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Physics Condensed Matter Materials Science

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Monday 23 February

Materials Science

Twist-induced altermagnetism in a metallic van der Waals antiferromagnet

Alberto M. Ruiz, Andrei Shumilin, Rafael González-Hernández et al. (+1 more)

Altermagnetism -a magnetic state characterized by spin-polarized electronic bands at zero net magnetization- offers a promising route for next-generation spintronic devices. In two-dimensional (2D) magnets, twist engineering enables its realization by breaking the combined inversion and time-reversal symmetry (PT) while preserving crystal symmetries that ensure the altermagnetic order. Here, by means of first-principles calculations and symmetry analysis, we demonstrate that twist engineering applied to the recently synthesized metallic van der Waals antiferromagnet Co-doped bilayer Fe$_3$GaTe$_2$ (Fe$_2$CoGaTe$_2$) provides a robust platform for altermagnetism. By twisting two layers of Fe$_2$CoGaTe$_2$, the PT symmetry between opposite spin sublattices is broken, resulting in a non-relativistic $i$-wave altermagnetic state with spin splitting up to 138 meV. In the absence of spin-orbit coupling (SOC), the electronic states are spin-degenerate along six high-symmetry directions, while the inclusion of SOC preserves this degeneracy along the three directions protected by twofold rotation axes. Furthermore, we unveil the microscopic mechanisms governing the magnetic behavior in twisted Fe$_2$CoGaTe$_2$. Our results establish twist engineering and metallic Fe-based van der Waals antiferromagnets as versatile platforms to realize 2D van der Waals altermagnetism, with potential for designing high-efficiency ultrathin nanodevices.
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All authors Alberto M. Ruiz, Andrei Shumilin, Rafael González-Hernández, José J. Baldoví

Materials Science

Strain- and Field-Tunable Nonrelativistic Spin Splitting and Wave-Symmetry-Dependent Spin Transport in Twisted Bilayer Altermagnets

Shantanu Pathak and Saswata Bhattacharya

Magnetism-driven nonrelativistic spin splitting (NRSS) provides a pathway toward efficient, spin-orbit-free spintronics. In centrosymmetric two-dimensional antiferromagnets, spin-polarized transport is symmetry-forbidden due to the combined space-time inversion ($PT$) symmetry. Here, by employing first-principles density functional theory and spin-group symmetry analysis, we demonstrate that twisting two antiferromagnetic or ferromagnetic monolayers of CoCl$_2$, AX$_2$ (A = Mn, V; X = Cl, Br, I), NiF$_2$, NiBr$_2$, FeS, CoS, MnTe$_2$, MnSe$_2$, and RuSe induces finite NRSS even in the absence of spin-orbit coupling. The relative twist breaks $[C_2||P]$ and $[E||C_{nz}]$ symmetries, giving rise to momentum-dependent spin polarization with distinct $d$-, $g$-, and $i$-wave altermagnetic patterns across the Brillouin zone. Using symmetry-invariant $k\cdot p$ modeling, we extract linear spin-splitting coefficients $\alpha^{(1)}$ ranging from 800-1100 meVÅ, comparable to SOC-induced Rashba-Dresselhaus strengths observed in noncentrosymmetric semiconductors. An out-of-plane electric field ($\mathcal{E}_z$) introduces Zeeman-type band splitting up to 110 meV at 10 MV/cm, while biaxial strain tunes the NRSS magnitude nearly linearly without altering symmetry. Crucially, the strain $u_{xx-yy}$ reduces the spin point group symmetry and drives reversible $g/i \rightarrow d$ wave-type transitions, resulting in finite spin conductivity and an enhanced spin-splitter angle (up to 18$^\circ$). These results extend the concept of altermagnetism to twisted bilayer geometries and establish a general route for realizing exchange-driven, nonrelativistic spin currents through symmetry engineering without requiring heavy elements or spin-orbit coupling.
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Materials Science · Cross-listed from cond-mat.mes-hall

Optical properties of single CsPbBr3 perovskite quantum dots synthesized by a modified ligand-assisted reprecipitation method

Marina Cagnon Trouche, Ernest Ruby, Margaux Cartier et al. (+5 more)

Colloidal perovskite quantum dots (pQDs) are promising quantum light emitters, and investigations at the single pQD scale have so far relied mostly on hot-injection synthesis, which requires precise temperature control and an inert atmosphere. While alternative synthesis routes under milder conditions are often associated with structural and surface defects that may have limited impact in ensemble measurements, demonstrating high optical quality at the level of individual pQDs constitutes the most stringent benchmark for a new synthesis protocol. Here, we demonstrate that a modified ligand-assisted reprecipitation (LARP) approach yields CsPbBr3 pQDs showing state-of-the-art optical properties at the scale of single emitters. By combining an amine-mediated post-synthetic size-trimming strategy with didodecyldimethylammonium bromide (DDAB) ligands for enhanced surface passivation and colloidal stability, we obtain isolated pQDs with stable emission and minimal spectral diffusion at cryogenic temperatures. Micro-photoluminescence experiments resolve the characteristic fine structure of the bright exciton, its low-energy optical phonon replicas, and the trion and biexciton states. Time-resolved and photon correlation measurements show a ~90 ps lifetime and high purity single photon emission, respectively. These results demonstrate that modified LARP synthesis constitutes an accessible alternative to hot injection, preserving the intrinsic excitonic and quantum optical properties of individual pQDs while offering greater flexibility for post-synthetic ligand engineering, as exemplified here by the use of DDAB for surface passivation.
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All authors Marina Cagnon Trouche, Ernest Ruby, Margaux Cartier, Christophe Voisin, Maxime Vallet, Yannick Chassagneux, Cédric R. Mayer, Carole Diederichs

