Materials Science · Cross-listed from cond-mat.mes-hall
Atul Rathor, Sahanawaj Akhtar and Arijit Haldar
We predict an in-plane, or hidden, Berry
curvature (BC) for
magnons in electrically insulating quasi-2D magnets and demonstrate that the hidden
magnon Berry
curvature (HMBC) gives rise to a previously unrecognized form of vertical, out-of-plane,
magnon transport. Combining a semiclassical framework with Boltzmann transport theory, we show that the vertical
magnon transport (VMT) currents respond both linearly and nonlinearly to the in-plane gradients of
magnetic field and temperature. The linear transport coefficients are tied to the total hidden
magnon BC, while the nonlinear (second-order) coefficients for the
magnetic field and temperature gradients are determined by the hidden
magnon BC dipole and the hidden extended
magnon BC dipole, respectively. Using linear
spin-wave theory, we find that the hidden
magnon BC over the Brillouin zone is given by the expectation value of a pseudo-${{\cal Z}}$ operator, representing vertical displacements, evaluated in the space of paraunitary
matrices that diagonalize the
magnon Hamiltonian. We estimate VMT in
spin models of realistic magnets with ferro- and
antiferromagnetic order, including the buckled honeycomb (BHC)
lattice and
bilayer Chromium trihalide (CrX$_3$; X = Cl, Br, I) systems. In BHC, both linear and nonlinear VMT arise when time-reversal
symmetry is broken by Dzyaloshinskii-Moriya interactions. In CrX$_3$ systems, the nonlinear coefficients dominate, while the linear responses vanish due to time-reversal
symmetry. Both systems exhibit distinctive features across a broad range of temperatures and parameters. Therefore, our prediction of VMT and its characteristic signatures is directly testable in present-day magnonic experiments, especially in atomically thin, few-layered van der Waals magnets.
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Materials Science · Cross-listed from cond-mat.mes-hall
Imaging Quantum Well States of Dirac Electrons in Exfoliated 3D Topological Insulators
Shreyashi Sinha, Shantanu Pathak, Saswata Bhattacharya et al. (+1 more)
We present a controlled mechanical exfoliation technique for bulk 3D topological
insulators that yields atomically clean ultrathin flakes, enabling
quantum well states (QWS) of Dirac electrons to be clearly resolved. Achieving reliable fabrication of pristine, high-quality two-dimensional layers suitable for atomic-scale spectroscopy remains a central experimental challenge in uncovering their emergent
quantum states and realizing device-relevant functionalities. Atomically resolved scanning probe microscopy and micro-Raman spectroscopy reveal a strong correlation between Raman intensity and film thickness, enabling rapid identification of (Bi\textsubscript{0.1}Sb\textsubscript{0.9})\textsubscript{2}Te\textsubscript{3} flakes with desired thickness. High resolution scanning tunneling spectroscopy on exfoliated flakes with atomically flat terraces reveals QWS, driven by
quantum confinement of Dirac electrons. This effect is rarely observed due to the electrons resistance to electrostatic confinement caused by Klein tunneling. The standard phase accumulation model accurately captures the characteristics of QWS and extracts the electronic band dispersion, showing excellent agreement with density functional theory calculations. Band structure calculation reveals that with increasing
quantum-layer thickness, the interlayer coupling enhances the electronic dispersion, progressively reducing subband splitting and giving rise to bulk-like continuous bands. Spatially resolved spectroscopy around surface defects further confirms that QWS of Dirac electrons in topological
insulators remains robust against defect scattering. This work paves the way for exploring diverse
quantum phenomena and device applications through
quantum confinement, surface-state engineering, and tunable topological phases.
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All authors Shreyashi Sinha, Shantanu Pathak, Saswata Bhattacharya, Sujit Manna
Materials Science
Artificial Intelligence Driven Workflow for Accelerating Design of Novel Photosensitizers
Hongyi Wang, Xiuli Zheng, Weimin Liu et al. (+2 more)
The discovery of high-performance photosensitizers has long been hindered by the time-consuming and resource-intensive nature of traditional trial-and-error approaches. Here, we present AI-Accelerated PhotoSensitizer Innovation (AAPSI), a closed-loop workflow that integrates expert knowledge, scaffold-based molecule generation, and
Bayesian optimization to accelerate the design of novel photosensitizers. The scaffold-driven generation in AAPSI ensures structural novelty and synthetic feasibility, while the iterative AI-experiment loop accelerates the discovery of novel photosensitizers. AAPSI leverages a curated database of 102,534 photosensitizer-solvent pairs and generate 6,148 synthetically accessible candidates. These candidates are screened via graph
transformers trained to predict singlet oxygen
quantum yield ($\phi_\Delta$) and absorption maxima ($\lambda_{max}$), following experimental validation. This work generates several novel candidates for photodynamic therapy (PDT), among which the hypocrellin-based candidate HB4Ph exhibits exceptional performance at the Pareto frontier of high
quantum yield of singlet oxygen and long absorption maxima among current photosensitizers ($\phi_\Delta$=0.85, $\lambda_{max}$=650nm).
