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Physics Condensed Matter Materials Science

cond-mat.mtrl-sci Watch Materials Science

Sunday 28 June

Materials Science · Cross-listed from physics.optics

Peculiarities Of High-Speed Dynamics Of Two-Photon Absorption In Si Nanowire Waveguides

Vadym Zayets, Siim Heinsalu and Akihiro Noriki

We investigate the complete dynamical pathway of photon-electron interactions involved in two-photon absorption (TPA) in a silicon nanowire waveguide using three independent high-speed measurement techniques. These methods probe different stages of the process: nonlinear photon absorption, electron excitation from the valence to the conduction band, and free-carrier generation. According to the conventional model of TPA, these three processes should occur at identical rates. However, our measurements reveal significant discrepancies between them. The measured nonlinear photon absorption is more than twice the value required to account for the measured TPA transitions, indicating the presence of additional absorption pathways or nontrivial TPA dynamics. Furthermore, the number of measured TPA transitions substantially exceeds the measured free-carrier density, indicating that long-lifetime free carriers represent only a small fraction of the TPA-excited electrons, while the majority recombine rapidly back to the valence band on a timescale shorter than 13 ps. In addition, the three stages of the TPA pathway exhibit distinct saturation behaviors at different photon densities, further indicating that the TPA process in silicon is more complex than described by the conventional model. These findings provide new insight into the physical mechanisms governing TPA, suggesting the existence of multiple competing pathways for this optical transition. A major obstacle to a complete understanding of TPA is the unclear physical origin of the virtual midgap level. The potential strategies for minimizing unwanted nonlinear losses in high-speed silicon photonic circuits, as well as for exploiting TPA in high-speed optical switching and photonic signal processing are investigated.
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Saturday 27 June

Materials Science

Oblate Spheroid Excitation Theory: A Unified, Lattice-Free Foundation for Plastic Deformation from Which Dislocations Emerge as Collective Excitations

Albert Linda and K.A. Padmanabhan

Dislocation theory has underpinned crystal plasticity for a century, yet its lattice-dependent definition cannot describe plastic flow in grain boundaries, glasses, ceramics, or nanocrystals near the glass transition, where no periodic lattice exists. We propose the Oblate Spheroid Excitation Theory (OSET): the elementary carrier of plastic deformation, in any solid, is a shear-eigenstrained oblate spheroid, the oblate-spheroidal transformation zone (OSTZ), treated within Eshelby's inclusion theory. The OSTZ requires no lattice and has a finite, non-singular, intrinsically thermally activated energy and stress fields. Three results are proved: a single OSTZ produces a non-singular elastic dipole, not a dislocation's singular field; a co-planar chain of N OSTZs is mathematically identical to a Peierls-Nabarro dislocation, core width and Burgers vector fixed by OSTZ geometry; and a genuine dislocation nucleates only once the chain reaches a host-lattice-set critical length. Dislocations emerge as a collective, large-$N$ limit of OSET rather than an assumed entity, and the theoretical shear strength, Peierls stress, core energy, Frank-Read critical stress, and stacking-fault energy follow as derived, parameter-free quantities. OSET is validated against grain-boundary-sliding data, independent literature spanning metals, ceramics, and bulk metallic glasses, and a recent 41-system compilation, reproducing the fitted dilatational and shear eigenstrains to within 2% and 15%, respectively. Because classical dislocation theory emerges from OSET but OSET does not require dislocations, it provides, in our view, a more fundamental, broadly applicable foundation for plastic deformation across material classes.
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Materials Science

Dislocation-loop formation is a first-order phase transition

Xiaoya Chang, Arsalan Hashemi, Nima Ghafari Cherati et al. (+3 more)

Dislocation loops are the elementary product of radiation damage in crystals, limiting reactor-component lifetimes, power-electronics reliability and the coherence of solid-state qubits. Their nucleation has been simulated for six decades but never reduced to a thermodynamic law. We show that dislocation-loop formation is a first-order phase transition, and construct its Ginzburg–Landau free energy, with the loop area as order parameter, entirely from atomistic simulation. In diamond, carbon self-interstitials condense into planar precursors that collapse abruptly into a prismatic $\tfrac{1}{2}\langle110\rangle$ loop across a 3.7-electronvolt barrier, with pressure–volume work supplying only 2% of the energy released. The reduced free energy proves material-independent: the vacancy platelet-to-loop collapse in body-centred-cubic iron falls on the same one-parameter family, placing loop nucleation on a transferable thermodynamic footing.
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All authors Xiaoya Chang, Arsalan Hashemi, Nima Ghafari Cherati, Mikko Karttunen, Adam Gali, Tapio Ala-Nissila

Materials Science · Cross-listed from cs.LG

On Surrogate Modeling of Static Response of AM Short-Fiber Thermoplastics Using Graph Neural Networks

Pharindra Pathak, Vipin Kumar, Trenton M. Ricks et al. (+2 more)

