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Physics Condensed Matter Materials Science

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Monday 27 July

Materials Science

A flexible kinetic Monte Carlo framework for GaN molecular beam epitaxy with adaptive on-the-fly barrier evaluation

Sajid Ali, Norbert Krause and Carla Verdi

We present a lattice-based kinetic Monte Carlo (KMC) framework for simulating GaN(0001) growth by molecular beam epitaxy. The framework captures the key microscopic processes governing epitaxial growth, including temperature-dependent surface diffusion, flux-driven deposition, Ehrlich–Schwoebel (ES) step-edge barriers, Ostwald ripening, and species-specific desorption, within a scalable architecture that enables systematic exploration of experimentally relevant growth conditions. In addition to predefined activation-energy catalogs, the framework supports adaptive on-the-fly barrier evaluation using machine-learned interatomic potentials. When previously unencountered local atomic configurations arise, activation barriers are computed via nudged elastic band, potential energy scans, or Brønsted–Evans–Polanyi methods, and cached for reuse. Predefined-barrier simulations reproduce compact triangular island formation, and further capture Ostwald ripening during growth interruptions and ES barrier-induced multilayer nucleation. At elevated temperatures, desorption drives an island “walking” regime, in which N–Ga exchange generates weakly bound Ga adatoms (AdGa) at trailing edges; preferential desorption of AdGa leads to asymmetric edge retreat and net island translation. Our KMC framework provides a flexible platform for predictive simulations of GaN epitaxy at the atomic scale and, more broadly, non-equilibrium growth of compound semiconductors.
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Materials Science

Growth and characterization of planar hexagonal Ge on CdS

Andrea Besana, Veronica Regazzoni, Marco Faverzani et al. (+13 more)

Hexagonal group-IV semiconductors have attracted increasing interest owing to their unconventional electronic and optical properties compared to the cubic diamond phase. However, the stabilization of these metastable allotropes in planar heterostructures remains a major challenge. In this work, we demonstrate the epitaxial growth of planar hexagonal germanium on non-basal $m$-plane CdS substrates by low-energy plasma-enhanced chemical vapor deposition. The role of growth temperature in the formation and stabilization of the hexagonal phase is investigated. X-ray diffraction, scanning transmission electron microscopy, and polarization-resolved Raman spectroscopy reveal the formation of epitaxial hexagonal germanium with the expected crystal symmetry. In particular, the Raman response exhibits the characteristic polarization selection rules of the E$_\text{2g}$ phonon mode of hexagonal Ge. Conversely, photoluminescence spectroscopy does not reveal any Ge-related emission feature. Combined transmission electron microscopy observations and atomistic modeling show that strain relaxation is governed by a limited set of dislocation mechanisms, which efficiently relieve most of the mismatch strain within a few nanometers from the interface and can involve localized cubic stacking insertions. At greater distances from the interface, the progressive loss of hexagonal order is increasingly dominated by stacking-fault disorder, particularly I3-type defects. These results establish CdS as a promising template for the planar stabilization of hexagonal Ge and provide insight into the defect mechanisms governing strain relaxation in metastable group-IV heterostructures.
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All authors Andrea Besana, Veronica Regazzoni, Marco Faverzani, Fabrizio Rovaris, Emiliano Bonera, Sonia Freddi, Elisa Brugaletta, Mohamed Zaghloul, Fabio Pezzoli, Francesco Montalenti, Monica Bollani, Daniel Chrastina, Anna Marzegalli, Antonio M. Mio, Emilio Scalise, Giovanni Isella

Materials Science

A General Model of Interfacial Chemical Equilibrium in Phase-Field Method

Yanzhou Ji and Long-Qing Chen

We report a new approach to interfacial equilibria among multiple solution phases in the phase-field method. It employs auxiliary non-conserved variables to describe the composition differences among different phases, and their temporal evolution is driven by the differences among different chemical diffusional potentials. It is reduced to the Wheeler-Boettinger-McFadden (WBM) model of equal interfacial chemical compositions and to the Kim-Kim-Suzuki (KKS) model of equal interfacial chemical diffusion potentials as two limiting cases. It can directly incorporate thermodynamic databases without any further approximations and simplifications. It is generally applicable to a wide range of problems involving composition evolution and chemical equilibria, including processes such as interdiffusion between two ordered phases not sharing any common composition range, which would pose difficulty to the existing treatments using WBM and KKS models.
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Materials Science

Quantum oscillation fingerprints of altermagnetism in hole-doped RuO2

Yuchi Yang and Yusheng Hou

Altermagnetism, characterized by its ferromagnetism-like spin-splitting band structure and antiferromagnetism-like magnetic order, has garnered considerable attention recently. Although hole doping may promote magnetism in the debated altermagnet candidate RuO2, the evolution of its electronic and magnetic properties under hole doping remains poorly understood. Based on first-principles calculations, we employ quantum oscillations to study hole-doped RuO2. We find that hole doping can enhance spin splitting and reconstruct the Fermi surface in RuO2, which is revealed by the angle-dependent quantum oscillation frequency. By tracking a pair of closed Fermi-surface pockets, we identify a meaningful correlation between the magnetic moment of Ru and a quantum-oscillation-based signature of spin splitting. This correlation follows a quasi-linear trend over a broad doping range, which can be captured by a minimal two-dimensional d-wave altermagnetic model. In addition, the hole-doped RuO2 exhibits a transition from a nonmagnetic to an altermagnetic state via an intermediate state. The quasi-linear correlation through quantum oscillation and the distinct quantum oscillation frequency of the stable altermagnetic state can serve as useful signatures for identifying the altermagnetic state in RuO2. Our results provide a comprehensive framework for understanding hole-doped RuO2, offering new insights into altermagnetic transitions and their identification.
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Materials Science · Cross-listed from cond-mat.str-el

Magnetic anisotropy in the near-stoichiometric van der Waals ferromagnet Fe$_3$GeTe$_2$

Riju Pal, Joyal John Abraham, Satyabrata Bera et al. (+5 more)

