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Physics Condensed Matter Materials Science

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Thursday 25 December

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

Proximity-Induced Spin-Orbit Torque in Graphene on a Trigonal CrSBr Monolayer

Maedeh Rassekh and Martin Gmitra

We present a first-principles and quantum transport study of proximity-induced spin-orbit torque (SOT) in graphene on a trigonal CrSBr monolayer. Density functional theory combined with nonequilibrium Green's function calculations shows that the CrSBr substrate induces spin polarization and a sizable exchange splitting in the graphene Dirac states. The resulting current-driven spin density in graphene generates a self-SOT on the Dirac electrons. The proximity-induced exchange field breaks time-reversal symmetry and gives rise to a purely odd SOT component, while the even contribution vanishes. The torque magnitude exhibits a strong angular dependence with phase shifts arising from the noncollinearity between the CrSBr magnetization and the induced magnetic moments in graphene. Monte Carlo simulations based on the calculated exchange parameters predict a Curie temperature of approximately 304 K, confirming the robustness of ferromagnetism in the trigonal CrSBr monolayer. These results identify graphene/CrSBr heterostructures as a promising platform for room-temperature two-dimensional spintronics.
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Materials Science · Cross-listed from cond-mat.soft

The Origin of Organic Magnetoresistance Using Time-Domain Magnetic Spectroscopy

Paul Ben Ishai, Roshlin Kiruba and Amos Bardea

Organic Magnetoresistance is defined as the change of resistance in an organic material, such as a conducting polymer, as a function of an imposed magnetic field. We demonstrate this effect in a Polypyrrole/ Polydimethylsiloxane complex by using a novel magnetic pulse system. The frequency spectrum of the current flowing through the sample reveals equally spaced reversed peaks in the current. We show that these peaks happen at the Lamour frequency for the dominant charge carrier of the system, namely polarons. This posits the origin Organic magnetoresistance as simple Rabi Oscillations rather than mechanisms based of bipolaron formation and singlet-triplet conversions. We directly estimate the effective polaronic mass in this complex. A semi classical theoretical approach is suggested to explain this effect as direct spin flipping in a time transient magnetic field. This is the first time that such an experimental approach has been applied in this field and the first time the effective polaronic mass has been measured in a conducting polymer.
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Materials Science · Cross-listed from cond-mat.str-el

Nonvolatile optical switching of surface metallicity in 1T-TaSe2

Junde Liu, Liwen Su, Pei Liu et al. (+12 more)

The surface of a quantum material provides a distinct electronic environment in which reconstruction can stabilize states absent from the bulk. However, whether light can persistently reconfigure this surface-bulk electronic contrast remains unclear. Here we demonstrate nonvolatile and reversible optical insulator-to-metal switching at the surface of the charge density wave (CDW) material 1T-TaSe2, whose bulk is metallic. Angle-resolved photoemission spectroscopy reveals that a single femtosecond pulse creates a persistent metallic surface state. Heating or optical pulse trains restore the insulating state. Comparison of the measured band structures with first-principles calculations associates the two electronic states with distinct near-surface CDW stacking configurations that modify interlayer-mediated electron hopping. Layer-resolved calculations further show insulating outer layers above a metallic interior in the equilibrium configuration, whereas metallic states reach the outermost layer in the configuration associated with the photoinduced state. Our results establish persistent optical control of surface metallicity and open a route to reconfiguring the effective surface-bulk electronic boundary in layered quantum materials.
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All authors Junde Liu, Liwen Su, Pei Liu, Hui Liu, Mojun Pan, Yuchong Zhang, Famin Chen, Yueqian Chen, Zhaoyang Xie, Stefan Mathias, Tianping Ying, Lin Hu, Tian Qian, Xun Shi, Yugui Yao

Materials Science

Account of the self-interaction energy correction in the first principles calculation of the fundamental absorption edge spectrum of LiCl

M.A.Bunin and I.I Geguzin

Within the framework of the local electron density functional theory, an ab-initio method is proposed that takes into account the self-interaction energy correction (SIC) for the crystal potential. The principle of dividing the unit cell into regions remained the same as for the ground-state potential. The expression for the self-consistent muffin-tin-SIC potential satisfies the condition of cell electroneutrality. This scheme was applied to calculate the spectrum of the fundamental absorption edge of LiCl. The obtained interband transition energies were not required for fitting to experimental values. In the calculated spectrum, the shape and position of the peaks above ~2.5 eV from the edge agree fairly well with the measured ones, which allows their intensity in this region to be attributed primarily to interband transitions. Closer to the edge, electron-hole interaction significantly alters the ground state, which must be taken into account when calculating the spectrum shape.
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Materials Science · Cross-listed from cond-mat.str-el

Kondo Effect in Nonreciprocal Response

Hajime Murata and Hiroaki Ishizuka

Chiral magnetic states give rise to rich phenomena, from the anomalous Hall effect and the nonlinear electrical current to multiferroics and magnetochiral dichroism. Most of the studies on electrical transport so far have focused on the cases where the magnetic moments are well approximated by classical local moments. Here, we reveal that the coexistence of quantum fluctuations and chiral spin correlations gives rise to a $\log(T)$ temperature dependence in the electrical magnetochiral effect, a nonreciprocal response. Using the Green's function method and a scattering theory approach, we show that the $\log(T)$ temperature dependence occurs through a scattering process similar to that of the Kondo effect. The electrical magnetochiral effect is sensitive to the sign of vector spin chirality and the magnetic field. The results demonstrate that local spin correlations and quantum fluctuations cooperatively induce nontrivial properties in transport phenomena.
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Materials Science

