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Physics Condensed Matter Materials Science

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Saturday 7 February

Materials Science

Positron annihilation lifetime and Doppler broadening spectral calculations of oxygen-doped 3C-SiC

Yi Zhao, Hongtao Zhang, Qiang Li et al. (+2 more)

Based on density functional theory (DFT), the formation energies of intrinsic vacancy defects (VC, VSi, and VSi+C) and oxygen-related defects (OC, OSi, OCVSi, and OSiVC) in 3C-SiC are systematically investigated. The results indicate that all defects considered, except for OC, possess neutral or negative charge states, thereby making them suitable for detection by positron annihilation spectroscopy (PAS). Furthermore, the electron and positron density distributions and positron annihilation lifetimes for the perfect 3C-SiC supercell and various defective configurations are computed. It is found that the OSi and OSiVC complexes act as effective positron trapping centers, leading to the formation of positron trapped states and a notable increase in annihilation lifetimes at the corresponding defect sites. In addition, coincidence Doppler broadening (CDB) spectra, along with the S and W parameters, are calculated for both intrinsic and oxygen-doped point defects (OC, OSi, OCVSi, and OSiVC). The analysis reveals that electron screening effects dominate the annihilation characteristics of the OSi defect, whereas positron localization induced by the vacancy is the predominant contributor in the case of OSiVC. This distinction results in clearly different momentum distributions of these two oxygen-related defects for different charge states. Overall, the PAS is demonstrated to be a powerful technique for distinguishing intrinsic vacancy-type defects and oxygen-doped composites in 3C-SiC. Combining the analysis of electron and positron density distributions, the electron localization and positron trapping behavior in defect systems with different charge states can be comprehensively understood. These first-principles results provide a solid theoretical foundation for identifying and characterizing the defects in oxygen-doped 3C-SiC by using PAS.
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All authors Yi Zhao, Hongtao Zhang, Qiang Li, Xian Tang, Guodong Cheng

Materials Science · Cross-listed from cs.AI

GraphAgents: Knowledge Graph-Guided Agentic AI for Cross-Domain Materials Design

Isabella A. Stewart, Tarjei Paule Hage, Yu-Chuan Hsu et al. (+1 more)

Large Language Models (LLMs) promise to accelerate discovery by reasoning across the expanding scientific landscape. Yet, the challenge is no longer access to information but connecting it in meaningful, domain-spanning ways. In materials science, where innovation demands integrating concepts from molecular chemistry to mechanical performance, this is especially acute. Neither humans nor single-agent LLMs can fully contend with this torrent of information, with the latter often prone to hallucinations. To address this bottleneck, we introduce a multi-agent framework guided by large-scale knowledge graphs to find sustainable substitutes for per- and polyfluoroalkyl substances (PFAS)-chemicals currently under intense regulatory scrutiny. Agents in the framework specialize in problem decomposition, evidence retrieval, design parameter extraction, and graph traversal, uncovering latent connections across distinct knowledge pockets to support hypothesis generation. Ablation studies show that the full multi-agent pipeline outperforms single-shot prompting, underscoring the value of distributed specialization and relational reasoning. We demonstrate that by tailoring graph traversal strategies, the system alternates between exploitative searches focusing on domain-critical outcomes and exploratory searches surfacing emergent cross-connections. Illustrated through the exemplar of biomedical tubing, the framework generates sustainable PFAS-free alternatives that balance tribological performance, thermal stability, chemical resistance, and biocompatibility. This work establishes a framework combining knowledge graphs with multi-agent reasoning to expand the materials design space, showcasing several initial design candidates to demonstrate the approach.
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All authors Isabella A. Stewart, Tarjei Paule Hage, Yu-Chuan Hsu, Markus J. Buehler

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

Complete electronic phase diagram and enhanced superconductivity in fluorine-doped PrFeAsO1-xFx

Priya Singh, Konrad Kwatek, Tatiana Zajarniuk et al. (+4 more)

Establishing a complete electronic phase diagram for REFeAsO (RE = rare earth, RE1111) ironbased superconductors has remained experimentally challenging. Here, we report a systematic investigation of PrFeAsO1-xFx over the full nominal fluorine-doping range 0 to 1 and construct the first comprehensive electronic phase diagram for this system. The evolution from the nonsuperconducting parent compound to the fluorine-rich limit reveals a broad dome shaped superconducting region. Structural refinement demonstrates a systematic lattice contraction with increasing fluorine content (x), corroborated by Raman spectroscopy through softening of the Pr(A1g) phonon mode and hardening of the Fe(B1g) mode, confirming effective fluorine incorporation at the oxygen sites. The maximum superconducting transition temperature (Tc) reaches up to 52.3 K, approximately 5 K higher than previous reports for Pr1111. Magnetotransport measurements yield large upper critical fields Hc2(0) exceeding 100 T, while analysis of resistive transition broadening reveals thermally activated flux flow with a crossover from single-vortex to collective pinning regimes. Specific-heat measurements exhibit a reduced jump deltaC/{\gamma}Tc < 1.43, reflecting strong superconducting fluctuations and multiband pairing. These results establish clear structure property correlations and provide a unified description of superconductivity across the entire doping range of the Pr1111 system.
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All authors Priya Singh, Konrad Kwatek, Tatiana Zajarniuk, Taras Palasyuk, Cezariusz Jastrzębski, A. Szewczyk, Shiv J. Singh

Materials Science

Benchmarking the plasmon-pole and multipole approximations in the Yambo Code using the GW100 dataset

