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Physics Condensed Matter Materials Science

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Friday 5 December

Materials Science

Compressional rate-dependent stability of ammonia hydrates crystallized from water-rich ammonia-water solutions

Anshuman Mondal, Katharina Mohrbach, Konstantin Glazyrin et al. (+2 more)

Understanding the crystallization pathways of water-rich ammonia-water (NH3-H2O) solutions and the stability of ammonia hydrates is key to unraveling the behavior of complex hydrogen-bonding networks as well as for planetary interior modelling. Yet, there are still inconsistencies in the crystallization sequence reported upon pressure-induced crystallization of H2O-rich NH3-H2O solutions at room temperature. Here, we investigate the effect of compression rates on the crystallization pathways of 25 wt% NH3 aqueous solutions at room temperature using dynamically compressed diamond anvil cells (dDAC) coupled with time-resolved X-ray diffraction. We show that compression rates exceeding 0.5 GPa/sec promote direct crystallization of a body-centered cubic (bcc) phase (DMA') with possible AMH stoichiometry coexisting with H2O ice VII, while rates below 0.2 GPa/sec stabilize monoclinic NH3-rich AHH-II and ice VII phases. Intermediate rates between 0.2-0.5 GPa/sec produce a mixture of both hydrates alongside ice VII, hence demonstrating the role of compression rate on the crystallization sequence of ammonia solutions. The compression behavior and phase stability of the distinct phase assemblies (AHH-II/DMA' + ice VII) are investigated further to place constraints on the composition of the DMA' phase, the effect of ice VII on the compressibility of ammonia hydrates, and the plausible incorporation of NH3 impurities within the lattice of high-pressure ice phases.
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All authors Anshuman Mondal, Katharina Mohrbach, Konstantin Glazyrin, Hanns-Peter Liermann, Carmen Sanchez-Valle

Materials Science

Direct demonstration of electric chirality control in a helimagnetic YMn$_6$Sn$_6$ by spin-polarized neutron scattering

Hidetoshi Masuda, Yutaro Yanagisawa, Kazuki Ohishi et al. (+3 more)

The spiral handedness of magnetic moments, referred to as chirality, gives rise to emergent electromagnetic phenomena in helimagnets. In insulating helimagnets, known as multiferroics, the cycloidal spin structure induces electric polarization by utilizing the inverse Dzyaloshinskii-Moriya mechanism. Spin-polarized neutron diffraction experiments, which directly probe circular spin arrangements, clearly demonstrated that an electric field controlled the chirality in multiferroic helimagnets. On the other hand, it was unclear until recently how the chirality could be controlled in metallic helimagnets where a large electric field cannot be applied, while the chirality control technique in metallic helimagnets should enable the exploration of chirality-dependent spintronic functionalities. Recently, Jiang et al. succeeded in controlling the chirality of a spiral structure by the simultaneous application of a magnetic field and electric current in a metallic helimagnet, utilizing the nonreciprocal electronic transport as an indirect probe of chirality, highlighting the need for a neutron diffraction experiment that directly probes the chirality. Here, we directly demonstrate the chirality control in a metallic helimagnet YMn$_6$Sn$_6$ by means of spin-polarized neutron diffraction, which should give rise to a firm basis for the development of future helimagnetic spintronics.
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All authors Hidetoshi Masuda, Yutaro Yanagisawa, Kazuki Ohishi, Yusuke Nambu, Yoichi Nii, Yoshinori Onose

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

Sub Band Gap Operation Limits for Perovskite Light Emitting Diodes

Pradeep R. Nair

Ultra low voltage operation of Perovskite light emitting diodes (PeLEDs) has been demonstrated in recent years as high radiance with minimal power consumption is a desired feature. However, the light output at such conditions from PeLEDs is typically very low, and the maximum in external quantum efficiency (EQE) and energy conversion efficiency (ECE) are achieved at large biases with significant power consumption. Here, we explore the possibility of achieving maximums in EQE and ECE at sub band gap voltages for PeLEDs. Our analysis consistently interprets otherwise scattered experimental data from literature, identifies the limits for low voltage operation, and elucidates optimization routes for sub band gap high radiance operation of PeLEDs.
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Materials Science

An evaluation of A15 Nb3Al superconducting thin films for application in quantum circuits

Joseph Falvo, Brooke Henry, Bernardo Langa Jr. et al. (+4 more)

A15 superconductors are distinguished by their high critical temperatures, magnetic fields, and current-carrying capabilities. Among them, Nb$_3$Al is of particular interest for superconducting quantum circuits as a means to extend device operating temperatures, provided that its electrodynamic properties are well understood. Here, we report on the synthesis of Nb$_3$Al thin films by magnetron co-sputtering followed by rapid thermal processing, yielding superconducting transition temperatures above 16~K. Microwire devices patterned from these films exhibit a coherence length of $3.2\,\mathrm{nm}$ and superfluid densities as low as $1.1\times 10^{26}\,\mathrm{m}^{-3}$, suggesting that Nb$_3$Al may enable high kinetic inductance in thinner films. Coplanar waveguide resonators fabricated on Nb$_3$Al demonstrate single-photon internal quality factors up to $2.26\times 10^{5}$. These results establish Nb$_3$Al as a promising material platform for the development of superconducting quantum circuits operating at elevated temperatures, contingent on appropriate control of interfacial chemistry and surface morphology.
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All authors Joseph Falvo, Brooke Henry, Bernardo Langa Jr., Rohit Pant, Ashish Alexander, Jason Dong, Kasra Sardashti

