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Physics Condensed Matter Materials Science

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Monday 20 October

Materials Science

Chiral soft mode transition driven by strain in ferroelectric bubble domains

Urmimala Dey, Natalya S. Fedorova, Jorge Íñiguez-González et al. (+1 more)

Chirality in solids is attracting growing attention as a potential ferroic order, yet virtually no paradigmatic example of a soft-mode achiral-to-chiral phase transition has been firmly established to date. Here we identify ferroelectric bubble domains as a model system that undergoes a strain-driven achiral-to-chiral transition exhibiting the hallmarks of spontaneous symmetry breaking. Using second-principles atomistic simulations, we uncover chiral phonon modes in ferroelectric/dielectric superlattices that soften under epitaxial strain following textbook soft-mode behaviour. The transition is accompanied by a change in topological character, highlighting an interplay between chirality and topology in these systems. This work provides a concrete step towards establishing chirality as a genuine ferroic order in solids.
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All authors Urmimala Dey, Natalya S. Fedorova, Jorge Íñiguez-González, Hugo Aramberri

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

Néel-Vector-Orientation Induced Direction-Robust Spin Filtering in Two-Dimensional Altermagnets

Xin Chen, Jin Zou, Lipeng Song et al. (+6 more)

Whether an antiferromagnet can host direction-robust spin-polarized transport without a conventional spin-selective band gap remains a central challenge in antiferromagnetic spintronics. Here we establish a gapless, direction-robust spin-filtering mechanism in a compensated two-dimensional altermagnetic Weyl semimetal that requires neither a spin-selective band gap nor a large velocity contrast between spin projections. Using Janus monolayer Ta$_2$TeSeO as a realistic platform, we combine symmetry analysis with first-principles calculations, full-Brillouin-zone Wannier interpolation, and semiclassical transport. Rotating the Néel vector removes a unitary-mirror constraint and shifts one Weyl-cone pair away from its parent high-symmetry line. For an in-plane Néel vector, the residual $C_{2z}\mathcal T$ symmetry forbids the independent $\sigma_y$ mass that would open a local gap, allowing the reconstructed cones to shift in momentum while remaining gapless. Breaking unitary $C_{2z}$ simultaneously lifts the energy equivalence of the remaining mirror-pinned Weyl cones. The resulting coexistence of a metallic spin-projected manifold and a low-DOS Weyl-derived manifold produces a predominantly DOS-driven conductance imbalance. At charge neutrality and 20~K, the longitudinal conductivity polarization for $\mathbf n\parallel x$ remains positive for every in-plane current direction and ranges from $76.4\%$ to $82.0\%$. The degenerate in-plane magnetic anisotropy facilitates reversible switching between symmetry-related spin-filtering states using strain or weak anisotropic fields. This Néel-vector-driven symmetry mechanism provides a general route to direction-robust gapless spin filtering in compensated altermagnets.
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All authors Xin Chen, Jin Zou, Lipeng Song, Wei Sun, Yiwen Wu, Luyao Zhu, Xu Cheng, Duo Wang, Biplab Sanyal

Materials Science

Hybridization in van der Waals epitaxy of PtSe2/h-BN and PtSe2/graphene heterostructures

Meryem Bouaziz, Samir El Masaoudi, Aymen Mahmoudi et al. (+10 more)

Van der Waals (vdW) heterostructures, which combine bi-dimensional materials of different properties, enable a range of quantum phenomena. Here, we present a comparative study between the electronic properties of mono- and bi-layer of platinum diselenide (PtSe2) grown on hexagonal boron nitride (h-BN) and graphene substrates using molecular beam epitaxy (MBE). Using angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT), the electronic structure of PtSe2/graphene and PtSe2/h-BN vdW heterostructures are investigated in systematic manner. In contrast to PtSe2/h-BN, the electronic structure of PtSe2/graphene reveals the presence of interlayer hybridization between PtSe2 and the graphene, which is evidenced by minigap openings in the {\pi}-band of graphene. Furthermore, our measurements show that the valence band maximum (VBM) of monolayer PtSe2 is located at the {\Gamma} point with different binding energies of about -0.9 eV and -0.55 eV relative to the Fermi level on h-BN and graphene and substrates, respectively. Our results represent a significant advance in the understanding of electronic hybridization between TMDs and different substrates, and they reaffirm the crucial role of the substrate in any nanoelectronic applications based on van der Waals heterostructures.
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All authors Meryem Bouaziz, Samir El Masaoudi, Aymen Mahmoudi, Eva Desgue, Marco Pala, Pavel Dudin, Mathieu G. Silly, Julien Chaste, Fabrice Oehler, Pierre Legagneux, Jose Avila, Iann C. Gerber, Abdelkarim Ouerghi

Materials Science

Electrical properties of PbS films doped with iodine by chemical bath deposition

T.B. Charikova, A.Yu. Pavlova, M.R. Popov et al. (+2 more)