Materials Science

Corrosion Evolution of T91 Steel in Static Lead-Bismuth Eutectic Under an Oxidising Environment

Minyi Zhang, Weiyue Zhou, Michael P. Short et al. (+3 more)

Understanding corrosion in liquid metal-cooled nuclear systems is essential in order be able to control it. While much literature exists detailing corrosion rates and mechanisms of structural materials in liquid metals, much still remains to be discovered in new regimes of temperature, chemistry, and impurity content. We focus on a less-studied set of conditions, specifically to investigate how liquid lead-bismuth eutectic (LBE) corrodes ferritic/martensitic steels under high-temperature oxidizing conditions. We find that the evolution of corrosion is determined by the formation of protection layer on the surface. The area without effective protection layer experiences oxidation along martensite grain boundaries, transitioning from intergranular attack to broader area corrosion as it progresses. The area a stable, coherent oxide scale will slow the corrosion process and then is oxidized along pre-austenite grain boundaries. Both chromium and oxygen diffusion play vital roles in this process. Most surprisingly, a layer of iron enriched body-centred cubic phase forms on the surface of LBE-corroded T91, contradicting previous studies, which reported only oxide-based surface layers.
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All authors Minyi Zhang, Weiyue Zhou, Michael P. Short, Paul A.J. Bagot, Michael P. Moody, Felix Hofmann

Materials Science

Impact of strain on electron-phonon coupling of quantum emitters

Vytautas Žalandauskas, Rokas Silkinis, Lukas Razinkovas et al. (+6 more)

Defects in semiconductors acting as optically active spin qubits are intriguing objects of fundamental study and future technological developments. These defect-based color centers are of particular interest for detection and response to physical variations such as pressure and strain, or conversely – as we demonstrate the possibility of herein – pressure and strain can be utilized to manipulate quantum emitter properties. To investigate how strain can alter the fundamental electron-phonon interaction of quantum defects, we employ the negatively charged silicon vacancy ($\mathrm{V_{Si}^{-}}$) in 4H-SiC as a use-case and study its vibrational structure under applied tensile and compressive uniaxial strain using first-principles calculations. We show that the strain variations of the emission spectrum can be explained by differing responses of bulk-like and quasi-localized vibrational modes. Importantly, the $\mathrm{V_{Si}^{-}}$ defect exhibits a strain-induced enhancement of the Debye-Waller factor under uniaxial tensile strain applied along the $a$-axis of 4H-SiC, thereby improving its performance as a quantum emitter. The strain-dependent changes in the phonon sideband enable distinguishing between compressive and tensile strain, opening up the possibility of magnetic-field-free strain detection using only spin-conserving transitions of solid-state quantum emitters.
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All authors Vytautas Žalandauskas, Rokas Silkinis, Lukas Razinkovas, Ali Tayefeh Younesi, Minh Tuan Luu, Ronald Ulbricht, Ulrike Grossner, Lasse Vines, Marianne Etzelmüller Bathen

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

Data-Driven Bath Fitting for Hamiltonian-Diagonalization Dynamical Mean-Field Theory

Taeung Kim, Jeongmoo Lee and Ara Go

We propose a machine-learning-based initialization method to overcome the nonlinear bath-fitting bottleneck in Hamiltonian-diagonalization-based dynamical mean-field theory (HD-DMFT). In HD-DMFT, the continuous hybridization function is approximated by a finite set of bath-site energies and hybridization amplitudes, determined by minimizing a highly non-convex multivariable cost function. As the number of bath sites increases, the optimization becomes more sensitive to the initial guess and more prone to suboptimal local minima, which can slow or destabilize the DMFT self-consistency loop. We reformulate bath fitting as a supervised regression problem and train a kernel ridge regression model to predict near-optimal discrete bath parameters directly from the target hybridization function on the Matsubara axis. To ensure physical relevance and data diversity, we construct the training dataset from tight-binding Hamiltonians of layered-perovskite-like ruthenate models across systematically deformed structures, instead of relying on naive random parameter sampling, and obtain high-quality labels through fully converged conventional bath fitting. Time-reversal symmetry is explicitly incorporated in both feature and target representations to reduce effective dimensionality and enforce physical consistency. Benchmarks in the non-interacting limit show that the learned initialization systematically reduces the initial fitting error, decreases the number of conjugate-gradient iterations, and improves robustness against local minima over a wide range of bath sizes. We further demonstrate transferability to interacting DMFT calculations for $\mathrm{Sr_{2}RuO_{4}}$ solved with an adaptive-truncation impurity solver, where the ML initialization yields consistently faster convergence than a symmetry-preserving heuristic baseline while preserving the final fitted solution.
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Materials Science · Cross-listed from physics.optics

Profiling THz Beams With Off-Label Use of Infrared Microbolometric Cameras

Gabriel Nagamine, Carlo Vicario, Tariq Leinen et al. (+6 more)

