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Physics Condensed Matter Materials Science

cond-mat.mtrl-sci Watch Materials Science

Monday 4 May

Materials Science

A Unified microscopic picture of cation and anion migration in MAPbI$_3$

Viren Tyagi, Geert Brocks and Shuxia Tao

Passivating defects and restricting defect mobilities in halide perovskites to increase device lifetimes has become a main field of research. Modeling structure and mobility of point defects is an essential contribution to this endeavor. We employ molecular dynamics, based on neural network potentials trained on density functional theory data, to model ion migration in MAPbI$_3$ triggered by I and MA vacancies or interstitials. Most of these species diffuse rapidly at room temperature, with migration barriers between 0.15 and 0.20 eV. MA interstitials are highly mobile despite their molecular nature, owing to a concerted migration mechanism involving multiple MA ions. No evidence of MA vacancy migration is obtained. Whereas diffusion of I-related defects appreciably depends on their charge state, diffusion of MA defects does not. These results revise the conventional picture of ion transport in hybrid perovskites and highlight the role of collective molecular motion in enabling fast ionic migration.
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Materials Science · Cross-listed from cond-mat.str-el

Exotic magnetism and persistent short-range spin correlations in a frustrated honeycomb lattice antiferromagnet

M. Barik, Q. Faure, F. Damay et al. (+3 more)

Two-dimensional high-spin bipartite honeycomb networks, where anisotropy, competing exchange interactions, and spin fluctuations interplay, provide an alternative platform to test theoretical models that distinguish between classical and quantum magnetism in the context of emergent many-body phenomena and exotic excitations. Here, we report the crystal structure, magnetization, specific heat, and inelastic neutron scattering measurements of the $S = 5/2$ distorted honeycomb magnet $\mathrm{CaZn_2Fe(PO_4)_3}$. Magnetization measurements reveal dominant antiferromagnetic interactions between the $\mathrm{Fe^{3+}}$ ($S = 5/2$) moments. The development and field evolution of a dip in the magnetic susceptibility under an external magnetic field indicate an unconventional field-induced transition, further supported by anomalies observed in magnetization isotherms. Zero-field specific heat measurements show an antiferromagnetic transition at $T_N \approx 1.67 \mathrm{K}$, which evolves under applied magnetic field, suggesting stabilization of a field-induced spin-canted state. Thermodynamic measurements reveal short-range spin correlations above the transition temperature. Inelastic neutron scattering results further corroborate antiferromagnetic ordering, consistent with specific heat data. Spin-wave calculations indicate competing exchange interactions that introduce magnetic frustration, along with weak Ising-like anisotropy. The interplay of competing interactions and anisotropy gives rise to exotic field-induced behavior and places the system in close proximity to a mean-field tricritical point in the $J_2/J_1$–$J_3/J_1$ phase diagram, opening a route to unconventional states in high-spin frustrated honeycomb magnets.
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All authors M. Barik, Q. Faure, F. Damay, J. P. Embs, S. Petit, P. Khuntia

Materials Science

Rigorous foundations of the effective mass approximation: analyticity and symmetry of band extrema

Jakob Kjærulff Svaneborg

The effective mass approximation is widely used across models of carrier transport, optical response, and excitons in semiconductors and insulators, but its validity hinges on the assumption that the band dispersion $E_n(\mathbf{k})$ at the relevant extremum is analytic. We prove that analyticity holds at any non-degenerate extremum for Kohn-Sham density functional theory (DFT) with local or hybrid exchange-correlation functionals, for Hartree-Fock, and for band-edge $G_0 W_0$ quasiparticle energies in gapped systems. Band non-analyticity (or warping) in these settings is therefore intrinsically tied to degeneracy. We then use group theory to determine the symmetry-allowed form of the effective mass tensor for each of the 32 crystallographic point groups, providing a stringent consistency check on first-principles calculations. As a representative application, we show that the electron and hole effective masses at the $K$ point of monolayer MoS$_2$ must be strictly isotropic at the DFT and $G_0W_0$ levels.
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Materials Science

Preliminary Structural Study of Chromium Coatings for Nuclear Applications

Michał A. Stróżyk, Jong-Dae Hong, JaeYong Kim et al. (+8 more)

Following the Fukushima Daiichi disaster, an increasing number of studies concentrate on the development of Accident Tolerant Fuel (ATF) cladding materials for nuclear fuel, aiming to prevent the oxidation of zirconium during incidents such as Loss of Coolant Accident (LOCA) and to effectively lower the amount of heat and hydrogen released during emergency core cooling (ECC). Zirconium alloy cladding with a protective chromium (Cr) coating is considered one of the promising candidates, largely due to its relatively short timeline for deployment in nuclear power plants. In this study, Cr-coated and uncoated Zircaloy-4 claddings were evaluated using high temperature X-ray diffraction (HT-XRD) in vacuum over a temperature range from RT to 1100oC. The temperatures corresponding to the formation of oxide phases are >200oC and >600oC for the uncoated and Cr-coated samples, respectively. SEM and TEM characterisation of the sub-surface in Cr-coated specimen after HT-XRD revealed Fe segregation, formation of Zr(Fe,Cr)2 Laves phase and nano-bubbles at the former Cr / Z4 interface.
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All authors Michał A. Stróżyk, Jong-Dae Hong, JaeYong Kim, Iwona Jóźwik, Ryszard Diduszko, Jerzy Zagórski, Witold Chromiński, Kinga Suchorab, Katarzyna Ciporska, Marcin Brykała, Jacek Jagielski