Short-fiber thermoplastic (SFT) composites are increasingly employed in lightweight aerospace and automotive structures owing to their favorable strength-to-weight ratio, high production rates, and recyclability. Unlike continuous-fiber systems, the mechanical response of SFTs is governed by mesoscale interactions among fiber orientation, spatial clustering, and manufacturing-induced porosity. These features exhibit significant spatial variability in manufactured components and influence stiffness, damage initiation, and nonlinear deformation. Although mesoscale finite element (FE) models can resolve such heterogeneity, their application to realistic three-dimensional microstructures remains computationally intractable. A data-driven surrogate framework is proposed to predict the mechanical behavior of additively manufactured, compression-molded (AM-CM) SFTs. Microstructures reconstructed from micro-computed tomography data were discretized into Voronoi-based cells representing distinct fiber-interaction neighborhoods. Each cell was homogenized via nonlinear FE simulations incorporating matrix damage, and the resulting stress-strain responses trained a hybrid Graph Neural Network-Long Short-Term Memory (GNN-LSTM) architecture encoding microstructural topology and history-dependent mechanical evolution. The surrogate accurately predicts stiffness and stress-strain behavior of unseen microstructures, achieving $R^2\approx 0.98$ relative to high-fidelity FE simulations with over two orders-of-magnitude reduction in computational cost. Coupling the framework with experimentally calibrated damage laws demonstrates that fiber orientation, clustering, and porosity collectively govern local effective stiffness. The approach provides a physics-informed, data-efficient pathway to identify mechanically weak microstructural cells and accelerate digital-twin development for SFT components.
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All authors Pharindra Pathak, Vipin Kumar, Trenton M. Ricks, Suhasini Gururaja, Siddhartha Srivastava

Materials Science

Structural properties of one-dimensional $\mathrm{Cs}_2\mathrm{CoCl}_4$ confined within single-walled carbon nanotubes

Jaskaran S. Mangat, Yu Lei, Matthew Weyland et al. (+9 more)

Crystals under one-dimensional (1D) confinement are well-known to exhibit drastic changes in metallicity, magnetic properties and chemical state, however, the intermediate phase space between binary metal halides and ternary metal halide perovskites remains poorly explored, especially in the context of the rich polymorphism exhibited by both families in the one-dimensional limit. Through aberration-corrected (scanning) transmission electron microscopy and multislice simulations, it is shown that the metal halide $\mathrm{Cs}_2\mathrm{CoCl}_4$ crystallizes in the tetragonal $\wp4/mcc$ and orthorhombic $\wp{mcm}$ rod groups under radial compression within single-walled carbon nanotubes (SWCNTs) of increasingly small diameter, with a massive re-entrant orthorhombic strain towards the $1$ $\mathrm{nm}$ extremum. The persistence of $\mathrm{Co}^{2+}$ is determined from fits to the d.c. magnetization, with a surprisingly small increase in the effective moment ($4.607(3)$ to $4.788(3) \mathit{{\mu}}_\mathrm{B}/\mathrm{f.u.}$) and Weiss constant ($-7.9(3)$ to $-4.09(7) \mathrm{K}$) after confinement in the SWCNTs, suggesting that the confined structure topologically preserves the core magnetic properties of the bulk. Both unconventional polymorphs observed are noticeably different to the high-pressure piezochromic polymorph previously shown to undergo a tetrahedral-to-octahedral coordination transition, highlighting 1D confinement as a unique tool for structural manipulation.
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All authors Jaskaran S. Mangat, Yu Lei, Matthew Weyland, Yisong Han, Kiran Bal, Martin R. Lees, Craig I. Hiley, Piotr Dłużewski, Sławomir Kret, Peng Wang, Richard I. Walton, Jeremy Sloan

Materials Science

Selenium direct doping obtained high-performance-n-type Bi2Te3-based thermoelectric materials with a wide temperature range

Zhiyuan Liu, Junjie Ma, Zhaopeng Zeng et al. (+5 more)

The article reports on a series of n-type Bi2Te3-based thermoelectric materials prepared via a high-temperature melting combined with annealing process. The effects of Se doping content and annealing process on the carrier concentration, suppression of the bipolar effect, and thermoelectric performance of the materials were systematically investigated. The experimental results provide valuable reference for researchers in this field.
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All authors Zhiyuan Liu, Junjie Ma, Zhaopeng Zeng, Ni Ma, Qian Ba, Di Zhang, Zhe Tao, Ailin Xia

Materials Science · Cross-listed from cs.LG

MALOQ: Massively Accelerated Learning of Operators for Quantum Transport

Manasa Kaniselvan, Alexander Maeder, Denghui Lu et al. (+2 more)

Machine-learned (ML) operator models can be trained to predict density functional theory (DFT) Hamiltonian/density matrices at significantly reduced computational cost, thus extending electronic-structure calculations to previously unfeasible scales. Here, we introduce MALOQ (Massively Accelerated Learning of Operators for Quantum Transport), an application built to train on and predict electronic-structure matrices for systems made of few to 100k atoms, described by large basis sets, and covering a wide range of atomic elements. Based on a state-of-the-art, SO(2)-equivariant backbone architecture, MALOQ provides (i) custom data-processing kernels to handle high-rank Hamiltonian matrix data and (ii) a scalable edge-wise distribution of atomic graph(s). Trained on the largest molecular Hamiltonian datasets available today, it reduces time-per-epoch by over 30% compared to a molecule-wise-distributed framework, and enables inference on material graphs of arbitrary size. We demonstrate scalable training and inference for 3,000-12,000 atoms on the Alps supercomputer, up to 192 GPUs and 256 GPUs, respectively.
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All authors Manasa Kaniselvan, Alexander Maeder, Denghui Lu, Alexandros Nikolaos Ziogas, Mathieu Luisier

Materials Science · Cross-listed from cs.CV

Projection-based coupling of infrared thermography and stereocorrelation-based digital image correlation

Jendrik-Alexander Tröger, Lutz Müller-Lohse and Stefan Hartmann

Full-field measurement techniques such as digital image correlation and infrared thermography are prevalent in experimental solid mechanics. Digital image correlation is used to analyze surface deformation, while infrared thermography quantifies surface temperature fields. However, sophisticated procedures are necessary to express both datasets in the same Lagrangian frame, especially when analyzing non-flat surfaces. In this study, we propose an external projection-based coupling that uses the pinhole camera model to relate two-dimensional temperature data measured by infrared thermography to three-dimensional point coordinates from stereocorrelation-based digital image correlation. Unlike existing multiview approaches, we utilize two independently calibrated industrial-grade systems and augment the experimental evaluation with the pinhole camera model. The projection matrix of the camera model is calibrated using a single image of a reference object. Through this projection, temperature fields are accurately represented at material points. Our method is particularly suited for, but not restricted to, curved surfaces and is straightforward to embed in existing experimental protocols, as the image registration is kept as is. Additionally, we use radial basis functions as a global interpolation ansatz in both space and time to compute in-plane temperature gradients and even temperature rates on curved surfaces, thereby providing an extensive and information-rich full-field dataset.
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Materials Science · Cross-listed from quant-ph

