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Physics Condensed Matter Materials Science

cond-mat.mtrl-sci Watch Materials Science

Wednesday 5 August

Materials Science

Unifying microscopic theories for the phono-magnetic effect

Natalia Shabala, Finja Tietjen, Ylva Liljegren et al. (+1 more)

Phonon angular momentum induces an effective magnetic field, a phenomenon called phono-magnetic effect, which has been measured to vary significantly in size. Here, we compare three approaches for deriving the effective magnetic field of a phonon, using electron-phonon coupling and orbital magnetism. The adiabatic approach assumes a slow ionic motion, keeping electrons in the ground state. The perturbative approach treats the electron-phonon interaction as a perturbation to the electronic ground state, and the Floquet approach is based on the time-periodicity of the circular ionic motion. We show that all three approaches lead to the same effective Hamiltonian in the low-frequency limit, which moves us closer towards a unified theory of the phono-magnetic effect. Furthermore, we identify two phononic contributions to the sample magnetization, spontaneous and induced. Thus, we clarify the role of the effective magnetic field in the phonon-induced magnetization. We conclude by providing a numerical estimate for the effective magnetic field in SrTiO$_3$.
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All authors Natalia Shabala, Finja Tietjen, Ylva Liljegren, R. Matthias Geilhufe

Materials Science

Atomic Scale Ordering of Sulfur Vacancies Enhances Charge Transport in Monolayer MoS$_2$

Alessandro Pecchia, Andrea Lorenzoni, Alexander Croy et al. (+2 more)

Defect engineering in two-dimensional semiconductors has primarily focused on controlling the nature and concentration of atomic defects. Here, we show that the spatial arrangement of defects can be equally decisive in determining electronic transport. Using sulfur vacancies in monolayer MoS$_2$ as a model system, we investigate the impact of vacancy ordering through density functional theory, density functional tight-binding calculations, and quantum transport simulations. We demonstrate that a periodic vacancy arrangement at a concentration of 11.1% transforms isolated defect states into a narrow dispersive in-gap miniband, whereas randomly distributed vacancies generate only localized electronic states. This electronic transition fundamentally alters charge transport, enabling band-like propagation through the defect network rather than transport limited by disconnected localized states. A systematic analysis of the complete symmetry-reduced ensemble of 94 non-adjacent four-vacancy configurations shows that the ordered pattern lies within a broad low-energy manifold and is not energetically anomalous, although it is not the thermodynamic ground state. Device-level simulations of Au/MoS$_2$/Au junctions reveal efficient alignment of the metal Fermi level with vacancy-derived states, promoting charge injection into the defect miniband. As a result, ordered vacancy arrays exhibit electrical currents up to five orders of magnitude higher than statistically equivalent random distributions and can approach, or locally exceed, the transport performance of pristine MoS$_2$. These findings establish atomic-scale defect ordering as a powerful design principle for two-dimensional materials, demonstrating that the organization of defects, beyond their concentration alone, provides a route to simultaneously preserve functionality and high electrical conductivity in highly defective semiconductors.
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All authors Alessandro Pecchia, Andrea Lorenzoni, Alexander Croy, Francesco Mercuri, Massimiliano Cavallini

Materials Science

Performance Analysis of Double Perovskite-Based Solar Cells Using SCAPS-1D Simulation: A brief review

H. Laltlanmawii, Lalrem Kima, Mahabur Rahman et al. (+7 more)

Lead-free double perovskites are among the rapidly developing next-generation solar cell technologies, providing the required low toxicity, stability, as well as high optoelectronic potential. So far, experimentally prepared lead-free perovskite solar cell devices are reported to have low power conversion efficiency (PCE) for practical application as compared to the lead-based perovskites. In recent years, numerical simulations have emerged as a cost-effective approach that plays a crucial role in expediting scientific research, can bridge the gap between experiment and theory, and provide predictive information regarding the preparation of solar cells and their PCEs without undergoing real-time experiments. The tools, such as 1D numerical simulation software SCAPS-1D, are now needed to test newer architectures and determine what exactly is holding them back. So far in the field of solar cell research, SCAPS-1D has been extensively used and looks like a powerful software due to its user-friendliness and simulation of results in a few seconds. The speed and ease of simulation make SCAPS-1D a very popular tool; as a result, it enables rapid optimization of a large number of photovoltaic devices and their performances without undergoing any experimental work, which can save time and money. However, one serious drawback is that the SCAPS-1D simulator works only for 1D configurations. It is ineffective in incorporating atomistic interactions and 3D effects. Hence, the efficacy of the SCAPS-1D simulator solely relies on the accuracy of the input parameters that the user provides, failing which may give wrong results and large deviations from accuracy.
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All authors H. Laltlanmawii, Lalrem Kima, Mahabur Rahman, Md. Ferdous Rahman, Dilshod Nematov, S. Bhattarai, C. V. M. Chaturvedi, Yazen M. Alawaideh, A. Laref, D. P. Rai

Materials Science

Nanomechanics of MXene flakes at gold interfaces

Pierluigi Bilotto, Sabine Schwarz, Marko Piljevic et al. (+3 more)

The demand for novel self-powering and sustainable electronics requires major efforts in identifying new advanced materials for nano-applications. MXene have gathered attention due to their electronic and mechanical properties, however, their nanomechanical compliance against a metal-like interface is still not clearly identified. In this work, we employ atomic force microscopy to characterize the nanomechanical properties of a self-assembled thin flake of titanium carbide MXene (Ti3C2Tx) against a gold probe. The investigation returns an interfacial shear stress of 399 MPa, and the observation of nano-wear localized in the center of the MXene flake. Nevertheless, MXene flakes retained their crystallinity in the tribofilms as confirmed by transmission electron microscopy and electron energy loss spectroscopy. The outcomes of this work set the basis for the use of Ti3C2Tx in novel nano harvesting systems involving metal interfaces (e.g., tribovoltaic nanogenerators) with large scope in nanoelectronics, wearable sensing, electric vehicles, and robotics.
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All authors Pierluigi Bilotto, Sabine Schwarz, Marko Piljevic, Iago Peters, Michael Stöger-Pollach, Carsten Gachot