Materials Science · Cross-listed from physics.optics

Spectrally-selective dynamic radiative thermoregulation via phase engineering

Qizhang Li, Yuanke Chen, Zhuang Luo et al. (+10 more)

Maintaining comfortable temperatures for buildings, humans, and devices consumes a substantial portion of global energy, underscoring the urgent need for energy-efficient thermoregulation technologies. Dynamic radiative thermal emitters that can switch between passive cooling and heating modes offer a promising solution, but most existing devices exhibit broadband optical responses, resulting in unwanted parasitic heat exchange and limited performance. Here, we introduce an elegant strategy that uses a dielectric cap to transform broadband metal-insulator transition (MIT) materials into spectrally selective dynamic emitters. This design creates a highly tunable Fabry-Perot cavity, enabling a tailored thermal emission spectrum by engineering the reflected-wave phase profile. Our Fresnel-formalism-based phasor diagram analysis reveals two key routes for realizing high spectral selectivity: a high-index dielectric cap and a low-loss metallic MIT state, which are further validated by Bayesian optimization. Following this principle, we demonstrated a wide-angle spectrally-selective thermoregulator operating in the atmospheric transparency window (8-13 um), where the thermal emittance can be electrically tuned from about 0.2 to 0.9 through reversible copper electrodeposition on a germanium cavity. Furthermore, this strategy can be extended to multispectral electrochromic windows, enabling switching between solar heating and spectrally-selective radiative cooling. Our work establishes a versatile and generalizable paradigm for spectral engineering of dynamic thermal emitters, opening opportunities in energy-efficient buildings, wearable thermal comfort, spacecraft thermoregulation, and multispectral camouflage.
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All authors Qizhang Li, Yuanke Chen, Zhuang Luo, Chenxi Sui, Xubing Wu, Ronghui Wu, Qingsong Fan, Ching-Tai Fu, Pei-Jan Hung, Gangbin Yan, Genesis Higueros, Ting-Hsuan Chen, Po-Chun Hsu

Materials Science

Awakening catalytically active surface of BaRuO3 thin film for alkaline hydrogen evolution

Jegon Lee, Dohyun Kim, Seulgi Ji et al. (+9 more)

The dynamic reconstruction of surfaces during electrochemical reactions plays a crucial role in determining the performance of electrocatalysts. However, because reconstructions occur at the atomic level, direct observation and elucidation of the underlying mechanism are challenging for conventional powder type catalysts with ill defined lattices. In this study, the catalytically active surface of 3C BaRuO3 (BRO) epitaxial thin films emerges upon the dynamic introduction of surface Ru clusters, for the alkaline hydrogen evolution reaction (HER). Based on the mass activity at overpotential 100 mV, the intrinsic HER performance increases dramatically from 0.11 to 7.72 A/mg immediately after the initial HER cycle and eventually saturates at 1.05 A/mg after continuous operation. The formation of Ru clusters on the catalyst surface, driven by selective Ba leaching under alkaline HER conditions, is observed experimentally. Density functional theory calculations demonstrate that HER activity increased with enhanced H* adsorption owing to the dynamic Ru6 cluster formation. A strategy for stabilizing the 'awakened' active surface of BRO is further proposed by validating that the atomic-scale control of the film thickness can effectively maintain the highly active state. This study offers fundamental insights into the design and stabilization of the highly active Ru-based electrocatalysts for the alkaline HER.
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All authors Jegon Lee, Dohyun Kim, Seulgi Ji, Sangmoon Yoon, Seung Hyun Nam, Jucheol Park, Jin Young Oh, Seung Gyo Jeong, Jong-Seong Bae, Sang A Lee, Heechae Choi, Woo Seok Choi

Materials Science

External-field-induced transition from altermagnetic metal to fully-compensated ferrimagnetic metal in monolayer $\mathrm{Cr_2O}$

San-Dong Guo, Qiqi Luo, Shi-Hao Zhang et al. (+1 more)

Altermagnets and fully-compensated ferrimagnets are two canonical classes of zero-net-moment magnets. An altermagnetic (AM) half-metal cannot exist due to its AM spin splitting, while a fully-compensated ferrimagnetic (FC-FIM) metal seems impossible to realize because both spin channels remain gapless. Here, we propose that an FC-FIM metal can be realized by breaking the rotational or mirror symmetry that links two spin-opposite magnetic atoms in an AM metal. We further demonstrate that charge-carrier doping is fundamentally unable to generate a net magnetic moment in an altermagnet, whereas such a net moment can be readily induced in a fully-compensated ferrimagnet. We use the AM monolayer $\mathrm{Cr_2O}$ as a concrete example to validate our proposal. Either electric field or uniaxial strain can break the $S_{4z}$ symmetry of $\mathrm{Cr_2O}$, thereby inducing a transition from an AM metal to an FC-FIM metal. Uniaxial strain plus carrier doping creates a net moment in an altermagnet, and the so-called piezomagnetism is essentially a strain-driven switch from altermagnetism to fully-compensated ferrimagnetism. By analogy, we advance the concept of electromagnetism: an electric field drives the transition from altermagnetism to fully-compensated ferrimagnetism, and subsequent charge-carrier doping stabilizes a net magnetization. Our work provides a roadmap for further exploring the connection and distinction between altermagnet and fully-compensated ferrimagnet, and confirms the feasibility of FC-FIM metal.
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All authors San-Dong Guo, Qiqi Luo, Shi-Hao Zhang, Peng Jiang

