Materials Science
Competing Hydrogenation Pathways to Metastable CaH$_6$ Revealed by Machine-Learning-Potential Molecular Dynamics
Ryuhei Sato, Peter I. C. Cooke, Maélie Caussé et al. (+7 more)
The synthesis of the high-$T_c$ superhydride CaH$_6$ has stimulated significant interest in understanding synthesis pathways for metastable hydrides. However, the microscopic mechanisms governing such hydrogenation reactions remain poorly understood. Here, we show that machine-learning potential molecular dynamics (MLP-MD) simulations can reproduce and distinguish competing reaction pathways leading to metastable and stable hydrides. By simulating hydrogenation reactions at CaH$_2$/H$_2$ and CaH$_4$/H$_2$ interfaces, we identify two distinct pathways that produce clathrate-type CaH$_6$ and A15-type CaH$_{5.75}$, respectively. CaH$_{5.75}$ lies on the convex hull but requires extensive Ca sublattice rearrangement and therefore forms only at elevated temperatures. In contrast, CaH$_6$ becomes kinetically accessible when CaH$_2$ is used as the precursor. The crystallographic compatibility between the Ca sublattice of CaH$_2$ and the bcc framework of CaH$_6$ enables a martensitic-like topotactic transformation that bypasses the reconstructive pathway leading to CaH$_{5.75}$. These results reveal how precursor structure and thermodynamic stability compete to determine superhydride formation pathways and demonstrate that machine-learning molecular dynamics can directly capture the kinetic selection of metastable phases in reactive materials systems.
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All authors Ryuhei Sato, Peter I. C. Cooke, Maélie Caussé, Hung Ba Tran, Seong Hoon Jang, Di Zhang, Hao Li, Shin-ichi Orimo, Yasushi Shibuta, Chris J. Pickard
Materials Science · Cross-listed from cond-mat.supr-con
Intrinsic magnetization of the superconducting condensate in Fe(Te,Se)
Mohammad Javadi Balakan, Shiva Heidari, Genda Gu et al. (+4 more)
A
spin-polarized superconducting condensate generates a net magnetization with measurable signatures. We present evidence for an intrinsic
magnetic field in mesoscopic Fe(Te,Se) rings. The intrinsic field, encoded in the phase of superconducting
quantum oscillations, scales linearly with the DC bias current, and its orientation exhibits an anomalous dependence on polarity and magnitude of the applied current. The magnetoresistance displays a dual flux-quantization effect with respect to the external
magnetic field and the DC current. A minimal model incorporating Rashba coupling with an effective anisotropic out-of-plane interaction accounts for the experimental observations. These results provide evidence for
spin-polarized superconductivity at the device scale and open new opportunities for superconducting spintronic and
quantum information platforms.
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All authors Mohammad Javadi Balakan, Shiva Heidari, Genda Gu, Qiang Li, Kenji Watanabe, Takashi Taniguchi, Ji Ung Lee
Materials Science
From Word2Vec to Transformers: Text-Derived Composition Embeddings for Filtering Combinatorial Electrocatalysts
Lei Zhang and Markus Stricker
Compositionally complex solid solution electrocatalysts span vast composition spaces, and even one materials system can contain more candidate compositions than can be measured exhaustively. Here we evaluate a label-free screening strategy that represents each composition using
embeddings derived from scientific texts and prioritizes candidates based on similarity to two property concepts. We compare a corpus-trained Word2Vec baseline with
transformer-based
embeddings, where compositions are encoded either by linear element-wise mixing or by short composition
prompts. Similarities to `concept directions', the terms conductivity and dielectric, define a 2-dimensional descriptor space, and a symmetric Pareto-front selection is used to filter candidate subsets without using electrochemical labels. Performance is assessed on 15 materials libraries including noble metal
alloys and multicomponent
oxides. In this setting, the lightweight Word2Vec baseline, which uses a simple linear combination of element
embeddings, often achieves the highest number of reductions of possible candidate compositions while staying close to the best measured performance.
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Materials Science
Machine-learning assistant DFT study of half-metallic full-Heusler alloy N2CaNa: structural, electronic, mechanical, and thermodynamics properties
E. B. Ettah, M.E. Ishaje, K. A. Minakova et al. (+2 more)
We studied the structural, electronic, mechanical, and thermodynamic properties of N2CaNa full Heusler
alloys using density functional theory (DFT). Results for the structural analysis establishes structural stability with a minimum formation energy of 29.90eV. The compounds is brittle and mechanically stable, having checked out with the Pugh criteria. B/G ratio for N2CaNa is 4.766 as computed in Tab.2, hence the material is ductile. N2CaNa
alloy is ductile in nature. Debye model correctly predicts the low temperature dependence of heat capacity, which is proportional to the debye T3 law. Just like the Einstein model, it also recovers the Dulong-Petit law at high temperatures, suggests thermodynamic stability of the compounds at moderate temperatures. The results demonstrate potential for applications in spintronics, structural engineering, and other fields requiring materials with tailored properties.
