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Physics Condensed Matter Materials Science

cond-mat.mtrl-sci Watch Materials Science

Thursday 25 June

Materials Science

Hydrogen segregation around a straight screw dislocation in bcc iron

Margot Lucas, Marie Landeiro Dos Reis, Sylvain Queyreau et al. (+1 more)

The interaction between hydrogen and screw dislocations in bcc iron is central to understanding hydrogen embrittlement. A major challenge lies in the high-dimensional parametric landscape governing this interaction. In this work, we perform a comprehensive set of molecular simulations using a reliable neural network interatomic potential, systematically exploring hydrogen binding across dislocation core structures (easy and hard cores), site types, and concentrations. From these energetics, we construct a thermodynamic framework that quantifies the statistical relevance of the various trapping configurations, thereby significantly reducing the complexity of the problem. Our results show good agreement with the limited density functional theory data available in the literature. We further delineate the validity domain of an elastic dipole description of hydrogen-dislocation interactions, providing a simplified yet physically grounded modeling approach. Finally, we demonstrate that the easy-core configuration plays a key role in rationalizing experimental hydrogen solubility limits. These findings establish a consistent multiscale foundation for incorporating hydrogen-dislocation interactions into larger-scale models of plasticity and embrittlement.
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All authors Margot Lucas, Marie Landeiro Dos Reis, Sylvain Queyreau, Xavier Feaugas

Materials Science

Controlled chemical vapor deposition for synthesis of emerging Mo(W)Te2 systems

Ya Deng, Zi-Yi Han, Yao Wu et al. (+6 more)

The Group-VI transition metal ditellurides offer a rich platform for correlated and topological phenomena, yet their structural polymorphism and instability complicate the creation of single crystals and heterointerfaces. Here, we introduce a confined-space chemical vapor deposition (CVD) strategy that lowers the growth temperature window and, when combined with tailored precursor configurations and stepwise thermal ramps, enables the deterministic synthesis of high-quality single crystals, alloys, and lateral/vertical heterostructures. High-resolution aberration-corrected STEM provides atomic characterization of lattice-matched Mo(W)Te2 lateral heterostructure, revealing nearly atomically sharp, compositionally well-defined seamless boundaries. This approach avoids the thickness nonuniformity and structural limitations commonly associated with exfoliated samples, enabling reproducible fabrication of clean heterointerfaces and establishing a nearly ideal in-situ experimental system. Furthermore, scanning tunneling microscopy and spectroscopy (STM and STS) enable direct imaging of the seamless boundaries in Mo(W)Te2 lateral heterostructures, while uncovering their distinct real-space distributions of the local density of states. Our results establish a scalable pathway for engineering crystalline Te-based structures with controlled geometry and stacking, providing an essential step toward quantum and topological device platforms based on the transition metal ditellurides family.
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All authors Ya Deng, Zi-Yi Han, Yao Wu, Kongyang Yi, Ya-Ning Ren, Dundong Yuan, Chao Zhu, Lin He, Zheng Liu

Materials Science

Thermal Rectification from Size-Dependent Phonon Confinement in Nanoparticle Assemblies

Megan J. Farrington, David J. Wasserman, Eden R. Josza et al. (+6 more)

Thermal insulation remains an important technological challenge across the vast number of applications, from living quarters to quantum technology. Here, we exploit the size-dependent modification of the phonon density of states arising from phonon confinement in nanoparticles to fabricate a simple phonon rectifier. The smaller of the two connected nanoparticles imposes stronger phonon confinement leading to rectifying phonon transport. This concept is extended to the macroscale by constructing two overlapping layers of differently sized nanoparticles, thereby realizing a macroscopic phonon diode. Following the localized heat deposition by laser light, the temperature profiles across a phonon-diode were measured by infrared imaging. Although the rectifying strength is moderate, the abundance of optimization possibilities makes this method promising for ultra-low volume thermal insulation at both the nano- and macroscale
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All authors Megan J. Farrington, David J. Wasserman, Eden R. Josza, Alessandro Podestà, Alessio Zaccone, Mariia Sidorova, Alexej D. Semenov, Nicola Ludwig, Marcel Di Vece

Materials Science

Preconditioning Magnetic Systems in Kohn-Sham Density Functional Theory

Clémentine Barat, Antoine Levitt and Marc Torrent

The convergence of the self-consistent field iterations in Kohn-Sham density functional theory can be significantly hindered by the presence of small eigenvalues in the dielectric matrix, which are often associated with electronic phase transitions in magnetic systems. In this work, we study this type of convergence issues and propose a new preconditioning scheme to mitigate them. Our preconditioning scheme is inspired by the Stoner model and based on a non-interacting susceptibility that neglects orbital variations. We demonstrate the effectiveness of our approach on a range of ferromagnetic systems, showing that it can significantly reduce the number of iterations required to achieve convergence in the vicinity of magnetic phase transitions.
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Materials Science · Cross-listed from cond-mat.soft