Materials Science

Platonic representation of foundation machine learning interatomic potentials

Zhenzhu Li and Aron Walsh

Foundation machine learning interatomic potentials (MLIPs) are trained on overlapping chemical spaces, yet their latent representations remain model-specific. Here, we show that independently developed MLIPs exhibit statistically consistent geometric organisation of atomic environments, which we term the Platonic representation. By projecting embeddings relative to a set of atomic anchors, we unify the latent spaces of seven MLIPs (spanning equivariant, non-equivariant, conservative, and non-conservative architectures) into a common metric space that preserves chemical periodicity and structural invariants. This unified framework enables direct cross-model optimal transport, interpretable embedding arithmetic, and the detection of representational biases. Furthermore, we demonstrate that geometric distortions in this space can indicate physical prediction failures, including symmetry breaking and incorrect phonon dispersions. Our results show that the latent spaces of diverse MLIPs present consistent statistical geometry shaped by shared physical and chemical constraints, suggesting that the Platonic representation offers a practical route toward interoperable, comparable, and interpretable foundation models for materials science.
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Materials Science

Phonon density of states of silica (SiO2) nanopore via molecular dynamics simulations

Pablo Galaviz, Dehong Yu, Nicolas de Souza et al. (+4 more)

This study presents a comprehensive computational investigation of the vibration density of states (VDOS) of a silica nanopore, systematically evaluating a range of force fields against inelastic neutron scattering results. We analyze the influence of temperature, crustal structure, and surface-adsorbed water molecules on the nanopore's structural and dynamic properties. We performed classical molecular dynamics simulations of nanopore and bulk silica, and used density functional theory (DFT) calculations for bulk silica for comparison. The resulting VDOS shows relatively good agreement with DFT and experimental data. The temperature has a relatively low effect on the dry nanopore. The inclusion of H2O molecules significantly affects the VDOS. The low-energy modes are dominated by H2O VDOS and increase with loading. This work is an essential step towards characterizing silica nanopores via molecular dynamics and provides a valuable reference for experimental comparison with X-ray and neutron scattering VDOS results.
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All authors Pablo Galaviz, Dehong Yu, Nicolas de Souza, Sho Kimura, Yoshitomo Kojima, Seiji Mori, Akira Yamaguchi

Thursday 4 December

Materials Science · Cross-listed from cond-mat.soft

Biomolecular crystallization in microfluidic devices

Nadine Candoni, Romain Grossier and Stéphane Veesler

This chapter presents an overview of microfluidic devices reported in the literature, used to develop methodologies for nucleation of biomolecules, with crystal size control, and for collecting thermodynamic and kinetic data. Part I is dedicated to the properties of microfluidic devices through materials used for their fabrication and for crystals analysis. Part II describes the variety of microfluidic devices available and how to handle them to produce flows, droplets and/or wells of micrometer size. These devices use crystallization methods inspired by batch processes and they are mainly used for protein crystallization. Part III focuses on fundamental properties of biomolecule crystallization determined using droplet-based microfluidics: nucleation kinetics, nucleation rate and effective interfacial energy crystal/solution. Part IV explains how the kinetic effect of confinement due to micrometer size, and so nanovolumes, leads to isolation of different phases. These latter are characterized by X-Ray Diffraction (XRD) and methods to minimize manual handling of crystals for XRD are also presented, with appropriate equipment to store the crystals.
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Materials Science

From orbital analysis to active learning: an integrated strategy for the accelerated design of TADF emitters

Jean-Pierre Tchapet Njafa, Steve Cabrel Teguia Kouam, Patrick Mvoto Kongo et al. (+1 more)

Thermally Activated Delayed Fluorescence (TADF) emitters must satisfy two competing requirements: small singlet-triplet energy gaps for thermal upconversion and sufficient spin-orbit coupling for fast reverse intersystem crossing. Predicting these properties accurately demands expensive calculations. We address this using a validated semi-empirical protocol (GFN2-xTB geometries, sTDA/sTD-DFT-xTB excited states) on 747 molecules, combined with charge-transfer descriptors from Natural Transition Orbital analysis. The hole-electron spatial overlap She emerges as a key predictor, accounting for 21% of feature importance for the triplet state alone. Our best model (Support Vector Regression) reaches MAE = 0.024 eV and R2 = 0.96 for $\Delta E_{ST}$. Active learning reduces the data needed to reach target accuracy by approximately 25% compared to random sampling. Three application domains are explored: NIR-emitting probes for bioimaging, photocatalytic sensitizers, and fast-response materials for photodetection.
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All authors Jean-Pierre Tchapet Njafa, Steve Cabrel Teguia Kouam, Patrick Mvoto Kongo, Serge Guy Nana Engo

Materials Science

Mapping vacancy and bonding electron distributions around aluminium nanovoids

Philip N. H. Nakashima, Yu-Tsun Shao, Zezhong Zhang et al. (+6 more)

