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Physics Condensed Matter Materials Science

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Thursday 18 December

Materials Science

On-demand phase-field modeling: Three-dimensional Landau energy for HfO2 through machine learning

Yusuke Tamura, Kairi Masuda and Yu Kumagai

The unexpected emergence of ferroelectricity in HfO2 at reduced dimensions has attracted considerable attention, as it provides a pathway toward the realization of ultrasmall ferroelectric devices. Ab initio calculations suggest that this effect arises from a unique mode coupling, in which an antipolar displacement mode stabilizes a robust polar distortion. Based on these insights, Landau-Devonshire energy models have been proposed using such lattice modes as order parameters. However, most existing models are limited to a simplified one-dimensional model because of the computational cost of ab initio calculations and the limitations of conventional Landau polynomials. Here, we constructed a three-dimensional Landau-Devonshire potential for HfO2 by employing the tetragonal, antipolar, and polar modes as coupled order parameters, based on the latest machine-learning technologies. We generated a large-scale dataset of energies over a three-dimensional structural space, with the computational cost drastically reduced through the use of machine-learning interatomic potentials, and trained a multilayer perceptron (MLP) to learn the relationship between the order parameters and the energy. The energy predicted by the MLP successfully captures the characteristic coupling behavior whereby the antipolar modes induce the polar mode. Furthermore, by extending this MLP-based Landau potential to a position-dependent functional, that is, to a phase-field modeling framework, we revealed that the polarization magnitude in thin films decreases compared with the bulk state, while the critical strain required for the onset of spontaneous polarization increases due to surface effects. This study presents a new framework for the on-demand construction of Landau energy and phase-field modeling using the latest machine-learning techniques, enabling multiscale analysis of complex ferroelectric phenomena.
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Materials Science · Cross-listed from cond-mat.str-el

Tunable Topological Phases in an Organic One-Dimensional Mott Chain: Odd-Haldane (S = 1/2) and Haldane (S = 1)

Khalid N. Anindya and Hong Guo

Establishing symmetry-protected topological (SPT) phases with interactions in chemically realistic systems remains an open challenge. We show that a single, synthetically plausible organic one-dimensional chain, tunable via chemical modification of its radical sites, hosts two such phases: an odd-Haldane phase of a dimerized $S=\tfrac{1}{2}$ Heisenberg chain and a Haldane phase of an $S=1$ chain realized when Hund coupling locks two $S=\tfrac{1}{2}$ spins per monomer into $S=1$. Density-functional theory places the active manifold deep in the Mott regime ($U/t\!\approx\!126$), justifying a spin-only Heisenberg description; a compact $(t,U)\!\to\!J$ mapping then fixes exchange couplings. Exact diagonalization and DMRG reveal a consistent SPT fingerprint across both phases, including a quantized many-body Zak phase, even-degenerate entanglement spectrum, protected edge spins, and characteristic triplon/Haldane features in $S^{+-}(q,\omega)$. Our results identify a chemically programmable molecular platform for interacting SPT physics in one dimension and suggest concrete spectroscopic routes to organic Haldane spin chains for nanoscale quantum devices.
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Materials Science

In situ XRD Study of Strain Evolution in AlGaN/GaN HEMT at High Temperatures up to 1000 {\deg}C

Botong Li, Bobby G. Duersch, Hunter Ellis et al. (+6 more)

The thermal stability and structural evolution of a GaN high-electron-mobility transistor (HEMT) heterostructure grown on a Si (111) substrate were investigated using in situ high-temperature X-ray diffraction (HT-XRD), reciprocal space mapping (RSM), Raman spectroscopy, and rocking-curve (RC) analysis at varying temperatures. The heterostructure, consisting of a p-GaN cap, an AlGaN barrier, and a GaN channel supported by two AlGaN/AlGaN superlattice (SL) buffer layers, maintained clear and periodic satellite peaks up to a temperature of 1000 deg C, confirming excellent structural integrity. Symmetric and asymmetric RSM results reveal that both the Si and GaN diffraction peaks shift to lower angles with increasing temperature, consistent with thermal expansion, and show no significant broadening or relaxation throughout the heating process. The c-lattice constant follows the theoretical expansion predicted by the multi-frequency Einstein model, whereas the a-lattice expansion is slower due to in-plane strain constraints imposed by the underlying Si substrate and buffer layers. Rapid lattice contraction during the fast-cooling stage induces a residual compressive strain of approximately 0.3 percent in the GaN channel after cooling. Raman spectra further confirm this strain state through a blue shift of approximately 1.5 cm-1 of the GaN E2 (high) phonon mode, corresponding to an in-plane strain of about 0.2 percent. Rocking-curve analysis reveals an increase in both screw and edge dislocation densities by 28 percent and 12 percent, respectively. These results collectively demonstrate that the GaN HEMT heterostructure exhibits robust crystalline stability up to 1000 deg C, with only minor strain redistribution and limited dislocation activity, providing experimental evidence for GaN device applications under high-temperature conditions.
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All authors Botong Li, Bobby G. Duersch, Hunter Ellis, Imteaz Rahaman, Aidan Belanger, Zlatan Aksamija, Brian Roy Van Devener, Kathy Anderson, Kai Fu

Materials Science

Artificial Intelligence-Enabled Holistic Design of Catalysts Tailored for Semiconducting Carbon Nanotube Growth

Liu Qian, Yue Li, Ying Xie et al. (+6 more)