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All authors Hongyi Wang, Xiuli Zheng, Weimin Liu, Zitian Tang, Sheng Gong
Materials Science · Cross-listed from physics.comp-ph
Electrified Metal-Water Interfaces at Constant Potential: Data-Efficient Transfer Learning for Machine-Learning-Based Molecular Dynamics
Michele Giovanni Bianchi, Michele Re Fiorentin, Francesca Risplendi et al. (+4 more)
Simulating electrified metal-water interfaces with explicit solvent under constant potential is essential for understanding electrochemical processes, yet remains prohibitively expensive with ab initio methods. We present TRECI, a data-efficient workflow for constructing machine learning force-fields (ML-FFs) that achieve ab initio-level accuracy in electronically grand-canonical molecular dynamics. By leveraging transfer learning from general-purpose and domain-specific models, TRECI enables stable and accurate simulations across a wide potential range using a reduced number of reference configurations. This efficiency allows the use of high-level meta-GGA functionals and rigorous surface-electrification schemes. Applied to Cu(111)-water, models trained on just one thousand configurations yield accurate molecular dynamics simulations, capturing bias-dependent solvent restructuring effects not previously reported. TRECI offers a general strategy for characterising diverse materials and interfacial chemistries, significantly lowering the cost of realistic constant-potential simulations and expanding access to quantitative electrochemical modelling.
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All authors Michele Giovanni Bianchi, Michele Re Fiorentin, Francesca Risplendi, Candido Fabrizio Pirri, Michele Parrinello, Luigi Bonati, Giancarlo Cicero
Materials Science
High-throughput validation of phase formability and simulation accuracy of Cantor alloys
Changjun Cheng, Daniel Persaud, Kangming Li et al. (+7 more)
High-throughput methods enable accelerated discovery of novel materials in complex systems such as high-
entropy alloys, which exhibit intricate phase stability across vast compositional spaces. Computational approaches, including Density Functional Theory (DFT) and calculation of phase diagrams (CALPHAD), facilitate screening of phase formability as a function of composition and temperature. However, the integration of computational predictions with experimental validation remains challenging in high-throughput studies. In this work, we introduce a quantitative confidence metric to assess the agreement between predictions and experimental observations, providing a quantitative measure of the confidence of machine learning models trained on either DFT or CALPHAD input in accounting for experimental evidence. The experimental
dataset was generated via high-throughput in-situ synchrotron X-ray diffraction on compositionally varied FeNiMnCr
alloy libraries, heated from room temperature to ~1000 {\deg}C. Agreement between the observed and predicted phases was evaluated using either temperature-independent phase classification or a model that incorporates a temperature-dependent probability of phase formation. This integrated approach demonstrates where strong overall agreement between computation and experiment exists, while also identifying key discrepancies, particularly in FCC/BCC predictions at Mn-rich regions to inform future model refinement.
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All authors Changjun Cheng, Daniel Persaud, Kangming Li, Michael J. Moorehead, Natalie Page, Christian Lavoie, Beatriz Diaz Moreno, Adrien Couet, Samuel E Lofland, Jason Hattrick-Simpers
Materials Science
E-coherent crystalline interfaces: coherency enhanced by discohesion arrays
Ryan B. Sills, Alejandro Hinojos, Trevor J. Murray et al. (+3 more)
Coherent crystalline interfaces form when a pair of joined
crystals share
lattice sites. Such interfaces are ubiquitous in materials, minerals, and compounds, with examples including grain boundaries in polycrystals and phase boundaries in multi-phase systems. Existing methodologies such as the topological model provide a framework for understanding the nature of coherency between two
crystals and the line defect content within an interface. However, these methods only consider states of coherency achieved via affine transformations. Here we show that in some interfaces, local relaxations in the form of non-affine transformations lead to the introduction of additional coincidence sites within the interface; we term this class of interfaces as e-coherent. These non-affine relaxations are topologically equivalent to inserting disconnection (or disclination) dipoles or loops into the interface. Unlike traditional interfacial line defects, the defects associated with e-coherency cannot have long-range stress fields and their motion alters the state of coherency between the
crystals. Given these unique properties, we differentiate them from other defects by referring to them as discohesions. Through atomistic simulations and transmission electron microscopy, we show that the energetics and kinetics of e-coherent interfaces are strongly affected by the discohesion content in the interface, leading to fundamentally different behaviors compared to non-e-coherent interfaces. We demonstrate e-coherency in grain, twin, and phase boundaries, and that a given interface can have multiple possible e-coherent states. These results suggest that e-coherency is likely to be pervasive in crystalline solids.