Materials Science

Raman Signatures of Lithium Ion Dynamics in LLZO Garnet Electrolytes: Atomistic Insights from MD-Raman Calculations

Takeru Miyagawa, Willis O'Leary, Manuel Grumet et al. (+4 more)

Lithium lanthanum zirconate (LLZO) garnets are among the most promising solid electrolytes for next-generation batteries owing to their high ionic conductivity, chemical stability, and compatibility with lithium metal. Raman spectroscopy is commonly employed to distinguish the highly conductive cubic phase from the poorly conductive tetragonal phase of LLZO, yet the atomistic origin of these spectral differences and their direct connection to Li-ion transport remain unresolved. Here, we close this gap by comparing computed and experimental Raman spectra for the tetragonal, cubic, and Ta-doped variants of LLZO, with the computed spectra obtained from the MD-Raman approach that combines machine-learning molecular dynamics with first-principles polarizability calculations. We show that the contrasting ionic transport behavior across these LLZO variants is encoded in the vibrational dynamics of the lithium sublattice and gives rise to distinct features in their Raman spectra. A symmetry-resolved analysis further reveals that experimentally observed Raman peaks do not correspond to individual normal modes, but instead arise from overlapping contributions of multiple symmetry-allowed vibrations, challenging conventional peak-assignment approaches. By explicitly connecting experimentally accessible Raman signatures to the underlying atomic-scale dynamics, our results show how Raman spectroscopy can move beyond empirical phase identification toward a microscopic probe of Li-ion dynamics in lithium garnet electrolytes.
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All authors Takeru Miyagawa, Willis O'Leary, Manuel Grumet, Hyunwon Chu, Jennifer L.M. Rupp, Waldemar Kaiser, David A. Egger

Materials Science · Cross-listed from cs.LG

Active Learning Guided Design Space Refinement for Scalable Multi-Objective Bayesian Optimization in Materials Discovery

Alexandros Ntagiantas, Panagiotis Tsilimidos, George Giannakopoulos et al. (+2 more)

Advanced materials discovery increasingly relies on machine learning and Bayesian optimization to explore large discrete design spaces under limited evaluation budgets. However, conventional Bayesian optimization (BO) can become inefficient as candidate spaces grow, often evaluating low-value regions before reaching informative areas. We propose an active-learning (AL)-guided adaptive search-space refinement framework combined with multi-objective BO to accelerate materials optimization while preserving Pareto-relevant regions. We evaluate the approach on CH4/N2 separation in covalent-organic frameworks and pressure-vessel design with material-direction stress components and thickness objectives. Results show that the AL-guided refinement reduces the candidate space by approximately half while preserving more than 99 percent of the original hypervolume. The reduced-space strategy improves early convergence and cumulative Pareto-front discovery from the BO, demonstrating efficient large-scale materials optimization across constrained autonomous materials discovery settings.
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All authors Alexandros Ntagiantas, Panagiotis Tsilimidos, George Giannakopoulos, Christoforos Rekatsinas, Panagiotis Krokidas

Materials Science

Nanosecond timescale plasticity in shock-compressed polycrystalline MgO: evidence for transition in mechanism above 100 GPa

A. Chakraborti, H. Ginestet, F. Bertrand et al. (+80 more)

The mechanical properties of ceramics under extreme conditions directly impact applications ranging from shielding spacecrafts, designing plasma facing materials in nuclear fusion to understanding the rheology of deep planetary interiors. Here, we use polycrystalline MgO as a model ceramic to understand the high-pressure-temperature mechanical behaviour of such materials under extreme strain rates. We use laser-driven shock compression up to 175(15) GPa on the principal Hugoniot along with ultrafast diagnostics at the European X-ray Free Electron Laser to probe the dominant deformation mechanisms with changing P -T conditions. These near-instantaneous time-resolved snapshots, coupled with elasto-viscoplastic self-consistent (EVPSC) simulations, strongly suggest that MgO attains plastic regime in the nanoseconds scale accompanied by a pressure-mediated change in dominant slip system between 95 and 175 GPa. This work provides a new direct window into the deformation dynamics of polycrystalline ceramics under high-velocity impacts.
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All authors A. Chakraborti, H. Ginestet, F. Bertrand, J. Chantel, S. Merkel, M. Harmand, S. Pandolfi, A. Amouretti, M. Andrzejewski, K. Appel, E. Barraud, A.B. Belonoshko, E. Brambrink, K. Buakor, C. Camarda, O. Castelnau, D.M. Cheshire, G. Collins, T.E. Cowan, C. Crépisson, J. Deng, X. Fang, M. Fitzgerald, A. Gleason, F. Hanby, N.J. Hartley, P.G. Heighway, H. Höppner, N. Jaisle, J. Kim, Z. Konopkova, D. Kraus, A. Krygier, L. Libon, C.M. Lonsdale, S-N. Luo, W. Lynn, M. Masruri, E.E. McBride, D. McGonegle, J.D. McHardy, M.I. McMahon, R.S. McWilliams, T. Michelat, B. Nagler, M. Nakatsutsumi, A-M. Norton, I. Ocampo, I.I. Oleynik, C. Otzen, S.E. Parsons, D.J. Peake, A. Pelka, A. Phelipeau, C. Prescher, T.R. Preston, N. Pulver, L. Rogal, J-P. Schwinkendorf, G. Shoulga, R.F. Smith, S. Singh, C.N. Somarathna, T. Stevens, C.V. Storm, C. Strohm, T-A. Suer, M. Tang, A. Tipeev, M. Toncian, T. Toncian, U. Trdan, T. Tschentscher, J.D. Umpleby-Thorp, L. Wang, J.S. Wark, A. Descamps, A. Higginbotham, T.M. Hutchinson, C. McGuire, A. Sollier, G. Morard, J.H. Eggert