Quasi-two-dimensional (2D) van der Waals (vdW) ferromagnets such as the series Fe$_{3-x}$GeTe$_2$, with a relatively high Curie temperature and robust metallicity, offer an ideal platform for investigating itinerant magnetism in reduced dimensions. Here, we present a comprehensive electron spin resonance (ESR) investigation of single-crystalline almost-stoichiometric Fe$_{3.03 \pm 0.03}$GeTe$_{2}$ across wide ranges of frequencies, temperatures, and magnetic fields to gain quantitative insights into its magnetic anisotropy and spin dynamics. Frequency-dependent ESR measurements establish Fe$_{3}$GeTe$_{2}$ as an easy-axis ferromagnet. Temperature-dependent high-field ESR reveals a large internal field that gradually decreases at higher temperatures. Remarkably, this internal field persists even above $T_\mathrm{C}$, evidencing short-range spin correlations in Fe$_{3}$GeTe$_{2}$. Analysis of spin-wave modes yields a strong uniaxial magnetocrystalline anisotropy $K_{\text{int}} \approx - 5 \times 10^6$ erg cm$^{-3}$ at 3 K and a large magnon gap $\Delta(3 \mathrm{K})$ $\approx$ 87.8 $\pm$ 13.7 GHz ($\approx$ 0.363 $\pm$ 0.057 meV). Our study highlights that small variations in Fe content in Fe$_{3}$GeTe$_{2}$ lead to substantial changes in the magnon gap at low temperatures, indicating the extreme sensitivity of spin dynamics to the chemical composition. These results establish Fe$_{3}$GeTe$_{2}$ as a model vdW ferromagnet for exploring tunable anisotropies and magnon excitations in metallic 2D magnets.
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All authors Riju Pal, Joyal John Abraham, Satyabrata Bera, Mintu Mondal, Atindra Nath Pal, Bernd Buchner, Vladislav Kataev, Alexey Alfonsov

Materials Science · Cross-listed from cond-mat.stat-mech

Amortized Posteriors for Estimation of Material Constitutive Parameters from Multimodal Measurements on Small Punch Tests

Mohammad Ali Seyed Mahmoud, Aditya Venkatraman, Raj Mahat et al. (+2 more)

Bayesian calibration of material constitutive parameters from multimodal mechanical test data is often limited by the need to specify a joint likelihood across measurement modalities that differ in dimensionality, noise structure, and physical units. The resulting posteriors are often broad or strongly correlated, causing standard Markov Chain Monte Carlo (MCMC) samplers to mix poorly. Here, we present an amortized, likelihood-free framework that combines Gaussian process (GP) surrogates with Conditional Flow Matching (CFM) to learn conditional posteriors over constitutive parameters directly from synthetic multimodal parameter–observation pairs, avoiding hand-crafted likelihoods and repeated MCMC sampling. Once trained, the GP–CFM model generates posterior samples for each new specimen at negligible cost. The utility of this novel approach is demonstrated in this paper by estimating the values of Young's modulus and yield strength from the early portion of the force–displacement ($F$–$D$) curve and a Digital Image Correlation (DIC)-based displacement field measured in a Small Punch Test (SPT). It is observed that the $F$–$D$ data alone produce broad posteriors, consistent with limited parameter discrimination in the global response. Adding the DIC-measured displacement field was seen to contract the posteriors and shift them towards the independently measured tensile reference values. This work establishes a robust likelihood-free framework for the inference of material constitutive parameters from multimodal data, demonstrated through SPT–DIC integration.
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All authors Mohammad Ali Seyed Mahmoud, Aditya Venkatraman, Raj Mahat, Samantha Mitra, Surya R. Kalidindi

Materials Science

Neural RHEED alignment with limited training data during CdTe MBE growth

Bartłomiej Turowski, Jakub J. Meixner, Róża Dziewiątkowska et al. (+5 more)

We introduce a data-efficient neural-vision assisted method to automate crystallographic alignment during molecular beam epitaxy (MBE) growth. Trained on reflection high-energy electron diffraction (RHEED) patterns from only 15 CdTe structures, our model - enabled by physics-aware postprocessing - reliably infers crystallographic directions, replacing manual frame-by-frame inspection. To this end, we design, test, and critically compare neural-network architectures based on 2D and 3D ResNet configurations, both with and without postprocessing that leverages the physical constraints of RHEED image acquisition. Our work delivers (i) a fully trained neural system ready for closed-loop deployment in future CdTe growth experiments and (ii) a generalizable pipeline for new materials where access to diverse RHEED datasets is limited. More broadly, this study represents a step toward AI-driven MBE growth and demonstrates the potential of machine-learning-assisted automation in thin-film synthesis.
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All authors Bartłomiej Turowski, Jakub J. Meixner, Róża Dziewiątkowska, Wojciech Zaleszczyk, Tomasz Wojciechowski, Valentine V. Volobuev, Marcin M. Wysokiński, Tomasz Wojtowicz

Materials Science

Hybrid-parity sliding multiferroics

Zhenzhou Guo, Jiangtao Yu, Shibo Fang et al. (+6 more)

In this work, we introduce a class of hybrid-parity sliding multiferroics in which the spontaneous ferroelectric polarization is coupled to certain nonrelativistic spin splitting components through interlayer sliding, allowing these components to be reversibly switched in an electrical way. Symmetry analysis identifies coplanar magnets as natural platforms for realizing this form of sliding multiferroicity. First-principles calculations establish bilayer VBr$_2$ as a representative example, demonstrating the coupled reversal of the out-of-plane ferroelectric polarization and the signs of both even- and odd-parity nonrelativistic spin splitting components via an interlayer-sliding pathway. The signs of these nonrelativistic spin splitting components are locked to the sliding-switchable ferroelectric polarization and encoded in the spin-current responses, providing a signature of the coupled ferroic switching. Our findings expand the scope of sliding multiferroics and the functionality of sliding ferroelectrics for low-energy, nonvolatile logic devices.
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All authors Zhenzhou Guo, Jiangtao Yu, Shibo Fang, Jin Cao, Xiaodong Zhou, Yee Sin Ang, Wenhong Wang, Zhenxiang Cheng, Xiaotian Wang

