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Physics Condensed Matter Materials Science

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Tuesday 24 February

Materials Science

Collective Phonon Mixing and Eigenvector Transport Under Isotope Substitution

Jeff Armstrong, Hamish Cavaye, Pankaj Sharma et al. (+1 more)

Isotopic substitution modifies nuclear masses without altering the electronic potential energy surface to first order and is therefore often interpreted as a simple rescaling of vibrational frequencies. In solids with dense phonon manifolds, however, mass substitution acts as a parametric Hermitian deformation of the mass-weighted dynamical matrix, generating a continuous family of eigenproblems whose eigenvectors can undergo substantial rotation within coupled subspaces. Here we investigate protiated and deuterated ZIF-8 using inelastic neutron scattering and density functional theory lattice-dynamics calculations. While many vibrational modes exhibit near-ideal mass scaling and preserve their character across isotopic endpoints, modes embedded in spectrally congested regions display pronounced redistribution of vibrational character that cannot be inferred from frequency shifts alone. Because inelastic neutron scattering intensity is directly weighted by hydrogen displacement amplitude, spectral sparsity and congestion provide experimental indicators of predictable frequency renormalisation or susceptibility to qualitative eigenvector reorganisation under deuteration. To establish physically meaningful mode correspondence, we develop an adiabatic eigenvector-continuation framework with overlap-based tracking and explicit stability diagnostics. These results show that vibrational identity in complex framework materials is best understood as a continuous trajectory in eigenvector space and provide a general framework for analysing isotope-induced spectral flow in dense phonon systems.
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All authors Jeff Armstrong, Hamish Cavaye, Pankaj Sharma, Matthew E. Potter

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

Efficient two-color Floquet control of the RKKY interaction in altermagnets

Mohsen Yarmohammadi, Pei-Hao Fu and James K. Freericks

Magnetic impurities in real materials can mask the intrinsic spin-dependent properties of hosts. They interact indirectly through the Ruderman-Kittel-Kasuya-Yosida (RKKY) mechanism, which limits the use of isolated impurity spins in applications such as qubits and spintronics. By suppressing the RKKY interaction, an effective control scheme should therefore enable access to the host's unperturbed behavior while simultaneously isolating the impurity spins for functional use. Although beyond static approaches, single-color laser driving can suppress the RKKY interaction, it typically requires strong fields that may be impractical or destabilizing. To overcome these limitations, we show that two-color laser driving provides efficient and tunable control over all components of the RKKY interaction using two weak laser fields. Focusing on two-dimensional Rashba altermagnets, we show that interference between one- and two-photon processes produces altermagnet-specific Floquet corrections. These include additional AC Stark shifts, magnetizations, spin-orbit renormalization, and emergent in-plane Zeeman fields, which are absent under single-color driving and in non-altermagnetic systems. Notably, two-color driving induces a finite $z$-component of the Dzyaloshinskii-Moriya (DM) interaction, stabilizing in-plane chiral magnetism and related textures in Rashba altermagnets. These effects enable tunable, near-complete on-off switching of the Heisenberg, Ising, and DM interactions through a Lifshitz-like modulation of the Fermi surface. We also show that the tuning process is highly sensitive to the chirality of both beams. We further map out phase diagrams for ferromagnetic and antiferromagnetic alignment of impurities with clockwise and counterclockwise canting as a function of Rashba coupling and altermagnetic order. Finally, we discuss candidate material platforms and experimental feasibility.
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Materials Science

Epitaxial Films as Model Platform for Understanding Compositionally Complex Electrocatalysts

Satyakam Kar, Alejandro E. Perez-Mendoza, Huixin Xiu et al. (+8 more)

Compositionally complex solid solutions provide a unique route for engineering high-performance electrocatalysts, where the polyelemental surface composition can be seamlessly tuned to optimize activity, selectivity, and stability. However, the mechanistic understanding of these electrocatalysts remains limited by the lack of a model system with a crystallographically-defined surface that is compatible with correlative, multi-scale characterization. Here, we present epitaxial films as a model platform for studying compositionally complex electrocatalysts. Using magnetron sputtering, we realize (111) epitaxial Ir-Pd-Pt-Rh-Ru films on (0001) sapphire substrate via a (111) Pt buffer layer, confirmed by X-ray diffraction and transmission electron microscopy. The growth approach is applicable across a broad composition range and produces smooth surfaces (root mean square roughness < 1 nm) with micrometer-sized grains in the nanoscale films. With these films, we demonstrate direct structure-activity mapping at the nanoscale through precise co-localization using micro-indents and performing correlative atomic force microscopy, electron backscatter diffraction, and scanning electrochemical cell microscopy. Our work establishes a model platform for fundamental scalebridging characterization and paves the way for rational design of compositionally complex electrocatalysts.
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All authors Satyakam Kar, Alejandro E. Perez-Mendoza, Huixin Xiu, Miran Joo, Kirill V. Yusenko, Ulrich Hagemann, Christoph Somsen, Janine Pfetzing-Micklich, Christina Scheu, Corina Andronescu, Alfred Ludwig

Materials Science

Machine Learning Modeling of Temperature-Dependent Optoelectronic Properties of Anharmonic Solid Solutions

Pol Benítez, Cibrán López, Edgardo Saucedo et al. (+1 more)