Materials Science

The Frequency-Dependent Spin Contribution to the Magnetoelectric Tensor of Cr$_2$O$_3$: A First-Principles Study

Torsten Geirsson, Davide Sangalli, Alberto García-Cristóbal et al. (+1 more)

The magnetoelectric (ME) effect provides a promising pathway for controlling magnetic functionalities using electric fields. While first-principles methods for the static linear ME response are well established, comparable approaches for the frequency-dependent response remain less developed, despite experiments showing pronounced finite-frequency resonances. Here, we investigate the dynamical spin-induced linear ME response from first principles and systematically compare the independent-particle approximation (IPA), random-phase approximation (RPA), time-dependent density functional theory (TDDFT), and the Bethe-Salpeter equation (BSE). We apply these methods to the prototypical ME material Cr$_2$O$_3$ and compare the results with available experimental and theoretical studies. We find that the IPA and RPA fail to reproduce the previously reported finite static limit of the spin-induced response. Within the BSE framework, pronounced excitonic resonances emerge in the ME spectrum, in qualitative agreement with experiment. We also identify a magnon-like peak that coincides with a pole of the transverse spin susceptibility while remaining essentially dark in optical absorption, highlighting the sensitivity of the ME response to spin excitations. TDDFT places this mode closer to the expected low-energy magnonic regime and yields a sizable static spin-induced response. Our results show that these frameworks capture complementary aspects of the dynamical ME response. Low-energy collective spin excitations are required to recover the static limit, whereas electron-hole interactions are essential for reproducing the excitonic resonances.
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All authors Torsten Geirsson, Davide Sangalli, Alberto García-Cristóbal, Alejandro Molina-Sánchez

Materials Science

Nano-scale visualization of magnetic vortices in metal nanoparticles

Satoko Toyama, Yoshiki O. Murakami, Ayako Nishikawa et al. (+4 more)

Magnetic vortices in individual nanoparticles are fundamental spin structures that govern the properties of next-generation magnetic devices and biomedical applications. However, directly imaging their complete structure, from the circulating in-plane magnetization to the nanometer scale out-of-plane core, remains challenging. Here, direct and quantitative visualization of magnetic vortex structures in individual cobalt nanoparticles is achieved by integrating a time-reversal methodology with tilt-scan-averaged differential phase contrast scanning transmission electron microscopy in a magnetic-field-free environment. This approach enables magnetic imaging in conjunction with atomic-scale analysis and reveals a correlation between particle geometry and internal demagnetizing fields. In addition, dynamic evolution of the vortex under in situ magnetic-field application is tracked, enabling unambiguous determination of the out-of-plane core polarity. This approach provides a powerful platform for correlating atomic-scale structure and magnetism within individual nanoparticles and for understanding the origins of nanoscale magnetic properties, thereby supporting the rational design of advanced nanomagnetic materials and devices.
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All authors Satoko Toyama, Yoshiki O. Murakami, Ayako Nishikawa, Takehito Seki, Akihito Kumamoto, Yuichi Ikuhara, Naoya Shibata

Materials Science

Monolithic integration of optically anisotropic GeSe-based films on GaAs by templated solid-phase epitaxy

Kira J. Martin, Autumn Y. Lee, Pranav Mahaadev et al. (+6 more)

Layered IV-VI semiconductors such as GeSe exhibit strong in-plane optical anisotropy, making them promising candidates for polarization-sensitive photonic devices. However, realizing these properties in scalable platforms requires heteroepitaxial integration on technologically relevant substrates like GaAs. Direct growth of GeSe is complicated by its glass formation at low temperatures and high vapor pressure at elevated temperatures. To overcome this, we develop a method for ex-situ solid-phase epitaxy utilizing a SnSe buffer and offcut GaAs substrate to enable single-orientation crystalline GeSe films. Using polarized reflection measurements, we find that stabilizing a single-in-plane-orientation results in a 2x increase in anisotropic response between the armchair and zigzag directions. This work provides a new integration route to harness the anisotropic optical properties of GeSe and its alloys for polarization-sensitive technologies.
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All authors Kira J. Martin, Autumn Y. Lee, Pranav Mahaadev, Pooja D. Reddy, Kelly Xiao, Tri Nguyen, Ashlee M. García, Aaron M. Lindenberg, Kunal Mukherjee

Materials Science

Unveiling the Role of Friction in Coarse-Grained Clay: A Hybrid Framework Integrating Long-Range Interactions and Granular Contact Mechanics

Wang-Qi Xu, Yijie Wang and Zhen-Yu Yin

Given the predominant role of inter-particle physicochemical forces in governing clay behavior, researchers have increasingly utilized coarse-grained molecular dynamics (CGMD) simulations. However, inter-particle friction has been historically overlooked due to methodological limitations, and the extent to which this omission influences simulation accuracy remains an unresolved question. This study proposes a novel hybrid CGMD framework explicitly coupling long-range Buckingham potential with Hertzian granular contact mechanics. A baseline model was validated via isotropic compression, where the resulting compressibility and derived compression index (Cc) aligned with macroscopic geotechnical observations. Parametric analyses revealed that viscoelastic damping of particle contacts governs structural evolution. Elevated damping suppresses densification, trapping platelets in disorganized, high-void-ratio configurations. Furthermore, evaluating the interplay with thermal fluctuations underscores the necessity of precise temperature control to prevent such unphysical kinetic trapping. Finally, uniaxial compression tests demonstrate the critical importance of inter-particle friction. Explicit friction locks sliding interfaces and sustains significantly higher loads compared to frictionless models; the latter rely solely on geometric interlocking and ultimately exhibit unphysical fluid-like yielding. By bridging atomistic potentials with contact mechanics, this framework highlights the fundamental role of the inter-particle friction and offers essential guidelines for future multi-scale simulations of clay assemblies.
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Tuesday 4 August

