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Physics Condensed Matter Materials Science

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Thursday 27 November

Materials Science

Generative Models for Crystalline Materials

Houssam Metni, Laura Ruple, Lauren N. Walters et al. (+13 more)

Understanding structure-property relationships in materials is fundamental in condensed matter physics and materials science. Over the past few years, machine learning (ML) has emerged as a powerful tool for advancing this understanding and accelerating materials discovery. Early ML approaches primarily focused on constructing and screening large material spaces to identify promising candidates for various applications. More recently, research efforts have increasingly shifted toward generating crystal structures using end-to-end generative models. This review analyzes the current state of generative modeling for crystal structure prediction and de novo generation. It examines crystal representations, outlines the generative models used to design crystal structures, and evaluates their respective strengths and limitations. Furthermore, the review highlights experimental considerations for evaluating generated structures and provides recommendations for suitable existing software tools. Emerging topics, such as modeling disorder and defects, integration in advanced characterization, incorporating synthetic feasibility constraints, and model explainability are explored. Ultimately, this work aims to inform both experimental scientists looking to adapt suitable ML models to their specific circumstances and ML specialists seeking to understand the unique challenges related to inverse materials design and discovery.
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All authors Houssam Metni, Laura Ruple, Lauren N. Walters, Luca Torresi, Jonas Teufel, Henrik Schopmans, Jona Östreicher, Yumeng Zhang, Marlen Neubert, Yuri Koide, Kevin Steiner, Paul Link, Lukas Bär, Mariana Petrova, Gerbrand Ceder, Pascal Friederich

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

Active flow-driven DNA remodeling generates millimeter-scale mechanical oscillations

Maya Levanon, Noa S. Goldberg, Dvir Cohen et al. (+3 more)

In living systems, DNA undergoes continuous and rhythmic mechanical remodeling through condensation, looping, and disentangling to regulate gene expression, segregate chromosomes, and guide morphogenesis. Here, we demonstrate a purely mechanical route to rhythmic DNA reorganization in a minimal active composite of microtubules, kinesin motors, and DNA. We embed a DNA polymer in an active turbulent microtubule-kinesin fluid, creating a self-morphing material. The active flows stretch and entangle the DNA, forming a self-organized viscoelastic network that resists active stresses and affects flow over large length scales. This mechanical feedback loop progressively amplifies velocity correlations and drives a nonequilibrium phase transition tuned by DNA contour length: from disordered flow to synchronized, millimeter-scale oscillations with vortices. We rationalize the phase transition with an active-gel model that predicts a growing length scale and an oscillatory instability emerging from the interplay between activity, orientational order, and self-generated viscoelasticity, rather than chemical signaling. The dependence of the oscillation frequency on system size and activity quantitatively agrees with experiment. Thus, flow-driven DNA remodeling provides a minimal physical route to autonomous, system-spanning oscillations in three dimensions and suggests design principles for programmable soft matter that coordinates, actuates, and reshapes itself.
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All authors Maya Levanon, Noa S. Goldberg, Dvir Cohen, Eran Bouchbinder, Ram M. Adar, Alexandra M. Tayar

Materials Science

Statistical characterization of the spin Hall magnetoresistance in YIG/Pt heterostructures

Denise Reustlen, Sebastian Sailler, Davina U. Schmidt et al. (+3 more)

The spin Hall magnetoresistance (SMR) is widely used to study the interplay between charge and spin currents in bilayers of a magnetic insulator and a normal metal. However, not much is known about the spatial variation of the SMR across the surface of one and the same sample. In this work, we investigate the statistical distribution of the SMR in hundreds of nominally identical Hall bar structures patterned into prototypical yttrium iron garnet (YIG)/Pt heterostructures. We find a Gaussian-distributed SMR with a narrow standard deviation of approximately 10$\,$% of the mean value in each YIG/Pt bilayer studied. However, the variation of the mean SMR between different YIG/Pt samples can be as large as ~30$\,$%, despite nominally identical fabrication conditions. This demonstrates that spatial variations of the SMR amplitude must not be neglected, in particular when comparing different heterostructures. On a microscopic level, local variations of the interface quality captured by the spin mixing conductance are the most likely origin for the observed SMR amplitude variations.
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All authors Denise Reustlen, Sebastian Sailler, Davina U. Schmidt, Richard Schlitz, Michaela Lammel, Sebastian T. B. Goennenwein

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

Accurate simulations of magnetic excitations in the neutron simulation package McStas

Silas B. Schack, Kristine M. L. Krighaar, Emma Y. Lenander et al. (+1 more)

A new component for the accurate simulation of neutron scattering from magnetic excitations has been developed for the neutron ray-tracing software McStas. The component SpinWave_BCO simulates inelastic neutron scattering from ferro-, antiferro-, and altermagnetic excitations in a body-centered orthorhombic crystal structure, where the dispersion relation and scattered neutron intensities are derived using linear spin wave theory. Data from a simulated Triple-Axis Spectrometer with an extremely high resolution have been verified by direct comparison with theory and by comparison to data simulated using the package SpinW. The component serves as a proof-of-concept for the implementation of a more general linear spin wave component in McStas.
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All authors Silas B. Schack, Kristine M. L. Krighaar, Emma Y. Lenander, Kim Lefmann