Leveraging strong optoelectronic responses to external stimuli, such as temperature and electric fields, is central to the development of advanced photonic technologies, including adaptive photodetectors and reconfigurable photovoltaic devices. However, only a limited number of semiconducting materials, typically characterized by strong electron-phonon coupling, are known to exhibit such pronounced responsiveness, and their equilibrium optoelectronic properties are often not optimally suited for targeted applications. Chemical engineering strategies, such as doping and solid-solution mixing, are therefore widely employed to fine-tune the electronic and optical properties of semiconductors. Predicting the impact of such modifications, however, remains highly challenging due to the intrinsic complexity of chemically disordered and anharmonic systems, as well as the computational limitations of conventional first-principles approaches. In this work, we introduce a new computational framework that combines ab initio electronic-structure methods with machine-learning techniques to achieve first-principles precision in the prediction of optoelectronic properties of anharmonic solid solutions at finite temperature. We apply this approach to silver chalco-halide solid solutions, an emergent class of optoelectronic materials that have been experimentally shown to exhibit large band-gap tunability and strong responses to thermal excitations. Our results provide quantitative insight into the interplay between chemical disorder, lattice dynamics, and electronic structure in these materials. More broadly, this study establishes a general strategy for the accurate modeling of optoelectronic functionality in chemically disordered semiconductors.
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All authors Pol Benítez, Cibrán López, Edgardo Saucedo, Claudio Cazorla

Materials Science

Beyond Point-like Defects in Bulk Semiconductors: Junction Spectroscopy Techniques for Perovskite Solar Cells and 2D Materials

Ivana Capan

Junction spectroscopy techniques (JSTs) are powerful tools for investigating electrically active defects in semiconductors. Originally developed to study point-like defects in bulk semiconductors, JSTs have since been extended to increasingly complex systems, providing valuable insights into defect energetics and interactions. This review paper outlines the fundamental principles of JSTs and critically examines their application to emerging materials, such as perovskite solar cells and two-dimensional (2D) materials. By highlighting both the capabilities and limitations of JSTs in these non-classical systems, the review demonstrates their continued relevance and important role in advancing next-generation semiconductor materials and devices.
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Materials Science

Phonon frequency comb close to an isolated Einstein mode in InSiTe3

Tea Belojica, Jovan Blagojević, Sanja Djurdjić Mijin et al. (+10 more)

The emergence of phonon frequency combs exemplifies a rare and intriguing phenomenon in quantum solids. Materials with distinctive phonon band structures are especially promising for hosting such states, as their vibrational dispersion landscape across the Brillouin zone can facilitate the formation of long-lived, collective lattice excitations. In the layered Van der Waals compound InSiTe$_3$, polarization-resolved Raman spectroscopy reveals a pronounced anharmonicity in symmetry-predicted modes and the formation of a self-organized frequency domain structure (coherent-like state), in the range of a localized highenergy A$_{1g}$ phonon mode near 500 cm$^{-1}$. This strong phonon-phonon coupling manifests itself as an anomalous temperature dependence around 200 K, coinciding with the appearance of higher-order excitations within the phonon density of states gap. These findings position InSiTe$_3$ as an unconventional platform where intrinsic highly structured phonon spectral correlations and unusually strong anharmonic effects coexist, opening new avenues for exploring emergent vibrational phenomena in low-dimensional materials.
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All authors Tea Belojica, Jovan Blagojević, Sanja Djurdjić Mijin, Andrijana Šolajić, Jelena Pešić, Emil S. Božin, Bojana Višić, Yu Liu, Cedomir Petrovic, Zoran V. Popović, Rudi Hackl, Ana Milosavljević, Nenad Lazarević

Materials Science

Disentangling the dynamics of transient spin and orbital magnetization in SrTiO$_3$ via the inverse Faraday effect from RT-TDDFT

Andri Darmawan, Markus E. Gruner and Rossitza Pentcheva

Motivated by recent evidence of ferroelectricity and even multiferroicity in the prototypical diamagnetic band insulator SrTiO$_3$ from terahertz experiments, we investigate the carrier and magnetization dynamics of SrTiO$_3$ excited optically by linearly (LPL) and circularly polarized light (CPL). Our RT-TDDFT simulations show a pronounced site- and orbital-dependent charge transfer from O$2p$ to Ti $3d$ states. With LPL anti-phase oscillations of the electron-density lobes at O and Ti resemble the soft transverse optical phonon mode and break dynamically inversion symmetry. CPL instead drives a coherent rotation of the charge dipoles around oxygen, producing a helicity-dependent transient magnetization of opposite sign at O and Ti, even without ionic motion. The dominant effect stems from the transfer of angular momentum (AM) from light to the electronic orbital AM, while spin-orbit coupling plays a key role in the transfer from orbital to spin AM, the former being an order of magnitude larger than the latter. The real-time resolution allows us to disentangle the inverse Faraday effect, which follows the pulse envelope, from optical orientation, which builds up during the pulse and saturates afterwards. The results demonstrate that purely electronic processes without lattice motion induce ultrafast magnetisation in a non magnetic insulator, offering a tunable route for light controlled magnetic functionalities.
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Materials Science

Large Photoelasticity in Topological Antiferromagnet Mn$_3$Sn Studied by Coherent Acoustic Phonon

Yuchen Wang, Takuya Matsuda, Yuta Murotani et al. (+5 more)