Dynamic heterogeneity in sodium silicate melts via machine-learning potential

Kumpei Shiraishi, Rikuta Nozawa and Emi Minamitani

We present a comprehensive characterisation of dynamic heterogeneity in sodium silicate melts using molecular dynamics simulation with machine-learning potentials. By studying sodium disilicate, tetrasilicate, and hexasilicate melts across a range of temperatures, mean squared displacement and a time-correlation function computed up to the nanosecond timescale provide a detailed account of how spatial mobility disparities emerge in a realistic multicomponent oxide glass. Within these timescales, the self-part of the van Hove function for sodium displays a bimodality, demonstrating that alkali transport is mediated by discrete displacement events consistent with a hopping mechanism. This distinct hopping allows sodium ions to decouple from the sluggish relaxation of the silicate matrix. Furthermore, evaluation of the non-Gaussian parameter reveals that, although all constituent species exhibit dynamic heterogeneity, the non-Gaussian behaviour is most pronounced for oxygen atoms. This trend reflects the intermittency of structural rearrangements, where framework atoms undergo rare and stochastic events compared to the frequent displacements of mobile ions. Our findings elucidate the microscopic mechanism of ion transport and its connection to dynamic heterogeneity in silicate melts, offering a new avenue to study fundamental glassy physics in realistic vitreous materials.
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Materials Science

Lattice Reconstruction and Orbital Hybridization Suppress Magnetism in TaCo$_2$Te$_2$

Ulysse Chazarin, Bharat C. Bathu, Zuned Ahmed et al. (+6 more)

Structural reconstruction in low-dimensional quantum materials can strongly modify electronic symmetry and magnetic stability through orbital hybridization. Here, we investigate the interplay between lattice reconstruction, electronic structure, and magnetic instability in the layered van der Waals compound TaCo$_2$Te$_2$ using scanning tunneling microscopy and spectroscopy (STM/STS), non-contact atomic force microscopy (nc-AFM), angle-resolved photoemission spectroscopy (ARPES), and density functional theory (DFT). While nc-AFM resolves a distorted hexagonal Te surface lattice, STM/STS reveal a pronounced square-like electronic symmetry that does not directly follow the atomic structure. ARPES further shows a strongly anisotropic Fermi surface and reconstructed low-energy states. Spatially resolved spectroscopy and orbital-projected DFT demonstrate that the bias-dependent STM contrast does not arise from a simple reversal between occupied and unoccupied states, but from the energy-integrated local density of states dominated by electronic states exhibiting opposite spatial contrast at selected energies. DFT calculations further show that reconstruction suppresses the magnetic instability present in the undistorted structure, stabilizing a nonmagnetic ground state through enhanced orbital hybridization. These results establish TaCo$_2$Te$_2$ as a model system in which lattice reconstruction reorganizes electronic symmetry and suppresses magnetism, highlighting structural reconstruction as a route for controlling correlated and magnetic phases in low-dimensional quantum materials.
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All authors Ulysse Chazarin, Bharat C. Bathu, Zuned Ahmed, Marta Zonno, Chiara Bigi, Francois Bertran, Adolfo O. Fumega, Orlando J. Silveira, Shawulienu Kezilebieke

Materials Science

Shape-Constrained Bayesian Active Learning of Self-Limiting Saturation Curves

Pouyan Navabi and Christos G. Takoudis

Saturation curves, monotone responses that rise from zero to a plateau, recur across gas adsorption, enzyme kinetics, and self-limiting growth. Learning one from a few costly, noisy measurements is a common bottleneck. We present a Bayesian active-learning platform built on a penalized monotone I-spline. Non-negative weights make every posterior draw rise from zero and never fall, a survey measurement anchors the plateau in place of a flatness prior, and the input reaching any saturation level is read from the posterior by level crossing, assuming no kinetic form. Each measurement is placed where the surrogate is least certain, which beats random designs and edges a log-spaced grid of the same budget. One fixed configuration recovers five kinetically distinct regimes from twenty measurements to a saturation error of 0.037 to 0.041, below the output noise, and averages 0.045 on 250 curves from an unseen distribution. Running unconstrained Gaussian processes (GPs) and two unpenalized monotone bases through the same loop separates each ingredient's contribution. Logarithmic resolution and the roughness penalty carry the accuracy, while monotonicity guarantees a level crossing unique on every draw. The penalty keeps the surrogate the most accurate model at every noise level, and its second-order form centers the recommended input for saturation where a first-order penalty reads late (a bias of +0.32 s with 94% coverage, against +0.44 s and 64% for the matched unconstrained GP). Because the recommended input lies where the curve is nearly flat, no method can pin it down precisely, and we report it as a cost-indexed quantile with the probability of falling short.
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2606.26577 · 25 Jun · v2: substantially revised and extended, with new analyses, figures and tables, and with the Supporting Information appended. 84 pages: main text with 8 figures and 4 tables, followed by the Supporting Information (5 figures, 19 tables). Code and cached results: https://github.com/pouyna/SplineBAL

Abstract PDF $BAYS ×2$QNTL ×1

Materials Science

Topological phase transition driven by in-plane spin rotation

Xinyue Zhu, Yu Xie, Yifei Hao et al. (+3 more)

