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Physics Condensed Matter Materials Science

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Tuesday 7 October

Materials Science

Magnon-Magnon Interaction Induced by Dynamic Coupling in a Hybrid Magnonic Crystal

Rawnak Sultana, Mojtaba Taghipour Kaffash, Gianluca Gubbiotti et al. (+3 more)

We report a combined experimental and numerical investigation of spin-wave dynamics in a hybrid magnonic crystal consisting of a CoFeB artificial spin ice (ASI) of stadium-shaped nanoelements patterned atop a continuous NiFe film, separated by a 5 nm Al2O3 spacer. Using Brillouin light scattering spectroscopy, we probe the frequency dependence of thermal spin waves as functions of applied magnetic field and wavevector, revealing the decisive role of interlayer dipolar coupling in the magnetization dynamics. Micromagnetic simulations complement the experiments, showing a strong interplay between ASI edge modes and backward volume modes in the NiFe film. The contrast in saturation magnetization between CoFeB and NiFe enhances this coupling, leading to a pronounced hybridization manifested as a triplet of peaks in the spectra - predicted by simulations and observed experimentally. This magnon-magnon coupling persists over a wide magnetic field range, shaping both the spin-wave dispersion and frequency-field response throughout the hysteresis loop. Our findings establish how ASI geometry can selectively enhance specific spin-wave wavelengths in the underlying film, identifying them as preferential channels for magnonic signal transport and manipulation.
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All authors Rawnak Sultana, Mojtaba Taghipour Kaffash, Gianluca Gubbiotti, Yi Ji, M. Benjamin Jungfleisch, Federico Montoncello

Materials Science

Effect of crystallographic texture on dealloying kinetics and composition of nanoporous gold surface

Ezgi Hatipoğlu, Ayman A. El-Zoka, Yujun Zhao et al. (+4 more)

Nanoporous metals allow for tailoring composition and surface-to-volume ratio, both aspects critical for applications in catalysis. Here, Ag70Au30 (2 at. %) films with a face-centered cubic structure were deposited at 400{\deg}C, either {111}-textured or randomly oriented. Upon chemical dealloying, atom probe tomography of the nanoporous structure reveals that the textured film retains a up to 2.7 times higher Ag concentration within the ligaments compared to the randomly oriented film that exhibits ligament coarsening, indicating faster dealloying kinetics. Our study highlights the potential of microstructure engineering in tailoring the properties of nanoporous metals for possible future catalytic and electrochemical applications.
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All authors Ezgi Hatipoğlu, Ayman A. El-Zoka, Yujun Zhao, Stanislav Mráz, Jochen M. Schneider, Baptiste Gault, Aparna Saksena

Materials Science

Autonomous interpretation of atomistic scattering data

Andy S. Anker, John L. A. Gardner, Louise A. M. Rosset et al. (+2 more)

Materials with bespoke properties have long been identified by computational searches, and their experimental realisation is now coming within reach through autonomous laboratories. Scattering experiments are central to verifying the atomic structures of autonomously synthesised materials. Yet, interpreting these measurements typically requires user expertise and manual processing, or machine learning (ML) models trained on predefined datasets, limiting fully autonomous materials discovery. Here, we introduce a differentiable optimisation framework that treats scattering calculations, energetics, and chemical constraints as a unified refinement problem. Capability demonstrations across molecules, crystal structures, nanoparticles, and amorphous matter show that this data-driven approach resolves structural degeneracies with multi-modal inputs - suggesting its usefulness for informing, and ultimately guiding, the operation of autonomous laboratories.
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All authors Andy S. Anker, John L. A. Gardner, Louise A. M. Rosset, Andrew L. Goodwin, Volker L. Deringer

Materials Science

Retardance of lab grown diamond substrates as a function of thickness: momentum-drift random walk model

Thanh Tran, Phuong Vo, Thomas Sheppard et al. (+2 more)

This work studies the correlation between mean retardance and thickness of diamond substrates grown homoepitaxially via microwave plasma-enhanced chemical vapor deposition (MPCVD). We measure the retardance of a diamond substrate in two orientations: perpendicular and parallel to the growth direction. Our experimental results demonstrate that the correlation between mean retardance and thickness differs for these orientations. When measured perpendicular to the growth direction, the mean retardance is approximately proportional to the square root of the substrate thickness. In contrast, when measured parallel to the growth direction, we observe a generally higher mean retardance and an approximately linear correlation with thickness. This anisotropy arises not from differences in stress magnitude but from differences in the interlayer correlation of the principal stress axes, as evidenced by correlation coefficients between the azimuth angles of consecutive layers in the diamond crystal. To simulate the integrated retardance of diamond wafers, we propose a two-dimensional random walk model with momentum drift, which captures the diamond crystal tendency to preserve the azimuth angle across the samples. By optimizing the momentum factor, we show that the model can closely match experimental data. The momentum factor is found higher along the growth direction, which is consistent with the calculated correlation coefficients. Furthermore, both the model and experiments indicate that retardance-to-thickness ratios of thin samples converge toward similar base retardances in both orientations. These findings establish a quantitative framework for interpreting birefringence in diamond substrates, with implications for material selection and development in thermal management, quantum sensing, high-power electronics, and optical applications.
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All authors Thanh Tran, Phuong Vo, Thomas Sheppard, Timothy Grotjohn, Paul Quayle