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All authors E. B. Ettah, M.E. Ishaje, K. A. Minakova, V.A. Sirenko, I. S. Bondar
Materials Science
Aligning van der Waals heterostructures using electron backscatter diffraction
R. Bangari, M. Mosayebi, J. Buchner et al. (+3 more)
Precise and accurate determination of crystallographic orientation is crucial for engineering van der Waals
heterostructures, where the twist angle between layers controls emergent electronic and optical properties. While Electron Backscatter Diffraction (EBSD) has been extensively used for bulk materials, its application to van der Waals materials remains largely unexplored. In this work, we demonstrate EBSD as a robust and versatile tool for determining crystallographic orientations of van der Waals materials with high precision. We show quantitative agreement between EBSD-determined orientations and facet orientations in orthorhombic {\alpha}-MoO3 flakes on silicon substrates. We use Grain Reference Orientation Distribution (GROD) and Kernel Average Misorientation (KAM) across the flakes to demonstrate precision better than 0.2{\deg}. We extend this technique to other low-
symmetry materials, specifically, monoclinic {\alpha}-As2Te3, monoclinic GaTe and triclinic ReSe2, demonstrating broad applicability across van der Waals materials with different
crystal structures. Finally, as a proof-of-concept application, we leverage EBSD-determined orientations to engineer twisted {\alpha}-MoO3
heterostructure with precisely controlled twist angle, enabling observation of recently reported canalized
phonon polaritons. Our results establish EBSD as a powerful characterization method for van der Waals materials, enabling precise orientation control essential for twistronics and twist-optics.
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All authors R. Bangari, M. Mosayebi, J. Buchner, J.D. Caldwell, N. Bassim, T. G. Folland
Materials Science
Uncovering the properties of homo-epitaxial GaN devices through cross-sectional infrared nanoscopy
Hossein Zandipour, Felix Kaps, Robin Buschbeck et al. (+19 more)
Validating material performance in electrical devices is crucial to product development. For Gallium Nitride (GaN) devices, evaluating material factors such as defects, dopant concentration, and overall production quality is essential to ensure their performance in advanced electronic and optoelectronic applications. This work demonstrates that scattering-type scanning near-field optical microscopy (s-SNOM) can meet the demanding performance requirements for characterizing homoepitaxial GaN devices. Specifically, we show that combining s-SNOM results in the mid-IR and terahertz (THz) spectral ranges can disentangle carrier and
lattice changes in a GaN p-i-n diode, which is not possible using one spectral range alone. We observe strong, resonant near-field signals near the LO
phonon mode of GaN that correlate well with point-dipole models. This data shows great sensitivity to the local carrier density, with changes on the order of 1018 cm-3 easily resolved experimentally. Further, we demonstrate high sensitivity to sub-surface defects, which remain a significant challenge for other non-destructive techniques. To validate the power of s-SNOM imaging, our results are compared to traditional metrologies, including micro-Raman mapping and Kelvin Probe Force Microscopy (KPFM). Our results show that s-SNOM shows superior resolution and sensitivity to perturbations, highlighting the power of this technique in
semiconductor device characterization.
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All authors Hossein Zandipour, Felix Kaps, Robin Buschbeck, Maximilian Obst, Aditha Senarath, Richarda Niemann, Niclas S. Mueller, Gonzalo Alvarez-Perez, Katja Diaz-Granados, Ryan A Kowalski, Jakob Wetzel, Raghunandan Balasubramanyam Iyer, Matthew Wortel, J. Michael Klopf, Travis Anderson, Alan Jacobs, Mona Ebrish, Lukas M. Eng, Alexander Paarman, Susanne C. Kehr, Joshua D. Caldwell, Thomas G. Folland
Materials Science
Ground-State Structure Search of Defective High-Entropy Alloys Using Machine-Learning Potentials and Monte Carlo Sampling
Siya Zhu and Raymundo Arroyave
Resolving the atomic-scale structure of defective high-
entropy alloys (HEAs) containing interstitial species remains a major computational challenge due to the vast configurational space and the limitations of existing methods. Here we introduce PAIPAI (Package for
Alloy Interstitial Predictions using Artificial Intelligence), a Monte Carlo framework coupled with machine-learning interatomic potentials (MLIPs) that searches for ground-state atomic configurations in HEAs with defects and interstitials. PAIPAI employs a dual-worker architecture-fast workers for rapid configurational screening and slow workers for high-accuracy refinement-coordinated through a shared waiting pool, enabling efficient parallel sampling. We demonstrate PAIPAI through three case studies: (i) surface segregation in a Ti-V-Cr-Re slab; (ii) interstitial oxygen and boron aggregation in bulk BCC Nb-Ti-Ta-Hf; and (iii) coupled metallic and interstitial segregation at grain boundaries in Nb-Ti-Ta-Hf. In all cases, Monte Carlo-optimized structures are significantly lower in energy than any configuration obtained by random sampling, and MLIP energy rankings are validated against density functional theory calculations. PAIPAI provides a general and efficient framework for predicting atomic ordering, segregation, and interstitial behavior in complex, defective HEA systems.
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Materials Science
Weyl-Transition-Driven Giant Reversible Orbital Hall Conductivity
Bo Zhao, Hao Wang, Wei Ren et al. (+1 more)
Orbital
Hall conductivity (OHC) is a central ingredient of orbitronics, yet how to control it microscopically remains largely unexplored. Here we identify a general mechanism in which tilted Weyl crossings formed by orbitally distinct bands generate a strongly asymmetric orbital Berry
curvature (OBC) distribution, whose imbalance survives Brillouin-zone integration and yields a sizable OHC already at zeroth order. Using first-principles calculations, we show that monolayer PtBi2 realizes this mechanism and hosts a
giant OHC dominated by a type-II Weyl point. A small biaxial tensile strain drives a type-II $\rightarrow$ type-I $\rightarrow$ type-II Weyl transition, leading to a reversible sign change of the OHC through the
evolution of the OBC imbalance. This process is governed by the
chiral orbital texture of the crossing bands and is further assisted by a strain-induced first-order structural phase transition through bonding reconstruction and polarization change. Our results establish Weyl engineering of orbital
quantum geometry as a powerful route to generating and reversibly controlling OHC in polar multi-orbital materials.