Catalyst design is crucial for materials synthesis, especially for complex reaction networks. Strategies like collaborative catalytic systems and multifunctional catalysts are effective but face challenges at the nanoscale. Carbon nanotube synthesis contains complicated nanoscale catalytic reactions, thus achieving high-density, high-quality semiconducting CNTs demands innovative catalyst design. In this work, we present a holistic framework integrating machine learning into traditional catalyst design for semiconducting CNT synthesis. It combines knowledge-based insights with data-driven techniques. Three key components, including open-access electronic structure databases for precise physicochemical descriptors, pre-trained natural language processing-based embedding model for higher-level abstractions, and physical - driven predictive models based on experiment data, are utilized. Through this framework, a new method for selective semiconducting CNT synthesis via catalyst - mediated electron injection, tuned by light during growth, is proposed. 54 candidate catalysts are screened, and three with high potential are identified. High-throughput experiments validate the predictions, with semiconducting selectivity exceeding 91% and the FeTiO3 catalyst reaching 98.6%. This approach not only addresses semiconducting CNT synthesis but also offers a generalizable methodology for global catalyst design and nanomaterials synthesis, advancing materials science in precise control.
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All authors Liu Qian, Yue Li, Ying Xie, Jian Zhang, Pai Li, Yue Yu, Zhe Liu, Feng Ding, Jin Zhang

Materials Science

Machine Learning Enabled Graph Analysis of Particulate Composites: Application to Solid-state Battery Cathodes

Zebin Li, Shimao Deng, Yijin Liu et al. (+1 more)

Particulate composites underpin many solid-state chemical and electrochemical systems, where microstructural features such as multiphase boundaries and inter-particle connections strongly influence system performance. Advances in X-ray microscopy enable capturing large-scale, multimodal images of these complex microstructures with an unprecedentedly high throughput. However, harnessing these datasets to discover new physical insights and guide microstructure optimization remains a major challenge. Here, we develop a machine learning (ML) enabled framework that enables automated transformation of experimental multimodal X-ray images of multiphase particulate composites into scalable, topology-aware graphs for extracting physical insights and establishing local microstructure-property relationships at both the particle and network level. Using the multiphase particulate cathode of solid-state lithium batteries as an example, our ML-enabled graph analysis corroborates the critical role of triple phase junctions and concurrent ion/electron conduction channels in realizing desirable local electrochemical activity. Our work establishes graph-based microstructure representation as a powerful paradigm for bridging multimodal experimental imaging and functional understanding, and facilitating microstructure-aware data-driven materials design in a broad range of particulate composites.
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All authors Zebin Li, Shimao Deng, Yijin Liu, Jia-Mian Hu

Materials Science

Tuning Carrier Type and Density in Highly Conductive and Infrared-Transparent (Bi1-xSbx)2Te3 films

Xiangren Zeng, Shenjin Zhang, Zhiheng Li et al. (+7 more)

Infrared transparent conductors have long been sought due to their broad optoelectronic applications in the infrared wavelength range. However, the search for ideal materials has been limited by the inherent trade-off between electrical conductance and optical transmittance. Band engineering offers an effective approach to modulate carrier type and density, enabling concurrent tuning of both conductance and transmittance. In this work, we present a band engineering strategy that enables effective tuning of both infrared transmittance and electrical conductance in topological insulator (Bi1-xSbx)2Te3, bridging the gap and paving the way for applying topological insulators to infrared photoelectric devices. More importantly, with the combination of high carrier mobility and a large optical dielectric constant as suggested by previous report, Sb2Te3 achieves a high electrical conductance (~1000 S/cm) and outstanding infrared transmittance (92.3%) in the wavelength range of 8~13 um, demonstrating strong potential as an infrared transparent conductor. Our findings reveal that concurrent enhancement of both carrier mobility and optical dielectric constant is key to overcoming the conductance-transmittance trade-off. This work provides valuable insight for the exploration of high-performance infrared transparent conducting materials.
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All authors Xiangren Zeng, Shenjin Zhang, Zhiheng Li, Weiyue Ma, Renjie Xie, Yanwei Cao, Fengguang Liu, Fengfeng Zhang, Haichao Zhao, Xiong Yao

Materials Science

Why planar cracks fragment into echelon cracks

Olivia Ward and Aditya Kumar

Predicting the path and shape of growing cracks is fundamental to understanding of fracture. Under out-of-plane shear loading, an initially planar crack may spontaneously fragment into multiple cracks, forming a striking echelon crack pattern. Explaining this crack morphogenesis in brittle materials has been a long-standing open problem essential to developing a complete theory of crack growth. Here, through comparison with classical experiments, we show that a strength-constrained minimization of the sum of elastic and surface energies explains echelon crack formation. Results are presented for both soft and hard materials, confirming the model's general applicability to any brittle material. As a corollary, we show that, contrary to prevailing views, a purely energetic minimization model is insufficient to predict the growth of large cracks. We identify two key non-dimensional parameters governing crack fragmentation and orientation, and demonstrate that these reconcile the various energy-based and stress-based empirical criteria proposed in the literature for crack path.
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Materials Science

Confinement-induced Ultrafast Conductivity in 2D Perovskites resolved by correlative Terahertz-NIR Spectroscopy

Lion Krüger, Fabian Brütting, Michael Baumann et al. (+5 more)