We investigate the role of ultrafast strain on the electronic and optical responses in topological antiferromagnet Mn$_3$Sn thin films using near-infrared femtosecond pump-probe spectroscopy. Coherent acoustic phonons are generated and exhibit remarkably large oscillations in differential transmission exceeding 1% in amplitude. Our quantitative analysis reveals that Mn$_3$Sn possesses an unusually large near-infrared photoelastic coefficient, several times greater than those in conventional materials, indicating a remarkable sensitivity of the electronic states to lattice distortions. This work establishes a quantitative framework for understanding and utilizing strain-induced responses in Mn$_3$Sn, paving a foundation for exploring coupled electron-phonon-magnon dynamics for ultrafast straintronics.
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All authors Yuchen Wang, Takuya Matsuda, Yuta Murotani, Hanyi Peng, Takumi Matsuo, Tomoya Higo, Satoru Nakatsuji, Ryusuke Matsunaga

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

Integrating Domain-Specialized Language Models with AI Measurement Tools for Deterministic Atomic-Resolution Experimentation

Zhuo Diao, Kouma Matsumoto, Linfeng Hou et al. (+3 more)

Self-driving laboratories based on large language models promise to transform scientific discovery through general experimental automation. However, realizing this vision on precision platforms remains challenging, requiring deterministic execution and effective domain adaptation under strict physical constraints. We address these requirements through a framework that specializes in small language models for autonomous control of scanning probe microscopy, coordinating task-specific models with AI-driven measurement tools. We demonstrate real-time, atomic-resolution SPM experiments at room temperature, achieving instruction-level control and multi-step experimental planning. Fine-tuning reduces perplexity from 1.44 to 1.20 and improves reliability, with the adapted model reaching 99.3% and 95.2% command accuracy, outperforming OpenAI o4-mini on domain-specific tasks. This architecture achieves lower computational cost while maintaining deterministic execution and enabling deployment on consumer-grade hardware. This work bridges probabilistic language models with deterministic experimental control through a modular, domain-specialized architecture, providing a generalizable pathway toward scalable and trustworthy self-driving laboratories across diverse scientific platforms.
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All authors Zhuo Diao, Kouma Matsumoto, Linfeng Hou, Masahiro Ohara, Hayato Yamashita, Masayuki Abe

Materials Science

Combining Quasiparticle Self-Consistent $GW$ and Machine-Learned DFT+$U$ to Assess Half-Metallicity in Co- and Ni-Based Heuslers

Zefeng Cai, Malcolm J. A. Jardine, Maituo Yu et al. (+6 more)

Half-metallic Heusler compounds are of significant interest for spintronics. For device fabrication, compounds that can be epitaxially grown on III-V semiconductors are particularly attractive. We present a first-principles investigation of four Co-based and two Ni-based Heusler compounds that are lattice-matched to InAs. The results of density functional theory (DFT) using semi-local and hybrid functionals are compared to quasiparticle self-consistent $GW$ (QP$GW$). We also consider DFT with machine-learned Hubbard $U$ corrections [npj Computational Materials 6, 180 (2020)] with a new Bayesian optimization (BO) objective function to determine the $U$ values that yield the closest agreement with the QP$GW$ band structure and magnetic moments. We find that DFT+$U$(BO) can adequately reproduce the key QP$GW$ features in most cases. Our results reveal a strong method dependence of the degree of spin polarization at the Fermi level and, in some cases, even the dominant spin channel (majority or minority). Of the materials studied here, Co$_2$TiSn and Co$_2$ZrAl are the most likely to be half-metals.
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All authors Zefeng Cai, Malcolm J. A. Jardine, Maituo Yu, Chenbo Min, Jiatian Wu, Hantian Liu, Derek Dardzinski, Christopher J. Palmstrøm, Noa Marom

Materials Science

Design of Skyrmion Bags with Tunable Topology in Symmetry-Broken 2D Lattices

Junhuang Yang, Kaiying Dou, Ying Dai et al. (+2 more)

Magnetic skyrmion bags, as high-order topological swirling spin textures, offer rich fundamental physics and distinct advantages for spintronic applications; however, their realization remains a formidable challenge, especially in two-dimensional (2D) systems. Here, through model analysis, we propose a novel design principle for engineering skyrmion bags with tunable topology in symmetry-broken 2D ferromagnetic lattices. The physics correlates to the delicate interplay of isotropic exchange interaction, Dzyaloshinskii-Moriya interaction and magnetic anisotropy, which can stabilize a rich variety of high-order topological spin states as well as the intriguing skyrmionium with zero topological charge. We further validate this mechanism in monolayer CrInTe2 using first-principles calculations and atomistic spin model simulations, revealing the existence of field-free skyrmion bags. Furthermore, we find that a weak magnetic field triggers a transition to skyrmioniums that exhibit remarkable thermal stability up to 240 K. Our results provide a compelling platform for exploring high-order topological magnetism.
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All authors Junhuang Yang, Kaiying Dou, Ying Dai, Baibiao Huang, Yandong Ma