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All authors Ryan B. Sills, Alejandro Hinojos, Trevor J. Murray, Shane H. Cooley, Xiaowang W. Zhou, Douglas L. Medlin
Materials Science
What is the signature of a trion in photoemission?
Jinyuan Wu, Zachary H. Withers, Thomas K. Allison et al. (+1 more)
Recent advances in time- and angle-resolved photoemission spectroscopy (tr-ARPES) allow for the probing of multiparticle excited-states in reciprocal space. While neutral two-particle excitations (
excitons) have been observed in tr-ARPES, signatures of trions – three-quasiparticle bound states – have only been probed via optical spectroscopy. Here, we develop a general theory for the ARPES signature of trions in the model system of a monolayer transition metal dichalcogenide (TMD). We simulate the ARPES signals of both positively and negatively charged trions and show that the interaction of the residual holes, or electron and hole, lead to large energy shifts, on the order of the
exciton binding energy, compared to the
exciton signal. For positive trions, the additional momentum degree of freedom of the residual particles removes any strict lower bound on the photoemission energy, leading to distinctive asymmetric spectral features. For negative trions, the photoemission process causes the tr-ARPES spectrum to reproduce inverted images of the
exciton band structure for multiple
exciton states, encompassing both
spin-allowed and
spin-forbidden states, providing a direct momentum-resolved probe of both trion and
exciton physics.
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All authors Jinyuan Wu, Zachary H. Withers, Thomas K. Allison, Diana Y. Qiu
Materials Science
Communication: Modeling layered mosaic perovskite alloy microstructures across length scales via a packing algorithm
Murray Skolnick and Salvatore Torquato
Layered "mosaic" metal-halide perovskite materials display a wide-variety of microstructures that span the order-disorder spectrum and can be tuned via the composition of their constituent B-site octahedral species. Such materials are typically modeled using computationally expensive ab initio methods, but these approaches are greatly limited to small sample sizes. Here, we develop a highly efficient hard-particle packing
algorithm to model large samples of these layered complex
alloys that enables an accurate determination of the geometrical and topological properties of the B-site arrangements within the plane of the inorganic layers across length scales. Our results are in good agreement with various experiments, and therefore our
algorithm bypasses the need for full-blown ab initio calculations. The accurate predictive power of our
algorithm demonstrates how our minimalist hard-particle model effectively captures complex interactions and dynamics like incoherent thermal motion, out of plane octahedral tilting, and bond compression/stretching. We specifically show that the composition-dependent miscibility predicted by our
algorithm for certain silver-iron and copper-indium layered
alloys are consistent with previous experimental observations. We further quantify the degree of mixing in the simulated structures across length scales using our recently developed sensitive "mixing" metric. The large structural snapshots provided by our
algorithm also shed light on previous experimentally measured
magnetic properties of a copper-indium system. The generalization of our
algorithm to model 3D perovskite
alloys is also discussed. In summary, our packing model and mixing metric enable one to accurately explore the enormous space of hypothetical layered mosaic
alloy compositions and identify materials with potentially desirable optoelectronic and
magnetic properties.
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Materials Science
Velocity dependence of kinetic friction by multi-scale Quantum Mechanics/Green's Function molecular dynamics
Alberto Pacini, Seiji Kajita, Gabriele Losi et al. (+1 more)
Atomistic simulations are powerful tools for investigating tribological phenomena at a fundamental level; however, simulating a tribological system remains challenging due to the multiscale nature of frictional processes. Recently, we introduced a hybrid method, QM-GF, that enables an accurate description of both interfacial chemistry and phononic dissipation in semi-infinite bulks. In this work, we apply this simulation scheme to study the dependence of kinetic friction on sliding velocity. Using a prototypical diamond interface with varying hydrogen coverages, we find that the friction force decreases with increasing sliding velocity, revealing two distinct sliding regimes at low and high speeds. We provide a physical interpretation of this velocity dependence based on the modulation of the frictional force by the sliding motion over the periodic potential energy surface of the interface. High velocities lead to force cancellation, while low velocities result in a net frictional force characterized by a distinctive sawtooth profile.