The intrinsic coupling between magnetism and nontrivial band topology in magnetic topological insulators makes external magnetic fields a powerful tool for manipulating topological states. However, conventional magnetic control mechanisms, such as driving magnetic phase transitions or fully reversing magnetization, typically demand large magnetic fields and lack continuous tunability. Here, we establish a symmetry framework for the reversible switching of topological states via continuous in-plane spin rotation, governed by magnetic point group constraints on the Berry curvature distribution. Using a two-dimensional kagome ferromagnetic Chern insulator as a prototype, we demonstrate that a 60{\deg}in-plane magnetization rotation reverses the sign of the Chern number, transitioning through a topologically trivial state. Crucially, micromagnetic simulations confirm that this spin-reorientation-driven switching operates under exceptionally small magnetic fields and on ultrafast timescales. This work provides a highly efficient, low-energy paradigm for the manipulation of topological states.
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All authors Xinyue Zhu, Yu Xie, Yifei Hao, Fei Gao, Yue Li, Junting Zhang

Materials Science

Ductility Design Rules for Tungsten based Refractory High Entropy Alloys from Sparse Experimental Datasets

Avik Mahata and Nick Beaver

Tungsten-based refractory high-entropy alloys (RHEAs) are promising materials for fusion applications but often remain brittle at room temperature because of tungsten's high ductile-to-brittle transition temperature (DBTT). To identify alloying strategies that improve ductility, we compiled a curated dataset of experimentally reported tungsten-containing alloys and developed composition-based, physics-informed descriptors for machine learning. Three classifiers were evaluated using nested cross-validation, and a support vector classifier (SVC) showed the best generalization for this sparse dataset. Shapley additive explanations identified the exchange-correlation parameter, valence electron concentration, pressure field, and electronegativity mismatch as the most influential features governing ductile-brittle behavior. Synthetic compositions generated within the convex interpolation domain of the training data were evaluated and visualized on pseudo-ternary diagrams to map predicted ductility trends. The model predicts that moderate additions of Ti, Ni, and Co increase the likelihood of room-temperature ductility, whereas high Cr contents promote brittle behavior. These predictions agree with published experimental observations, including Ti-assisted ductility through solid-solution and electronic effects and embrittlement in Cr-rich refractory alloys. Agreement between DFT-derived elastic descriptors and SVC decision-function margins further supports the physical relevance of the learned classification boundary. Although the available mechanical dataset remains limited and heterogeneous, the model captures experimentally consistent trends and provides an interpretable, physics-informed framework for screening tungsten-based RHEAs for targeted simulation and experimental validation in fusion environments.
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Wednesday 24 June

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

Odd-parity electronic order near the semiconductor limit

Jack Tregidga, Dibyata Rout, Johannes Hielscher et al. (+3 more)

Identifying materials platforms in which dilute carriers experience strong Coulomb interactions is a central challenge in the search for interaction-driven quantum phases. In such systems, weak carrier screening can promote a variety of collective instabilities beyond the conventional Fermi liquid paradigm, including superconductivity, Wigner crystallization, and odd-parity electronic order. Experimental realizations of such dilute, strongly interacting electronic systems remain rare in crystalline materials. Here we report a spontaneous odd-parity phase transition in the phosphide semiconductor family $\textit{Ln}$Cd$_3$P$_3$ ($\textit{Ln}$ = La, Ce, Pr, Nd). Using optical second harmonic generation, we observe the onset of bulk inversion and rotational symmetry breaking accompanied by the emergence of an in-plane polar axis. Second harmonic microscopy reveals three domain variants related by 120$^\circ$ rotations, while ultrafast transient reflectivity measurements uncover a pronounced electronic reconstruction across the transition. Remarkably, the ordered phase appears only in lightly self-hole-doped compounds and is absent in insulating SmCd$_3$P$_3$, indicating an essential role for itinerant carriers despite their extremely low concentration. Guided by density functional theory, we develop a four-band model of the valence states and show that modest interactions can stabilize odd-parity electronic order. The resulting phase combines a spontaneous Fermi surface distortion with a momentum-dependent bilayer polarization that breaks inversion symmetry. Our results establish a route to interaction-driven parity breaking in dilute-carrier semiconductors and identify honeycomb bilayer systems as a promising platform for odd-parity electronic phases.
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All authors Jack Tregidga, Dibyata Rout, Johannes Hielscher, Josiah Turner, Stephen D. Wilson, John W. Harter

Materials Science

Nanoscale Phase Distribution Governs Exchange Bias in Multiphase Magnetic Nanoparticles

Murtaza Bohra, Stefanos Giaremis, Vidyadhar Singh et al. (+3 more)

Exchange bias at ferromagnet-antiferromagnet interfaces underpins magnetic memory, spintronic devices, and nanoscale electromagnetic technologies, yet its behaviour in complex nanoscale heterostructures remains poorly understood. Here we uncover how exchange bias emerges in functional multiphase metal-oxide nanoparticles by combining gas-phase synthesis, advanced magnetic characterisation, and first-principles-informed spin-dynamics simulations. Using Ni-Cr/NiO nanoparticles as a model system, we show that exchange bias is governed not simply by the presence of ferromagnetic and antiferromagnetic phases, but critically by their nanoscale spatial distribution and interfacial topology. At 10 K, significant negative exchange bias (0.8 kOe) and coercivity enhancement (1.4 kOe) was exhibited; both decreased due to either Cr-segregation (at low Cr content) or to Cr accumulation inside the core (at high Cr content). The resulting competition between magnetic phases produces a temperature-driven inversion from negative to positive exchange bias and a crossover from exchange-dominated to dipolar interactions. By linking density-functional-theory calculations directly to spin-dynamics simulations of nanoparticle ensembles, we establish a predictive framework for designing exchange-coupled nanomagnets capable of operating beyond the superparamagnetic limit.
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All authors Murtaza Bohra, Stefanos Giaremis, Vidyadhar Singh, Joseph Kioseoglou, Stephan Steinhauer, Panagiotis Grammatikopoulos