Materials Science

Aligning Heterogeneous DFT Datasets: A Graph Neural Network Approach to Cross-Functional Formation Energies

Yidong Huang, Tenglong Lu, Hanwen Kang et al. (+3 more)

Heterogeneous density functional theory (DFT) calculations, particularly plane-wave implementations, introduce systematic formation energy errors ranging from tens to hundreds of meV/atom, depending on the selection of exchange-correlation functionals, kinetic energy cutoffs, pseudopotentials, and dispersion corrections. As demonstrated by the MatPES dataset, identical structures can exhibit an average energy discrepancy of 107 meV/atom between PBE and r2SCAN calculations. Such method-dependent discrepancies hinder the integration of multi-source DFT data, greatly limiting the scale and quality of datasets for training robust materials AI models. Here, we resolve this fundamental data silo barrier via graph-based transfer learning. Leveraging 380,190 structurally paired PBE-r2SCAN entries from the MatPES database, we train a structure-aware graph neural network to predict cross-functional energy residuals and align inconsistent DFT energy scales. By adopting GPTFF model architecture, the model converts conventional PBE energies to r2SCAN-level accuracy with a mean absolute error of 14.3 meV/atom, compared with 18.2 meV/atom achieved by CHGNet. This versatile approach effectively upgrades massive legacy PBE datasets to high-precision r2SCAN standards. It enables reliable predictions of phase stability, battery voltage profiles, and reaction thermodynamics, while allowing the integration of multi-source DFT data to advance the development of high-performance materials foundation models.
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All authors Yidong Huang, Tenglong Lu, Hanwen Kang, Junfeng Huang, Sheng Meng, Miao Liu

Materials Science

Catalyst Diffusion Transformer: Generative Inverse Design of Heterogeneous Catalysts

Hayoung Doo, Dong Hyeon Mok, Seoin Back et al. (+1 more)

The vast chemical design space and complex, interdependent design variables make catalyst discovery for targeted properties highly labor- and resource-intensive. Although generative models have emerged as a promising solution, existing approaches are generally limited to single-property conditioning or narrow chemical spaces. Here, we present Catalyst Diffusion Transformer (CatDiT), a unified framework for inverse catalyst design that generates valid and novel structures ranging from intermetallic alloys to oxide surfaces. By learning compressed latent representations, CatDiT enables efficient training and rapid sampling while supporting simultaneous conditioning on adsorbate type, binding energy, and catalyst class. The model provides reliable control of discrete properties and directional control of continuous properties, enriching candidate pools for reaction-specific catalyst discovery. As a representative application, multi-conditional generation for the nitrogen reduction reaction (NRR) yields 28 density functional theory (DFT)-relaxed alloy candidates that satisfy the target activity window and lie above the pure-metal *N-*H scaling line, corresponding to a ~1.5-fold enrichment over the source distribution. These results establish CatDiT as a practical and scalable approach for property-directed catalyst inverse design and targeted catalyst generation.
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All authors Hayoung Doo, Dong Hyeon Mok, Seoin Back, Jonggeol Na

Materials Science

Square Net TaSiAs Nanowires with Topological Surface Conduction and Linear Magnetoresistance

Anand Roy, Ofek Goldreich, Guy Ohad et al. (+7 more)

Square-net topological materials exhibit various interesting properties arising from the interplay of their structural diversity, topologically protected bands and different possible electronic features, including intrinsic magnetic ordering and superconductivity. However, little is known about the low-dimensional structures of these materials and their properties. Realization of low-dimensional topological square-net materials has the potential of enabling enhancement of their quantum behavior arising from quasi-compact 1D geometry and high surface-to-volume ratio, and their integration into functional devices. This work reports on the synthesis and properties of high-quality, single-crystal nanowires of Si square-net material TaSiAs. The chemical-vapor-transport produces TaSiAs nanowires that are chemically encapsulated with a thin dielectric shell of SiO2, enabling remarkable ambient stability and a pristine surface, which are critical for observing robust topologically protected surface states. Atomic-resolution structural analysis reveals a sharp core-shell interface, and a Si square-net lattice extending along the nanowire axis. These chemically protected nanowires allowed us to observe rich electrical and magnetotransport features, including 4 to 11 times lower room-temperature resistivity than the closest bulk analogues and non-saturating linear magnetoresistance, revealing topologically protected coherent surface transport. First-principles calculations show a wide-range (4 eV) linear band dispersion, alongside different Dirac cones that are protected by either symmorphic (C4) or non-symmorphic symmetries, predicting transport features consistent with our results. The findings demonstrate the unique properties of low-dimensional square-net topological materials and their potential applications, including next-generation interconnects, spintronic devices and quantum computing.
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All authors Anand Roy, Ofek Goldreich, Guy Ohad, Barun Barick, Olga Brontvein, Ora Bitton, Katya Rechav, Yishay Feldman, Leeor Kronik, Ernesto Joselevich

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

Accessing Few-Layer CrI$_3$ Magnetoelasticity Through Bulk Single Crystals

J. Arneth, M. Möller, D. A. S. Kaib et al. (+9 more)

The persistence of ferromagnetic long-range order in monolayers of the van der Waals semiconductor CrI$_3$ opens new routes for spintronic applications based on two-dimensional quantum magnets. In the fabrication of such devices, the constituent materials inevitably experience anisotropic strain, which modifies their intrinsic electronic properties. At the same time, strain can serve as a powerful tuning parameter, driving the material to desired regimes. While several theoretical studies have investigated the effect of biaxial in-plane strain on CrI$_3$ numerically, experiments are widely limited to the application of hydrostatic pressure. Here, we perform high-resolution magnetostriction experiments on bulk CrI$_3$ samples, and ab-initio-based magnetoelastic calculations, to elucidate the role of uniaxial lattice strain on the magnetic properties. Our data show that magnetostriction in CrI$_3$ is unexpectedly sensitive to surface effects, which enables us to investigate the influence of in-plane and out-of-plane strain separately, in both the bulk ferromagnetic (BFM) phase emerging at $T_{\rm C}=61\,\mathrm{K}$ and the surface antiferromagnetic (SAFM) phase below $T^* \simeq 50\,\mathrm{K}$. In particular, we quantify the uniaxial strain dependence of the surface interlayer coupling $J^{\rm SAFM}_{\perp}$ and the surface spin-flip field $B^*$, which drastically exceed the strain effects in the BFM phase by a factor of $\sim 30$. The large magnetostrictive response allows us to study the magnetoelastic coupling in few-layer CrI$_3$ through experiments on bulk single crystals, without requiring exfoliation.
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All authors J. Arneth, M. Möller, D. A. S. Kaib, P. P. Stavropoulos, A. Razpopov, M. Jonak, S. Spachmann, M. Abdel-Hafiez, S. Biswas, K. Riedl, R. Valentí, R. Klingeler