Materials Science

Propagation of elastic waves in a flexomagnetic solid

Swarnava Ghosh

Flexomagnetism is the coupling between magnetism and strain gradients and is a technologically relevant phenomenon. We present a theory of elastic wave propagation in a linear elastic flexomagnetic material with microstructure and strain gradient elastic interactions. The expressions of frequency, phase velocity, and group velocity of longitudinal and transverse waves are derived and are shown to depend on the flexomagnetic coefficient and microstructure. We also show that the effect of flexomagnetism and microstructure can lead to some interesting phenomena in wave propagation, which are not observed in classical linear elasticity theory of waves. Specifically, in contrast to classical linear elastic materials, where wave propagation is non-dispersive, flexomagnetic materials with microstructure can exhibit both normal and abnormal dispersion. It is also noteworthy that, in flexomagnetic materials with gradient elasticity, the phase velocities of transverse waves can exceed those of longitudinal waves, which is atypical in classical elasticity. Furthermore, waves can also attenuate for a certain range of wavenumbers that depend on the flexomagnetic coefficient and microstructural parameters. Finally, we explore the possibility of waves exhibiting zero group velocity modes, where waves are non-propagating but have strong local energy confinement, negative group velocity modes, where the wave packet moves in the opposite direction to that of wave propagation, and the phenomenon of wave freezing, where a propagating wave stops in space without diffusing or spreading.
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Materials Science

Autonomous epitaxial atomic-layer synthesis via real-time computer vision of electron diffraction

Haotong Liang, Yunlong Sun, Ryan Paxson et al. (+8 more)

Autonomous science platforms which make decisions on the fly are fundamentally changing the outlook for materials development. AI-driven schemes can effectively reduce the total number of iterations needed to arrive at the best stoichiometry for desired properties or optimum synthesis parameters by significant margins. Here, we demonstrate real-time closed-loop autonomous navigation of a multi-dimensional synthesis parameter space for fabricating phase-pure epitaxial films of a metastable functional oxide phase using pulsed laser deposition. Sequential growth iterations in search of the optimized recipe to stabilize the desired crystal phase were performed using frame-by-frame quantitative computer vision of electron diffraction images at the unit-cell level. Our scheme regularly resulted in > 30-fold reduction in the number of experiments compared to comprehensive parameter-space mapping. The real-time workflow developed here can be readily extended to other thin film synthesis platforms opening the door for self-driving atomic-level materials design as well as autonomous semiconductor manufacturing.
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All authors Haotong Liang, Yunlong Sun, Ryan Paxson, Chih-Yu Lee, Alex T. Hall, Zoey Warecki, John Cumings, Hideomi Koinuma, Aaron Gilad Kusne, Mikk Lippmaa, Ichiro Takeuchi

Monday 23 February

Materials Science

Band-Like Transport and Cation Off-Centring in Ag/Bi-Based Solar Absorbers

Yi-Teng Huang, Yixin Wang, Georgia Fields et al. (+19 more)

Ag(I)-Bi(III)-based semiconductors have gained substantial attention as nontoxic, stable alternatives to lead-halide perovskites for optoelectronics, but are widely limited by carrier localization, which severely restricts diffusion lengths. The most efficient Ag/Bi solar absorber is AgBiS2, but diffusion lengths in nanocrystal films are <50 nm. Carrier localization in this rock-salt (Fm-3m) system is believed to arise from cation disorder, and so we herein investigate the layered cation-ordered analogue. Through beyond-DFT simulations combined with neutron and X-ray powder diffraction, we reveal that off-centring of Ag+ and Bi3+ cations is energetically-favoured in this cation-ordered phase. Despite local distortions in the AgS6 and BiS6 octahedra, band-like transport takes place, which, surprisingly, also occurs in the cation-disordered rock-salt phase when these materials are made as bulk powders. The cubic-phase powders have the same degree of cation disorder as the nanocrystals that have carrier localization, which suggests that extrinsic factors play a determining role. We ascribe the intrinsic band-like transport of both phases of AgBiS2 to its close packing, ensuring high electronic dimensionality. These insights offer pathways for designing solar absorbers avoiding carrier localization limitations, and call for future efforts to enhance the efficiency of AgBiS2 photovoltaics to focus on large-grained thin films, or improved nanocrystal surface passivation.
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All authors Yi-Teng Huang, Yixin Wang, Georgia Fields, Peixi Cong, Yongjie Wang, Jack E. N. Swallow, Avari Roy, Jack M. Woolley, Victoria Rotaru, Maxim Guc, Lars van Turnhout, Mohamed Aouane, Emmanuelle Suard, Dominik Kubicki, Alejandro Pérez-Rodríguez, Aditya Sadhanala, Akshay Rao, Dennis Friedrich, Robert S. Weatherup, Simon J. Clarke, Seán R. Kavanagh, Robert L. Z. Hoye

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

Electronic dynamics in long linear and cyclic polyynes towards the carbyne limit

Soumyadip Bhunia, Yueze Gao, Jack Woolley et al. (+3 more)

Carbyne-the one-dimensional sp-hybridised allotrope of carbon-has long been predicted to exhibit unique properties, yet its synthesis remains elusive. To probe its behaviour, finite sp-carbon chains such as cumulenes and polyynes have been studied, but work to date has focused almost exclusively on short, linear systems far from the infinite carbyne limit and without considering topology. Here, we investigate long (48-carbon) linear and cyclic polyynes using steady-state and ultrafast, temperature- and polarization-resolved optical and vibrational spectroscopy. We find highly delocalized ground states in both topologies, with Peierls distortions markedly weaker than in short chains. In contrast, excited states undergo rapid self-localisation, with the localisation pathway and subsequent intersystem crossing strongly dependent on chain length and topology. Unlike shorter polyynes, excited-state structural rearrangements are minimal, and comparison with theoretical predictions shows that properties, such as Huang-Rhys factors, have plateaued by 48 carbons. Our results reveal how topology influences the electronic dynamics of long polyynes and refines our understanding of sp-carbon systems approaching the carbyne limit
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All authors Soumyadip Bhunia, Yueze Gao, Jack Woolley, Ross Milverton, Harry L Anderson, Raj Pandya