Materials Science

Challenging the $p$-type Paradigm: Intrinsic $n$-type Mobility in Antiferromagnetic Cr$_2$O$_3$

Á. A. Carrasco Álvarez and S. Poncé

Chromium oxide (Cr$_2$O$_3$) is widely considered a $p$-type transparent conducting oxide despite ongoing debate regarding its intrinsic transport character. Here, we resolve this question by computing phonon-limited electron and hole mobilities using the ab initio Boltzmann transport equation. We find that electron mobility systematically exceeds hole mobility over a wide temperature range, demonstrating that Cr$_2$O$_3$ is intrinsically $n$-type. Analysis of scattering mechanisms reveals that scattering with phonons affects electrons and holes similarly, and that the mobility asymmetry originates from the electronic structure, namely the larger effective mass and multi-valley character of the valence band. The intrinsic $n$-type character, combined with moderate hole mobility, enables bipolar transport and revises the role of Cr$_2$O$_3$ in transparent electronics. Additionally, our results on mobility complement previous studies on defect formation indicating that the commonly observed $p$-type behavior is extrinsic. These insights provide a complete chemical-transport paradigm for Cr$_2$O$_3$, re-evaluating its role in functional transparent electronic and magneto-optoelectronic applications
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Materials Science · Cross-listed from cond-mat.supr-con

New Superconductors in the PtPb$_3$Bi Structure Type

Lior Verbitsky, Amira Merino, Scott B. Lee et al. (+8 more)

The quest for new superconductors is of both fundamental and technological importance. Recently, an artificial intelligence method correctly predicted PtPb$_3$Bi to be a superconductor. In this work, we find superconductivity in the newly synthesized $M$Pb$_{4-x}$Bi$_x$ ($M$ = Au, Pd, and Rh), of which PtPb$_3$Bi is a member. When $M$ = Ni, whose radius is considerably smaller, the structure instead collapses into the different, Pb-substituted NiBi$_3$ type. Interestingly, the stoichiometric parameter $x$ shifts across the three compounds to keep the total valence electron count close to 20 per formula unit. The superconducting transitions occur at 4.9, 4.2, and 3.4 K, for $M$ = Au, Pd, and Rh, respectively. Using electrical resistivity, magnetization, and specific heat measurements, we establish the bulk nature of the superconducting state and determine the critical fields, characteristic length scales, and anisotropy ratios. All three compounds are moderately anisotropic type-II superconductors, with modest upper critical field anisotropies of $H_{c2}^{\parallel c}/H_{c2}^{\perp c} \approx 1.2$ to $1.5$. These results establish $M$Pb$_{4-x}$Bi$_x$ as a family of anisotropic superconductors and a platform for studying how site disorder and Pb-Bi mixing govern superconductivity in heavy-element intermetallics.
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All authors Lior Verbitsky, Amira Merino, Scott B. Lee, Jaime M. Moya, Sigalit Aharon, Fatmagül Katmer, Sudipta Chatterjee, Grigorii Skorupskii, Josh Leeman, Gabrielle Carrel, Leslie M. Schoop

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

Anomalous Hall viscosity of altermagnets

Iksu Jang, Rui Aquino, Jörg Schmalian et al. (+1 more)

We show that the phonon Hall viscosity at zero magnetic field is a natural probe of altermagnetism. First, we demonstrate that the finite elements of the Hall viscosity tensor unambiguously distinguish altermagnets from ferromagnets and conventional antiferromagnets. We then microscopically compute the Hall viscosity in models for d-wave and g-wave altermagnets, and find a strong sensitivity to electronic spectrum features such as gapped Dirac points and Lifshitz transitions. This sensitivity reflects a strain-space Berry curvature monopole, which contrast to the multipolar character of the standard momentum-space Berry curvature in altermagnets. Since the Hall viscosity can be probed experimentally through magneto-acoustic measurements, it provides a compelling method to probe the broken symmetries and topology of insulating altermagnets.
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All authors Iksu Jang, Rui Aquino, Jörg Schmalian, Rafael M. Fernandes

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

Strong photogalvanic effect in Weyl materials due to magnetic resonances

Vahid Hassanzade and Vladyslav Kozii

We study the photogalvanic effect in Weyl semimetals under a magnetic field, focusing on the shift current. Using the Kubo formalism for an ideal, clean Weyl node at zero temperature, we derive a general analytic expression valid for arbitrary light frequency, Fermi energy, and magnetic field strength. We identify a series of resonances that can be probed experimentally. To complement the microscopic analysis, we employ the semiclassical Boltzmann approach, which allows us to incorporate finite scattering phenomenologically. Unlike most previous studies using this method, we do not treat the magnetic field perturbatively; instead, we solve the Boltzmann equation for a Weyl node exactly within the limits of validity of the semiclassical theory. Our solution reproduces the low-frequency resonances and elucidates the role of finite scattering.
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Materials Science · Cross-listed from cond-mat.mes-hall

Quantum Back-Action Expands the Excitonic Hilbert Space in a Soft Polar Semiconductor