Materials Science

Atomic-scale phase-field modeling with dopants: Stochastic self-consistent harmonic approximation with fractional site occupancy

Kairi Masuda and Yu Kumagai

Phase-field modeling has achieved great success in predicting pattern formation in materials, such as the formation of ferroelectric domains. However, because it is typically based on continuum mechanics, conventional phase-field modeling cannot be straightforwardly applied to atomic-scale pattern formation, such as dopant segregation and vacancy ordering, which are driven by chemical potentials. Here, we extend the phase-field concept to the atomic scale by formulating the free energy of atomic systems within stochastic self-consistent harmonic approximation (SSCHA) theory to allow fractional site occupations. Our methodology enables us to directly calculate the derivative of the free energy with respect to site occupation and thereby obtain the chemical potential, successfully reproducing Ag distributions in bulk Cu as well as the resulting lattice expansion. Furthermore, we applied our methodology to investigate dopant segregation around a {\Sigma}5(310)[001] Cu grain boundary doped with Ag atoms. We found that Ag atoms preferentially segregate at the vertices of the triangular motif of a grain boundary. As the number of dopants increases, excess Ag atoms segregate near the vertices and then at the bottom sites of the triangular motif. This study extends the phase-field concept to discrete atomic systems, enabling the identification of preferential dopant-segregation sites and thereby visualizing atomic-scale pattern formation.
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Materials Science · Cross-listed from cond-mat.soft

Response-Selected Hidden Hyperuniformity in Hydrodynamic Active Matter

Liyu Zhong and Yang Jiao

Hyperuniformity in active matter is usually treated as a property of a prescribed density or continuum field. This view misses a basic feature of hydrodynamic active matter: an incompressible fluid does not respond equally to every microscopic force. Longitudinal forcing is absorbed into pressure, whereas transverse forcing drives flow. The relevant question is therefore not only whether particles are uniformly arranged or whether the total activity is small, but which sector of the active forcing is selected by the physical response. Here we introduce response-selected hyperuniformity, in which long-wavelength order is a property of a source-response pair. In a reversible valence-one fluid with no prescribed partners, locally neutral clusters screen the signed active-moment sector that controls transverse flow, producing a first-moment spectrum that vanishes quadratically at low wavenumber. Locally unscreened moments instead generate a nonzero infrared plateau. The resulting transverse-force spectrum has a universal crossover from fourth- to sixth-order scaling, with the crossover set by the ratio of the unscreened residual to the screened analytic contribution. Complete partner renewal preserves this normal form, establishing exchangeable multipole inheritance, while turnover tunes the residual through an independently measured local defect density. The zero-residual limit yields strictly hyperuniform velocity fluctuations; any finite residual causes defect-controlled infrared leakage and sets a finite screening length. Thus microscopic exchange need not destroy hidden hyperuniform flow order, but rare unscreened moments determine how far the quiet-flow regime survives.
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Materials Science

Interaction-driven electronic ferroelectricity in van der Waals heterostructures

Ziying Wang, Ana Vera Montoto, Mohammad Amini et al. (+5 more)

Strong electronic correlations in narrow-band systems provide a promising route to realize emergent quantum phases. While ferroelectricity in van der Waals materials is typically associated with inversion symmetry breaking driven by lattice distortions, interlayer sliding, or moiré reconstruction, the possibility of generating ferroelectricity directly from electronic interactions remains largely unexplored. Here, using molecular beam epitaxy, scanning tunneling microscopy, and ab initio calculations, we investigate two stacking geometries of bilayer 1T-TaSe$_2$, A-C and A-C$'$, formed by coupled Star-of-David charge density wave phases. We show that both stackings realize quasi-one-dimensional interacting chains, but are governed by distinct interaction mechanisms. In the A-C stacking, strong interlayer hybridization leads to dimerization and the formation of a band insulating state. In contrast, the A-C$'$ stacking is dominated by interlayer Coulomb interactions, producing a spontaneous charge imbalance between layers that gives rise to an out-of-plane ferroelectric polarization. Furthermore, we demonstrate that ferroelectric and antiferroelectric interchain configurations can be stabilized and electrically switched by an external field. Our results prove that bilayer 1T-TaSe$_2$ is a platform for interaction-driven electronic ferroelectricity, establishing an overlooked family of charge-ordered correlated states in 1T-TaSe$_2$ multilayers.
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All authors Ziying Wang, Ana Vera Montoto, Mohammad Amini, Yuxiao Ding, Jose L. Lado, Robert Drost, Adolfo O. Fumega, Peter Liljeroth

Materials Science

Recrystallisation phenomena in an ultrafine-grained Al-Mg-Si alloy revealed by correlative in situ EBSD and TEM heating

Moritz Theissing, Sandra Gonzaga, Patrick Willenshofer et al. (+5 more)