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

Transient Plastic Spin Labeling with Chlorine Dioxide

Bence G. Márkus, Sándor Kollarics, Kristóf Kály-Kullai et al. (+5 more)

Plastic waste, being one of the most important problems for humankind, poses severe threats to ecosystems, wildlife, and human health. Tracing, quantifying, and identifying types of plastic waste is of crucial importance to understand its environmental pathways and develop targeted strategies for reduction, recycling, and remediation. To contribute to addressing this global issue, we investigated the spin-labeling capabilities of aqueous chlorine dioxide (ClO$_2$) radicals upon introduction into poly(ethylene terephthalate) and utilized electron spin resonance spectroscopy for detection. The technique is capable of identifying plastic species as the unpaired electron of the radical molecule is strongly sensitive to its local environment through its coupling parameters. Temperature-dependent measurements revealed that the molecules are immobilized at low temperatures and exhibit well-resolved anisotropic and hyperfine spectra that are quantitatively described by a model spin Hamiltonian. Even above the melting point of water, some degrees of freedom remain blocked as a result of the polymer matrix. Furthermore, employing a time-series measurement at room temperature enabled us to determine the diffusion coefficient of the molecule in the polymer.
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All authors Bence G. Márkus, Sándor Kollarics, Kristóf Kály-Kullai, Bernadett Juhász, Dávid Beke, László Forró, Zoltán Noszticzius, Ferenc Simon

Materials Science

Demonstration of High-Performance Ultra-Wide Bandgap SrSnO$_3$ Top-Gated MOSFETs

Junghyun Koo, Weideng Sun, Donghwan Kim et al. (+6 more)

We report the demonstration of high-performance top-gated metal-oxide-semiconductor field-effect transistors (MOSFETs) based on the ultra-wide bandgap perovskite oxide SrSnO$_3$ (SSO). Using hybrid molecular beam epitaxy-grown SSO channels and ALD-deposited HfO$_2$ gate dielectrics, the devices exhibit field-effect mobility exceeding 65 cm$^2$/V$\cdot$s, an on-state current up to 194 mA/mm, an on/off current ratio above $10^8$, and a contact resistance of 0.66 $\Omega\cdot$mm. The devices also show a near-ideal subthreshold slope of 68 mV/dec and negligible hysteresis, indicating a high-quality dielectric/semiconductor interface. These results establish SrSnO$_3$ as a promising ultra-wide bandgap oxide semiconductor platform for high-performance power electronic applications.
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All authors Junghyun Koo, Weideng Sun, Donghwan Kim, Hongseung Lee, Chengyu Zhu, Kiyoung Lee, Hagyoul Bae, Bharat Jalan, Gang Qiu

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

Altermagnetic spin textures: Emergent electrodynamics, quantum geometry, and probes

Constantin Schrade and Mathias S. Scheurer

Emergent electrodynamics arising from spatially and temporally varying magnetic textures provides a framework for spin control in quantum materials. While this principle is established for ferromagnetic and antiferromagnetic textures, its consequences for altermagnets – magnetic orders with vanishing net magnetization but finite spin splitting – remain largely unexplored. In this work, we develop an effective low-energy theory of itinerant electrons coupled to smoothly varying altermagnetic spin textures. In the adiabatic regime, we show that altermagnetic textures generate additional emergent electromagnetic fields and quantum-geometric effects that are absent in conventional magnetic systems. These effects include emergent Zeeman fields that encode the structure of the altermagnetic order parameter, enabling local spin manipulation and a way to distinguish different altermagnetic orders. Moreover, we demonstrate a quantum-metric-induced, spin-dependent electron lensing effect that provides a mechanism for spin filtering, and discuss the local admixture of effective odd-parity magnetic components. Our results suggest that textured altermagnets could serve as a versatile resource for spintronics functionalities and a probe of altermagnetism.
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Materials Science

Role of octahedral tilting induced acoustic softening on limiting thermal transport in SrSnO3

Yuzhou Hao, Turab Lookman, Xiangdong Ding et al. (+2 more)

Octahedral tilting is a fundamental structural distortion in perovskites, governing key phenomena such as lattice stabilizing, soft phonon dynamics, group-theoretical analysis, phase transitions, ferroelectricity, and even for tunable electronic band gap. However, its influence on lattice thermal conductivity (kL) remains poorly understood. In the archetypal perovskite SrTiO3, tilting in the low-temperature tetragonal phase is known to enhance kL by suppressing specific phonon scattering channels around 200 cm-1. Here, we investigate the thermal transport in strontium stannate (SrSnO3), another perovskite oxide that undergoes temperature-driven phase transitions, and reveal a completely opposite effect. Through a systematic study across its orthorhombic, tetragonal, and cubic phases, we demonstrate that octahedral tilting in the tetragonal phase of SrSnO3 anomalously triggers acoustic phonon softening. This softening manifests as reduced frequencies and group velocities in low-frequency (<3 THz) acoustic modes, creating a large decrease for heat transport, particularly along the c-axis. Consequently, kL is significantly suppressed, decreasing from 7.48 W m-1 K-1 to 6.06 W m-1 K-1 as the tilting angle increases by a mere 1 degree. These findings identify tilting-induced acoustic softening as a pivotal mechanism for limiting and controlling anisotropic thermal transport in SrSnO3, presenting a stark contrast to the established behavior in SrTiO3.
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All authors Yuzhou Hao, Turab Lookman, Xiangdong Ding, Jun Sun, Zhibin Gao