Materials Science

Designing explicit functionals for the charge density in terms of a potential

Muhammed H. Güneş, Ayoub Aouina, Vitaly Gorelov et al. (+2 more)

One of the most powerful strategies to address properties of real many-body systems is to incorporate data obtained for models, for example, to use data of the homogeneous electron gas in order to build the Local Density Approximation for the Kohn-Sham exchange-correlation potential. In the present work, we examine to what extent we can use model data to design functionals directly for observables of materials. In particular, we study different approximations for the charge density of real inhomogeneous materials expressed as a simple, explicit functional of a given Kohn-Sham potential, using as central building block the Lindhard density-density response function of the homogeneous electron gas. Our increasingly realistic set of approximations includes a fully nearsighted expression equivalent to the Thomas-Fermi approximation, functional Taylor expansions, and different approximations to the Connector Theory developed in [Aouina et al., npj Computational Materials 11, 242 (2025)]. In all cases, the charge density is obtained without ever solving the Kohn-Sham Schrödinger equation. Results for cubic helium, a prototypical strongly inhomogeneous material, as well as the covalent semiconductor silicon and metallic aluminum, systematically improve with higher levels of approximation. At the present stage, the results may be used for qualitative discussions or as optimized starting point for a self-consistent Kohn-Sham cycle. More generally, their quality indicates that this is a promising route to obtain functional expressions for observables that are relatively simple to calculate and to analyze.
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All authors Muhammed H. Güneş, Ayoub Aouina, Vitaly Gorelov, Matteo Gatti, Lucia Reining

Materials Science

A unified equation for saturation magnetization and spin transport in weakly disordered ferromagnets

Sumanta Mukherjee

In this report, a unified description of the loss of saturation magnetization in the presence of a distribution of finite-size effects is provided for weakly disordered spin-1/2 ferromagnets. This description allows us to derive a unified form of the Bloch equation for these systems. We further extend this approach to obtain a unified expression for spin transport in such systems.
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Materials Science

Investigation of filamentation in a-Si/Ag/Cu memristors with atomic force microscope

Alena Samsonova, Viacheslav Dremov, Oleg Klimenko et al. (+2 more)

Cation-based Ag/Cu filaments formed in an insulating $\alpha$-Si matrix are widely used as memristors in crossbar arrays for efficient in-memory computing. However, the stochastic nature of filament formation and rupture gives rise to device-to-device and cycle-to-cycle variation. Despite successful implementation of large-scale memristor arrays, systematic studies of filament parameters and their spatial distribution in the memristors are scarce. In this work, we use conductive atomic force microscopy (c-AFM) to probe the spatial distribution of conductive filaments in $\alpha$-Si memristors. The charge transport is dominated by a limited number of discrete filaments rather than by uniform conduction across the device area. The systematic analysis of the experiment gives the mean surface density of the conductive filaments $\sim$3200 per $\mu\text{m}^2$. Both volatile and non-volatile filaments can be found within one memristor. The experimental data and the nature of volatile and non-volatile filaments may be explained within the model of multiple trap assisted tunnelling. The model yields reasonable estimates for physical properties for both types of filaments.
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All authors Alena Samsonova, Viacheslav Dremov, Oleg Klimenko, Nikolai Brilliantov, Vladimir N. Antonov

Materials Science

Photovoltaic creation of charged domain walls in barium titanate

P.S. Bednyakov, P.V. Yudin, A.K. Tagantsev et al. (+1 more)

The optical control of domain structures in ferroelectrics is of great interest. In the present work, we demonstrate the reliable creation of charged domain walls - conductive channels in otherwise insulating barium titanate - by a combined effect of light and electric field. We propose a scenario for the documented process, in which the bulk photovoltaic effect plays the key role, providing charge screening at the walls. Our scenario is supported by the results of phase-field simulations. The results are of interest for future reconfigurable electronic and opto-electronic devices.
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All authors P.S. Bednyakov, P.V. Yudin, A.K. Tagantsev, J. Hlinka

Materials Science

Persistent Spin Texture and Spin-Orbital Hall Responses on the AgI (110) Surface

Manish Kumar Mohanta

A systematic investigation of the structural, electronic, and spin-orbital transport properties of the AgI (110) surface is presented using first-principles calculations combined with analytical modelling. The non-centrosymmetric and nonsymmorphic nature of the system gives rise to a robust persistent spin texture (PST), characterized by a unidirectional spin configuration and suppressed spin relaxation, enabling an effectively infinite spin lifetime. Unlike previously reported PST materials, which are predominantly based on chalcogen compounds, this work demonstrates that a halide semiconductor can host PST, thereby significantly expanding the materials platform for spintronic applications. The underlying mechanism is captured using an effective spin-orbit coupled Hamiltonian, which reproduces the anisotropic spin splitting and momentum shift observed in the band structure. This work introduces two new analytical models describing PST and compares them with existing models, offering new perspectives on PST arising from spin-orbit interaction. In addition, the system exhibits sizable intrinsic spin Hall conductivity (SHC) and orbital Hall conductivity (OHC), highlighting its potential for efficient charge-to-spin and charge-to-orbital conversion. The PST is found to be robust against biaxial strain, structural distortion, and multilayer formation, while a vertical electric field breaks the symmetry protection and drives a transition to a Rashba-type spin texture. These findings establish AgI (110) as a promising platform for realizing long-lived spin transport and tunable spin-orbit functionalities in the low-dimensional halide systems.
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Materials Science