Materials Science

Electron correlation in semiconductors and insulators via symbolic regression

Nick Pant, Viet-Anh Ha, Donghwan Kim et al. (+1 more)

Predicting quasiparticle energies in materials requires expensive numerical evaluations of the electron self-energy. This limits calculations to ordered systems with small unit cells. Here, using symbolic regression, we show that the GW self-energy can be accurately approximated with compact analytical functions of physically motivated Kohn-Sham descriptors. These expressions can be learned from a single GW calculation in the ordered phase and remain accurate under symmetry breaking induced by quantum and thermal fluctuations, elastic deformations, and amorphous disorder. This development enables routine GW calculations of complex materials with thousands of atoms at a computational cost comparable to semi-local density functional theory. We demonstrate the accuracy of this approach for covalent semiconductors, ionic insulators, and two-dimensional materials. These results establish symbolic regression as a viable route to predictive, interpretable, and transferable many-body electronic structure models.
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All authors Nick Pant, Viet-Anh Ha, Donghwan Kim, Feliciano Giustino

Materials Science

First-principles predictions of carrier mobility with record accuracy using GW perturbation theory

Nick Pant, Sabyasachi Tiwari, Steven G. Louie et al. (+2 more)

Accurate prediction of carrier mobility is critical for the discovery and design of next-generation electronic materials. Despite sustained progress, state-of-the-art ab initio methods remain limited by the approximate treatment of electron-phonon interactions at the density functional theory level. Here, we demonstrate that incorporating many-body GW corrections to both the electronic band structure and electron–phonon couplings when solving the ab initio Boltzmann transport equation yields a mean absolute relative error of just 11% for electron mobilities across benchmark semiconductors, including Si, GaAs, GaP, diamond, and SiC. The common practice of neglecting GW corrections to the electron–phonon interaction can lead to mobility errors exceeding 50%. The present findings highlight the importance of many-body GW self-energy effects in carrier transport simulations, and provides fundamental insights into how many-body electron–phonon interactions govern charge transport in crystalline solids.
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All authors Nick Pant, Sabyasachi Tiwari, Steven G. Louie, Zhenglu Li, Feliciano Giustino

Materials Science

Computational Microstructure Analysis of Sintered Ceramics

Nikhil Dhanankam, Deeksha Kodangal, Rajendra K. Bordia et al. (+1 more)

Characterizing materials through manual extraction of physical properties from microstructure images is a laborious process. This work presents a workflow to extract porosity, solid fraction, grain size distribution, and pore size distribution from scanning electron microscopy (SEM) images of sintered ceramic samples using an automated pipeline. The primary challenge for extracting physical properties from SEM images is the presence of unimodal histograms in SEM images as a result of the overlapping intensity ranges for the grain and pore phases. We evaluated several different methods for noise reduction and local thresholding of SEM images. We find that topological filtering in combination with Sauvola thresholding enables segmentation and extraction of physical property data from SEM images. We validated the automated pipeline by comparing our results with the results of manual analyses performed for samples sintered at 1200$^o$C and 1400$^o$C and achieved an Intersection over Union (IoU) score of 95.14% and 99.85%, respectively. The workflow provides an efficient means to automatically extract microstructure properties from SEM images as a crucial step in generating materials datasets for machine learning.
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All authors Nikhil Dhanankam, Deeksha Kodangal, Rajendra K. Bordia, Ulf D. Schiller

Materials Science

Green's function theory of magnetism in Bi$_2$CuO$_4$: anisotropic Heisenberg XYZ model

R.O. Kuzian and E.E. Krasovskii

The Green's function theory of Lymar' and Rudoi [Theor. Math. Phys. vol. 21, 990 (1974)] is generalized to the case of multiple intra- and inter-sublattice magnetic interactions in a collinear spin-half antiferromagnet and applied to magnetic excitations in Bi$_2$CuO$_4$. The spin Hamiltonian includes both the out-of-plane and in-plane symmetric anisotropy terms and the Zeeman term, which describes the interaction with an external magnetic field $B$ applied along the Néel vector. Within the spin-wave approximation we calculate spin excitation dispersion, antiferromagnetic resonance frequencies and the critical field $B_c$ of the spin-flop metamagnetic transition. A weak in-plane anisotropy is shown to result in a gap in the acoustic-like branch of the excitations. The gap nonlinearly depends on the external field and closes at $B=B_c$. An expression for the Neel temperature in the Tyablikov random phase approximation at zero field is derived. For the parameters derived from the recent inelastic neutron scattering study by Yuan et al. [Phys. Rev. B vol. 103, 134436 (2021)] it gives $T_{\rm N}\approx 52$~K.
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Materials Science

Pressure-Tunable Electronic and Magnonic Transport in Altermagnet La$_2$O$_3$Mn$_2$Se$_2$

Nafise Rezaei, Alireza Qaiumzadeh, Artem R. Oganov et al. (+1 more)