Properties of 2D Electron or Hole Gases at Tailored s-Si/SiGe Interfaces: A First-Principles Investigation

Garima Ahuja, Farha S, Tanuja Shridhar Joshi et al. (+2 more)

We have performed first-principles hybrid density functional theory calculations to study the formation and properties of two-dimensional electron or hole gases (2DEG or 2DHG) at s-Si/SiGe interfaces. For small Ge concentrations $x < 0.25$, we find a novel type of band alignment with no offset in the conduction bands, implying that a 2DEG cannot be formed, though a 2DHG can. In contrast, for $x > 0.25$ the band alignment suggests that either a 2DEG or 2DHG can be formed. The electronic band structure features two nearly degenerate 2DEG states at the bottom of the conduction bands, and two 2DHG states at the top of the valence band. These states can be accessed by appropriate doping and gating. Charge density plots of these states show that they feature carriers confined to the near vicinity (2–3 atomic layers) of the interface. Calculated effective masses are anisotropic, being markedly so for the 2DHG states, and in excellent agreement with experiment. This property can be exploited to create a 1D carrier gas. Our results are especially important for s-Si/SiGe-based semiconducting spin qubits for quantum computing applications.
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All authors Garima Ahuja, Farha S, Tanuja Shridhar Joshi, Anil Shaji, Shobhana Narasimhan

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

Anomalous Behavior of the Ni$^{1+}$ moment in bi-infinite-layered La$_3$Ni$_2$O$_5$F

Young-Joon Song, W. E. Pickett and K.-W. Lee

The discovery of superconductivity in hole-doped Ni$^{1+}$ systems with “infinite layer" NiO$_2$ square-lattices analogous to the Cu$^{2+}$ CaCuO$_2$ cuprate has renewed conflicting pictures of the Cu$^{2+}$$-$Ni$^{1+}$ similarities and distinctions. Recent synthesis of formal Ni$^{1+}$ La$_3$Ni$_{2}$O$_{5}$F with two infinite NiO$_{2}$ layers per cell provides a novel member of this class. We find that Ni$^{1+}$ in this material is unusually flexible. First principles density functional theory studies reveal a single partially occupied electron band derived from density in three interstitial layers that provides self-doping to a Ni$^{1.09+}$ charge state. The blocking La(O/F)La layer provides isolation of the NiO$_2$ bilayer to strictly two-dimensional electronic and magnetic systems. Fixed spin moment calculations of magnetic tendencies reveal behavior unlike previous nickelates, including an essentially vanishing differential susceptibility up to a large magnetic field and a metastable ferromagnetic (FM) state. Antiferromagnetic (AFM) order is favored energetically as expected; surprisingly both FM and AFM states are obtained without correlation (Hubbard $U$) corrections. Doping away from half-filling by an interstitial density derived band and likely two-dimensional fluctuations account for the lack of observation of any magnetic transition.
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Materials Science

Emergence of half-semiconductor behavior and tunable magnetism via carrier doping in Janus VXSe (X=Cl, Br, I) monolayers

Zhixiang Wang, Aining Wang, Zikui Ye et al. (+5 more)

Two-dimensional ferromagnetic semiconductors with high Curie temperature, large magnetic anisotropy, and electrically tunable properties are highly sought for nanoscale spintronics. Here, using first-principles calculations, we predict a new class of Janus VXSe (X=Cl, Br, I) monolayers. All three compounds are intrinsic ferromagnetic semiconductors with indirect band gaps of 1.66-2.33 eV, Curie temperatures up to 128 K, and magnetic anisotropy energies up to 910 ueV per V, leading to easy-plane magnetization for VClSe and VBrSe and an out-of-plane easy axis for VISe. The robust ferromagnetic order originates from the competition between superexchange and itinerant magnetism. Remarkably, VISe is a pristine half-semiconductor, whereas carrier doping unlocks fully spin-polarized states in the initially non-ideal VClSe and VBrSe. We also find that hole doping can switch the magnetic easy axis of VBrSe from in-plane to out-of-plane, enabling a spin-field-effect transistor with giant magnetoresistance. Our findings highlight carrier doping as a key to unlock hidden half-semiconducting behavior and establish Janus VXSe as a promising platform for 2D spintronics and magnetoelectric devices.
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All authors Zhixiang Wang, Aining Wang, Zikui Ye, Yuqiao Qu, Qian Zheng, Yiding Liu, Qiang Fan, Gang Yao

Materials Science · Cross-listed from quant-ph

High-Fidelity Hole Spin Qubits Reveal Quadrupolar Nuclear-Bath Dynamics in Isotopically Purified Planar Germanium

Jian Zeng, Xiangjun Tan, Hongzhang Wang et al. (+10 more)