We present the results of measurements of bulk current-voltage (I-V) characteristics and local surface I-V characteristics by atomic force microscopy (AFM) of iodine-doped PbS films. It is established that bulk I-V curves of both undoped and iodine-doped PbS films demonstrate a linear (ohmic) U(I) dependence. The tipe of local surface I-V characteristics is ohmic at the concentration range of the dopant 0<[NH4I]<=0.10 M and becomes rectifying at [NH4I]>=0.15 M, which is determined by a decrease in the size and a change in the shape of the film grains, as well as a decrease in the surface roughness of the film. An increase in the iodine content in the PbS(I) films leads to nonlinear dependences of the microscopic characteristics and photoelectric parameters of the PbS(I) films. A sharp decrease in the diffusion coefficient, the beginning of an increase in the charge carrier lifetime, a maximum in voltage sensitivity and specific detectability are observed in the PbS(I) film chemically deposited from a reaction mixture containing [NH4I] = 0.15 M. This indicates that the optimalconcentration of iodine in the film is 2.7 at.%.
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All authors T.B. Charikova, A.Yu. Pavlova, M.R. Popov, A.V. Pozdin, L.N. Maskaeva

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

Large-scale stochastic propagation method beyond the sequential approach

Zhichang Fu, Yunhai Li, Weiqing Zhou et al. (+1 more)

The $O(N)$ stochastic propagation method, which relies on the numerical solution of the time-dependent Schrödinger equation using random initial states, is widely used in large-scale first-principles calculations. In this work, we eliminate the conventional sequential computation of intermediate states by introducing a concurrent strategy that minimizes information redundancy. The new method, in its state-, moment-, and energy-based implementations, not only surpasses the time step constraint of sequential propagation but also maintains precision within the framework of the Nyquist-Shannon sampling theorem. Systematic benchmarking on one billion atoms within the tight-binding model demonstrates that our new concurrent method achieves up to an order-of-magnitude speedup, enabling the rapid computation of a wide range of electronic, optical, and transport properties. This performance breakthrough offers valuable insights for enhancing other time-propagation algorithms, including those employed in large-scale stochastic density functional theory.
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All authors Zhichang Fu, Yunhai Li, Weiqing Zhou, Shengjun Yuan

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

Geometry-Driven Charge and Spin Transport in $\beta12$ Borophene Quantum Dots

Seyed Mahdi Mastoor and Amirhossein Ahmadkhan Kordbacheh

Theoretical research has been conducted to study how geometry affects charge and spin transport in $\beta\mathrm{12}$ borophene quantum dots, which are confined systems. The study examined two distinct central regions, which included a circular disc and a regular hexagonal area that connected to semi-infinite zigzag and armchair borophene nanoribbon leads. The system was described by a five-band tight-binding Hamiltonian parameterized using first-principles data, and the transport properties were calculated within the non-equilibrium Green's function framework. Spin resolved transmissions and spin polarization were computed for a range of lead widths and proximity-induced exchange field strengths. The analysis revealed distinct transport characteristics determined by geometry and edge configuration: armchair-connected structures exhibited broader and more stable fully spin-polarized windows compared with zigzag-connected counterparts. Furthermore, critical lead-width thresholds ($\approx 1.01$ nm for zigzag and $\approx 0.87$ nm for armchair) and a moderate exchange field above which complete spin filtering occurs were identified. The results highlight the strong influence of edge termination and confinement geometry on transport properties and provide useful design guidelines for developing borophene-based nanoscale spintronic devices.
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Materials Science

A Computational Study for Screening High-Selectivity Inhibitors in Area-Selective Atomic Layer Deposition on Amorphous Surfaces

Gijin Kim, Purun-hanul Kim, Suk Gyu Hahm et al. (+4 more)

Area-selective atomic layer deposition (AS-ALD) is an emerging technology in semiconductor manufacturing. However, accurately understanding inhibitor reactivity on surfaces remains challenging, particularly when the substrate is amorphous. In this study, we employ density functional theory (DFT) to investigate reaction pathways and quantify the reactivity of (N,N-dimethylamino)trimethylsilane (DMATMS) and ethyltrichlorosilane (ETS) at silanol (-OH), siloxane (-O-), amine (-NH2), and imide (-NH-) sites on both amorphous and crystalline silicon oxide and silicon nitride surfaces. Notably, both molecules exhibit greater reactivity toward terminal sites (-OH and -NH2) on amorphous surfaces compared to crystalline counterparts. For bridge sites, -O- and -NH-, multiple reaction pathways are identified, with bridge-cleavage reactions being the predominant mechanism, except for DMATMS reactions with nitride surfaces. The reactivity of DMATMS with -NH- sites is comparable to that with -NH2, with both reactions yielding volatile products. This study underscores the importance of amorphous surface modeling in reliably predicting inhibitor adsorption and reactivity on realistic surfaces. Moreover, we outline a computational screening approach that accounts for site-specific precursor-inhibitor interactions, enabling efficient and rational theoretical design of AS-ALD precursor-inhibitor pairs.
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All authors Gijin Kim, Purun-hanul Kim, Suk Gyu Hahm, Myongjong Kwon, Byungha Park, Changho Hong, Seungwu Han

Materials Science

Chemically tailored planar defect phases in the Ta-Fe {\mu}-phase

Christina Gasper, Nisa Ulumuddin, Siyuan Zhang et al. (+5 more)