Hydrostatic pressure provides a symmetry-preserving route to engineer electronic and magnonic transport in the correlated insulating altermagnet La$_2$O$_3$Mn$_2$Se$_2$. Using first-principles calculations combined with spin-Hamiltonian modeling, we show that compression from 0 to 40 GPa markedly enhances the inequivalence between the competing second-neighbor exchange interactions, increasing $|J_{2a}-J_{2b}|$ from 1.97 to 9.38 meV while preserving the compensated antiferromagnetic ground state. The resulting exchange anisotropy amplifies the momentum-dependent splitting between the two chiral magnon branches, yielding a nearly fourfold enhancement of the longitudinal magnon-driven spin Seebeck response at 100 K, from $3.68\times10^{-1}$ to $1.36$ meV/K. In contrast, hydrostatic pressure preserves the magnetic-symmetry selection rules governing the anomalous Hall effect while redistributing the electronic Berry curvature, producing pronounced energy-dependent sign reversals in the anomalous Hall conductivity. These results identify exchange anisotropy as the microscopic mechanism underlying the pressure-enhanced magnon response and establish hydrostatic pressure as an effective means of simultaneously controlling electronic and magnonic transport in insulating altermagnets.
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All authors Nafise Rezaei, Alireza Qaiumzadeh, Artem R. Oganov, Mojtaba Alaei

Materials Science · Cross-listed from physics.ins-det

A table-top few-femtosecond broadband extreme-ultraviolet absorption spectrometer with cryogenic cooling

Sheng-Chih Lin, Alfred Zong, Emma Berger et al. (+5 more)

We present a table-top cryogenic ultrafast broadband XUV absorption spectroscopy (c-UBXAS) beamline designed for temperature-dependent and time-resolved investigations of quantum materials. The instrument combines a broadband high-harmonic generation source spanning 22-73 eV with automated image registration and cryogenic sample control down to 20 K, enabling element-specific measurements under both equilibrium and nonequilibrium conditions. The beamline provides sub-50-meV energy resolution and a sub-10-fs instrument response function, while maintaining long-term stability suitable for extended, ultrasensitive measurements. Benchmark experiments on NiI$_2$, a van der Waals multiferroic, reveal temperature-dependent spectral evolution across its magnetic phase transitions, demonstrating the ability to identify element-specific contributions to the ground state and its transient response in the femtosecond regime. This instrument introduces the opportunity to unravel the very first response of a solid-state material during light-matter interaction, paving the way for understanding the complex phase space of nonequilibrium dynamics and emergent states in strongly correlated systems where the few-femtosecond electronic response has eluded most other types of time-resolved techniques.
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All authors Sheng-Chih Lin, Alfred Zong, Emma Berger, Bailey R. Nebgen, Marcus Hui, Shuaiwei Pan, Jackson McClellan, Michael W. Zuerch

Materials Science

Emergence of Chiral Helimagnetic Order in Chromium-intercalated Tantalum Disulfide CrTa$_3$S$_6$ Powders with Controlled Intercalation

Ayaka Toshima, Keigo Mizutani, Yusuke Kousaka et al. (+3 more)

We report a highly sensitive change in magnetic properties of a chiral Cr-intercalated transition-metal dichalcogenide Cr$_{x}$Ta$_{3}$S$_{6}$. Magnetization curves and small-angle neutron scattering data revealed that the powder samples exhibit chiral helimagnetism with a Cr intercalation quantity $x$ below 0.996, while they show ferromagnetism above 1.000. The emergence and temperature-dependent evolution of the helimagnetic period are argued in terms of sample dimensions of powders and microfabricated crystals.
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All authors Ayaka Toshima, Keigo Mizutani, Yusuke Kousaka, Kazuki Ohishi, Hiroaki Shishido, Yoshihiko Togawa

Materials Science

Understanding and Designing Phase Change Materials: Insights from Atom Probe Tomography

Jan Köttgen, Nils von den Driesch, Alexander Pawlis et al. (+1 more)

Phase Change Materials (PCMs) can be rapidly and reversibly switched between their amorphous and crystalline state; a transition which is accompanied by a pronounced change of optoelectronic properties. Here progress is reviewed to explain these property changes, focusing on advances by atom probe tomography (APT). This technique classifies bonding by providing two crucial bonding descriptors. Most important is the Probability of Multiple Events (PME), which is related to the likelihood that more than one ion is dislodged per successful laser pulse in laser assisted field evaporation. Crystalline PCMs are characterized by a PME above 55%, not found for metals or iono-covalent solids. This confirms that crystalline PCMs employ a unique bonding mechanism coined metavalent bonding (MVB). While crystalline PCMs employ MVB, amorphous PCMs behave as covalent solids characterized by a much lower PME. PCMs thus change their bonding upon crystallization, consistent with quantum-chemical calculations of bonding. Crystalline solids with a high PME lie in a narrow conductivity range between metals and iono-covalent solids, indicative for a competition between electron localization and delocalization. A map quantifying chemical bonding locates metavalent solids in a region where approximately one electron is shared between adjacent atoms and bonding is not too ionic. This quantum chemical bonding map is now used to find and explain property trends relevant for PCMs in various application domains.
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All authors Jan Köttgen, Nils von den Driesch, Alexander Pawlis, Matthias Wuttig

Materials Science

Phonon-based determination of elastic coefficients in the Weyl semimetal TaAs

Fabián Jofré-Parra, Debankita Ghosh and Enrique Muñoz

Reliable determination of elastic properties in topological semimetals is essential for understanding strain-related effects, but is often hindered by methodological and computational limitations. In this work, we combine first-principles phonon calculations with an elastic continuum model to determine the elastic properties of Weyl semimetal TaAs. The sound velocities extracted from the acoustic phonon branches are used to obtain the full set of elastic moduli, which show good agreement with previously reported values in the literature. From these results, we derive standard elastic parameters such as bulk, shear, and Young's moduli, and the Poisson ratio. This approach highlights a computationally efficient alternative to conventional strain-based methods, avoiding the need for large supercells when shear is applied, and thus possible strain-induced inconsistencies in the electronic basis, while providing a possibility to connect with experimental characterizations (e.g. Raman or Brillouin-Mandelstam scattering) of the lattice dynamics in Weyl semimetals.
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Materials Science