Quantum wells made of two-dimensional organic-inorganic hybrid perovskites (2D-PKs) offer a high degree of flexibility in tailoring optoelectronic properties through carrier confinement and functional interlayers. Compared to their 3D counterparts, 2D-PKs exhibit tunable photoluminescence, excitonic binding at room temperature and enhanced structural stability. However, the dynamics of photo-induced charge carriers and their transport properties are highly intertwined due to the interplay of diverse excitation species, charge carrier cooling, transport, and radiative and non-radiative recombination. In this study, we employ optical-pump terahertz-probe spectroscopy (OPTP) to analyze the local conductivity dynamics of 2D and 3D methylammonium lead iodide (MAPI) perovskites at timescales down to picoseconds. Remarkably, we observe an intensity-dependent, 2D-specific buildup of an ultrafast, few-picosecond decay in local conductivity. By combining OPTP with transient absorption (TA) and picosecond time-resolved photoluminescence (TRPL), we demonstrate the disentanglement of photoconductivity and carrier population. This allows us to attribute the 2D-specific ultrafast THz response to delayed hot-carrier cooling and subsequent exciton formation, which effectively reduces the free-carrier conductivity. This intensity-dependent, ultrafast THz response is a signature of the recently identified hot-carrier bottleneck in 3D MAPI, and this effect manifests itself in a unique form in the 2D material. These results encourage further investigations on the impact of functional organic interlayers and provide insights into designing tunable carrier responses for ultrafast devices via adapted heterostructures and confinement.
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All authors Lion Krüger, Fabian Brütting, Michael Baumann, Moritz B. Heindl, Maximilian Spies, Anna Köhler, Alexander JC Kühne, Georg Herink

Wednesday 17 December

Materials Science · Cross-listed from cs.LG

Leakage-Aware Bandgap Prediction on the JARVIS-DFT Dataset: A Phase-Wise Feature Analysis

Gaurav Kumar Sharma

In this study, we perform a systematic analysis of the JARVIS-DFT bandgap dataset and identify and remove descriptors that may inadvertently encode band-structure information, such as effective masses. This process yields a curated, leakage-controlled subset of 2280 materials. Using this dataset, a three-phase modeling framework is implemented that incrementally incorporates basic physical descriptors, engineered features, and compositional attributes. The results show that tree-based models achieve R2 values of approximately 0.88 to 0.90 across all phases, indicating that expanding the descriptor space does not substantially improve predictive accuracy when leakage is controlled. SHAP analysis consistently identifies the dielectric tensor components as the dominant contributors. This work provides a curated dataset and baseline performance metrics for future leakage-aware bandgap prediction studies.
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Materials Science

Pulse-Mode Operation and Reliability of BEOL-Compatible Ferroelectric Non-Volatile Capacitive Memories with Amorphous Oxide Semiconductor Channels

Junmo Lee, Chengyang Zhang, Tae-Hyeon Kim et al. (+2 more)

Non-volatile capacitive memories (nvCAPs) exhibiting AC small-signal capacitance on/off ratio (Con/Coff) with non-destructive read have emerged as a promising device for next-generation memory paradigms. Recently, BEOL-compatible ferroelectric nvCAPs with an amorphous oxide semiconductor channel have been reported, suggesting the possibility of monolithic 3D integration of nvCAPs on top of CMOS. So far, the characterization studies on oxide-channel ferroelectric nvCAPs have been done using dual DC sweep C-V measurements which are typically performed over a time scale of a few seconds. However, non-volatile memory arrays typically require nvCAPs to operate under pulse-mode. It is thus crucial to advance understanding of the behavior of oxide-channel ferroelectric nvCAPs under pulse-mode operation, governed by the unique interplay between ferroelectric layer and oxide channel physics. In this study, we provide a systematic study of the pulse-mode operation of ferroelectric nvCAPs with an amorphous oxide semiconductor channel, including its pulse-based write characteristics and reliability characteristics. We examine overlap area, wake-up and pulse-width dependent Con and Coff writing characteristics under pulse-mode. Further, we suggest the importance of optimizing ferroelectric depolarization for Con retention, while reducing read-after-delay for Coff retention under pulse-mode. Lastly, non-destructive read operation for >10^9 read stress cycles at |Vread|=1V is demonstrated.
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All authors Junmo Lee, Chengyang Zhang, Tae-Hyeon Kim, Suman Datta, Shimeng Yu

Materials Science

Transient Surface Oxides Form on Pt(111) - But Vanish During Ammonia Oxidation

David Simonne, Allesandro Coati, Alina Vlad et al. (+4 more)

Ammonia oxidation on platinum catalysts is pivotal for industrial nitric acid production and environmental abatement, yet the role of surface oxides in this process remains debated. Using operando surface X-ray diffraction (SXRD), crystal truncation rod (CTR) analysis, and near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS), we reveal that Pt(111) does not form stable surface oxides under ammonia oxidation conditions. Instead, transient hexagonal monolayers and a Pt(111)-(8x8) superstructure emerge under oxygen-rich atmospheres and above the catalyst light-off temperature, but vanish upon ammonia exposure. Real-time mass spectrometry and NAP-XPS demonstrate that the reaction proceeds via a Langmuir-Hinshelwood mechanism, where adsorbed NHx and O species availability dictate selectivity toward NO or N2. Reducing the oxygen pressure by an order of magnitude slows the kinetics of oxide growth, only detected after 24 hr, and facilitated by transient and precursor structures.
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All authors David Simonne, Allesandro Coati, Alina Vlad, Yves Garreau, Benjamin Voisin, Marie-Ingrid Richard, Andrea Resta

Materials Science

Magneto-optical Kerr effect in pump-probe setups

Amir Eskandari-asl and Adolfo Avella

We develop a general theoretical framework for computing the time-resolved magneto-optical Kerr effect in ultrafast pump-probe setups, formulated within the Dynamical Projective Operatorial Approach (DPOA) and its application to the generalized linear-response theory for pumped systems. Furthermore, we exploit this formalism to express the post-pump optical conductivity and consequently the Kerr rotation in terms of the time-evolved single-particle density matrix (SPDM), providing a transparent and computationally efficient description of photo-excited multi-band systems. This extension, in addition to its lower computational cost, has the advantage of allowing the inclusion of phenomenological damping. We illustrate the formalism using both (i) a two-band tight-binding model, which captures the essential physics of ultrafast spin-charge dynamics and the Kerr rotation, and (ii) weakly spin-polarized germanium, as a realistic playground with a complex band structure. The results demonstrate that, by exploiting DPOA and/or its SPDM extension, one can reliably reproduce both the short-time features under the pump-pulse envelope and the long-time dynamics after excitation, offering a versatile framework for analyzing time-resolved magneto-optical Kerr effect experiments in complex materials. Moreover, this analysis clearly shows that the Kerr rotation can be used to deduce experimentally the relevant n-photon resonances for a given specific material.
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Materials Science