Ultrafine-grained (UFG) aluminium alloys are promising lightweight structural materials for space applications, where a high grain-boundary density can act as sinks for irradiation-induced defects. Their deployment, however, is contingent on thermal stability: aluminium components in low-Earth orbit can reach $\sim$200 $^\circ$C under solar irradiation, close to where severely deformed aluminium alloys recrystallise. Accurate, bulk-representative determination of recrystallisation onset is therefore essential, yet conventional in situ transmission electron microscopy (TEM) heating is compromised by thin-film effects, ambiguous grain-boundary contrast, and small sampling volumes. Here, a UFG AA6061 (Al-Mg-Si) alloy produced by high-pressure torsion was studied by a direct comparison of in situ TEM heating and in situ electron backscatter diffraction (EBSD) heating, complemented by differential scanning calorimetry (DSC), analytical scanning transmission electron microscopy (STEM-EDX) and microhardness. In situ EBSD sampled $\sim 10^{3}$ grains from bulk material and resolved the microstructural evolution into sequential recovery, recrystallisation and grain-growth regimes, placing the onset of instability at $\sim$198 $^\circ$C. Calorimetry, microhardness and nanoscale elemental mapping showed that grain refinement suppresses GP-zones formation and shifts precipitation to lower temperatures, with precipitation neither retarding recrystallisation nor restoring strength once the UFG structure is consumed. Revisiting the Brailsford-Bullough-Hayns sink-strength theory with a KAM-informed, temperature-dependent internal sink strength, we show that the grain-boundary sink strength collapses as recovery and recrystallisation proceed. We establish in situ EBSD heating as an in operando method for bulk-representative determination of microstructural instabilities in advanced metallic systems.
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All authors Moritz Theissing, Sandra Gonzaga, Patrick Willenshofer, Thomas M. Kremmer, Stefan Pogatscher, Stefan Mitsche, Frank F. Sene, Matheus A. Tunes

Materials Science

Nature of Cr Segregation in FCC to BCC quenched Fe-12Cr Alloy, with Post-Quenching Heat Treatment: A Positron Annihilation Study

S. Hari Babu and V. R. Reddy

The Fe-Cr binary alloy is a model system for ferrtitic/martensitic steels. Cr clustering instigated loss of ductility, is one of the influential factors to deteriorate the alloys, and its severity depends on Cr concentration. This study aims at understanding (i) the nature of Cr precipitation in Fe-12Cr alloy using positron lifetime spectroscopy, transmission electron microscopy and Mössbauer spectroscopy studies and (ii) comparing the precipitation concentration and size with respect to Fe-9Cr alloy. The quenched alloys (from the high temperature FCC phase) are found to contain a high density of dislocations along with the sub-grain structure which lead to Cr rich {\sigma}-phase precipitation during post-quenching heat treatment at 748 K, while the alloy prepared with no dislocations and the sub-grain structure shows no precipitation, except at grain boundaries, consistent with observations in Fe-9Cr alloys. Selected area diffraction confirmed that the precipitates are tetragonal intermetallic {\sigma}-phase, contradicting the majority of the literature on Fe-Cr ferritic alloys which reported it as Cr rich BCC {\alpha}'-phase. Mossbauer spectroscopy studies showed signatures of Cr depletion from the rest of the matrix, indicating that the precipitates are Cr rich compared to the alloy composition. The kinetics of Cr precipitation is found to be higher in the Fe-12Cr alloy compared to the Fe-9Cr alloy. The average precipitate size is observed to be higher and the number density is lower in the Fe-12Cr alloy compared to the Fe-9Cr alloy. In addition, the experimental positron lifetime in defect-free lattice of Fe-(9, 12)Cr alloys is reported for the first time with substantial evidence.
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Materials Science · Cross-listed from cond-mat.str-el

Symmetry Criterion for Van Hove Criticality at Non-Time-Reversal-Invariant Momenta

Min-Quan Kuang and Hua-Yu Li

At non-time-reversal-invariant momenta (non-TRIMs), time-reversal symmetry does not constrain the linear term of the band dispersion. Whether $\nabla E$ vanishes is therefore determined entirely by the representation theory of the little group. For nondegenerate bands, $\nabla E$ is forced to zero if and only if the vector representation $\Gamma_{\mathrm{vec}}$ of the little group does not contain the trivial representation $\Gamma_1$. When $\Gamma_{\mathrm{vec}}$ does contain $\Gamma_1$, $\nabla E$ is not forced to vanish for any nondegenerate band; the classification instead depends on the multiplicity of $\Gamma_1$ in $\Gamma_{\mathrm{vec}}$. For degenerate bands, the Wigner-Eckart theorem and Clebsch–Gordan coefficients determine whether linear couplings vanish, with classification performed at the subband level. Applied to space group 225, the criterion explains why the $W$ point is critical for all nondegenerate bands, the degenerate $E$ bands are generically noncritical, and the $K$ and $U$ points host parameter-dependent criticality. Supporting phase diagrams reveal a two-tier hierarchy: symmetry enforces $\nabla E=0$, while band parameters determine higher-order character. We extend this classification to all space groups hosting non-TRIMs in the single-group limit, providing a symmetry-dictated, parameter-independent framework for engineering Van Hove singularities in three-dimensional quantum materials.
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Materials Science · Cross-listed from physics.optics

Non-Centrosymmetric $\gamma$-Phase GaS Nanobelts for On-Chip Nonlinear Photonic Applications

Yukihiro Endo, Masato Takiguchi, Yoshiaki Sekine et al. (+2 more)

Second-order nonlinear optical processes in van der Waals (vdW) semiconductors offer a compelling route toward compact, integrable photon-conversion platforms. Among III-VI vdW semiconductors, GaS is particularly attractive owing to its wide bandgap suppressing two-photon absorption under near-infrared laser excitation. However, bulk GaS typically crystallizes in the centrosymmetric $\beta$ phase, which eliminates second-order nonlinearity and severely limits its application in nonlinear photonics. Here, we demonstrate that GaS nanobelts synthesized via self-catalyzed vapor-liquid-solid growth predominantly crystallize in non-centrosymmetric $\gamma$-phase stacking. This behavior originates from edge-selective growth kinetics at the Ga catalyst interface, which stabilizes the $\gamma$ phase and enables deterministic in-plane dipole moment alignment. The GaS nanobelts exhibit strong second-harmonic generation (SHG) with intensities comparable to those of GaSe, a widely used nonlinear optical material. Furthermore, we integrate the nanobelts onto SiN waveguides and demonstrate efficient on-chip SHG and sum-frequency generation. These results establish $\gamma$-GaS nanobelts as a transferable one-dimensional nonlinear materials well suited for on-chip photonic integration and indicate their strong potential for nonlinear optical applications.
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All authors Yukihiro Endo, Masato Takiguchi, Yoshiaki Sekine, Hisashi Sumikura, Yoshitaka Taniyasu