M. Bonacci, D. A. Leon, N. Spallanzani et al. (+4 more)

Verification and validation of electronic structure codes are essential to ensure reliable and reproducible results in computational materials science. While density functional theory has been extensively benchmarked, systematic assessments of many-body perturbation theory methods such as the GW approximation have only recently emerged, most notably through the GW100 dataset. In this work, we assess the numerical accuracy and convergence behavior of the GW implementation in the yambo code using both the Godby-Needs plasmon-pole model and the recently introduced multipole approximation. Quasiparticle energies are compared against GW100 reference data to evaluate the performance, numerical stability, and consistency of these approaches.
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All authors M. Bonacci, D. A. Leon, N. Spallanzani, E. Molinari, D. Varsano, A. Ferretti, C. Cardoso

Materials Science

Spontaneous Symmetry Breaking and Collective Higgs-Goldstone Dynamics in Solid-State Phononic Frequency Combs

Murtaza Rangwala and Adarsh Ganesan

We investigate the generation of phononic frequency combs arising from nonlinear coupling between Higgs-like and Goldstone-like phonon modes in hexagonal InMnO3. The Higgs-like mode, an infrared-active optical phonon, is resonantly driven by a short, high-electric field terahertz pulse, while the optically inactive Goldstone-like mode is indirectly excited through intrinsic nonlinear mode coupling. Using a nonlinear phononics model, we numerically solve the coupled equations of motion governing the lattice dynamics and analyze the resulting time- and frequency-domain responses. By systematically varying key drive and material parameters-including electric field amplitude, pulse width, driving frequency, and mode damping-we identify the conditions under which stable phononic frequency combs emerge. Our results reveal clear threshold behaviors for comb formation, tunability of comb spacing and spectral bandwidth through external control parameters, and a breakdown of coherent comb structure at high drive strengths or weak damping. These findings demonstrate how nonlinear Higgs-Goldstone interactions enable controllable phononic frequency comb generation and provide insight into ultrafast lattice dynamics in symmetry-broken materials.
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Materials Science

First-principles study on the high-$T_\text{c}$ superconductivity of Mg-Ti-H ternary hydrides up to the liquid-nitrogen temperature range under high pressures

Pan Min, Wang Yujie, Hu Kaige et al. (+1 more)

Ternary hydrides have emerged as the primary focus of the new wave of research into superconducting hydrides. In this work, Mg-Ti-H ternary hydrides are explored under high pressures up to 300 GPa using the prediction method of the particle swarm optimization algorithm combined with first-principles calculations. Two new structures, $P4/nmm$-MgTiH$_6$ and $Pmm2$-Mg$_3$TiH$_6$, are identified to be thermodynamically stable at both 200 GPa and 300 GPa. Thermodynamically stable structures of Mg$_3$TiH$_{12}$ are also identified, whose space groups are $R3/m$ at 200 GPa and $Pm\bar{3}m$ at 300 GPa, respectively. Among these Mg-Ti-H structures, $P4/nmm$-MgTiH$_6$ achieves a record-high $T_\text{c}$ of 81.9 K at 170 GPa, exceeding the boiling point of liquid nitrogen. Such a high $T_\text{c}$ is primarily attributed to strong electron-phonon coupling (EPC) driven by low-frequency acoustic phonon modes, with the EPC strength reaching a large value of 1.54. The $T_\text{c}$ of $Pm\bar{3}m$-Mg$_3$TiH$_{12}$ is predicted to be 40 K at 300 GPa. Furthermore, element substitution of Zr(Hf) for Ti achieves considerable enhancement of superconducting properties in our predicted hydrogen-rich and high-symmetric crystal structures, i.e., $P4/nmm$-MgTiH$_6$ and $Pm\bar{3}m$-Mg$_3$TiH$_{12}$. The high pressure required for dynamical stability is lowered to 100 GPa in both $Pm\bar{3}m$-Mg$_3$ZrH$_{12}$ and $Pm\bar{3}m$-Mg$_3$HfH$_{12}$, and to 90 GPa and 120 GPa for $P4/nmm$-MgZrH$_6$ and $P4/nmm$-MgHfH$_6$, respectively. Particularly, the electronic structure near the Fermi level is significantly modified in the $P4/nmm$-MgHfH$_6$ phase, and pronounced softening of low-frequency acoustic phonon modes occurs. As a result, the EPC strength is enhanced to 1.72, leading to a higher $T_\text{c}$ of 86 K.
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All authors Pan Min, Wang Yujie, Hu Kaige, Deng Huiqiu

Materials Science

Efficient and Robust p-type Transistor based on Ultra-wide-bandgap Semiconductor

Kaijian Xing, Zherui Yang, Weiyao Zhao et al. (+17 more)

The p-type transistor is an indispensable component of semiconductor technology, enabling complementary operation with n-channel transistors for computation, storage, and communication. Achieving both high robustness and high efficiency is highly desirable but challenging for p-type transistors due to limited semiconductors with reliable hole transport and their high activation energies. Here, we achieved a robust yet efficient p-type transistor by heterogeneously integrating an ultra-wide-bandgap semiconductor and a high-k dielectric layer through van der Waals integration. The p-type transistor employs a two-dimensional hole channel on hydrogenated diamond (bandgap 5.6 eV) combined with a high-k (30.5) SrTiO3 perovskite membrane. At room temperature, the transistor exhibits stable operation with a high on-current (~200 mA/mm), low subthreshold swing (70 mV/dec), high hole mobility (566 cm^2/Vs to 572 cm^2/Vs) and high on-off ratio (~10^9). Furthermore, tuning annealing temperature allows operation in either enhancement or depletion mode. The robust p-type transistor with high efficiency holds great potential for future power electronics, UV optoelectronics, and harsh-environment electronic applications.
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All authors Kaijian Xing, Zherui Yang, Weiyao Zhao, Yuefeng Yin, Huiping Han, Shanhu Wang, Shifan Wang, James Bullock, Alastair Stacey, James A. Belcourt, Sergey Rubanov, Hang Yin, David A. Broadway, Jean-Philippe Tetienne, Xinmao Yin, Liang Wu, Dong-Chen Qi, Michael S. Fuhrer, Qingdong Ou, Xiao Renshaw Wang