All materials have defects and many contain nanostructures, both of which disrupt chemical bonding - the basis of materials properties. No experimental measurements of bonding electron distributions associated with defects and nanostructures have ever been possible. We present a method enabling such measurements and interrogate nanovoids surrounded by vacancies - the most fundamental of nanostructures and defects - in aluminium. We measure the volume of a vacancy with 3% uncertainty and map vacancy concentrations surrounding nanovoids with nanometre resolution in three dimensions where previously only two-dimensional mapping was possible. We discover that radiation-damaged voids can "heal". Our bonding measurements are depth-resolved, vacancy-sensitive, and agree with density functional theory. This work opens bonding electron density measurements to inhomogeneous nanostructured multi-phased materials so that the electronic origins of phenomena such as strengthening, weakening, interface functionality, solute diffusion and phase transformations within them may be revealed.
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All authors Philip N. H. Nakashima, Yu-Tsun Shao, Zezhong Zhang, Andrew E. Smith, Tianyu Liu, Nikhil V. Medhekar, Joanne Etheridge, Laure Bourgeois, Jian-Min Zuo

Materials Science

Benchmarking Universal Machine Learning Interatomic Potentials for Supported Nanoparticles: Decoupling Energy Accuracy from Structural Exploration

Jiayan Xu, Abhirup Patra, Amar Deep Pathak et al. (+3 more)

Supported nanoparticle catalysts are widely used in the chemical industry. Computational modeling of supported nanoparticles based on density functional theory (DFT) often involves structural searches of stable local minimum energy configurations and molecular dynamics simulations at finite temperature. These are computationally demanding tasks that are intractable within DFT for large systems. In the last two decades, machine learning interatomic potentials (MLIPs) have been successfully used to substantially increase the size and time scales accessible to simulations approximating DFT accuracy. However, training reliable MLIPs is non-trivial as it requires many costly DFT calculations. Recently, several universal MLIPs (uMLIPs) have been developed, which are trained on large datasets that cover a wide range of molecules and materials. Here, we benchmark the accuracy and the efficiency of these uMLIPs in describing Cu nanoparticles supported on Al$_2$O$_3$ surfaces against our domain-specific DP-UniAlCu model. We find that the MACE-OMAT can reproduce reasonably well the low-energy structures found in global optimization at an energy accuracy comparable to DP-UniAlCu. Interestingly, the MatterSim-v1.0.0-1M model, which exhibits larger deviations in the binding energies, can find even more stable configurations than the other two models in some supported nanoparticle sizes, showing its capability in structure exploration. For MD simulations, MACE-OMAT and MatterSim-v1.0.0-1M can qualitatively reproduce the mean-squared displacements of Cu atoms (MSD$_\mathrm{Cu}$) predicted by DP-UniAlCu, albeit at roughly two orders of magnitude higher cost. We demonstrate that the uMLIPs can be very useful in simulating supported nanoparticles even without any fine-tuning, though their reduced efficiency remains a limiting factor for large-scale simulations.
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All authors Jiayan Xu, Abhirup Patra, Amar Deep Pathak, Sharan Shetty, Detlef Hohl, Roberto Car

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

Fluorine-substitution-dependent phase diagram and superconducting properties of Sm-based oxypnictides synthesized by a high-pressure growth technique

Mohammad Azam, Tatiana Zajarniuk, Ryszard Diduszko et al. (+5 more)

A series of SmFeAsO1-xFx (Sm1111) bulk samples (x = 0.05 to 0.40) are synthesized by an in-situ cubic-anvil high-pressure technique at 4 GPa and systematically characterized through structural, microstructural, Raman, transport, and magnetic measurements. Structural analysis confirms that the tetragonal Sm1111 phase remains dominant across the entire substitution range, with lattice parameters decreasing smoothly as fluorine content increases, demonstrating effective incorporation of F even in the overdoped regime (x = 0.4). Raman spectroscopy provides complementary, local-phase-sensitive evidence that supports the structural analysis and confirms fluorine substitution in the main Sm1111 phase. In the underdoped region, the superconducting transition temperature (Tc) is enhanced by 10-17 K and the critical current density (Jc) is increased by up to an order of magnitude compared with conventionally synthesized samples at the ambient pressure. The upper critical field, estimated using the WHH model, reaches values approaching 200 T, reflecting strong paramagnetic limitation and multiband effects. TAFF analysis reveals a power-law field dependence of the activation energy, consistent with collective vortex pinning in polycrystalline iron-based superconductors. The superconducting phase diagram constructed from Tc and Jc versus fluorine content reveals a dome-like trend, with a maximum Tc of 57 K and Jc of 10^4 A cm^-2 at the optimal doped region. Comparison with conventionally synthesized Sm1111 samples demonstrates that high-pressure growth significantly extends the effective fluorine substitution range and enhances the superconducting performance, particularly in the underdoped regime. These results establish high-pressure fluorine substitution as an effective materials-engineering approach for tuning the phase diagram and optimizing the superconducting properties of Sm1111.
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All authors Mohammad Azam, Tatiana Zajarniuk, Ryszard Diduszko, Taras Palasyuk, Cezariusz Jastrzebski, Andrzej Szewczyk, Hiraku Ogino, Shiv J. Singh

Materials Science

Modeling Compressive Instability in Two-Dimensional Ti2COx MXenes

Hossein Darban

In practical applications, MXenes are often subjected to a variety of loads, including compression. While their mechanical response under different loading conditions, such as tensile loading, has been extensively studied, their compressive instability remains largely unexplored. The compressive and post-buckling behavior of Ti2C and Ti2CO2 MXene nanosheets is studied using molecular dynamics (MD) simulations and a nonlocal formulation. The employed interatomic potential is first validated against experimental and density functional theory (DFT) data for structural and mechanical properties. The results indicate that classical continuum mechanics underestimates the buckling strains, whereas the nonlocal formulation adequately captures the observed response. A systematic examination of various defect types up to a defect fraction of 3% reveals that while isolated point defects primarily reduce the critical buckling stress, vacancy clusters significantly alter the buckling mode shapes. Lateral confinement pressure and oxygen surface termination substantially increase the buckling stress. Atomistic analysis reveals opposite stress states in the top and bottom Ti layers due to curvature-induced strain gradients. Under biaxial compression, the nanosheet buckles in a dome-like shape, whereas shear loads produce elliptical deflection modes. The presented findings may stimulate future studies on MXene morphological transformations, such as the development of nanotube, nanoscroll, and folded architectures.
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Materials Science · Cross-listed from physics.optics