Visualizing the spatial profile of light beams is essential for evaluating irradiance, characterizing beam quality, and achieving precise alignment. In the optical spectral range, this is readily performed using silicon-based CCD and CMOS cameras. In the terahertz (THz) range, however, it typically requires specialized detectors with prohibitive costs. Here, we show that an infrared (IR) camera can be used outside of its labeled specifications to achieve similar performance as a dedicated microbolometric THz camera, at under 1% of the THz camera's cost. We compared the cameras by characterizing THz beam profiles from two sources: a pulsed broadband THz beam produced through optical rectification in organic crystals, and a narrowband quasi-continuous-wave (quasi-CW) THz beam emitted by a quantum cascade laser. For the broadband THz radiation, the beam width measured by the two cameras differed by only ~ 6%, well within the pixel resolution limit, and in the narrowband quasi-CW case by just ~ 1.3%. Additionally, the IR camera exhibits a lower minimum detectable power (down to 1.5 THz) than the THz camera, while also maintaining a linear and polarization-independent responsivity. These results expand the applicability of conventional IR cameras to the THz range, suggesting that they will become routine tools for high-fidelity THz beam diagnostics and imaging in scientific and industrial applications.
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All authors Gabriel Nagamine, Carlo Vicario, Tariq Leinen, Guy Matmon, Marco Raffa, Mattias Beck, Giacomo Scalari, Adrian L. Cavalieri, Flavio Giorgianni

Materials Science

Coherent Phonon-Driven Band Renormalizations in 1T$'$-MoTe$_2$

Carl E. Jensen, Christoph Emeis, Stephan Jauernik et al. (+3 more)

Here, we investigate phonon mode- and electron band-selective electron-phonon couplings in centrosymmetric 1T$'$-MoTe$_2$ using time- and angle-resolved photoemission spectroscopy combined with frequency-domain analysis. Femtosecond near-infrared pulses excite coherent $A_g$-symmetric phonon modes at 2.34 THz, 3.34 THz, and 3.86 THz, which manifest as oscillatory modulations in photoemission intensity and binding energy across the valence bands. Pixel-wise Fourier analysis using recently developed methodologies reveals pronounced band selectivity with distinct coupling strengths for different electronic states and phonon modes, enabling the evaluation of band-renormalization amplitudes in the range of few meV. Ab initio calculations qualitatively reproduce the experimentally observed coupling patterns and relative trends, demonstrating the capability of combined experimental and theoretical approaches to resolve ultrafast electron-phonon interactions in quantum materials.
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All authors Carl E. Jensen, Christoph Emeis, Stephan Jauernik, Petra Hein, Fabio Caruso, Michael Bauer

Materials Science

Rotational anisotropy Raman spectrometer for high-sensitivity crystallographic symmetry analysis

Di Cheng, Junxiang Li, Shizhuo Luo et al. (+2 more)

Raman spectroscopy stands as a cornerstone technique for probing collective excitations and emergent quantum phases in solids. While polarization-resolved Raman scattering has been widely used to extract symmetry information of eigenmodes, its conventional geometry suffers from significant limitations: it accesses only a subset of Raman tensor elements, enforces {\pi}-periodic intensity patterns that obscure intrinsic crystalline symmetries, and lacks sensitivity to wavevector-dependent anisotropy. To overcome these constraints, here we introduce rotational-anisotropy Raman spectroscopy (RA-Raman). By measuring scattering intensity during full azimuthal rotation of the optical scattering plane at oblique incidence, this geometry enables complete reconstruction of the Raman tensor and reveals rich rotational anisotropy patterns essential for accessing subtle symmetry information elusive to conventional methods. We developed a prototype instrument to validate this approach experimentally. For centrosymmetric crystals, RA-Raman unambiguously identifies phonon symmetry representations and determines crystallographic axes. In noncentrosymmetric crystals, it resolves directional anisotropy and angular dispersion of phonon-polaritons, enabling quantitative determination of the Faust-Henry coefficient and the separation of deformation-potential and electro-optic scattering contributions. By demonstrating unprecedented symmetry-resolving power in standard benchmark crystals, we establish RA-Raman as a powerful tool with far-reaching potential to discover and characterize symmetry-breaking phases and topological excitations in quantum materials.
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All authors Di Cheng, Junxiang Li, Shizhuo Luo, Zehao Chen, Xinwei Li

Materials Science

Improving Reliability of Machine Learned Interatomic Potentials With Physics-Informed Pretraining

Qianyu Zheng and Victor Fung

Machine learned interatomic potentials (MLIPs) have emerged as powerful tools for molecular dynamics (MD) simulations with their competitive accuracy and computational efficiency. However, MLIPs are often observed to exhibit un-physical behavior when encountering configurations which deviate significantly from their training data distribution, leading to simulation instabilities and unreliable dynamics, thus limiting the reliability of MLIPs for materials simulations. We present a physics-informed pretraining strategy that leverages simple physical potentials which can improve the robustness and stability of graph-based MLIPs for MD simulations. We demonstrate this approach by deploying a pretraining-finetuning pipeline where MLIPs are initially pretrained on data labelled with embedded atom model potentials and subsequently finetuned on the quantum mechanical ground truth data. By evaluating across three diverse material systems (phosphorus, silica, and a subset of Materials Project) and three representative MLIP architectures (CGCNN, M3GNet, and TorchMD-NET), we find that this physics-informed pretraining consistently improves both prediction accuracy as well as stability in MD compared to the baselines.
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Sunday 22 February

Materials Science

Phase field as a front propagation method for modeling grain growth in additive manufacturing