Materials Science

ELAS3D-Xtal: An OpenMP-accelerated crystal elasticity solver with automated experiment-driven microstructure generation

Juyoung Jeong and Veera Sundararaghavan

This paper introduces ELAS3D-Xtal, a high-performance Fortran/OpenMP upgrade of the NIST ELAS3D voxel-based finite element solver for computing 3D elastic fields in polycrystals with defects. The code supports crystal anisotropy by precomputing rotated stiffness tensors from user-specified orientations and solves the equilibrium problem with a matrix-free, OpenMP-parallel preconditioned conjugate-gradient (PCG) method using a point-block Jacobi preconditioner. On a single shared-memory multicore PC, OpenMP threading accelerates the baseline CG solver by ~10X, while the block-preconditioned CG solver achieves 53-61X speedup relative to the serial CG baseline for meshes from 100^3 to 500^3 voxels (scaling to domains up to 800^3 voxels). Accuracy is validated against the analytical Eshelby inclusion solution. ELAS3D-Xtal also integrates microstructure construction, including statistically calibrated polycrystal generation via spatial filtering and parallel voxel-to-grain assignment, direct pore insertion from XCT centroid/radius data, and texture assignment. Full-field phase, orientation, and stress outputs are written in HDF5 to enable scalable post-processing and defect-mechanics workflows. Applications are demonstrated for (i) anisotropy-controlled defect-scale stress fields and (ii) LPBF SS316L microstructures with gas, lack-of-fusion, and keyhole pore morphologies.
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Materials Science

AtomMOF: All-Atom Flow Matching for MOF-Adsorbate Structure Prediction

Nayoung Kim, Honghui Kim, Sihyun Yu et al. (+3 more)

Metal-organic frameworks (MOFs) are promising materials for applications such as direct air capture, where performance depends on how adsorbates bind within the framework. Accurately predicting MOF-adsorbate configurations is therefore important for screening candidate materials. However, traditional approaches are computationally expensive and require a known host structure, while existing generative models rely on rigid-body assumptions and do not explicitly model adsorbates. We introduce AtomMOF, a scalable all-atom flow-matching model that jointly predicts MOF-adsorbate structures from discrete building blocks and adsorbates. Built on a Diffusion Transformer with a building block-based pairwise attention bias, AtomMOF operates in an unconstrained all-atom space and exhibits clear scaling behavior. To improve the structural validity of flexible all-atom models, we also propose Feynman-Kac (FK) steering with machine-learned interatomic potentials (MLIPs). On the BW dataset, AtomMOF achieves a 58.17% relative increase in match rate and a 31.84% relative reduction in RMSD compared with prior work. MLIP-guided steering further improves validity by 28.7% and reduces formation energy error by 86.5%. On ODAC25, AtomMOF generates adsorption configurations faster than GCMC and, when combined with MLIP relaxation, identifies lower-energy configurations than those in the reference dataset.
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All authors Nayoung Kim, Honghui Kim, Sihyun Yu, Minkyu Kim, Seongsu Kim, Sungsoo Ahn

Materials Science · Cross-listed from physics.optics

High-Throughput In-Situ Fabrication of Fibrous Membranes Enables Scalable Passive Radiative Cooling

Hanzhuo Shao, Xiaoli Huang, Xuemei Huang et al. (+5 more)

Deploying fibrous membranes for passive daytime radiative cooling (PDRC) on large and irregular surfaces is highly desirable but remains challenging, owing to the slow deposition rates and the need for electrically conductive substrates in conventional electrospinning. Here, we demonstrate a high-throughput in-situ strategy for fabricating nanocomposite PDRC fibrous membranes via solution blow spinning. This method achieves deposition rates 8-12 times faster than electrospinning and can be applied directly onto nonplanar, nonconductive objects. The resulting membranes, composed of styrene-ethylene-butylene-styrene (SEBS) fibers embedded with Y2O3 nanoparticles, achieve sub-ambient cooling of up to 7.0 {\deg}C outdoors, effectively delaying ice melting. Moreover, they are fully recyclable through simple cleaning, dissolution, and reprocessing. This scalable and sustainable fabrication route provides a versatile and practical platform for integrating PDRC fibrous membranes across diverse surfaces, paving the way toward real-world thermal management applications.
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All authors Hanzhuo Shao, Xiaoli Huang, Xuemei Huang, Jin Zhao, Nailin Xing, Hua Xu, Weijie Song, Yuehui Lu

Materials Science

Ordering Mixed-Q Topological Magnetism into Lattice via Moire Engineering

Xiudong Wang, Zhonglin He, Kaiying Dou et al. (+3 more)