Quantum Limits of Electronic Transport in Nanostructured Macroscopic Conductors

Agnieszka E. Lekawa-Raus, John S. Bulmer, Teresa Kulka et al. (+7 more)

Macroscopic assemblies of one- and two-dimensional materials promise to translate nanoscale electronic properties into device-scale performance, yet the microscopic principles governing charge transport in such networks remain unresolved. In these systems, conductivity is often interpreted using phenomenological models that do not explicitly connect electronic structure to macroscopic magnetotransport. Here we develop a unified atomistic framework that links quantum-coherent transport, thermal disorder and magnetic-field effects, and combine it with ultrahigh-field magnetotransport measurements up to 60 T over a broad temperature range on carbon nanotube fibres. We show that positive magnetoresistance is controlled by junction overlap length, whereas negative magnetoresistance arises predominantly from lattice-mismatched heterojunctions rather than weak localisation alone. Statistical analysis of a large-scale numerical dataset reveals that the experimentally observed positive quadratic magnetoresistance originates from junction transport. These results show that macroscopic transport in disordered low-dimensional networks is governed primarily by junction-level quantum interference rather than solely by defects or doping.
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All authors Agnieszka E. Lekawa-Raus, John S. Bulmer, Teresa Kulka, Magdalena Marganska, Nick Papior, Dwight G. Rickel, Fedor F. Balakirev, Jacek A. Majewski, Krzysztof Koziol, Karolina Z. Milowska

Materials Science

Composition-Weighted Symbolic Regression for General-Purpose Property Prediction

Yang Huang and Jingrun Chen

We introduce a composition-weighted symbolic regression framework for interpretable prediction of materials properties directly from chemical composition. The method jointly learns analytical functional forms and task-dependent elemental weightings without predefined descriptors. By incorporating max/min operators, it naturally enforces constraints such as non-negative band gaps and bounded classification probabilities, unifying regression and classification tasks. Efficient search is achieved through a hybrid Monte Carlo tree search–genetic programming algorithm with gradient-based refinement and parallel computation. Benchmarks on MatBench tasks show competitive accuracy relative to state-of-the-art black-box models while yielding explicit analytical expressions. Applied to III–V semiconductor alloys, the model produces smooth composition-dependent trends and learned elemental weights with chemically meaningful periodic behavior. This framework provides a scalable and interpretable route for materials discovery and property screening.
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Materials Science · Cross-listed from cond-mat.mes-hall

3D Quantum Hall Effect with Two Distinct Plateaus

Jun-Hong Li, Yi-Yuan Chen, Peng-Lu Zhao et al. (+2 more)

The recent discovery of the 3D quantum Hall effect in $\mathrm{HfTe_5}$ has also revealed puzzling signatures of possible 3D fractionalization. Beyond the first plateau associated with the lowest Landau band, Hall conductivity exhibits a second plateau with a value of about $3/5$ of the first, accompanied by a suppressed longitudinal resistivity. Here, we attribute this second plateau to an insulating ground state arising from spin-density-wave order. We show that a magnetic-field-driven Lifshitz transition causes the spin-down holelike zeroth Landau band to cross the Fermi energy and that the resulting nesting between the lowest spin-up and spin-down Landau bands induces a spin-density wave. We calculate the Hall and longitudinal resistivity and reproduce the experimental behaviors. Our renormalization-group analysis further supports this insulating ground state. Our work reveals that the tunability of Landau bands along the magnetic-field direction endows the 3D quantum Hall effect with a broader phenomenology than its 2D counterpart and merits further exploration.
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All authors Jun-Hong Li, Yi-Yuan Chen, Peng-Lu Zhao, Hai-Zhou Lu, X. C. Xie

Materials Science

Metastable MnBi$_2$Te$_4$ enabled by magnetic-field-assisted synthesis

Abhinna Rajbanshi, G. M. Zills, Alexander M. Donald et al. (+7 more)

Magnetic topological insulators provide a unique platform to explore the interplay between magnetism and topology. MnBi$_2$Te$_4$, known for its A-type antiferromagnetic (AFM) ground state, undergoes a striking transformation when single crystals are grown in an applied magnetic field. Despite retaining the same crystal structure, field-grown MnBi$_2$Te$_4$ exhibits a ferromagnetic (FM) ground state with a Curie temperature of $\sim$ 12.5 K, confirmed by magnetization, magnetic torque, electrical resistivity, and specific heat measurements. First-principles calculations support these findings, revealing that magnetic-field-assisted synthesis can effectively reconfigure the ground-state spin order and thereby modify the material's electronic properties, as reflected in the de Haas-van Alphen oscillation seen in the magnetic torque.
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All authors Abhinna Rajbanshi, G. M. Zills, Alexander M. Donald, Daniel Duong, David Graf, James J. Hamlin, Mark W. Meisel, I. Vekhter, Williams A. Shelton, Rongying Jin