Murali Uddagiri, Pankaj Antala and Ingo Steinbach

A mesoscopic grain-envelope model applying a phase-field front-propagation method is developed to simulate grain growth under additive manufacturing process conditions. The envelope represents the outer surface of dendritic grains through a diffuse interface. While a modified heat-conduction model that incorporates moving heat sources and latent-heat release provides the evolution of local thermal field. Envelope propagation is determined from microscopic-solvability-based kinetic law. The model is validated through two- and three-dimensional simulations and subsequently applied to examine the influence of material and process parameters on microstructure evolution. The results demonstrate that the proposed mesoscopic model offers an efficient and predictive approach for modeling grain growth during multi-pass and multi-layer build-up in additive manufacturing.
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Materials Science

OrgFlow: Generative Modeling of Organic Crystal Structures from Molecular Graphs

Mohammadmahdi Vahediahmar, Matthew A. McDonald and Feng Liu

Crystal structure prediction is a long-standing challenge in materials science, with most data-driven methods developed for inorganic systems. This leaves an important gap for organic crystals, which are central to pharmaceuticals, polymers, and functional materials, but present unique challenges, such as larger unit cells and strict chemical connectivity. We introduce a flow-matching model for predicting organic crystal structures directly from molecular graphs. The architecture integrates molecular connectivity with periodic boundary conditions while preserving the symmetries of crystalline systems. A bond-aware loss guides the model toward realistic local chemistry by enforcing distributions of bond lengths and connectivity. To support reliable and efficient training, we built a curated dataset of organic crystals, along with a preprocessing pipeline that precomputes bonds and edges, substantially reducing computational overhead during both training and inference. Experiments show that our method achieves a Match Rate more than 10 times higher than existing baselines while requiring fewer sampling steps for inference. These results establish generative modeling as a practical and scalable framework for organic crystal structure prediction.
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Materials Science

Dual energy-differentiated topological transition in artificial red phosphorus chains

Vít Jakubský, B. Manjarrez-Montañez, Rafael A. Méndez-Sánchez et al. (+1 more)

We investigate the spectral and transport properties of an atomic chain of red phosphorus. We reveal the separation of flat-band states from the rest of the system and calculate its energy bands analytically. The topological properties of the system are established through the evaluation of the Berry (Zak) phase of the energy bands, revealing nontrivial topology. The Berry phase depends on the relative strength of the hopping parameters and exhibits dual energy-dependent topological phase transitions. Remarkably, the emergence of inert band edges provides a direct spectral signature of these transitions, acting as energy-resolved indicators of the redistribution of topological charge between bands. The existence of the associated edge states is proved numerically for finite lattices. The theoretical predictions, particularly the band structure and the existence of edge states, are further confirmed by numerical simulations of red phosphorus through a phononic lattice in the form of a highly structured aluminum plate.
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All authors Vít Jakubský, B. Manjarrez-Montañez, Rafael A. Méndez-Sánchez, Yonatan Betancur-Ocampo

Materials Science

Unraveling the temperature-responsive charge-disproportionation in BaBiO$_3$

Sumit Sarkar, Priyanka Yadav, Sourav Chowdhury et al. (+2 more)

This study shows that the charge disproportionation at the Bi site in BaBiO$_3$ alters as a function of temperature. Decreasing the temperature from 300K down to 160K leads to a significant modification of the density of states corresponding to the Bi-O hybridized band near the Fermi level (E$_\text{F}$). This modification indicates reduction of Bi 6$sp$ - O 2$p$ hybridization and O 2$p$ spectral weight near E$_\text{F}$. The strong decrement of covalency at lower temperatures is accompanied by a decrement in O 2$p$ hole density due to possible charge transfer from Bi 6$s$ to the O 2$p$ band. Bi-charge state analysis from Bi-4$f$ core-level spectra showed that at 300K, $\delta$ (charge difference between alternate Bi sites) value in 4$\pm\delta$ is much less than at 160K, which reveals the transition towards the ionic nature of CD or static CD in BBO at low temperature. On the other hand, O 1$s$ core-level spectra displayed an asymmetric shape, and temperature-dependent modifications of the asymmetric shape and intensity have been observed. This highlights the significant influence of the O 2$p$ band hole on the dynamical CD at the Bi site.
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All authors Sumit Sarkar, Priyanka Yadav, Sourav Chowdhury, Rajamani Raghunathan, Ram Janay Choudhary

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

Microscopic origin of hard-plane antiferromagnetism in the Kondo lattice Ce2Rh3Ge5

Rajesh Tripathi, Ewan Scott, D. T. Adroja et al. (+16 more)

Hard plane antiferromagnetic order where ordered moments lie perpendicular to the single-ion crystal electric field easy axis is rare in Ce-based Kondo lattices and is a subject of active interest. Here we show that Ce$_2$Rh$_3$Ge$_5$ realizes a hard-plane antiferromagnetic state in which partial delocalization of the local moment gives rise to an RKKY exchange that overturns the single-ion easy-axis preference. Neutron diffraction reveals moments in the $ab$ plane, while inelastic neutron scattering and susceptibility establish a magnetic easy axis along $c$ in the paramagnetic regime, highlighting a clear inversion between single-ion and ordered-state anisotropies. In this work, we establish a unified microscopic framework to consistently account for partial $4f$-moment delocalization, enhanced in-plane RKKY exchange, and the resulting hard-plane antiferromagnetic order. Ce$_2$Rh$_3$Ge$_5$ thus provides a benchmark system in which single-ion anisotropy, Kondo screening, and RKKY exchange compete on comparable energy scales, revealing a cooperative route to hard-axis ordering in strongly hybridized Kondo lattices.
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All authors Rajesh Tripathi, Ewan Scott, D. T. Adroja, D. Das, C. Ritter, Huanzhi Hu, Michal P. Kwasigroch, Nicholas Corkill, Gheorghe Lucian Pascut, T. Masuda, S. Asai, T. Takabatake, T. Onimaru, T. Shiroka, Francis Pratt, A. M. Strydom, S. Langridge, A. Sundaresan, S. Patil