Nonvolatile Electrical Control of Spin via Sliding Fractional Quantum Multiferroics

Jiajun Lu, Mu Tian, Chaoxi Cui et al. (+3 more)

We propose a fractionally quantized polarization induced by interlayer sliding in bilayer altermagnets, unveiling a previously unrecognized multiferroic phase termed sliding fractional quantum multiferroicity (SFQM). This unconventional magnetic phase uniquely integrates sliding ferroelectricity with fractional quantum ferroelectricity, enabling highly efficient switching and nonvolatile electrical control of spin.~Unlike conventional multiferroics, SFQM simultaneously exhibits lattice-scale atomic displacements, ultralow switching barriers, and spin splitting, giving rise to a large fractionally quantized polarization and strong magnetoelectric coupling. Through symmetry analysis and first-principles calculations, we identify bilayer altermagnet Ca(CoN)$_2$ and its family materials as promising candidates hosting SFQM. In contrast to gate-controlled schemes, the spin-layer coupling in SFQM is intrinsically induced by spontaneous electrical and layer polarization, requiring no sustained gate field and exhibiting nonvolatile character. This mechanism enables nonvolatile electrical control of spin through biaxial sliding, where displacements along the x- and y-axes generate opposite polarization directions in the layer-dependent electrical polarization. Furthermore, SFQM exhibits a fully switchable anomalous Hall effect and a pronounced magneto-optical response, which can be utilized for its detection and distinction. These findings highlight the promising role of sliding-mediated couplings among unconventional magnetism, fractional quantum ferroelectricity, and stacking order in realizing electrically controllable two-dimensional multiferroics.
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All authors Jiajun Lu, Mu Tian, Chaoxi Cui, Zhi-Ming Yu, Run-Wu Zhang, Yugui Yao

Materials Science

Hidden layered structures from carbon-analog metastability in metal dichalcogenides

Shota Ono

Carbon exhibits both a layered ground state structure that produces two-dimensional (2D) nanosheets and a non-layered diamond structure created under high pressure conditions. Motivated by this metastability relationship, we revisit the ground state structure of metal dichalcogenides that are known to have non-layered pyrite-type structure. Ultrathin films of pyrite-type ZnSe$_2$ spontaneously transform into a layered phase. This phase is identified as a ground state, and the monolayer exhibits strong elastic anisotropy and a semiconducting bandgap larger than that of the pyrite phase by a factor of two. We demonstrate that a two-valued but directional potential energy surface exists along a Bain-like distortion path, hiding the layered ground state. This work implies that many 2D materials are hidden in non-layered materials and connects 2D materials science with surface and high-pressure science.
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Wednesday 24 December

Materials Science · Cross-listed from cs.ET

Ovonic switches enable energy-efficient dendrite-like computing

Unhyeon Kang, Jaesang Lee, Seungmin Oh et al. (+9 more)

Over the last decade, dendrites within individual biological neurons, which were previously thought to generally perform information pooling and networking, have now been shown to express complex temporal dynamics, Boolean-like logic, arithmetic, signal discrimination, and edge detection for image and sound recognition. Mimicking this rich functional density could offer a powerful primitive for neuromorphic computing, which has sought to replace the aging digital computing paradigms using biological inspirations. Here, using electrically driven Ovonic threshold switching in Sb-Te-doped GeSe, we demonstrate a single two-terminal component capable of self-sustained dynamics and universal Boolean logic, in addition to XOR operations (which is traditionally thought to require a network of active components). We then employ logic-driven dynamics in a single component to detect and estimate the gradients of edges in images, a task that otherwise requires elaborate circuits. A network of Ovonic switches exhibits properties of a half adder and a full adder, in addition to discriminative logic accommodating inhibitory and excitatory signals. We show that this computational primitive is not only seemingly simpler, but also offers many orders of magnitude improved energy efficiency compared to prevailing digital solutions. As such, this work paves the path for potentially emulating dendrites for efficient post-digital neuromorphic computing.
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All authors Unhyeon Kang, Jaesang Lee, Seungmin Oh, Hanchan Song, Jongkil Park, Jaewook Kim, Seongsik Park, Hyun Jae Jang, Sangbum Kim, Su-in Yi, Suhas Kumar, Suyoun Lee

2512.23736 · 24 Dec · 17 pages, 5 figures, 1 table. Published in Nano Letters (2025). This document is the unedited Author's version of a Submitted Work that was subsequently accepted for publication in Nano Letters, copyright (c) American Chemical Society after peer review. To access the final edited and published work see https://pubs.acs.org/doi/full/10.1021/acs.nanolett.5c04348

Abstract PDF $CRCT ×1$NRN ×1

Materials Science

Toward reducing the formation temperature of diopside via wet-chemical synthesis routes using chloride precursors