Materials Science

Screening novel cathode materials from the Energy-GNoME database using MACE machine learning force field and DFT

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

The development of new battery materials, particularly novel cathode chemistries, is essential for enabling next generation energy storage technologies. In this work, we employ a multi-fidelity screening protocol combining the Energy-GNoME confident criteria, foundational MACE machine-learning force fields (MLFF), and physically motivated heuristic filters to identify novel intercalation cathodes for post-lithium batteries, namely: Na-, K-, Mg-, and Ca-ion batteries. Foundational MACE models are used to efficiently asses dynamical stability, thermodynamical stability, average voltage, and theoretical specific energy, enabling a rapid screening of candidates. For the most promising cathodes, voltage predictions are refined using DFT+U calculations. This work delivers three key outcomes: i) establishing and validating a robust high-throughput screening approach for cathode materials with foundational MLFF models; ii) suggestions for cathode candidates for the development of next-generation of batteries; iii) a fair comparison between the MACE predictions and the readily available figures of merit reported in the Energy-GNoME database on the examined materials.
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All authors Nada Alghamdi, Paolo de Angelis, Pietro Asinari, Eliodoro Chiavazzo

Materials Science

Reststrahlen band and optical bandgaps in semiconducting CrN films

Duc V. Dinh, Xiang Lü, Oliver Brandt et al. (+8 more)

We present a comprehensive optical characterization of 200-nm-thick CrN(111) films grown simultaneously on Al$_2$O$_3$(0001) and AlN/Al$_2$O$_3$(0001) using plasma-assisted molecular beam epitaxy. Spectroscopic ellipsometry, spanning the far-infrared to ultraviolet range (0.04 - 5.5 eV), is conducted at room temperature to determine the optical constants $n$ and $k$ of the films. Spectral fits reveal two interband transitions at approximately 0.35 and 0.60 eV. In the infrared range, the ellipsometry data also reveals a pronounced Reststrahlen band stemming from transversal and longitudinal optical phonons at approximately 403 and 629 cm$^{-1}$, respectively. The relative static and high-frequency permittivities are estimated to be about 39 and 15, respectively. A Born effective charge of approximately 2.7, extracted from the far-infrared region, indicates that CrN is partially ionic.
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All authors Duc V. Dinh, Xiang Lü, Oliver Brandt, Dilara Sen, Olivia Fairlamb, Frank Peiris, Farihatun Lima, Alexander Bordovalos, Suresh Chaulagain, Ambalanath Shan, Nikolas J. Podraza

Materials Science · Cross-listed from physics.optics

Integration of 2D Materials in Radial van der Waals Heterostructure Metasurfaces

Connor Heimig, Jonas Biechteler, Cristina Cruciano et al. (+12 more)

Two-dimensional semiconductors, such as monolayer transition metal dichalcogenides (TMDC), exhibit strong excitonic transitions at room temperature and offer a unique platform for exploring light-matter interactions in nanoscale photonic systems. In this work, we demonstrate a compact and polarization-invariant photonic metasurface, fabricated from hexagonal boron-nitride (hBN) and based on radial bound states in the continuum (BIC), which are formed by radially distributed pairs of structurally asymmetric resonators. The metasurface employs multiple symmetry-breaking perturbations to support high quality-(Q-)factor resonances within a footprint smaller than 8 x 8 $\mu m^2$ - one-sixth of the area of previous approaches. Compared to established hBN metasurface designs, the radial geometry furthermore achieves significantly higher Q-factors with a reduced footprint. By integrating the hBN photonic structure with a WS$_2$ monolayer, we observe enhanced photoluminescence when its resonance is spectrally aligned with the exciton resonance, accompanied by signatures of discrete momentum-space patterns that identify the orbital-angular-momentum-carrying ring eigenmodes. These features persist over a wide range of excitation powers and show minimal linewidth broadening, indicating robust and spatially modulated exciton-photon coupling. This work establishes a scalable approach for generating hybrid photonic-excitonic states with momentum-space structure, offering new opportunities for exciton localization, valley emission, spatially programmable light-matter interaction in two-dimensional material platforms and compact luminescent devices based on 2D material-integrated metasurfaces.
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All authors Connor Heimig, Jonas Biechteler, Cristina Cruciano, Armando Genco, Thomas Weber, Michael Hirler, Dmytro Gryb, Alexander A. Antonov, Leonardo de S. Menezes, Gianluca Valentini, Cristian Manzoni, Giulio Cerullo, Stefan A. Maier, Luca Sortino, Andreas Tittl

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

Is the atomic quadrupole moment of a carbon atom in graphene zero?: The case for a rational definition of the properties of atoms in a molecule