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All authors Bo Zhao, Hao Wang, Wei Ren, Hongbin Zhang
Materials Science
A Kagome-Derived Mosaic Lattice Family A3V9Te13 (A = Cs, Rb) with Tunable Strong Electronic Correlations
Yusen Xiao, Zhibin Qiu, Qingchen Duan et al. (+5 more)
The pursuit of geometrically frustrated
lattices beyond conventional paradigms remains a central challenge in the design of
quantum materials. Herein, we report the discovery of the A3V9Te13 (A = Cs, Rb) family of vanadium-based intermetallic compounds, which host a unique two-dimensional Mosaic
lattice derived from the Kagome network, composed of an ordered tessellation of triangles, squares, and pentagons. The Cs compound (CVT) exhibits strong electronic correlations, characterized by non-
Fermi liquid behavior at low temperatures, an exceptionally large Sommerfeld coefficient, and a bulk phase transition at T* $\approx$ 47 K with possible charge- or
spin-related origin. Inspired by pressure-tuning in related Kagome systems, we demonstrate that the electronic ground state of this
lattice is exquisitely tunable via chemical pressure. Systematic substitution of Cs with smaller Rb ions suppresses the T* phase transition and the correlated electronic response, ultimately driving the system into a highly frustrated semiconducting ground state without long-range
magnetic order down to 60 mK. This work unveils a new structural platform for exploring the interplay between
geometric frustration and strong electron correlations, providing a chemically controllable platform for exploring the phase space between distinct correlated electronic states.
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All authors Yusen Xiao, Zhibin Qiu, Qingchen Duan, Zhaoyi Li, Xiaotong Xu, Hengxin Tan, Shu Guo, Ruidan Zhong
Materials Science
Au and Ag nanoparticles produced by ion implantation in single-crystalline $\beta$-Ga$_2$O$_3$
Duarte Magalhães Esteves, Ana Sofia Sousa, Inês Freitas et al. (+5 more)
This work reports the successful formation of Ag and Au nanoparticles in $\beta$-Ga$_2$O$_3$ single-
crystals by ion implantation and annealing at 550 {\deg}C. X-ray diffraction measurements revealed that nanoparticles were formed after the annealing step, presenting a highly-ordered crystalline structure and conforming to a crystallographic relation with respect to the
matrix: $\left(0\overline{1}0\right)_\beta \parallel \left(110\right)_{\mathrm{Ag/Au}}$ and $\left[102\right]_\beta \parallel \left[1\overline{1}2\right]_{\mathrm{Ag/Au}}$. The presence of these nanoparticles was also confirmed via absorbance measurements revealing the localised surface plasmon resonance peaks associated with these particles. Considering the multiple advantages and the versatility of metallic nanoparticles, their combination with the exceptional properties of $\beta$-Ga$_2$O$_3$ paves the way for a wide range of applications.
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All authors Duarte Magalhães Esteves, Ana Sofia Sousa, Inês Freitas, Ângelo Rafael Granadeiro da Costa, Joana Madureira, Sandra Cabo Verde, Katharina Lorenz, Marco Peres
Materials Science
Anisotropic implantation damage build-up and crystal recovery in $\beta$-Ga$_2$O$_3$
Duarte Magalhães Esteves, Sérgio Magalhães, Ângelo Rafael Granadeiro da Costa et al. (+2 more)
The present work aims at investigating the defect accumulation and recovery dynamics in the inherently anisotropic $\beta$-Ga$_2$O$_3$
lattice. A systematic Rutherford Backscattering Spectrometry in Channelling mode (RBS/C) analysis of Cr-implanted samples was performed across multiple surface orientations and channelling directions. Distinct apparent defect accumulation and annealing rates were observed along different channelling axes, mainly attributed to the shadowing of certain types of defects along some directions. The efficient defect removal observed after annealing was correlated with the strain relaxation observed via High-Resolution X-ray diffraction (HRXRD) at temperatures as low as 500 {\deg}C, which is attributed to the removal of point defects. Annealing at higher temperature further improves crystalline quality but at a slower rate. In short, this work enhances the understanding of the effect of structural anisotropic properties of $\beta$-Ga$_2$O$_3$ during ion implantation, as well as the
crystal recovery during thermal annealing, highlighting the interplay between crystallography and defect dynamics.
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All authors Duarte Magalhães Esteves, Sérgio Magalhães, Ângelo Rafael Granadeiro da Costa, Katharina Lorenz, Marco Peres
Materials Science
Defect-induced multiferroicity in bulk solid solutions of WSe$_2$ and WTe$_2$
H. Rojas-Páez, G. Villabón-Linares, J. Pazos et al. (+5 more)
Transition metal dichalcogenides provide a versatile platform for tunable ferroic phenomena at the atomic scale owing to their reduced dimensionality. Here we investigate the structural,
magnetic, and ferroelectric properties of bulk solid solution W(Se1-xTex)2(1-delta) single
crystals synthesized by chemical vapor transport. The room temperature behavior is analyzed as a function of tellurium concentration (x) and chalcogen defect fraction (delta). X ray diffraction and Raman spectroscopy reveal
lattice expansion and
symmetry reduction with increasing x, consistent with a 2H to 1Td structural transition above a critical composition xc about 18 percent. Piezoresponse force microscopy identifies piezoelectricity near stoichiometric compositions (delta less than 5 percent) and switchable ferroelectricity in the chalcogen deficient regime (delta greater than 20 percent). Magnetometry measurements show a corresponding
evolution from paramagnetic to
ferromagnetic behavior with increasing delta. Near stoichiometric Te poor samples exhibit piezoelectric and paramagnetic responses, whereas multiferroic states characterized by the coexistence of ferroelectric and
ferromagnetic responses emerge at high vacancy concentrations. The performed characterizations indicate that x primarily governs structural
symmetry, while delta controls the emergence of both
ferromagnetic and ferroelectric responses. These trends are summarized in a configurational phase diagram highlighting the cooperative influence of dopants and defects on ferroic behavior. Overall, controlled stoichiometry and vacancy engineering offer an effective strategy to tailor ferroic responses in transition metal dichalcogenides.