Topological magnetic lattices offer a fertile ground for exploring fundamental physics and developing novel spintronic devices. However, current research is predominantly confined to single-Q topologies hosting uniform type of quasiparticle. The realization of exotic mixed-Q states, where distinct topological quasiparticles co-assemble into an ordered lattice, remains largely unexplored. Here, we propose a generic mechanism to order disordered mixed-Q topological magnetism into periodic lattice via moire engineering. By leveraging the synergy between spatially modulated interlayer coupling and intrinsic intralayer magnetic frustration, we demonstrate that moire potential can effectively regularize skyrmions, antiskyrmions, and magnetic bubbles into a hybrid lattice. Combining first-principles with atomistic spin simulations, we validate this mechanism in twisted bilayer CrGaTe3, identifying it as an exemplary platform for hosting these complex ordered textures. We systematically map the phase evolution as a function of twist angle and biaxial strain, unveiling the critical role of moire potential in stabilizing mixed-Q lattice. Our findings significantly advance the frontier of topological and moire spintronics.
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All authors Xiudong Wang, Zhonglin He, Kaiying Dou, Ying Dai, Baibiao Huang, Yandong Ma

Materials Science

Analyzing Band Gaps in Ensemble Density Functional Theory using Thermodynamic Limits of Finite One-Dimensional Model Systems

Gregory G. V. Kenning, Remi J. Leano and David A. Strubbe

Ensemble Density Functional Theory (EDFT) is a promising extension to Density Functional Theory (DFT) for calculating excited states. While Kohn-Sham eigenvalue differences underestimate gaps, EDFT has been shown to provide more accurate excitation energies in atoms, molecules and isolated model systems. However, it is unclear whether EDFT is capable of calculating band gaps of periodic systems – and what an appropriate theoretical formulation would be to describe periodic systems. We explored how EDFT could calculate band gaps by estimating the thermodynamic limit with increasingly wide finite versions of the one-dimensional Kronig-Penney (KP) periodic model. We use Octopus, an ab initio, open-source, real-space DFT code, as in our previous work [R. J. Leano et al., Electron. Struct. 6, 035003 (2024)] in which we found with "particle in a box" models that EDFT can provide a reasonable effective mass correction for the homogeneous electron gas. Now, we use a periodic reference that is gapped. We find that the finite systems' Kohn-Sham gap approaches the same periodic limit for each of three ways of terminating the finite system, though the appropriate states corresponding to the valence band maximum and conduction band minimum have to be carefully identified in each case. Finally, our EDFT results, using a simple ensemblized LDA approximation, have a reasonable nonzero correction to the bandgap in the periodic limit. The results indicate that EDFT is promising for periodic systems, to motivate further work on developing a suitable formalism.
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Materials Science

The impact of spurious imaginary phonon modes on thermal properties of Metal-organic Frameworks

Prathami Divakar Kamath and Kristin A. Persson

Metal-organic Frameworks (MOFs) have emerged as potential candidates for direct air capture (DAC) of green house gases and water. Thermal properties of MOFs, such as their heat capacity, are used to determine the energy penalty associated with the adsorbent retrieval during the Temperature Swing Adsorption process. To aid exploration of the vast experimental design space of MOFs for such applications, computational methods like Density Functional Theory (DFT) or surrogate machine learning models trained on DFT data have been developed for obtaining phonon-derived heat capacities of MOFs. However, the high cost of explicit phonon computation in large and flexible nanoporous MOFs often necessitates the use of small supercells or lower convergence criteria which decrease predictive accuracy. These approximations often result in spurious imaginary phonon modes which are commonly ignored in practice. At present, there is no clear consensus in the literature on what magnitude of negative frequency or what fraction of imaginary modes can be considered acceptable. Here, we systematically demonstrate that spurious imaginary phonon modes can introduce substantial errors in heat capacity estimates, leading to incorrect ranking of MOFs in thermal-property-based screening. We further show that benchmarking machine learning interatomic potentials (MLIPs) against DFT datasets containing spurious imaginary modes can misrepresent models that predict physically meaningful phonon spectra for dynamically stable MOFs. Finally, we introduce a simple, rapid post-processing workflow that can be applied to standard phonon calculations to effectively correct heat capacity estimates and account for spurious imaginary modes in MOFs.
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Materials Science

A Low-Cost, Strong, and Ductile Single-Phase Nb-Based Refractory Complex Concentrated Alloy

Ayeman M. Nahin, Jacob Pustelnik, Jessica Dong et al. (+2 more)

The development of structural materials capable of sustained operation above 1200 {\deg}C is critical for next-generation energy and aerospace systems; however, Ni-based superalloys are fundamentally constrained by their melting temperatures, while conventional Nb-based refractory alloys are limited by modest specific strength and high cost. Here, we report on the design and mechanical performance of a cost-effective, non-equiatomic refractory complex concentrated alloy (RCCA), Nb45Ta15Ti20V20, engineered to overcome these limitations. Specifically, its specific strength surpasses wrought C-103 and rivals additively manufactured (AM) C-103 at temperatures up to 1300 {\deg}C while maintaining extensive room temperature tensile ductility (>10 %). Coupled with a high melting point (~2167 {\deg}C), reduced density (8.67 g/cc), and a raw material cost of ~$130/kg compared to >$500/kg for wrought C-103 and >$2,500/kg for AM C-103, this alloy delivers superior specific strength-cost efficiency, highlighting the promise of non-equiatomic RCCAs as viable alternatives to commercial refractory alloys.
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All authors Ayeman M. Nahin, Jacob Pustelnik, Jessica Dong, Tamanna Zakia, Mingwei Zhang

Materials Science

Constitutive theory for mechanics of amorphous thermoplastic polymers under extreme dynamic loading