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All authors Alberto Pacini, Seiji Kajita, Gabriele Losi, Maria Clelia Righi
Materials Science
Effect of FABr Over-Stoichiometry on the Morphology and Optoelectronic Properties of Wide-Bandgap FAPbBr_3 Films
G. Ammirati, F. Martelli, F. Toschi et al. (+9 more)
In this study, we investigate the impact of formamidinium bromide (FABr) over-stoichiometry in the precursor solution on the optoelectronic properties and morphology of the resulting films of formamidinium lead bromide (FAPbBr_3). Optical characterization, including steady-state absorption, photoluminescence (PL), and femtosecond transient absorption spectroscopy, reveals a systematic blueshift in emission energy with increasing FABr content, attributed to the passivation of bromine vacancies and to the reduction of defect-assisted recombination. Power-dependent PL confirms this interpretation: the stoichiometric film exhibits a PL band due to donor-acceptor pair (DAP) recombination as identified by the typical excitation-dependent blueshift, whereas FABr-enriched samples show no evidence of DAP emission, indicating effective defect passivation. Additionally, morphological characterization shows a reduction in grain size with increasing FABr excess, indicating a trade-off between improved electronic quality and enhanced structural disorder. The film synthesized with a 5% excess of FABr provides the optimal balance, yielding the highest power conversion efficiency (6.26%), average visible transmittance (61.6%), and light utilization efficiency (3.85%). These results demonstrate that fine-tuning the precursor stoichiometry through controlled FABr addition represents a simple yet effective strategy to enhance the optoelectronic quality and performance of semitransparent perovskite
solar cells.
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All authors G. Ammirati, F. Martelli, F. Toschi, S. Turchini, P. O'Keeffe, A. Paladini, F. Matteocci, J. Barichello, D. Di Girolamo, S. Piccirillo, A. Di Carlo, D. Catone
Materials Science
Micrometer thick single crystal iron-garnet films on a diamagnetic buffer layer for cryogenic applications
A.N. Kuzmichev, P.M. Vetoshko, E.I. Pavluk et al. (+8 more)
This work advances the frontier of low-damping
magnetic materials, directly addressing the demand for ultra-low-loss components in
quantum computing and cryogenic electronics. Here we demonstrate a new approach to get single
crystal micrometer-thick yttrium iron garnet (YIG) films with low damping through isolating and mitigating interfacial paramagnetic contributions of a paramagnetic substrate by a buffer-layer. The YIG films with the diamagnetic yttrium scandium gallium garnet buffer layer grown by
liquid phase epitaxy on a gadolinium gallium substrate demonstrate homogeneity unprecedented for the thin planar YIG structures, yielding
ferromagnetic resonance linewidths of 4.9 MHz at 4 K and 5.9 MHz at 16 mK, the lowest values reported to date. These results underscore the critical role of interfacial engineering in overcoming intrinsic material limitations, opening avenues for further optimization in
spin-based technologies.
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All authors A.N. Kuzmichev, P.M. Vetoshko, E.I. Pavluk, A.A. Holin, G.A. Knyazev, A.S. Kaminskiy, S.S. Demirchan, R. Tyumenev, D.S. Kalashnikov, V.S. Stolyarov, V.I. Belotelov
Materials Science
Observation of a phonon bottleneck effect on the thermal depopulation from a photoexcited shallow defect in silicon
Sergio Revuelta, Hai I. Wang, Mischa Bonn et al. (+1 more)
We report the observation of a
phonon bottleneck effect impacting the thermal depopulation of photoexcited shallow defects in high-resistivity silicon. Using time-resolved terahertz (THz) spectroscopy, near-band-gap excitation produces a pronounced temporal delay in photoconductivity, indicating that a fraction of photogenerated charge carriers is temporarily trapped immediately after excitation. By analyzing the frequency-resolved complex photoconductivity as a function of pump-probe delay and
photon energy, we attribute this delay to the presence of a localized shallow state situated approximately 40 meV from the band edge, which competes with silicon's indirect band-to-band absorption. The zero-order kinetic profile of the temporal delay, its invariance with respect to
photon flux, and its temperature dependence collectively support the existence of a
phonon bottleneck that hinders the thermal release of electrons from this shallow trap. this represents experimental evidence of a
phonon bottleneck effect associated with the thermal activation of shallow traps in photoexcited silicon. These findings provide microscopic insight into carrier relaxation dynamics in silicon and highlight the significance of electron-
phonon interactions in the ultrafast processes governing materials used in optoelectronic applications.