Anisotropic Response in Metamaterials with Elliptically Perforated Plates: Applications to Near-Field Radiative Heat Transfer

J. E. P'erez-Rodr'iguez, R. Esquivel-Sirvent and A. Camacho de la Rosa

Metamaterials with tunable optical properties provide a versatile platform for controlling electromagnetic interactions at the nanoscale. This study explores the anisotropic thermal behavior of metamaterials composed of planar plates perforated with periodic arrays of cylinders possessing elliptical cross sections. In contrast to conventional circular perforations, elliptical geometries inherently break rotational symmetry, introducing anisotropy in the effective electromagnetic and thermal response of the structure. Using a fluctuation electrodynamics framework combined with full-wave numerical simulations, we quantify the near-field radiative heat transfer between such elliptically perforated plates as a function of ellipse orientation, aspect ratio, and separation distance. The results reveal that elliptical perforations enable enhanced spectral and directional control of evanescent mode coupling and surface polariton excitation, leading to significant modulation of the near-field heat flux. These findings highlight the potential of geometrically engineered anisotropy for advanced thermal management and energy conversion applications, and offer new design strategies for the development of thermally functional metamaterials operating in the near-field regime.
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Materials Science

Prediction of Novel Li-AgII-F Compounds using Evolutionary Algorithms

Katarzyna Kuder and Wojciech Grochala

This work provides a theoretical exploration of the thermodynamic stability and magnetic behaviour of previously unknown ternary Li AgII F compounds. Convex-hull analysis shows that all predicted structures lie slightly above the LiF plus AgF2 decomposition line, indicating a natural tendency toward phase separation; nevertheless, their negative formation energies relative to AgF, LiF, and F2 or F suggest that alternative synthetic pathways may be feasible for these compounds. All studied structures show preference for antiferromagnetic ground state. Notably, the triclinic LiAgF3 type2 is predicted to exhibit an exceptionally large superexchange constant, J equal to minus 358 meV, within Ag2F7 dimers, placing it above the strongest known magnetic exchange interactions reported to date.
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Materials Science · Cross-listed from math.MG

Geometric Data Science

Olga D Anosova and Vitaliy A Kurlin

This book introduces the new research area of Geometric Data Science, where data can represent any real objects through geometric measurements. The first part of the book focuses on finite point sets. The most important result is a complete and continuous classification of all finite clouds of unordered points under rigid motion in any Euclidean space. The key challenge was to avoid the exponential complexity arising from permutations of the given unordered points. For a fixed dimension of the ambient Euclidean space, the times of all algorithms for the resulting invariants and distance metrics depend polynomially on the number of points. The second part of the book advances a similar classification in the much more difficult case of periodic point sets, which model all periodic crystals at the atomic scale. The most significant result is the hierarchy of invariants from the ultra-fast to complete ones. The key challenge was to resolve the discontinuity of crystal representations that break down under almost any noise. Experimental validation on all major materials databases confirmed the Crystal Isometry Principle: any real periodic crystal has a unique location in a common moduli space of all periodic structures under rigid motion. The resulting moduli space contains all known and not yet discovered periodic crystals and hence continuously extends Mendeleev's table to the full crystal universe.
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Materials Science

A meta-GGA perspective on the altermagnetism of RuO2

Markus Meinert

The metallic oxide RuO$_2$ hosts a fascinating edge case of magnetism: while nonmagnetic in ideal bulk material, density functional theory (DFT) predicts an altermagnetic ground state within the DFT$+U$ method. The magnetic state of strained or doped thin films remains controversial, but evidence for a nontrivial magnetic state is ample. Here, I study the altermagnetic ground state of RuO$_2$ on a higher rung of Jacob's ladder of density functional approximations, the meta-GGA level including the kinetic energy density and the density Laplacian. While the workhorse functional of solid-state physics is a generalized gradient approximation (GGA), the modern r$^2$SCAN-L functional has been established as a general-purpose functional which can replace GGA, while systematically improving solid-state properties without introducing spurious errors like erroneous magnetic ground states. Comparison of LSDA+U, GGA+U, and meta-GGA+U results on RuO$_2$ shows systematic enhancement of the exchange interaction, leading to a reduction of the onset value of the Hubbard $U$ parameter at different levels of density functional approximation. However, the magnetic ground state, studied at the experimental lattice constants, remains nonmagnetic with r$^2$SCAN-L. I demonstrate that altermagnetism is easily formed upon lattice expansion, hole doping, and uniaxial strain on the c-axis. The r$^2$SCAN-L calculations set conservative thresholds for distortions and doping levels for the onset of altermagnetism in a parameter-free framework.
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Materials Science

Tuning the Electronic States of Bi2Se3 Films with Large Spin-Orbit Interaction Using Molecular Heterojunctions