Intermetallics often exhibit complex crystal structures, which give rise to intricate defect structures that critically influence their mechanical and functional properties. Despite studies on individual defect types, a comprehensive understanding of the defect landscape in {\mu}-phases, a class of topologically close-packed phases, remains elusive. In this study, we investigated the planar defect structures in the Ta-Fe {\mu}-phase across a compositional range of 46 to 58 at.% Ta using electron microscopy and density functional theory calculations. Electron backscatter diffraction and high-resolution scanning transmission electron microscopy reveal a transition from basal twin boundaries and planar faults containing C14 TaFe2 Laves phase layers at a low Ta content to pyramidal {1\bar{1}02} twins at a higher Ta content. Density functional theory calculations of defect formation energies confirm a chemical potential-driven stabilisation of Laves phase lamellae. The prevalence of pyramidal twins in Ta-rich {\mu}-phase samples is attributed to the competitive nature of different planar defects during solidification. A defect landscape for {\mu}-phases is proposed, illustrating the interplay between site occupancy, dislocation types and planar faults across the chemical potential space. These findings provide fundamental insights into defect engineering in structurally complex intermetallics and open pathways for optimising material properties through chemical tuning.
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All authors Christina Gasper, Nisa Ulumuddin, Siyuan Zhang, Sang-Hyeok Lee, Christina Scheu, Benjamin Berkels, Zhuocheng Xie, Sandra Korte-Kerzel

Materials Science

Ab-initio force prediction for single molecule force spectroscopy made simple

Pooja Bhat, Wafa Maftuhin and Michael Walter

Bond rupture under the action of external forces is induced by temperature fluctuations. We show that measured forces from single molecule force spectroscopy experiments can be predicted from two quantities describing the bond that are the barrier to break the bond in absence of force as well as the maximal force the bond can withstand. The former can be obtained by a force free transition state calculation and the latter is determined by a simple constrained ge- ometry simulates forces (COGEF) calculation. Considering experimental temperature and force loading rate allows the prediction of measured bond rupture forces from a closed expression with very good accuracy.
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Materials Science

Exploration of the hysteresis of martensite-austenite transition in bulk e̱ta}-Cu-Zn-Al single crystals

O. Goisot, H. vanLandeghem, R. Haettel et al. (+4 more)

Improvement of functional and structural fatigue endurance for applications of ferroelastic materials requires an optimization of their composition. A strategy for finding alloy compositions that minimize the transformation hysteresis is necessary. We propose an experimental high throughput methodology to explore the model e̱ta}-Cu-Zn-Al system. It is based on an original route to process bulk gradient composition single crystals to investigate fine variation of composition range coupled with local measurements of the austenite-martensite microstructure by light microscopy during the transformation. The latter method is compared with differential scanning calorimetry measurements. The methodology is applied in an Al-richer range of composition of standard CuZnAl SMA where a minimum of transformation hysteresis is observed.
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All authors O. Goisot, H. vanLandeghem, R. Haettel, F. Robaut, O. Robach, L. Porcar, M. Verdier

Materials Science

Micro-crystal GaAs array sub-cells for Si tandem solar cells

James P. Connolly, Ahmed Nejim, Alexandre Jaffré et al. (+9 more)

This work reports optical and electronic numerical modelling of a novel emerging structure which is the GaAs nanocrystal on Si tandem solar cell by epitaxial lateral overgrowth, a technique which allows defect free material growth. The techniqueconsists of creating nucleation sites in a silicon surface SiO2 layer and initiating growth of nanoscalescale seeds, whereby strain energy remains below the Matthews-Blakeslee strain relaxation limit. This leads to AlxGaAs growth in micro-crystals without generation of material defects. The focus of this presentation is optical and electrical modelling of nanocrystals for applications in the very active field of silicon based multijunction solar cells, and design of a AlxGaAs/Si two terminal tandem, for compositions ranging from x=0 to x=30% in absorber layers. We present a model of the complete structure in two dimensions, consisting of a Al xGaAs high bandgap subcell connected with a tunnel junction to the low bandgap Si junction. The elaboration of models is described, with an emphasis on the AlxGaAs crystal featuring a non-planar pn-junction, and a focus on the optical properties of this lattice of micrometric AlGaAs crystals and in particular their light trapping properties from the resulting surface texture. The question of AlxGaAs surface coverage is addressed, given that neighbouring AlxGaAs crystals have different crystal orientations on a (111) Si surface, such that any coalescence of neighbour AlxGaAs crystals leads to crippling defects at their interface. The result is that some high energy incident light above the AlxGaAs bandgap is nevertheless transmitted directly to the Si cell, such that the resulting photogenerated carriers thermalise to the Silicon bandgap, and result in a loss of efficiency. The interface between AlxGaAs and Si subcells is addressed, with an emphasis on current transport efficiency through the nanoseeds and tunnelling currents through appropriately designed SiO2 buffer layers. This work therefore presents a theoretical framework for evaluating the potential of AlxGaAs nanocrystal growth on Si for light trapping, for GaAs silicon two terminal tandem cell performance including tunnel junctions, and provides models and design rules for efficient AlxGaAs microcrystal arrays as high bandgap subcells for tandem solar cells on silicon.
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All authors James P. Connolly, Ahmed Nejim, Alexandre Jaffré, J Alvarez, Kleider J.P., Denis Mencaraglia, Laurie Dentz, Geraldine Hallais, Frédéric Hamouda, Laetitia Vincent, Daniel Bouchier, Charles Renard