Materials Science · Cross-listed from math-ph

Some effective operators for graphene monolayer superlattices, from variational perturbation theory

Louis Garrigue

Our goal is to provide precise effective operators for monolayer graphene at Fermi energy. We consider the microscopic potential created by a lattice, and add a macroscopic potential with the same periodicity but varying at a scale $\varepsilon^{-1} \in \mathbb{N}$, creating a superlattice. Our approach consists in coupling the variational approximation, perturbation theory together with a multiscale method. At the effective level the usual massless Dirac operator is replaced by other operators, and we provide simulations in the case of graphene.
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Materials Science

Specific features of the magnetic-field dependences of electrical resistivity in Bi–Mn solid solutions with low Mn content

A.V. Terekhov, V.M. Yarovyi, Yu.A. Kolesnichenko et al. (+4 more)

For the first time, the field dependences of the magnetoresistance of a textured polycrystalline Bi$_{88.08}$Mn$_{11.92}$ sample were investigated in the $H \perp I$ and $H \parallel I$ configurations, and the results were compared with those previously obtained for the solid solution Bi$_{95.69}$Mn$_{3.69}$Fe$_{0.62}$ with a lower manganese concentration. It was demonstrated that the magnetoresistance field dependencies for Bi$_{88.08}$Mn$_{11.92}$ differ significantly from those of Bi$_{95.69}$Mn$_{3.69}$Fe$_{0.62}$ below 100 K, becoming nearly identical as the temperature approaches room temperature. It was established that the maximum relative magnetoresistance values for Bi$_{88.08}$Mn$_{11.92}$ are observed at a magnetic field of 90 kOe and a temperature of 100 K in both the $H \perp I$ and $H \parallel I$ configurations, amounting to 3170% and 380%, respectively. These values are significantly lower than those previously reported for the solid solution containing less manganese (Bi$_{95.69}$Mn$_{3.69}$Fe$_{0.62}$), which were 3877% and 742%, respectively. Analysis of the results indicates that differences in field dependence among the studied materials are associated with varying amounts of magnetic $\alpha$-BiMn. The anomalous magnetoresistance behavior, as compared with that of pure bismuth, is attributed to the influence of internal magnetism on electrical transport within the bismuth matrix of the studied solid solutions.
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All authors A.V. Terekhov, V.M. Yarovyi, Yu.A. Kolesnichenko, K. Rogacki, E. Lähderanta, E.V. Khristenko, A.L. Solovjov

Materials Science

Constrained Diffusion for Accelerated Structure Relaxation of Inorganic Solids with Point Defects

Jingyi Cui, Jacob K. Christopher, Ankita Biswas et al. (+2 more)

Point defects affect material properties by altering electronic states and modifying local bonding environments. However, high-throughput first-principles simulations of point defects are costly due to large simulation cells and complex energy landscapes. To this end, we propose a generative framework for simulating point defects, overcoming the limits of costly first-principles simulators. By leveraging a primal-dual algorithm, we introduce a constraint-aware diffusion model which outperforms existing constrained diffusion approaches in this domain. Across six defect configuration settings for Bi2Te3, the proposed approach provides state-of-the-art performance generating physically grounded structures.
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All authors Jingyi Cui, Jacob K. Christopher, Ankita Biswas, Prasanna V. Balachandran, Ferdinando Fioretto

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

Precompression engineering of metal-insulator transition and magnetism in designed breathing kagome systems

Qingzhuo Duan, Hongdao Zhuge, Ying Liang et al. (+1 more)

Kagome materials featuring dispersive Dirac cones and topological flat bands exhibit unique electronic and magnetic properties. However, kagome compounds with tunable electrical conductivity remain scarce, which severely impedes their device applications. Here, based on density functional theory (DFT) and Boltzmann transport theory, we introduce the breathing effect into kagome materials $\mathrm{Nb_3XCl_7}$ (X = F, Cl, Br, I) via chemical precompression, thereby inducing a metal-insulator transition and magnetic variation. We determine that the band structures, optical absorption spectra and magnetic ground states agree well with experimental results at the effective correlation strength $U_{\text{eff}} = 2$ eV. The calculated conductivity and magnetic properties reveal that the monolayer $\mathrm{Nb_3Cl_8}$ and $\mathrm{Nb_3XCl_7}$ undergoes transitions from paramagnetic metals to Mott insulators at $U_{\text{eff}} = 1$ eV and $t_{\text{out}}/t_{\text{in}} = 0.6674$, respectively. Our detailed analysis establishes that the stronger breathing effect corresponds to enhanced chemical precompression, which reduces the region of free electron gas between intercell Nb atoms and facilitates the metal-insulator transition. Finally, we propose several viable synthesis routes for $\mathrm{Nb_3FCl_7}$, $\mathrm{Nb_3BrCl_7}$, and $\mathrm{Nb_3ICl_7}$, providing predictive guidance for experimental studies. Our study establishes a practical framework for investigating the breathing effect in correlated kagome systems and yields valuable insights into the mechanisms underlying metal-insulator transition and magnetic properties in real breathing kagome materials.
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All authors Qingzhuo Duan, Hongdao Zhuge, Ying Liang, Tianxing Ma