Sunday 3 May

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

Lattice-Spring Analogy for Isotropic Elasticity

D. M. LI and Meng-Cheng HE

This study introduces an innovative Isotropic Elastic Lattice Spring Model (IELSM) that addresses the fundamental limitation of classical lattice spring models: the constraint of fixed Poisson's ratio. By amending the total strain energy within the Lattice Spring Model (LSM), IELSM provides a self-consistent formulation for simulating isotropic elastic materials with arbitrary Poisson's ratios. The model's core innovation lies in augmenting classical axial spring frameworks with additional volumetric constraints, establishing a direct and exact mapping between IELSM's parameters and macroscopic elastic constants. This enables simulation across the full admissible Poisson's ratio: -1 < {\nu} < 1 under plane stress and -1 < {\nu}< 0.5 under plane strain conditions. Eigenvalue analysis indicates that the IELSM has better numerical stability compared to the standard bilinear quadrilateral element and the constant strain triangular element. The characteristic of the numerical implementation lies in directly decomposing the additional volumetric constraints into an equivalent combination of standard mechanical components (axial, shear and rotational springs), laying the foundation for the realization of fracture simulation based on discrete methods. Comprehensive validation through uniaxial tension, pure shear, stress concentration around a circular hole, and stress singularity analyses for central and crucifix-shaped cracks demonstrates IELSM's exceptional accuracy, convergence and computational robustness. The model exhibits excellent performance in stress intensity factor calculations at crack tips, validating its effectiveness for singular stress field analysis. This work bridges the gap between LSM and continuum mechanics, establishing an analog framework that maintains theoretical consistency while offering computational accuracy for the solution of elastic boundary value problems.
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Materials Science

Meta-LegNet: A Transferable and Interpretable Framework for Surface Adsorption Prediction via Self-Defined Adsorption-Environment Learning

Yifan Li, Arravind Subramanian, Xiaoqing Liu et al. (+3 more)

A central challenge in computational catalysis is the identification of low-energy and chemically plausible adsorption configurations, as these directly affect adsorption energies, reaction pathways, and catalytic performance. Existing approaches generally rely on enumerating candidate adsorption sites followed by iterative refinement through density functional theory calculations or machine-learning-based relaxations. However, such workflows remain computationally expensive and are difficult to scale to complex surfaces or multi-adsorbate systems. Here, we introduce Meta-LegNet, a graph learning framework that combines SE(3)-equivariant atom-level message passing with voxel-based multiscale aggregation and cross-domain meta-learning to learn transferable representations of local adsorption environments across diverse catalyst–adsorbate systems. Rather than following a conventional regression-only paradigm, Meta-LegNet encodes local chemical environments using invariant radial features and equivariant directional information, and further incorporates broader structural context through coordinate-frame voxel pooling, assignment-based upsampling, and gated feature fusion. The resulting local-global decomposition produces atom-resolved attribution maps, which are processed to identify adsorption-relevant local environments in an interpretable manner. Based on the learned representations, we further construct an adsorption-environment database and develop a template-matching strategy to propose likely adsorption sites on previously unexplored surfaces without exhaustive site enumeration. Overall, our results suggest that learning transferable adsorption environments provides an accurate, interpretable, and practical route for accelerating catalyst screening.
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All authors Yifan Li, Arravind Subramanian, Xiaoqing Liu, Qiujie Lyu, Sergey Kozlov, Lei Shen

Materials Science

Ab initio evidence for a framework-preserving spin-polarized high-DOS state in D-type carbon schwarzite C136

Eugene Yashin

Negative Gaussian curvature provides an unusual route for designing electronic structure in extended sp2 carbon networks. Here I report ab initio density-functional calculations on the D-type carbon schwarzite C136, focusing on the response of the ideal high-symmetry framework to spin polarization and fixed-cell ionic distortion. A partial spin-polarized fixed-cell relaxation lowers the total energy by approximately 0.213 eV per 136-atom cell over six completed ionic steps. The distortion remains moderate: the RMS atomic displacement is approximately 0.098 Angstrom, the maximum atomic displacement is approximately 0.200 Angstrom, the RMS C-C bond-length change for the 170 reference bonds shorter than 1.80 Angstrom is only approximately 0.0107 Angstrom, and no unphysically short C-C contacts below 1.20 Angstrom are found. A separate clean from-scratch spin-polarized SCF calculation on the last saved distorted geometry converges successfully to a magnetic state with total energy -2490.35442340 Ry, total magnetization 10.63 muB/cell, and absolute magnetization 12.94 muB/cell. Spin-resolved DOS calculations further show that the distorted geometry retains a high density of states near the Fermi level. A 3x3x3 diagnostic DOS gives N(EF) approximately 42.84 states/eV/cell, while a 4x4x4 validation DOS gives N(EF) approximately 42.85 states/eV/cell, demonstrating that the high-DOS character is robust with respect to this k-point refinement. These results support the interpretation of C136 as a negative-curvature carbon parent phase near coupled spin-lattice and high-DOS electronic instabilities. Superconductivity is not established here; rather, the results motivate a search for stabilized, distorted, doped, or intercalated descendants of D-type carbon schwarzites.
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Materials Science

Multiscale computational approaches to magnetic behaviour in Cobalt Ferrite (CoFe$_2$O$_4$) nanostructures