Materials Science

Microstructural Evolution and Crystallization Behavior of Amorphous Medium-Entropy Ti-Nb-Zr-Ag Thin Films

Anna Benediktová, Lucie Nedvědová, Michal Procházka et al. (+6 more)

Improving the performance of metallic implants increasingly relies on the development of multifunctional surface modifications that combine structural stability, bioactivity, and prevention of bacterial colonization. Medium-entropy alloys (MEAs) represent a promising approach for such coatings, as their chemical complexity allows the formation of structurally stable matrices with tunable properties. In this study, Ti-Nb-Zr and Ti-Nb-Zr-Ag thin films were deposited by magnetron sputtering and subjected to annealing at temperatures of up to 1100 $^{\circ}$C to evaluate the influence of Ag, added for its antibacterial potential, on structural evolution. The as-deposited Ag-free film was fully amorphous, whereas the Ag-containing film exhibited a predominantly amorphous matrix with finely dispersed crystalline nanoparticles, indicating that Ag promoted early-stage crystallization. Both films displayed a fine columnar morphology (column diameter $\sim$15 nm) with dome-like protrusions, a hierarchical surface structure favorable for protein adhesion. Upon annealing, the Ag-free film recrystallized into a granular, loosely packed morphology, while the Ag-containing film retained a compact structure, demonstrating the stabilizing role of Ag. These findings underscore the potential of Ag-containing amorphous MEAs for forming multifunctional coatings with enhanced thermal stability, antibacterial functionality, and biointerface-relevant surface features for advanced biomedical applications.
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All authors Anna Benediktová, Lucie Nedvědová, Michal Procházka, Zdeněk Jansa, Štěpánka Jansová, Christopher D. Woodgate, David Redka, Julie B. Staunton, Ján Minár

Materials Science

Tunable dislocations overcome mechano-functional tradeoff in perovskite oxides

Jiawen Zhang, Wenjun Lu and Xufei Fang

Recent advancements in dislocation engineering are reshaping the traditional view towards ceramics being brittle. Here, we use KTaO3 (KTO), a perovskite oxide that is newly discovered with room-temperature bulk plasticity, and demonstrate that the seeded dislocations can effectively tune both mechanical and functional properties. We uncover a novel brittle-ductile-brittle (BDB) transition: low dislocation densities lead to brittle failure, intermediate densities (~10*14 m-2) enable superior ductility with strains over 20%, and high dislocation densities (~10*15 m-2) induce again brittle fracture. This dislocation density-dependent non-monotonic mechanical response challenges the traditional behavior of ceramics and offers new design opportunities. Furthermore, dislocation densities can monotonically decrease thermal conductivity, revealing a tradeoff between mechanical strength and functionality. The findings reveal a critical threshold of dislocation density in optimizing the performance of functional oxides, and provide a new framework for using dislocations to design advanced materials where mechanical durability and enhanced functionality are intertwined.
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Materials Science

The effect of the A-site cation on the phase transition temperature of metal halide perovskites

Tom Braeckevelt, Sander Vandenhaute, Sven M. J. Rogge et al. (+2 more)

A key challenge for the practical application of metal halide perovskites (MHPs) is the instability of the desired perovskite phase relative to the optically non-active $\delta$ phase. To determine the phase stability, we previously developed a procedure to compute the harmonic free energy as a function of temperature, which was suited for CsPbI$_3$ but fails when Cs is replaced by organic cations due to their rotational freedom. Herein we propose a multistep thermodynamic integration (TI) approach that corrects the harmonic free energy to obtain the Gibbs free energy. Given the abundance of local minima in these materials, we employ replica exchange to prevent simulations from getting trapped, while introducing an intermediate potential energy surface to improve convergence and reduce computational cost. Benchmarking energy and forces from different exchange-correlation functionals and dispersion methods against high-level RPA+HF calculations identifies PBE+D3(BJ) as the best trade-off between accuracy, computational efficiency, and precision. To perform molecular dynamics simulations within the TI framework, it was necessary to train a machine learning potential using the MACE architecture on ab initio data calculated with density functional theory. Our results show that, for all three materials, the free energy difference between the $\gamma$ and $\delta$ phases exhibits a very similar temperature dependence. This suggests that phase stability is primarily governed by differences in ground-state energy, rather than by material-specific thermal effects. Beyond these three materials, our methodology provides a robust framework for investigating the phase behavior of other MHPs, paving the way for the discovery of more stable perovskites.
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All authors Tom Braeckevelt, Sander Vandenhaute, Sven M. J. Rogge, Johan Hofkens, Veronique Van Speybroeck

Materials Science

Cr3+ spin dynamics under the octahedral crystal field in van der Waals antiferromagnets

Rabindra Basnet, Subhashree Chatterjee, Paul Kigaya et al. (+3 more)