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All authors Sergio Revuelta, Hai I. Wang, Mischa Bonn, Enrique Canovas
Materials Science
Atomistic Origin of Photoluminescence Quenching in Colloidal MoS2 and WS2 Nanoplatelets
Surender Kumar, Markus Fröhlich, Stefan Velja et al. (+8 more)
Large chemical tunability and strong light-matter interactions make colloidal transition metal dichalcogenide (TMD) nanostructures particularly suitable for light-emitting applications. However, ultrafast
exciton decay and quenched photoluminescence (PL) limit their potential. Combining femtosecond transient absorption spectroscopy with first-principles calculations on MoS2 and WS2 nanoplatelets, we reveal that the observed sub-picosecond
exciton decay originates from edge-located optically bright hole traps. These intrinsic trap states stem from the metal d-orbitals and persist even when the sulfur-terminated edges are hydrogen-passivated. Notably, WS2 nanostructures show more localized and optically active edge states than their MoS2 counterparts, and zigzag edges exhibit a higher trap density than armchair edges. The nanoplatelet size dictates the competition between ultrafast edge-trapping and slower core-
exciton recombination, and the states responsible for
exciton quenching enhance catalytic activity. Our work represents an important step forward in understanding
exciton quenching in TMD nanoplatelets and stimulates additional research to refine physicochemical protocols for enhanced PL.
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All authors Surender Kumar, Markus Fröhlich, Stefan Velja, Marco Kögel, Onno Strolka, André Niebur, Samuell Ginzburg, Muhammad Sufyan Ramzan, Jannik C. Meyer, Jannika Lauth, Caterina Cocchi
Materials Science
A Combined Theoretical and Experimental Study of Oxygen Vacancies in Co$_3$O$_4$ for Liquid-Phase Oxidation Catalysis
Amir Omranpour, Lea Kämmerer, Catalina Leiva-Leroy et al. (+12 more)
In the present work, we investigate oxygen vacancies (V$_\mathrm{O}$) in Co$_3$O$_4$, both in the bulk phase and under
liquid-phase ethylene glycol oxidation, by combining theoretical and experimental techniques. Density functional theory calculations for bulk Co$_3$O$_4$ show that introducing an oxygen vacancy reduces two adjacent Co$^{3+}$ ions to Co$^{2+}$ and narrows the band gap. The newly formed Co$^{2+}$ ions adopt high-
spin configurations in distorted octahedral sites and remain stable in this state in ab initio molecular dynamics simulations at $300$ K. Computed O and Co K-edge X-ray absorption spectra for ideal and vacancy-containing Co$_3$O$_4$ show excellent agreement with the experimental data and serve as references to analyze the
liquid-phase ethylene glycol oxidation. The comparison with experimental O K-edge spectra of fresh and post-reaction catalysts shows that fresh samples resemble the vacancy-containing reference, whereas post-reaction spectra shift toward the ideal reference. These results suggest that under
liquid-phase ethylene glycol oxidation conditions, Co$_3$O$_4$ becomes more oxidized rather than reduced, by refilling preexisting oxygen vacancies. This is further supported by the observation that higher O$_2$ pressures increase the conversion and that the catalyst remains stable and active over several cycles.
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All authors Amir Omranpour, Lea Kämmerer, Catalina Leiva-Leroy, Anna Rabe, Takuma Sato, Soma Salamon, Joachim Landers, Benedikt Eggert, Eugen Weschke, Jean Pascal Fandré, Ashwani Kumar, Harun Tüysüz, Martin Muhler, Heiko Wende, Jörg Behler
Materials Science
Fostering Innovation: Streamlining Magnetocaloric Materials Research by Digitalization
Simon Bekemeier, Moritz Blum, Luana Caron et al. (+15 more)
Refrigeration based on the magnetocaloric effect (MCE) can contribute to energysaving, environmentally friendly cooling in private households, or industrial application. The cooling is based on the reversible heat release or uptake during a phase-transformation of the materials that can be controlled by a
magnetic field. This process could replace conventional compression-based refrigeration, which often relies on environmentally harmful refrigerants. Here we show, how to digitalize the process chain for the synthesis, theoretical and experimental characterization, and prototypical application of magnetocaloric
alloy. Different Heusler
alloys are examined experimentally as model systems for potential application in
magnetic cooling. OTTR templates are used for the acquisition and semantic representation of knowledge in the development of an ontology. The ontology, when combined with unstructured data, can be exploited to train a model that can then be used to predict missing facts, which can help to gain new insights and to generate new hypotheses. Furthermore, tools are developed that automate data acquisition into ontological structures and workflows are implemented that provide an easy-to-use theoretical and experimental evaluation of the MCE from first principles and raw data.