Soham Chandra and Soumyajit Sarkar

Cobalt ferrite (CoFe$_2$O$_4$) is a prototypical ferrimagnetic spinel oxide whose exceptional magnetic anisotropy, magnetoelastic coupling, and thermal stability underpin applications in spintronics, magnetic hyperthermia, energy harvesting, and catalysis. This chapter presents a comprehensive computational framework that integrates electronic$-$structure calculations with atomistic spin modeling, statistical mechanics, and continuum micromagnetics to predict magnetic functionality across length and time scales. Starting from density functional theory with Hubbard corrections (DFT$+$U), we derive exchange constants J$_{ij}$, magnetocrystalline anisotropy K$_1$, and magnetoelastic coefficients B$_1$, accounting for cation inversion, strain, and correlation effects. These parameters feed into generalized Heisenberg Hamiltonians, enabling Monte Carlo and Landau-Lifshitz-Gilbert simulations of finite-size effects, hysteresis, coercivity, and hyperthermia response in nanoparticles and thin films. Coarse-graining strategies bridge to micromagnetic modeling, ensuring consistent parameter flow without empirical fitting. Computational case studies demonstrate size-dependent anisotropy enhancement, surface spin disorder, strain-tunable switching, and doping trends, revealing design principles inaccessible to experiment alone. Validation against benchmarks, e.g. Curie temperature, anisotropy constants, coercivity, magnetostriction, confirms predictive accuracy. Current challenges, e.g., U$-$parameter sensitivity, realistic surface chemistry, spin-lattice coupling, and large-scale integration are discussed alongside emerging directions including DFT$+$DMFT, coupled dynamics, and machine-learned potentials.
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Materials Science

Dynamic Mechanical Response of Spinodal Architectures Across Length and Time Scales

Vatsa Gandhi, Rishi Kommalapati, Carlos M. Portela et al. (+1 more)

High-throughput characterization of architected materials across a wide range of length scales enables rapid screening of topologies for engineering applications. Scaled-down specimens manufactured and evaluated in laboratory environments enable this iteration, but application scenarios may involve differing length scales and loading conditions that complicate direct comparisons. Here, we use a spinodal architected morphology to determine the interplay among the constituent material's strain-rate sensitivity, the topological length scale, and the imposed deformation rates. We report characterization spanning strain rates from $10^{-3}$ s$^{-1}$ to $10^{4}$ s$^{-1}$ on spinodal architected specimens with length scales of 100 $\mu$m (microscale) and 30 mm (macroscale). The experiments show that while microscale specimens exhibit moderate increase in strength at high strain rates, macroscale specimens exhibit a nearly tenfold increase in strength at equivalent strain rates. Finite element calculations reveal that this increase is linked to a transition from a response governed by constituent material strain-rate sensitivity to inertia-dominated behavior in macroscale specimens, a transition not observed in microscale specimens at the strain rates investigated here. Using extensive finite element calculations, we develop maps to establish the parameters governing the regimes of behavior, illustrating that the transition from behavior governed by constituent material rate sensitivity to inertia-dominated behavior has analogies to fluids in that it depends on a structural length scale. Our findings provide insights into the physical parameters that govern responses across length and time scales, towards the development and design of new laboratory experiments that extract relevant dynamic properties for structural applications.
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All authors Vatsa Gandhi, Rishi Kommalapati, Carlos M. Portela, Vikram Deshpande

Materials Science · Cross-listed from physics.optics

Excited states engineering maximizes singlet generation by triplet fusion in conjugated systems

Alessandra Ronchi, Masashi Mamada, Michel Frigoli et al. (+1 more)

Photon upconverters are anti Stokes emitters capable of generating photons with higher energy than those absorbed. This behavior can be achieved through different mechanisms, which are extensively studied for applications in imaging, anticounterfeiting, information encryption and most importantly, solar energy technologies. Among these mechanisms, photon upconversion based on sensitized triplet-triplet annihilation (sTTA-UC) is particularly attractive because it operates under low intensity, incoherent light. In sTTA UC, two optically dark triplet states of a conjugated annihilator fuse upon collision to populate a higher energy fluorescent singlet, with the triplets initially generated via energy transfer from a lower energy sensitizer. Here we introduce a general molecular design strategy to maximize singlet generation through TTA. By selective substitution, we engineered a naphthalene derived annihilator with an excited state energy landscape that strongly favors singlet formation, achieving yields up to 0.83. When combined with an appropriate triplet sensitizer, the system delivers stable UV/visible upconversion peaking at 390 nm, with an absolute upconversion yield about 0.19 and an activation excitation intensity threshold lower than 0.1 suns under non-coherent broadband excitation fully compatible with the requirements of solar powered technologies.
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All authors Alessandra Ronchi, Masashi Mamada, Michel Frigoli, Angelo Monguzzi

Materials Science

Mid-infrared photo-induced force microscopy (IR-PiFM/PiF-IR) – Answers to some questions

Daniela Täuber

Mid-infrared photo-induced force microscopy (IR-PiFM/PiF-IR) enables high-resolution chemical imaging of surfaces with lateral resolution less than 5 nm. Here are some answers to questions about the physical background, practical handling and potential applications of PiF-IR including its use in the context of studying antimicrobial interaction. Such questions had been addressed to me during the Faraday Discussions on Vibrations at Interfaces which took place in April 2026 in Manchester/UK. The discussion was part of the theme "What is the question, what is the technique?" in the context of which I presented our recent work [James et al., Faraday Discussions, 2026, doi: 10.1039/d6fd00003g]. A modified version of this manuscript will be published in the themed collection "Vibrations at Interfaces" in Faraday Discussions.
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Materials Science

Chirality in BaTiOCu$_4$(PO$_4$)$_4$

Alex Hallett and Nicola A. Spaldin

We present a first principles study of the ferrochiral phase transition in chiral BaTiOCu$_4$(PO$_4$)$_4$, which has been shown using X-ray diffraction to proceed via the antiferroaxial rotation of antipolar structural units. We analyze the atomic-site electric and magnetic multipole moments to identify connections between these multipoles and chirality and corroborate the previous experimental interpretation. We show that antiferroically ordered atomic-site electric toroidal dipole moments act as an order parameter for the antiferroaxial rotations, and that the overall chiral order is parameterized by the composite order of the antipolar electric dipole and electric toroidal dipole moments. Finally, we evaluate the suitability of various proposed order parameters for chirality, show that some can be excluded and suggest the most promising directions for future exploration.
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Materials Science