Visualizing metal-mediated nucleation and growth of GaN

Abby Liu, Zhucong Xi, Xiaobo Chen et al. (+6 more)

Understanding the atomic-scale mechanisms governing metal-mediated nucleation and growth of gallium nitride (GaN) and related alloys is critical for tailoring their structural and functional properties in advanced electronic, optoelectronic, and quantum devices. Using real-time environmental transmission electron microscopy (E-TEM) in conjunction with Gibbs free energy calculations, we elucidate the distinct processes of GaN nucleation and growth from Ga droplet arrays with and without GaN pre-nuclei. For the lowest temperatures, although GaN nucleation at Ga droplet arrays is not observed, GaN growth occurs preferentially at pre-existing GaN nuclei, presumably due to the reduced Gibbs free energy for NH3 decomposition at Ga/GaN interfaces. For intermediate to high temperatures, E-TEM reveals nucleation and growth of GaN from Ga droplets with and without GaN nuclei, with enhanced crystallinity for the GaN nuclei, due to epitaxial templating. These results highlight the critical role of the Ga/GaN interface in facilitating NH3 decomposition and GaN growth, offering fundamental insights into metal-mediated nucleation and growth of GaN and related materials.
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All authors Abby Liu, Zhucong Xi, Xiaobo Chen, Catherine Huang, Meng Li, Judith C. Yang, Liang Qi, Dmitri N. Zakharov, Rachel S. Goldman

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

Exploring Overlapping Mechanisms of Dynamic Nuclear Polarization in Type 1b HPHT Diamond

Brendan C. Sheehan, Margaret Hubble, Daphna Shimon et al. (+1 more)

The inhomogeneous distribution of P1 centers in type 1b HPHT diamond samples allows multiple DNP mechanisms to occur within the same crystal, resulting in complex DNP spectra. At some crystal orientations, different DNP mechanisms can compete to drive hyperpolarization with different signs at the same applied microwave frequency. We perform microwave-irradiated DNP using both monochromatic and frequency-modulated microwave excitation to explore the competition between these DNP mechanisms in diamond at room temperature. We demonstrate that frequency-modulated DNP is a tool for suppressing certain DNP mechanisms while enhancing others in a single-crystal diamond sample. Frequency modulation also enables higher enhancement of the NMR signal beyond traditional monochromatic DNP under some conditions. In a powder sample, competing enhancement mechanisms can also arise from different crystallite orientations in the powder. We observe that at certain microwave frequencies the DNP signal changes sign during the polarization build-up, even with monochromatic microwave irradiation. We do not observe this phenomenon in any single-crystal spectrum. We discuss both methods of investigating competing mechanisms of DNP as a means of selectively enhancing different DNP mechanisms driving $^{13}$C NMR signal enhancement.
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All authors Brendan C. Sheehan, Margaret Hubble, Daphna Shimon, Chandrasekhar Ramanathan

Materials Science

Active Learning Discovery of High Temperature Oxidation Resistant Refractory Complex Concentrated Alloys

Akhil Bejjipurapu, Sharmila Karumuri, Joseph C. Flanagan et al. (+5 more)

Refractory complex concentrated alloys (RCCAs) are of significant interest for advanced high-temperature applications, owing to their broad compositional range and potential for attractive mechanical properties and oxidation resistance. However, their compositional complexity poses significant challenges to conventional alloy discovery methodologies. In this study, an active learning framework is introduced that integrates Gaussian process regression with Bayesian global optimization to accelerate identification of oxidation-resistant RCCAs. Focusing on aluminum-containing quaternary systems, alloy and oxide descriptors were used to predict oxidation performance at 1000$^\circ$C. Beginning with a dataset of 81 experimentally validated RCCAs, this framework was used to iteratively select alloy batches (five alloys per batch) with optimization based on a balance between exploration and exploitation to minimize associated experimental costs. After six iterations, two alloys were identified (nominal Al$_{30}$Mo$_5$Ti$_{15}$Cr$_{50}$ and Al$_{40}$Mo$_5$Ti$_{30}$Cr$_{25}$) that exhibited specific mass gains less than 1 mg/cm$^2$ at 1000$^\circ$C in air. Both of these alloys formed adherent external $\alpha$-Al$_2$O$_3$ scales and exhibited parabolic oxidation kinetics consistent with diffusion-limited scale growth. Furthermore, our multiobjective analysis demonstrates that these alloys simultaneously achieve high specific hardness ($>0.12$ HV$_{0.5}$m$^3$/kg) and thermal expansion compatibility with thermal barrier coating systems, positioning them as promising bond coat candidates. This work underscores the efficacy of active learning in traversing complex compositional landscapes, and offers a scalable strategy for the development of advanced materials suitable for extreme environments.
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All authors Akhil Bejjipurapu, Sharmila Karumuri, Joseph C. Flanagan, Victoria Tucker, Ilias Bilionis, Alejandro Strachan, Kenneth H. Sandhage, Michael S. Titus

Materials Science

Machine-Learned Interatomic Potential for Predictive Simulation of MoS2 Epitaxy

Emir Bilgili, Nicholas Taormina, Richard Hennig et al. (+2 more)