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All authors Simon Bekemeier, Moritz Blum, Luana Caron, Alisa Chirkova, Philipp Cimiano, Basil Ell, Inga Ennen, Michael Feige, Maik Gaerner, Thomas Hilbig, Andreas Hütten, Günter Reiss, Tapas Samanta, Sonja Schöning, Christian Schröder, Lennart Schwan, Chris Taake, Martin Wortmann
Materials Science
Light-induced photomechanical patterning of ferroelectric polarization
Alban Degezelle, Jonas Strobelt, Sarah Loebner et al. (+10 more)
Tailoring at will polar textures in ferroelectrics is critical for the development of nanoscale electronics and functional
oxide technologies. Freestanding ferroelectric membranes have enabled studies of strain-induced polarization responses, but the control over membrane shape and local polarization typically remains limited to spontaneous buckling or uniaxial mechanical deformations. In this work, we employ a versatile photosensitive-
polymer patterning approach to impose programmable bending strain profiles in ferroelectric membranes. Using BaTiO3 as a model system, we demonstrate deterministic 90{\deg} polarization rotation driven by engineered in-plane strain, and 180{\deg} polarization reversal arising from flexoelectric coupling through a controlled strain gradient. These results establish this programmable bending as a powerful approach to investigate strain-dependent domain structures, leverage flexoelectric effects, and engineer custom ferroelectric landscapes across a wide range of
oxide membranes.
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All authors Alban Degezelle, Jonas Strobelt, Sarah Loebner, Moussa Mebarki, Stephane Fusil, Vincent Garcia, Bruno Berini, Vincent Polewczyk, Yves Dumont, Sylvia Matzen, Philippe Lecoeur, Svetlana Santer, Thomas Maroutian
Materials Science
Growth driven phase transitions in Zinc Oxide nanoparticles through machine-learning assisted simulations
Quentin Gromoff, Magali Benoit, Jacek Goniakowski et al. (+2 more)
This study investigates the formation of zinc
oxide (ZnO) nanoparticles, a material of significant technological interest with complex structural properties, through atom-by-atom deposition modeling a process common in bottom-up synthesis. Our findings demonstrate that, although the body-centered tetragonal (BCT) structure is thermodynamically stable at
equilibrium for small particle sizes, the deposition process induces a
crystal-to-
crystal phase transition into the more stable wurtzite (WRZ) phase. This transformation is facilitated by a specific redistribution of the nanoparticle ions, which effectively compensates the emerging polar facets at the moment of transition. These insights offer a deeper understanding of
oxide nanoparticle formation, which should ultimately help the design of materials with targeted structural features.
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All authors Quentin Gromoff, Magali Benoit, Jacek Goniakowski, Carlos R. Salazar, Julien Lam
Materials Science · Cross-listed from cond-mat.str-el
Clustering-Enhanced Time- and Angle-Resolved Photoemission Study of LaTe$_3$: Absence of a Photoinduced Secondary CDW in the Electronic Structure
Gesa-R. Siemann, Davide Curcio, Anders S. Mortensen et al. (+9 more)
Optical control offers a compelling route for tailoring material properties on an ultrafast time scale. Ordered states such as charge density waves (CDWs) can be transiently melted by an ultrafast light excitation. This is also the case for the rare-earth tritelluride LaTe$_3$, a prototypical CDW compound. For this material it has recently been reported that the suppression of the primary CDW allows the transient formation of a second CDW, whose wave vector is orthogonal to the primary one. This creates the intriguing scenario where light enables switching between two distinct ordered phases of the material. While the second CDW has so far been observed by structural techniques, it remains an open question how the interplay of the two CDW phases is reflected in the material's electronic structure. We investigate this via time- and angle-resolved photoemission measurements of LaTe$_3$. The complex
Fermi contour is probed using a FeSuMa analyzer, which records the photoemission intensity of the entire
Fermi contour at once. The dynamics revealed by the FeSuMa analyzer are complemented by measurements using a conventional hemispherical electron analyzer. We combine conventional data analysis with $k$-means clustering, an unsupervised machine learning technique, demonstrating its strong potential for disentangling large
datasets. While we do not find any features that cannot be explained by the melting and re-establishment of the primary CDW, distinct dynamics and coherent oscillations are observed in the different branches of the
Fermi contour.