John D. Clayton

A geometrically nonlinear continuum mechanical theory is formulated for deformation and failure behaviors of amorphous polymers. The model seeks to capture material response over a range of loading rates, temperatures, and stress states encompassing shock compression, inelasticity, melting, decomposition, and spallation. Thermoelasticity, viscoelasticity, viscoplasticity, ductile failure with localized shear yielding, and brittle fracture with crazing can all emerge under this ensemble of intense loading conditions. Known prior theories have considered one or more, but not all, such physical mechanisms. The present coherent formulation invokes thermodynamics with internal state variables for dynamic molecular and network configurational changes affecting viscoelasticity and plastic deformation, and it uses order parameters for more abrupt structural changes across state-dependent glass-transition and shock-decomposition thresholds. A phase-field order parameter captures material degradation from ductile or brittle fracture, including evolving porosity from crazing. The theory is applied toward polymethyl methacrylate (PMMA) under intense dynamic loading. The high-pressure equilibrium response, with shear strength and temperature over known ranges, is well represented along the principal Hugoniot to pressures far exceeding shock decomposition. Predicted release wave velocities agree with experiment. A semi-analytical solution for steady waves describes the relatively lower-pressure viscoelastic setting, providing insight into relaxation times. One-dimensional calculations assess suitability of the model for representing spall fracture strengths seen in experiments over a range of initial temperatures and loading rates.
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Friday 6 February

Materials Science

Impact of crystallinity on the circular and linear dichroism signals in chiral perovskite

Reshna Shrestha and Wanyi Nie

Chiral perovskites owing to their broken mirror symmetry exhibit selective absorption of circularly polarized light manifesting a strong circular dichroism (CD). CD spectroscopy has been a key technique to understand chiral perovskites and how these semiconductors achieve chirality at the molecular level. However, there is a debate on whether the observed CD is intrinsic to the chiral crystal structures or is modulated by extrinsic phenomena particularly linear dichroism (LD) and linear birefringence (LB) effects. This work investigates the chiroptical properties of $(R-/S-\text{MBA})_2\mathrm{CuCl}_4$ (MBA = Methylbenzyl ammonium) series by thoroughly studying the contribution from LD and LB to the observed CD signals. The comparison of highly oriented and randomly oriented films exhibits notable LD and LB contributions to the observed CD, which are caused by orientation-dependent electric-field interactions and local anisotropy. Both randomly and highly oriented films exhibit distinct CD responses, with LD–LB effects largely dominating the CD in highly oriented films. This has been revealed by the obvious shift in the observed CD signals to the below-absorption-edge ($\sim 430$ nm) regime and broadening of features. Our findings demonstrate that careful consideration of crystal orientation and structural effects is necessary for appropriate interpretation of CD spectra in chiral perovskite thin films.
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Materials Science · Cross-listed from physics.comp-ph

Event-Chain Monte Carlo: The global-balance breakthrough

E.A.J.F. Peters

The seminal 2009 paper by Bernard, Krauth, and Wilson marked a paradigm shift in Monte Carlo sampling. By abandoning the restrictive condition of detailed balance in favor of the more fundamental principle of global balance, they introduced the Event-Chain Monte Carlo (ECMC) algorithm, which achieves rejection-free, deterministic sampling for hard spheres. This breakthrough demonstrated that persistent, directional dynamics could dramatically accelerate equilibration in dense particle systems. In this commentary, we review this foundational work and elucidate its underlying mechanism using the broader Event-Driven Monte Carlo (EDMC) framework developed in subsequent years. We show how the original hard-sphere concept naturally generalizes to continuous potentials and modern lifted Markov chain formalisms, transforming a surprising specific result into a powerful general class of sampling algorithms.
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Materials Science

Sensitivity of grain-averaged elastic strain and orientation predictions on the mesh density and boundary conditions in crystal plasticity finite element simulations

Jeremiah Lethoba, Romain Quey, Darren C. Pagan et al. (+1 more)

Combined high-energy X-ray diffraction microscopy (HEDM) and crystal plasticity finite element (CPFE) modeling studies have emerged as a preferred paradigm to shed insight into the evolution of elasticity and plasticity at the intragrain scale of polycrystals. In particular, far-field HEDM measures the deformation response of upwards of thousands of individual grains simultaneously in situ during mechanical loading, though measurements are primarily limited, however, to the average state of each grain – i.e., the grain's full strain tensor, crystallographic orientation, spatial location and volume. CPFE is utilized to shed information on the intragrain deformation response, via the sub-discretization of each grain into many finite elements, though the direct point of comparison to HEDM remains the grain-averaged response. We thus seek to find the minimum simulation conditions necessary to provide consistent grain-averaged predictions in an attempt to limit computational cost. In this study, we perform a suite of simulations and systematically study the effects of mesh density and boundary conditions, and consider different materials. We discuss these results and show that accurate prediction of grain-averaged elastic strains in a given region of interest typically requires a mesh with 250 elements per grain on average and a buffer layer of at least three grains between the region of interest and the control surfaces.
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All authors Jeremiah Lethoba, Romain Quey, Darren C. Pagan, Matthew Kasemer

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

Microscopic origin of Rashba coupling from first principles: Layer-resolved orbital asymmetry in transition metal dichalcogenides

Miguel Morales Cócera, Marta Prada, Franz Fischer et al. (+1 more)