Planar Germanium has emerged as a promising platform to build spin-based large scale quantum computers. By exploiting the anisotropic hyperfine interaction of holes in Ge, qubits with long T2* have been recently realized. While the performance of single qubits is still more or less limited by 73Ge nuclear spin fluctuations, the site-to-site variation of qubit sweet spot becomes obstacles to maintaining high fidelity of each qubit across the whole wafer. To achieve high performance Ge-based quantum circuit, it is therefore essential to eliminate the origin source of hyperfine noise. In its Silicon counterparts, reduction of 29Si abundance enables exceptional high-fidelity operation. In contrast, hole qubits based on isotopically purified Ge have not been demonstrated. Here, we report the synthesis of high quality 2-dimensional hole gas (2DHG) with enriched 70GeH4 precursor. Due to the suppression of nonzero spin nucleus, the qubits' T2* on the sweet spot is moderately extended beyond 20 us, surpassing the previous best reported Ge hole qubits. More importantly, the qubits' T2* off the sweet spot is enhanced to above 3 us, enabling single qubit gate fidelity exceeding 99.9% in both operating regimes. Hahn-echo spectroscopy further resolves a finite-frequency nuclear-noise channel that is distinct from the conventional Larmor-linked hyperfine response. We associate this channel with quadrupole-modified dynamics of residual 73Ge nuclei sampling local electric-field gradients near the Ge/SiGe interface. Its field scaling and angle-dependent visibility are consistent with a qubit-visible quadrupolar nuclear-noise component transduced through the anisotropic hyperfine interaction of Ge holes. These results establish isotopically purified planar Ge as a high-coherence scalable platform for hole spin qubits and provide a spectroscopic probe of interfacial quadrupolar nuclear dynamics.
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All authors Jian Zeng, Xiangjun Tan, Hongzhang Wang, Yulei Zhang, Wendong Bian, Lingting Wu, Chenggang Yang, Zhengshan Guo, Jiankun Li, Yongfeng Wang, Jun Lu, Jun-Wei Luo, Tian Pei

Materials Science

Strain engineering of ultrafast magnetism in the room-temperature vdW ferromagnet Fe3GaTe2

Fan Fei, Yangchen He, Wuzhang Fang et al. (+6 more)

Controlling ultrafast magnetic dynamics is critical to understanding nonequilibrium spin interactions and advancing high-speed spintronics. However, a lack of efficient in situ tuning strategies leaves most ultrafast studies largely dependent on the intrinsic properties of the individual materials. Here we demonstrate continuous strain tuning of both the equilibrium magnetic response and ultrafast demagnetization dynamics in the room-temperature van der Waals ferromagnet Fe3GaTe2. Applying up to 4.2% uniaxial tensile strain increases the coercive field from nearly zero to 100 Oe, consistent with an enhancement of the effective perpendicular magnetic anisotropy. Time-resolved magneto-optical Kerr effect measurements further reveal strain-accelerated ultrafast demagnetization, with 1.2% tensile strain reducing the characteristic demagnetization time by approximately 20%. Remarkably, strain accesses an accelerated demagnetization regime that cannot be reached simply by increasing pump fluence in the unstrained sample. Combined with first-principles calculations, our results resolve that the applied strain modifies the spin-lattice energy transfer, leading to the observed accelerated demagnetization. These findings establish mechanical strain as an effective route for on-demand control of ultrafast magnetic dynamics while reducing the required optical energy by reconfiguring the magnetic energy landscape and associated spin-relaxation pathways.
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All authors Fan Fei, Yangchen He, Wuzhang Fang, Jessica Kienbaum, Jacopo Simoni, Robert Boyd, Yuan Ping, Daniel A. Rhodes, Jun Xiao

Materials Science

Structural relaxation and stagnation of grain boundary during migration

Tingting Yu

Instability is a major bottleneck in nanomaterials due to grain boundary (GB) activities under thermal or mechanical stimuli. The relaxation of GB will stabilize the properties of materials by structure modification of GBs to lower energy states. However, lack of understanding of mechanisms limits the application of GB relaxation. In this study, we certify that GB can realize relaxation through defect emission including vacancy, twinning and dislocations, etc, which lower the average atomic energy of GB. In particular, we found stagnation and shear-coupling migration accompany the relaxation process, where a lower average atomic energy and lower average atomic volume of grain boundaries can be the reasons for stagnation. In this study, we found when simulated by ramped energy-conserving orientated driving force, under a specific driving force, the grain boundary suddenly stops migrating during the migration process, and as the driving force increases to a certain value, the grain boundary continues to migrate. Even at fixed driving forces, the grain boundary migration process can stall. This phenomenon has been found in many grain boundaries, and it has been found that the reason for the stagnation is the change of the average atomic energy of grain boundaries and the average atomic volume of grain boundaries. The discovery of this stagnation phenomenon is helpful to better understand the migration characteristics of grain boundaries and lays a foundation for improving the strength of materials by designing grain boundary microstructures.
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Friday 26 June

Materials Science · Cross-listed from cond-mat.stat-mech

Brownian Bridge for Coherent State Path Integral Monte Carlo

Riccardo Fantoni

We propose a new Brownian bridge construction for our newly devised Coherent States Path Integral Monte Carlo algorithm. We apply it to the numerically exact calculation of the thermodynamic properties of the Helium fluid on a plane at low non zero temperature. We find very good agreement with the conventional plane waves path integral Monte Carlo results.
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Materials Science · Cross-listed from cond-mat.str-el

Magnetic symmetry implications of the zero- and applied-field Hall effect of UNi$_4$B