A machine-learned interatomic potential (MLIP) for multilayer MoS2 was developed using the ultra-fast force field (UF3) framework. The UF3 MLIP reproduces key properties in strong agreement with DFT including lattice constants, interlayer binding energies, and phase-stability. Furthermore, the potential reasonably captures the phonon spectra and the highly anisotropic elastic tensor across monolayer (1H) and bulk (2H, 3R) MoS2 phases. Critically, defect and edge formation energies are captured with high fidelity, exhibiting a strong correlation with DFT (R^2 = 0.91) across ten defective monolayers and reproducing the relative difference between the free energies of zigzag and armchair edges within 5% of DFT. Non-equilibrium molecular dynamics simulations reveal layered homoepitaxial growth consistent with experimental observations, demonstrating the formation of van der Waals gaps between successive epilayers and triangular domains bounded by zigzag edges. The robust UF3 MLIP, which is only ~2X slower than the fastest empirical potentials, enables large-scale atomistic simulations of MoS2 epitaxial growth.
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All authors Emir Bilgili, Nicholas Taormina, Richard Hennig, Simon R. Phillpot, Youping Chen

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

Spin-dependent quasiparticle lifetimes in altermagnets

Kristoffer Leraand, Kristian Mæland and Asle Sudbø

We investigate many-body effects on the spin-split electron bands in altermagnets by computing the electron self-energy arising from interactions with magnons, phonons, and hybridized magnon-phonon modes. These interactions lead to band broadening, which can obscure the intrinsic spin-splitting in spectroscopic measurements. We consider a $d$-wave Lieb lattice altermagnet as a representative example. Our results reveal that the spin-splitting remains spectroscopically resolvable and provide theoretical estimates of lifetime effects relevant for experimental detection. For electron-magnon coupling, we find a distinct difference between spectral function broadening for up and down spins close to the Fermi surface, which is not present in the case of electron-phonon coupling. We relate it to the spin splitting of the magnon modes in altermagnets. The results, including magneto-elastic coupling, are very similar to the pure magnon case. This provides insights into quasiparticle dynamics in altermagnets and contributes to the broader understanding of many-body interactions in spin-split systems. By including the temperature dependence of the self-energies, we also quantify how thermal fluctuations influence the broadening of the electronic states.
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Materials Science

High-throughput discovery of moiré homobilayers guided by topology and energetics

Naoto Nakatsuji, Jennifer Cano and Valentin Crépel

Van der Waals heterostructures promise on-demand designer quantum phases through control of monolayer composition, stacking, twist angle, and external fields. Yet, experimental efforts have been narrowly focused, leaving much of this vast moiré landscape unexplored and potential promises unrealized. Here, we present a scalable workflow for high-throughput characterization of twisted homobilayers and apply it to $K$-valley semiconductors. Combining small-scale density functional theory with perturbation theory, we efficiently extract moiré band gaps, valley Chern numbers, magic angles, and the threshold for lattice relaxation. Beyond this rapid high-throughput characterization, we parameterize a continuum model for each material, which provides a starting point for more detailed study. Our survey delivers an actionable map for systematic exploration of correlated and topological phases in moiré homobilayers, and identifies promising new platforms: chromium-based transition metal dichalcogenides for high-temperature quantum anomalous Hall effects, transition metal nitride halides for intertwined superconducting and moiré physics, and atomically thin $\rm{III-V}$ semiconductors for room-temperature-scale moiré effects.
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Materials Science

Examination of Hydrogen Evolution Bubble Trapping in Ordered Porous 3D Printed Metal and Metal Oxide-Coated Microlattice Electrodes

Matthew Ferguson, Alex Lonergan, Christopher Kent et al. (+2 more)

Determining the nature of surface roughness and electrode pore structure on H2 bubble evolution rate and quantity, and bubble trapping under electrolytic conditions is important for quantifying useful gas production during total water splitting and hydrogen evolution reactions. Controlled electrode systems involving the design of geometry, surface area, and porosity provides options to understand trapped/redissolved gas bubble evolution and improve overall efficiency. In this study, we use vat polymerization (Vat-P) 3D-printing to create ordered microlattice electrode structures from metal and metal-oxide coated photopolymerized methacrylate-based resins. These micro-lattice structures are designed with various geometries to influence bubble traffic from gas nucleation and evolution during electrochemical HER processes. Using cyclic and linear sweep voltammetry, and chronopotentiometry, this work analyzes the response of metallized (NiO/Ni(OH)2 and Au) microlattice HER electrodes as a function of geometric structure, to gauge influence of material activity, small scale surface roughness, and the larger substrate pore network on the traffic or larger bubbles formed during HER. This work also uses broadband acoustic resonance dissolution spectroscopy (BARDS) to quantify bubble evolution and reabsorption in the electrolyte during electrolysis. The results show that coated 3D printed electrodes are robust HER electrodes, allow efficient transport of small bubbles, but significant limitations are found for larger bubble transport through ordered porous microlattice shown through model simulations and experimental measurements.
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All authors Matthew Ferguson, Alex Lonergan, Christopher Kent, Dara Fitzpatrick, Colm O'Dwyer

Materials Science

Ab initio study of mechanical and functional properties of novel CaZnC and CaZnSi half-Heusler materials