Sunday 26 July

Materials Science

Harnessing X-ray Absorption Spectroscopy Data through Multimodal Mining of Battery Literature

Tanjin He, Aikaterini Vriza, Logan Ward et al. (+7 more)

X-ray absorption spectroscopy (XAS) is central to understanding the local electronic and atomic structure of materials, yet most published spectra remain inaccessible to data-driven analysis because they are embedded in figures and described through fragmented textual context in the literature. Here, we use multimodal (image and text) literature mining to transform this dispersed knowledge into an AI-ready experimental data resource. We developed a scalable spectroscopy data digitization pipeline that identifies XAS figures in full-text articles, digitizes spectral curves, and links each spectrum to accompanying metadata on the measured edge and material. Applying this pipeline to the battery literature produced an open dataset of 13,740 XAS spectra, spanning 66 absorbing elements and diverse battery chemistries, with expert validation confirming accurate extraction of spectral and metadata information. By converting literature-embedded spectra into structured numerical data, this dataset provides a foundation for large-scale XAS analysis, cross-laboratory comparison, high-throughput characterization, and autonomous discovery of advanced materials.
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All authors Tanjin He, Aikaterini Vriza, Logan Ward, Xu Huang, Yiming Chen, Anubhav Jain, Gerbrand Ceder, Rajeev S. Assary, Ian T. Foster, Maria K. Y. Chan

Materials Science · Cross-listed from physics.optics

Dispersion models describing coupled systems in doped crystals. II. Infrared phonon excitations in GaAs single crystals and phonon-plasmon coupling in Zn-doped GaAs

Daniel Franta, Beata Hroncova, Mihai-George Muresan et al. (+1 more)

We present a temperature-dependent infrared study of intrinsic and Zn-doped GaAs within a generalized dispersion framework describing coupled phonon and free-carrier excitations. Reflectance, transmittance, and ellipsometric data measured from 300 to 440 K are fitted simultaneously using Gaussian-broadened distributions for single-phonon modes and triangle-delta-triangle representations for multiphonon absorption bands. The formalism provides a self-consistent description of lattice and carrier contributions to the dielectric response and reproduces the evolution of phonon-plasmon coupling with doping and temperature. In heavily doped samples, a pronounced Fano-type asymmetry of the TO-phonon resonance is observed and attributed to interference between phonon excitations and the Drude background of light and heavy holes. The resulting optical constants remain physically consistent across the far- and mid-infrared range and demonstrate the applicability of the framework to coupled bound and free excitations in doped semiconductors. The approach provides a quantitative tool for modeling infrared optical properties of GaAs and related photonic materials.
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All authors Daniel Franta, Beata Hroncova, Mihai-George Muresan, Stefan Zollner

Materials Science

Elemental Germanium Phase-Change Memory

Till Zellweger, Marko Mladenović, Kevin Portner et al. (+7 more)

Phase-change memory (PCM) is a mature technology for fast, scalable, non-volatile data storage, with applications spanning embedded memory, as well as in-memory and neuromorphic computing. PCM predominantly relies on chalcogenide alloys, with $\mathrm{Ge_2Sb_2Te_5}$ (GST) as the industry standard. Yet in these alloys, the individual Ge, Sb, and Te atoms redistribute upon cycling, causing stochastic operation and ultimately device failure. To address this issue, elemental antimony was proposed as a PCM material, but it exhibits a metastable amorphous state that prevents reliable data retention. Moreover, tellurium and antimony can contaminate complementary metal-oxide-semiconductor (CMOS) production lines or act as unintended dopants, restricting manufacturing of PCM to dedicated fabs. Here we introduce elemental germanium (Ge) as a CMOS-native phase-change material that overcomes these fundamental limitations. In a vertical PCM cell architecture, Ge enables sub-nanosecond crystallization (240 ps, 40 times faster than GST), non-volatile data storage with excellent thermal stability ($>$110 {\deg}C for 10 years vs. $\sim$87 {\deg}C for GST), and a resistance drift coefficient approximately 60% lower than in GST. These results establish pure Ge, a standard semiconductor, as an alternative to chalcogenide phase-change materials, achieving superior performance in key metrics and enabling phase-change memory to be fabricated in standard semiconductor facilities.
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All authors Till Zellweger, Marko Mladenović, Kevin Portner, Christoph Weilenmann, Michael Stiefel, Hanglin He, Klemens Bauer, Luiz Felipe Aguinsky, Mathieu Luisier, Alexandros Emboras

Materials Science

Observation of correlation-driven topological transport and robust ferromagnetism in 2D CrS$_2$

Sk Md Obaidulla, M. Nur Hasan, Dayal Das et al. (+10 more)

The realization of correlated layered magnets hosting robust ferromagnetism with emergent topological transport remains a key challenge in quantum materials. Here we report the first catalyst-free chemical vapour deposition growth of layered 1T-CrS$_2$, establishing a highly stable vdWs ferromagnet with an out-of-plane easy-axis anisotropy and a Curie temperature above room temperature. Transport measurements reveal a semimetal–insulator crossover near 80 K and pronounced negative magnetoresistance up to 350 K. A topological Hall effect emerges below 30 K, a rare signature of correlated transport in layered transition-metal dichalcogenide ferromagnets. First-principles calculations show that spin–orbit coupling gaps Dirac-like crossings, while electronic correlations reconstruct the Fermi surface by suppressing electron pockets and reducing the carrier density, enhancing momentum-dependent out-of-plane spin polarization. Magnetic measurements, supported by Heisenberg exchange calculations, reveal strong nearest-neighbour ferromagnetic exchange that stabilizes long-range ferromagnetism. Our results establish 1T-CrS$_2$ as a rare correlated 3$d$ layered ferromagnet in which electronic correlations and spin–orbit coupling cooperatively drive emergent topological transport.
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All authors Sk Md Obaidulla, M. Nur Hasan, Dayal Das, Rafiqul Alam, Antonio Supina, Muhammad Awais Aslam, Sherif Kamal, Iva Šarić Jankovic, Aleksandar Matkovic, Christian Teichert, Atindra Nath Pal, Heike C. Herper, Marko Kralj