Devin M. Mulvey, Kenneth D. Jordan and Alston J. Misquitta

It is generally assumed that the carbon atoms of graphitic samples and their finite analogs have sizable quadrupole moments, with the out-of-plane component ($Q^{\rm C}_{20}$ in traceless spherical coordinates) being the dominate contribution. However, there is no consensus on what the quantity is for such carbon-based systems and values reported in the literature range from $Q^{\rm C}_{20} \sim -1.14$ to $+0.79$ a.u. In this work we propose a theoretical framework in which well-defined statements can be made about properties of atoms-in-a-molecule (AIMs) even when these properties are not experimentally observable. Using this framework and the distributed multipole method basis-space iterated Stockholder atoms (BS-ISA), we show that the atomic quadrupole moment of a carbon atom in graphene is essentially zero within the limits of precision of the numerical method used. We explain how the experimentally measured atomic quadrupole moment of a graphite sample determined by Whitehouse & Buckingham likely originated almost entirely from edge dipoles, and we propose a more realistic electrostatic model for finite graphene nanoflakes.
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Materials Science

Symmetry-Breaking Phenomena in MnPS3/TMDC Heterostructures: Non-relativistic Spin Splitting, Altermagnetism and Spin-Valley Effects

Kamil Wrzos, Magdalena Birowska and Milosz Rybak

We explore symmetry-breaking phenomena in MnPS3/TMDC (MoS2, WS2, MoSe2, WSe2) heterostructures using first-principles calculations, considering two high-symmetry stacking configurations, S1 and S2, which differ not only by their interfacial registry but also by a 30{\deg} twist between the layers. Depending on the stacking geometry, the systems exhibit two distinct types of nonrelativistic spin splitting (NRSS): S2 hosts altermagnetic-like band crossings, while S1 shows global spin splitting characteristic of symmetry-breaking NRSS. Magnetic exchange and anisotropy parameters indicate that the intrinsic magnetic properties of MnPS3 are largely preserved upon interfacing. Including spin-orbit coupling, we find tunable conduction-valley splitting controlled by the MnPS3 spin orientation. Our results identify MnPS3 as a symmetry-tunable antiferromagnetic substrate capable of inducing and controlling spin and valley effects in 2D heterostructures without relying on net magnetization or strong SOC, offering a route toward nonvolatile valleytronic functionalities.
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Materials Science · Cross-listed from cond-mat.soft

Complex network analysis of pore structures in monodisperse granular materials with varied grain shapes

Jie Qi, Wenbin Fei and Guillermo A. Narsilio

Understanding how pore structure influences flow and transport behaviour in granular materials is essential for addressing a wide range of geotechnical, hydraulic, and environmental challenges. These processes are largely shaped by the microscopic arrangement of particles and interconnections between pores within the material. However, detailed insights considering granular assemblies with diversified grain shapes remain scarce. This study introduces a comprehensive framework incorporating Discrete Element Method (DEM) simulations, image processing, pore-network modelling, and complex network theory to investigate the links between particle morphology and their hydraulic behaviours. Mono-disperse assemblies of natural sand particles with varied shapes are constructed in DEM, and pore networks are extracted through image processing and pore-network modelling. Complex network analysis is then applied to calculate structural metrics that reveal intrinsic relationships between pore microstructures and hydraulic properties. Our results demonstrate that particle morphology significantly impacts pore network characteristics, including pore and throat sizes, closeness centrality, pore structure anisotropy, providing valuable insights into how pore structure influence transport properties.
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Materials Science

Patterning perovskite colour converters for AR/VR microdisplays

Ruairi Baker, Maria Pervez, Angus Hawkey et al. (+3 more)

Colour conversion offers the clearest path to achieve RGB colours in high resolution microdisplays for AR/VR. With resolutions beyond 5000 ppi (i.e. RGB pitch of 5 um), the thickness of the conversion layers is critical for efficiency and manufacturing. Perovskites outperform other conversion materials (quantum dots or phosphors) with their high absorption coefficients for blue light. In this contribution, we show how perovskite materials, engineered for high optical density and colour purity, can be patterned to produce colour converting pixels. We demonstrate patterning using three approaches (lift-off, negative photoresist and dry etch), and discuss their advantages and disadvantages. The results consolidate the choice of perovskites for AR/VR applications by demonstrating their robustness and compatibility with multiple patterning strategies suitable for high resolution microdisplays.
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All authors Ruairi Baker, Maria Pervez, Angus Hawkey, Nobuya Sakai, Valerie Berryman-Bousquet, Bernard Wenger

Materials Science · Cross-listed from physics.optics

2D MoS$_2$/Au interfaces for enhanced opto-electronic response with sub-bandgap photons

Jinlei Wu, Wei Huang, Ahsan Ali et al. (+5 more)

