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Physics Condensed Matter Materials Science

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Thursday 4 December

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

Demonstration of surface-engineered oxidation-resistant Nb-Nb thermocompression bonding toward scalable superconducting quantum computing architectures

Harsh Mishra, Yusuke Kozuka, Sathish Bonam et al. (+4 more)

Scalable quantum computing currently requires a large array of qubit integration, but present two-dimensional interconnects face challenges such as wiring congestion, electromagnetic interference, and limited cryogenic space. To overcome this challenge, implementing three-dimensional (3D) vertical architectures becomes crucial. Niobium (Nb), due to its excellent superconducting characteristics and strong fabrication process compatibility, stands out as a prime material choice. The main challenge in Nb-Nb bonding is the presence of an oxide layer at the interface, even after post-bonding annealing across various bonding methods. The native Nb oxide forms rapidly in air, creating a resistive barrier to supercurrent flow and introducing two-level system losses that degrade qubit coherence while increasing the overall thermal budget. These issues show the need for effective surface engineering to suppress oxidation during bonding. This study introduces an ultrathin gold (Au) capping layer as a passivation strategy to prevent oxygen incorporation at the Nb surface. This approach enables low-temperature Nb-Nb thermocompression bonding at 350 {\deg}C under a reduced bonding pressure of 0.495 MPa. Detailed microstructural and interfacial analyses confirm that Au passivation effectively suppresses oxide formation and hence enhances bonding uniformity and strength with keeping the superconductivity, establishing a robust route toward low-temperature, low-pressure Nb-Nb bonding for scalable 3D superconducting quantum computing architectures.
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All authors Harsh Mishra, Yusuke Kozuka, Sathish Bonam, Jun Uzuhashi, Praveenkumar Suggisetti, Tadakatsu Ohkubo, Shiv Govind Singh

Materials Science

Boosting the Memory Window of Memristive Stacks via Engineered Interfaces with High Ionic Mobility

José Diogo Costa, Daniel Veira-Canle, Noa Varela-Domínguez et al. (+8 more)

Realizing the potential of oxide-based memristive devices for high-density data storage and energy-efficient computing still relies on overcoming key technical challenges, including the need for a larger number of stable resistance states, faster switching speeds, lower SET/RESET voltages, improved endurance, and reduced variability. Addressing these limitations requires innovative material design strategies. Here, we demonstrate that introducing a thin layer of oxide-ion conductor SrCoO3-x between the metal and the SrTiO3-based memristive elements expands the number of distinguishable resistance states from about 8 to about 22. This modification also reduced the SET/RESET voltage by 50% and markedly improved device endurance, albeit with a trade-off of reduced state retention. To assess the performance of this architecture, we trained a two-layer fully connected neural network using the experimental SrTiO3/SrCoO3-x memristor characteristics on the MNIST handwritten digit dataset. Networks with hidden-layer sizes between 64 and 256 neurons achieved classification errors below 7%. Finally, we confirmed the transferability of this interface-engineering approach by applying it to HfOx-based devices, achieving a consistent enhancement in the resistive state window.
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All authors José Diogo Costa, Daniel Veira-Canle, Noa Varela-Domínguez, Nicholas Davey, Victor Leborán, Rafael Ramos, Fèlix Casanova, Luis E. Hueso, Victor M. Brea, P. López, Francisco Rivadulla

Materials Science · Cross-listed from physics.optics

Exceptionally high Verdet constant in gold nanodisc arrays

Gajendra Mulay, Shraddha Choudhary, Ashwin A Tulapurkar et al. (+4 more)

Magneto-optical effects in non-magnetic noble metals can be greatly enhanced by leveraging the in-plane Lorentz force at engineered plasmonic resonances. We demonstrate a 2D array of gold nanodiscs designed to host a hybrid resonance of localized plasmon and surface lattice modes. The structure exhibits a Verdet constant of 1.98e6 deg/T.m, corresponding to a Faraday rotation of -0.15 deg at a 1 T magnetic field. This Verdet constant represents a 15-fold enhancement over unpatterned gold and is highly competitive with many plasmon-enhanced diamagnetic nanostructures. These findings offer new opportunities for harnessing strong magneto-plasmonic effects in optoelectronic devices by patterning common non-magnetic metals.
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All authors Gajendra Mulay, Shraddha Choudhary, Ashwin A Tulapurkar, Daria O Ignatyeva, Vladimir I Belotelov, Shriganesh S Prabhu, Venu Gopal Achanta

Materials Science

Ab initio study of Coulomb drag driven electron-hole bifluidity in doped graphene

Dwaipayan Paul, Elena Trukhan and Nakib H. Protik

Motivated by the notion that a preponderance of Coulomb interactions might lead to hydrodynamics, we carry out an ab initio calculation of the charge carrier transport properties of the electron-hole plasma of doped graphene. We include both the phonon and Coulomb interactions within a momentum and band resolved Boltzmann transport formalism. We find that, under suitable conditions, the strong Coulomb drag effect induces effects like negative conductivity and joint electron-hole hydrodynamics (bifluidity) in the plasma. We also identify the exclusive electron or hole hydrodynamics. We find that there is a strong violation of the Wiedemann-Franz law in the low doped regimes. Our work elucidates the roles of the microscopic scattering mechanisms that drive these hydrodynamic phenomena.
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Materials Science · Cross-listed from cond-mat.str-el