P. K. Kamlesh, U. K. Gupta, S. Verma et al. (+3 more)

This research work introduces the DFT through FP-LAPW+lo technique in WIEN2k software to obtain information about structural, thermoelectric, and optoelectronic characteristics of CaZnC and CaZnSi materials. The structural optimization was performed using PBE-GGA functional, while the rest of the characteristics were obtained with the PBE-GGA + TB-mBJ approach. The thermoelectric parameters were evaluated using BoltzTraP software. The elastic constants and other mechanical parameters were computed by utilizing the ELAST code within the WIEN2k software, while the thermodynamic characteristics were evaluated using the Gibbs2 program. The findings show a correlation between atomic composition and lattice dimensions while finding that CaZnC has a direct ($\Gamma$-$\Gamma$) band gap of $1.186$ eV, whereas CaZnSi has an indirect ($\Gamma$-$X$) band gap of $1.067$ eV. The optical studies of the compounds show potential applications for photovoltaics while the thermoelectric results find optimized power factors and figure of merit values for energy conversion performance. The elastic parameters of CaZnC and CaZnSi demonstrate material stability and brittleness. Lastly, the thermodynamic evaluations provide information about the thermal mechanism and disorder of the materials. As a result, this research work provides significant advancements in the understanding of the fundamentals of these compounds and highlights their promising applications in renewable energy technologies.
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All authors P. K. Kamlesh, U. K. Gupta, S. Verma, M. Rani, Y. Toual, A. S. Verma

Materials Science

A new multiscale modeling approach to unravel the influence of interlayer sp3 bonds on the nonlinear large-deformation and fracture behaviors of 2D carbon nanostructures under tension

Xiangyang Wang, Huibo Qi, Biao Xu et al. (+2 more)

To delve deeply into the nonlinear large-deformation and fracture behaviors of 2D carbon nanostructures (2D CNs), including bilayer graphene, diamane, and their transitional structures, this paper introduces a multiscale auxiliary nodes (MAN) method rooted in atomic structures and potentials. This approach simulates 2D CNs by constructing two virtual continuum sheets with high-order continuity. The moving least squares (MLS) approximation is employed to facilitate the transformation between atomic displacements and nodal displacements, thereby converting atomic potential energy into strain energy within the continuum model. Through iterative solutions of nonlinear stiffness equations, the equilibrium configuration of the system under specified loading conditions can be obtained. The flexibility in the density and arrangement of nodes allows for a smooth and seamless cross-scale transition from discrete atomic structures to a continuum model. Numerical simulations demonstrate that MAN method accurately predicts the nonlinear large-deformation and fracture behaviors of 2D CNs. The Young's modulus and shear modulus of diamane in both zigzag and armchair directions closely approach those of diamond and are notably higher than those of graphene. Furthermore, the quantity and distribution of interlayer sp3 bonds significantly influence the fracture behavior of 2D CNs, with strategic placement of these bonds effectively enhancing the tensile strength of the structures.
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All authors Xiangyang Wang, Huibo Qi, Biao Xu, Shichao Dai, Jiqiang Li

Materials Science

Anisotropic Band-Split Magnetism in Magnetostrictive CoFe$_2$O$_4$

Harry Lane, Guratinder Kaur, Masahiro Kawamata et al. (+9 more)

Single crystal spinel CoFe$_2$O$_4$ exhibits the largest room-temperature saturation magnetostriction among non-rare-earth compounds and a high Curie temperature ($T_c \sim 780$ K), properties that are critical to a wide range of industrial and medical applications. Neutron spectroscopy reveals a large band splitting ($\sim$ 60 meV) between two ferrimagnetic magnon branches, which is driven by site mixing between Co$^{2+}$ and Fe$^{3+}$ cations, and a significantly weaker magnetocrystalline anisotropy ($\sim$ 3 meV). Central to this behavior is the competition between extremely large mismatched molecular fields on the tetrahedral $A$-site and octahedral $B$-site sublattices and the single-ion anisotropy on the $B$-site. This creates a strong energetic anisotropy that locks the magnetic moment within each structural domain in place. As a result of these differing energy scales, switching structural domains is energetically favored over a global spin reorientation under applied magnetic fields, and this is what amplifies the magnetostrictive nature of CoFe$_2$O$_4$.
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All authors Harry Lane, Guratinder Kaur, Masahiro Kawamata, Yusuke Nambu, Lukas Keller, Russell A. Ewings, David J. Voneshen, Travis J. Williams, Helen C. Walker, Dwight Viehland, Peter M. Gehring, Chris Stock

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

Pressure-Induced Changes in Structure, Magnetic Order and Development of Superconductivity in the Ferromagnetic Topological Insulator MnBi8Te13

S. Huyan, T. Qian, L. Wang et al. (+14 more)

We report a comprehensive study of pressure-induced evolution of the magnetism and development of superconductivity (SC) in MnBi8Te13, a promising ambient pressure, ferromagnetic (FM) topological insulator candidate. By employing high-pressure electrical transport, magnetoresistance, DC magnetic susceptibility, and X-ray diffraction measurements, we construct a detailed temperature-pressure phase diagram. At ambient pressure, MnBi8Te13 exhibits FM ordering with an easy-axis along the c-axis which is progressively suppressed under pressure and replaced by an antiferromagnetic (AFM) order. Density functional theory calculations predicted an evolution from FM to a G-type AFMg2 phase near 5 GPa. Above 16.6 GPa, a bulk SC state emerges with a maximum transition temperature ~6.8 K, as confirmed by resistance and magnetic susceptibility measurements. This pressure-induced SC may co-exist with another AFM dome that shows a weak anomaly in the transport data. In contrast, our work on MnBi6Te10 shows no SC up to 40 GPa. Indeed, in contrast to MnBi8Te13, the MnBi2Te4(Bi2Te3)n compounds with n < 3, didn't exhibit SC, highlighting the crucial role of Mn concentration in stabilizing SC. The observation of pressure-induced FM-AFM-SC transitions in MnBi8Te13 not only establishes it as a rare Mn-based SC but also provides a platform to study the interplay between magnetism, SC, and potentially nontrivial band topology in correlated magnetic materials.
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All authors S. Huyan, T. Qian, L. Wang, W. Bi, F. Xue, D. Zhang, C. Hu, B. Kalkan, Y. Huang, Z. Li, A. Das, J. Schmidt, R. A. Ribeiro, T. J. Slade, N. Ni, P. C. Canfield, S. L. Bud'ko