Monolayer MoS$_2$ is a direct band gap semiconductor with potential applications in optoelectronics and photonics. MoS$_2$ also has a large optical nonlinearity. However, the atomic thickness of the monolayer limits the strength of the measured functional signals, such as the photocurrent or photoluminescence, in optoelectronic devices. Here, we show that photocurrent in monolayer MoS$_2$ can be induced by sub-band gap photons by depositing Au nanoparticles on it. In this system, the nonlinear light-matter interaction in Au nanoparticles enhanced by the localized surface plasmons results in the generation of supercontinuum, which is reabsorbed by MoS$_2$ due to efficient resonant energy transfer. Au nanoparticle assisted photocurrent is more than an order of magnitude larger than two-photon photocurrent in monolayer MoS$_2$. Optimization of the shape, size and composition of the nanoparticle has the potential to enhance the photocurrent significantly with the prospect of applications in the detection of NIR photons, and related technologies including optical telecommunication.
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All authors Jinlei Wu, Wei Huang, Ahsan Ali, Yuhong Chen, Elissaios Stavrou, Zuoti Xie, Jinhui Zhong, Khadga Jung Karki

Materials Science

Unraveling UV Stability in Metal Halide Perovskites: From Degradation Mechanisms to Molecular Passivation

Xin Wen, Zhiyi Yao, Wenzhuo Li et al. (+2 more)

Understanding the mechanisms of UV-induced degradation is crucial for enhancing the UV stability of perovskite solar cells. The UV-driven structural dynamics of CH3NH3PbI3 (MAPbI3) are investigated using real-time TDDFT simulations, revealing that under the electron and hole excitation, the distortion of the inorganic framework (PbI) is primarily driven by the electron occupation of Pb-p and I-p antibonding states, whereas in the hole case, it is mainly governed by the direct cooling induced distortion. We also find that UV accelerates the rotation of MA+ molecules. Further, a BDO molecule is introduced as a passivant, which suppresses structural distortions and provides multi-phonon channels to dissipate carrier cooling energy. Experimental results confirm the UV-protective role of BDO, with suppressed PbI2 formation and improved device stability. These results clarify the mechanism of the UV-induced degradation in the MAPbI3 perovskite and further elucidate how passivation molecules enhance UV stability.
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All authors Xin Wen, Zhiyi Yao, Wenzhuo Li, Zhijun Ning, Fan Zheng

Materials Science

Scalable Synthesis of Large-Area WS2 Thin Films from Tungsten Precursors by Thermal CVD and Their Application in Schottky-Barrier Solar Cells

Jisun Kim, Yoosuk Kim, Seung-Ho Park et al. (+2 more)

Two-dimensional tungsten disulfide (WS2) is a promising semiconductor for next-generation optoelectronic and photovoltaic devices, but scalable routes to uniform, large-area films remain challenging. In this study, a systematic thermal chemical vapor deposition (T-CVD) strategy is presented to synthesize centimeter-scale WS2 thin films by sulfurizing tungsten (W) precursors in a controlled sulfur vapor environment. High-purity sputtered W thin films on SiO2/Si and W foils were sulfurized at temperatures between 400 and 1000 C, with Raman spectroscopy identifying 800 C as the optimal growth temperature. Under these conditions, the films exhibit the characteristic E12g (349.7 cm-1) and A1g (416.8 cm-1) modes with narrow full-width at half-maximum values, indicative of high crystallinity and controlled thickness. Optical microscopy, scanning electron microscopy, atomic force microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy collectively validate the creation of stoichiometric, layered WS2 with significantly enhanced uniformity and diminished roughness when utilizing sputtered W thin films in contrast to W foils. Leveraging this optimized process, WS2 films grown on W foils were transferred onto target substrates, including indium tin oxide (ITO)-coated glass, using a PMMA-assisted wet-transfer method that preserves structural integrity over large areas. Schottky-barrier solar cells with an Au/WS2/ITO architecture fabricated from these films deliver a short-circuit current density of 7.91 mA cm-2, an open-circuit voltage of 0.495 V, and a power conversion efficiency of 1.45 percent. These results demonstrate that sulfurization of W thin films and foils via T-CVD provides a scalable, substrate-compatible platform for integrating WS2 into practical optoelectronic and low-cost photovoltaic technologies.
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All authors Jisun Kim, Yoosuk Kim, Seung-Ho Park, Yong Hun Ko, Chong-Yun Park

Materials Science

Collective Magnetic Excitations in a Photo-excited Electron-doped Cuprate Superconductor

Daniel Jost, Jiarui Li, Jordyn Hales et al. (+21 more)

Elucidating the microscopic behavior of cuprates under ultrafast photoexcitation offers critical insights into their highly correlated out-of-equilibrium states. Although quasiparticle dynamics have been investigated extensively, the behavior of collective magnetic excitations remains comparatively unexplored. Here, we use time-resolved resonant inelastic X-ray scattering (trRIXS) at the Cu $L_3$-edge to track the collective magnetic excitations (paramagnons) in an optimally electron-doped cuprate driven out-of-equilibrium by a femtosecond pump laser pulse. Upon pumping, we observed an anti-Stokes signal associated with paramagnon generation, which modifies the paramagnon dispersion near the zone center, although the bandwidth remained unchanged. Moreover, the spectral weight exhibits a momentum-dependent variation across the Brillouin zone. The light-driven boost of the paramagnon population and the resulting spectral-weight transfer could provide new leverage to manipulate the properties of cuprates.
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All authors Daniel Jost, Jiarui Li, Jordyn Hales, Jonathan Sobota, Giacomo Merzoni, Leonardo Martinelli, Shuhan Ding, Kejun Xu, Justine Schlappa, Andreas Scherz, Robert Carley, Benjamin E. Van Kuiken, Teguh C. Asmara, Le Phuong Hoang, Laurent Mercadier, Sergii Parchenko, Martin Teichmann, Patrick S. Kirchmann, Giacomo Ghiringhelli, Brian Moritz, Zhi-Xun Shen, Thomas P. Devereaux, Yao Wang, Wei-Sheng Lee