Z. W. Riedel, W. S. Simeth, S. M. Thomas et al. (+2 more)

The zero-field and applied-field Hall effects in noncollinear antiferromagnets provide evidence for topological states of matter and are tied to materials' magnetic symmetry. For UNi$_4$B, the antiferromagnetic state with $T_\mathrm{N}=20$ K at zero and low magnetic field is debated due to recent magnetoelectric measurements and theory work calling into question the proposed toroidal arrangement of magnetic dipole moments. For a magnetic field applied within the plane of uranium magnetic moments, the field-dependent Hall resistivity of UNi$_4$B shows a curved response for $\rho_{yz}$ ($H{\parallel}x$, $I{\parallel}z$) and $\rho_{zx}$ ($H{\parallel}y$, $I{\parallel}x$) up to $\sim$8 T at 2 K, while an out-of-plane field results in linear behavior of $\rho_{yx}$ ($H{\parallel}z$, $I{\parallel}x$) up to 16 T. Analysis using conventional empirical relationships for the Hall effect indicate that an intrinsic effect from momentum-space Berry curvature contributes significantly to the curved transverse resistivity. Moreover, a finite zero-field Hall effect emerges at the onset of magnetic order for $\rho_{yz}$ and $\rho_{zx}$, further supporting an intrinsic origin of the Hall response. Symmetry arguments for a finite Berry curvature, an observable magnetoelectric effect, and reported magnetic structures suggest that the previously proposed magnetic space groups for the zero-field magnetic structure cannot account for the observed finite zero-field effect for two Hall orientations. Instead, we propose that $Cm'$ or $Pm'$ magnetic symmetry, depending on the parent nonmagnetic space group, is consistent with Hall resistivity, neutron diffraction, and magnetoelectric effect measurements.
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All authors Z. W. Riedel, W. S. Simeth, S. M. Thomas, F. Ronning, E. D. Bauer

Materials Science

Surrogate-Gated Generation and Foundation-Model Embeddings for Bayesian Materials Design

Sk Md Ahnaf Akif Alvi, Jan Janssen, Danny Perez et al. (+2 more)

Closed-loop materials discovery iterates between proposing candidate structures and evaluating their properties, and property evaluation dominates the cost. In the generative variant, a learned prior proposes candidate crystals and a property oracle scores them; we ask whether a cheap probabilistic surrogate can triage the generator's output, and what such a surrogate must do well. Across three architecturally distinct pretrained diffusion priors (MatterGen, CrystalFlow, ADiT) and two targets (room-temperature heat capacity and bulk modulus), we insert a Gaussian process acquisition gate between structure generation and the oracle in an RL-steered generative workflow. The gate matches or exceeds ungated fine-tuning of the generative model while capping oracle calls at a fixed per-cycle budget. Budget-matched ablations isolate the mechanism. At an identical four-call budget, ranking-based selection outperforms arbitrary selection, confirming that the gain comes from the surrogate's choice; the gate comes within $\sim$9% of exhaustive oracle spending at roughly one-fifth of the calls. A density-functional-theory check of the bulk-modulus discoveries confirms the learned oracle to within 2.5% on average and the surrogate's ranking of the generated structures at Spearman $\rho = 0.94$. A cross-factorial benchmark of surrogate performance spanning mechanical, electronic, and vibrational properties identifies pretrained ORB embeddings with a Gaussian process as the most reliable combination, which we adopt as the building blocks of the proposed workflow. The complete pipeline is released as open-source software.
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All authors Sk Md Ahnaf Akif Alvi, Jan Janssen, Danny Perez, Douglas Allaire, Raymundo Arroyave

Materials Science · Cross-listed from quant-ph

High yield creation of germanium vacancy centers in diamond by focused ion beam implantation and high temperature annealing

S. Dietel, E. Scattolo, J. Fuhrmann et al. (+5 more)

Negatively charged germanium vacancy centers (GeV) in diamond are a promising platform for quantum computing and quantum communication. However, these applications require the precise incorporation of GeV centers with good optical properties inside of nanophotonic structures. In this work, we demonstrate the highly efficient local creation of GeV centers in diamond via focused-ion-beam implantation, followed by high-temperature annealing. We report the successful creation of GeV centers over the depth range of 5.5 - 30 nm. Implantation at low fluence enables the creation of single GeV centers. The formation yield strongly depends on implantation energy and fluence, reaching up to 33% at energies of 35 and 70 keV. This method, therefore, enables the efficient creation of GeV centers within a small, well-defined local sample volume and offers a potential means of incorporating them into photonic structures.
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All authors S. Dietel, E. Scattolo, J. Fuhrmann, L. Kazak, A. Cian, E. Missale, D. Giubertoni, F. Jelezko

Materials Science

Microscopic and macroscopic characterization: MBE-grown versus sputter-deposited Au/Co/Au thin films for CISS and MIPAC effect studies

Lokesh Rasabathina, Thi Ngoc Ha Nguyen, Aleksandr Kazimir et al. (+12 more)

Chirality-induced spin selectivity (CISS) enables spin-dependent transport at chiral molecule/Au(111) interfaces and is used in spintronics when combined with ferromagnetic thin films in spin-valve-type hybrids. However, the influence of substrate microstructure on CISS and the related magnetization induced by the proximity of adsorbed chiral molecules (MIPAC) effect is still not well understood. In this study, we compare the effects of the adsorption of L-chiral alpha-helical alanine-rich peptides on Au/Co/Au ferromagnetic thin films fabricated by molecular beam epitaxy (MBE) and magnetron sputtering. X-ray reflectivity and X-ray diffraction show sharper interfaces and a narrower Au(111) rocking-curve width for the MBE-grown sample. However, atomic force microscopy and scanning tunneling microscopy images reveal that both sample types have locally smooth Au(111) surface regions suitable for peptide adsorption, despite clear differences in larger-scale morphology. Microscopic scanning tunneling spectroscopy after peptide exposure yields similar magnetization-direction-dependent tunneling currents in both sample types, confirming a similar magnitude CISS effect on the molecular scale. In contrast, macroscopic magneto-optical Kerr effect hysteresis loops and effect microscopy reveals that only sputter-deposited samples show slight coercivity enhancements and a consistent reduction in domain wall velocity after peptide exposure. These results suggest that microscopic CISS signatures are robust for both sample types, whereas macroscopic MIPAC-type magnetic responses are more sensitive to the substrate microstructure.
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All authors Lokesh Rasabathina, Thi Ngoc Ha Nguyen, Aleksandr Kazimir, Rico Ehrler, Julia Krone, Franziska Schölzel, Zihao Liu, Peter Heinig, Markus Gößler, Irene Coin, Christina Lamers, Georgeta Salvan, Lech Tomasz Baczewski, Christoph Tegenkamp, Olav Hellwig