Materials Science

Dissociative Mechanism from NH3 and CH4 on Ni-Doped Graphene: Tuning Electronic and Optical Properties

A. Aligayev, U. Jabbarli, U. Samadova et al. (+3 more)

In this study, we employ a multi-scale computational modeling approach, combining density functional theory (DFT) and self-consistent charge density functional tight binding (SCC-DFTB), to investigate hydrogen (H2) production and dissociation mechanisms from ammonia (NH3) and methane (CH4) on pristine and nickel-doped graphene. These two-dimensional materials hold significant potential for applications in advanced gas sensing and catalysis. Our analysis reveals that Ni-doped graphene, validated through work function calculations, is a promising material for gas separation and hydrogen production. The samples with adsorbed molecules are characterized by calculating chemical potential, chemical hardness, electronegativity, electrophilicity, vibrational frequencies, adsorbtion and Gibbs energies by DFT calculations. Methane molecules preferentially adsorb at the hexagonal ring centers of graphene, while ammonia inter-acts more strongly with carbon atoms, highlighting distinct molecular doping mechanisms for CH4 and NH3. Dynamic simulations show that CH4 splits into CH3+H, with Ni-doped graphene facilitating enhanced hydrogen transmission, while NH3 dissociates into NH2+H, which may lead to N2H4 formation. Our non-equilibrium Green's function (NEGF) simulations demonstrate increased H-atom transmission on Ni-doped graphene during gas interactions. These findings suggest that Ni-doped graphene is superior to pristine graphene for applications in gas separation, hydrogen production, and high-sensitivity sensors.
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All authors A. Aligayev, U. Jabbarli, U. Samadova, F. J. Dominguez-Gutierrez, S. Papanikolaou, Qing Huang

Materials Science

Achieving Empirical Potential Efficiency with DFT Accuracy: A Neuroevolution Potential for the $\alpha$-Fe–C–H System

Fan-Shun Meng, Shuhei Shinzato, Zhiqiang Zhao et al. (+4 more)

A neuroevolution potential (NEP) for the ternary $\alpha$-Fe–C–H system was developed based on a database generated from spin-polarized density functional theory (DFT) calculations, achieving empirical potential efficiency with DFT accuracy. At the same power consumption, simulation speeds using NEP are comparable to, or even faster than, those with bond order potentials. The NEP achieves DFT-level accuracy across a wide range of scenarios commonly encountered in studies of $\alpha$-Fe- and $\alpha$-Fe–C under hydrogen environments. The NEP enables large-scale atomistic simulations with DFT-level accuracy at the cost of empirical potentials, offering a practical tool to study hydrogen embrittlement in steel.
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All authors Fan-Shun Meng, Shuhei Shinzato, Zhiqiang Zhao, Jun-Ping Du, Lei Gao, Zheyong Fan, Shigenobu Ogata

Materials Science

Deep Learning-Based Extraction of Promising Material Groups and Common Features from High-Dimensional Data: A Case of Optical Spectra of Inorganic Crystals

Akira Takahashi, Yu Kumagai, Arata Takamatsu et al. (+1 more)

We report an interpretation method for deep learning models that allows us to handle high-dimensional spectral data in materials science. The proposed method uses feature extraction and clustering analysis to categorize materials into classes based on similarities in both spectral data and chemical characteristics such as elemental composition and atomic arrangement. As a demonstration, we apply this method to an atomistic line graph neural network (ALIGNN) model trained on first-principles calculation data of 2,681 metal oxides, chalcogenides, and related compounds for optical absorption spectrum prediction. Our analysis reveals key elemental species and their coordination environments that influence optical absorption onset characteristics. The method proposed herein is broadly applicable to the classification and interpretation of diverse spectral data, extending beyond the optical absorption spectra of inorganic crystals.
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All authors Akira Takahashi, Yu Kumagai, Arata Takamatsu, Fumiyasu Oba

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

Zero resistance when metals mixed with insulators

Ya-Dong Gu, Ji-Hai Yuan and Zhi-An Ren

A false zero resistance behavior was observed during our study on the search of superconductivity in Ge-doped GaNb4Se8. This zero resistance was proved to be caused by open-circuit in multi-phase samples comprised of metals and insulators by measuring with four-probe method. The evidence strongly suggests that the reported superconductivity in hydrides should be carefully re-checked.
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Materials Science

Exploring transition pathways in the Landau-Brazovskii model

Zhiyi Zhang, Gang Cui, Kai Jiang et al. (+4 more)

The Landau-Brazovskii model provides a theoretical framework for describing various phases arising from competing short- and long-range interactions in many physical systems. In this work, we investigate phase transitions among various ordered phases within the three-dimensional Landau-Brazovskii model. We construct the phase diagram of this model, which encompasses eight distinct phases, and systematically compute the transition pathways connecting various metastable and stable states using the Landau-Brazovskii saddle dynamics. Along each transition pathway, the critical nucleus is identified with some detailed analyses of its shape, energy barrier, and Hessian eigenvalues. Furthermore, we explore how the transition state is influenced by model parameters, revealing systematic trends in critical nucleus sizes and energy barrier heights. Our results provide a comprehensive characterization of the nucleation mechanisms within the Landau-Brazovskii model and offer valuable insights into the structural transformations of modulated-phase systems.
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All authors Zhiyi Zhang, Gang Cui, Kai Jiang, An-Chang Shi, Pingwen Zhang, Jianyuan Yin, Lei Zhang