Materials Science · Cross-listed from cond-mat.dis-nn

Local micromechanics in a mean-field model of glasses reveal key properties of its non-equilibrium RSB phase

Makoto Suda, Edan Lerner and Eran Bouchbinder

A recently formulated mean-field model of glasses features an equilibrium, zero-temperature Replica-Symmetry-Breaking (RSB) transition in some parameter range. In this range, the model's solution in the Replica-Symmetric phase is expressed in terms of an effective, self-consistent random potential for uncoupled degree of freedoms, where the transition to the RSB phase is characterized by the emergence of spectral-edge localized modes and a pseudogapped quartic vibrational spectrum, resulting in a finite spin-glass susceptibility. These properties are preserved in numerical solutions of the model under non-equilibrium conditions, i.e., upon an instantaneous quench. Inspired by recent advances in computer glasses, we define a micromechanical response function — the linear response to local force monopoles — in the framework of the mean-field model. We establish exact relations between the force monopole stiffness and global susceptibilities, which suggest a close correspondence between the non-equilibrium RSB phase of the model and the above-mentioned effective random potential description. As such, the obtained micromechanical observables constitute a concrete realization of the collective degrees of freedom of the model, offering a bridge between a glassy mean-field model and finite-dimensional glasses. We show that the model's vibrational spectrum can be computed solely from the monopole response statistics and, by employing a marginal stability criterion, we extract a characteristic frequency/stiffness scale of soft glassy modes, which is related to the boson peak in finite-dimensional, laboratory glasses.
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Materials Science

Universal temperature-dependent electrical resistivity in actinides

E.F. Talantsev

Temperature-dependent electrical resistivity $\rho(T)$ is one of the most common types of experimental data analyzed in condensed matter physics. For one group of pure metals, the actinides, experimental $\rho(T)$ curves differ radically from one another to the point that there is no unified theoretical approach to understanding and fitting $\rho(T)$ data in these elements. First-principles calculations result in $\rho(T)$ curves that differ from experimental data, even qualitatively. In an attempt to unravel this long-standing problem, here I propose a simple model that accurately fits the $\rho(T)$ data for nine phases of elemental actinides (from thorium (Th) to curium (Cm)) for which experimental data are publicly available to date. The model is based on the concept of two parallel conduction channels: one is described by the Bloch-Grüneisen equation, which is associated with the classical electron-phonon dissipation mechanism, and the other by the Arrhenius equation, which is associated with the nearest-neighbor hopping (NNH) conductivity. Debye temperatures $\Theta_D$ obtained by applying the model to the $\rho(T)$ data for nine elemental actinide phases are in good agreement with published values deduced from heat capacity measurements. For neptunium (Np) a maximum Arrhenius activation energy (among all actinides) of $E_a=15.9$ $meV$ was derived. The model was also successfully applied to $\rho(T)$ data measured on $\delta$-phase plutonium-based alloys Pu-Ce and Pu-Ce-Ga.
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Materials Science

Molecular dynamics with a first-principles-validated universal machine-learning potential reveals dynamic elementary processes of growth-related adspecies on GaN(0001)

Yoshito Takaesu, Akira Kusaba, Junko Ishii et al. (+2 more)

Atomic-scale understanding of the surface elementary processes in metalorganic vapor phase epitaxy (MOVPE) of GaN has so far relied on static density-functional-theory (DFT) energetics and on first-principles molecular dynamics (FPMD) limited to a few tens of picoseconds. Here we combine FPMD with a universal machine-learning interatomic potential (MLIP), UMA, to follow the dynamics of growth-related adspecies on GaN(0001) over time scales inaccessible to purely first-principles approaches. FPMD simulations of a GaNH admolecule coexisting with H adatoms reveal a hitherto unrecognized diffusion mode, in which the N atom lifts the Ga atom of the GaNH unit off the surface layer during migration, and show that the lifted Ga abstracts an H adatom from the surface, events invisible to static DFT. Single-point UMA calculations on FPMD snapshots reproduce the first-principles relative energies along the trajectory (RMSE of about 8.5 meV/atom) without any retraining. Long-time MLIP-based MD (150 ps) then reveals dynamics never observed within the FPMD window: site-to-site H-adatom hopping, which gates the migration paths of the growth unit, and reversible dissociation of the GaNH unit into independently migrating Ga and NH adspecies. This work constitutes, to our knowledge, the first application of an MLIP to the molecular dynamics of GaN MOVPE.
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All authors Yoshito Takaesu, Akira Kusaba, Junko Ishii, Shigenori Matsushima, Yoshihiro Kangawa

Materials Science

Fe-doping-induced band structure modification and cryogenic phase stability in Cs2AgBiBr6 single crystals

Yanan Li, Xuejiao Wu, Jidong Deng et al. (+1 more)

Despite its promise as a lead-free alternative, the practical application of Cs2AgBiBr6 in optoelectronics is limited by its wide band gap and detrimental intrinsic defects. To overcome these challenges, we synthesized Cs2AgBi0.5Fe0.5Br6 single crystals via a modified hydrothermal method. While both pristine and Fe-doped crystals undergo a structural phase transition near 125 K, Fe incorporation fundamentally alters its impact. The dopant simultaneously narrows the band gap in the high-temperature phase and suppresses the associated cryogenic structural instability. Our optical and X-ray structural studies establish Fe doping as a powerful strategy for tailoring the properties of Cs2AgBiBr6 , advancing its potential for high-performance, low-temperature optoelectronic and spintronic devices.
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All authors Yanan Li, Xuejiao Wu, Jidong Deng, Jinbao Zhang