Optical centers in cubic boron nitride and diamond: remarkable similarities

Konstantin Iakoubovskii, Andrey Katrusha, Weihua Peng et al. (+1 more)

We present a comparative study of optical absorption and luminescence from cubic boron nitride (cBN) and diamond grown by the high-pressure high-temperature technique in the same cubic press. We note remarkable similarities in spectral and spatial dependences for these two materials. Using the previous identification of defects in diamond, we tentatively assign the optical center responsible for yellow color in some cBN crystals to substitutional oxygen at the nitrogen site, the RC1 and RC3 centers to a defect comprising substitutional oxygen and a boron vacancy in the neutral and negative charge states, respectively, the GC1 center to a nickel-related defect, the 1.816 eV (683 nm) luminescence peak to a Si-vacancy complex, and the BN1 center to an interstitial-related defect.
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All authors Konstantin Iakoubovskii, Andrey Katrusha, Weihua Peng, Jianguo Peng

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

Local magnetic order in vacancy-disrupted spin ice Ho2TiO5

Raju Baral, Haidong Zhou, Qiang Zhang et al. (+2 more)

We investigate how local magnetic correlations evolve when the classical pyrochlore spin-ice Ho2Ti2O7 is transformed into the partially disordered stuffed compound Ho2TiO5. Neutron scattering measurements were analyzed using real-space magnetic pair distribution function, reciprocal-space reverse Monte Carlo, and half-polarized neutron powder diffraction methods to connect the average crystal structure with local magnetic correlations. Both compounds retain long-range $Fd-3m symmetry, but in Ho2TiO5 this average structure distorts locally through Ho-O, Ti-O and O-O bond-length disorder associated with partial Ho/Ti occupancy. Despite this disorder, half-polarized neutron powder diffraction shows that the Ho moments retain local <111> rangle Ising anisotropy and form spin-in/spin-out configurations on the tetrahedral network. In Ho2Ti2O7, real- and reciprocal-space analyses reveal a nearly ideal two-in/two-out spin-ice state at 0.3K, with 95% of tetrahedra satisfying the ice rule, followed by progressive thermal disordering on warming. In Ho2TiO5, most tetrahedra are magnetically incomplete because some tetrahedral vertices are occupied by nonmagnetic Ti rather than Ho; nevertheless, the dominant incomplete configurations are 2-in/1-out and 1-in/2-out, which are locally compatible with the ice rule if the missing Ho spin is restored. The single-ion <111> Ising anisotropy is therefore retained throughout, while the collective ice ordering is strongly disrupted: Ho2TiO5 is a vacancy-disrupted spin ice in which a local ice-rule tendency persists on a topologically incomplete magnetic network.
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All authors Raju Baral, Haidong Zhou, Qiang Zhang, Benjamin A. Frandsen, Stuart Calder

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

Wavelength-Selective control of Atomic Scale Au Contacts

Werner Brämer-Escamilla, Floralba Lopez, Laila Procel et al. (+3 more)

We demonstrate wavelength-selective control of atomic motion in a mechanically controllable Au break junction. Excitation at $\lambda_{\mathrm{form}}\simeq 530~{\rm nm}$ drives gap closure and metallic bridge formation, whereas excitation at $\lambda_{\mathrm{rup}}\simeq 407~{\rm nm}$ drives neck thinning, bridge rupture, and subsequent gap opening. Unlike conventional optical switching in metallic contacts, where illumination primarily acts via thermal expansion, the present experiment reveals oppositely directed atomic drift at different wavelengths. Time-resolved conductance traces allow us to distinguish two dynamical regimes. In the tunneling regime, exponential conductance transients measure the drift velocity of the gap coordinate for both gap closure and gap opening. In the metallic regime, the Sharvin relation converts linear $\sqrt{G/G_0}$ transients into radial neck-growth and neck-thinning velocities of comparable magnitude. These results establish optically selected atomic drift as a mechanism for reversible control of metallic nanocontacts and provide a quantitative route to follow plasmon-assisted atomic rearrangements in real time.
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All authors Werner Brämer-Escamilla, Floralba Lopez, Laila Procel, David Llerena, Carlos Sabater, Ernesto Medina

Materials Science · Cross-listed from physics.optics

Self-Focusing Control for Depth-Precise Wafer Slicing of 4H-SiC in Femtosecond Laser Processing

Dong Hee Kang, Jaeseung Lim, Mishfaqur Rahman et al. (+4 more)

4H-SiC has emerged as a third-generation chip material because its superior thermal conductivity and high breakdown field enable the material to achieve high power density and higher switching frequencies in power-electronics applications. As chip architectures evolve toward 3D and heterogeneous integration, the mechanical and thermal design space tightens while yield risks grow. In particular, advanced packages require mid-process wafer thinning to < 100 $\mu$m to shorten interconnects and control thermo-mechanical stress. Femtosecond laser slicing for 4H-SiC wafers offers a non-contact processing approach to produce thin layers with low defects, while strong optical nonlinearities obscure the relationship between the laser parameters and the resulting slicing quality. Here, we systematically investigate Kerr-induced self-focusing using a femtosecond laser in 4H-SiC slicing by combining experiments, a semi-empirical analytical model, and numerical ray optics simulations. We demonstrate that the interplay between pulse energy and processing depth governs the self-focusing behavior, which directly correlates with post-separation surface texture parameters and separation stress, thereby linking nonlinear beam propagation to slicing quality. Based on this relationship, we define a processability map in the pulse energy with self-focusing depth space over a normalized irradiance background. Analytically, the model extends the Marburger formula to focused beams by replacing the power ratio with a normalized irradiance. Ray optics simulations capture the geometric features at the self-focusing point and are validated against experimental observations. Within physically defined thresholds, the processability map directly connects laser parameters to separation stress and surface texture metrics, providing practical guidance for depth control beyond trial-and-error.
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All authors Dong Hee Kang, Jaeseung Lim, Mishfaqur Rahman, Seongheum Han, Jae-Hak Lee, Seungman Kim, Jihoon Jeong