The magnetic moment in van der Waals (vdW) materials containing 3d transition metals originates from unpaired d-electron spins and their interaction with surrounding ligands. The interplay between exchange interactions and magnetic anisotropy stabilizes long-range ordering of such moments. The compound CuCrP2S6 (CCPS) presents an interesting class of vdW solids where the coupling of Cr3+ moments and ordering of Cu1+ ions give rise to a multiferroic ground state. Here we investigate the spin dynamics of Cr3+ ions in CCPS through magnetization and broadband as well as single sub-THz magnetic resonance measurements. The orbital moment of Cr3+ is quenched under the octahedral crystal field of surrounding chalcogen ions, resulting in negligible magnetic anisotropy-a feature common to Cr-based vdW antiferromagnets (AFM). Resonance spectra over a wide frequency-field-temperature range reveal quasi-2D AFM dynamics governed mainly by isotropic Cr-Cr exchange interactions, which determine the magnetic order, spin reorientation, and damping. Sub-THz resonance spectra also uncover a field-induced ferromagnetic polarization, highlighting the universal role of Cr-Cr exchange in layered Cr compounds. Moreover, persistent magnetic correlations far above the Néel temperature (TN ~ 32 K) points to short-range magnetic order in CCPS and motivates future studies of a possible interplay between AFM and antiferroelectric orders. These results establish CCPS as an exemplary system for exploring 2D magnetism and electric-field-tunable spintronic functionalities in layered multiferroics.
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All authors Rabindra Basnet, Subhashree Chatterjee, Paul Kigaya, Ezana Negusse, J. van Tol, Ramesh C. Budhani

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

Non Fermi liquid signatures across strain engineered metal-insulator transition in line-graph lattices

Shashikant Singh Kunwar and Madhuparna Karmakar

Controlling the properties and thus the functionalities of correlated electron systems via externally tunable perturbations has always remained a cherished goal in quantum condensed matter physics. Recently, straintronics has proved to be one such external control which can dictate the quantum phases and transitions in materials via the reconstruction of their electronic band structure. A particularly intriguing scenario arises in the context of flat band line-graph lattices wherein straintronics is found to bring forth non trivial phase transitions. This paper reports the phase transitions and thermal scales across the Lieb/Kagome interconversion in the electronic interaction-strain-temperature space. Based on the thermodynamic, spectroscopic and transport signatures across the strain tuned interconversion of these line-graph lattices we have mapped out the low temperature phases and thermal transition scales, numerically determined using non perturbative calculations. While at the low temperatures, interaction-strain plane is spanned by magnetically correlated insulators, flat band induced weak transiently localized insulators and non Fermi liquid metallic phases, thermal fluctuations aid in to stabilize coexistent magnetic correlations. Apart from quantifying the magnetic transition scales in this system our results on the spectroscopic and transport signatures distill the strain tuned metal-insulator transition and crossover scales which exhibit variable transport scaling exponents.
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Materials Science

Electronic structure of Graphene/Co interfaces

Daniela Pacilé, Simone Lisi, Iolanda Di Bernardo et al. (+12 more)

Photoemission, from core levels and valence band, and low-energy electron diffraction (LEED) have been employed to investigate the electronic and structural properties of novel graphene-ferromagnetic (G-FM) systems,obtained by intercalation of one mono-layer (1ML) and several layers (4ML) of Co on G grown on Ir(111). Upon intercalation of 1ML of Co, the Co lattice is resized to match the Ir-Ir lattice parameter, resulting in a mismatched G/Co/Ir(111) system. The intercalation of further Co layers leads to a relaxation of the Co lattice and a progressive formation of a commensurate G layer lying on top. We show the C 1s line shape and the band structure of G in the two artificial phases, mismatched and commensurate G/Co, through a comparison with the electronic structure of G grown directly on a Co thick film. Our results show that while the G valence band mainly reflects the hybridization with the d states of Co, regardless of the structural phase, the C 1s line shape is very sensitive to the rumpling of the G layer and the coordination of carbon atoms with the underlying Co. Even in the commensurate (1x1) G/Co phase, where graphene is in register with the Co film, from the angular dependence of the C 1s core level we infer the presence of a double component, due to in-equivalent adsorption sites of carbon sub-lattices.
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All authors Daniela Pacilé, Simone Lisi, Iolanda Di Bernardo, M. Papagno, L. Ferrari, Michele Pisarra, Marco Caputo, S. K. Mahatha, P. M. Sheverdyaeva, P. Moras, P. Lacovig, Silvano Lizzit, Alessandro Baraldi, Maria Grazia Betti, Carlo Carbone

Materials Science

Automated structure discovery for Tip Enhanced Raman Spectroscopy

Harshit Sethi, Markus Junttila, Orlando J Silveira et al. (+1 more)

Tip-Enhanced Raman Spectroscopy (TERS) provides nanoscale chemical fingerprints alongside high-resolution topographic mapping of molecules, offering a powerful tool for materials discovery. However, TERS image datasets are challenging to interpret and typically demand time-consuming, computationally intensive quantum-chemistry calculations. To overcome this problem, we present an encoder-decoder model trained and evaluated on simulated TERS images of planar molecules, enabling direct prediction of molecular structures from spectral simulated data with high accuracy. Our approach demonstrates the feasibility of automating molecular structure identification from TERS images, bypassing traditional manual analysis. These findings provide a foundation for extending machine learning methods to experimental TERS datasets, potentially accelerating molecular discovery by integrating nanoscale spectroscopy with automated computational analysis.
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All authors Harshit Sethi, Markus Junttila, Orlando J Silveira, Adam S Foster