Materials Science · Cross-listed from quant-ph

Multiconfigurational Mixed Quantum-Classical Approach for Correlated Many-Body Dynamics

Pritha Ghosh, Rajanya Sarkar and Arkajit Mandal

In this work, we introduce a multiconfigurational mixed quantum-classical many-body approach for simulating the finite-temperature correlated multi-exciton dynamics in the presence of phonon-induced static and dynamic disorder. In this mixed quantum-classical approach, the excitonic subsystem is described using a multiconfigurational wavefunction that extends beyond the mean-field limit, while the phonons are evolved quasi-classically. Using this approach, we simulate a multi-excitonic dissipative system and show how the interplay between phonon-induced dynamic disorder and exciton-exciton many-body interactions determines excitation-dependent excitonic transport and spatial correlations. Our results show that while the mean-field approach produces semi-quantitatively accurate diffusive dynamics, it does not capture the spatial correlations as expected. We find that a mean-field path approximation, where we generate pre-computed trajectories using our mean-field mixed quantum-classical approach and then perform multiconfigurational dynamics, can reproduce the spatial correlations to a good accuracy, positioning this approach as an efficient method for capturing spatial correlations in complex systems.
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Materials Science

Tailoring the Frequency-Dependent Optical Response of Hematite through Mono- and Co-Doping: A First-Principles Study

Abdul Ahad Mamun and Muhammad Anisuzzaman Talukder

Understanding the effects of doping on the crystal structure and optical properties of semiconductor materials is crucial for advancing next-generation semiconductor and photonic technologies. Although various studies have focused on doped hematite ($\alpha$-Fe$_2$O$_3$), the relationship between dynamical stability and optical properties remains insufficiently explored. This study presents a comprehensive first-principles investigation that simultaneously evaluates the phonon dispersion characteristics and frequency-dependent optical response of B-doped, Y-doped, and (B, Y)-co-doped $\alpha$-Fe$_2$O$_3$, providing deeper insights into the underlying mechanisms. We examined the finite-temperature vibrational properties, dielectric function, and optical characteristics to comprehend the lattice dynamics and light-matter interactions under electromagnetic radiation. Vibrational thermodynamics reveal that pristine and Y-doped hematite maintain dynamic stability, while B-doped hematite exhibits imaginary phonon modes indicating lattice instability due to distortions in the Fe–O framework. Notably, Y co-doping with B helps suppress these soft modes, restoring structural stability through lattice relaxation and improved interatomic forces. B doping enhances low-energy absorption by introducing additional states in the valence band, while Y doping alters orbital hybridization, leading to a broader dispersion. In the optical regime, doped hematite displays dominant interband transitions below $2$ eV and strong absorption between 1.80 eV and 4 eV. The (B, Y) co-doping combines the low-energy benefits with an improved optical response profile. In summary, doping significantly enhances lattice vibrations, light-matter interactions, and optical responses, providing an effective strategy for tailoring hematite for diverse applications in photoactive, optoelectronic, and photonic technologies.
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Materials Science · Cross-listed from physics.plasm-ph

Comparative qualification of advanced plasma-facing materials for fusion pilot plants through public- and private-sector experiments in DIII-D

Florian Effenberg, Jonathan D. Coburn, Luca Cappelli et al. (+44 more)

A coordinated DIII-D campaign exposed and comparatively assessed 44 advanced plasma-facing materials from 12 institutions, including four public-private fusion partnerships, to support fusion pilot plant wall and divertor material down-selection. Samples were exposed using the Divertor Materials Evaluation System (DiMES) under Ohmic, L-mode, and H-mode conditions with edge-localized modes, at 0.2-2.5 MW m$^{-2}$ on flush geometries and 10-15 MW m$^{-2}$ on 10$^{\circ}$ angled geometries. Engineered tungsten architectures retained integrity; long-fiber Wf/W showed the clearest crack-arrest behavior. W-Re and K-doped W showed near-ITER-W-like responses, while additively manufactured W-Ta showed heat-flux-sensitive mass losses of 0.64 mg for the flat sample and 2.19-2.87 mg for angled samples. After irradiation to 0.3 dpa at 550$^{\circ}$C, neutron-irradiated ITER-grade W retained 2.8 times more deuterium than pristine W, while TiB$_2$ showed the lowest D$_2$ release in the Ohmic set. VTaHfMo was the most stable refractory multi-principal-element alloy. NbC and (Nb$_{0.5}$Ta$_{0.5}$)C retained integrity with 0.02-0.03 mg mass loss, whereas ZrC lost 7 mg. CVD SiC retained macroscopic integrity but exhibited an effective Si erosion yield of 0.5, about 5-10 times above prior DIII-D trends. Renewable boron pebble rods underwent controlled recession; 13% of released boron was ionized near the outer strike point and up to 50% was recovered locally. Initial in-situ chromium gross-erosion measurements yielded values of order $10^{-2}$. Together, these results provide cross-material benchmarks for fusion pilot plant down-selection and future AI/ML-assisted plasma-facing-material development.
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All authors Florian Effenberg, Jonathan D. Coburn, Luca Cappelli, Simon D. Corah, Amoolya Grandhi, Jerome Guterl, Charlie Hirst, Mike Jackson, Dylan A. Kohler, Rob Kolasinski, Erick Martinez-Loran, Ria Meston, Lauren Nuckols, Angelica Ottaviano, Zana Popovic, Sergey Tsurkan, Tessa Van Volkenburg, Daniel Velazquez, Aaliyah Zuniga, Arunodaya Bhattacharya, Jose Boedo, Kent Christian, Giacomo Dose, Eric Hollmann, Mykola Ialovega, Florian M. Laggner, Samara Levine, Xavier Navarro, Andrew Shone, Lance L. Snead, David Sprouster, Jason Trelewicz, Jayson Barr, Ryan Hood, Charlie Lasnier, Yan-Ru Lin, Vincent Mazon, Jun Ren, Gilson Ronchi, Dmitry Rudakov, Dinh Truong, Cedric Tsui, Shawn Zamperini, Weicheng Zhong, Tyler Abrams, Anthony W. Leonard, Roberto Maurizio