Materials Science

A Magnetism-Aware Crystal Graph Neural Network with Explicit Magnetic-Sublattice and Exchange-Geometry Representations

Sourav Mal and Satadeep Bhattacharjee

Magnetic order in crystals is governed by moment-carrying sublattices and ligand-mediated exchange pathways, yet standard crystal graph neural networks do not explicitly distinguish magnetic sublattices and encode bonds primarily through pair distances. We propose mCGCNN, a magnetism-aware crystal graph neural network that augments the full structural graph with a dedicated magnetic subgraph. The magnetic stream performs angle-aware message passing between magnetic centers using metal–ligand–metal exchange-path descriptors motivated by Goodenough–Kanamori–Anderson physics, while layer-wise cross-coupling transfers structural and ligand-environment information from the full crystal graph. Separate pooling of the magnetic sublattice preserves magnetism-specific information alongside the global structural representation. We evaluate mCGCNN for total magnetic moment prediction on 49,462 crystalline materials with ferromagnetic spin configurations from the Materials Project and find that the proposed model improves agreement with DFT magnetic moments over established crystal-graph approaches without requiring a large increase in model complexity. Material-level analysis shows that the largest gains are concentrated predominantly in complex transition-metal and oxygen-containing magnetic compounds. Our results show that explicitly incorporating magnetic sublattices and exchange geometry provides a physically motivated route to improved crystal-graph representations for magnetic materials.
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Materials Science

Radiation Damage of TF-1 and PbWO$_4$ Crystals with 20 MeV Electrons

Hamlet G. Mkrtchyan, Arthur H. Mkrtchyan, Vardan H. Tadevosyan et al. (+9 more)

We studied the radiation hardness of two types of crystals, lead glass TF-1 and lead tungstate PbWO$_4$, using a 20 MeV electron beam from the LUE-75 linear accelerator at AANL. The transmittance of the crystals in the wavelength range of 200-1000 nm was measured before and after irradiation, and after thermal annealing. % The irradiation was performed in two stages. First, both crystals were irradiated with beam current of 0.125 $\mu$A, each for a total exposure time of 720 s and absorbing $5.6 \times 10^{14}~e^-$. % Strong degradation of the optical properties of TF-1 caused by this amount of radiation was observed, while the effect on PbWO$_4$ was negligible (it is a few tens of times radiation harder than TF-1). % In the second stage, only the PbWO$_4$ crystal was exposed to radiation, with a beam current of 0.28 $\mu$A and an exposure time of 1200 s, absorbing an additional $2.1 \times 10^{15}e^-$, still no notable effect. % Thermal annealing was performed in the temperature range of 160 to 250$^\circ$C (isochronal) for 10-12 hours. The transmittance of the annealed crystals increased with the annealing temperature and time.
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All authors Hamlet G. Mkrtchyan, Arthur H. Mkrtchyan, Vardan H. Tadevosyan, Hrachya H. Marukyan, Argine S. Hakobyan, Ashot S. Hakobyan, Arthur A. Hoghmrtsyan, Diana G. Khurshudyan, Nina N. Prazyan, Albert H. Shahinyan, Adelina S. Stepanyan, Lusine R. Vahradyan

Materials Science

Moiré Phonons and Emergent Exciton-Phonon Coupling in a Moiré Heterobilayer

Can B. Uzundal, Woochang Kim, Zhiyuan Cui et al. (+13 more)

Moiré superlattices have emerged as a new platform for engineering electronic and optical properties in van der Waals heterostructures, enabling control over correlated and excitonic phenomena. Yet the impact of moiré superlattices on exciton-phonon coupling remains largely unexplored. Here we demonstrate emergent, layer-selective coupling between moiré phonons and moiré excitons in angle-aligned WS2/WSe2 heterobilayers. Using a broadband terahertz phonon transducer, we coherently launch moiré phonons that resonantly perturb the excitonic states. We show that the exciton-phonon coupling is intrinsically modified by the moiré superlattice in a layer-selective manner. A driven oscillator model captures the dynamics, revealing three moiré phonon resonances with distinct coupling to the moiré excitons. First principles calculations show that many moiré phonon modes can arise with distinct strongly hybridized in-plane and out-of-plane vibrations in the moiré unit cells. The calculations further identify the three experimentally observed moiré phonons and their emergent characteristic coupling to the moiré excitons.
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All authors Can B. Uzundal, Woochang Kim, Zhiyuan Cui, Yuxuan Wei, Zheyu Lu, Qixin Feng, Francis L. Hong, Indrajit Maity, Takashi Taniguchi, Kenji Watanabe, Manish Jain, Mit H. Naik, Yoseob Yoon, Michael F. Crommie, Steven G. Louie, Feng Wang

Materials Science

Uniaxial compression of crystalline HCP titanium: an atomistic modelling study of size effects

Fatemeh Safari and Konstantinos Konstantinou

Understanding the deformation behaviour of titanium is important not only for technological advances associated with industrially-relevant applications, but also essential to achieve a fundamental understanding of the mechanical properties of the relevant alloys. In this computational study, molecular-dynamics simulations are employed to ascertain the impact of model size on the mechanical response of alpha-titanium under compression. The deformation behaviour of the crystalline models is investigated as a function of different system sizes (varied by four orders of magnitude, up to 32 million atoms), and strain rates (down to 10^8 s^-1). The results show that the elastic properties remain independent of both system size and strain rate, whereas marked size effects emerge during plastic deformation. Increasing the system size of the titanium model reduces stress fluctuations, results in more homogeneous structural evolution, and stabilizes dislocation activity. Decreasing the applied strain rate requires correspondingly a larger system size to achieve equilibration and to ensure a stable behaviour for the simulated structure. The modelling results demonstrate that system size and strain rate are strongly coupled, and their combined effect governs the simulated deformation behaviour of the compressed crystalline material.
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Materials Science · Cross-listed from physics.ins-det