Matthew Rogers, Craig Knox, Bryan Hickey et al. (+13 more)

An electric bias can shift the Fermi level along the Dirac cone of a topological insulator and modify its charge transport, but tuning the electronic states and spin-orbit interaction (SOI) without destroying the surface topology is challenging. Here, we show that thin film Bi2Se3/n-p (p-n) molecular diodes form ordered interfaces where charge transfer and orbital re-hybridisation result in a decrease (increase) of the carrier density and improved mobility. In Bi2Se3 the spin-orbit lifetime, t_so, is 0.13 ps, which is comparable to the strongest spin-orbit materials. This lifetime drops further to 0.06 ps (0.09 ps) with the addition of p-n (n-p) molecular diodes, at the limit of measurable values. This strengthened spin-orbit interaction occurs even though molecules are made of light elements and increase the mean free path of the charge carriers by almost 50%, indicating changes to the Berry curvature and/or Rashba splitting around the hybridisation points. Raman spectroscopy gives evidence that the coupling effect may be controlled by optical irradiation, opening a pathway towards the design of heavy-light element hybrids with optically tunable quantum transport.
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All authors Matthew Rogers, Craig Knox, Bryan Hickey, Lida Ansari, Farzan Gity, Timothy Moorsom, Mairi McCauley, Gilberto Teobaldi, Manuel dos Santos Dias, Hari B. Vasili, Manuel Valvidares, Mannan Ali, Gavin Burnell, Ahmet Yagmur, Satoshi Sasaki, Oscar Cespedes

Materials Science

In search of the electron-phonon contribution to total energy

Samuel Poncé and Xavier Gonze

The total energy is a fundamental characteristic of solids, molecules, and nanostructures. In most first-principles calculations of the total energy, the nuclear kinetic operator is decoupled from the many-body electronic Hamiltonian and nuclear potential, and the dynamics of the nuclei is reintroduced afterward. This two-step procedure introduced by Born and Oppenheimer (BO) is approximate. Energies beyond the electronic and vibrational (or phononic) main contributions might be relevant when small energy differences are important, such as when predicting stable polymorphs or describing magnetic energy landscape. We clarify the different flavors of BO decoupling and give an exact formulation for the total energy in the basis of BO electronic wavefunctions. Then, we list contributions, beyond the main ones, that appear in a perturbative expansion in powers of $M_0^{-1/4}$, where $M_0$ is a typical nuclear mass, up to sixth order. Some of these might be grouped and denoted the electron-phonon contribution to total energy, $E^{\textrm{elph}}$, that first appears at fourth order. The electronic inertial mass contributes at sixth order. We clarify that the sum of the Allen-Heine-Cardona zero-point renormalization of eigenvalues over occupied states is not the electron-phonon contribution to the total energy but a part of the phononic contribution. The computation of the lowest-order $E^{\textrm{elph}}$ is implemented and shown to be small but non-negligible (3.8~meV per atom) in the case of diamond and its hexagonal polymorph. We also estimate the electronic inertial mass contribution and the quasi-harmonic one. We confirm the size consistency of all computed terms.
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Materials Science

Controlling Carbon Nanostructure Synthesis in Thermal Plasma Jet: Correlation of Process Parameters, Plasma Characteristics, and Product Morphology

Taki Aissou, Jerome Menneveux, Fanny Casteignau et al. (+2 more)

Thermal plasma has emerged as an effective approach for producing carbon nanostructures without the need for specific catalysts nor substrates. While efforts have focused on the effect of process parameters such as reaction pressure, input power or carbon source, the intricate role and relationship with plasma characteristics like density and temperature are often overlooked due to the complexity of the environment. This study addresses this gap by establishing a correlation between process parameters, plasma characteristics, and product morphology, essential for controlling the growth of carbon nanostructures. We explored the impact of carbon precursor type (CH4 and C2H2), hydrogen, pressure, and flow rate on nanostructure formation. Using in situ optical emission spectroscopy (OES), we mapped the distribution of both temperature and dicarbon molecule (C2) density within the plasma jet. We demonstrate that the growth of low-density nanostructures, such as carbon nanohorns (CNHs), is favoured at dilute C2 local densities and high temperatures, while denser nanostructures, such as onion-like polyhedral graphitic nanocapsules (GNCs), are favoured at higher C2 densities and lower temperatures. The carbon density can be controlled by the flow rate and the pressure, which in turn significantly influence the nanostructure morphology, evolving from graphene nanoflakes (GNFs) to GNCs as either parameter increases. Increasing the H/C ratio from 1 to 8 resulted in a morphological transition from CNHs to GNFs. During the synthesis, the plasma jet temperature surpassed 3,000 K, with crystalline growth occurring 50 to 100 mm below the nozzle.
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All authors Taki Aissou, Jerome Menneveux, Fanny Casteignau, Nadi Braidy, Jocelyn Veilleux

Materials Science

Valley Splittings in Si/SiGe Heterostructures from First Principles

Lukas Cvitkovich, Tancredi Salamone, Christoph Wilhelmer et al. (+3 more)