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

Superconducting phase diagram of multi-layer square-planar nickelates

Grace A. Pan, Dan Ferenc Segedin, Sophia F. R. TenHuisen et al. (+21 more)

The discovery of superconductivity in square-planar nickelates has offered a rich materials platform to explore the origins of cuprate-like superconductivity. Experimental investigations however have largely been limited to the infinite-layer $R$NiO$_2$ ($R$=rare-earth) nickelates. Here, we construct a phase diagram of multi-layer square-planar Nd$_{n+1}$Ni$_n$O$_{2n+2}$ compounds and discover signatures of superconductivity for $n$ = 4 - 8. Upon decreasing the dimensionality $n$, the superconducting anisotropy evolves due to 4$f$ electron effects, and electronic structure characteristics approach cuprate-like behavior. Magnetic fluctuations persist from within the superconducting regime and into the over-doped, non-superconducting regime. Remarkably, the superconducting regime overlaps with that of chemically-doped infinite-layer nickelates, demonstrating underlying commonalities and distinct differences across varying structural realizations of square-planar nickelates. Our work establishes this layered template for creating new nickel-based superconductors.
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All authors Grace A. Pan, Dan Ferenc Segedin, Sophia F. R. TenHuisen, Lopa Bhatt, Harrison LaBollita, Abigail Y. Jiang, Qi Song, Ari B. Turkiewicz, Denitsa R. Baykusheva, Abhishek Nag, Stefano Agrestini, Ke-Jin Zhou, Jonathan Pelliciari, Valentina Bisogni, Hua Zhou, Mark P. M. Dean, Hanjong Paik, David A. Muller, Lena F. Kourkoutis, Charles M. Brooks, Matteo Mitrano, Antia S. Botana, Berit H. Goodge, Julia A. Mundy

Materials Science

Nonperturbative Magnetic Orbital Hall Effect in Altermagnets

Xukun Feng, Jin Cao, Lay Kee Ang et al. (+3 more)

Recent studies on altermagnets have focused considerable attention on nonrelativistic effects that persist in the absence of spin-orbit coupling (SOC). As a result, the relative importance of various phenomena in altermagnets has commonly been judged by their dependence on SOC. Here, we challenge this common wisdom by uncovering the magnetic orbital Hall effect, which is nonperturbative in SOC strength. We establish the symmetry properties of this effect, demonstrating that it is strictly forbidden in conventional collinear antiferromagnets yet universally allowed in all ten spin-Laue classes of collinear altermagnets. Counterintuitively, although SOC-induced, it reaches giant magnitudes in altermagnets-comparable to or even exceeding the nonrelativistic spin Hall effect. Moreover, altermagnetic symmetry enables unconventional collinear-polarized orbital currents, allowing field-free manipulation of perpendicular magnetization. Our first-principles calculations predict strong room-temperature responses in the experimentally established altermagnets CrSb and FeSb2. These findings reveal the previously overlooked potential of altermagnetic orbitronics and broaden the horizons for altermagnets in high-performance magnetic memory applications.
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All authors Xukun Feng, Jin Cao, Lay Kee Ang, Shengyuan A. Yang, Cong Xiao, X. C. Xie

Materials Science

Hidden Chiral Ferroelectricity in AgNbO$_3$ Perovskite

Ying Song, Lingzhi Cao, Jinming Zhai et al. (+4 more)

AgNbO$_3$ is a lead-free perovskite with considerable potential for energy storage and optoelectronic applications, yet its low-temperature crystal structure has remained controversial. In this Letter, we revisit its low-energy structural landscape using a systematic first-principles structural search based on symmetry-adapted phonon-mode theory. We uncover a previously unreported chiral ferroelectric phase with space group $R3$, which exhibits a large spontaneous polarization and a low polarization switching barrier, enabling polarization reversal under electric fields. Crucially, the structural chirality of this phase is intrinsically locked to the ferroelectric polarization, allowing electrical control of the chiral handedness. Consequently, chiral optical responses–including circular dichroism, circular photogalvanic effect, optical activity, and second-order nonlinear optics–can be reversibly switched by an external electric field. These results not only clarify the complex low-temperature structural behavior of AgNbO$_3$ but also establish a rare purely inorganic platform for electric-field-tunable chirality, opening a pathway toward ultrafast, electrically controlled chiral optoelectronics.
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All authors Ying Song, Lingzhi Cao, Jinming Zhai, Zhilong Yang, Yali Yang, Laurent Bellaiche, Jiangang He

Saturday 21 February

Materials Science

A computationally efficient approach for predicting the transport properties of transition-metal alloys at elevated temperatures

Akshay Korpe, Manish Sudan, Ishtiaque K. Robin et al. (+5 more)