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All authors H. Rojas-Páez, G. Villabón-Linares, J. Pazos, E. Ramos, R. Moreno, O. Herrera-Sandoval, J. A. Galvis, P. Giraldo-Gallo
Materials Science · Cross-listed from cond-mat.soft
Modeling the Slow Arrhenius Process (SAP) in Polymers
Valeriy V. Ginzburg, Oleg V. Gendelman, Simone Napolitano et al. (+2 more)
Amorphous glass-forming
polymers exhibit multiple relaxation processes, including the structural {\alpha}-relaxation associated with the glass transition and faster secondary relaxations that typically follow Arrhenius behavior. Recently, a distinct slow Arrhenius process (SAP) has been observed at frequencies well below the {\alpha}-process. Although Arrhenian in its temperature dependence, the SAP involves much longer relaxation times and its microscopic origin remains unclear. Here, we extend the two-state, two-timescale (TS2) theory to describe both the {\alpha}-relaxation and the SAP within a unified framework. We propose that the SAP represents the high-temperature limit of an {\alpha}-like process in a coarse-grained fluid of dynamically correlated clusters. With renormalized interaction energies and coordination parameters, the same model quantitatively reproduces both {\alpha} and SAP data across multiple
polymers without additional adjustable parameters and explains the observed Meyer-Neldel compensation behavior. The theory further predicts that the SAP should deviate from Arrhenius behavior at sufficiently low temperatures, transitioning to Vogel-Fulcher-Tammann-Hesse-like dynamics, thereby offering a physically transparent interpretation of cluster-scale relaxation in glass-forming
polymers.
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All authors Valeriy V. Ginzburg, Oleg V. Gendelman, Simone Napolitano, Riccardo Casalini, Alessio Zaccone
Materials Science
Second harmonic study of thermally oxidized mono- and few-layer 2H-MoS2
Katharina Burgholzer, Henry Volker Hübschmann, Gerhard Berth et al. (+5 more)
A comprehensive study of second harmonic generation on thermally oxidized MoS2 flakes with thickness ranging from monolayer up to seven layers is presented. Observing the fundamental nonlinear behavior for non-treated and oxidized MoS2 reveals that oxidation causes significant changes in the second harmonic (SH) response for all investigated structures. Excitation power dependent measurements to analyze the nonlinear behavior with respect to the oxidation time show progressive oxidation within the maximum oxidation time of six hours, under the considered oxidation conditions. Here, polarization dependent measurements reveal the structural changes due to oxidation. Additionally, it is found that the oxidation depth is restricted to the top most layer and the oxidation behavior exhibits a layer dependency. These findings are supported by theoretical band structure calculations. The results demonstrate that the thermal oxidation progress of two dimensional MoS2 can be monitored with non-resonant and non-invasive SH microscopy, by following distinct fingerprints of structural modification in the nonlinear response.
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All authors Katharina Burgholzer, Henry Volker Hübschmann, Gerhard Berth, Adriana Bocchini, Uwe Gerstmann, Wolf Gero Schmidt, Klaus D. Jöns, Alberta Bonanni
Materials Science
Unveiling the Thermal and Aqueous Stability of 1D Lepidocrocite Titania
Risha A. Iythichanda, Sukanya Maity, Mustafa M. Aboulsaad et al. (+3 more)
One dimensional lepidocrocite titanium dioxide filaments are investigated with respect to their thermal and aqueous stability. Structural and phase
evolution are examined using in situ heating in
vacuum within transmission electron microscopy combined with electron energy loss spectroscopy, and at ambient conditions using Raman spectroscopy. The filaments retain their lepidocrocite structure up to 300 degree and above which localized sintering and amorphization occur at filament overlap junctions. With further heating, the amorphous regions crystallize into anatase, with Raman spectroscopy corroborating the onset of structural disorder. Long term aqueous storage up to 100 days at ambient conditions induces transformation into flake like anatase nanoparticles. This process is strongly suppressed under refrigerated storage, where no structural changes are observed over the same period. These results establish critical thermal and environmental stability thresholds that define operational advantages and limits for emerging applications of 1D lepidocrocite filaments.
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All authors Risha A. Iythichanda, Sukanya Maity, Mustafa M. Aboulsaad, Tomas Edvinsson, Johanna Rosen, Per O.Å. Persson
Materials Science
Structural phase transitions in double perovskite crystals studied by Brillouin light scattering
D. O. Horiachyi, M. O. Nestoklon, I. A. Akimov et al. (+5 more)
Inorganic lead-free double perovskites represent particular interest as non-toxic and stable material platform for optoelectronic applications. Here, we employ Brillouin light scattering spectroscopy to investigate the elastic properties and structural phase transitions in single
crystals of Cs2AgBiBr6 and Cs2AgBiCl6. A complete set of elastic constants is determined from the Brillouin scattering measurements performed along three different crystallographic directions. Both materials exhibit similar elastic constants and weak elastic anisotropy in the cubic phase. At low temperatures, the lifting of degeneracy of transverse acoustic
phonon modes is attributed to a lowering of
crystal symmetry. From the temperature dependence of the acoustic
phonon frequencies, we determine the structural phase transition temperature of about 43 K for Cs2AgBiCl6, compared to 122 K for the cubic-to-tetragonal phase transition in Cs2AgBiBr6.