Materials Science

High- and medium-entropy nitride coatings from the Cr-Hf-Mo-Ta-W-N system: properties and high-temperature stability

Pavel Souček, Stanislava Debnárová, Šárka Zuzjaková et al. (+9 more)

High- and medium-entropy nitride coatings from the Cr-Hf-Mo-Ta-W-N system were studied using ab initio calculations and experiments to clarify the role of entropy and individual elements in phase stability, microstructure, and high-temperature behaviour. Formation energy calculations indicated that nitrogen vacancies stabilise the cubic (fcc) phase, with hafnium and tantalum acting as strong stabilisers, while tungsten destabilises the lattice. Coatings were deposited by reactive magnetron sputtering at approx. 50C (AT) and approx. 580C (HT). All exhibited columnar fcc structures; high-temperature deposition produced denser coatings, lower nitrogen content, and larger crystallites, resulting in higher hardness and elastic modulus. Thermal stability was tested up to 1200C on Si and oxidation at 1400C on sapphire. AT coatings failed early, while most HT coatings endured. Nitrogen loss less than 10 at.% at 1000C was critical for survival. TEM revealed tungsten segregation and HfO2 formation, while fcc nitride remained dominant. Ta enrichment proved essential for superior thermal and oxidation stability.
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All authors Pavel Souček, Stanislava Debnárová, Šárka Zuzjaková, Shuyao Lin, Matej Fekete, Zsolt Czigány, Katalin Balázsi, Lukáš Vrána, Tatiana Pitoňáková, Ondřej Jašek, Petr Zeman, Nikola Koutná

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

Hidden phonon-assisted charge density wave transition in BaFe2Al9 revealed by ultrafast optical spectroscopy

Lei Wang, Mingwei Ma, Jiangxu Li et al. (+10 more)

The interplay between electronic and lattice degrees of freedom is fundamental to charge density wave (CDW) formation, yet the microscopic origin often remains elusive. Here, we investigate the transient optical response of the intermetallic compound BaFe2Al9 using polarization-resolved ultrafast optical spectroscopy. We identify a discontinuous sign reversal in the transient reflectivity at Tc ~ 110 K, providing unambiguous evidence for the first-order transition. The anisotropic quasiparticle relaxation establishes the three-dimensional nature of the ordered state. Below Tc, a single coherent 1.6 THz oscillation appears abruptly and remains confined to the CDW phase. This mode exhibits weak temperature dependence with negligible softening and is absent in Raman spectra. First-principles calculations imply that it is a precursor phonon at the CDW wave vector with strong electron-phonon coupling. Our results indicate that the CDW in BaFe2Al9 arises from intertwined electronic and lattice instabilities, assisted by a displacive mechanism mediated by a hidden strongly coupled phonon, distinct from conventional amplitude-mode softening scenarios.
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All authors Lei Wang, Mingwei Ma, Jiangxu Li, Liucheng Chen, Bingru Lu, Xiang Li, Feng Jin, Elbert E. M. Chia, Jianlin Luo, Rongyan Chen, Peitao Liu, Fang Hong, Xinbo Wang

Materials Science

Confinement-Controlled Morphology and Stability of One-Dimensional CrI3 Nanotubes

Ihsan Caha, Aqrab ul Ahmad and Francis Leonard Deepak

Integrating monolayers derived from 2D van der Waals (vdW) magnetic materials into next-generation technological applications remains a significant challenge due to their structural and magnetic instability issues. Template-assisted encapsulation is a potential route for the growth of stable 2D monolayers aimed at designing novel 1D heterostructures, opening new avenues for studying low-dimensional quantum effects and spin-related phenomena. In this study, we explored the diameter-dependent encapsulation of 2D CrI3 crystals using multi-walled carbon nanotubes as nanoscale host templates. Advanced microscopic analysis revealed distinct structural transitions, ranging from internal nanorod encapsulation to external shell formation, directly influenced by the host nanotube diameter. Furthermore, statistical analysis of structural morphologies indicates that CrI3 nanorods preferentially form within MWCNTs with inner diameters up to 5 nm, while single-walled CrI3 nanotubes are stabilized in CNTs with diameters up to 8 nm. For host CNTs exceeding 10 nm in diameter, CrI3 predominantly forms surface coatings rather than confined one-dimensional structures. In situ electron beam irradiation demonstrates the superior structural stability of single-walled CrI3 confined within MWCNTs, while externally coated CrI3 undergoes decomposition into metallic Cr clusters. Prolonged irradiation induces a morphological transformation of CrI3 nanotubes into nanorods. These insights lay the groundwork for engineering robust, tunable 1D magnetic heterostructures of CrI3 for spintronic and data storage applications.
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Materials Science · Cross-listed from cond-mat.mes-hall

Giant and robust Josephson diode effect in multiband topological nanowires

Bao-Zong Wang, Zi-Kai Li, Zhong-Da Li et al. (+1 more)