Sunday 19 October

Materials Science

Coherent terahertz control of metastable magnetization in FePS3

Batyr Ilyas, Tianchuang Luo, Honglie Ning et al. (+7 more)

The crystal lattice governs the emergent electronic, magnetic, and optical properties of quantum materials, making structural tuning through strain, pressure, or chemical substitution a key approach for discovering and controlling novel quantum phases. Beyond static modifications, driving specific lattice modes with ultrafast stimuli offers a dynamic route for tailoring material properties out of equilibrium. However, achieving dynamic coherent control of the nonequilibrium phases via resonant excitation of lattice coherences remains largely unexplored. Such manipulation enables non-volatile, on demand amplification and suppression of order parameters on femtosecond timescales, necessary for next generation optoelectronic ultrafast computation. In this study, we demonstrate coherent phononic control of a newly discovered, light-induced metastable magnetization in the van der Waals antiferromagnet FePS3. By using a sequence of terahertz (THz) pulses, we modulate the magnetization amplitude at the frequencies of phonon coherences, whose infrared-active nature and symmetries are further revealed by polarization- and field-strength-dependent measurements. Furthermore, our two-dimensional THz spectroscopy, in tandem with first-principles numerical simulations, shows that these phonons nonlinearly displace a Raman active phonon, which induces the metastable net magnetization. These findings not only clarify the microscopic mechanism underlying the metastable state in FePS3 but also establish vibrational coherences in solids as a powerful tool for ultrafast quantum phase control, enabling manipulation of material functionalities far from equilibrium.
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All authors Batyr Ilyas, Tianchuang Luo, Honglie Ning, Emil Vinas Bostrom, Alexander von Hoegen, Jaena Park, Junghyun Kim, Je-Geun Park, Angel Rubio, Nuh Gedik

Materials Science

A first-principles investigation of the diffusivities of oxygen and oxygen defects in ThO$_2$

Maniesha Singh and Anter El-Azab

A comprehensive analysis is presented for the diffusivity of oxygen defects and oxygen self-diffusion in ThO$-2$. The migration energy and diffusivity of oxygen defects with nominal charges have been investigated using density functional theory and phonon simulations. The pathway for the lowest migration energy barrier of oxygen vacancies was found to be along the $\langle 100 \rangle$ direction. Neutral and non-neutral oxygen interstitials exhibited direct (interstitial) and indirect (interstitialcy) migration, respectively. The vacancy migration barrier was found to be lowest for the highest charge, while for interstitials, it is lowest when the charge is lowest. The attempt frequencies of defects were calculated using the Eyring and Vineyard theories. These frequencies displayed a similar dependence on the defect charge as the activation barriers. The charge-averaged diffusivity of vacancies and interstitials were also computed. Across all temperatures, the average vacancy diffusivity was found to be greater than that of interstitial, indicating that oxygen vacancies are more mobile than interstitials. Oxygen self- and chemical diffusion coefficients were analyzed by combining the defect diffusivities with the concentrations computed using an equilibrium defect thermodynamics. The self-diffusion coefficient of oxygen was found to rise with temperatures and lower oxygen pressures. The contributions of various defects to self-diffusion of oxygen were subsequently examined. In the normal to high oxygen pressure range, at all temperatures, it is found that interstitials contribute most to oxygen diffusion in ThO2. At low oxygen pressures, vacancies with highest charge state were found to dominate oxygen diffusion. The chemical diffusion coefficient of oxygen was further computed, which was found to increase with temperature and decrease with hypo-stoichiometry in ThO2 to a plateau value.
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Materials Science

Intermediate-Band Formation in Tm3+-doped Ca2SnO4: A Wide-Gap Oxide Host for Visible-Light Absorption and Energy Applications

Shah Hussain, Sikander Azam, Umme Habiba et al. (+4 more)

Rare earth doping is an effective way to convert chemically stable oxides into multifunctional materials with coupled electronic, optical, and magnetic properties. We present first principles calculations of pristine and Tm3+ doped Ca2SnO4 to understand how localized 4f states change the structural, electronic, magnetic, and optical behavior of the host. Pristine Ca2SnO4 is a mechanically stable, wide band gap insulator with mostly ionic covalent bonding and diamagnetic character. Replacing Ca2+ with Tm3+ introduces several key changes: (i) localized Tm 4f states create intermediate levels inside the wide gap, reducing the optical band gap; (ii) exchange and spin orbit interactions generate strong local magnetic moments and spin asymmetry near the conduction band; (iii) electron localization function analysis shows enhanced covalency and electron pockets that stabilize luminescent centers; and (iv) the optical response shows visible range absorption, refractive index features, and low energy plasmon peaks while maintaining high energy dielectric stability. These effects make Tm doped Ca2SnO4 a mechanically robust, optically tunable, and magnetically active oxide phosphor suitable for red emission, intermediate band photovoltaics, and spin photon coupling. More broadly, our results show how targeted rare earth substitution can enable multifunctionality in wide gap stannates and guide the design of next generation spintronic photonic oxides.
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All authors Shah Hussain, Sikander Azam, Umme Habiba, Qaiser Rafiq, Amin Ur Rahman, Hamada H. Amer, Yasir Saeed