Arnab Ghosh, Patrick Brosseau, Priya Nagpal et al. (+4 more)

Electronic excitations in solids are commonly described within a hierarchy in which the excitonic Hamiltonian is defined first and the lattice acts later through renormalization, relaxation, and dephasing. This picture assumes that the optically accessible excitonic manifold is already present at the moment of photoexcitation. Here we show that this assumption fails in a soft polar semiconductor. Using femtosecond coherent multidimensional spectroscopy on lead-halide perovskite nanocrystals, we observe quantum back-action between an electronic excitation and a collective lattice-polarization field that expands the excitonic Hilbert space in real time. The optical pulse first prepares an excitonic polarization, X1. A second configuration, X2, emerges only after the polaron field develops, while coherent X1-X2 coupling appears at later times. State formation and coherence formation are therefore resolved as distinct stages of quasiparticle formation. In contrast, CdSe quantum dots exhibit the conventional limit in which excitonic states and couplings are present at time zero and are only weakly perturbed by phonons. The observed diagonal and anti-diagonal splittings increase with nanocrystal size and correlate with radiative oscillator strength, opposite to expectations from simple quantum confinement. A dynamical polaron-field model describes the lattice polarization as an order parameter that expands the optically accessible manifold and generates time-dependent coherent coupling. These results show that strong system-bath coupling can actively create excitonic states and the coherent manifold in which they evolve.
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All authors Arnab Ghosh, Patrick Brosseau, Priya Nagpal, Dmitry N. Dirin, Rui Tao, Maksym V. Kovalenko, Patanjali Kambhampati

Materials Science

Epitaxial Strain Activates Altermagnetic Spin-Splitting Torques in RuO2(100)

Qi Jia, Seung Gyo Jeong, Seungjun Lee et al. (+10 more)

The altermagnetic nature of rutile RuO2 remains under active debate: bulk measurements indicate a nearly nonmagnetic ground state, whereas thin-film studies have reported symmetry-dependent transport signatures consistent with altermagnetism. Here, we provide experimental evidence that altermagnetic spin splitting in RuO2 is a strain-stabilized emergent state rather than an intrinsic bulk property. Angular-resolved spin-torque measurements reveal a symmetry-selected spin Hall response characteristic of altermagnetic spin splitting, which is strongest in the strained regime but progressively suppressed as the lattice relaxes toward the bulk limit. Complementary magnetic measurements further reveal enhanced coercivity and exchange-bias behavior exclusively in strained films, indicating the emergence of a strain-stabilized magnetic state. First-principles calculations reproduce the strain-dependent evolution of the Neel order and spin-split electronic structure, supporting the experimental observations. Together, these results establish altermagnetic spin splitting in RuO2 as a strain-stabilized emergent state and provide a unified explanation for the long-standing discrepancy between bulk and thin-film observations.
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All authors Qi Jia, Seung Gyo Jeong, Seungjun Lee, Denis Tonini, Anand Santhosh, Yifei Yang, Xiangrui Li, Brahmdutta Dixit, Shuang Liang, Yu-Chia Chen, Tony Low, Bharat Jalan, Jian-Ping Wang

Materials Science

Primary damage and mechanical degradation of WTaCrV refractory high-entropy alloy: effects of solid-solution and chemical ordering

Yihan Wu, Pengfei Yu, Yaohong Suo et al. (+1 more)

As advanced nuclear reactors demand novel irradiation-tolerant materials, this study investigates the radiation damage and mechanical degradation of the promising WTaCrV refractory high-entropy alloy (RHEA). To isolate complex nanoscale chemical effects, we propose an atomistic modeling strategy comparing Average-Atom (AA), random solid-solution (RSS), and local chemical order (LCO) configurations using newly developed interatomic potentials. Collision cascades simulations reveal that the number of Frenkel pairs follow NRSS > NLCO > NAA at the same radiation dose. While the RSS effect accelerates defect generation due to rugged energy landscapes, LCO enhances lattice cohesion to mitigate radiation damage. Despite more primary defects in the RSS and LCO configurations compared with the AA configurations, the RSS and LCO effects can suppress radiation-induced mechanical degradation. Irradiation severely degrade the homogenized AA model but exert a limited impact on the strength and flow stress of the RSS and LCO models. This exceptional resistance is driven by inherent lattice distortion resulting from interactions among different alloy elements, which outweighs point defect induced lattice disruptions. Moreover, the complex interactions between deformation twins and point defects cause confined plastic flow, elevating flow stress in the RSS and LCO models. The findings provide atomistic guidance for performance assessment of next-generation structural materials for extreme nuclear environments.
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All authors Yihan Wu, Pengfei Yu, Yaohong Suo, Lei Zhang

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

Residual orbital magnetization governs the anomalous Hall effect in altermagnets

Yufei Zhao, Yiyang Jiang, Kamal Das et al. (+2 more)