We theoretically predict the giant and robust Josephson diode effect in quasi-one-dimensional topological Majorana nanowires in the regime with multiple subbands, which is expected to be relevant for the real experiment. In the multiband regime, the Majorana bound states and conventional Andreev bound states can naturally coexist, and respectively contribute to the fractional and conventional parts in the Josephson effect, with the former/latter having 4$\pi$/2$\pi$-periodicity. We show that the interplay between the two types of bound modes can produce a robust and giant diode effect in the deep topological phase regime. Notably, we unveil a novel spin parity exchange mechanism, occurring only in the multiband regime, which leads to a robust high efficiency plateau of the giant diode effect. This effect is a nontrivial consequence of the balanced Fermi moment shifts of the multiple subbands in tuning the external magnetic field. Our finding highlights the subband engineering as a powerful tool to optimize the Josephson diode effect realistically and provides a new feasible signature to identify topological phase regime in superconducting nanowires.
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All authors Bao-Zong Wang, Zi-Kai Li, Zhong-Da Li, Xiong-Jun Liu

Materials Science

From High-Entropy Ceramics (HECs) to Compositionally Complex Ceramics (CCCs) and Beyond

Jian Luo

Over the past decade, the field of high-entropy ceramics (HECs) has expanded rapidly to encompass a broad range of oxides, borides, silicides, and other ceramic solid solutions. In 2020, we proposed extending HECs to compositionally complex ceramics (CCCs), where non-equimolar compositions and the presence of long- or short-range order, although reducing configurational entropy, create new opportunities to tailor and enhance properties, often surpassing those of higher-entropy counterparts. Along these lines, several fundamental scientific questions arise. Is the entropy in HECs truly high? Is maximizing entropy always desirable? In this perspective article, I revisit key concepts and terminologies and highlight emerging directions, including dual-phase CCCs, ultrahigh-entropy phases, and novel processing routes such as ultrafast reactive sintering. I propose that exploring compositional complexity across vast non-equimolar spaces, together with exploiting correlated disorder (coupled chemical and structural short-range order), represents a transformative strategy for designing ceramics with superior performance.
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Materials Science · Cross-listed from cond-mat.mes-hall

Engineering Magnetic States and Magnetoresistance in Bisegmented Co-Ni Jellyfish Nanowires via Interplay of Shape and Magnetocrystalline Anisotropies

M.I. Sobirov, K.A. Rogachev, M.A. Bazrov et al. (+9 more)

Expanding the spectrum of 3D magnetic nanostructures requires mastering the interplay between different anisotropy contributions. Here, we fabricate bisegmented jellyfish nanowires with tailored arrangements of Co (strong magnetocrystalline anisotropy) and Ni (dominant shape anisotropy) segments. We uncover a unique magnetic duality: Co segments can be tuned to exhibit either a flux-closing multidomain state or a shape-anisotropy-dominated vortex configuration, directly governed by their geometry. This local domain structure, imaged by MFM and simulated micromagnetically, dictates the global magnetic response-suppressing magnetostatic interactions in arrays and enabling the programming of the anisotropic magnetoresistance (AMR) in single nanowires. Our work provides a blueprint for designing functional magnetic nanomaterials where magnetoresistive properties are engineered through strategic anisotropy control.
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All authors M.I. Sobirov, K.A. Rogachev, M.A. Bazrov, Zh. Zh. Namsaraev, I.M. Sapovskii, T.R. Rakhmatullaev, N.V. Ilin, A.O Lembikov, S.M. Pisarev, A.V. Ognev, A.S. Samardak, A.Yu. Samardak

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

Field Free Spin-Orbit Torque Controlled Synapse and Stochastic Neuron Devices for Spintronic Boltzmann Neural Networks

Aijaz H. Lone, Meng Tang, Camelia Florica et al. (+4 more)

Spintronics offers a promising approach to energy efficient neuromorphic computing by integrating the functionalities of synapses and neurons within a single platform. A major challenge, however, is achieving field-free spin orbit torque SOT control over both synaptic and neuronal devices using an industry-adopted spintronic materials stack. In this study, we present field-free SOT spintronic synapses utilizing a CoFeB ferromagnetic thin film system, where asymmetrical device design and specifically added lateral notches in the CoFeB thin film facilitate effective domain wall DW nucleation, movement, and pinning and depinning. This method yields multiple analog, nonvolatile resistance states with enhanced linearity and symmetry, resulting in programmable and stable synaptic weights. We provide a systematic measurement approach to improve the linearity and symmetry of the synapses. Additionally, we demonstrate nanoscale magnetic tunnel junctions MTJs that function as SOT-driven stochastic neurons, exhibiting current-tunable, Boltzmann-like probabilistic switching behavior, which provides an intrinsic in-hardware Gibbs sampling capability. By integrating these synapses and neurons into a Boltzmann machine complemented by a classifier layer, we achieve recognition accuracies greater than 98 percent on the MNIST dataset and 86 percent on Fashion MNIST. This work establishes a framework for field free synaptic and neuronal devices, setting the stage for practical, materials-compatible, and all spintronic neuromorphic computing hardware.
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All authors Aijaz H. Lone, Meng Tang, Camelia Florica, Bin He, Jingkai Xu, Xixiang Zhang, Gianluca Setti

Materials Science

Co-evaporated Formamidinium tin triiodide with suppressed p-type self-doping

Junhyoung Park, Andrea Olivati, Mirko Prato et al. (+2 more)