N. Namvar, E. Salahinejad, A. Saberi et al. (+3 more)

Reducing the formation temperature of single-phase multioxides is one of the classic challenges in ceramic processing, including wet-chemical synthesis routes. Toward pursuing this aim for diopside (MgCaSi2O6), the merit of different sol-gel and coprecipitation processes using the related chloride precursors followed by calcination was compared from the viewpoints of crystallinity and homogeneity. In accordance to the results, the use of the sol-gel techniques, directed with/without an alkaline catalyst, gave rise to the unfavorable creation of multiphase and low-crystallinity structures. Regarding the coprecipitation methods, the one-step addition of a precipitant agent is accompanied by an indirect low-temperature formation of nano-diopside, while a direct crystallization into this phase was explored in the dropwise condition, albeit with a lower crystallinity. Thus, by employing a suitable synthesis processing, it is feasible to take control of a wide range of nanoparticulate diopside-based structures achieved after a low-temperature calcination.
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All authors N. Namvar, E. Salahinejad, A. Saberi, M.J. Baghjeghaz, L. Tayebi, D. Vashaee

Materials Science

Representations of the symmetry groups of infinite crystals

Bachir Bekka and Christian Brouder

We investigate the representations of the symmetry groups of infinite crystals. Crystal symmetries are usually described as the finite symmetry group of a finite crystal with periodic boundary conditions, for which the Brillouin zone is a finite set of points. However, to deal with the continuous crystal momentum $\mathbf{k}$ required to discuss the continuity, singularity or analyticity of band energies $\epsilon_n(\mathbf{k})$ and Bloch states $\psi_{\mathbf{k}}$, we need to consider infinite crystals. The symmetry groups of infinite crystals belong to the category of infinite non-compact groups, for which many standard tools of group theory break down. For example, character theory is no longer available for these groups and we use harmonic analysis to build the group algebra, the regular representation, the induction of irreducible representations of the crystallographic group from projective representations of the point groups and the decomposition of a representation into its irreducible parts. We deal with magnetic and non-magnetic groups in arbitrary dimensions. In the last part of the paper, we discuss Mackey's restriction of an induced representation to a subgroup, the tensor product of induced representations and the symmetric and antisymmetric squares of induced representations.
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Materials Science

Structure, biomineralization and biodegradation of Ca-Mg oxyfluorosilicates synthesized by inorganic salt coprecipitation

E. Salahinejad and M. Jafari Baghjeghaz

In this research, a novel group of Ca-Mg oxyfluorosilicates containing different levels of fluoride substituting for oxide was synthesized by an inorganic salt coprecipitation process followed by calcination/sintering. The effects of the incorporation of fluoride on the resultant structural characteristics, apatite-forming ability and biodegradability were evaluated by X-ray diffraction, transmission electron microscopy, scanning electron microscopy/energy-dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy, inductively coupled plasma spectroscopy and pH measurements. According to the results, the samples containing up to 2 mol% F present a single-phase structure of diopside (MgCaSi2O6) doped with F. It was also found that to meet the most biomineralization characteristic, the optimal value of fluoride in the homogeneous samples is 1 mol%. In this regard, on the one hand, the partial incorporation of fluoride into apatite (via forming fluorohydroxyapatite) and, on the other hand, the absence of fluorite (CaF2) as a consumer of Ca in the deposits are responsible for achieving the most apatite-forming ability circumstance controlled by an ion-exchange reaction mechanism. In conclusion, this study reflects the merit of the optimization of fluoride-doping into Ca-Mg silicates for development in biomedicine.
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Materials Science

Channel-last gate-all-around nanosheet oxide semiconductor transistors

Fabia F. Athena, Xiangjin Wu, Nathaniel S. Safron et al. (+18 more)

As we move beyond the era of transistor miniaturization, back-end-of-line-compatible transistors that can be stacked monolithically in the third dimension promise improved performance for low-power electronics. In advanced transistor architectures, such as gate-all-around nanosheets, the conventional channel-first process involves depositing dielectrics directly onto the channel. Atomic layer deposition of gate dielectrics on back-end-of-line compatible channel materials, such as amorphous oxide semiconductors, can induce defects or cause structural modifications that degrade electrical performance. While post-deposition annealing can partially repair this damage, it often degrades other device metrics. We report a novel channel-last concept that prevents such damage. Channel-last gate-all-around self-aligned transistors with amorphous oxide-semiconductor channels exhibit high on-state current ($>$ 1 mA/$\mu$m) and low subthreshold swing (minimum of 63 mV/dec) without the need for post-deposition processing. This approach offers a general, scalable pathway for transistors with atomic layer deposited channel materials, enabling the future of low-power three-dimensional electronics.
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All authors Fabia F. Athena, Xiangjin Wu, Nathaniel S. Safron, Amy Siobhan McKeown-Green, Mauro Dossena, Jack C. Evans, Jonathan Hartanto, Yukio Cho, Donglai Zhong, Tara Peña, Paweł Czaja, Parivash Moradifar, Paul C. McIntyre, Mathieu Luisier, Yi Cui, Jennifer A. Dionne, Greg Pitner, Iuliana P. Radu, Eric Pop, Alberto Salleo, H.-S. Philip Wong

Materials Science

Adhesion Energy of Phosphorene on Different Pristine and Oxidized Metallic Substrates