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All authors Gesa-R. Siemann, Davide Curcio, Anders S. Mortensen, Charlotte E. Sanders, Yu Zhang, Jennifer Rigden, Paulina Majchrzak, Deepnarayan Biswas, Emma Springate, Ratnadwip Singha, Leslie M. Schoop, Philip Hofmann
Materials Science
Al$_{1-x}$Hf$_{x}$N Thin Films with Enhanced Piezoelectric Responses for GHz Surface Acoustic Wave Devices
Laura I. Wagner, Verena Streibel, Esperanza Luna et al. (+12 more)
Ternary compounds obtained by alloying wurtzite AlN with transition metals have emerged as promising materials with significantly enhanced piezoelectric characteristics relative to binary AlN. The increased electromechanical coupling in these compounds boosts the performance of high-frequency acoustic devices. So far, progress has largely focused on Al$_{1-x}$Sc$_x$N, which is costly and poorly compatible with complementary metal-
oxide-
semiconductor (CMOS) technologies. Here, we investigate aluminum hafnium nitride (Al$_{1-x}$Hf$_{x}$N) as a scalable and potentially CMOS-compatible alternative to Al$_{1-x}$Sc$_x$N. Using reactive co-sputtering on both Si and sapphire substrates, we demonstrate wurtzite Al$_{1-x}$Hf$_{x}$N thin films ($x \leq 0.17$) with strong $c$-axis texture and nearly isotropic
lattice expansion upon Hf incorporation. X-ray absorption spectroscopy indicates cross-gap hybridization between N 2$p$ and Hf 5$d$ states, which can enhance the Born effective charge and, thereby, the piezoelectric response. Correspondingly, we observe a multifold enhancement in the piezoelectric coefficient, $d_{33}$, relative to AlN, despite increasing structural disorder in Al$_{1-x}$Hf$_{x}$N. Building on this finding, we demonstrate Al$_{1-x}$Hf$_{x}$N GHz surface acoustic wave (SAW) resonators that exhibit enhanced performance, as well as efficient excitation of bulk acoustic waves with low propagation losses. These results establish Al$_{1-x}$Hf$_{x}$N as a promising platform for next-generation high-frequency electromechanical devices, with prospects for further piezoelectric enhancements through improved epitaxy.
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All authors Laura I. Wagner, Verena Streibel, Esperanza Luna, Katarina S. Flashar, Valentin Walbrunn, Walid Anders, Nicole Volkmer, Doreen Steffen, Katrin Morgenroth, Frans Munnik, Tsedenia A. Zewdie, Saswati Santra, Jutta Schwarzkopf, Ian D. Sharp, Mingyun Yuan
Materials Science · Cross-listed from cond-mat.supr-con
Giant Domain Walls and Intrinsic Heterogeneity in 214 Cuprate Superconductors
Evie Ladbrook, Mark S. Senn and Jon Wright
Structural phase transitions generate complex microstructures that often govern material functionality, yet directly resolving their three-dimensional organisation in bulk samples remains challenging. Here we employ scanning three-dimensional X-ray diffraction to resolve the bulk microstructure of La$_{1.675}$Eu$_{0.2}$Sr$_{0.125}$CuO$_{4}$, a prototypical 1/8-doped cuprate in which structural and electronic heterogeneity is well established. We reveal remarkably broad tetragonal-like domain wall regions within the nominally orthorhombic
crystal structure, and, upon cooling to 100 K, a fine microstructure of orthorhombic-like stripes embedded within the tetragonal
matrix that has significant consequences for interpreting the interplay between structural and electronic heterogeneity in this class of materials. More broadly, this work establishes 3DXRD as a powerful approach for resolving bulk microstructures and understanding their role in emergent functionality.
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Materials Science
Device-Scale Atomistic Simulations of Heat Transport in Advanced Field-Effect Transistors
Ke Xu, Gang Wang, Ting Liang et al. (+8 more)
Self-heating in next-generation, high-power-density field-effect transistor limits performance and complicates fabrication. Here, we introduce NEP-FET, a machine-learned framework for device-scale heat transport simulations of field-effect transistors. Built upon the neuroevolution potential, the model extends a subset of the OMat24
dataset through an active-learning workflow to generate a chemically diverse, interface-rich reference set. Coupled with the FETMOD structure generator module, NEP-FET can simulate realistic field-effect transistor geometries at sub-micrometer scales containing millions of atoms, and delivers atomistic predictions of temperature fields, per-atom heat flux, and thermal stress in device structures with high fidelity. This framework enables rapid estimation of device-level metrics, including heat-flux density and effective thermal conductivity. Our results reveal pronounced differences in temperature distribution between fin-type and gate-all-around transistor architectures. The framework closes a key gap in multiscale device modeling by combining near-
quantum-mechanical accuracy with device-scale throughput, providing a systematic route to explore heat transport and thermo-mechanical coupling in advanced transistors.