Materials Science

Electric Current Control of Helimagnetic Chirality from a Multidomain State in the Helimagnet MnAu$_2$

Yuta Kimoto, Hidetoshi Masuda, Jun-ichiro Ohe et al. (+3 more)

In this paper, we study the domain wall dynamics under electric current in the helimagnet MnAu$_2$. We have found that the threshold electric current of the transition from a multidomain state to a single-chiral domain state in a magnetic field is much lower than that of chirality reversal from a single-chiral domain within certain ranges of temperature and magnetic field. The chirality after the transition depends on whether the magnetic field and electric current were parallel or antiparallel. Numerical calculations based on the Landau-Lifshitz-Gilbert equation reproduced the experimental observations. These results indicate that the domain walls are highly mobile in the helimagnet.
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All authors Yuta Kimoto, Hidetoshi Masuda, Jun-ichiro Ohe, Shoya Sakamoto, Takeshi Seki, Yoshinori Onose

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

Evidence for Anion-Free-Electron Duality and Enhanced Superconducting Role of Interstitial Anionic Electrons in Electrides

Zhao Liu, Xiang Wang, Yin Yang et al. (+10 more)

The discovery of superconducting electrides, characterized by interstitial anionic electrons (IAEs) residing in lattice cavities, has established a distinctive platform for investigating superconductors. Yet the superconducting origin and the fundamental role of IAEs in Cooper pairing formation remain poorly understood due to the challenges in directly observing IAEs. Here, combining angle-resolved photoemission spectroscopy (ARPES), transport measurements, and first-principles calculations, we certify that the IAEs in electride La3In (Tc = 9.4 K) exhibit a dual nature as both anions and free electrons. With the finite-depth potential well model, we trace that IAEs originate from electronic states near the Fermi level located above potential barriers, forming a Fermi sea susceptible to scattering by La-derived phonons, triggering superconductivity. ARPES combined with high-resolution XRD measurements on oxygen-treated samples directly reveals IAEs' spatial distribution and energy dispersion from interstitial sites with the consistent energy value predicted by our theory model. The concomitant diminution of free electrons upon oxygen treatment, leading to a marked reduction in superconductivity, further provides compelling experimental evidence that IAEs actively participate in electron-phonon coupling. Our findings resolve the long-standing ambiguity regarding the electronic nature of IAEs, elucidate their enhancing superconductivity in the phonon-mediated mechanism, and provide a foundation for exploring advanced electride-based superconductors.
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All authors Zhao Liu, Xiang Wang, Yin Yang, Pengcheng Ma, Zhijun Tu, Xinyu Wang, Donghan Jia, Wenju Zhou, Huiyang Gou, Hechang Lei, Qiang Xu, Zhonghao Liu, Tian Cui

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

Pervasive electronic nematicity as the parent state of kagome superconductors

Muxian Xu, Siyu Cheng, Andrea Capa Salinas et al. (+10 more)

Kagome superconductors $A$V$_3$Sb$_5$ ($A$ = Cs, K, Rb) have developed into an exciting playground for realizing and exploring exotic solid state phenomena. Abundant experimental evidence suggests that electronic structure breaks rotational symmetry of the lattice, but whether this may be a simple consequence of the symmetry of the underlying 2 $\times$ 2 charge density wave phase or an entirely different mechanism remains intensely debated. We use spectroscopic imaging scanning tunneling microscopy to explore the phase diagram of the prototypical kagome superconductor CsV$_3$Sb$_5$ as a function of doping. We intentionally suppress the charge density wave phase with chemical substitutions selectively introduced at two distinct lattice sites, and investigate the resulting system. We discover that rotational symmetry breaking of the electronic structure – now present in short-range nanoscale regions – persists in all samples, in a wide doping range long after all charge density waves have been suppressed. As such, our experiments uncover ubiquitous electronic nematicity across the $A$V$_3$Sb$_5$ phase diagram, unrelated to the 2 $\times$ 2 charge density wave. This further points towards electronic nematicity as the intrinsic nature of the parent state of kagome superconductors, under which other exotic low-temperature phenomena subsequently emerge.
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All authors Muxian Xu, Siyu Cheng, Andrea Capa Salinas, Ganesh Pokharel, Alexander LaFleur, Hong Li, Hengxin Tan, Brenden R. Ortiz, Qinwen Deng, Binghai Yan, Ziqiang Wang, Stephen D. Wilson, Ilija Zeljkovic

Wednesday 26 November

Materials Science

Superionicity in Ammonium Polyhydrides at Extreme Pressures

Kyla de Villa, Xiaoyu Wang, Eva Zurek et al. (+1 more)