Magnetocaloric effect measurements in ultrahigh magnetic fields up to 120 T

Reon Ogawa, Masaki Gen, Kazuyuki Matsuhira et al. (+1 more)

We report proof-of-concept measurements of the magnetocaloric effect (MCE) in ultrahigh magnetic fields up to 120 T for the classical spin-ice compound Ho$_{2}$Ti$_{2}$O$_{7}$. Radio-frequency resistivity measurements using an Au$_{16}$Ge$_{84}$ thin-film thermometer enable us to detect a rapid change in the sample temperature associated with a crystal-field level crossing in the high-field region in addition to a giant MCE at low fields. We discuss a possible delay in the temperature response and outline prospects for more precise MCE measurements in destructive pulsed fields.
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All authors Reon Ogawa, Masaki Gen, Kazuyuki Matsuhira, Yoshimitsu Kohama

Materials Science · Cross-listed from math.AP

Irreversibility condition and stability of equilibria in the inverse-deformation approach to fracture

Arnav Gupta

We derive an irreversibility condition for fracture in the inverse-deformation approach using the second law of thermodynamics. We consider the problem of brittle failure in one-dimensional, homogeneous, elastic bars undergoing quasistatic motion. Despite the presence of a non-zero interfacial/surface energy, the third derivative of the inverse deformation is discontinuous at crack faces. This is due to the inequality constraint allowing fracture while ensuring that the orientation of matter is preserved. We show that any change in the location of open cracks in their reference configuration result in negative entropy production. We disallow these changes according to the second law of thermodynamics, fixing the location of cracks in their reference domain. The inequality constraint and the irreversibility condition limit the space of admissible variations. We prove second-order conditions for local stability incorporating these restrictions. Their numerical implementation shows that all broken equilibria of the elastic bar under hard loading are locally stable.
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Materials Science

General spin models from noncollinear spin density functional theory and spin-cluster expansion

Tomonori Tanaka and Yoshihiro Gohda

We present a data-efficient framework for constructing general classical spin Hamiltonians by combining the spin-cluster expansion (SCE) with fully self-consistent noncollinear spin density functional theory (DFT). The key idea is to fit the SCE model to magnetic torques rather than to total energies. Because torques are site-resolved vectors, each spin configuration provides many informative regression targets, improving conditioning and substantially reducing the number of required DFT calculations, especially for large supercells. Applied to the B20-type chiral magnets ${\rm Mn}_{1-x}{\rm Fe}_{x}{\rm Ge}$ and ${\rm Fe}_{1-y}{\rm Co}_{y}{\rm Ge}$, the resulting SCE models determine full pairwise exchange tensors – including isotropic exchange, symmetric anisotropic exchange, and the Dzyaloshinskii–Moriya interaction – and predict the helical spin period via a micromagnetic mapping. The composition trends and the divergence of the period at the chirality sign-change point are well reproduced, in agreement with experiment. Moreover, the systematic nature of SCE enables controlled assessment of interaction order: as the training spin configurations become more disordered, the lowest-order model loses torque accuracy, whereas including higher-order interactions restores predictive power. These advances enable near-DFT-accurate spin models for finite-temperature magnetism and complex spin textures at modest computational cost, providing an extensible route to quantitative first-principles parameterization and predictive materials design. An open-source implementation is available as a Julia package, Magesty.jl.
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Materials Science · Cross-listed from physics.comp-ph

GPU-Portable Real-Space Density Functional Theory Implementation on Unified-Memory Architectures

Atsushi M. Ito

We present a GPU-portable implementation of a real-space density functional theory (DFT) code “QUMASUN” and benchmark it on the new Plasma Simulator featuring Intel Xeon 6980P CPUs, and AMD MI300A GPUs. Additional tests were performed on an NVIDIA GH200 GPU. In particular MI300A supports unified memory and GH200 supports coherent memory interconnect, simplifying GPU porting. A lightweight C++ lambda-based layer enables CPU, CUDA, and HIP execution without OpenMP/OpenACC preprocessor directives. For diamond (216 atoms) and tungsten (128 atoms) systems, MI300A and GH200 achieve 2.0-2.8 $\times$ and 2.3-2.4 $\times$ speedups over a 256-core Xeon node. The compute-bound kernels, which are fast Fourier transforms (FFT), dense matrix-matrix multiplications (GEMM) and eigenvalue solver, show substantial acceleration on both GPUs, indicating that the present GPU-portable approach can benefit a wide range of plasma-fusion simulation codes beyond DFT.
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Materials Science

Preparation and magnetic properties of (Ln0.2La0.2Nd0.2Sm0.2 Eu0.2)MnO3 (Ln = Dy, Ho, Er) high-entropy perovskite ceramics containing heavy rare earth elements

Jiedong Qin, Xingmin Feng, Zhiqin Wen et al. (+3 more)