Co-evaporation of formamidinium tin triiodide (FASnI3) precursors, without any additives or reducing agents, leads to the growth of a highly crystalline thin film which shows a bandgap around 1.31 eV, closely matching the theoretical value predicted from the ideal single crystal structure of FASnI3. The polycrystalline thin film presents a lower tendency of Sn2+ to Sn4+ oxidation and highly reduced tendency to self-doping, demonstrating, overall, an improved resistance to defects formation. These findings suggest solvent-free co-evaporation processes as a promising route for high quality Sn-based perovskite polycrystalline thin films.
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All authors Junhyoung Park, Andrea Olivati, Mirko Prato, Min Kim, Annamaria Petrozza

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

Probing orbital currents through inverse orbital Hall and Rashba effects

E. Santos, J. L. Costa, R.L. Rodriguez-Suarez et al. (+2 more)

We report a comprehensive experimental investigation of orbital-to-charge conversion in metallic and semiconductor materials, emphasizing the fundamental roles of the inverse orbital Hall effect (IOHE) and the inverse orbital Rashba effect. Using spin pumping driven by ferromagnetic resonance (SP-FMR) and the spin Seebeck effect (SSE), we demonstrate efficient orbital current generation and detection in YIG/Pt/NM structures, where NM is either a metal or a semiconductor. A central finding is the dominance of orbital contributions over spin-related effects, even in systems with weak spin-orbit coupling. In particular, a large enhancement of the SP-FMR and SSE signals is observed in the presence of naturally oxidized Cu in different heterostructures. Furthermore, we identify positive and negative IOHE signals in Ti and Ge, respectively, and extract orbital diffusion lengths in both systems using a diffusive model. Our results confirm the presence of orbital transport and offer valuable insights that may guide the further development of orbitronics.
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All authors E. Santos, J. L. Costa, R.L. Rodriguez-Suarez, J. B. S. Mendes, A. Azevedo

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

Memory behavior of a randomly driven model glass

Roni Chatterjee, Smarajit Karmakar, Muhittin Mungan et al. (+1 more)

We investigate by atomistic simulations the memory behavior a model glass subjected to random driving protocols. The training consists of a random walk of forward and/or backward shearing sequences bounded by a maximal shear strain of absolute value {\gamma}T . We show that such a stochastic training protocol is able to record the training amplitude. Different read-out protocols are also tested and are shown to be able to retrieve the training amplitude. We then emphasize the ten- sorial character of the memory encoded in the glass sample and then characterize the anisotropic mechanical behavior of the trained samples.
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All authors Roni Chatterjee, Smarajit Karmakar, Muhittin Mungan, Damien Vandembroucq

Monday 6 October

Materials Science

A Wigner Matrix Based Convolution Algorithm For Matrix Elements in the LCAO Method

Tyler C. Sterling

The linear combination of atomic orbitals (LCAO) method uses a small basis set in exchange for expensive matrix element calculations. The most efficient approximation for the matrix element calculations is the two-center approximation (2CA) in tight binding (TB). In the 2CA, a variety of matrix elements are neglected with only "two-center integrals" (2CI) remaining. The 2CI are calculated efficiently by rotating to symmetrical coordinates where the integral is parameterized. This makes TB fast in exchange for diminished transferability. An ideal electronic structure method has both the efficiency of TB and the transferability of ab-initio methods. In this work, I expand the full crystal potential into multipoles where the resulting matrix elements are transformed into the form of 2CI between high angular momentum functions. The usual Slater-Koster formulae for TB are limited to $l\leq3$; to enable efficient evaluation of the full crystal potential 2CI, I derive a Wigner matrix based convolution algorithm (WMCA) that works for arbitrary angular momentum. Given a suitable method for generating a local ab-initio Kohn-Sham potential, the algorithm for calculating matrix elements is applicable to fully ab-initio LCAO methods (this is the subject of forthcoming work). In this paper, I apply the WMCA to silicon using a model crystal potential.
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Materials Science · Cross-listed from physics.ins-det

Non-destructive diagnostics of fiber orientation in large planar fiber-reinforced concrete specimens

Václav Papež, Karel Künzel, Petr Konrád et al. (+2 more)

The mechanical performance of fiber-reinforced concrete is critically dependent on the orientation and concentration of its reinforcing fibers. This paper presents a non-destructive method for diagnosing fiber parameters in large planar specimens. The proposed technique utilizes an electromagnetic measurement system using electrical coils with commercial LCR meters, but also a novel custom-made meter suitable for factory environments, offering a practical solution for quality control in prefabrication of high-performance structural elements. This study details the theoretical background, experimental setup and methodology, and provides an evaluation of the diagnostic system's effectiveness and accuracy. The results demonstrate a strong correlation between the electromagnetic measurements and the actual fiber parameters, confirming the method's reliability.
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All authors Václav Papež, Karel Künzel, Petr Konrád, Kristýna Carrera, Petr Konvalinka

Materials Science

Design and Modeling of CdGa2Te4 and ZnGa2Te4 Chalcogenide Compound-Based Photovoltaic Devices: A DFT Study along with SCAPS-1D Simulation

Md Hasan Shahriar Rifat, Tanvir Khan, Md Arafat Hossain Shourov et al. (+2 more)