Polyhydrides have been shown to form novel structures at high pressure, which may be found in the interiors of giant planets. With density functional molecular dynamics simulations we studied the behavior of ammonium polyhydride compounds with stoichiometries of NH$_7$, NH$_9$, NH$_{10}$, NH$_{11}$, NH$_{14}$, NH$_{20}$, and NH$_{24}$ which were predicted with crystal structure search methods to be metastable at 100-300~GPa. For every compound, we performed simulations at a range of temperatures (and for several compounds, pressures) covering the solid, superionic and liquid phases. We show that when heated, high pressure ammonium polyhydride compounds exhibit hydrogen superionic diffusion. We demonstrate a number of metrics by which the solid-to-superionic and superionic-to-liquid transitions can be detected from simulation data, including changes in the internal energy and pressure, formation of new chemical species, and atomic diffusion rates. We find that both the solid-to-superionic and the superionic-to-liquid transitions decrease in temperature as proton fraction increases. These trends indicate that above a proton fraction of $\sim$0.97, ammonium hydride structures are likely to directly melt instead of first exhibiting a superionic phase. Our observed melting trend further indicates that at the extreme conditions of ice giant interiors, hydrogen rich ammonium hydrides such as those studied in this work would exist predominantly as liquids rather than exhibiting a superionic phase.
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All authors Kyla de Villa, Xiaoyu Wang, Eva Zurek, Burkkhard Militzer

Materials Science

Anomalous spin-lattice coupling in a 2D antiferromagnetic semiconductor revealed by surface acoustic Rayleigh waves

Zahra Ebrahim Nataj, Md. Sabbir Hossen Bijoy, Vladislav Korostelev et al. (+5 more)

Magnetic order in van der Waals magnets can strongly influence their lattice dynamics, yet how this interaction manifests across different phonon length scales remains unclear. Optical phonons probe bond-scale exchange modulation and short-range spin correlations, whereas long-wavelength acoustic modes couple to uniform strain fields and are sensitive to the renormalization of the macroscopic elastic tensor associated with long-range magnetic order. Experimentally accessing these low-energy acoustic excitations in low-dimensional crystals is challenging due to their low energies and the small lateral dimensions of exfoliated samples. Here, we employ angle-resolved Brillouin-Mandelstam scattering spectroscopy to investigate the surface acoustic phonon spectrum of exfoliated NiPS3 thin films across their antiferromagnetic transition temperature. Our results show a single Rayleigh surface mode whose phase velocity exhibits a pronounced 5.5% softening upon cooling through the Neel temperature. This anomaly reflects a giant magnetoelastic renormalization of the long-wavelength elastic constants triggered by the onset of zigzag antiferromagnetic order. First-principles calculations of the full elastic tensor, combined with continuum finite-element modelling of the NiPS3/SiO2/Si heterostructure, reproduce both the Rayleigh-wave dispersion and its magnetic-order-induced shift. The obtained results reveal how microscopic exchange interactions shape macroscopic mechanical properties in two-dimensional antiferromagnetic semiconductors, providing a basis for lattice-controlled magnetism and magnetically tunable phononic, magnonic, and strain-mediated spintronic device concepts.
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All authors Zahra Ebrahim Nataj, Md. Sabbir Hossen Bijoy, Vladislav Korostelev, Dylan Wright, Mohammad Zeinolabedini, Konstantin Klyukin, Fariborz Kargar, Alexander A. Balandin

Materials Science

Demonstration of the ODMR activity of the telecom range ClV center in SiC: a wavefunction theory analysis

Zsolt Benedek, Oscar Bulancea-Lindvall, Joel Davidsson et al. (+2 more)

Recently, density functional theory-based high-throughput screening of point defects in 4H-SiC revealed the positively charged chlorine-vacancy (ClV) defect to be a promising quantum bit candidate emitting at telecom wavelengths, with an electronic structure analogous to the well-known NV center in diamond. Furthermore, recent infrared photoluminescence (PL) measurements on chlorine-implanted 4H-SiC have revealed new PL lines associated with the ClV defect. While the defect possesses a high-spin ground state, there is a lack of evidence of optically detected magnetic resonance (ODMR), a key ingredient for optical spin initialization and readout. In this Letter, we employ a multireference wavefunction-based quantum chemistry method, specifically, second-order perturbation theory (NEVPT2) on top of a defect-localized many-body wavefunction (CASSCF), to explore the many-body electronic structure of the ClV center. We estimate photoluminescence, internal conversion, and intersystem crossing rates to investigate the possibility of spin polarization and ODMR activity. Our findings establish the ClV center in 4H-SiC as an optically addressable spin qubit with fiber optics compatibility in the technologically mature 4H-SiC host material, enabling the development of large-scale quantum networks.
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All authors Zsolt Benedek, Oscar Bulancea-Lindvall, Joel Davidsson, Viktor Ivády, Igor Abrikosov

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

On the Generation of Phononic Frequency Combs Using Defect Modes of Phononic Crystals