We compute valley splittings in Si/SiGe superlattices using ab initio density functional theory (DFT). This first-principle approach is expected to provide an excellent description of interfaces, strains, and atomistic disorder without empirically fitted parameters. We benchmark atomistic tight-binding (TB) and the “$2k_0$” theory within the effective mass (EM) approximation against DFT. We show that DFT supports the main conclusions of the 2$k_0$ theory, but reveals some limitations of semi-empirical methods such as the EM and TB, in particular about the description of atomistic disorder. The DFT calculations also highlight the effects of strong valley-orbit mixing at large valley splittings. Nevertheless, TB and the 2$k_0$ theory shall provide reasonable valley splitting statistics in many heterostructures of interest for spin qubit devices.
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All authors Lukas Cvitkovich, Tancredi Salamone, Christoph Wilhelmer, Biel Martinez, Tibor Grasser, Yann-Michel Niquet

Materials Science

Isostructural phase transition and equation of state of type-I and type-VIII metallic sodium borosilicide clathrates

M. Demoucron, S. Pandolfi, Y. Guarnelli et al. (+10 more)

Electronic properties of silicon-based clathrates can be tuned by boron incorporation into the silicon cage network. Sodium borosilicides clathrate outstands with uncommon stoichiometry and expected metallic properties, in contrast to other alkali metal semiconductive Zintl borosilicides. In this study, we report an experimental investigation of the high-pressure behavior of type-I and type-VIII sodium borosilicide clathrates. An isostructural phase transition, marked by an abrupt volume collapse at 13 GPa, is observed exclusively in type-I sodium borosilicide clathrates. This transition is attributed to the pressure-induced diffusion of silicon atoms from the Si(6c) site. This mechanism provides the first experimental validation of a transition predicted theoretically for this class of materials. Isostructural phase transitions were only observed in type-I borosilicide. In contrast, the type-VIII borosilicide phase exhibits conventional elastic compression. The metallic character was established using reflectance spectroscopy over a wide energy range, in good agreement with crystallographic data on the boron content.
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All authors M. Demoucron, S. Pandolfi, Y. Guarnelli, B. Baptiste, P. Chauvigne, N. Guignot, D. Portehault, T.A. Strobel, M. Bykov, T.B. Shiell, W.A. Crichton, Y. Le Godec, A. Courac

Materials Science

Interband and kinetic corrections to the electronic Boltzmann transport equation

Elena Trukhan and Nakib H. Protik

Interband effects such as coherence/tunneling have recently been shown to give an important contribution to the charge and heat transport properties under certain conditions. These can be captured by adding corrective terms to the semiclassical Boltzmann transport equation. In recent derivations of this type of transport equations that are based on the density matrix formalism, there remain, however, certain omissions. These derivations also rely on a particular type of relaxation time approximation and a band-diagonal form of the interaction self-energies. In this work we derive the interband terms of the electronic Boltzmann transport equation starting from the Keldysh formulation of the quantum kinetic equation and considering the band non-diagonality of the electron-impurity and electron-phonon self-energies. We introduce a minimally modified Kadanoff-Baym Ansatz, and find a quantum-corrected, matrix Boltzmann transport equation that is well beyond the current state of the art theory. We show that the occupations and coherences are interdependent, and that the kinetic corrections due to the included interactions cannot, in general, be ignored. This work clarifies the various approximations that must be introduced in an ab initio derivation of Boltzmann-like equations and finds a new matrix Boltzmann transport equation that is suitable for parameters-free numerical implementations.
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Materials Science

Interfacial Synergy in Ag-Doped CuO-AgCl-g-C3N4 Composites for Efficient Charge Separation and Low-power Methylene Blue Degradation

Suresh Chandra Baral, Uttama Kumar Saint, Dilip Sasmal et al. (+5 more)

An Ag-doped CuO-AgCl-g-C3N4 heterostructure has been designed to achieve rapid Methylene Blue (MB) degradation through a synergistic photo-Fenton mechanism driven by low-power UV illumination. The composite integrates narrow-bandgap CuO, plasmonic Ag/AgCl, and visible-responsive g-C3N4 into a dual Z-scheme configuration that promotes efficient interfacial charge transfer while preserving strong redox potentials. Diffuse reflectance UV-Vis spectra ascertained the bandgap positions of the composite corresponding to those of its constituents: 2.9 eV (g-C3N4) and 1.42 eV (Ag-doped CuO-AgCl), indicating enhanced absorption and efficient charge carrier generation. BET analysis confirmed the presence of mesoporosity and revealed an effective surface area, ensuring the availability of abundant adsorption and reaction sites. A commercial 11 W UV irradiation was used for the photocatalytic test. Almost complete degradation of MB occurred within 10 min, following pseudo-first-order kinetics with a high apparent rate constant of 0.45/min. The remarkable activity arises from the synergistic interplay of Fenton-like redox cycling and efficient photoinduced charge carrier generation and separation. In addition, it has been demonstrated that intentionally incorporated AgCl plays an active role as a plasmonic-semiconducting interface, strengthening charge separation and catalyst stability under neutral conditions, rather than acting as a passive chloride byproduct. Overall, by linking defect engineering, heterojunction design, and photo-Fenton synergy, this study establishes a low-power, catalytic platform offering a viable pathway towards sustainable dye wastewater remediation.
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All authors Suresh Chandra Baral, Uttama Kumar Saint, Dilip Sasmal, Sradhanjali Lenka, Ashish Kalkal, A. Mekki, Sudhagar Pitchaimuthu, Somaditya Sen

Materials Science · Cross-listed from physics.chem-ph

Teaching a Transformer to Think Like a Chemist: Predicting Nanocluster Stability

João Marcos T. Palheta, Octavio Rodrigues Filho, Mohammad Soleymanibrojeni et al. (+6 more)