Equimolar ratio high-entropy perovskite ceramics (HEPCs) have attracted much attention due to their excellent magnetization intensity. To further enhance their magnetization intensities, (Ln0.2La0.2Nd0.2Sm0.2Eu0.2)MnO3 (Ln = Dy, Ho and Er, labeled as Ln-LNSEMO) HEPCs are designed based on the configuration entropy Sconfig, tolerance factor t, and mismatch degree. Single-phase HEPCs are synthesized by the solid-phase method in this work, in which the effects of the heavy rare-earth elements Dy, Ho and Er on the structure and magnetic properties of Ln-LNSEMO are systematically studied. The results show that all Ln-LNSEMO HEPCs exhibit high crystallinity and maintain excellent structural stability after sintering at 1250 degree centigrade for 16 h. Ln-LNSEMO HEPCs exhibit significant lattice distortion effects, with smooth surface morphology, clearly distinguishable grain boundaries, and irregular polygonal shapes. The three high-entropy ceramic samples exhibit hysteresis behavior at T = 5 K, with the Curie temperature TC decreasing as the radius of the introduced rare-earth ions decreases, while the saturation magnetization and coercivity increase accordingly. When the average ionic radius of A-site decreases, the interaction between their valence electrons and local electrons in the crystal increases, thereby enhancing the conversion of electrons to oriented magnetic moments under an external magnetic field. Thus, Er-LNSEMO HEPC shows a higher saturation magnetization strength (42.8 emu/g) and coercivity (2.09 kOe) than the other samples, which is attributed to the strong magnetic crystal anisotropy, larger lattice distortion (0.00652), smaller average grain size (440.49 plus or minus 22.02 nm), unit cell volume (229.432 A3) and A-site average ion radius (1.24 A) of its magnet. The Er-LNSEMO HEPC has potential applications in magnetic recording materials.
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All authors Jiedong Qin, Xingmin Feng, Zhiqin Wen, Li Tang, Defeng Long, Yuhong Zhao

Materials Science

Evidence of a two-dimensional nitrogen crystalline structure on silver surfaces

Xuegao Hu, Haijun Cao, Zhicheng Gao et al. (+11 more)

Nitrogen, the most abundant element in Earth's atmosphere, exists as a diatomic gas under standard temperature and pressure. In the two-dimensional (2D) limit, atomically thin nitrogen, termed nitrogene, has been theoretically predicted to form crystalline materials with various polymorphic configurations, exhibiting diverse chemical and physical properties. However, the synthesis of nitrogene has remained elusive due to the strong nitrogen-nitrogen triple bonds. Here, we report experimental evidence of the formation of nitrogen-based crystalline structures compatible with nitrogene on silver surfaces via ion-beam-assisted epitaxy. Through a combination of scanning tunneling microscopy, angle-resolved photoemission spectroscopy, and first-principles calculations, we demonstrate that the nitrogene-like structure adopts a puckered honeycomb lattice. Notably, our calculations predict a nitrogene band gap of up to 7.5 eV, positioning it as a promising candidate for ultraviolet optoelectronic devices and high-k dielectric applications.
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All authors Xuegao Hu, Haijun Cao, Zhicheng Gao, Hui Zhou, Daiyu Geng, Dong Li, Jisong Gao, Qiaoxiao Zhao, Zhihao Cai, Peng Cheng, Lan Chen, Sheng Meng, Kehui Wu, Baojie Feng

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

In-plane anomalous features in the 3D quantum Hall regime

Ming Lu and Xiao-Xiao Zhang

Studies of the 3D quantum Hall effect (QHE) have primarily emphasized transport features that mimic the well-established 2D QHE. In this work, we show that qualitatively new features arise when an in-plane magnetic field is applied to a 3D Weyl semimetal in the quantum Hall regime. An unexpected Hall quantum oscillation, distinct from the Weyl-orbit oscillation, coexists with the QHE, along with an unquantized two-terminal magnetoresistance. Moreover, unconventional antichiral transmission enables a peculiar disorder-robust negative longitudinal resistance. Quantization tunable by the lead configuration is further found in this transport geometry. A unique type of nonlocal quantum backscattering channels underlies these phenomena. Our work demonstrates a breakdown of the topological characterization of transport even with 3D Chern numbers and reveals hidden 3D QHE transport properties. It opens a new class of transport measurements and phenomena.
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Wednesday 3 December

Materials Science

On the relation between microstructure and impact toughness of 17-4__PH stainless steel produced by powder bed fusion laser beam (PBF-LB)

Renata de Oliveira Melo, Jean-Bernard Vogt, Eric Nivet et al. (+6 more)

This work investigated the effects of aging heat treatments on the microstructure and, consequently, the quasi-static (tensile properties) and dynamic (impact toughness) mechanical behaviour of a 17-4 PH stainless steel produced by powder bed fusion laser beam (PBF-LB). Multiscale microstructural characterization, using X-ray diffraction, scanning and transmission electron microscopy and electron backscatter diffraction, was conducted to establish quantitative correlations between microstructural evolution and mechanical performance, providing insight into the mechanisms governing plasticity and fracture. The PBF-LB specimens exhibited tensile strengths comparable to or exceeding those of conventionally manufactured counterparts but consistently showed significantly lower impact toughness, regardless of heat treatment conditions. Within the complex microstructure, strain-induced transformation of reversed austenite was found to enhance ductility and impact toughness. SiO2 inclusions, originating from the starting powder, were identified as nucleation sites for micro-cavities and proved detrimental to impact toughness. Meanwhile, the distribution of Cu-rich nanoprecipitates could be tailored to favour either tensile strength through Orowan strengthening or impact toughness by enhancing local plasticity, but not both simultaneously. This work highlights the pronounced strength-toughness trade-off inherent in PBF-LB-produced 17-4 PH alloys and reveals the interplay between strength, ductility, and toughness. These findings underscore the need for further research into dynamic loading mechanical properties, especially for demanding applications such as those in the nuclear sector.
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All authors Renata de Oliveira Melo, Jean-Bernard Vogt, Eric Nivet, Flore Villaret, Christophe Grosjean, Eric Baustert, Nhu-Cuong Tran, Jeremie Bouquerel, Gang Ji