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All authors D. O. Horiachyi, M. O. Nestoklon, I. A. Akimov, D. R. Yakovlev, V. Vasylkovskyi, O. Trukhina, V. Dyakonov, M. Bayer
Materials Science · Cross-listed from cond-mat.soft
A thermodynamic metric quantitatively predicts disordered protein partitioning and multicomponent phase behavior
Zhuang Liu, Beijia Yuan, Mihir Rao et al. (+2 more)
Intrinsically disordered regions (IDRs) of
proteins mediate sequence-specific interactions underlying diverse cellular processes, including the formation of biomolecular condensates. Although IDRs strongly influence condensate compositions, quantitative frameworks that predict and explain their phase behavior in complex mixtures remain lacking. Here we introduce a thermodynamic model that quantitatively predicts the behavior of arbitrary combinations of IDRs across a wide range of concentrations, with accuracy comparable to state-of-the-art simulations. The model learns low-dimensional, context-independent representations of IDR sequences that combine to form mixture representations, producing context-dependent interactions. These representations define a thermodynamic metric space in which distances between IDRs correspond directly to differences in their thermodynamic properties. We show that the model predicts multicomponent phase diagrams in quantitative agreement with molecular simulations without being trained on free-energy or phase-coexistence data. The metric space provides geometrically intuitive predictions of IDR partitioning, multicomponent condensation, and context-dependent mutational effects, addressing several central problems in IDR biophysics within a single model. Systematic interrogation of the learned representations reveals how amino-acid composition and sequence patterning jointly determine mixture thermodynamics. Together, our results establish a unified and interpretable framework for predicting and understanding the behavior of complex mixtures of IDRs and other sequence-dependent biomolecules.
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All authors Zhuang Liu, Beijia Yuan, Mihir Rao, Gautam Reddy, William M. Jacobs
Materials Science
Atomic-resolution imaging of gold species at organic liquid-solid interfaces
Sam Sullivan-Allsop, Nick Clark, Wendong Wang et al. (+26 more)
Understanding solid-
liquid interfaces at the atomic-scale is key to improved performance of heterogeneous catalysts, electrodes and membranes. Here we combine unique specimen design, record atomic resolution in situ electron microscopy, and artificial intelligence-enabled analysis to achieve a step change in quantitative understanding of interfacial atomic behaviour. We create the first
graphene liquid cells with organic solvents and employ them to track over 106 gold adatoms and clusters at a
graphene surface immersed in acetone and cyclohexanone. We reveal dynamic correlated behaviour of gold adatom monomers, dimers, trimers and clusters, strongly influenced by each other, the solvent properties, and the atomic
lattice of the substrate, in good agreement with theoretical calculations. We use the results to interpret differences in catalytic activity towards the industrially important acetylene hydrochlorination reaction. This new capability for exploration of atomic scale chemistry could enable rational design of future catalysts, membranes and electrodes with improved functionality.
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All authors Sam Sullivan-Allsop, Nick Clark, Wendong Wang, Rongsheng Cai, William Thornley, David G. Hopkinson, James G. McHugh, Ben Davies, Samuel Pattisson, Nicholas F. Dummer, Rui Zhang, Matthew Lindley, Gareth Tainton, Jack Harrison, Hugo De Latour, Joseph Parker, Joshua Swindell, Eli G. Castanon, Amy Carl, David J. Lewis, Natalia Martsinovich, Christopher S. Allen, Mohsen Danaie, Andrew J. Logsdail, Vladimir Falko, Graham J. Hutchings, Alex Summerfield, Roman Gorbachev, Sarah J. Haigh
Materials Science · Cross-listed from physics.optics
Large differential attosecond delays in solid state photoemission
Andreas Gebauer, Walter Enns, Sergej Neb et al. (+11 more)
Time-resolved photoelectron spectroscopy provides access to the electronic structure and non-
equilibrium electron dynamics in matter. At solid surfaces photoemission dynamics can be investigated on its natural time scale by measuring attosecond time delays of emitted electrons. Photoelectrons with final state energies of several tens of eV need tens to hundreds of attoseconds to be released into the
vacuum. Competing effects determine the emission dynamics and, hence, the full picture of the process is still under debate. The rather large energy differences between the final states probed in commonly reported relative photoemission delays obscure their complex fine structure and hinders the interpretation of the measurements. Here we report differential attosecond delays $\tau_{\mathrm{DAD}}$, i.e., relative photoemission delays for energetically close-lying
spin-orbit split states. Differential attosecond delays on the order of 30 to 100 as for Bi 5d, Te 4d, and Se 3d core level photoemission from Bi$_2$Te$_3$ and Bi$_2$Se$_3$ can neither be attributed to intra-atomic delays, nor to ballistic transport and subsequent emission. Instead, calculations based on the one-step photoemission theory reveal that photoemission delays vary strongly on the energy scale of the
spin-orbit splitting and quantitatively match experimental observations. This strong variation arises from multiple scattering at the surface leading to final states that involve both evanescent and propagating Bloch waves. Their relative amplitudes vary strongly affecting thereby the timing of the photoemission event since evanescent and propagating components exhibit inherently different dynamics.