Materials Science

Three-Stage Synthesis of a Cobalt-Embedded Graphene-like Carbon Framework with Long-Range Atomic Order

G.G. Ryzhkova, I.Yu. Kurochkin, T.N. Rudneva et al. (+5 more)

We report a three-stage synthesis of a hybrid metal-carbon 2D material, in which cobalt atoms are covalently embedded in the graphene-like carbon (GLC) matrix. The resulting material (CoGLC) exhibits a distinctive XRD pattern indicative of the ordered arrangement of cobalt atoms in the layers. Furthermore, we demonstrate the fabrication of surfactant-free conductive inks from CoGLC via electrochemical exfoliation, making it a promising candidate for applications in in flexible electronics, spintronics or electrocatalysis.
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All authors G.G. Ryzhkova, I.Yu. Kurochkin, T.N. Rudneva, A.V. Zotov, A.V. Moiseenko, G.M. Zirnik, D.A. Vinnik, V.I. Korepanov

Materials Science

Fractional Quantum Multiferroics from Coupling of Fractional Quantum Ferroelectricity and Altermagnetism

M. Q. Dong, B. Liu, Z. H. Dai et al. (+3 more)

Multiferroics, which combine ferroelectric and magnetic order, offer a transformative platform for next-generation electronic devices. However, the intrinsic competition between the mechanisms driving ferroelectricity and magnetism in single-phase materials severely limits their performance, typically resulting in weak magnetoelectric coupling at room temperature. Here, we propose a solution to this long-standing challenge through the novel concept of fractional quantum multiferroics (FQMF), where strong magnetoelectric coupling is naturally realized by coupling fractional quantum ferroelectricity (FQFE) with altermagnetism (AM). Symmetry analysis shows that reversing the FQFE polarization necessarily inverts the AM spin splitting under parity-time ($\mathcal{PT}$) or time-reversal ($\mathcal{T}\tau$) operations. A minimal tight-binding model reproduces this effect, demonstrating electrically driven spin control without rotating the Néel vector. First-principles calculations further identify a broad family of candidate materials in two and three dimensions including bulk MnTe, Cr$_2$S$_3$, Mn$_4$Bi$_3$NO$_{15}$ and two-dimensional AB$_2$ bilayers such as MnX$_2$ (X=Cl, Br, I), CoCl$_2$, CoBr$_2$, and FeI$_2$. Notably, MnTe exhibits a high Néel temperature ($\sim$300 K) and a large electrically switchable spin splitting ($\sim$0.8 eV), demonstrating room-temperature magnetoelectric performance that surpasses that of conventional multiferroics. To further showcase the technological potential, we propose an electric-field-controlled FQMF tunnel junction based on MnTe that achieves tunneling magnetoresistance exceeding 300%. This work establishes FQMF as a distinct and promising route to achieving room-temperature strong magnetoelectric coupling, opening a new avenue for voltage-controlled spintronics.
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All authors M. Q. Dong, B. Liu, Z. H. Dai, Zhi-Xin Guo, Hongjun Xiang, Xin-Gao Gong

Materials Science

Efficient small-cell sampling for machine-learning potentials of multi-principal element alloys

Yan Liu, Jiantao Wang, Hongkun Deng et al. (+3 more)

Multi-principal element alloys (MPEAs) exhibit exceptional properties but face significant challenges in developing accurate machine-learning potentials (MLPs) due to their vast compositional and configurational complexity. Here, we introduce an efficient small-cell sampling (SCS) method, which allows for generating diverse and representative training datasets for MPEAs using only small-cell structures with just one and two elements, thereby bypassing the computational overhead of iterative active learning cycles and large-cell density functional theory calculations. The efficacy of the method is carefully validated through principal component analysis, extrapolation grades evaluation, and root-mean-square errors and physical properties assessment on the TiZrHfCuNi system. Further demonstrations on TiZrVMo, CoCrFeMnNi, and AlTiZrNbHfTa systems accurately reproduce complex phenomena including phase transitions, chemical orderings, and thermodynamic properties. This work establishes an efficient one-shot protocol for constructing high-quality training datasets across multiple elements, laying a solid foundation for developing universal MLPs for MPEAs.
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All authors Yan Liu, Jiantao Wang, Hongkun Deng, Yan Sun, Xing-Qiu Chen, Peitao Liu

Saturday 18 October

Materials Science

Semiconducting nanotubes derived from a rectangular graphyne: a DFT study

Wjefferson Henrique da Silva Brandão, Anderson Gomes Vieira, Jonathan da Rocha Martins et al. (+3 more)