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

The effects of H blistering and ELMs on the thermal fatigue cracking of W by strikepoint sweeping

J. Hargreaves, J. Vernimmen, J. Scholten et al. (+1 more)

Cyclic thermal loads imposed on a W divertor by strikepoint sweeping may induce low-cycle thermal fatigue cracking of its plasma-facing surfaces. This cracking may be accelerated by plasma-material interactions such as H implantation, blistering, fuzz and void formation. Fatigue cracking may also synergise with ELM cracking. To explore these novel forms of environmentally assisted fatigue, FEA modelling was used to design a uniaxial fatigue experiment for Magnum-PSI that represents strikepoint sweeping at 1 Hz across a 100 mm span of a divertor target. Magnum-PSI was used to combine cyclic thermal loading of W with H implantation and two forms of ELM like pre-cracking. Quantitative SEM analysis of fatigue-cracked W revealed that H implantation significantly delayed crack initiation, with preimplanted targets requiring 450-600 cycles before failure compared to 150 cycles for non-implanted samples. This was attributed to hydrogen-induced dislocation pinning, which produces a case-hardening effect that inhibits persistent slip band formation. ELM-like pre-cracking combined with strikepoint sweeping was found to give rise to localised melting and the formation of 30 micromete diameter droplets, caused by thermal isolation of W regions by fatigue cracks. The implications for the fatigue lifetime of tokamak divertors are also discussed.
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All authors J. Hargreaves, J. Vernimmen, J. Scholten, T.W Morgan

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

An Orbifold Framework for Classifying Layer Groups with an Application to Knitted Fabrics

Sonia Mahmoudi, Elizabeth J. Dresselhaus and Michael S. Dimitriyev

Entangled structures such as textiles and architected materials are often doubly periodic. Due to this property and their finite transverse thickness, the symmetries of these materials are described by the crystallographic layer groups. While orbifold notation provides a compact topological description and classification of the planar wallpaper groups, no analogous framework has been available for the spatial layer groups. In this article we develop an orbifold theory in three dimensions and introduce a complete set of Conway-type symbols for all layer groups. To illustrate its applicability, we analyze several knitted fabric motifs and show how their layer-group symmetries are naturally expressed in this new orbifold notation. This work establishes a foundation for the topological classification of doubly periodic structures beyond the planar setting.
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Materials Science

Local expression of fractional corner charges in obstructed atomic insulators and relationship with the fractional disclination charges

Hidetoshi Wada and Shuichi Murakami

In obstructed atomic insulators, fractionally quantized charges appear at the corners of the crystals in the shapes of vertex-transitive polyhedra, and are given by the filling anomaly divided by the number of corners. Recent studies reveal that the filling anomaly for the cases with genus $0$ is universally given by the total charge at the Wyckoff position $1a$. In this study, we rewrite the formula in terms of the degree of sharpness of the corner, and show that the corner charge formula also holds for cases with arbitrary genus. We also extend our formula to vertex-transitive shell polyhedra, which are closed or open polyhedra without the bulk region, with all the vertices related by symmetry. Then, we show that the corner charges of such shell polyhedra are equal to the two-dimensional disclination charges of the corresponding disclinations. By identifying it with the disclination charge under the Wen-Zee action, we show that the coupling constant of the Wen-Zee action for a crystalline insulator is given by the total charge at the Wyckoff position at the disclination core.
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Materials Science

Electrical Conductivity of Copper-Graphene (Cu-Gr) Composites: The Underlying Mechanisms of Ultrahigh Conductivity

Jiali Yao, Uschuas Dipta Das, Hamid Safari et al. (+4 more)

Copper-graphene composite (CGC) conductors are widely considered as a potential alternative to pure copper (Cu). Yet, the effect of graphene (Gr) on the electrical conductivity of CGCs remains elusive, and their electrical performance is still controversial. This work addresses these unresolved questions by unambiguously quantifying how the electrical properties of CGCs depend on the characteristics of Gr and Cu. Gr is synthesized on Cu foils, foams, and wires by utilizing a wide range of chemical vapor deposition conditions to independently control their characteristics. Then the Gr-enhanced electrical conductivity ({\Delta}{\sigma}) is characterized for CGCs with different Cu geometries and Gr qualities. This study confirms that unprecedented electrical conductivity ({\Delta}{\sigma} = 17.1%) can be achieved only when both Gr and Cu are carefully optimized. Specifically, the study reveals three key factors: (1) {\Delta}{\sigma} is positively correlated with continuity of Gr; (2) CGCs with a continuous monolayer Gr exhibit a strong {\Delta}{\sigma}-A_s linear relation where A_s is the specific surface area of a CGC; and (3) {\Delta}{\sigma} becomes more pronounced when a Cu matrix has a curved cross-section. This work reveals the fundamental mechanisms of how Gr influences the overall electrical conductivity of CGCs and, therefore, is a crucial step toward designing and manufacturing high-performance CGC conductors for emerging applications.
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All authors Jiali Yao, Uschuas Dipta Das, Hamid Safari, Md Ashiqur Rahman Laskar, Junghoon Yeom, Umberto Celano, Wonmo Kang