In altermagnets that exhibit anomalous Hall effect, the small remanent magnetization exists but has been treated as too small to be relevant to the Hall response. In this work, we point out that this dismissal is incomplete because the generalized Středa relation ties the intrinsic anomalous Hall conductivity ($\sigma_{xy}$) to the orbital magnetization ($M_z$, the topological component from the modern orbital magnetization) by $\sigma_{xy}=-e\frac{\partial M_z}{\partial \mu}$. We reveal a microscopic mechanism to generate net orbital moment from the interplay of local crystal field and spin-orbit coupling for MnTe-type altermagnets, in which the magnetic anisotropy generates weak net magnetization without invoking exchange between neighboring spins (e.g., Dzyaloshinskii-Moriya interaction). Our work indicates that residual orbital and spin magnetization is an intrinsic thermodynamic property that governs anomalous transport in unconventional antiferromagnets, including altermagnets and noncollinear antiferromagnets.
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All authors Yufei Zhao, Yiyang Jiang, Kamal Das, Chao-Xing Liu, Binghai Yan

Materials Science

Capturing Nuclear Quantum Effects in Hydrogen Diffusion through MoS2 via Machine-Learning-Enhanced Path-Integral Simulations

Ismail Eren, Ege Yigit Erbil, Maria-Judith Caisachana-Lozada et al. (+3 more)

Hydrogen transport through layered two-dimensional (2D) materials is central to technologies such as hydrogen storage, fuel cells, and isotope separation. Among these materials, MoS2 exhibits tunable interlayer diffusion properties, whose accurate theoretical description requires accounting for nuclear quantum effects (NQEs), including zero-point motion and tunneling. Here, we present a machine-learning-enhanced atomistic study of hydrogen and deuterium diffusion in layered MoS2 based on interatomic potentials trained on r2SCAN+rVV10 density-functional-theory data. Combining well-tempered metadynamics with path-integral molecular dynamics, we investigate diffusion across multiple MoS2 polytypes and twisted bilayer structures while explicitly incorporating NQEs. Our simulations show that NQEs substantially lower free-energy barriers for hydrogen diffusion at 300 K, significantly increasing the hydrogen self-diffusion coefficient compared to classical nuclei simulations. We further identify a pronounced kinetic isotope effect, with a 35 meV difference between hydrogen and deuterium quantum free-energy barriers. In twisted bilayer MoS2, hydrogen transport exhibits strong spatial variations governed by the local stacking environments within the moiré superlattices. These results highlight the critical role of NQEs in hydrogen transport through layered materials and provide atomistic insight intoisotope-selective diffusion in structurally complex 2D systems.
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All authors Ismail Eren, Ege Yigit Erbil, Maria-Judith Caisachana-Lozada, Hossein Mirhosseini, Thomas D. Kühne, Agnieszka B. Kuc

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

High-Performance Nanophononic Resonators in Self-Suspended WSe$_2$ Domes and Drums

Jens-Christian Drawer, Bo Han, Edson Rafael Cardozo de Oliveira et al. (+7 more)

Van der Waals materials are ideally suited for the implementation of high-frequency nanophononic resonators with atomically flat interfaces. Here, we present two versatile van der Waals-based nanophononic architectures: First, we introduce self-supporting nano-domes of WSe$_2$ as a scalable platform for the simultaneous generation of hundreds of high-quality nanoacoustic resonators with resonance frequencies in the 100 GHz range. Second, we engineer self-supporting nano-drums that reach record-high working frequencies for 2D-semiconductor transducers beyond 1 THz. Through optical pump-probe spectroscopy experiments and photoelastic linear chain model calculations, we gain a detailed understanding of the intricate interplay between phononic mode hybridization across heterostructures, the differences between modes close to the center and edge of the acoustic Brillouin zone, and the temporal structure of the photoelastic response. Both architectures have potential applications in low-cost nanoacoustic probing and the ultrafast modulation of quantum emitters in two-dimensional semiconductors. While nano-drums surpass the THz frequency barrier, nano-domes appear as an accessible, low-cost alternative for developing scalable nanophononic technologies.
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All authors Jens-Christian Drawer, Bo Han, Edson Rafael Cardozo de Oliveira, Chushuang Xiang, Vita Solovyeva, Kenji Watanabe, Takashi Taniguchi, Norberto Daniel Lanzillotti-Kimura, Christian Schneider, Martin Esmann

Materials Science

Impact of sintering conditions on the dielectric properties of TiO2 ceramics for metamaterialsapplications at terahertz frequencies

Djihad Amina Djemmah, Delphine Gourdonnaud, Faycal Bouamrane et al. (+3 more)

Titanium dioxyde (TiO2) is a promising dielectric material for the realization of metamaterials operating in the terahertz (THz) range. Indeed, these necessitate a high permittivity and low loss material. In this paper, we compare the processes of fabrication and the results of characterisation of bulk TiO2 pellets. From the results of this characterization, we have numerically designed 2D all dielectric metamaterials (ADM) showing that they may exhibit negative or near-zero effective index. Our previous simulations show that the relative permittivity epsilon has to be around 100, while the loss tangent has to be lower than 0.02. We have thus compared conventional sintering (CS) vs spark plasma sintering (SPS), and investigated the effect of post-sintering annealing on the loss to satisfy these two criteria. The samples were characterized by THz Time Domain Spectroscopy (THz-TDS). One of the samples exhibits a loss tangent as low as 0.006, with a permittivity epsilon = 103. These results highlight the importance of the fabrication process on the EM properties of bulk TiO2, and demonstrate that it is a promising material for the development of metamaterial in the THz band.
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All authors Djihad Amina Djemmah, Delphine Gourdonnaud, Faycal Bouamrane, Jean-Francois Roux, Pierre-Marie Geffroy, Eric Akmansoy