Matteo Vezzelli, Carsten Gachot and Maria Clelia Righi

Black phosphorus and its single-layer constituent, phosphorene, have emerged as promising two-dimensional materials with remarkable tribological properties. However, recent experimental investigations revealed that the their lubricating capabilities can change with the substrate. The present computational study employs density functional theory calculations to quantify the adhesion energy of both pristine and oxidized phosphorene monolayers on various metallic substrates (aluminum, copper, iron, and chromium) and their corresponding oxides ($\mathrm{Al_2O_3}$, $\mathrm{Cu_2O}$, $\mathrm{Fe_2O_3}$, and $\mathrm{Cr_2O_3}$), correlating these interfacial property with experimentally observed tribological performance. Results demonstrate that oxidized phosphorene presents higher adhesion to all substrates with respect to pristine phosphorene, attributed to favorable interactions between oxygen non-bonding states and substrate empty states. Adhesion is systematically more favorable on pristine metals than on their corresponding oxides, with chromium and iron showing particularly strong interactions due to partially filled 3d orbitals. This result is consistent with the coefficient of friction decrease observed in tribological experiments after scratching the iron substrate, thus removing the outermost oxide layer. Charge redistribution correlates with the adhesion and electronic structure analyses reveal system-dependent interfacial bonding characteristics, with some configurations inducing metallic character in phosphorene. These findings provide fundamental insights into substrate-dependent lubricating properties of black phosphorus, highlighting the key role of layer-substrate adhesion.
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Materials Science · Cross-listed from cs.LG

MiST: Understanding the Role of Mid-Stage Scientific Training in Developing Chemical Reasoning Models

Andres M Bran, Tong Xie, Shai Pranesh et al. (+9 more)

Large Language Models can develop reasoning capabilities through online fine-tuning with rule-based rewards. However, recent studies reveal a critical constraint: reinforcement learning succeeds only when the base model already assigns non-negligible probability to correct answers – a property we term 'latent solvability'. This work investigates the emergence of chemical reasoning capabilities and what these prerequisites mean for chemistry. We identify two necessary conditions for RL-based chemical reasoning: 1) Symbolic competence, and 2) Latent chemical knowledge. We propose mid-stage scientific training (MiST): a set of mid-stage training techniques to satisfy these, including data-mixing with SMILES/CIF-aware pre-processing, continued pre-training on 2.9B tokens, and supervised fine-tuning on 1B tokens. These steps raise the latent-solvability score on 3B and 7B models by up to 1.8x, and enable RL to lift top-1 accuracy from 10.9 to 63.9% on organic reaction naming, and from 40.6 to 67.4% on inorganic material generation. Similar results are observed for other challenging chemical tasks, while producing interpretable reasoning traces. Our results define clear prerequisites for chemical reasoning training and highlight the broader role of mid-stage training in unlocking reasoning capabilities.
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All authors Andres M Bran, Tong Xie, Shai Pranesh, Jeffrey Meng, Xuan Vu Nguyen, Jeremy Goumaz, David Ming Segura, Ruizhi Xu, Dongzhan Zhou, Wenjie Zhang, Bram Hoex, Philippe Schwaller

Materials Science

PhononBench:A Large-Scale Phonon-Based Benchmark for Dynamical Stability in Crystal Generation

Xiao-Qi Han, Ze-Feng Gao, Wen-Kao Li et al. (+2 more)

In recent years, generative artificial intelligence has made significant advances in the design of crystalline materials, giving rise to approaches based on graph neural networks, diffusion models, and large language models. Existing evaluations commonly follow the stability-uniqueness-novelty (S.U.N.) framework, where stability is primarily assessed using thermodynamic criteria, which do not fully capture the dynamical stability essential for a material's practical existence. Dynamical stability is a key determinant of whether a material can be synthesized and persist, with phonon spectrum calculations serving as the standard for its evaluation. However, the high computational cost of such calculations has prevented large-scale assessment of dynamical stability in generated crystals. In this work, we introduce PhononBench, the first large-scale benchmark for dynamical stability in AI-generated crystals. Leveraging the recently developed MatterSim interatomic potential, which achieves density-functional-theory (DFT)-level accuracy in phonon predictions across more than 10,000 materials, PhononBench enables efficient phonon calculations and dynamical-stability analysis for 133,838 crystal structures generated by 7 leading crystal generation models. PhononBench reveals a widespread limitation of current generative models: unless otherwise specified, all reported dynamical-stability metrics are evaluated at a phonon-frequency threshold of -0.1 THz, with the average dynamical-stability rate across all generated structures being only 32.15%, and the top-performing model, MatterGen, reaching just 45.05%.In addition, we identify 32,995 crystal structures that are phonon-stable across the entire Brillouin zone under a strict threshold of -0.001 THz. In addition, a web-based service is accessible at http://phononbench.cn/, enabling minute-level ultra-fast phonon predictions.
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All authors Xiao-Qi Han, Ze-Feng Gao, Wen-Kao Li, Peng-Jie Guo, Zhong-Yi Lu

Materials Science

Features of the Electronic and Magnetic Properties of Heusler Alloys in the States of a Half-Metallic Ferromagnet and a Spin-Gapless Semiconductor