Materials Science

More is different: How chemical complexity influences stability in high entropy oxides

Ksenia Khoroshun, Mario U. González-Rivas and Alannah M. Hallas

Tailoring the chemical composition of a high entropy oxide (HEO) is a powerful approach to enhancing desirable material properties. However, the targeted synthesis of HEO materials is often hindered by competing stabilizing and destabilizing factors, which are difficult to predict. This work examines the effects of increased configurational entropy on the phase formation and stability of four notable complex oxide families: perovskite ($AB$O$_3$), pyrochlore ($A_2B_2$O$_7$), Ruddlesden-Popper ($A_2B$O$_4$), and zirconium tungstate ($AB_2$O$_8$). Each of these structures has a tetravalent cation site, which we attempt to substitute with an entropic mixture of four cations, benchmarked by the parallel synthesis of a non-disordered reference compound. While all four target high entropy materials can be expected to form based on ionic radii criteria, only the high entropy perovskite Ba(Ti,Zr,Hf,Sn)O$_3$ is successfully synthesized. In the case of the pyrochlore, an entropy-stabilized defect fluorite is formed instead, while the Ruddlesden-Popper phase co-exists with multiple competing phases. For the tungstate, an unexpected deep eutectic point between the precursors results in melting that precedes the formation of a high entropy phase. Our case studies therefore illustrate that the stability of HEOs cannot be straightforwardly predicted based on ionic radii, lattice geometry, and charge-balancing considerations alone due to the underlying complexity of the interactions between the many chemical constituents.
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Materials Science

Enhancing solar cell efficiency of AlxIn1-xN/Si heterojunctions using an a-Si buffer: A study of material, interface and device properties

M. Sun, R. G. Cornejo, M. de la Mata et al. (+4 more)

This study explores the impact of an optimized amorphous silicon (a-Si) buffer layer on AlxIn1-xN-on-Si(100) heterojunction solar cells, with Al content varying from 0% (InN) to 55%. The buffer layer improves the structural quality of the AlInN layer, as evidenced by reduced full width at half maximum values in X-ray diffraction rocking curves around the AlInN (0002) peak. Atomic force microscopy reveals that the buffer layer does not alter surface roughness. The effectiveness of the a-Si buffer is demonstrated by an enhancement of the conversion efficiency under AM1.5G illumination from 3.3 % to 3.9 % for devices with 35 % Al. Looking at the effect of the Al content in devices with the a-Si buffer, the device with 22% Al shows the best photovoltaic performance, with a conversion efficiency of 4.1 % and a VOC of 0.42 V, JSC of 15.4 mA/cm2, and FF of 63.3%. However, performance declines for Al contents above 36% due to increased resistivity and reduced carrier concentration. These findings highlight the critical role of the novel a-Si buffer layer developed by RF-sputtering and the Al content in optimizing AlInN/Si heterojunction solar cell performance.
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All authors M. Sun, R. G. Cornejo, M. de la Mata, S. I. Molina, B. Damilano, S. Valdueza-Felip, F. B. Naranjo

Materials Science

Characterizing Defect Dynamics in Silicon Carbide Using Symmetry-Adapted Collective Variables and Machine Learning Interatomic Potentials

Soumajit Dutta, Cunzhi Zhang, Gustavo Perez Lemus et al. (+4 more)

Silicon carbide (SiC) divacancies are attractive candidates for spin defect qubits possessing long coherence times and optical addressability. The high activation barriers associated with SiC defect formation and motion pose challenges for their study by first-principles molecular dynamics. In this work, we develop and deploy machine learning interatomic potentials (MLIPs) to accelerate defect dynamics simulations while retaining ab initio accuracy. We employ an active learning strategy comprising symmetry-adapted collective variable discovery and enhanced sampling to compile configurationally diverse training data, calculation of energies and forces using density functional theory (DFT), and training of an E(3)-equivariant MLIP based on the Allegro model. The trained MLIP reproduces DFT-level accuracy in defect transition activation free energy barriers, enables the efficient and stable simulation of multi-defect 216-atom supercells, and permits an analysis of the temperature dependence of defect thermodynamic stability and formation/annihilation kinetics to propose an optimal annealing temperature to maximally stabilize VV divacancies.
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All authors Soumajit Dutta, Cunzhi Zhang, Gustavo Perez Lemus, Juan J. de Pablo, Francois Gygi, Giulia Galli, Andrew L. Ferguson

Materials Science

Double Perovskites K2NbTaO6 and Rb2NbTaO6 from First-Principles: Towards Efficient Materials for Green Energy

Ouendadji Salima, Aissani Ali, El Haj Hassan Fouad et al. (+1 more)