Materials Science

Orientation Mapping via Dictionary Indexing of AC-STEM Kikuchi patterns using Open-source Software

Shamail Ahmed, Johannes Haust, Felix Gruber et al. (+5 more)

The properties of polycrystalline materials are strongly influenced by the spatial arrangement and orientations of individual grains within the microstructure, making nanoscale characterization of grain orientation essential. This is also often the case for small grains in the nm regime explored using scanning transmission electron microscopy (STEM). Automated crystal orientation mapping (ACOM) is traditionally performed using spot-like diffraction patterns. In contrast, orientation mapping based on transmission Kikuchi diffraction (TKD) using an aberration-corrected (AC) convergent STEM probe remains relatively underexplored, despite its superior orientation sensitivity and higher spatial resolution. In this work, we present an open-source software-based template-matching approach for orientation mapping using AC-STEM TKD. A master pattern (a simulated angular distribution of Kikuchi band intensities on the unit sphere) is first generated through a dynamical simulation implemented in open-source software. This resulting pattern is subsequently imported into another open-source package for geometric simulations and orientation indexing. We demonstrate the capability of the proposed method by applying it to orientation mapping in BaZr0.4Ce0.4Y0.1Yb0.1O3-{\delta} (BZCYYb4411) fuel-cell material and LiNiO2 (LNO) lithium-ion battery cathode material. The best-matched simulated patterns exhibit strong agreement with experimental data, even under the challenging conditions with limited diffraction space available for matching.
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All authors Shamail Ahmed, Johannes Haust, Felix Gruber, Celina Becker, Juergen Belz, Andreas Beyer, Laura-Alena Schaefer, Kerstin Volz

Materials Science

The interplay of interfaces, supramolecular assembly, and electronics in organic semiconductors

Belinda J. Boehm, Huong T.L. Nguyen and David M. Huang

Organic semiconductors, which include a diverse range of carbon-based small molecules and polymers with interesting optoelectronic properties, offer many advantages over conventional inorganic semiconductors such as silicon and are growing in importance in electronic applications. Although these materials are now the basis of a lucrative industry in electronic displays, many promising applications such as photovoltaics remain largely untapped. One major impediment to more rapid development and widespread adoption of organic semiconductor technologies is that device performance is not easily predicted from the chemical structure of the constituent molecules. Fundamentally, this is because organic semiconductor molecules, unlike inorganic materials, interact by weak non-covalent forces, resulting in significant structural disorder that can strongly impact electronic properties. Nevertheless, directional forces between generally anisotropic organic-semiconductor molecules, combined with translational symmetry breaking at interfaces, can be exploited to control supramolecular order and consequent electronic properties in these materials. This review surveys recent advances in understanding of supramolecular assembly at organic-semiconductor interfaces and its impact on device properties in a number of applications, including transistors, light-emitting diodes, and photovoltaics. Recent progress and challenges in computer simulations of supramolecular assembly and orientational anisotropy at these interfaces is also addressed.
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Materials Science

Sensitivity-optimal coplanar waveguide design for broadband magnetic resonance spectroscopy: a Beer–Lambert framework

Scott Dietrich and Artur Solodovnyk

Coplanar waveguide (CPW) transmission spectroscopy is used to probe spin dynamics, ferromagnetic resonance, and complex conductivity across a wide range of materials, yet no systematic framework connects waveguide geometry to measurement sensitivity when sample volume and concentration are fixed. We show that the shared geometric scaling of sample coupling and conductor loss maps CPW design onto the Beer–Lambert optimization problem of optical spectrophotometry, reducing it to a universal one-parameter problem whose solution depends only on sample geometry and the dominant noise source – not on sample properties, operating frequency, or system losses. The framework predicts a near-universal $1\,\mathrm{Np}$ optimum across the full range of sample thicknesses and design geometries. Benchmarking against seven published broadband FMR instruments reveals two fabrication-delimited classes: PCB-milled designs are bounded by a ceiling imposed by their minimum slot width, while photolithographic designs approach the additive-noise optimum. For large-area PCB samples a meander geometry offers a direct path to near-optimal sensitivity without interferometric compensation; for sub-millimeter samples, a single lithographic straight pass suffices.
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Materials Science

A Combined Tight Binding with Machine Learning Potential Model for Magnesium Compounds

Jiwen Yu, Arash A. Mostofi and Andrew Horsfield

We present a model for magnesium-based systems that combines density functional tight binding (DFTB) with MACE, a machine learning interatomic potential (DFTB+MACE). In this model, the conventional repulsive potential, pair potential, is replaced by a many-body MACE potential. The MACE component of the model is trained on the difference between density functional theory (DFT) energies and forces and the corresponding DFTB values, but neglecting the pair potential contribution. Using this model we performed structural relaxation of MgO-CO2 adsorption systems, molecular dynamics calculations of water clusters and phonon spectrum calculations of stable fcc-MgO and metastable bcc-MgO structures. We compare the performance of our model with a pure MACE model and with DFT. We demonstrate that the DFTB+MACE model achieves improved accuracy relative to DFTB with a pair potential, in many cases with only a moderate increase in computational cost. In addition, it can provide electronic structures that most of the machine learning potentials cannot. The training dataset, originally developed for MACE, may not fully represent all regions of the potential surface we may encounter during simulations. Expanding the dataset for a wider potential surface is expected to further enhance predictive accuracy of DFTB+MACE model. Overall, the resulting DFTB+MACE framework enables simulations at length and time scales beyond the reach of first-principles methods while retaining an explicit description of electronic structures, making it particularly attractive for studying charge-transfer in materials.
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Materials Science