V. V. Marchenkov, V. Yu. Irkhin and Yu. A. Perevozchikova

The review treats Heusler alloys that display distinctive functional properties, including shape-memory behavior and magnetocaloric effects. Particular emphasis is placed on Heusler systems in which half-metallic ferromagnetism and spin-gapless semiconductor state are realized. Although these compounds are crystallographically rather "ordinary", peculiarities of their electronic structure and magnetic state lead to unconventional kinetic and magnetic properties. Their magnetic and transport characteristics are highly sensitive to external stimuli, and changes in alloy composition or external parameters can induce transitions between the states considered. This tunability provides further opportunities for controlling the electronic and magnetic properties of Heusler alloys and for exploiting them in applications such as spintronics and micro- and nanoelectronics.
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Materials Science

Substrate Role in Polaron Formation on Single-layer Transition Metal Dihalides

Affan Safeer, Oktay Güleryüz, Guangyao Miao et al. (+3 more)

Single-layer transition metal dihalides grown on conducting substrates were shown to host stable polarons. Here, we investigate polarons in insulating single-layer MnBr$_2$ grown by molecular beam epitaxy on three different substrates, namely graphene on Ir(110), graphene on Ir(111), and Au(111). The number densities and species of polarons observed vary strongly as a function of the substrate. For MnBr$_2$ grown on Ir(110) the largest number of polaron species is observed, namely four, of which three show clear similarities with the species observed for CoCl$_2$ on graphite. Polarons in single-layer MnBr$_2$ are observed up to 300K. They can be created, converted, and moved by the STM tip when a tunneling current flows at a proper bias voltage. For graphene on Ir(110) as a substrate, mobile polarons in MnBr$_2$ are guided through the periodic potential imposed from the super-moiré resulting from the interaction of MnBr$_2$ with graphene and Ir(110). Our findings indicate that modeling of polarons in such single-layer insulators in contact with a conducting substrate requires to take the substrate explicitly into account.
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All authors Affan Safeer, Oktay Güleryüz, Guangyao Miao, Wouter Jolie, Thomas Michely, Jeison Fischer

Materials Science · Cross-listed from quant-ph

Thermodynamic sampling of materials using neutral-atom quantum computers

Bruno Camino, Mao Lin, John Buckeridge et al. (+1 more)

Neutral-atom quantum hardware has emerged as a promising platform for programmable many-body physics. In this work, we develop and validate a practical framework for extracting thermodynamic properties of materials using such hardware. As a test case, we consider nitrogen-doped graphene. Starting from Density Functional Theory (DFT) formation energies, we map the material energetics onto a Rydberg-atom Hamiltonian suitable for quantum annealing by fitting an on-site term and distance-dependent pair interactions. The Hamiltonian derived from DFT cannot be implemented directly on current QuEra devices, as the largest energy scale accessible on the hardware is two orders of magnitude smaller than the target two-body interaction in the material. To overcome this limitation, we introduce a rescaling strategy based on a single parameter, $\alpha_v$, which ensures that the distribution sampled by the hardware is well described by Boltzmann-like weights corresponding to those of the material at an effective temperature $T^{\prime} = \alpha_v T$, where $T$ is the device sampling temperature. This rescaling also establishes a direct correspondence between the global laser detuning $\Delta_g$ and the grand-canonical chemical potential $\Delta\mu$. We validate the method on a 28-site graphene nanoflake using exhaustive enumeration, and on a larger 78-site system where Monte Carlo sampling confirms preferential sampling of low-energy configurations.
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All authors Bruno Camino, Mao Lin, John Buckeridge, Scott M. Woodley

Materials Science · Cross-listed from quant-ph

Photoexcitation of moiré-trapped interlayer excitons via chiral phonons

A. Borel, T. V. Ivanova, J. Cervantes-Villanueva et al. (+7 more)

Moiré superlattices in transition-metal dichalcogenide semiconductor heterobilayers enable the quantum confinement of interlayer excitons with large out-of-plane permanent electric dipoles and spin-valley control. Here, we report a novel phonon-assisted excitation mechanism of individual moiré-trapped interlayer excitons in 2H-stacked MoSe$_2$/WSe$_2$ heterobilayers via chiral $E^{\prime\prime}$ in-plane optical phonons at the {\Gamma}-point. This excitation pathway preserves valley-selective optical selection rules and enables deterministic generation of individual interlayer excitons with defined helicity, emitting within a spectrally narrow energy spread. Through photoluminescence excitation spectroscopy in both the ensemble and quantum emitter regimes, we identify a fixed phonon energy of $\sim$23 meV mediating the process. First-principles calculations corroborate the symmetry and energy of the relevant phonon mode and its coupling to interlayer excitons, providing microscopic support for the observed valley-selective phonon-assisted excitation mechanism. Our results highlight the utility of chiral phonons as a tool for controlled excitation of quantum emitters in TMD moiré systems, opening new opportunities for valleytronic and quantum photonic applications.
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All authors A. Borel, T. V. Ivanova, J. Cervantes-Villanueva, P. Thor, H. Baek, T. Taniguchi, K. Watanabe, A. Molina-Sanchez, B. D. Gerardot, M. Brotons-Gisbert

Materials Science

Ferromagnetic interface engineering of spin-charge conversion in RuO$_2$

Dongchao Yang, Zhaoqing Li, Yu Dai et al. (+4 more)