Microscale bending plasticity and fracture behavior of amorphous aluminum oxide films

Nidhin George Mathews, Erkka J. Frankberg, Vivek Devulapalli et al. (+12 more)

Recent work has demonstrated microscale compressive plasticity in pulse laser deposited (PLD) amorphous alumina (a-Al2O3). This work explores microscale bending plasticity and fracture behavior of a-Al2O3 films deposited using three different methods-PLD, atomic layer deposition (ALD) and sputter deposition (SD). The three deposition routes produced amorphous films with similar stoichiometric compositions. We demonstrate, for the first time, bending plasticity in PLD and ALD a-Al2O3 films at microscale using in situ microcantilever bending experiments at room temperature. All tested PLD a-Al2O3 microcantilevers showed substantial ductile behavior in bending by accommodating total strains >10% without fracture. Half of the tested ALD a-Al2O3 cantilevers exhibited elastic brittle fracture while the other half showed bending plasticity, indicating that the observed deformation behavior is strongly influenced by the presence and distribution of defects within the tested volume. All SD a-Al2O3 microcantilevers showed elastic brittle failure attributed to their columnar growth microstructure. The microscale bending response was found to be highly dependent on the film deposition method highlighting the role of defects in suppressing plasticity mechanisms. Notched microcantilever bending tests on all three films showed brittle failure with similar fracture toughness value of 3.1 +/- 0.2 MPa.m0.5, effectively ruling out any localized crack tip plasticity. These findings underscore the importance of minimizing defects during fabrication in order to develop damage tolerant amorphous oxides. Nonetheless, the observation of bending plasticity in both PLD and ALD microcantilevers, which include a tensile stress component as well, suggests that the plastic deformation mechanisms in amorphous alumina are more general and are not exclusively governed by the deposition method.
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All authors Nidhin George Mathews, Erkka J. Frankberg, Vivek Devulapalli, Chandan Kumar, Barbara Putz, Aloshious Lambai, Sergei Khakalo, Mattia Cabrioli, Bjarke Holl Christensen, Janne-Petteri Niemelä, Arnold Milenko Müller, Fabio Di Fonzo, Ivo Utke, Erkki Levänen, Gaurav Mohanty

Materials Science

Liquid-phase encapsulation of $\pi$-conjugated dyes in boron nitride nanotubes: Ensemble and single-nanotube optical characterization

Friedrich Schöppler, Lukas Stumpf, Lucas Fuhl et al. (+5 more)

Boron nitride nanotubes (BNNTs) provide wide-bandgap, optically transparent one-dimensional hosts for molecular dyes, limiting direct electronic participation of the host. Whether dye@BNNT systems produce bright, well-defined J- or H-aggregates or instead heterogeneous emissive ensembles whose character depends on chain length and local packing remains only partly resolved. We address this question using ensemble extinction, photoluminescence, quantum-yield measurements, and TCSPC-derived radiative and non-radiative rates, together with polarization-resolved single-nanotube microscopy on encapsulated quaterthiophene, sexithiophene, octithiophene, and Nile Red, selected from a ten-dye screening. In the oligothiophene series, confinement modifies spectra and excited-state dynamics in a length-dependent manner, with all three oligothiophenes forming weakly emissive ensembles with suppressed effective radiative rates and 6T showing the strongest redistribution between effective radiative and non-radiative decay. The absence of radiative-rate enhancement or fluorescence-lifetime shortening across the series disfavors bright J-aggregate assignments. Polarization-resolved single-nanotube microscopy reveals strongly polarized emission, but with tube-to-tube and intratube variations, identifying oligothiophene@BNNTs as ordered yet structurally heterogeneous confined ensembles. Nile Red provides a complementary case in which the dominant response is dielectric tuning of a solvatochromic charge-transfer state rather than oligothiophene-like aggregate formation. These findings establish dye-filled BNNTs as optically quiet nanoconfined systems in which molecular ordering, dielectric confinement, and guest-guest coupling can be distinguished through combined ensemble and single-nanotube spectroscopy.
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All authors Friedrich Schöppler, Lukas Stumpf, Lucas Fuhl, Elena Behr, Charlotte Allard, Christoph Lambert, Richard Martel, Tobias Hertel

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

Two distinct superconducting regimes in Ti4Co2O under pressures

Lifen Shi, Keyuan Ma, Binbin Ruan et al. (+8 more)