The electronic and optical properties of CdGa2Te4 and ZnGa2Te4 were studied using first-principles DFT calculations. Band gaps were calculated using the GGA-PBESol functional. Both materials show promise for photovoltaic applications because of their large, near-unity absorption efficiencies (10^4 cm^-1) in the visible region. They exhibit low exciton binding energies (18.85-26.81 meV), large Bohr radii (23-34.3 Angstrom), and moderate exciton temperatures (218-311 K), which are favorable for photovoltaic applications. Their performance as solar cells was simulated using the SCAPS-1D tool for thin-film devices with Pt/CdS/CdGa2Te4/Cu2O/Ti and Pt/CdS/ZnGa2Te4/Cu2O/Ti structures. We investigated the effects of layer thickness, donor and acceptor concentrations (shallow donors/acceptors), and defect density on device performance. The ideal absorber thickness for XGa2Te4 (X = Cd, Zn) was found to be 1000-1800 nm, and the CdS buffer layer around 100 nm. To obtain an efficiency above 20%, the defect density in the CdGa2Te4 and ZnGa2Te4 absorber layers should be kept below 1.772 x 10^13 cm^-3. The best simulations show efficiencies of 18.46% and 17.35% for CdGa2Te4- and ZnGa2Te4-based solar cells, respectively.
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All authors Md Hasan Shahriar Rifat, Tanvir Khan, Md Arafat Hossain Shourov, Md Sahat Bin Sayed, Md Saiful Islam

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

The roles of elasticity and dimension in liquid-gel phase separation

Shichen Wang and Peter D. Olmsted

We compare six elastic models for polymer networks in the context of phase separation within a gel, including a new model that combines the finite extensible Arruda-Boyce model and the slip tube model for entangled chains. We study incompressible uniaxial stretch and compression, and three volume-changing constrained-dimension deformations, in which the material can only deform in the designated dimensions(s) while the constrained direction(s) remain(s) the same. Each model responds differently to large deformations, and our proposed model successfully describes both strain softening and strain hardening, which are both present in well-entangled elastomers. When considering phase separation, we show that the commonly-used neo-Hookean model fails to admit a common tangent construction for phase coexistence for 3D deformations. This can be resolved by using a model with finite extension, such as the Arruda-Boyce model. In constrained-dimension deformations, where the gel's volume is allowed to change, for elastic models in which phase coexistence is possible, the critical temperatures increases and the critical concentration decreases with increasing deformation dimensions. This strong dependence of the phase diagram on spatial dimension and geometry distinguishes phase separation elastic media from conventional phase separation.
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Materials Science

Imaging Nanoscale Carrier, Thermal, and Structural Dynamics with Time-Resolved and Ultrafast Electron Energy-Loss Spectroscopy

Wonseok Lee, Levi D. Palmer, Thomas E. Gage et al. (+1 more)

Time-resolved and ultrafast electron energy-loss spectroscopy (EELS) is an emerging technique for measuring photoexcited carriers, lattice dynamics, and near-fields across femtosecond to microsecond timescales. When performed in either a specialized scanning transmission electron microscope or ultrafast electron microscope (UEM), time-resolved and ultrafast EELS can directly image charge carriers, lattice vibrations, and heat dissipation following photoexcitation or applied bias. Yet recent advances in theoretical calculations and electron optics are often required to realize the full potential of ultrafast EEL spectrum imaging. In this review, we present a comprehensive overview of the recent progress in the theory and instrumentation of time-resolved and ultrafast EELS. We begin with an introduction to the technique, followed by a physical description of the loss function. We outline approaches for calculating and interpreting ground-state and transient EEL spectra spanning low-loss plasmons to core-level excitations analogous to X-ray absorption. We then survey the current state of time-resolved and ultrafast EELS techniques beyond photon-induced near-field electron microscopy, highlighting abilities to image carrier and thermal dynamics. Finally, we examine future directions enabled by emerging technologies, including electron beam monochromation, in situ and operando cells, laser-free UEM, and high-speed direct electron detectors. These advances position time-resolved and ultrafast EELS as a critical tool for uncovering nanoscale dynamic processes in quantum materials and solar energy conversion devices.
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All authors Wonseok Lee, Levi D. Palmer, Thomas E. Gage, Scott K. Cushing

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

Quantum oscillations and anisotropic magnetoresistance in the quasi-two-dimensional Dirac nodal line superconductor $\mathrm{YbSb_2}$

Yuxiang Gao, Kevin Allen, Rose Albu Mustaf et al. (+8 more)

Recent interest in quantum materials has focused on systems exhibiting both superconductivity and non-trivial band topology as material candidates to realize topological or unconventional superconducting states. So far, superconductivity in most topological materials has been identified as type II. In this work, we present magnetotransport studies on the quasi-two-dimensional type I superconductor $\mathrm{YbSb_2}$. Combined ab initio DFT calculations and quantum oscillation measurements confirm that $\mathrm{YbSb_2}$ is a Dirac nodal line semimetal in the normal state. The complex Fermi surface morphology is evidenced by the non-monotonic angular dependence of both the quantum oscillation amplitude and the magnetoresistance. Our results establish $\mathrm{YbSb_2}$ as a candidate material platform for exploring the interplay between band topology and superconductivity.
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All authors Yuxiang Gao, Kevin Allen, Rose Albu Mustaf, Yichen Zhang, Sanu Mishra, Christopher Lane, Marta Zonno, Sergey Gorovikov, Jian-Xin Zhu, Ming Yi, Emilia Morosan

Materials Science

Photoluminescence excitation spectroscopy of quantum wire-like dislocation states in ZnS