The structural flexibility and multifunctional nature of double perovskite oxides make them attractive for applications requiring coupled optical, mechanical, and thermal performance. Using first-principles computations, this study examines the structural, electronic, elastic, optical, and thermoelectric stability of K2NbTaO6 and Rb2NbTaO6. The two compounds combine to form a cubic double perovskite structure with ordered Nb$^{5+}$ and Ta$^{5+}$ cations. The calculated elastic constants satisfy the Born stability criteria, confirming mechanical stability; however, both K2NbTaO6 and Rb2NbTaO6 exhibit brittle behavior according to Pugh's ratio, reflecting limited ductility. Semiconducting behavior is revealed by band structure analysis with energy gaps of 2.79 eV for K2NbTaO6 and 2.63 eV for Rb2NbTaO6. Optical spectra show noticeable absorption in the high-energy region near the UV, indicating relevance for theoretical studies of optoelectronic and photocatalytic processes, without implying practical device efficiency. Therm
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All authors Ouendadji Salima, Aissani Ali, El Haj Hassan Fouad, Benahmedi Lakhdar

2512.04134 · 3 Dec · The author requests withdrawal of this submission because the manuscript requires significant revision to improve clarity, accuracy, and completeness of the presented results. A fully revised version will be submitted in the future. This withdrawal ensures that readers only have access to a rigorously validated version of the work

Abstract PDF $OXIDE ×1

Materials Science

Machine Learning Pipeline for Denoising Low Signal-To-Noise Ratio and Out-of-Distribution Transmission Electron Microscopy Datasets

Brian Lee, Meng Li, Judith C Yang et al. (+2 more)

High-resolution transmission electron microscopy (HRTEM) is crucial for observing material's structural and morphological evolution at Angstrom scales, but the electron beam can alter these processes. Devices such as CMOS-based direct-electron detectors operating in electron-counting mode can be utilized to substantially reduce the electron dosage. However, the resulting images often lead to low signal-to-noise ratio, which requires frame integration that sacrifices temporal resolution. Several machine learning (ML) models have been recently developed to successfully denoise HRTEM images. Yet, these models are often computationally expensive and their inference speeds on GPUs are outpaced by the imaging speed of advanced detectors, precluding in situ analysis. Furthermore, the performance of these denoising models on datasets with imaging conditions that deviate from the training datasets have not been evaluated. To mitigate these gaps, we propose a new self-supervised ML denoising pipeline specifically designed for time-series HRTEM images. This pipeline integrates a blind-spot convolution neural network with pre-processing and post-processing steps including drift correction and low-pass filtering. Results demonstrate that our model outperforms various other ML and non-ML denoising methods in noise reduction and contrast enhancement, leading to improved visual clarity of atomic features. Additionally, the model is drastically faster than U-Net-based ML models and demonstrates excellent out-of-distribution generalization. The model's computational inference speed is in the order of milliseconds per image, rendering it suitable for application in in-situ HRTEM experiments.
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All authors Brian Lee, Meng Li, Judith C Yang, Dmitri N Zakharov, Xiaohui Qu

Materials Science

Quantum theory of nonlinear phononics

Francesco Libbi and Boris Kozinsky

The recent capability to use THz pulses to control the nuclear quantum degrees of freedom in crystals has opened promising avenues for the advanced manipulation of material properties. While numerical approaches exist for studying the time evolution of the quantum nuclear density matrix, an interpretable analytical framework to explicitly analyze the influence of quantum fluctuations on nuclear dynamics remains lacking. In this work, we present an analytical quantum theory of nonlinear phononics. This framework is a basis for deriving models of realistic materials, allowing for exact solutions of the nuclear time evolution with full consideration of quantum fluctuations. This is accomplished by treating for all possible third- and fourth-order phonon couplings and expressing forces as analytic functions of such fluctuations. We provide an analytic proof that, in general, a strong pulse displacing a phonon mode from equilibrium induces the quenching, or squeezing, of its quantum lattice fluctuations. This finding, which establishes a systematization of the mechanism observed in Ref. 1, introduces a new paradigm in nonlinear phononics, harnessing this cooling effect to drive symmetry breaking in quantum paraelectric materials.
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Materials Science

The Loss Landscape of Powder X-Ray Diffraction-Based Structure Optimization Is Too Rough for Gradient Descent

Nofit Segal, Akshay Subramanian, Mingda Li et al. (+2 more)

Solving crystal structures from powder X-ray diffraction (XRD) is a central challenge in materials characterization. In this work, we study the powder XRD-to-structure mapping using gradient descent optimization, with the goal of recovering the correct structure from moderately distorted initial states based solely on XRD similarity. We show that commonly used XRD similarity metrics result in a highly non-convex landscape, complicating direct optimization. Constraining the optimization to the ground-truth crystal family significantly improves recovery, yielding higher match rates and increased mutual information and correlation scores between structural similarity and XRD similarity. Nevertheless, the landscape may remain non-convex along certain symmetry axes. These findings suggest that symmetry-aware inductive biases could play a meaningful role in helping learning models navigate the inverse mapping from diffraction to structure.
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All authors Nofit Segal, Akshay Subramanian, Mingda Li, Benjamin Kurt Miller, Rafael Gomez-Bombarelli

Materials Science

Anisotropic Phonon Dynamics and Directional Transport in Actinide van der Waals Semiconductor USe$_3$

Aljoscha Söll, Valentino Jadrisko, Sourav Dey et al. (+13 more)