Suhas Suresh Bharadwaj, Murtaza Rangwala and Adarsh Ganesan

This paper proposes a method for generating phononic frequency combs (PFCs) using defect-localized modes in a two-dimensional hexagonal phononic crystal. Localized vibration modes from a singular point defect produce evenly spaced spectral lines corresponding to PFCs. Numerical modelling reveals robust energy transfer under a single-tone drive, generating spectral sidebands. These results demonstrate defect engineering in phononic crystals as a tunable platform for PFC generation with significant applications in high-resolution sensing, timing, and quantum-acoustic technologies.
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Materials Science

EPW-VASP interface for first-principles calculations of electron-phonon interactions

Danylo Radevych, Aidan Thorn, Manuel Engel et al. (+5 more)

We present an interface between the Vienna Ab initio Simulation Package (VASP) and the EPW software for calculating materials properties governed by electron-phonon (e-ph) interactions. Computation of the e-ph matrix elements with the finite-difference supercell approach in VASP and their fine-grid interpolation in EPW enable accurate modeling of temperature-dependent materials properties and phonon-assisted quantum processes with VASP's extensive library of exchange-correlation functionals and pseudopotentials. We demonstrate the functionality of the EPW-VASP interface by examining the superconducting gap and critical temperature in MgB$_2$ using the anisotropic Migdal-Eliashberg equations, and the carrier mobility in cubic BN using the ab initio Boltzmann transport equation.
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All authors Danylo Radevych, Aidan Thorn, Manuel Engel, Aleksey N. Kolmogorov, Sabyasachi Tiwari, Georg Kresse, Feliciano Giustino, Elena R. Margine

Materials Science

Perspective: Magnon-magnon coupling in hybrid magnonics

Wei Zhang, Yuzan Xiong, Jia-Mian Hu et al. (+10 more)

The internal coupling of magnetic excitations (magnons) with themselves has created a new research sub-field in hybrid magnonics, i.e., magnon-magnon coupling, which focuses on materials discovery and engineering for probing and controlling magnons in a coherent manner. This is enabled by, one, the abundant mechanisms of introducing magnetic interactions, with examples of exchange coupling, dipolar coupling, RKKY coupling, and DMI coupling, and two, the vast knowledge of how to control magnon band structure, including field and wavelength dependences of frequencies, for determining the degeneracy of magnon modes with different symmetries. In particular, we discuss how magnon-magnon coupling is implemented in various materials systems, with examples of magnetic bilayers, synthetic antiferromagnets, nanomagnetic arrays, layered van der Waals magnets, and (DMI SOT materials) in magnetic multilayers. We then introduce new concept of applications for these hybrid magnonic materials systems, with examples of frequency up/down conversion and magnon-exciton coupling, and discuss what properties are desired for achieving those applications.
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All authors Wei Zhang, Yuzan Xiong, Jia-Mian Hu, Joseph Sklenar, Mitra Mani Subedi, M.Benjamin Jungfleisch, Vinayak S. Bhat, Yi Li, Luqiao Liu, Qiuyuan Wang, Yunqiu Kelly Luo, Youn Jue Bae, Benedetta Flebus

Materials Science

Effect of Magneto-Mechanical Synergism in the Process-Structure Correlation in Fe-C Alloys: A Phase-Field Modeling Approach

Soumya Bandyopadhyay, Sourav Chatterjee, Dallas R. Trinkle et al. (+4 more)

Applied magnetic fields can alter phase equilibria and kinetics in steels; however, quantitatively resolving how magnetic, chemical, and elastic driving forces jointly influence the microstructure remains challenging. We develop a quantitative magneto-mechanically coupled phase-field model for the Fe-C system that couples a CALPHAD-based chemical free energy with demagnetization-field magnetostatics and microelasticity. The model reproduces single- and multi-particle evolution during the alpha to gamma inverse transformation at 1023 K under external fields up to 20 T, including ellipsoidal morphologies observed experimentally at 8 T. Chemically driven growth is isotropic; a magnetic interaction introduces an anisotropic driving force that elongates gamma precipitates along the field into ellipsoids, while elastic coherency promotes faceting, yielding elongated cuboidal or “brick-like" particles under combined magneto-elastic coupling. Growth kinetics increase with C content, and decrease with field strength and misfit strain. Multi-particle simulations reveal dipolar interaction-mediated coalescence for field-parallel neighbors and ripening for field-perpendicular neighbors. Incorporating field-dependent diffusivity from experiment slows kinetics as expected; a first-principles-motivated anisotropic diffusivity correction is estimated to be small (<2%). These results establish a process-structure link for magnetically assisted heat treatments of Fe-C alloys and provide guidance for microstructure control via chemo-magneto-mechanical synergism.
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All authors Soumya Bandyopadhyay, Sourav Chatterjee, Dallas R. Trinkle, Richard G. Hennig, Victoria Miller, Michael S. Kesler, Michael R. Tonks

Materials Science

IRSSG: An Open-Source Software Package for Spin Space Groups

Sheng Zhang, Ziyin Song, Zhong Fang et al. (+2 more)