Alexander Blackston, Alexandra Fonseca Montenegro, Sevim Polat Genlik et al. (+2 more)

Recent ab initio calculations predict 1D dispersive electronic bands confined to the atomic scale cores of dislocations in the wide bandgap (3.84 eV) semiconductor ZnS. We test these predictions by correlating sub-bandgap optical transitions with the density of dislocations formed during strain relaxation in epitaxial ZnS grown on GaP. The densities for four predicted partial dislocations are quantified using scanning electron microscopy-based electron channeling contrast imaging. Room-temperature ellipsometry reveals absorption peaks that scale with dislocation density and align with theoretical predictions. Low-temperature photoluminescence spectra show deep emission peaks matching dislocation 1D band-to-band transitions. Photoluminescence excitation spectroscopy reveals six distinct emission lines with contrasting excitation dependence. Four peaks (2.78, 2.41, 2.20, 1.88 eV), assigned to dislocations, exhibit only modest suppression ($\leq$5$\times$) when excited below the ZnS bandgap, while two other peaks (3.11, 1.53~eV) are strongly quenched ($>$10$\times$). These findings support the existence of efficient, 1D band-to-band radiative transitions within quantum wire-like dislocation core states in ZnS, distinct from typical non-radiative deep-level defects in wide-gap semiconductors.
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All authors Alexander Blackston, Alexandra Fonseca Montenegro, Sevim Polat Genlik, Maryam Ghazisaeidi, Roberto C. Myers

Materials Science

Machine Learning Interatomic Potentials Enable Molecular Dynamics Simulations of Doped MoS2

Abrar Faiyad and Ashlie Martini

Dopants can tune the performance of MoS2 in various applications, but use of molecular dynamics simulations for doped MoS2 materials discovery is limited by the lack of multi-dopant interatomic potentials. Universal machine learning interatomic potentials (MLIPs) could be a solution, but the accuracy of these potentials must first be evaluated. Here, we evaluate the accuracy of a recently developed MLIP, META's Universal Model for Atoms (UMA), for 25 different MoS2 dopants spanning metals, non-metals, and transition metals in Mo substitution, S substitution, and intercalated positions by benchmarking the MLIP-predicted formation energy and the dopant-induced structural change against density functional theory calculations. The computational framework for MLIP validation and simulations are described in detail and the source code is made open source. The MLIP is then demonstrated by performing heating-cooling simulations of MoS2 supercells with all 25 dopants. These simulations capture complex phenomena including dopant clustering, MoS2 layer fracturing, interlayer diffusion, and chemical compound formation at orders-of-magnitude reduced computational cost compared to density functional theory. This work provides a computational workflow for application-oriented design of doped-MoS2, enabling high-throughput screening of dopant candidates and optimization of compositions for targeted tribological, electronic, and optoelectronic performance. Github Repo Link: https://github.com/AbrarFaiyad/Machine-Learning-Interatomic-Potential-UMA-Enables-MD-Simulations-of-MoS2-Doped
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Materials Science

Cation vacancies mediate thermochemical water splitting with iron aluminates

Nathan J. Szymanski, Kent J. Warren, Alan W. Weimer et al. (+1 more)

Solar thermochemical water splitting enables hydrogen production by cycling metal oxides between reduced and oxidized states, typically through an oxygen vacancy mechanism. However, recent experimental work suggests that cation vacancies have a greater influence on the redox behavior of iron aluminate spinels used in water splitting. This remains debated, as calculations predict that such cation vacancies are thermodynamically unfavorable. In the current work, we show that Fe vacancies in (Fe$\zeta$Al1-$\zeta$)3O4 become accessible only when facilitated by inversion between Fe and Al. This antisite disorder lowers the formation energy of octahedral Fe vacancies in Al-rich spinels ($\zeta$ = 1/3) from over 3 eV to just 0.62 eV when one third of the cation sites are inverted, allowing high Fe vacancy concentrations under oxidizing conditions. This mechanism supports high H2 yields up to 361 $\mu$mol/g, consistent with experimental observations. Our findings support the notion that solar thermochemical water splitting can occur through a cation vacancy mechanism. They also clarify how site inversion, vacancy energetics, and defect interactions each contribute to redox performance, offering general design principles for identifying and optimizing materials that operate through cation vacancy cycling.
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All authors Nathan J. Szymanski, Kent J. Warren, Alan W. Weimer, Christopher J. Bartel

Materials Science

Thermodynamics of proton insertion across the perovskite-brownmillerite transition in La0.5Sr0.5CoO3-{\delta}