Materials Science

Generalized Space Groups from Internal Configuration Spaces

Zeying Zhang, Zhenye Li, Zhi-Ming Yu et al. (+2 more)

We develop a unified construction of generalized space groups for crystals with unconventional internal degrees of freedom. Starting from the full group $G_P$ of allowed internal transformations and the stabilizer $P$ of a reference object, we determine the pointwise and setwise symmetries, $J$ and $K$, of the allowed configuration set. Goursat's lemma then couples the internal quotient $K/J$ to a spatial quotient. The framework includes ordinary, magnetic, spin, and color space groups as special cases. As an example, we consider a dodecahedral object with $P=I\simeq A_5$, for which we obtain the nontrivial pair $T\triangleleft O$ with $O/T\simeq\mathbb Z_2$. The resulting generalized space group hosts a point node with topological charge $|C|=12$.
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All authors Zeying Zhang, Zhenye Li, Zhi-Ming Yu, Gui-Bin Liu, Yugui Yao

Materials Science

Physics-Informed Machine Learning for Refractory Alloy Design

Blaise Awola Ayirizia, Bimal K C and Jorge A. Munoz San Martin

Refractory complex concentrated alloys (CCAs) exhibit exceptional mechanical properties at elevated temperatures, but their vast compositional space (approximately 10^15 possible combinations) poses significant challenges for experimental exploration. We develop a physics-informed machine learning framework combining quantile gradient boosting with Born stability constraints to predict eight mechanical properties (bulk modulus B, shear modulus G, Vickers hardness Hv, elastic constants C11, C12, C44, Young's modulus E, and Poisson's ratio nu) of refractory CCAs. Using 393 density functional theory (DFT)-computed alloys from a 10-element design space (Cr, Hf, Mo, Nb, Re, Ta, Ti, V, W, Zr), we achieve coefficient of determination (R^2) values of 0.89 to 0.97 with mean absolute errors of 0.85 to 14.74 GPa across six properties on a held-out test set (n = 59). Remarkably, all test predictions satisfy the Born stability criteria (100% compliance), demonstrating the efficacy of physics-informed constraints. Compositional screening in valence electron concentration (VEC)-atomic size mismatch (delta) space identifies high-performance candidates, with MoReW exhibiting the highest predicted shear rigidity (C44 = 139 GPa). Elemental analysis reveals that rhenium appears in 100% of the top-performing alloys, molybdenum in 90%, and chromium in 80%, establishing quantitative design rules: VEC = 6.0 to 6.4, delta < 10%, and Re-Mo-Cr ternary systems maximize shear resistance. This physics-informed screening framework enables accelerated discovery of mechanically stable, high-performance refractory alloys from the trillion-scale compositional space.
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Materials Science

From phase transformation to amorphization: damage accumulation in Yb-implanted $\beta-Ga_2O_3$

Joanna Matulewicz, Renata Ratajczak, Ewa Grzanka et al. (+11 more)

This study provides a comprehensive analysis of the radiation response and structural evolution of differently oriented$\beta-Ga_2O_3$ single crystals subjected to Yb ion implantation over a wide fluence range from $5 \times 10^{12}$ to $1 \times 10^{16}$~cm$^{-2}$ ($0.04$–$74$~dpa). A multi-technique approach (RBS/c, PAS, HRTEM, and HRXRD) was employed to investigate the mechanisms of damage accumulation. The results reveal a multi-stage process of defect evolution. At a critical threshold of around $0.4$~dpa, the accumulation of lattice strain triggers a phase transformation from monoclinic $\beta$-Ga$_{2}$O$_{3}$ to a defective spinel structure of $\gamma$-Ga$_{2}$O$_{3}$. Notably, the formation of this new phase is accompanied by strain relaxation. With further irradiation, defects develop within the crystal structure of $\gamma$-Ga$_{2}$O$_{3}$. The associated atomic reorganization at this stage is reflected by a distinct dip in the damage accumulation curve and the appearance of stacking faults in the subsurface region of the implanted layer. In contrast to previous reports suggesting high radiation stability of this phase, the present study clearly demonstrates that continuous defect accumulation results in a significant increase in both displaced atoms and vacancy-type defects, with a strong depth dependence in their type and density. Ultimately, at an irradiation level of approximately $7$~dpa, the surface layer amorphizes. With further irradiation, the amorphous layer expands, gradually replacing the transient $\gamma$-Ga$_{2}$O$_{3}$ phase. These findings reveal that the radiation tolerance of gallium oxide is highly sensitive to ion-specific interactions and strain-induced instabilities, thereby challenging the previously assumed robustness of this material under high-fluence ion irradiation.
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All authors Joanna Matulewicz, Renata Ratajczak, Ewa Grzanka, Maciej Oskar Liedke, Damian Kalita, Eric Hirschmann, Andreas Wagner, Mikolaj Grabowski, Michal A. Strozyk, Cyprian Mieszczynski, Przemyslaw Jozwik, Ulrich Kentsch, Rene Heller, Frederico Garrido