We report on the pressure dependence of superconducting transition temperature Tc and upper critical field Bc2(0) through electrical transport of the Ti4Co2O superconductor (eg.,the superconducting transition temperature Tc = 2.5 K and the Bc2(0)=7.2T=2.9Tc). We find that the Tc exhibits non-monotonic pressure dependence:it rises monotonically at first with a pressure coefficient of dTc/dP=0.034 K/GPa, but rapidly decreases around 10-20 GPa, and then increases with the dTc/dP = 0.023 K/GPa, up to= 4.31 K at 69.7 GPa. Concurrently, the Bc2(0)exhibits a dome shaped pressure dependence, with its maximum at 5 GPa of almost twice the value at ambient pressure, exceeding the weak-coupling Pauli paramagnetic limit Bp throughout the whole pressure range. By comparing the normal-state and superconducting properties, we identify two distinct superconducting regimes, with a low-pressure superconducting phase characterized by an enhanced Bc2(0)values and Fermi-liquid normal-state electrical transport (the exponent n = 2), and a high-pressure superconducting phase with a monotonically increased Tc and an enhancement in phonon scatterings (the exponent n = 4). Room-temperature synchrotron X-ray diffraction indicates that there is no structural transition up to 55.8 GPa, which gives a relatively large bulk modulus of 192 GPa in comparison with other alloy superconductors. First-principles calculations suggest that the nonmonotonic Tc maybe closely related to the evolution of the density of states of Ti4Co2O upon compression, which is different from those of isostructural superconductors Ti4Ir2O and Nb4Rh2C. Our results show that even in the Ti4Co2O with weak spin-orbit coupling, superconductivity remains highly sensitive to the external stimuli such as pressure.
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All authors Lifen Shi, Keyuan Ma, Binbin Ruan, Zhen Wang, Pengtao Yang, Zhian Ren, Jianping Sun, Gang Li, Fabian O. von Rohr, Bosen Wang, Jinguang Cheng

Materials Science

Polymorphic crystallites model for monolayer amorphous materials

Le-Ye Zhu, Xi Zhang, Yun-Peng Wang et al. (+4 more)

Establishing atomistic models of amorphous materials remains a central challenge in materials science. Here, we examine three representative multicomponent monolayer systems using first-principles-trained machine-learning potentials (MLPs) and an energy-guided Monte Carlo structural-search workflow. For monolayer amorphous boron nitride (maBN), independent density-functional-theory (DFT) single-point calculations confirm that the ensemble generated by the MLP-driven search is lower in energy on average than an extended-Tersoff-generated ensemble. Across the sampled maBN structures, local-ring classification combined with strict connected-domain analysis identifies h-BN-like, o-B$_2$N$_2$-like, and o-B$_4$N$_4$-like crystallite domains together with mixed medium-range order. Only individual samples containing at least two distinct phase-pure crystallite-domain types are classified as polymorphic crystallites. The same analysis finds coexisting hexagonal and tetragonal domains in monolayer amorphous LiCl and graphene-like, h-BN-like, borophene-like, and mixed-order regions in monolayer amorphous BCN. Five-nanosecond MLP molecular-dynamics trajectories at 300 K show that the candidate structures are dynamically persistent on the simulated timescale. These results support a polymorphic-crystallite description for the selected model systems, while experimental discrimination from alternative continuous-random-network descriptions will require medium-range-order-sensitive measurements.
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All authors Le-Ye Zhu, Xi Zhang, Yun-Peng Wang, Jieheng Shi, Junwei Zhang, Shixuan Du, Yu-Yang Zhang

Materials Science

Reversible fully spin polarization in strain-engineered two-dimensional fully compensated magnets

Xiuli Zhang, Peng Jiang, Yurui Ma et al. (+6 more)

Achieving controllable spin polarization and its reversal in symmetry-compensated magnets. Here we demonstrate, using symmetry analysis and a minimal tight-binding model, that uniaxial strain removes these constraints by inducing inequivalence between magnetic sublattices in two-dimensional (2D) system, driving an altermagnetic (AM) state into a fully compensated ferrimagnetic (fFIM) state and enabling fully spin polarization. Furthermore, strain along orthogonal directions gives rise to two energetically degenerate fFIM states with opposite spin polarization, enabling reversible spin switching. More importantly, the two symmetry-related fFIM states can be regarded as distinct ferroelastic variants, suggesting that this model or mechanism can be extended to ferroelastic fFIM systems. The generality of this mechanism is confirmed by combining spin-group analysis, first-principles calculations, and Boltzmann transport theory in representative candidates, including AM Mn$_2$SeO and ferroelastic fFIM V$_2$SO. Our results reveal a universal symmetry-driven framework for strain-controlled and -reversible fully spin-polarized transport and identify strain-engineered AM and ferroelastic fFIM systems as a promising platform for volatile and nonvolatile spintronic applications.
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All authors Xiuli Zhang, Peng Jiang, Yurui Ma, Xiaodong Zhou, Linlin Liu, Hong-Mei Huang, San-Dong Guo, Tengfei Cao, Yan-Ling Li

Materials Science

Surface segregation of liquid metal plasma-facing component alloys: A ReaxFF investigation

Md Adnan Mahathir Munshi, Abdul Aziz Shuvo, Mike Kotschenreuther et al. (+2 more)

Engineering liquid metal alloys offers a transformative pathway for plasma-facing components (PFC) by enabling chemically tailored surfaces that can simultaneously optimize plasma-material interactions, reduce divertor heat flux, and enhance core plasma confinement, thereby advancing the commercial viability of nuclear fusion power plants. This study, employing an atomistic simulation framework, provides direct evidence that incorporating non-metal surface-active agents (such as O and H, or their combination) enables strong surface segregation. This capability makes tin-aluminum (Sn-Al) and tin-lithium (Sn-Li) alloys, with suitable compositions, good candidates for PFC applications. Specifically, the presence of low-Z solutes (Li, Al) leads to preferential surface enrichment, which imparts low-Z sputtering characteristics, while the Sn solvent maintains thermophysical stability. To systematically examine this behavior, we first optimized ReaxFF parameter sets for Sn-Al, Sn-Al-O, Sn-Li, Sn-Li-O, Sn-Li-H, and Sn-Li-O-H systems. We validated them using formation energies and elastic constants. We then employed reactive molecular dynamics simulations to resolve the coupled effects of surface segregation and impurity-driven chemistry at fusion-relevant temperatures. We also introduced an overlap-based segregation index that captures interfacial compositional separation directly from atomistic density distributions. This metric reveals a clear hierarchy of segregation regimes across all systems and presents a unified view of segregation across all observations reported herein. Together, these findings establish a mechanistic link between non-metal chemistry and interfacial structure, providing a predictive framework for designing self-adaptive, low-sputtering liquid metal alloys for fusion applications.
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All authors Md Adnan Mahathir Munshi, Abdul Aziz Shuvo, Mike Kotschenreuther, Adri C.T. van Duin, Bladimir Ramos-Alvarado