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All authors Andreas Gebauer, Walter Enns, Sergej Neb, Tillmann Schabbehard, Luis Maschmann, Stefan Muff, J. Hugo Dil, Ulrich Heinzmann, Stephan Fritzsche, Ricardo Diez Muiño, Pedro M. Echenique, Nikolay M. Kabachnik, Eugene E. Krasovskii, Walter Pfeiffer
Materials Science
Strain-driven magnetic anisotropy and spin reorientation in epitaxial Co V 2 O 4 spinel oxide thin films
Lamiae El Khabchi, Laurent Schlur, Jérôme Robert et al. (+9 more)
CoV___O___ (CVO) stands out among spinel vanadates for its ultra-short V-V distances, placing it at the brink of itinerant electron behaviour-an ideal playground for strain engineering. In this work, we exploit this sensitivity by growing high-quality epitaxial CVO thin films on SrTiO___ (001) and MgO (001), inducing compressive and tensile strain, respectively. Using pulsed
laser deposition under ultra-low oxygen pressure, we achieve high crystalline quality and straincontrolled tetragonal distortions: c > a under compression (STO) and c < a under tension (MgO). Resonant elastic X-ray scattering confirms a normal spinel structure, with cobalt occupying tetrahedral sites and vanadium octahedral ones. Both strain types reduce charge transport, driving the system into a highly resistive state.
Magnetic measurements reveal strain-driven anisotropy switching: STO films transition from out-of-plane to in-plane easy axis below 90 K, while MgO films flip from in-plane to out-of-plane below 45 K. These results highlight CVO's exceptional responsiveness to
lattice strain, unlocking a path to finely tunable electronic and
magnetic properties. With its strong
spin-
lattice coupling and potential in
spin Hall magnetoresistance, strained CVO emerges as a compelling platform for next-generation lowpower spintronic devices.
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All authors Lamiae El Khabchi, Laurent Schlur, Jérôme Robert, Marc Lenertz, Cédric Leuvrey, Gilles Versini, François Roulland, Gilbert Chahine, Nils Blanc, Daniele Preziosi, Christophe Lefèvre, Nathalie Viart
Materials Science · Cross-listed from cond-mat.mes-hall
Layer-Dependent Orbital Magnetization in Graphene-Haldane Heterostructures
Sovan Ghosh and Bheema Lingam Chittari
Rhombohedral multilayer
graphene (RMG) proximity-coupled to a Haldane substrate provides a platform to investigate the interplay between band
topology, layer number, and electric-field control of orbital magnetism. Using a tight-binding model and the modern theory of orbital magnetization, we study the layer-dependent
magnetic response in
bilayer, trilayer, tetralayer and pentalayer
graphene under Haldane proximity. While monolayer
graphene develops a global topological gap with quantized magnetization slope, multilayer systems remain metallic due to protected low-energy bands associated with unperturbed sublattices. Despite the absence of a global gap, finite valley-contrasting Berry
curvature produces non-trivial layer-dependent Chern numbers. We decompose the total orbital magnetization into self-rotation ($M_{\mathrm{SR}}$) and center-of-mass ($M_C$) contributions, revealing their distinct behaviors across doping and applied interlayer bias. In
bilayer graphene, magnetization remains negative and monotonic. Remarkably, trilayer and tetralayer
graphene display a bias-induced sign reversal of orbital magnetization beyond critical thresholds ($\Delta \simeq -55$ meV for 3LG, $-50$ meV for 4LG) in the hole-doped regime, a feature completely absent in the
bilayer. It is further established in the case of 5LG, that the magnetization reversal is independent of the topological transition, and depends on the direction of bias and hole doping. The effect persists across both hole and electron doping, demonstrating that layer count serves as a key tuning parameter for orbital magnetism. Our findings establish topologically proximitized multilayer
graphene as a versatile platform for electric-field-manipulable orbitronic and valleytronic devices.
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Materials Science · Cross-listed from physics.class-ph
Unimode material based low-frequency underwater acoustic isolation
Yu Wei, Binghao Zhao, Fen Du et al. (+2 more)
Extremal materials are a specific class of Cauchy materials whose elasticity tensor has one or more zero
eigenvalues. Each zero
eigenvalue corresponds to a soft mode requiring zero strain energy, while non-zero
eigenvalues correspond to hard modes that cost energy. According to the number, N, of zero
eigenvalues, these materials can be referred to as unimode (N=1), bimode (N=2), etc. Extremal materials have enabled novel functions beyond conventional Cauchy media, e.g.,
phonon polarizers, Rayleigh wave isolators and
underwater acoustic cloaks. These functions typically require a single extremal material. Interfaces between two extremal materials exhibit rich wave behaviors, yet have been seldom explored. Here, we proposed the concept of complementary extremal materials, i.e., the soft mode of one extremal material is a hard mode of the other. As one example, we study the interface between an isotropic unimode material and an isotropic bimode material. We show that the interface allows perfect mode conversion from longitudinal waves to transverse waves. A low-frequency
underwater acoustic
insulator based on complementary extremal materials is proposed. Our finding has been verified with designed metamaterials and using effective-medium modeling. This work demonstrates the potential of complementary extremal materials in controlling elastic wave polarization and waterborne sound.