Armand J. Lannerd, Nathan J. Szymanski and Christopher J. Bartel

La$_{1-x}$Sr$_{x}$CoO3-$\delta$ is a promising off-stoichiometric metal oxide that undergoes a topotactic perovskite ($\delta$ = 0) to brownmillerite ($\delta$ = 0.5) transition under electrochemical and thermochemical stimuli, with concomitant variations in its electrical, magnetic, thermal, and optical properties. Recent studies on thin-film cycling in electrochemical devices show incomplete reversibility of this transition, with significant acid-etching serving as a degradation mechanism. While earlier investigations examined the protonation of brownmillerite SrCoO2.5, the thermodynamics of protonation across the perovskite-to-brownmillerite transition remain poorly understood. In this work, we combine density functional theory calculations with predictions from universal machine-learning interatomic potentials to elucidate the energetics and implications of protonation across the transition for La0.5Sr0.5CoO3-$\delta$. These calculations reveal negative hydrogen insertion energies and strong competition with oxygen vacancy formation across the transition for a wide range of conditions. The extent of protonation is primarily limited by the availability of Co 3d states to accommodate reduction by inserted hydrogen. Although hydrogen insertion is often thermodynamically favorable within a defect picture, a convex hull analysis of the resulting HyLa0.5Sr0.5CoO3-$\delta$ phases reveals them to be unstable against decomposition into hydroxides among other products. This instability increases with hydrogen content and provides a thermodynamic basis for the acid-etching observed during electrochemical cycling. This work advances the fundamental understanding of protonation in La0.5Sr0.5CoO3-$\delta$ and contextualizes experimental observations of related materials in the presence of moisture or H2.
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Materials Science

Fermi surface and Berry phase analysis for Dirac nodal line semimetals: cautionary tale to SrGa$_2$ and BaGa$_2$

Yuxiang Gao, Yichen Zhang, Shiming Lei et al. (+8 more)

A Berry phase of odd multiples of $\pi$ inferred from quantum oscillations (QOs) has often been treated as evidence for nontrivial reciprocal space topology. However, disentangling the Berry phase values from the Zeeman effect and the orbital magnetic moment is often challenging. In centrosymmetric compounds, the case is simpler as the orbital magnetic moment contribution is negligible. Although the Zeeman effect can be significant, it is usually overlooked in most studies of QOs in centrosymmetric compounds. Here, we present a detailed study on the non-magnetic centrosymmetric $\mathrm{SrGa_2}$ and $\mathrm{BaGa_2}$, which are predicted to be Dirac nodal line semimetals (DNLSs) based on density functional theory (DFT) calculations. Evidence of the nontrivial topology is found in magnetotransport measurements. The Fermi surface topology and band structure are carefully studied through a combination of angle-dependent QOs, angle-resolved photoemission spectroscopy (ARPES), and DFT calculations, where the nodal line is observed in the vicinity of the Fermi level. Strong de Haas-van Alphen fundamental oscillations associated with higher harmonics are observed in both compounds, which are well-fitted by the Lifshitz-Kosevich (LK) formula. However, even with the inclusion of higher harmonics in the fitting, we found that the Berry phases cannot be unambiguously determined when the Zeeman effect is included. We revisit the LK formula and analyze the phenomena and outcomes that were associated with the Zeeman effect in previous studies. Our experimental results confirm that $\mathrm{SrGa_2}$ and $\mathrm{BaGa_2}$ are Dirac nodal line semimetals. Additionally, we highlight the often overlooked role of spin-damping terms in Berry phase analysis.
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All authors Yuxiang Gao, Yichen Zhang, Shiming Lei, Neil Harrison, Mun Keat Chan, Jonathan D. Denlinger, Sergey Gorovikov, Sanu Mishra, Yan Sun, Ming Yi, Emilia Morosan

Materials Science

Tunable electronic energy level alignment and exciton diversity in organic-inorganic van der Waals heterostructures

Aurélie Champagne, Olugbenga Adeniran, Jonah B. Haber et al. (+3 more)

van der Waals stacking of two-dimensional (2D) materials offers a powerful platform for engineering material interfaces with tailored electronic and optical properties. While most van der Waals multilayers have featured inorganic monolayers, incorporating molecular monolayers introduces new degrees of tunability and functionality. Here, we investigate hybrid bilayers composed of atomically thin perylene-based molecular crystals interfaced with monolayer transition metal dichalcogenides (TMDs), specifically MoS2 and WS2. Using ab initio many-body perturbation theory within the GW approximation and the Bethe-Salpeter equation approach, we predict emergent properties beyond those of the isolated constituent systems. Notably, we find substantial renormalization of monolayer molecular crystal band gap due to TMD-induced polarization. Furthermore, by varying the TMD monolayer, we demonstrate tuning of the energy level alignment of the bilayer and subsequent control over a diversity of lowest-energy excitons, which include strongly bound hybrid excitons and long-lived charge-transfer excitons. These findings establish organic-inorganic van der Waals heterostructures as a promising class of materials for tunable optoelectronic devices and quantum excitonic phenomena, expanding the design space for low-dimensional systems.
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All authors Aurélie Champagne, Olugbenga Adeniran, Jonah B. Haber, Antonios M. Alvertis, Zhen-Fei Liu, Jeffrey B. Neaton

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

Variational and field-theoretical approach to exciton-exciton interactions and biexcitons in semiconductors