Materials Science · Cross-listed from cs.CE

On the role of crack electrolyte wetting in the degradation and performance of battery active particles

S. Luza-Vega, Y. Zhao and E. Martínez-Pañeda

Cathode particle fracture is widely recognised as a major degradation mechanism in lithium-ion batteries, yet cracking also permits electrolyte wetting of newly exposed internal surfaces, modifying interfacial reaction pathways. The mechanistic role of electrolyte wetting in redistributing reactions within cracked particles remains unclear. Here, we isolate this effect through a controlled comparison between (i) a fully coupled electro-chemo-mechanical model resolving lithium concentration, electrostatic potential, and stress fields in both the active material and the electrolyte inside and outside cracks, and (ii) a single-particle chemo-mechanical model employing the conventional uniform flux assumption. The coupled model predicts strong spatial heterogeneity in interfacial reaction rates, with flux amplification approximately 8x relative to the imposed uniform flux at the crack tip. Reaction redistribution, and thus lithium flux, is governed predominantly by local solid-state lithium concentration and stress variations, while electrolyte potential gradients inside cracks remain secondary under the conditions considered. Uniform flux models can underpredict delivered capacity by 25% at 1C-rate; this discrepancy increases at higher rates. They also underestimate tensile stresses throughout the delithiation process by 10%, directly affecting crack driving conditions. These results demonstrate that neglecting crack-electrolyte coupling leads to systematic underestimation of both utilisation limits and fatigue-relevant stress histories.
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Materials Science · Cross-listed from cond-mat.str-el

Field-induced metal-insulator transition, Chern insulators, and topological semimetals in a clean magnetic semiconductor GdGaI

Kazuki Guzman and Hiroaki Ishizuka

Non-coplanar magnetic order in low-carrier-density semiconductors provides a platform on which spin-charge coupling can reshape the electronic structure and induce nontrivial topological phases. Motivated by the recent discovery of the four-sublattice triple-$q$ order in the magnetic semiconductor GdGaI, we study an effective theory that couples a Ga $4p$ hole pocket at the $\Gamma$ point to three Gd $5d$ electron pockets at the $M$ points through four exchange channels. For the antiferromagnetic umbrella state with zero net magnetization, the model hosts trivial ($C = 0$) and $C = \pm 4$ Chern insulator phases separated by metallic regions; by deriving an analytical low-energy theory at the $\Gamma$ point, we show that the topological phase boundary is described by two degenerate double-Weyl semimetals, naturally explaining the $\Delta C = 4$ jump in the Chern number. In addition, a nodal-line-like state pinned near the Fermi level emerges in the absence of the $p$-$d$ exchange coupling, which separates the $C=\pm4$ phases for $\theta=\arccos(1/3)$ into two. Tuning the canting angle by an external magnetic field drives an insulator-to-metal transition out of the Chern insulator phase while leaving the trivial insulator largely intact, and stabilizes an additional $C = \pm 2$ Chern insulator phase when the uniform-magnetization exchange couplings become appreciable. These results identify GdGaI and its sister compounds as highly tunable platforms for realizing topological phases and field-induced metal-insulator transitions in clean magnetic semiconductors.
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Materials Science

Compositionally tuned phase transformations enhance pyroelectric energy harvesting from low-grade heat

Ruiheng Geng, Ka Hung Chan, Xinyue Huang et al. (+6 more)

Phase-transforming pyroelectric materials have emerged as promising candidates for low-grade thermal energy harvesting. However, whether first-order transformations with large pyroelectric coefficient or second-order transformations with better reversibility are preferable remains unclear. Here we report compositionally tunable phase transformations in Ba$_{1-x}$Sr$_x$TiO$_3$ ($x \in [0, 0.3]$), revealing evolution from first-order to second-order character. We identify a transitional regime between Sr$_{0.15}$ and Sr$_{0.22}$ where transformation mechanism fundamentally changes. Within this regime, Sr$_{0.19}$ achieves optimal lattice compatibility, exhibiting electrical leakage suppressed by over two orders of magnitude while retaining substantial polarization response. Energy conversion demonstrations show the multilayer Sr$_{0.19}$ device delivers pyroelectric current of $\sim$1.6 $\mu$A at 64$~^\circ$C with an energy density of 1.6 mJ/cm$^3$ per cycle and 5.5% conversion efficiency. Remarkably, this composition operates stably over 10,000 full energy conversion cycles without external bias field or recharging, demonstrating that transitional regime compositions provide the optimal balance between energy density and operational durability for practical low-grade heat harvesting.
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All authors Ruiheng Geng, Ka Hung Chan, Xinyue Huang, Nobumichi Tamura, Faqiang Zhang, Wanjia Han, Yang Zhang, Chenbo Zhang, Xian Chen