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All authors Yu Wei, Binghao Zhao, Fen Du, Yi Chen, Gengkai Hu
Materials Science
Unexpected Planar Dislocation Boundary Formation in FCC Metals Captured by Dark-Field X-ray Microscopy and Continuum Dislocation Dynamics
Adam André William Cretton, Khaled SharafEldin, Axel Henningsson et al. (+8 more)
Validating dislocation patterning models against in situ imaging experiments is a longstanding goal in materials physics. Here, we provide the first direct morphological comparison of such models. Using in situ Dark-Field X-ray Microscopy (DFXM), we map the local orientations in high-purity aluminium deformed along [100] and find unexpected planar dislocation boundaries aligned with {111} slip planes that form prior to the development of a conventional dislocation
cell structure. To explain this behaviour, we generate synthetic DFXM contrast images from a continuum dislocation dynamics (CDD) simulation. This mesoscale model, using nickel as a high stacking fault energy (SFE) FCC analogue, independently predicts the formation of the same {111} planar boundary types. This correspondence demonstrates that state-of-the-art CDD and DFXM experimental data can be used synergistically - despite differences in strain rates and length scales - as a practical route for refining continuum theories of plasticity.
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All authors Adam André William Cretton, Khaled SharafEldin, Axel Henningsson, Felix Frankus, Can Y{\i}ld{\i}r{\i}m, Carsten Detlefs, Flemming Bjerg Grumsen, Albert Zelenika, Anter El-Azab, Grethe Winther, Henning Friis Poulsen
Materials Science
Role of photonic interference in exciton-mediated magneto-optic responses
Güven Budak, Christian Riedel, Akashdeep Kamra et al. (+4 more)
Coupled optical and
magnetic excitations can give rise to remarkably strong magneto-optic responses. This is particularly evident in van der Waals magnets, such as the antiferromagnet CrSBr, where
excitons and
magnons emerge from the same electronic orbitals. While previous work has primarily focused on uncovering the magneto-electric origin of the resulting
exciton-
magnon interactions, the influence of photonic effects has received comparatively little
attention. Here, we use numerical simulations to disentangle
exciton-
magnon coupling from the
exciton-mediated
magnon-
photon interactions observed in optical experiments. Our simulations show the strong dependence of these interactions on photonic interference and dispersion effects near excitonic resonances. Such effects shape the optical response to coherent
magnons and make it intrinsically non-linear in the
magnon-induced
exciton energy shift. Thermal
magnons, which have a particularly pronounced impact on
excitons, are found to even produce qualitatively different trends in optical signatures. Depending on weak or strong coupling of
excitons and
photons, the same
exciton-
magnon interaction can lead to a red-shift of optical modes, a nearly vanishing response, or their blue-shift. Finally, we demonstrate first steps towards optimizing the multi-parameter problem of efficient
magnon-
photon transduction using a machine-learning approach.
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All authors Güven Budak, Christian Riedel, Akashdeep Kamra, Patrick Rinke, Christian Back, Matthias Stosiek, Florian Dirnberger
Materials Science
Shape Selection in Nanopillar Formation
Marta A. Chabowska and Magdalena A. Załuska-Kotur
Crystal growth processes produce a diverse array of surface formations, primarily distinguished by their
geometric shapes. While some structures strictly adhere to the underlying
crystal symmetry, others exhibit universal circular or oval geometries. Utilizing Vicinal Cellular Automata (VicCA) modeling, we demonstrate that these morphological differences depend on the spatial distribution of the growth potential. Specifically, local potential variations concentrated around surface steps drive the formation of the
lattice symmetry - following structures, whereas global potentials - often originating from defects-generate universal spherical or oval shapes. Furthermore, we illustrate how these morphologies are influenced by the growth parameters such as sticking coefficient or
diffusion coefficient. Although the positioning of surface defects is difficult to control, we show that temperature and external particle flux can be effectively used to steer and manipulate surface pattern formation.
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Materials Science
Rethinking Charge Transport and Recombination in Donor-diluted Organic Solar Cells
Chen Wang, Christopher Wöpke, Toni Seiler et al. (+12 more)
We systematically investigate PM6:Y12 bulk-heterojunction
solar cells with donor fractions ranging from 1% to 45%, linking morphology, charge transport, and recombination to device performance. Complementary structural and spectroscopic methods reveal that a percolating PM6 network forms even at below 5% donor content, with lamellar stacking and vertical composition gradients that do not hinder the charge extraction. The reduction of the effective active layer conductivity towards low donor fractions obeys a three-dimensional percolation model, indicating that charge transport is governed by network
topology rather without a pronounced percolation threshold. A transition from nongeminate Langevin recombination to a dispersive Smoluchowski-type loss occurs below 5% donor fraction. The latter regime is also nongeminate, i.e., pertains to recombination of the total charge carrier density. Correspondingly, we observe that the Langevin reduction in the higher donor fractions - mostly dominated by redissociation of electron-hole pairs after encounter - changes towards low donor fractions: in these cases, the nongeminate loss rate exceeds the prediction of the Langevin model. This regime coincides with increasing transport resistance due to
topology-limited hole conduction, leading to reduced fill factors despite a high retained charge-generation efficiency. Our results demonstrate that strong donor dilution preserves photogeneration if a continuous donor network is maintained, and unveil how
topology-controlled transport and non-Langevin recombination jointly define the performance limits of donor-diluted organic
solar blends.