Peter A. Noordman, Lucas Maisel Licerán and Henk T. C. Stoof

Bound electron-hole pairs in semiconductors known as excitons are the subject of intense research due to their potential for optoelectronic devices and applications, especially in the realm of two-dimensional materials. While the properties of free excitons in these systems are well understood, a general description of the interactions between these quasiparticles is complicated due to their composite nature, which leads to important exchange processes that can take place between the identical fermions of different excitons. In this work, we employ a variational approach to study interactions between Wannier excitons and obtain an effective interaction potential between two ground-state excitons in a system of spin-degenerate electrons and holes. This potential is in general nonlocal in position space and depends on the combined spin configurations of the electrons and holes. When particularized to the case of hydrogen-like excitons with a heavy hole, this potential becomes local and exactly reproduces the Heitler-London result for two interacting hydrogen atoms. Thus, our result can be interpreted as a generalization of the Heitler-London potential to the case of arbitrary masses. We also show how including corrections due to excited states into the theory results in a van der Waals potential at large distances, which is expected due to the induced dipole-dipole nature of the interactions. Our approach is also applicable to more complicated systems with nonhydrogenic exciton series, such as layered semiconductors with Rytova-Keldysh interactions. Additionally, we use a path-integral formalism to develop a many-body theory for a dilute gas of excitons, resulting in an excitonic action that formally includes many-body interactions between excitons. While in this approach the field representing the excitons is exactly bosonic, we clarify how the internal exchange processes arise in...
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Materials Science

Role of chromium oxides and carbides in strengthening CoCrFeNi multi-principle element alloys

Artur Olejarz, Wenyi Huo, Anna Kosinska et al. (+7 more)

Multi-principal element alloys (MPEAs) can potentially offer exceptional material properties, but their complex, costly manufacturing limits their scalability. Chemical complexity and complex manufacturing processes lead to the formation of some secondary phases, which have a significant impact on the final properties. In this work, chromium compound dispersoid enhancements (Cr- oxides and carbides) were formed in CoCrFeNi MPEAs to enhance their microstructural and high-temperature mechanical properties. A single FCC phase was observed in the arc melted (AM) samples, chromium oxides were detected in the gas-atomized (GA) samples, and Cr2O3 with Cr23C6 or Cr7C3 was found in the mechanically alloyed (MA)samples depending on the sintering temperature. Mechanical tests at room temperature and 575{\deg}C, where no phase evolution is expected, showed that the GA samples with oxides achieved enhanced mechanical properties at 575{\deg}C. This was co-induced by precipitation strengthening, recrystallization suppression, and twinning-induced plasticity. The MA samples with carbides exhibited high strength but low ductility, with Cr7C3 outperforming Cr23C6 because of its lower hardness and twinning effects. This work links chromium compound evolution to mechanical performance of MPEAs, offering insights to optimize HEA production for high-temperature applications through controlled phase formation.
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All authors Artur Olejarz, Wenyi Huo, Anna Kosinska, Maciej Zielinski, Tomasz Stasiak, Marcin Chmielewski, Wojciech Chmurzynski, Max Rae Chu, Michael Patrick Short, Lukasz Kurpaska

Materials Science

Comparing fine-tuning strategies of MACE machine learning force field for modeling Li-ion diffusion in LiF for batteries

Nada Alghamdi, Paolo de Angelis, Pietro Asinari et al. (+1 more)

Machine learning force fields (MLFFs) are transforming materials science and engineering by enabling the study of complex phenomena, such as those critical to battery operation. In this work, we explore the predictive capabilities of pre-trained and fine-tuned MACE MLFF and compare different fine-tuning strategies for predicting interstitial lithium diffusivity in LiF, a key component in the solid electrolyte interphase in Li-ion batteries. Our results demonstrate that the MACE-MPA-0 foundational model achieves comparable accuracy to well-trained DeePMD, in predicting key diffusion properties based on large scale molecular dynamics simulation, while requiring minimal or no training data. For instance, the MACE-MPA-0 predicts an activation energy $E_a$ of 0.22eV, the fine-tuned model with only 300 data points predicts $E_a =$ 0.20eV, both of which show good agreement with the DeePMD model reference value of $E_a = $ 0.24eV. In this work, we provide a solid test case where fine-tuning approaches, whether using data generated for DeePMD or data produced by the foundational MACE model itself, yield similar robust performance to the DeePMD potential trained with over 40,000 actively learned data, albeit requiring only a fraction of the training data.
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All authors Nada Alghamdi, Paolo de Angelis, Pietro Asinari, Eliodoro Chiavazzo

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

Controlling an altermagnetic spin density wave in the kagome magnet CsCr3Sb5

Zihao Huang, Chenchao Xu, Yande Que et al. (+8 more)

The interplay of charge and spin orders lies at the heart of correlated electron physics and plays a critical role in the emergence of unconventional quantum phases. Kagome magnets provide a particularly promising platform to investigate these phenomena, owing to their geometrically frustrated lattice structure. However, resolving spin and charge orders microscopically and establishing ways to control them remain fundamental challenges. Here, we demonstrate magnetic field control of an altermagnetic spin density wave order intertwined with charge density wave order in kagome magnet CsCr3Sb5. Scanning tunneling microscopy down to deep cryogenic temperature of 50 mK reveals two previously unreported charge density wave orders in the Sb surface. Density functional theory confirms that one of them is coupled to a spin density wave with an altermagnetic ground state. The charge density waves can be tuned in both amplitude and phase by an external magnetic field, reflected in domain switching and stripe sliding of the charge density wave. Our findings deepen the understanding of symmetry-breaking in kagome systems, providing a tunable platform to explore the interplay of electronic correlation with emergent quantum magnetism.
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All authors Zihao Huang, Chenchao Xu, Yande Que, Yi Liu, Ranjith Shivajirao, Zheng Jue Tong, Amit Kumar, Chao Cao, Guang-Han Cao, Hong-Jun Gao, Bent Weber