A novel phenomenological framework for an efficient estimation of the thermo-electric properties at room temperature and elevated temperatures of body-centered cubic (BCC) transition metal concentrated alloys is proposed in this work. The methodology is used to predict the electrical resistivity of BCC systems with our predictions showing excellent correlation with experimental data. This framework is further extended to predict the electrical resistivity $\rho$, thermal conductivity $\kappa$ and the specific heat capacity Cp of BCC alloys in the temperature range of 300-1300 K and the results are validated against experimental data. We demonstrate the capabilities of this model by using it to predict the thermo-electric properties of a concentrated W53Ta42V5 alloy which shows a saturation in the electrical resistivity $\rho$ in the temperature range 300K-1300K. This model is then used to predict the properties of another concentrated Nb$_4$0Mo$_4$0Ta$_2$0 alloy in the same temperature regime.
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All authors Akshay Korpe, Manish Sudan, Ishtiaque K. Robin, Bikram Bhatia, Garrett Pataky, Thomas Berfield, Osman El-Atwani, Enrique Martinez

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

Exact expression for the Berry connection in the projection gauge

Trey Cole and David Vanderbilt

The Berry connection encodes the momentum-space geometry of occupied Bloch states in gapped insulators and plays a central role in topological materials. While gauge-invariant quantities can be evaluated from overlap matrices between neighboring $k$ points, accessing the Berry connection itself as a smooth field requires specifying a continuous gauge over the Brillouin zone. Wannier-based workflows achieve this through projection onto localized orbitals, enabling stable evaluation of geometric quantities and response functions. In this setting, the Berry connection enters directly in Wannier-interpolated calculations of polarization, Berry curvature, and related response functions. In practical implementations, however, the projection-gauge Berry connection is typically constructed from finite-difference overlaps between neighboring $k$ points, discretizing momentum derivatives and introducing errors tied to $k$-mesh spacing and gauge alignment. These effects can become numerically delicate in systems with small band gaps or when evaluating higher-order responses such as the Chern-Simons axion angle. Here, we derive an exact expression for the non-Abelian Berry connection in the projection gauge that is local in crystal momentum. Starting from projected and orthonormalized Bloch-like states, we obtain a closed-form equation expressed entirely in terms of $k$-local quantities. We validate the formulation in one and three dimensions by computing the Berry phase and Chern-Simons axion angle in tight-binding models. The resulting framework provides a stable route to evaluating geometric properties within Wannier interpolation schemes and future first-principles implementations.
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Materials Science

Unravelling the mechanisms underlying crack initiation in additively manufactured steel

KenHee Ryou, Yaozhong Zhang, James A. D. Ball et al. (+7 more)

Metal additive manufacturing (AM) is increasingly adopted for safety-critical applications across the biomedical, aerospace, and energy sectors. However, many AM alloys exhibit substantially lower fracture toughness and shorter fatigue lives than their wrought counterparts, limiting their structural reliability. The microscale mechanisms governing this deficit remain obscured because the evolution of crack-tip deformation cannot be directly resolved using conventional characteriza-tion techniques. Here, we combine in situ multimodal synchrotron X-ray diffraction with phase-contrast tomography to directly observe the three-dimensional evolution of crack-tip plasticity dur-ing loading. We find that wrought steel develops a localized crack-tip process zone characterized by extensive geometrically necessary dislocation (GND) accumulation, effective stress relaxation, and crack-tip blunting. This plastic zone provides shielding of the crack tip by redistributing defor-mation and reducing the local driving force for crack initiation. In contrast, the AM alloy exhibits suppressed GND evolution, limited crack-tip blunting, and persistent elevated stresses over an ex-tended region ahead of the crack tip, indicating ineffective stress relaxation and premature crack initiation. These findings demonstrate that fracture resistance is governed by the spatial evolution of crack-tip plasticity, providing a mechanistic framework for improving the damage tolerance of AM structural alloys.
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All authors KenHee Ryou, Yaozhong Zhang, James A. D. Ball, Dan Rubio-Ejchel, Dillon Jobes, Buhari Ibrahim, Charles Romain, Minh-Son Pham, Henry Proudhon, Jerard V. Gordon

Materials Science

Layer-number parity induced topological phase transition

Kai Chen, Junyan Guan, Jiamin Guo et al. (+3 more)

We demonstrate that stacking topologically trivial layers, under enforced symmetry restrictions, yields emergent topological phases with protected boundary states. Remarkably, the number of layers itself acts as a topological switch, enabling the system to host topological bound states in the continuum (BICs). Analytically, we reveal that an odd-layer configuration not only renders the spectrum gapless but also supports BICs protected by interlayer reflection symmetry. Combined with entanglement-spectrum calculations, this confirms that odd-layer systems indeed support topological BICs. We provide experimental confirmation of these topological states in stacked acoustic lattices. Our findings establish a previously overlooked pathway to topology and demonstrate a readily applicable strategy for realizing exotic states in a wide range of artificial material systems.
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All authors Kai Chen, Junyan Guan, Jiamin Guo, He Gao, Zhongming Gu, Jie Zhu

Materials Science

Beam-Offset Thermoreflectance with Bayesian Optimization to Measure the Anisotropic Thermal Properties of Semiconductor Superlattices