Materials Science

Guided Synthesis of EMT Zeolites by Machine Learning

Emmanuel A. Olanrewaju, Santosh Adhikari, Zhiyin Niu et al. (+4 more)

Zeolites are microporous crystalline materials with diverse frameworks, widely used in industrial applications such as petroleum refining and molecular separation. Unlike most zeolites, EMT can be synthesized under mild conditions (at low temperatures and without the use of organic structure-directing agents), making it attractive for cost-effective and environmentally sustainable production. However, the specific synthesis conditions that selectively produce EMT rather than similar frameworks like FAU are not yet well established. In this work, we develop machine learning (ML) models to guide the discovery of synthesis conditions for EMT zeolites. Our dataset comprises 174 experimental synthesis attempts, recording reaction time, temperature, silica and alumina sources, Si/Al stoichiometric ratio, and other synthesis parameters. We apply both classical ML methods and pretrained foundation models to predict zeolite framework outcomes from these synthesis parameters. Feature importance analysis identifies critical parameters for EMT formation, validating known synthesis principles. Leveraging the ML models, we explore the synthesis space and identify six promising new conditions for EMT formation. Experimental validation confirms EMT crystallization in five cases, including two with Si/Al stoichiometric ratios outside the training dataset's range. Evaluation on independent literature-reported synthesis conditions further demonstrates the generalizability of the model. This work demonstrates a data-driven approach to accelerating zeolite synthesis, closing the loop between ML prediction and experimental validation.
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All authors Emmanuel A. Olanrewaju, Santosh Adhikari, Zhiyin Niu, Michael Nikolaou, Jeremy C. Palmer, Jeffrey D. Rimer, Mingjian Wen

Materials Science

Machine Learning-Guided Screening of Advantageous Solvents for Solid Polymer Electrolytes in Lithium Metal Batteries

Jiadong Shen, Junjie Chen, Xiaosa Xu et al. (+8 more)

Trace residual solvents in solid polymer electrolytes (SPEs) significantly affect electrolyte and interface properties, where optimal selection enhances ionic conductivity and transference numbers. However, solvent complexity hinders general screening methods. We establish a universal criterion linking electronic (HOMO, LUMO) and macroscopic properties (dielectric constant, dipole moment, polarizability) via machine learning on an approximately 10,000-solvent dataset from high-throughput DFT. Two solvents, N-methoxy-N-methyl-2,2,2-trifluoroacetamide and 2,2,2-trifluoro-N,N-dimethylacetamide, were identified. Experimental incorporation of trace N-methoxy-N-methyl-2,2,2-trifluoroacetamide into a poly(vinylidene fluoride-co-hexafluoropropylene) matrix achieves a 4.5 V window, 5.5x10^-4 S cm^-1 conductivity (30 C), and 0.78 Li+ transference number. The cell retains 86.7% capacity over 500 cycles (LiFePO4) and 98.7% after 200 cycles at 2C (LiNi0.9Co0.05Mn0.05O2), outperforming 2,2,2-trifluoro-N,N-dimethylacetamide, dimethylformamide, N-methyl-2-pyrrolidone, and dimethyl sulfoxide. This synergy enables balanced ion transport, wide stability, and cycling durability, advancing safer, high-energy lithium metal batteries. Our integrated approach establishes a solvent screening paradigm for rational SPE design, accelerating next-generation battery development.
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All authors Jiadong Shen, Junjie Chen, Xiaosa Xu, Jin Li, Zhenyu Wang, Pengzhu Lin, Zixiao Guo, Yu Wang, Jing Sun, Baoling Huang, Tianshou Zhao

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

Microscopic Origin of Random Singlet Behavior in B-site Disordered Spin-1/2 Perovskite BaCu_1/3Nb_2/3O_3 Revealed by EXAFS and Thermodynamics

Sagar Mahapatra, Francesco De Angelis, Martin Etter et al. (+5 more)

We report a combined structural and thermodynamic study of the ABO$_3$-type disordered perovskite BaCu$_{1/3}$Nb$_{2/3}$O$_3$ (BCNO), whose B site is jointly occupied by Cu and Nb in the $1:2$ ratio. Using synchrotron powder x-ray diffraction (XRD) and x-ray absorption fine structure (XAFS) spectroscopy, we investigate the microscopic nature of Cu$^{2+}$/Nb$^{5+}$ disorder on the pseudo-cubic B-sublattice and its relation to the emergent random-singlet (RS) behavior evidenced at low temperatures. While XRD reveals no long-range Cu/Nb ordering and average site occupancy consistent with stoichiometry, XAFS reveals a peculiar local chemical order characterized by preferential heteroatomic Cu$:$Nb correlations. This local arrangement strongly suppresses direct Cu$:$Cu linkages, despite the Cu concentration being close to the percolation threshold of a cubic lattice. The resulting exchange network explains the absence of spin-glass freezing or long-range magnetic order in the presence of substantial antiferromagnetic interactions, as indicated by a Curie-Weiss temperature $\Theta_{CW}\approx -50$ K. Instead, the magnetic susceptibility $\chi(T)$ and specific heat $c_p(T)$ exhibit power-law behavior and characteristic single-parameter $T/H$ scaling over broad temperature and magnetic-field ranges, consistent with random-singlet phenomenology. Notably, at very low temperatures, the specific heat behavior transitions from $T^{1-\gamma}$ ($\gamma \approx 0.6$ from the $T/H$ scaling) in zero-field to a T-linear dependence under high field, indicating a crossover to a distinct low-energy regime whose microscopic origin remains to be established.
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All authors Sagar Mahapatra, Francesco De Angelis, Martin Etter, Edmund Welter, M. P. Saravanan, Rajeev Rawat, Carlo Meneghini, Surjeet Singh