Spin-orbit torque efficiency is conventionally fixed by bulk materials. $D$-wave altermagnets introduce an additional nonrelativistic spin-charge conversion channel beyond inverse spin-Hall effect. Using prototypical candidate RuO$_2$ as an example, we show that the adjacent ferromagnet alone can dictate both the magnitude and sign of spin-charge conversion. Spin-pumping measurements on RuO$_2$/Y$_3$Fe$_5$O$_{12}$ (YIG) and RuO$_2$/Ni$_{80}$Fe$_{20}$ (Py) bilayers yield opposite effective spin-Hall angles that persist across crystalline and polycrystalline RuO$_2$. Inserting an ultrathin Au spacer at the RuO$_2$/YIG interface reverses the signal, envidencing a dominant interfacial inverse Rashba-Edelstein effect, whereas RuO$_2$/Py is governed by bulk inverse spin-Hall effect. First-principles calculations trace this dichotomy to interface-selective band hybridization: Rashba surface states survive at the insulating YIG contact yet are quenched by metallic Py. Our findings establish ferromagnetic interfacing as a deterministic knob for tailoring spin-charge conversion in altermagnetic oxides, paving the way to field-free, low-dissipation spintronic memory devices.
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All authors Dongchao Yang, Zhaoqing Li, Yu Dai, Lili Lang, Zhong Shi, Zhe Yuan, Shi-Ming Zhou

Materials Science

Active Learning Guided Computational Discovery of 2D Materials with Large Spin Hall Conductivity

Abhijeet J. Kale, Sanjeev S. Navaratna, Pratik Sahu et al. (+3 more)

Two-dimensional (2D) materials are promising candidates for next-generation spintronic devices due to their tunable properties and potential for efficient spin-charge interconversion. However, discovering materials with intrinsically high spin Hall conductivity (SHC) is hindered by the vast chemical space and expensive nature of conventional experimental and first-principles methods. In this work, we employ an active learning framework to accelerate the discovery of high-SHC 2D materials. Machine learning (ML) models were trained on SHC values computed from density functional theory calculations, incorporating the Kubo formalism via tight-binding Hamiltonians constructed from maximally localized Wannier functions, with explicit treatment of spin-orbit coupling. Starting from random but chemically diverse 24 2D systems, the dataset was expanded to 41 cases (from an overall pool of around 2000 materials) over three active learning loops using an expected improvement acquisition strategy. The ML technique successfully identified several high SHC candidates with the best candidate exhibiting a SHC of 271.52 (hbar/e) Ohm^-1, nearly 23 times higher than the top performer in the initial round. Beyond candidate discovery, several features such as orbital symmetry near the Fermi energy, types of atomic species, material composition, covalent radii, and electronegativity of constituent atoms were found to play critical role in shaping the spin Hall response in 2D systems. The data generated is made publicly available to facilitate further advances in 2D spintronics.
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All authors Abhijeet J. Kale, Sanjeev S. Navaratna, Pratik Sahu, Henry Chan, B. R. K. Nanda, Rohit Batra

Materials Science

From cluster to nanocrystal: the continuous evolution and critical size of copper clusters revealed by machine learning

Hongsheng Liu, Luneng Zhao, Yaning Li et al. (+5 more)

The evolution of cluster structure with size and the critical size for the transition from cluster to nanocrystal have long been fundamental problems in nanoscience. Due to limitations of experimental technology and computational methods, the exploration of the continuous evolution of clusters towards nanocrystal is still a big challenge. Here, we proposed a machine learning force field (MLFF) that can generalize well to various copper systems ranging from small clusters to large clusters and bulk. The continuous evolution of copper clusters CuN towards nanocrystal was revealed by investigating clusters in a wide size range (7 <= N <= 17885) based on MLFF simulated annealing. For small CuN (N < 40), electron counting rule plays a major role in stability. For large CuN (N > 80), geometric magic number rule plays a dominant role and the evolution of clusters is based on the formation of more and more icosahedral shells. For medium size CuN (40 <= N <= 80), both rules contribute. The critical size from cluster to nanocrystal was calculated to be around 8000 atoms (about 6 nm in diameter). Our work terminates the long-term challenge in nanoscience, and lay the methodological foundation for subsequent research on other cluster systems.
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All authors Hongsheng Liu, Luneng Zhao, Yaning Li, Yuan Chang, Shi Qiu, Xiao Wang, Junfeng Gao, Feng Ding

Materials Science

Realization of Insulating Buffer Layers via MOCVD-Grown Nitrogen-Doped (010) e̱ta}-Ga2O3

Rachel Kahler, Carl Peterson and Sriram Krishnamoorthy

We present MOCVD-grown, nitrogen-doped e̱ta}-Ga2O3 films as an insulating buffer layer on Fe-doped (010) e̱ta}-Ga2O3 substrates in lieu of 49% HF treatment to remove unintentional silicon at the substrate-epitaxial layer growth interface. N-doped layer thickness and NH3 flow were systematically varied to experimentally determine the lowest nitrogen concentration and thickness of the buffer layer needed to fully compensate the interfacial silicon peak. The NH3 molar flow rate was varied from 200 sccm to 1800 sccm. Results showed fully insulating N-doped layers for samples with NH3 flow rates greater than or equal to 1200 sccm and a thickness of 50 nm. This study demonstrates the efficacy of in-situ, controllably doped nitrogen buffer layers as a mitigation method for unintentional interfacial silicon at the substrate-epitaxial layer growth interface.
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Materials Science