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All authors Chen Wang, Christopher Wöpke, Toni Seiler, Jared Faisst, Mathias List, Meike Kuhn, Bekcy Joseph, Alexander Ehm, Dietrich R. T. Zahn, Yana Vaynzof, Eva M. Herzig, Roderick C. I. Mackenzie, Uli Würfel, Maria Saladina, Carsten Deibel
Materials Science
The giant anomalous Hall and Nernst effects in Kagome permanent magnets RCo5
Weian Guo, Pengyu Zheng, Rui Liu et al. (+3 more)
Kagome
lattice materials have attracted considerable
attention due to their intriguing topological properties and potential applications in next-generation
quantum and spintronic technologies. In particular, rare-earth permanent magnets with Kagome structure provide an ideal platform that combines robust magnetism with nontrivial
quantum phenomena. However, their anomalous transport properties, particularly thermoelectric responses, remain insufficiently explored. In this work, we perform systematic first-principles calculations on the anomalous
Hall and anomalous Nernst effects in Kagome permanent magnets RCo5 (R = Ce, La, Sm, Gd). We find that CeCo5 exhibits a pronounced anomalous
Hall conductivity of about 1500 Omega^-1 cm^-1 while GdCo5 displays a substantial anomalous Nernst conductivity of 11 A m^-1 K^-1 within +/- 0.1 eV of the
Fermi energy, both comparable to or surpassing the measured intrinsic values reported in many typical Weyl and Heusler magnets. These exceptional anomalous transport properties originate from Berry
curvature hotspots near
spin-orbit coupling induced band gaps. If validated, these theoretical predictions would be important for Berry-
curvature-driven transport in
magnetic intermetallics. Our results establish RCo5 compounds as versatile platforms for exploring Berry
curvature-driven transport in tunable
magnetic topological materials.
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All authors Weian Guo, Pengyu Zheng, Rui Liu, Yiran Peng, Ying Yang, Zhiping Yin
Materials Science
Non-Markovian heat production in ultrafast phonon dynamics
Fredrik Erikssonm Yulong Qiao, Erik Fransson, R. Matthias Geilhufe et al. (+1 more)
High-intensity THz
laser pulses enable the light-mediated control of
lattice vibrations by resonantly driving selected
phonon modes. On ultrafast timescales,
memory effects influence the
phonon dynamics and must be accounted for to describe the heat production associated with energy dissipation. Here, we establish a microscopic framework for non-Markovian
phonon dynamics by deriving the noise and dissipation kernels governing a driven
phonon mode. Using large-scale molecular dynamics simulations, we reconstruct these kernels directly from the many-body
lattice dynamics and determine the corresponding heat production rate. Our results provide a quantitative picture of the crossover between Markovian and non-Markovian dynamics on picosecond timescales and show how the finite bandwidth of the driving field limits the dynamically relevant bath spectrum. Furthermore, we demonstrate that thermodynamic quantities such as heat production can be inferred directly from the dynamics of an individual
phonon mode, enabling their experimental measurement using time-resolved spectroscopy.
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All authors Fredrik Erikssonm Yulong Qiao, Erik Fransson, R. Matthias Geilhufe, Paul Erhart
Materials Science · Cross-listed from cond-mat.mes-hall
Quantum metric senses a persistent spin helix
Awadhesh Narayan
Persistent
spin helices are a manifestation of
symmetry-protected
spin textures in systems with balanced
spin-orbit coupling. They enable long-lived
spin structures that are of interest for spintronics and coherent
spin manipulation. The
quantum metric has recently emerged as a promising tool for characterizing the
geometric structure of
quantum states. Here, we demonstrate that the
quantum metric provides a sensitive
geometric probe of the persistent
spin helix. Within the Rashba-Dresselhaus
Hamiltonian, we analytically evaluate the
quantum metric components and uncover a divergent
geometric contribution that emerges precisely at the persistent
spin helix condition. We reveal that this divergence originates from a hidden line degeneracy that forms when the strengths of Rashba and Dresselhaus
spin-orbit coupling become equal. We further study the role of higher-order cubic
spin-orbit interactions and determine how these corrections regularize the
geometric response and control the scaling behavior of the
quantum metric. Our results establish
quantum geometry as a powerful framework for identifying and characterizing persistent
spin helices and related
symmetry-protected
spin textures.
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Materials Science
Melting behavior and dynamical properties of Cr2Ge2Te6 phase-change material
Suyang Sun, Yihui Jiang, Riccardo Mazzarello et al. (+1 more)
Cr2Ge2Te6 (CrGT) is known as an intrinsic
ferromagnetic semiconductor and a promising candidate for phase-change
memory applications. In amorphous CrGT, Cr atoms form non-defective octahedral motifs with Te atoms, similar to those in the crystalline phase. The abundance of Cr[Te6] octahedra is regarded as the key structural factor in reducing the resistance drift coefficient of amorphous CrGT. However, the stage at which these octahedra emerge during melt-quench amorphization remains unclear. Here, we present ab initio molecular dynamics (AIMD) simulations to model the melting process of crystalline CrGT and to investigate the dynamical properties of
liquid and supercooled
liquid CrGT in detail. Upon heating, Ge atoms are observed to leave their
lattice sites earlier than Cr and Te atoms, diffusing into the van der Waals gap and initiating the collapse of the layered structure. The Cr[Te6] octahedra are more robust, maintaining their structural pattern up to 1400 K despite continuous rupture and re-formation of Cr-Te bonds. At higher temperatures, Cr and Te atoms start to migrate independently. In supercooled
liquid CrGT at 550 K, most Cr-centered octahedra remain intact, with only limited Cr-Te bond breaking. The collective motion of these octahedra in this temperature regime helps explain why crystallization in CrGT devices can be accomplished in tens of nanoseconds.
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All authors Suyang Sun, Yihui Jiang, Riccardo Mazzarello, Wei Zhang