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

Mechanical and Structural Contributions to Anisotropy in Granular Materials

Mehdi Pouragha, Gertraud Medicus, Selvarajah Premnath et al. (+1 more)

Anisotropy in granular materials arises from both the internal fabric and the directionality of the stress state, yet separating these effects experimentally remains challenging. This study develops a first-order linearisation of the incremental stress-strain response that isolates mechanical anisotropy from structural anisotropy using two independent orientation measures. The formulation enables both contributions to be quantified directly from macroscopic laboratory data. The method is applied to hollow-cylinder tests with systematically varied loading directions. Results show that both anisotropy components intensify as the stress state becomes more deviatoric; mechanical anisotropy is consistently stronger; and its relative dominance decreases with increasing deviatoric stress. Comparison with an isotropic hypoplastic model confirms that mechanically induced directional effects are captured even without fabric anisotropy. The framework offers a practical and physically transparent means for quantifying and comparing anisotropy mechanisms in granular materials.
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All authors Mehdi Pouragha, Gertraud Medicus, Selvarajah Premnath, Siva Sivathayalan

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

A Physics-Regularized Neural Network and Kirchhoff Markov Random Field Framework for Inferring Internal Electrochemical States from Operando Spectromicroscopy

Naoki Wada, Yuta Kimura, Masaichiro Mizumaki et al. (+3 more)

Quantitative understanding of coupled reaction-transport dynamics in lithium-ion battery (LIB) composite electrodes is limited by the inaccessibility of key internal electrochemical states. Here we present a physics-integrated, data-driven framework to reconstruct latent states from operando microscopic X-ray absorption fine structure ($\mu$ -XAFS) hyperspectral data of LIB cathodes. Normalized-local-lithium-stoichiometry (NLLS) maps derived from Co K-edge spectra are refined using a physics-regularized neural network that enforces spatial continuity and current conservation to resolve ambiguities in the weak-sensitivity region. The reconstructed NLLS dynamics are embedded in a Kirchhoff-based Markov random field incorporating Kirchhoff's laws, Ohm's law, and Butler-Volmer kinetics to infer interfacial current density, ionic current, electrolyte potential, and effective ionic conductivity. Application to electrodes with differential initial electrolyte concentrations reveals distinct reaction-propagation modes that are consistent with a mechanism involving the non-monotonic concentration dependence of electrolyte conductivity. The inferred electrolyte concentration profiles qualitatively resemble the spatially extended concentration changes observed by independent operando X-ray transmission imaging using a 1 M LiAsF$_6$ electrolyte.
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All authors Naoki Wada, Yuta Kimura, Masaichiro Mizumaki, Koji Amezawa, Ichiro Akai, Toru Aonishi

Materials Science

Interface stability of beta-Ga2O3 (100) on oxidized Si- and C-terminated 3C-SiC (001) substrates: a first-principles investigation

Marica Licciardi, Aldo Ugolotti, Emilio Scalise et al. (+1 more)

We provide a first-principles modeling of the beta-Ga2O3/3C-SiC interface that takes into account the reconstructions occurring at the 3C-SiC (001) surface by oxidation, aiming to mimic the actual deposition process under the best structural and thermodynamic conditions. Using density functional theory calculations, we systematically investigate the interface configurations between beta-Ga2O3 (100) and both Si- and C-terminated 3C-SiC (001) substrates, considering realistic oxidation states that form at the SiC surface prior to epitaxial growth. Our analysis evaluates different stacking sequences and atomic-scale bonding arrangements, computing adhesion energies for various interface geometries to determine their relative thermodynamic stability. This work addresses the critical need for understanding beta-Ga2O3 integration on substrates with superior thermal conductivity, providing a theoretical framework for optimizing heteroepitaxial growth conditions in ultra-wide-bandgap power electronics applications.
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All authors Marica Licciardi, Aldo Ugolotti, Emilio Scalise, Leo Miglio

Materials Science

Bismuth-substituted Lutetium Iron Garnet Films with Giant Visible-Range Magneto-Optical Sensitivity

Megan H. Dransfield, Lukáš Flajšman, Matthijs H. J. de Jong et al. (+4 more)

Magneto-optical materials are indispensable across modern physics, serving as the foundation for precision magnetic sensing, nonreciprocal photonics, and optical isolation technologies. The continual pursuit of materials with high Verdet constants has driven the development of garnet-based compounds exhibiting giant magneto-optical sensitivity. In this work, we report the growth and comprehensive magneto-optical characterization of bismuth-substituted lutetium iron garnet (LuBiIG), a material that combines the large spin-orbit coupling of bismuth with the lattice stability of lutetium iron garnet. LuBiIG films with thicknesses between 85-220nm were grown by pulsed laser deposition and characterized at room temperature over the 500-820nm wavelength range. The films exhibit an exceptionally high Verdet constant of up to -1.37e8 degree/meter/Tesla, peaking in the visible spectral range near 520nm. These results position LuBiIG as a highly sensitive magneto-optical material suitable for advanced cryogenic detection and hybrid quantum applications.
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All authors Megan H. Dransfield, Lukáš Flajšman, Matthijs H. J. de Jong, Lars Peeters, Rhodri Mansell, Sebastiaan van Dijken, Laure Mercier de Lépinay