A single platform with van der Pauw geometry for measurement of Seebeck coefficient, resistivity, and Hall effect of thin films

Niraj Kumar Singh, Martin Falk, Per-Anton Schön Wallander et al. (+3 more)

A modular thermoelectric properties measurement setup in van der Pauw configuration was developed for a straightforward and simultaneous measurement of electrical resistivity and Seebeck coefficient in an extensive temperature range of 25{\deg}C - 600{\deg}C and can also perform Hall measurements at room temperature. The setup is optimized for accurate measurement of voltages and temperatures gradients by minimizing possible errors from offset voltages, wire contributions and thermal contact resistances which helps getting reliable data. The setup is user friendly, and the measurements are fully automated and controlled using a LabVIEW program. The detachable modules make this setup quite versatile and provide an all-in-one (except thermal conductivity) solution for thermoelectric measurements.
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All authors Niraj Kumar Singh, Martin Falk, Per-Anton Schön Wallander, Per Sandström, Per Eklund, Arnaud le Febvrier

Materials Science

Tuning the optoelectronic properties of wide bandgap perovskites: Data-driven insights from combinatorial synthesis and high-throughput experimentation

Alexander Wieczorek, Sergey Tsarev, Nathan Rodkey et al. (+4 more)

The discovery and optimization of wide-bandgap lead halide perovskites (LHPs) is hindered by solution-based workflows with limited scalability. Large compositional parameter spaces present an additional challenge for materials optimization. Here, we establish an integrated, combinatorial workflow based on sequential physical vapor deposition that enables independent tuning of cation (Cs/Pb) and anion (Br/Cl) compositions. Applying automated structural, compositional, and optical characterizations across >500 samples regions of interest are rapidly screened in the quaternary Cs-Pb-Br-Cl space. From the screening, we establish a practical Cs/Pb window of 1.05-1.20 for wide bandgap perovskites, within which elevated PL yields were observed. Through in-depth analysis of the data set, we uncover a high-energy optical transition as a robust determinant for high PL yields. By combining mechanistic insight into the compositional origins of high PL efficiency with a fully integrated, high-throughput screening framework, and by openly releasing the complete multi-modal dataset, this work provides a broadly accessible benchmark to accelerate data-driven discovery of wide-bandgap perovskites.
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All authors Alexander Wieczorek, Sergey Tsarev, Nathan Rodkey, Oleksandr Pshyk, Stefanie Frick, Maksym V. Kovalenko, Sebastian Siol

Materials Science

Room-temperature magnon-phonon transduction in high-damping Co/Pt structures

Gauravkumar Patel, Takuma Sato, Maximilian Frenzel et al. (+8 more)

Quantum communication and information processing strongly benefit from the coupling between different quasi-particles, offering complementary advantages. Magnetoelastic materials inherently allow for direct coupling between magnetization dynamics and quantized lattice vibrations, called phonons. Near the ferromagnetic resonances, phonons may thus trade energy and angular momentum with uniformly precessing magnetization, called magnons, and enable transduction of information from magnetic to phononic modes, thereby paving the way for long-range transport of magnetic information without the need of magnetic material. Here, we employ tailored magnetic-nonmagnetic heterostructures, which simultaneously act as cavities for standing shear waves, to bring selective phonons and magnons into resonance. These Co films with Pt seed layers show extended linewidth and reduced amplitude of the phonon-resonant FMR lines, providing a hallmark of energy and angular momentum exchange. Complementarily, by theoretical modeling and ultra-fast coherent phonon spectroscopy, we identify the responsible transverse acoustic phonons as standing shear waves in the combined Co and Pt structure. We find a high crystal quality in conjunction with a large magnetoelastic coupling constant as a prerequisite for efficient magnon-phonon coupling of this type. Such resonant enhancement of magnon-phonon coupling in CMOS-compatible material provides an ideal material platform for future quantum transducers.
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All authors Gauravkumar Patel, Takuma Sato, Maximilian Frenzel, Prakriti P. Joshi, Ruslan Salikhov, Ievgeniia Korniienko, Dominik Legut, Olav Hellwig, Sebastian F. Maehrlein, Kilian Lenz, Jürgen Lindner

Materials Science

In situ synchrotron X-ray diffraction study of flash austenitization and process design insights in medium-Manganese steels for energy applications

Bowen Zou, Mathias Zapf, Thea Kannenberg et al. (+5 more)

Medium Mn steels (MMnSs) are promising candidates for energy-related infrastructure because their multiphase microstructures and austenite stability can be tailored to improve failure resistance under demanding service conditions. Flash austenitization (FA) provides a rapid route to form austenite while limiting prior austenite grain coarsening and substitutional solute homogenization, but the related short-time transformation kinetics remain insufficiently quantified. In the present work, the effects of FA temperature and initial microstructure on austenitization kinetics were investigated in an Fe-6Mn-1.5Si-1Cr-0.3Mo-0.05Nb-0.2C (wt.%) MMnS using dilatometry-integrated in situ synchrotron X-ray diffraction. Two initial microstructures produced by austenite reversion treatment (ART) were heated at 100 degrees C/s to 850 degrees C, 900 degrees C, or 950 degrees C and then held isothermally. Rapid heating alone is insufficient for full austenitization, even above the reference Ac3 temperature determined under slow heating. Full austenitization, defined by bcc fraction (f_alpha) <= 1 wt.%, requires short holding, decreasing from about 8 s at 850 degrees C to about 2 s at 950 degrees C. The final stage of austenitization is less sensitive to FA temperature than the early holding stage. The initial ART state mainly shifts the starting austenite fraction, whereas both states show comparable kinetic trends at higher FA temperatures.
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All authors Bowen Zou, Mathias Zapf, Thea Kannenberg, Daniel Schneider, Yixu Wang, Xiao Shen, Ulrich Prahl, Wenwen Song