Spin-orbit coupling in two-dimensional materials gives rise to a Rashba spin splitting when inversion and mirror symmetries are broken, yet its microscopic origin and quantitative characterization in transition metal dichalcogenides remains incomplete. Both symmetries are broken in certain bilayer structures, enabling Rashba splittings in the absence of external electric fields. We determine this zero-field offset and the Rashba parameters that dictate the spin splitting in the linear regime. Surprisingly, the splitting is substantially smaller in bilayers than in monolayers at typical fields. This is clarified within a perturbative microscopic model, revealing that the spin splitting results from a competition between internal polarization and interlayer hybridization. We further introduce the orbital polarization imbalance as an order parameter that captures the asymmetry of the valence bands and determines the spin ordering of the Rashba-split states. Our results are both quantitative and qualitative, as they clarify the nature and origin of Rashba coupling in transition metal dichalcogenides.
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All authors Miguel Morales Cócera, Marta Prada, Franz Fischer, Gabriel Bester

Materials Science · Cross-listed from physics.optics

High-resolution spectroscopy of Pr3+ions in YAl3(BO3)4:Pr3+. Crystal-field, hyperfine,and electron-deformation interactions

B. Z. Malkin, T. A. Igolkina, K. N. Boldyrev et al. (+3 more)

Optical transmission spectra of YAl3(BO3)4 crystals doped with the Pr3+ ions in concentrations 1 and 2.5 at. % were studied by high-resolution (up to 0.05 cm-1) Fourier spectroscopy, including in magnetic field parallel to the trigonal c axis of the crystal. The g factors of several crystal-field doublets of Pr3+ were determined. The crystal-field calculations performed using the exchange-charge model and high-resolution spectroscopy data allowed us to obtain a physically reasonable set of crystal-field parameters. The observed splitting of a number of doublets in zero magnetic field is explained by the presence of random lattice deformations. Simulation of the profiles of observed deformational doublets was carried out taking into account both hyperfine and electron-deformation interactions. The width of the distribution function of random strains was estimated. The main sources of random strains in YAl3(BO3)4:Pr3+ crystals are discussed.
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All authors B. Z. Malkin, T. A. Igolkina, K. N. Boldyrev, A. Baraldi, E. P. Chukalina, M. N. Popova

Materials Science

From Symmetry to Stability: Structural and Electronic Transformation in Cs$_2$KInI$_6$

Mohammad Bakhsh, Victor Trinquet, Rogério Almeida Gouvêa et al. (+2 more)

Cs$_2$KInI$_6$ is a promising lead-free halide double perovskite with a calculated direct band gap of 1.94 eV, ideal for solar cell applications. Our first-principles calculations reveal that its cubic phase (Fm$\bar{3}$m) is dynamically unstable. Using an accelerated machine learning approach, we identify 42 dynamically stable structures and further validate these findings using first-principles calculations on 11 of these. The most stable phase has Cmc$2_1$ symmetry with 20 atoms/unit cell. It lies 13 meV/atom above the convex hull but lacks octahedral cation coordination. The most stable perovskite-like structure has P$\bar{3}$ symmetry with 10 atoms/unit cell and low octahedral connectivity. Structure-property trade-offs are highlighted, with calculated distortions generally widening the band gap, shifting it from direct to indirect, and flattening the band edges. This work showcases the synergy of genetic algorithms, machine-learned potentials, and first-principles validation for discovering stable, complex materials.
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All authors Mohammad Bakhsh, Victor Trinquet, Rogério Almeida Gouvêa, Gian-Marco Rignanese, Samuel Poncé

Materials Science

The prospects of nonthermal magnetization switching in near-compensated rare earth iron garnets

N. I. Gribova, D. O. Ignatyeva, N. A. Gusev et al. (+2 more)

Ultrafast spin dynamics in a magnetically compensated rare earth iron garnet film driven by femtosecond optical pulses through the inverse Faraday effect is theoretically investigated. Numerical simulations based on the equations of motion for the Néel vector reveal the temporal evolution of the system and its trajectories in the effective potential landscape tuned by external field and temperature. The results demonstrate a clear threshold behavior: weak pulses induce only oscillations around the initial equilibrium state, while a stronger excitation results in a deterministic magnetization switching. The switching threshold is determined by the magnetic state of the sample on its phase diagramme as well as on the laser pulse helicity. This mechanism demonstrates a non-thermal and even non-absorptive pathway towards optomagnonic logic and memory devices.
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All authors N. I. Gribova, D. O. Ignatyeva, N. A. Gusev, A. K. Zvezdin, V. I. Belotelov

Materials Science

Damage accumulation induced metal-insulator transition through ion implantation of ScN thin films

Charlotte Poterie, Marc Marteau, Per Eklund et al. (+3 more)

Ion implantation is a powerful approach for tuning the electrical properties of materials through controlled doping and defect engineering, with applications in thermoelectrics and microelectronics. Scandium nitride (ScN) is particularly sensitive to irradiation-induced disorder, with transport properties spanning several orders of magnitude and multiple conduction mechanisms involved. In this study, we investigate the evolution of electrical transport in epitaxial ScN thin films undergoing accumulated irradiation damage at an initial defect state. A phenomenological damage-accumulation model was successfully combined with temperature dependent resistivity and Hall effect measurements to elucidate the impact of defect buildup on electrical transport and to provide physically grounded, quantitative insight into the nature and accumulation of irradiation-induced defects. It reveals two distinct defects-generation regimes of electrically active defects. At low doses, direct-impact damage produces stable and isolated acceptor-type complex defects, (VSc-X) with VSc a scandium vacancy and X denoting residual impurities, leading to a gradual increase in resistivity. At higher doses, defect accumulation dominates through a multi-hit process, giving rise to point-defect buildup and carrier localization, resulting in hopping-dominated transport. This localized regime is thermally unstable and recovers upon low-temperature annealing. We further demonstrate that the residual defect landscape strongly influences both the critical dose for the metal-insulator transition and the localization strength: films grown on Al2O3 exhibit an earlier transition and weaker localization than those grown on MgO. These results highlight ion implantation as an effective route for engineering disorder-induced localization in ScN, with the initial film quality playing a decisive role.
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All authors Charlotte Poterie, Marc Marteau, Per Eklund, Thierry Cabioch, Jean-Francois Barbot, Arnaud le Febvrier