Materials Science

Understanding the effect of drying time in process-structure-performance relationships for PM6-Y6 organic solar cells

Marc Steinberger, Maxime Siber, Hans-Joachim Egelhaaf et al. (+13 more)

Making solution-cast organic solar cells industrially available generally comes at the cost of significant performance losses compared to device prototypes manufactured under laboratory conditions. Adjusting solvent evaporation kinetics is postulated to recover efficiency. Yet, a comprehensive characterization of their effect, independently of other property-defining parameters, is lacking. Thus, the present objective is to isolate the influence of the solvent drying rate on solution-deposited organic active layer nanomorphologies and performances. To this end, a specially designed gas quenching technique is employed to fabricate PM6:Y6 donor-acceptor films under systematic variations of evaporation conditions. Using an extensive investigation protocol that combines insights from numerical simulations and experimental measurements, process-structure-performance relationships are unraveled. It is found that higher drying rates imply finer and more dispersed nanomorphologies with increased fractions of amorphous material. This enhances electric charge generation, thereby improving short-circuit current density and overall cell performance. The open-circuit voltage is also boosted under accelerated evaporation due to changes in the aggregation mode of the Y6 small molecule that induce higher effective bandgaps. The results demonstrate that the developed gas-quenching technique is a valuable tool for optimizing the performance of upscaled organic photovoltaics, as it is readily compatible with high-throughput equipment, such as roll-to-roll coating machines.
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All authors Marc Steinberger, Maxime Siber, Hans-Joachim Egelhaaf, Mingjian Wu, Irene Kraus, Johannes Will, Xianqiang Xie, Laju Bu, Jonas Graetz, Tobias Unruh, Larry Lüer, Erdmann Spiecker, Andreas Distler, Jens Harting, Christoph J. Brabec, Olivier J.J. Ronsin

Materials Science

Polaritonic Bloch's Theorem beyond the Long-Wavelength Approximation

Giovanna Bruno, Rosario Roberto Riso, Henrik Koch et al. (+1 more)

Cavity quantum electrodynamics provides a powerful tool to manipulate material properties, yet it remains a matter of debate whether and how quantized fields affect the periodicity of crystals. Here, we extend Bloch's theorem to crystals under strong light-matter coupling, revealing that polariton quasiparticles preserve lattice periodicity. We introduce a general framework to incorporate multimode cavity fields in a simple and tractable way, showing that additional modes contribute small energy corrections noticeable only at low frequencies. Within the single-photon approximation, these contributions reduce to a spatially uniform effective field in the crystal plane, providing a formal justification for the single-mode and long-wavelength approximations commonly used in molecular polaritonics. Together, these results establish a rigorous framework for describing polaritonic states in crystalline solids.
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All authors Giovanna Bruno, Rosario Roberto Riso, Henrik Koch, Enrico Ronca

Materials Science

From Complex Magnetic Ground States to Magnetocaloric Effects: A Review of Rare Earth R$_2$In Intermetallic Compounds

Anis Biswas, Ajay Kumar, Prashant Singh et al. (+1 more)

R2In (R = rare earth) intermetallics exhibit unusual magnetic and magnetocaloric properties, driven by subtle electronic effects, lattice distortions, and spin-lattice coupling. Most of these binary compounds adopt the hexagonal Ni2In-type structure at room temperature, with Eu2In and Yb2In stabilizing in the orthorhombic Co2Si-type lattice. Lighter lanthanide compounds Eu2In, Nd2In, and Pr2In undergo first-order magnetic transitions with negligible hysteresis and minimal lattice volume change and exhibit giant cryogenic magnetocaloric effects, while heavy lanthanide R2In compounds including Gd2In show second-order transitions with moderate magnetocaloric effect. No lanthanide-based R2In compound exhibits symmetry-breaking structural transition, while Y2In transforms from hexagonal to orthorhombic structure near 250 K. Secondary low-temperature transitions, including spin reorientation or antiferromagnetic ordering, further enrich the magnetic phase landscape in these compounds. Integrating theoretical descriptors such as charge-induced strain and electronic structure provides predictive insight into phase stability and magnetocaloric performance, guiding the design of rare-earth intermetallics with tunable magnetic properties for cryogenic applications
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All authors Anis Biswas, Ajay Kumar, Prashant Singh, Yaroslav Mudryk

Materials Science

First-principles simulation of spin diffusion in static solids using dynamic mean-field theory

Timo Gräßer, Götz S. Uhrig and Matthias Ernst

The dynamics of disordered nuclear spin ensembles are the subject of nuclear magnetic resonance studies. Due to the through-space long-range dipolar interaction generically many spins are involved in the time evolution, so that exact brute force calculations are impossible. The recently established spin dynamic mean-field theory (spinDMFT) represents an efficient and unbiased alternative to overcome this challenge. The approach only requires the dipolar couplings as input and the only prerequisite for its applicability is that each spin interacts with a large number of other spins. In this article, we show that spinDMFT can be used to describe spectral spin diffusion in static samples and to simulate zero-quantum line shapes which eluded an efficient quantitative simulation so far to the best of our knowledge. We perform benchmarks for two test substances that establish an excellent match with published experimental data. As spinDMFT combines low computational effort with high accuracy, we strongly suggest to use it for large-scale simulations of spin diffusion, which are important in various areas of magnetic resonance.
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Materials Science · Cross-listed from cs.AI