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

Hidden valley dynamics behind vanishing circular polarization in moiré excitons

Yuto Urano, Lata Chouhan, Nurul Fariha Ahmad et al. (+4 more)

Optically addressable valley degrees of freedom in transition-metal dichalcogenide heterostructures provide a powerful platform for valleytronic and quantum-optical functionalities. In moiré superlattices, interlayer excitons inherit valley-contrasting optical selection rules while acquiring long lifetimes, electric dipoles, and site-dependent optical responses. However, because conventional measurements typically probe time-integrated valley polarization, the dynamical origin of vanishing polarization has remained elusive. Here, we show that a nearly zero steady-state valley polarization in electrically tunable moiré excitons does not necessarily indicate fast valley relaxation. Helicity-resolved time-resolved photoluminescence reveals a temporal crossing between co- and cross-circularly polarized emission, indicating that helicity-opposite dynamical components coexist and compensate after time integration. A minimal two-channel model, representing A-like and B-like moiré emission channels with opposite optical selection rules and distinct effective decay/depolarization rates, reproduces the observed helicity crossing without invoking a single rapid valley relaxation process. Furthermore, two-dimensional gate-field maps show that the crossing time evolves systematically with electrostatic tuning, demonstrating that the hidden valley dynamics are electrically controllable. These results show that time-integrated circular polarization can give a false-negative indication of valley polarization in multichannel valley emitters.
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All authors Yuto Urano, Lata Chouhan, Nurul Fariha Ahmad, Kenji Watanabe, Takashi Taniguchi, Daichi Kozawa, Ryo Kitaura

Materials Science

Symmetry-Selective Strain Control of Spin-Momentum Locking and Spin Transport in Two-Dimensional Pentagonal Altermagnets

ShuaiYu Wang, Sheng Chen, Xiao-Ping Li et al. (+1 more)

Altermagnets are compensated magnets featuring momentum-dependent nonrelativistic spin splitting generated by nontrivial operations connecting opposite-spin sublattices. A direct symmetry-based route to control this spin splitting is to modify the real-space operations that define the altermagnetic spin-momentum locking (SML). Here, we develop a strain-resolved symmetry framework for two-dimensional pentagonal altermagnets, classifying whether uniaxial and shear strain tensors preserve, reconstruct, or eliminate the SML. Using the above criterion combined with first-principles screening, we identify 94 stable altermagnetic candidates from 3330 materials. These candidates cover all type-III spin Laue groups of orthorhombic lattices and are classified into three strain-response types: Type-I preserves the SML; Type-II reconstructs the SML through partial symmetry breaking while retaining essential altermagnetic features; and Type-III destroys the altermagnetic SML. Representative materials further demonstrate this classification: ferroelastic $\alpha$-CoS$_2$ exhibits ferroelastically switchable SML and reverses the sign of the off-diagonal spin conductivity; shear-strained \(\alpha\)-CoP\(_2\) undergoes a \(g\)- to \(d\)-wave reconstruction of the SML, activating off-diagonal spin conductivity; and uniaxially strained FeSSe realizes strain-selected spin-valley transport. This work provides theoretical and material guidance for strain-controlled transport in two-dimensional orthorhombic altermagnets.
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All authors ShuaiYu Wang, Sheng Chen, Xiao-Ping Li, Lei Wang

Materials Science

Unlocking Cryogenic Energy Storage by Constructing Dipole Glass with Unit-cell-level Polar Disorder

Yangyang Si, Denan Li, Yijie Li et al. (+21 more)

Cryogenic energy storage is vital for frontier technologies including deep-space exploration and quantum computing, yet conventional electrochemical energy systems fail below ~230 K due to frozen ion migration. While relaxor-based dielectric capacitors provide high efficiency at room temperature, the intrinsic freezing/growth of polar nanodomains at extended cryogenic regime limits their applications with deteriorated hysteresis losses. Here, we realize superior cryogenic energy-storage performance by designing unit-cell-level disordered dipole-glass state in Pb0.6Sr0.4ZrO3 thin films with composition near antiferroelectric-paraelectric phase boundary. The antiferroelectric-derived dipole-glass introduces enhanced unit-cell-level complexity of dipole interaction that suppresses long-range ferroelectric order. This enables ultralow-hysteresis operation (efficiency > 88%) down to 4 K, delivering record-high energy density (211 J/cm^3) at 9 MV/cm, stability over 10^8 charge/discharge cycles and microsecond-scale charge/discharge capability. This work establishes a dipole-glass paradigm for cryogenic dielectric capacitors, opening a new avenue to highly-efficient energy-storage systems with broad applications in frontier nanoelectronics.
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All authors Yangyang Si, Denan Li, Yijie Li, Changsheng Chen, Jingxuan Li, Chao Zhou, Hao Xiong, Tianfu Zhang, Wenjin Liao, Zhongqi Ren, Huaicheng Yuan, Dong Li, Jing-Kai Qin, Cheng-Yan Xu, Ye Zhu, Yunlong Tang, Sujit Das, Jieun Kim, Junling Wang, Hao Pan, Fei Li, Zhen Chen, Shi Liu, Zuhuang Chen