Materials Science

Optimized Photoemission from Organic Molecules in 2D Layered Halide Perovskites

Muhammad S. Muhammad, Dilruba A. Popy, Hamza Shoukat et al. (+12 more)

In recent years, hybrid organic-inorganic metal halides have been at the forefront of materials research. Typically, the functional (e.g., optoelectronic) properties of hybrid halides are derived from the inorganic structural part, whereas the organic structural units can add extra advantages in terms of stability, rigidity, and processability. Here, we report the design, synthesis, and characterization of two new hybrid materials in which the outstanding photophysical properties originate from the organic structural part. The new compounds, (C15H16N)2CdCl4 and ((Br)C15H15N)2CdCl4, have 2D layered Ruddlesden-Poppertype perovskite structures. These hybrids are blue-white light emitters just like their corresponding pure organic salts, but with much improved emission efficiencies. Optical spectroscopy and density functional theory (DFT) studies confirm that photoemission comes from the trans-stilbene organic cations. The photoluminescence quantum yield (PLQY) values of these new materials are among the highest known, 50.83 % and 26.60 % for (C15H16N)2CdCl4 and ((Br)C15H15N)2CdCl4, respectively. This is up to a 5-fold increase as compared to the light emission efficiency of the precursor salt C15H16NCl (PLQY of 10.33 %). Alongside their outstanding optical properties, their environmental and thermal stability allow their consideration for potential practical applications such as radiation detection. This work shows that hybrid metal halides can be compositionally and structurally engineered to have highly efficient photoemission originating from the organic components for fast scintillation applications.
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All authors Muhammad S. Muhammad, Dilruba A. Popy, Hamza Shoukat, John M. Lane, Neeraj Rai, Vojtech Vanecek, Zdeneek Remes, Romana Kucerkova, Vladimir Babin, Chenjia Mi, Yitong Dong, Mark D. Smith, Novruz G. Akhmedov, Daniel T. Glatzhofer, Bayram Saparov

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

Investigation of the Electronic Structure and Spin-State Crossover in LaCoO3 Using Photoemission Spectroscopy

Sayari Ghatak, Abhishek Das, Andrei Gloskovskii et al. (+1 more)

Photoemission spectroscopy is a powerful technique for studying electronic structure and spin-state transitions, as it reveals changes in the orbital configuration accompanying a spin-state crossover. In this report, we combine excitation-energy-, temperature-, and geometry-dependent photoemission measurements to probe the electronic structure of LaCoO3 across its thermally driven spin-state transition. By systematically comparing valence-band spectra across a wide photon-energy window - from surface-sensitive soft x-ray photoemission spectroscopy (SXPS) to bulk-sensitive hard x-ray photoemission spectroscopy (HAXPES) - we identify the Co 3d-derived feature (A) along with the O 2p-dominated features (B and C), and explain their relative evolution in terms of photon-energy-dependent photo-ionization cross-section ratios. The thermally induced spin-state crossover is demonstrated using temperature-dependent SXPS valence-band spectra, which show a progressive suppression of the feature A with heating. Geometry-dependent HAXPES measurements further clarify how the signature of the spin-state transition in LaCoO3 is intricately linked to the orbital-selective response of the t2g and eg states. Additionally, angular-dependent photo-ionization cross-section analysis provides a consistent description of the polarization dependence observed in HAXPES. Finally, configuration-interaction analysis of the Co 2p core-level spectra reveals that LaCoO3 evolves from a predominantly low-spin ground state at low temperature to a mixed low-spin/high-spin configuration at elevated temperatures, with the high-spin fraction reaching about 30 percent at 400 K. The temperature evolution of the core-level line shape thus establishes Co 2p photoemission as a sensitive quantitative probe of spin-state transitions in LaCoO3.
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All authors Sayari Ghatak, Abhishek Das, Andrei Gloskovskii, Dinesh Topwal

Materials Science

Bridgman method grown $Cs_2Li_3I_5$: an inter-alkali metal scintillator with high lithium content

Katerina Krehlikova, Vojtech Vanecek, Robert Kral et al. (+5 more)