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All authors Ke Xu, Gang Wang, Ting Liang, Yang Xiao, Dongliang Ding, Haichang Guo, Xiang Gao, Lei Tong, Xi Wan, Gang Zhang, Jianbin Xu
Materials Science
Observation of cooperative strong coupling between optical phonon and crystal-field excitations in a pseudo Jahn-Teller system
Fangliang Wu, Xiaoxuan Ma, Zhongwei Zhang et al. (+6 more)
Cooperative interactions between localized electronic excitations and
crystal lattice are central to the emergence of complex structural phases in materials. However, the scaling relations governing these collective behaviors remain largely unexplored. Here, using magneto-Raman spectroscopy, we report the direct observation of the strongly coupled optical
phonon and non-degenerate
crystal-field excitations (CFEs) in ErFeO3. By independently tuning the effective population of Jahn-Teller-active erbium ions through temperature and chemical dilution with Jahn-Teller-inactive yttrium ions, we identify the coupling strength varies linearly with the square root of electronic excitations population. Notably, Y-doping reveals the hybridization gap reduces significantly faster than predicted by density scaling alone, indicating
phonon coherence is essential for establishing this cooperative interaction. Our findings highlight the role of optical
phonons in mediating short-range interactions that drive cooperative Jahn-Teller effect, evidencing the pathway for tailoring electronic and vibrational properties of Jahn-Teller materials through population control.
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All authors Fangliang Wu, Xiaoxuan Ma, Zhongwei Zhang, Motoaki Bamba, Haowen Wu, Jian Sun, Yuan Wan, Shixun Cao, Qi Zhang
Materials Science
Detecting Linear Dichroism with Atomic Resolution
Roger Guzman, Ján Rusz, Ang Li et al. (+3 more)
X-ray linear dichroism has been pivotal for probing electronic anisotropies, but its inherent limited spatial resolution precludes atomic-scale investigations of orbital polarization. Here we introduce a versatile electron linear dichroism methodology in scanning transmission electron microscopy that overcomes these constraints. By exploiting momentum-transfer-dependent electron energy-loss spectroscopy with an atomic-sized probe, we directly visualize orbital occupation at individual atomic columns in real space. Using strained La0.7Sr0.3MnO3 thin films as a model system, we resolve the Mn-3d eg orbital polarization with sub-angstrom precision. We show that compressive strain stabilizes 3z2-r2 occupation while tensile strain favors x2-y2. These results validate our approach against established X-ray measurements while achieving the ultimate single atomic-column sensitivity. We further demonstrate two optimized signal extraction protocols that adapt to experimental constraints without compromising sensitivity. This generalizable platform opens unprecedented opportunities to study
symmetry-breaking phenomena at individual defects, interfaces, and in
quantum materials where atomic-scale electronic anisotropy governs emergent functionality.
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All authors Roger Guzman, Ján Rusz, Ang Li, Juan Carlos Idrobo, Wu Zhou, Jaume Gazquez
Materials Science
WSe2 p-MOSFETs with Nb-Doped WS2 Contacts Deposited using Atomic Layer Deposition
Shivanshu Mishra, Ruixue Li, Dongjea Seo et al. (+5 more)
WSe2 p-MOSFETs with Nb-doped WS2 contacts formed using atomic layer deposition are demonstrated. The devices are fabricated using a technique that aligns the contact metallization with the Nb-doped WS2 contacts using a selective oxidation process. Devices with source/drain spacing of 0.15 um have on-state current of 103 uA/um at VDS = -1 V at a channel carrier concentration of ~ 7.5 x 1012 cm-2. The results provide a promising CMOS-compatible pathway to create low-resistance contacts to 2D-channel transistors.
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All authors Shivanshu Mishra, Ruixue Li, Dongjea Seo, Anil Adhikari, Lucas G. Cooper, Rebecca A. Dawley, Ageeth A. Bol, Steven J. Koester
Materials Science
Quantized Polarization Redefines Polar Interfaces
Hongsheng Pang and Lixin He
In crystalline solids, the electronic polarization follows the generalized Neumann's principle, under which all crystallographic point groups can, in principle, support ferroelectric polarization. However, in high-
symmetry structures, polarization is constrained by
symmetry operations and becomes quantized into discrete values. We demonstrate that this quantized polarization (QP) is not a mathematical artifact but a
symmetry-protected invariant that encodes intrinsic information about a material's
symmetry and electronic structure. Because of its discrete and non-continuous nature, when two materials with different QPs form an interface, their bulk polarization states cannot be connected adiabatically, compelling the system to develop pronounced interfacial responses: such as metallic states, bound charges, or strong
lattice distortions. This theoretical framework provides a unified reinterpretation of classical systems such as the LaAlO$_3$/SrTiO$_3$ interface, revealing it as a prototypical case of QP mismatch. By establishing QP as a fundamental bulk invariant, our work uncovers a universal mechanism governing interfacial electronic phenomena and opens new pathways for the design of functional
quantum materials through engineered polarization mismatch.
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