A. Chatterjee, N. Spitzer, T. Kruck et al. (+5 more)

The directional nature of heat conduction in semiconductor superlattices–marked by significant differences between in-plane and cross-plane pathways–poses substantial challenges for precise thermal property assessment. Conventional frequency-domain thermoreflectance (FDTR) techniques, while proficient at evaluating cross-plane thermal conductivity, suffer from restricted capability in resolving in-plane transport due to inherent phase-delay constraints and inadequate lateral resolution. In this investigation, we establish a non-contact beam-offset FDTR (BO-FDTR) approach that concurrently determines both directional thermal conductivities within layered semiconductor architectures. Our methodology implements spatial separation between excitation and detection beams while utilizing coupled normalized amplitude and phase responses as analytical inputs, thereby improving discrimination between anisotropic thermal parameters. We combine this experimental configuration with a Bayesian optimization scheme incorporating Gaussian Process Regression (BO-GPR) to reduce estimation inaccuracies, attaining measurement uncertainties under 1% to 2% at 95% confidence intervals. This technique demonstrates particular efficacy for intricate multilayer nanostructures, furnishing a structured protocol for superlattice thermal evaluation. Experimental characterization of an AlAs/GaAs superlattice (period thickness 52 nm) delivers thermal conductivity values of 14.7 W m-1 K-1 (cross-plane) and 37.4 W m-1 K-1 (in-plane). Our findings indicate that integrating frequency sweeps with varied beam offset locations yields superior measurement precision, exceeding conventional single-variable methods and confirming thermal assessment validity across both geometric arrangements.
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All authors A. Chatterjee, N. Spitzer, T. Kruck, P. Song, A. Ludwig, A. D. Wieck, J. Ordonez-Miranda, M. Pawlak

Materials Science

EXAFS studies of the local environment of lead and selenium atoms in PbTe$_{1-x}$Se$_x$ solid solution

A. I. Lebedev, I. A. Sluchinskaya, V. N. Demin et al. (+1 more)

EXAFS spectroscopy is used to study the local environment of lead and selenium atoms in PbTe$_{1-x}$Se$_x$ solid solution. In addition to a bimodal distribution of bond lengths in the first shell, unusually large Debye–Waller factors for the Pb–Pb interatomic distances in the second shell and a substantial deviation of these distances from Vegard's law are observed. Valence force field (VFF) calculations show that these observations are due to the complex structure of the distribution function for Pb–Pb distances. It is found that the number of Se–Se pairs in the second shell surpasses the statistical value, which indicates that chemical factors play an important role in the structure of the solid solution. The contribution of these chemical factors to the enthalpy of mixing of the solid is estimated to be approximately 0.5 kcal/mole, which is comparable to the strain contribution.
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All authors A. I. Lebedev, I. A. Sluchinskaya, V. N. Demin, I. Munro

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

Acoustic Manipulation of Tangible Janus Icons on Liquid Droplets

Yusuke Koroyasu, Yoichi Ochiai and Tatsuki Fushimi

Interfaces that couple digital information with physical matter enable computation to be expressed through tangible motion and touch, yet typically rely on embedded actuators, rigid mechanisms, or enclosed environments. Consequently, contactless manipulation and interaction with centimeter-scale tangible elements in open settings remain difficult to achieve. Here, we present PolygonWave, a solid–fluid acoustic interface that enables transport and tangible interaction by coupling airborne ultrasound with liquid-mediated support. The system employs lightweight Janus icons with asymmetric wettability: a superhydrophobic upper surface permits dry touch interaction, while a hydrophilic lower surface couples to a water droplet resting on a superhydrophobic mesh. Focused acoustic fields generated by a 256-element phased array induce lateral forces, enabling programmable motion without mechanical contact. Systematic characterization demonstrates transport of payloads up to 525 mg across variations in icon size, droplet volume, and applied load. Beyond translation, the liquid layer functions as a reconfigurable mechanical element, enabling button-like input with self-recovery and resonance-driven vibro-visual feedback, exhibiting a peak response near 22 Hz for 200 \textmu L droplets. Liquid-mediated acoustic coupling provides a unified mechanism for mechanically expressive, touch-accessible tangible interfaces bridging acoustics, soft matter physics, and physical human–computer interaction.
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Materials Science

A ReaxFF-based thermomechanical analysis of N-carbophenes: phase-change, thermal expansion, and high temperature synthesis pathway

Chad E. Junkermeier, Kat Lavarez, R. Martin Adra et al. (+6 more)

N-carbophenes are a class of two-dimensional covalent organic frameworks with potential for solid-state gas storage and as 2D topological materials. Previous studies have demonstrated that variations in their bonding, topology, and functionalization enable the tuning of their chemical, electrical, and mechanical properties. Yet, the thermal stability and high-temperature behavior of pristine and functionalized N-carbophenes remain unexplored. Using ReaxFF-based reactive molecular dynamics (RMD) simulations with extensive statistical validation, we performed temperature-ramp MD simulations of pristine and functionalized N-carbophenes. We demonstrate that N-carbophenes remain stable up to temperatures above 1000 K. The phase-change onset temperatures decrease as the N-phenylene chain length increases in pristine N-carbophenes, attributed to increasing antiaromaticity in the central phenylene segments, thereby contributing to the foundational understanding of aromatic versus antiaromatic bonding in 2D carbon networks, a topic of considerable interest in theoretical chemistry. Pristine N-carbophenes exhibit negative area thermal expansion (NATE), whereas functional groups modulate this, leading to either negative or positive expansion. Functional groups remain stably bonded well above the transition temperature. We also show that a temperature-induced phase transition from graphenylene (2-carbophene) to {\gamma}-graphyne is possible. Our results provide upper bounds on N-carbophene stability, clarify the relationships between structure and thermal properties, and identify a new transformation pathway. These results will have applications in tunable band gaps, porous architectures, or chemically accessible sites.
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All authors Chad E. Junkermeier, Kat Lavarez, R. Martin Adra, Valeria Aparicio Diaz, Heather Osterstock, Pal Casinto, M. Verano, Ricardo Paupitz, Adri C. T. van Duin