Proposing new ways to organize carbon in 2D nanomaterials has been a relevant strategy in the search for systems with targeted properties for different applications. One focus is the study of fully sp$^2$ non-graphitic networks, with successfully synthesized examples. Hybrid sp-sp$^2$ systems of the graphyne family are a related approach, and many systems have the honeycomb lattice as a base model. However, other examples have been inspired by other lattices as the recently proposed r$\gamma$GY sheet, which features a semiconducting behavior with highly localized quasi-1D states. Here, we investigate how to tune r$\gamma$GY properties by folding this sheet into nanotube forms. We elucidate mechanisms that determine their electronic structure by means of density functional theory calculations, as well as we identify the interplay involving chirality, diameter, and the emergence of dispersive/localized frontier states on gap modulation through simple extrapolated methods.
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All authors Wjefferson Henrique da Silva Brandão, Anderson Gomes Vieira, Jonathan da Rocha Martins, Andrea Latgé, Marcelo Lopes Pereira Junior, Eduardo Costa Girão

Materials Science

Structure and stability of 7:3 rare earth oxide-phosphates: a combined ab initio and experimental study

Ligen Wang, Konrad Burkmann, Sergey V. Ushakov et al. (+9 more)

Rare earth oxide-phosphates (REOPs) form a largely unexplored family of refractory lanthanides and yttrium compounds with general formula RExOy(PO4)z. They are of interest for applications ranging from thermal barrier coatings to catalysts and magnetic materials. At least four REOPs phases were experimentally identified with RE/P ratios from 7:3 to 6:1, however the structures were solved only for 3:1 phases (RE3O3(PO4)). In this work we report the structure for the 7:3 phases (RE7O6(PO4)3) derived by ab initio analysis of models based on previously reported oxide-vanadate analogues. The most stable structures for all 7:3 REOPs were found to be isotypic, adopting monoclinic symmetry with space group P21/c. The structures were validated by comparison of their powder X-ray diffraction patterns to those of synthesized La, Pr, Nd, Sm, Eu, Gd and Tb 7:3 phases (Rietveld refinement for all except Tb). Ab initio analysis of thermodynamic stability showed that all 7:3 REOPs are unstable at 0 K toward decomposition to REPO4 and RE3PO7 or RE2O3. The entropy contribution stabilizes RE7O6(PO4)3 phases for light rare earth elements above 1000 K, however, starting with Dy, computationally predicted stabilization temperature is higher than estimated melting points of RE7O6(PO4)3, which is consistent with observed synthesis pattern.
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All authors Ligen Wang, Konrad Burkmann, Sergey V. Ushakov, Edric X. Wang, Jared Matteucci, Mara Scheuermann, Erik Melnitschuk, Robert Glaum, Hongwu Xu, Elizabeth J. Opila, Alexandra Navrotsky, Qi-Jun Hong

Materials Science

Understanding the Structural Origin of Chirality in Magic-Size Semiconductor Nanoclusters through Self-Assembly Simulations

Hongjin Du, Ellery J. Hendrix, Richard D. Robinson et al. (+1 more)

Semiconductor magic-size clusters (MSCs) are atomically precise nanoparticles exhibiting unique size-dependent properties, but their ultrasmall dimensions hinder structural characterization, limiting our understanding of their formation and stability. A few MSC structures have been fully resolved, revealing either bulk-like zincblende-type structures or a range of non-bulk-like motifs. Here we use a computational model to investigate the relationship between cluster size and atomic structure in zincblende-forming II-VI and III-V semiconductors. Firstly, we find that all non-bulk-like MSCs in these systems exhibit the same distorted icosahedral motif that is intrinsically chiral. Secondly, we reproduce these MSC geometries in small-cluster self-assembly simulations and discover that their chirality emerges from the geometric frustration and symmetry breaking in arranging tetrahedral bonding environments into an icosahedral topology. Overall, this work reproduces experimentally reported motifs without system-specific parameterization, establishes the structural origin of chirality in MSCs, and provides design principles for predicting new cluster geometries.
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All authors Hongjin Du, Ellery J. Hendrix, Richard D. Robinson, Julia Dshemuchadse

Materials Science

Graph Neural Network for Unified Electronic and Interatomic Potentials: Strain-tunable Electronic Structures in 2D Materials

Moon-ki Choi, Daniel Palmer and Harley T. Johnson

We introduce UEIPNet, an equivariant graph neural network designed to predict both interatomic potentials and tight-binding (TB) Hamiltonians for an atomic structure. The UEIPNet is trained using density functional theory calculations followed by Wannier projection to predict energies and forces as node-level targets and Wannier-projected TB matrices as edge-level targets. This enables physically consistent modeling of coupled mechanical electronic responses with near-DFT accuracy. Trained on bilayer graphene and monolayer MoS2 DFT data, UEIPNet captures key deformation-electronic effects: in twisted bilayer graphene, it reveals how interlayer spacing, in-plane strain, and out-of-plane corrugation drive isolated flat-band formation, and further shows that modulating substrate interaction strength can generate flat bands even away from the magic angle. For monolayer MoS2, the UEIPNet accurately reproduces phonon dispersions, strain-dependent band-gap evolution, and local density of states modulations under non-uniform strain. The UEIPNet offers a generalized, efficient, and scalable framework for studying deformation-electronic coupling in large-scale atomistic systems, bridging classical atomistic simulations and electronic-structure calculations.
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Materials Science

Femtosecond photo-induced displacive phase transition in Sb$_{2}$Te (group 2) phase-change material

Zhipeng Huang, Xinxin Cheng, Hazem Daoud et al. (+3 more)

