermoelectrics into High-Performance HER Catalysts

Gemeda Jemal Usa, Caique C. Oliveira, Varinder Pal et al. (+7 more)

With the increase in the complexity of the materials used in various sophisticated electronic devices, recycling of E-waste is becoming challenging. In the present study, we have converted thermoelectric (TE) waste into functional HER electrocatalyst by considering circular-economy and low-carbon approach. The as received TE waste was processed through ball milling (TE waste-BM) and melting casting (TE waste-M) routes. Morphological and structural evaluations revealed that the formation of BiSbTe3/ZnTe heterostructure in TE-waste-M promote HE efficiency when compared to the presence of Bi2Te3/BiSbTe3 heterostructure (TE-waste-BM). TE waste-M exhibited lower overpotential (641 mV at 10 mA/sq.cm), smaller Tafel slope (233 mV/dec) and stable operation for 5.5 h with negligible current decay than that of TE waste-BM, attributed to the accelerated charge transfer, fast water dissociation steps and rapid hydrogen adsorption in TE waste-M, originated from the presence of BiSbTe3/ZnTe heterostructure, defect enriched interfaces. Density functional theory calculations supported the experimental findings, revealing that heterostructures strengthens the bonding states near the Fermi level, thereby enhancing the HER activity of BiSbTe3/ZnTe heterostructure. This work simultaneously integrates waste management with green hydrogen production by offering an economically viable, scalable and low-carbon approach for HER catalysts.
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All authors Gemeda Jemal Usa, Caique C. Oliveira, Varinder Pal, Suman Sarkar, Gebisa Bekele Feyisa, Moumita Kotal, Emmanuel Femiolu, Pedro A. S. Autreto, Temesgen Debelo Desissa, Chandra Sekhar Tiwary

Materials Science · Cross-listed from cs.LG

The Infinite-Dimensional Nature of Spectroscopy and Why Models Succeed, Fail, and Mislead

Umberto Michelucci and Francesca Venturini

Machine learning (ML) models have achieved strikingly high accuracies in spectroscopic classification tasks, often without a clear proof that those models used chemically meaningful features. Existing studies have linked these results to data preprocessing choices, noise sensitivity, and model complexity, but no unifying explanation is available so far. In this work, we show that these phenomena arise naturally from the intrinsic high dimensionality of spectral data. Using a theoretical analysis grounded in the Feldman-Hajek theorem and the concentration of measure, we show that even infinitesimal distributional differences, caused by noise, normalisation, or instrumental artefacts, may become perfectly separable in high-dimensional spaces. Through a series of specific experiments on synthetic and real fluorescence spectra, we illustrate how models can achieve near-perfect accuracy even when chemical distinctions are absent, and why feature-importance maps may highlight spectrally irrelevant regions. We provide a rigorous theoretical framework, confirm the effect experimentally, and conclude with practical recommendations for building and interpreting ML models in spectroscopy.
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