Ab initio Approach to Collective Excitations in Excitonic Insulators

Fengyuan Xuan, Jiexi Song and Zhiyuan Sun

An ab initio approach is presented for studying the collective excitations in excitonic insulators, charge/spin density waves and superconductors. We derive the Bethe-Salpeter-Equation for the particle-hole excitations in the quasiparticle representation, from which the collective excited states are solved and the corresponding order parameter fluctuations are computed. This method is demonstrated numerically for the excitonic insulating phases of the biased WSe2-MoSe2 bilayer. It reveals the gapless phase-mode, the subgap Bardasis-Schrieffer modes and the above-gap scattering states. Our work paves the way for quantitative predictions of excited state phenomena from first-principles calculations in electronic systems with spontaneous symmetry breaking.
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Materials Science

Deciphering the lattice vibrational behaviors of CuInP2S6 by angle-resolved polarized Raman scattering

Yiqi Hu, Jun-Jie Zhang, Zhou Zhou et al. (+12 more)

The layered van der Waals (vdW) ferroelectric CuInP2S6 (CIPS) exhibits unique cation hopping-driven phenomena that bring about unconventional properties with intriguing mechanisms and hold promises for advanced applications in nanoelectronics. However, an explicit analysis of its lattice dynamics and vibrational symmetries, pivotal for understanding the material's peculiar ferroelectric and ferroionic behaviors, remains incomplete. Here, we employ angle-resolved polarized Raman spectroscopy in concert with first-principles calculations to systematically unravel the anisotropic lattice vibrations of CIPS single crystals. By analyzing the polarization-dependent Raman intensities, we determine the symmetry assignments and Raman tensors of all major vibrational modes, revealing good agreement with theoretical predictions. Furthermore, we demonstrate the utility of Raman spectroscopy as a sensitive and non-invasive probe for structural and ferroelectric order evolution, by examining temperature-driven phase transitions and thickness-dependent polarization suppression in CIPS. Our findings establish a foundational framework for correlating lattice dynamics with functional properties in CIPS and provide a methodological blueprint for studying other vdW ferroelectrics.
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All authors Yiqi Hu, Jun-Jie Zhang, Zhou Zhou, Shun Wang, Qiankun Li, Yanfei Hou, Tianhao Ying, Lingling Yang, Jingyao Zhang, Shuzhong Yin, Yuyan Weng, Shuai Dong, Jianlin Yao, Liang Fang, Lu You

Materials Science

A voltage-responsive strongly dipolar-coupled macrospin network with emergent dynamics for computing

Xinglong Ye, Zhibo Zhao, Qian Wang et al. (+15 more)

Emergent behavior, which arises from local interactions between simple elements, is pervasive in nature. It underlies the energy-efficient computing in our brains. However, realizing such dynamics in artificial materials, particularly under low-energy stimuli, remains a fundamental challenge. While dipole-dipole interactions are typically suppressed in magnetic storage, here we harness and amplify them to construct a strongly dipolar-coupled network of SmCo5 macrospins at wafer scale, which can exhibit intrinsic interaction-driven collective dynamics in response to voltage pulses. The network combines three essential ingredients: strong dipolar coupling by large single-domain macrospin, giant voltage control of coercivity over nearly 1000-fold, and disordered network topology with frustrated Ising-like energy landscape. When stimulated by 1 V pulses, the network enters a regime where interaction-driven magnetic behaviors emerge, including spontaneous demagnetization, greatly enhanced magnetization modulation, reversible freeze and resume evolution and stochastic convergence toward low-energy magnetic configurations. All these behaviors are completely absent at the single-nanomagnet level. Furthermore, by constructing micromagnetic models of the strongly dipolar-coupled macrospin networks, we show that the resulting nonlinear, high-dimensional collective dynamics, intrinsic to strongly-interacting systems, can enable accurate chaotic Mackey-Glass prediction and multiclass drone-signal classification. Our work establishes the voltage-responsive strongly-coupled SmCo5 network as a mesoscopic platform for probing emergent magnetic dynamics previously inaccessible under ambient conditions. It also suggests a fundamental distinct route towards scalable, low-voltage computing, one rooted in native physical interaction-driven collective dynamics at the network level.
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All authors Xinglong Ye, Zhibo Zhao, Qian Wang, Jiangnan Li, Fernando Maccari, Ning Lu, Christian Dietz, Esmaeil Adabifiroozjaei, Leopoldo Molina-Luna, Yufeng Tian, Lihui Bai, Guodong Wang, Konstantin Skokov, Yanxue Chen, Shishen Yan, Robert Kruk, Horst Hahn, Oliver Gutfleisch

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

Drastic field-induced resistivity upturns as signatures of unconventional magnetism in superconducting iron chalcogenides