Harnessing electrostatics through temperature modulations to control ferroelectrics

Cameron A.M. Scott, Xabier Diaz de Cerio and Jorge Íñiguez-González

Temperature modulations provide an alternative method for dynamically controlling the ferroelectric state. In this paper, we use scale-independent Landau potentials and predictive atomistic simulations to explore how temperature modulation can harness the depolarizing field to obtain non-electrical poling. We further predict that temperature gradients can be combined with strain to induce persistent polar textures such as multidomain states.
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Materials Science

Spontaneous spin splitting and tunable valley polarization in a two-dimensional fully compensated ferrimagnet

Hongyan Lv, Yueli Li, Yunfan Zhang et al. (+3 more)

Materials with controllable valley polarization and anomalous valley Hall (AVH) effect are highly desired in valleytronic applications. While current AVH studies primarily focus on ferromagnetic materials, two-dimensional (2D) antiferromagnets are more attractive for valleytronics since they possess zero net magnetization, negligible stray fields, and ultrafast spin dynamics. Nevertheless, the joint space-inversion and time-reversal ($PT$) symmetry in conventional collinear antiferromagnets prohibits the occurrence of AVH response. The recently proposed fully compensated ferrimagnets break $PT$ symmetry, and the spin-opposite sublattices are not related by crystal symmetry, providing a natural platform for the coexistence of spontaneous spin splitting, valley polarization, and anomalous-Hall compatible symmetry. Herein, we demonstrate that such compensated ferrimagnetism can be realized in a Janus Mn$_{2}$BrI monolayer, with a Néel temperature above room temperature. Spontaneous spin splitting is observed due to the built-in layer-dependent electrostatic potential. When SOC is considered, valley polarization emerges for an out-of-plane Néel vector. Moreover, proper hole doping stabilizes the perpendicular magnetic anisotropy and the two valleys exhibit markedly different Berry curvatures, thereby making AHE responses allowed. Furthermore, the valence band extrema of Mn$_{2}$BrI monolayer can be effectively tuned by external biaxial strain and giant piezomagnetism can be achieved. Our results identify Janus Mn$_{2}$BrI monolayer as a promising fully compensated ferrimagnetic platform for 2D valleytronics and spintronics.
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All authors Hongyan Lv, Yueli Li, Yunfan Zhang, Jiayu Dai, Zhongjun Li, Ding-Fu Shao

Materials Science

Thermoelectric response of a ferroelectric insulator

Ryo Iguchi, Takashi Teranishi, Sakyo Hirose et al. (+4 more)

Thermoelectric effects enable the conversion between heat and electricity without moving parts. While conventionally associated with mobile charges, we report thermoelectricity caused by bound charges in the form of temperature changes measured by multi-harmonic lock-in thermography of a ferroelectric under an ac electric field. The observed temperature gradient depends on the field-induced displacement current, a Peltier effect in a dielectric material. Its coefficient exceeds 100 V around the ferroelectric-paraelectric phase transition, which is several orders of magnitude greater than reported values in conductors. Our findings uncover previously hidden functionalities of ferroelectric materials for thermal management by directional heat transport in ferroelectrics.
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All authors Ryo Iguchi, Takashi Teranishi, Sakyo Hirose, Ping Tang, Jun Kano, Gerrit E.W. Bauer, Ken-ichi Uchida

Materials Science

Barocaloric phase transformation from data efficient fine-tuning of machine learned interatomic potentials

Ludwig Hedin and Johan Klarbring

Solid-state cooling based on the barocaloric (BC) effect has emerged as promising environmentally friendly prospective alternative to conventional vapor-compression refrigeration. The search for suitable BC materials relies on efficient atomistic simulations of their phase behavior. Machine-learned interatomic potentials (MLIPs) enable such simulations at near density functional theory (DFT) accuracy, but generating the required DFT training data remains computationally demanding, which motivates development of strategies that reduce the amount of data needed to train accurate models. In this work, we use a prototypical BC material, ammonium sulfate, as a model system and investigate how small a training set can be while still reproducing the temperature-driven structural phase transformation that underlies its BC response. We train a series of MLIPs based on the MACE architecture using three strategies: training from scratch, and naive- and multihead replay fine-tuning of the MACE-MPA-0 foundation model. These strategies are evaluated on their ability to reproduce the phase transformation of ammonium sulfate in molecular dynamics simulations across a range of training-set sizes. We find that, while the MACE-MPA-0 foundation model itself fails to reproduce the transformation, and models trained from scratch break down for small datasets, fine-tuned models reproduce the transformation using as few as 5 to 10 60-atom DFT configurations. Both fine-tuning protocols yield similarly accurate results for ammonium sulfate, but we also find some indications that multihead replay fine-tuning is more robust on configurations outside the fine-tuning domain. Exploiting this data efficiency, we further show that models can be trained on small datasets and the hybrid-DFT level, and that some form of inclusion of dispersion correction is necessary to describe the phase behavior correctly.
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