Evaluating Large Language Models in Scientific Discovery

Zhangde Song, Jieyu Lu, Yuanqi Du et al. (+53 more)

Large language models (LLMs) are increasingly applied to scientific research, yet prevailing science benchmarks probe decontextualized knowledge and overlook the iterative reasoning, hypothesis generation, and observation interpretation that drive scientific discovery. We introduce a scenario-grounded benchmark that evaluates LLMs across biology, chemistry, materials, and physics, where domain experts define research projects of genuine interest and decompose them into modular research scenarios from which vetted questions are sampled. The framework assesses models at two levels: (i) question-level accuracy on scenario-tied items and (ii) project-level performance, where models must propose testable hypotheses, design simulations or experiments, and interpret results. Applying this two-phase scientific discovery evaluation (SDE) framework to state-of-the-art LLMs reveals a consistent performance gap relative to general science benchmarks, diminishing return of scaling up model sizes and reasoning, and systematic weaknesses shared across top-tier models from different providers. Large performance variation in research scenarios leads to changing choices of the best performing model on scientific discovery projects evaluated, suggesting all current LLMs are distant to general scientific "superintelligence". Nevertheless, LLMs already demonstrate promise in a great variety of scientific discovery projects, including cases where constituent scenario scores are low, highlighting the role of guided exploration and serendipity in discovery. This SDE framework offers a reproducible benchmark for discovery-relevant evaluation of LLMs and charts practical paths to advance their development toward scientific discovery.
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All authors Zhangde Song, Jieyu Lu, Yuanqi Du, Botao Yu, Thomas M. Pruyn, Yue Huang, Kehan Guo, Xiuzhe Luo, Yuanhao Qu, Yi Qu, Yinkai Wang, Haorui Wang, Jeff Guo, Jingru Gan, Parshin Shojaee, Di Luo, Andres M Bran, Gen Li, Qiyuan Zhao, Shao-Xiong Lennon Luo, Yuxuan Zhang, Xiang Zou, Wanru Zhao, Yifan F. Zhang, Wucheng Zhang, Shunan Zheng, Saiyang Zhang, Sartaaj Takrim Khan, Mahyar Rajabi-Kochi, Samantha Paradi-Maropakis, Tony Baltoiu, Fengyu Xie, Tianyang Chen, Kexin Huang, Weiliang Luo, Meijing Fang, Xin Yang, Lixue Cheng, Jiajun He, Soha Hassoun, Xiangliang Zhang, Wei Wang, Chandan K. Reddy, Chao Zhang, Zhiling Zheng, Mengdi Wang, Le Cong, Carla P. Gomes, Chang-Yu Hsieh, Aditya Nandy, Philippe Schwaller, Heather J. Kulik, Haojun Jia, Huan Sun, Seyed Mohamad Moosavi, Chenru Duan

Materials Science

Fabrication of (In,Ga)N pseudo-substrates by a three-step growth protocol without ex-situ processing

Huaide Zhang, Aidan F. Campbell, Jingxuan Kang et al. (+3 more)

We fabricate (In,Ga)N pseudo-substrates with a total thickness of ~1 um grown on GaN templates using plasma-assisted molecular beam epitaxy. In a three-step process, we change growth conditions from N-rich to metal-rich in order to sequentially form a roughened GaN layer, relaxed (In,Ga)N nanostructures, and a coalesced, smooth (In,Ga)N layer. Samples are analyzed by scanning electron and atomic force microscopy, X-ray diffraction, as well as photo- and cathodoluminescence spectroscopy. Compared to a reference layer grown directly on GaN, the pseudo-substrate exhibits a higher In content (~0.3), strain relaxation degree (~80%), narrower photoluminescence linewidth, and larger area fraction of bright regions in cathodoluminescence maps, showing the benefits of the three-step growth protocol. This straightforward approach does not necessitate any ex-situ processing and could enable the scalable fabrication of (In,Ga)N pseudo-substrates for high-efficiency red-emitting (In,Ga)N devices.
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All authors Huaide Zhang, Aidan F. Campbell, Jingxuan Kang, Jonas Laehnemann, Oliver Brandt, Lutz Geelhaar

Materials Science

Exciton radiative lifetimes in hexagonal diamond Ge and Si$_x$Ge$_{1-x}$ alloys

Michele Re Fiorentin, Michele Amato and Maurizia Palummo

Recent reports of strong room-temperature photoluminescence in hexagonal diamond (2H) germanium stand in marked contrast to theoretical predictions of very weak band-edge optical transitions. Here we address radiative emission in 2H-Ge and related materials through a comprehensive investigation of their excitonic properties and radiative lifetimes, performing Bethe-Salpeter calculations on pristine and uniaxially strained 2H-Ge, 2H-Si$_x$Ge$_{1-x}$ alloys with $x=\frac{1}{6},\,\frac{1}{4},\,\frac{1}{2}$, and wurtzite GaN as a reference. Pristine 2H-Ge features sizable exciton binding energies ($\sim\!30$ meV) but extremely small dipole moments, yielding radiative lifetimes above $10^{-4}$ s. Alloying with Si reduces the lifetime by nearly two orders of magnitude, whereas a 2% uniaxial strain along the $c$ axis induces a band crossover that strongly enhances the in-plane dipole moment of the lowest-energy exciton and drives the lifetime down to the nanosecond scale. Although strained 2H-Ge approaches the radiative efficiency of GaN, its much lower exciton energy prevents a full match. These results provide the missing excitonic description of 2H-Ge and 2H-Si$_x$Ge$_{1-x}$, demonstrating that, even when excitonic effects are fully accounted for, the strong photoluminescence reported experimentally cannot originate from the ideal crystal.
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