Z. Zajicek, I. Paulescu, P. Reiss et al. (+5 more)

Electronic scattering is a powerful tool to identify underlying changes in electronic behavior and incipient electronic and magnetic orders. The nematic and magnetic phases are strongly intertwined under applied pressure in FeSe, however, the additional isoelectronic substitution of sulphur offers an elegant way to separate them. Here we report the detailed evolution of the electronic and superconducting behaviour of FeSe$_{0.96}$S$_{0.04}$ under applied pressure via longitudinal magnetoresistance studies up to 15T. At intermediate pressures, inside the nematic phase, the resistivity displays an upturn in zero magnetic field, which is significantly enhanced in the magnetic field, suggesting the stabilization of a spin-density wave phase, which competes with superconductivity. At higher pressures, beyond the nematic phase boundaries, the resistivity no longer displays any clear anomalies in the zero magnetic field, but an external magnetic field induces significant upturns in resistivity reflecting a field-induced order, where superconductivity and magnetic anomalies are enhanced in tandem. This study highlights the essential role of high magnetic fields in stabilizing different electronic phases and revealing a complex interplay between magnetism and superconductivity tuned by applied pressure in FeSe$_{1-x}$S$_{x}$.
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All authors Z. Zajicek, I. Paulescu, P. Reiss, R. M. Abedin, K. Sun, S. J. Singh, A. A. Haghighirad, A. I. Coldea

Tuesday 23 December

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

New RVE concept and FFT methods in micromechanics of composites subjected to body force with compact support

Valeriy A. Buryachenko

We consider static linear elastic composite materials (CMs) with periodic structure. The core of the proposed methodology is the generation of a novel dataset using specially designed body force fields with compact support (BFCS), enabling a new RVE concept that reduces the infinite periodic medium to a finite domain without boundary artifacts. This functionally reduced RVE is used for translated averaging of direct numerical simulations (DNS) results, efficiently computed via a newly developed FFT-based solver for BFCS loading. The resulting dataset captures localized field responses and is used to train machine learning (ML) and neural networks (NN) models to learn effective nonlocal surrogate operators. These operators accurately predict macroscopic responses while reflecting microstructural features and nonlocal interactions. By accounting for field localization while simultaneously eliminating influences from finite sample size and boundary effects, it provides a physically grounded and data-driven framework for constructing accurate surrogate models for the homogenization of complex materials.
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Materials Science

Interfacial Charge Transfer and Electronic Structure Modulation in Ultrathin Graphene P3HT Hybrid Heterostructures

Yosra Mater, Salih Demirci and V. Ongun Özçelik

Ultrathin polymer-graphene heterostructures are promising materials for next generation optoelectronic and photovoltaic technologies, while the influence of the polymer's structural variation on interfacial charge transfer remains unclear. Here, using ab initio quantum mechanical calculations we show how different forms of Poly(3-hexylthiophene) (P3HT), a widely used organic semiconductor, interact with graphene. We analyze the effects of molecular chain length, end-group termination, periodicity, and the distinction between ordered and random P3HT arrangements. For isolated P3HT, the band gap decreases with increasing chain length and layer thickness, while structural disorder leads to slightly larger gaps due to reduced electronic coupling. When P3HT is deposited on graphene, all configurations exhibit spontaneous charge transfer, with electrons accumulating on graphene and holes remaining in the polymer. This effect is significantly enhanced in ordered and fully periodic structures and is noticeably weaker in disordered ones. Charge density analyses further show that thicker and more ordered P3HT layers improve electron hole separation across the interface. Our results reveal how molecular structure governs charge transfer in P3HT-graphene heterojunctions and provide practical guidelines for designing high efficiency polymer-graphene photovoltaic devices.
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Materials Science

Epitaxial growth and semiconductor properties of NiGa2O4 spinel for Ga2O3/NiO interfaces

Kingsley Egbo, Emily M. Garrity, Shivashree Shivamade Gowda et al. (+8 more)

Unintentionally formed interfacial layers are ubiquitous in semiconductor devices that operate at extreme conditions. However, these layers' structure and properties often remain unknown due to the thinness of these naturally formed interphases. Here, we report on the intentional epitaxial growth and semiconductor properties of NiGa2O4 spinel layers that form at Ga2O3/NiO interfaces used in high-power and high-temperature electronic devices. Cubic spinel NiGa2O4 films of 10-50 nm thicknesses and low surface roughness (~ 2 nm) were grown using pulsed laser deposition at a substrate temperatures in the 300-900 {\deg}C range on {\alpha}-Al2O3 and {\beta}-Ga2O3 substrates of different orientation. The optical absorption onset (3.6-3.9 eV) and thermal conductivity (4-9 W m-1 K-1) vary systematically with substrate temperature, consistent with theoretical predictions of varying Ni and Ga cation ordering on the spinel lattice. The valence band offset between NiGa2O4 and {\beta}-Ga2O3 is determined to be 1.8 eV. The NiGa2O4-based p-n heterojunction devices on Ga2O3 (001) substrates exhibit a rectification ratio of 10^8 (for +/-2V) and a turn-on voltage of 1.4 V, maintaining diode behavior up to 600 {\deg}C. These results highlight the potential of NiGa2O4 as a p-type interlayer in Ga2O3-based devices and shows a new approach to investigate such interfacial layers.
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All authors Kingsley Egbo, Emily M. Garrity, Shivashree Shivamade Gowda, Saman Zare, Ethan A. Scott, Glenn Teeter, Brooks Tellekamp, Vladan Stevanovic, Patrick E. Hopkins, Andriy Zakutayev, Nancy Haegel