Direction-dependent charge transport and optical responses are characteristic of van der Waals (vdW) materials with strong in-plane anisotropy. While transition-metal trichalcogenides (TMTCs) exemplify this behavior, heavier analogs remain largely unexplored. In this study we examine USe$_3$ as an anisotropic vdW material and a heavier analog of the well-studied TMTCs. We reveal strong in-plane anisotropy using polarization-resolved Raman spectroscopy, investigate strain-induced shifts of phonon modes, and quantify direction-dependent charge-carrier mobility through transport measurements on field-effect devices. First-principles calculations based on density-functional theory corroborate our findings, providing a theoretical basis for our experimental observations. Casting USe$_3$ as an actinide analog of a TMTC establishes a platform for exploring low-dimensional semiconductors that combine strong in-plane anisotropy with f-electron physics.
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All authors Aljoscha Söll, Valentino Jadrisko, Sourav Dey, Nassima Benchtaber, Kalyan Sarkar, Borna Radatovic, Jan Luxa, Fedor Lipilin, Kseniia Mosina, Vojtech Kundrat, Jakub Zalesak, Jana Vejpravova, Martin Zacek, Christoph Gadermaier, José J. Baldoví, Zdeněk Sofer

Materials Science

Construction of minimal integrity basis for anisotropic hyperelasticity via structural tensors

Brain M. Riemer, Jörg Brummund, Karl A. Kalina et al. (+3 more)

We present minimal integrity bases for all common anisotropies in hyperelasticity via the structural tensor concept, which can be used to formulate any algebraic invariant function in the elements of the respective bases. Hence, the provided minimal integrity bases are of great interest for formulating a concise but general anisotropic material model. Our work covers results for the 11 types of anisotropy that arise from the classical 7 crystal systems, as well as findings for 4 additional non-crystal anisotropies derived from the cylindrical, spherical, and icosahedral symmetry systems. By using well-known results from literature about structural tensors, isotropic invariants, and isotropic extension, functional bases are directly determined. A simple analytical-numerical approach is employed to identify polynomial relations between the invariants of these functional bases, thereby enabling the construction of functional bases of reduced cardinality. After that, we show that the determined reduced functional bases are also minimal integrity bases by identifying polynomial relations to known integrity bases from literature. Furthermore, fundamental concepts from invariant theory, including the Hironaka decomposition of invariant rings and the closely related Hilbert series, are employed to further validate the results. Alongside the presented findings, this article also aims to provide an introductory overview of the complex field of modeling anisotropic materials, especially for researchers with an engineering background.
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All authors Brain M. Riemer, Jörg Brummund, Karl A. Kalina, Abel H. G. Milor, Franz Dammaß, Markus Kästner

Materials Science

Non-radiative energy transfer between boron vacancies in hexagonal boron nitride and other 2D materials

Jules Fraunié, Mikhail M. Glazov, Sébastien Roux et al. (+13 more)

Boron vacancies ($V_B^-$) in hexagonal boron nitride (hBN) have emerged as a promising platform for two-dimensional quantum sensors capable of operating at atomic-scale proximity. However, the mechanisms responsible for photoluminescence quenching in thin hBN sensing layers when placed in contact with absorptive materials remain largely unexplored. In this Letter, we investigate non-radiative Förster resonance energy transfer (FRET) between $V_B^-$ centers and either monolayer graphene or 2D semiconductors. Strikingly, we find that the FRET rate is negligible for hBN sensing layers thicker than 3 nm, highlighting the potential of $V_B^-$ centers for integration into ultra-thin quantum sensors within van der Waals heterostructures. Furthermore, we experimentally extract the intrinsic radiative decay rate of $V_B^-$ defects.
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All authors Jules Fraunié, Mikhail M. Glazov, Sébastien Roux, Abraao Cefas Torres-Dias, Cora Crunteanu-Stanescu, Tom Fournier, Maryam S. Dehaghani, Tristan Clua-Provost, Delphine Lagarde, Laurent Lombez, Xavier Marie, Benjamin Lassagne, Thomas Poirier, James H. Edgar, Vincent Jacques, Cedric Robert

Materials Science

Influence of a generative parameter on the mechanical performance of topological interlocking assemblies of a hexagonal block

Lukas Schnelle, Meike Weiß, Reymond Akpanya et al. (+2 more)

A topological interlocking assembly is an arrangement of blocks, where all blocks are kinematically constrained by their neighboring blocks and a fixed frame. This concept has been known for a long time, attracting recent interest due to its advantageous mechanical properties, such as reusability, redundancy and limited crack propagation. New mathematical methods enable the generation of vast numbers of new topologically interlocking blocks. A natural next question is the quantification of the mechanical performance of these new blocks. We conduct a numerical study of topological interlocking assemblies whose blocks are constructed based on the hexagonal grid. By varying a design parameter used in the generation of these blocks, we study its influence on the structural performance of the entire assembly. The results improve our understanding of the link between the block parameters and the mechanical performance. This enhances the ability to custom design blocks for certain mechanical requirements of the topological interlocking assemblies.
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All authors Lukas Schnelle, Meike Weiß, Reymond Akpanya, Kai-Uwe Schröder, Alice C. Niemeyer

Materials Science

Resolving Structural Transitions in Lanthanide High-Entropy Oxides

Billy E. Yang, Saeed S. I. Almishal, Sai Venkata Gayathri Ayyagari et al. (+6 more)