Two classes of Phase Change Materials (PCMs) have emerged as the best candidates for applications requiring the fast reading and writing of data: GeTe-Sb$_{2}$Te$_{3}$ pseudobinary alloys (group 1) and doped Sb-Te compounds near the eutectic composition Sb$_{70}$Te$_{30}$ (group 2). Both material classes undergo reversible switching between a low-resistance opaque crystalline phase and a high-resistance but less absorbing amorphous phase through heating, electrical, or optical pulses, achieving (sub-)nanosecond switching speeds. While group 1 compounds are employed in current generation devices and relatively well studied, model systems in group 2 compounds have been found to crystallize more rapidly and thus offer the perspective of improved devices. Despite their superior crystallization speed (SET process), to this point there have been no ultrafast experimental studies on crystallized PCMs of group 2 for the RESET process. Here we perform ultrafast electron diffraction and femtosecond resolved sum frequency non-linear spectroscopy on Peierls distorted Sb$_{2}$Te crystallized thin films (PCM of group 2) following femtosecond optical pulse irradiation. We observe a pump-induced structural change on two distinct timescales: responses with characteristic timescales of $\approx$ 300 fs and 2~ps. We quantified the experimental result by a coherent displacement and the Debye-Waller effect. In particular, the $\approx$ 300 fs UED signal results from the ultrafast release of the Peierls distortion through non-thermal coherent Sb displacement, while the 2~ps response reflects electron-lattice equilibrium. These results reveal the ultrafast non-thermal structural dynamics of Sb$_{2}$Te and suggest energy-efficient switching of group 2 PCMs should be possible on femtosecond time scales.
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All authors Zhipeng Huang, Xinxin Cheng, Hazem Daoud, Wen-Xiong Song, R. J. Dwayne Miller, R. Kramer Campen

Materials Science

Growth, discovery and characterization of single crystalline Eu$_{0.8}$Pt$_6$Al$_{16.4}$

Juan Schmidt, Oliver Janka, Jutta Kösters et al. (+2 more)

We report the discovery of a ternary compound, Eu$_{0.8}$Pt$_6$Al$_{16.4}$. We determine its chemical and structural characteristics based on energy-dispersive X-ray spectroscopy as well as both powder and single-crystal X-ray diffraction, demonstrating that it crystallizes in a hexagonal structure type EuPt$_6$Al$_{17}$ with no reported structural analog. The electronic and magnetic properties are characterized by temperature- and field-dependent magnetization, and temperature-dependent resistance measurements, revealing that the Eu$^{2+}$ magnetic moments order antiferromagnetically below 2.8 K.
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All authors Juan Schmidt, Oliver Janka, Jutta Kösters, Sergey L. Bud'ko, Paul C. Canfield

Materials Science

Surface Reactivity in Low Temperature Deposited Amorphous/Crystalline SnO2 Thin Films: Chemisorbed Oxygen Activity and CO Oxidation Pathways Revealed by In Situ XPS and Mass Spectrometry

Engin Ciftyurek, Zheshen Li and Klaus Schierbaum

This study investigates two critical aspects of the gas sensing mechanism in metal oxide sensors: (1) the conditions that maximize chemisorbed oxygen concentration as a function of temperature and oxygen partial pressure, and (2) which surface oxygen species (chemisorbed or lattice-bound) are primarily responsible for interaction with carbon monoxide (CO). SnO2 thin films, deposited at temperatures as low as 60 C and exhibiting mixed amorphous-crystalline phases with open, tortuous porosity, were evaluated for CO sensing at 200 C. Comprehensive characterization using EIS, MS, XPS, TEM, and sensor tests revealed a strong correlation between high sensing performance and the structural/electronic features of the defect rich low-temperature-deposited SnO2. Electrochemical impedance spectroscopy (EIS) was employed to identify the optimal sensing temperature. Mass spectroscopy (MS) used to analyze the exhaust gases after sensing reactions. The films exhibited oxygen under-stoichiometry and high concentrations of chemisorbed oxygen species. In situ XPS under 1 mbar (10000 ppm) O2 and CO exposures showed that chemisorbed oxygen, not lattice oxygen, was actively involved in CO oxidation, as further confirmed by CO2 detection via Mass spectroscopy (MS). Quantitative analysis revealed dynamic surface chemical status alternations, emphasizing the pivotal role of chemisorbed oxygen in the sensing mechanism at 200 C.
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Materials Science

Vacancy-concentration-dependent thermal stability of fcc-(Ti,Al)Nx predicted via chemical-environment-sensitive diffusion activation energies

Ganesh Kumar Nayak, David Holec and Jochen M. Schneider

Thermal decomposition of metastable fcc-(Ti,Al)Nx limits the lifetime of coated components. While energetic decomposition aspects can be modelled reliably, the inherent variability of chemical environment-dependent diffusion activation energies remains systematically unexplored. Here, we predict an activation energy range (envelope) for mass transport in varying chemical environments, reflecting the vacancy concentration range fcc-(Ti0.5Al0.5)1-xNx with x = 0.47, 0.5, 0.53. The stoichiometric compound shows maximum thermal stability, consistent with experimental data. Metal vacancies decrease the average migration energy, while metal and nitrogen vacancies reduce barriers via lattice strain relaxation, enhancing mobility. The strong chemical environment dependence challenges conclusions from single-point activation energy data.
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