Atomically precise metal nanoclusters bridge the molecular and bulk regimes, but designing bimetallic motifs with targeted stability and reactivity remains challenging. Here we combine density functional theory (DFT) and physics-grounded predictive artificial intelligence to map the configurational landscape of 13-atom icosahedral nanoclusters X$_{12}$TM, with hosts X = (Ti, Zr, Hf), and Fe and a single transition–metal dopant spanning the 3$d$-5$d$ series. Spin-polarized DFT calculations on 240 bimetallic clusters reveal systematic trends in binding and formation energies, distortion penalties, effective coordination number, d-band centre, and HOMO-LUMO gap that govern the competition between core-shell (in) and surface-segregated (out) arrangements. We then pretrain a transformer architecture on a curated set of 2968 unary clusters from the Quantum Cluster Database and fine-tune it on bimetallic data to predict formation energies and in/out preference, achieving mean absolute errors of about $0.6-0.7$eV and calibrated uncertainty intervals. The resulting model rapidly adapts to an unseen Fe-host domain with only a handful of labelled examples. At the same time, attention patterns and Shapley attributions highlight size mismatch, $d$-electron count, and coordination environment as key descriptors. All data, code, and workflows follow FAIR/TRUE principles, enabling reproducible, interpretable screening of unexplored nanocluster chemistries for catalysis and energy conversion.
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All authors João Marcos T. Palheta, Octavio Rodrigues Filho, Mohammad Soleymanibrojeni, Alexandre Cavalheiro Dias, Diego Guedes-Sobrinho, Wolfgang Wenzel, Roland Aydin, Celso R. C. Rêgo, Maurício Jeomar Piotrowski

Materials Science

Bulk photovoltaic effect in MoSe$_2$ and Janus MoSSe sliding ferroelectrics

Roumita Roy, Giuseppe Cuono and Silvia Picozzi

We present a first-principles study of the nonlinear optical properties of sliding ferroelectric bilayers based on MoSe$_2$ and Janus MoSSe. Two Janus configurations are considered: i) one bilayer where the two intralayer polarizations caused by Janus chemical asymmetry cancel each other out, yielding photocurrent spectra comparable to pristine MoSe$_2$ bilayers; ii) another bilayer where the intralayer polarizations add up, for which the photoresponses are strongly enhanced. Our results show that photocurrent generation in the polar Janus structures is predominantly governed by vertical chemical asymmetry, with limited dependence on the sliding direction. These findings highlight complementary design strategies: interlayer sliding enables sensitivity to external tuning, while the Janus intralayer polarization enhances photoresponses in the visible range. The interplay between composition and stacking therefore provides a versatile platform for tailoring light-matter interactions in 2D ferroelectric materials.
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Materials Science

Heat transport in superionic materials via machine-learned molecular dynamics

Wenjiang Zhou, Benrui Tang, Zheyong Fan et al. (+3 more)

Precise modeling and understanding of heat transport in the superionic phase are of great interest. Although simulations combining Green-Kubo (GK) molecular dynamics with machine-learned potentials (MLPs) stand as a promising approach, substantial challenges remain due to the crucial impact of atomic diffusion. Here, we first show that the thermal conductivity (${\kappa}$) of superionic materials calculated via conventional GK integral of the energy flux varies notably with the MLP model. Subsequently, we highlight that reliable, model-independent $\kappa$ values can be obtained by applying Onsager's reciprocal relations to correctly capture the coupled heat and mass transport. Remarkably, an anomalously invariant $\kappa$ can be observed over a wide temperature range, distinct from the characteristic trends in traditional crystals and glasses. In addition, we illustrate that conventional $\kappa$ decompositions into kinetic, potential, and cross terms suffer from ambiguities in the physical interpretation, despite their mathematical rigor. Finally, we propose a criterion for the necessity of the Onsager correction and reveal the underlying mechanism as a competition between thermally and chemically driven ion fluxes.
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All authors Wenjiang Zhou, Benrui Tang, Zheyong Fan, Federico Grasselli, Stefano Baroni, Bai Song

Materials Science

Accelerating discovery of infrared nonlinear optical materials with large shift current via high-throughput screening

Aiqin Yang, Dian Jin, Mingkang Liu et al. (+4 more)

Discovering nonlinear optical (NLO) materials with strong shift current response, particularly in the infrared (IR) regime, is essential for next-generation optoelectronics yet remains highly challenging in both experiments and theory, which still largely relies on case by case studies. Here, we employ a high-throughput screening strategy, applying a multi-step filter to the Materials Project database (>154,000 materials), which yielded 2,519 candidate materials for detailed first-principle evaluation. From these calculations, we identify 32 NLO materials with strong shift current response ($\sigma$ > 100 $\mu A/V^2$). Our work reveals that layered structures with $C_{3v}$ symmetry and heavy $p$-block elements (e.g. Te, Sb) exhibit apparent superiority in enhancing shift current. More importantly, 9 of these compounds show shift current response peaks in the IR region, with the strongest reaching 616 $\mu A/V^2$, holding significant application potential in fields such as IR photodetection, sensing, and energy harvesting. Beyond identifying promising candidates, this work establishes a comprehensive and high-quality first-principles dataset for NLO response, providing a solid foundation for future AI-driven screening and accelerated discovery of high-performance NLO materials, as demonstrated by a prototype machine-learning application.
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All authors Aiqin Yang, Dian Jin, Mingkang Liu, Daye Zheng, Qi Wang, Qiangqiang Gu, Jian-Hua Jiang