In this study, we report on the growth of ternary caesium lithium iodide ($Cs_2Li_3I_5$, CLI) bulk crystals, both undoped and doped with thallium (Tl) and indium (In), using the miniaturised vertical Bridgman method (mVB). X-ray Powder Diffraction (XRPD) confirmed the presence of the ternary CLI phase in all three crystals, with CLI : In appearing homogeneous structure-wise throughout the entire ingot. Measurements of radioluminescence (RL), photoluminescence emission (PL), photoluminescence excitation (PLE) spectra, and photoluminescence decay kinetics (PL decay) demonstrated that the primary luminescence centers originate from the matrix itself. When doped with thallium, the efficiency of the luminescence was significantly increased. Furthermore, CLI : Tl and CLI : In crystals exhibited emission spectra similar to those of their doped caesium iodide counterparts, CsI : Tl and CsI : In, respectively. The main component of the PL decay was 523 ns, 557 ns, and 554 ns for the undoped, Tl+-doped, and In+-doped crystals, respectively. It is worthy of note that only CLI : Tl exhibited a single exponential decay. Differential Scanning Calorimetry (DSC) measurements revealed two endothermic peaks corresponding to the eutectic and liquidus temperature for CLI and CLI : In, indicating that this ternary compound has a congruent melting behaviour. Finally, the melting point of CLI was estimated to be approximately 220 {\deg}C.
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All authors Katerina Krehlikova, Vojtech Vanecek, Robert Kral, Romana Kucerkova, Petra Zemenova, Jan Rohl{\i}cek, Petr Prusa, Katerina Rubesova

Materials Science

In-depth study of spectroscopic properties of new $Pr^{3+}$-ion doped low-phonon sesquisulfide $Lu_2S_3$ material for mid-IR laser sources

Martin Fibrich, Jan Sulc, Lubomír Havlak et al. (+6 more)

$Lu_2S_3$ material from the family of sesquisulfide hosts appears to be a promising low-phonon material for use as a laser gain medium allowing for a laser emission over a broad spectral range from ultraviolet to far mid-infrared wavelengths. In this paper, a praseodymium-ion doped $Lu_2S_3$ single crystal grown by micro-pulling down technique is investigated with focus on its spectroscopic properties. The Raman, excitation, and luminescence spectra are presented. By selective excitation of the main energy levels of $Pr^{3+}$-ions, 26 luminescence transitions of the $Pr:Lu_2S_3$ crystal spanning the 0.49 to 5.5 $\mu$m wavelength range have been identified. The correct assignment of the observed luminescence spectra to the respective $Pr^{3+}$ energy-level transitions was confirmed by the calculation of the optimized squared reduced-matrix elements for the tensor operators U(k) and L + gS.
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All authors Martin Fibrich, Jan Sulc, Lubomír Havlak, Vítezslav Jarý, Robert Kral, Vojtech Vanecek, David Vyhlidal, Helena Jelinkova, Martin Nikl

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

Physical properties of RhGe and CoGe single crystals synthesized under high pressure

Shangjie Tian, Xiangjiang Dong, Bowen Zhang et al. (+4 more)

Chiral topological semimetals hosting multifold fermions and exotic surface states represent a frontier in topological materials research. Among them, noncentrosymmetric cubic B20 compounds-notably transition-metal silicides and germanides-offer a unique platform for realizing symmetry-protected topological phases and unconventional optoelectronic responses. Here, we report the physical properties of RhGe and CoGe single crystals with B20 structure in detail. Transport measurements reveal metallic behavior with characteristic Fermi-liquid scaling at low temperatures, while magnetization results confirm paramagnetism in both compounds. In addition, both of materials exhibit low carrier concentrations with small electronic specific heat coefficient, indicating their semimetal feature with weak electronic correlations. Such high-quality CoGe and RhGe single crystals provide a material platform to explore the evolution of multifold fermions and the instability of helicoid-arc surface states with spin-orbit coupling and surface environment in B20 material systems.
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All authors Shangjie Tian, Xiangjiang Dong, Bowen Zhang, Zhijun Tu, Runze Yu, Hechang Lei, Shouguo Wang

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

Chirality Driven Ratchet Currents in Two-Dimensional Tellurene with an Asymmetric Grating

M. D. Moldavskaya, L. E. Golub, Chang Niu et al. (+2 more)

The emergence of the terahertz (THz) ratchet effect is a rapidly expanding field of research that utilizes broken spatial symmetry in low-dimensional materials to rectify alternating current (AC) induced by THz fields into direct current (DC). This mechanism is highly promising for next-generation, room-temperature terahertz applications, particularly in high-speed, sensitive detection and imaging. In this work, we explore a ratchet effect generated in two dimensional tellurene, a novel promising semiconductor material consisting of helical atomic chains, creating a structure with inherent chirality. As a key result, the DC circular ratchet current flowing in the chiral axis direction $c$ is determined by the helicity of the radiation and can be reversed by switching the helicity from right to left handed. The circular ratchet effect excited by THz laser radiation is demonstrated for room temperature. The effect is demonstrated at various gate voltages when the Fermi level lies in vicinity of the Weyl point in the conduction band, in the band gap, and in the valence band with almost parabolic energy dispersion. The results are described by the developed microscopic theory based on the Boltzmann kinetic equation approach.
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All authors M. D. Moldavskaya, L. E. Golub, Chang Niu, Peide D. Ye, S. D. Ganichev

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

Resonant absorption and linear photovoltaic effect in ferroelectric moiré heterostructures

V. V. Enaldiev and Z. Z. Alisultanov

Twisted bilayers, featuring interfacial ferroelectricity in the form of array of polar domains, combined with incommensurate two-dimensional layers in a single van der Waals heterostructures allows for generation of purely electrostatic moiré superlattice potentials in the latter. We study electronic and optoelectronic properties of such heterostructures composed of graphene stacked with the twisted ferroelectric bilayers and show that doping of graphene substantially affects mini-band structures because of screening of free carriers. We demonstrate that formation of van Hove singularities in density of states modifies linear and second-order responses of the structures leading to resonant absorption and linear photovoltaic effect, respectively. The latter is generated solely by a shift photocurrent, arising only with account of virtual optical transitions, whereas an injection photocurrent is forbidden by symmetry.
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