We report a temperature-composition phase diagram for the chemically disordered and CeO2-LA2O3 high entropy oxides (HEOs), where LA denotes equimolar Y, La, Sm, and Pr, delineating stability regions for bixbyite, disordered fluorite, and intermediate vacancy-ordered fluorite phases. The diagram is constructed from a characterization package applied to bulk ceramics including X-ray diffraction (XRD), transmission electron microscopy (TEM) electron diffraction, Raman spectroscopy, energy-dispersive spectroscopy, X-ray absorption near-edge structure spectroscopy, and ultraviolet-visible spectroscopy, to quantify crystal structure at multiple length-scales, local coordination environments, and electronic structures across the formulation space. This comprehensive measurement suite is critical to identify boundaries between the closely related phases. For example, Raman scattering reveals local structural and defect environments unique to bixbyite local order that persist to ~50% Ce under equilibrium synthesis conditions but are invisible to XRD and TEM. We also report a companion thin film study to demonstrate that quenched kinetic energy from a physical deposition process can metastabilize the high symmetry, and thus high entropy, fluorite phase with only 20% Ce. This is noteworthy because electroneutrality constraints demand an exceptionally vacated oxygen sublattice; we estimate 16.7%, approaching that of delta-Bi2O3. Together, our equilibrium ceramics and far-from-equilibrium thin films show that when synthesis is coupled with rigorously chosen, multi-length-scale characterization, now one can identify the phase stability thermodynamic drivers and simultaneously derive practical guidelines for experimentally realizing targeted phases and structures - and thereby deliberately engineer properties in CeO2-LA2O3 HEOs, whose broad defect chemistries demand such an approach.
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All authors Billy E. Yang, Saeed S. I. Almishal, Sai Venkata Gayathri Ayyagari, Mary Kathleen Caucci, Gerald Bejger, Christina M. Rost, Nasim Alem, Susan B. Sinnott, Jon-Paul Maria

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

Super-hard and superconducting boron clathrates in the prediction of U-B compounds

Juefei Wu, Dexi Shao, Junjie Wang et al. (+6 more)

The binary metal borides provide a promising platform for searching unique materials with superconductivity and super-hardness under high pressure, owing to the distinctive bonding characters of boron. In this work, combined the first-principles calculations and crystal structure predictions, we predicted 4 exotic stoichiometries and 8 unique U-B compounds under high pressure. The predicted compounds have layered or caged structure units and 4 of them host high hardness under ambient pressure. By removal of the U atoms, we predicted three meta-stable boron clathrates at ambient pressure. Remarkably, the Vickers hardness of the predicted C2/m-B6 is estimated to be 49-53 GPa, and the C2/m-B12 is superconducting with the Tc value of 16.12 K. Our calculations enrich the phase diagram of binary metal borides and boron allotropes, providing insights for the future theoretical and experimental studies on unique materials.
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All authors Juefei Wu, Dexi Shao, Junjie Wang, Yu Han, Bangshuai Zhu, Cuiying Pei, Qi Wang, Jian Sun, Yanpeng Qi

Materials Science

Interfacial Control of Orbital Occupancy and Spin State in LaCoO$_3$

Ellen M. Kiens, Nicolas Gauquelin, Arno Annys et al. (+11 more)

Transition metal oxides exhibit a wide range of tunable electronic properties arising from the complex interplay of charge, spin, and lattice degrees of freedom, governed by their $d$ orbital configurations, making them particularly interesting for oxide electronics and (electro)catalysis. Perovskite oxide heterointerfaces offer a promising route to engineer these orbital states. In this work, we tune the Co $3d$ orbital occupancy in LaCoO$_3$ from a partial $d^7$ to a partial $d^5$ state through interfacial engineering with LaTiO$_3$, LaMnO$_3$, LaAlO$_3$ and LaNiO$_3$. Using X-ray absorption spectroscopy combined with charge transfer multiplet calculations, we identify differences in the Co valence and spin state for the series of oxide heterostructures. LaTiO$_3$ and LaMnO$_3$ interfaces result in interfacial charge transfer towards LaCoO$_3$, resulting in a partial $d^7$ orbital occupancy, while a LaNiO$_3$ interface results in a partial Co $d^5$ occupancy. Strikingly, a LaAlO$_3$ spacer layer between LaNiO$_3$ and LaCoO$_3$ results in a Co $d^6$ low spin state. These results indicate that the Co spin state, like the valence state, is governed by the interfacial environment. High-resolution scanning transmission electron microscopy imaging reveals a clear connection between strain and spin configuration, emphasizing the importance of structural control at oxide interfaces. Overall, this work demonstrates that interfacial engineering simultaneously governs orbital occupancy and spin state in correlated oxides, advancing spin-engineering strategies in correlated oxides and offering new insights for the rational design of functional oxide heterostructures.
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All authors Ellen M. Kiens, Nicolas Gauquelin, Arno Annys, Emma van der Minne, Iris C.G. van den Bosch, Matthijs A. van Spronsen, Zezhong Zhang, Annick de Backer, Sandra van Aert, Jo Verbeeck, Gertjan Koster, Bastian Mei, Frank M.F. de Groot, Christoph Baeumer