We present an open-source software package IRSSG for investigating magnetic systems with spin space groups (SSGs). The package works within the density functional theory (DFT) framework and requires wavefunctions from DFT codes, such as VASP, Quantum ESPRESSO, as well as any other code that has an interface to Wannier90. We introduce a set of compact SSG international symbols by combining non-crystallographic point groups with the 230 crystallographic space groups. The program first identifies all SSG operations and determines the SSG international symbol for a given magnetic system. It then generates the SSG character tables of little groups at any $k$ point. Finally, it computes the traces of matrix representations of SSG operations and assigns irreducible corepresentation labels to magnetic energy bands. The program is not only timely but also essential for advancing research on the study of magnons, altermagnetism, magnetic topology, and novel high-degeneracy excitations in SSG systems.
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All authors Sheng Zhang, Ziyin Song, Zhong Fang, Hongming Weng, Zhijun Wang

Materials Science

On the generalized Keffer form of the Dzyaloshinskii constant: its consequences for the spin, momentum and polarization evolution

Pavel A. Andreev

Different analytical features of the Dzyaloshinskii-Moriya interaction are related to different contributions to the Dzyaloshinskii constant in the microscopic Hamiltonian. Consequences appear in the macroscopic Landau–Lifshitz–Gilbert equation. It leads to various phenomena. Three contributions to the Dzyaloshinskii constant are reviewed and combined in the generalized Keffer form of the Dzyaloshinskii constant. The fourth possible form of the Dzyaloshinskii constant is suggested as well. Macroscopic consequences of these three mechanisms are well-known, but further possible generalizations of the Keffer form of the Dzyaloshinskii constant are suggested. Consequences for the spin evolution equations, the momentum balance equations, and polarization evolution equations are considered. Some analog of the Keffer form is suggested for the exchange integral in symmetric Heisenberg Hamiltonian demonstrating the nontrivial contribution of the ligands in this regime.
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Materials Science · Cross-listed from cond-mat.mes-hall

Phonon-tunable THz magnonic emission in multiferroic heterostructures

Sylvain Massabeau, Amr Abdelsamie, Florian Godel et al. (+13 more)

Collective excitations such as magnons and polar phonons provide natural access to the terahertz (THz) regime, but efficient generation and tunability remain elusive. Multiferroic BiFeO3 combines both orders at room temperature, offering a unique platform for narrowband THz emission. Here, we achieve efficient sub-bandgap optical rectification of coupled phonon-polaritons near 2 THz in bare epitaxial thin films. In Pt/BiFeO3 bilayers, we demonstrate that coupling the electromagnon branch with ultrafast strain waves, optically generated in Pt layers with various thicknesses, can produce tunable and narrowband emission between 0.4-0.8 THz. These results uncover the intertwined role of phonons, magnons, and magneto-acoustic dynamics in antiferromagnetic multiferroics, and establish these hybrid platforms as versatile engineered narrowband THz sources.
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All authors Sylvain Massabeau, Amr Abdelsamie, Florian Godel, Filip Miljevic, Noela Rezi, Pascale Gemeiner, Karim Bouzehouane, Thomas Buttiens, Sukhdeep Dhillon, Thomas Maroutian, Jean-Marie George, Henri Jaffres, Brahim Dkhil, Stephane Fusil, Vincent Garcia, Romain Lebrun

Materials Science · Cross-listed from physics.optics

High resolution 3D imaging of diamonds with multiphoton microscopy

Elana G. Alevy, Samuel D. Crossley, Lam T. Nguyen et al. (+2 more)

Diamonds offer unique benefits for optical technology development due to their optical, chemical, electrical, mechanical, and thermal properties. These attributes also contribute to their aesthetic appeal, high commercial value, and utility in geological studies. Thus, there is high demand for nondestructive techniques that enable rapid analysis of natural and synthetic diamonds as well as diamond-like simulants. Here, we demonstrate sub-micrometer, nondestructive, three-dimensional imaging and spectral analysis of diamonds using multiphoton microscopy (MPM). This approach stimulates nonlinear optical emissions to provide unique insights into the interior structure, fluorescent defects, and formational conditions of diamonds. As a result, MPM can be used to investigate gemstone quality, vacancy centers used in quantum technologies, and the various inclusions and fluorescent emitters that may trace gemstone provenance and treatment history.
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All authors Elana G. Alevy, Samuel D. Crossley, Lam T. Nguyen, Vu D. Phai, Khanh Q. Kieu

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

Symmetries of excitons

Muralidhar Nalabothula, Davide Sangalli, Fulvio Paleari et al. (+2 more)

Excitons, bound electron-hole pairs, are responsible for strong optical resonances near the bandgap in low-dimensional materials and wide-bandgap insulators. Although current ab initio methods can accurately determine exciton energies and eigenstates, their symmetries have been much less explored. In this work, we employ standard group-theory methods to analyse the transformation properties of excitonic states, obtained by solving the BSE, under crystal symmetry operations. We develop an approach to assign irreducible-representation labels to excitonic states, providing a state-of-the-art framework for analysing their symmetries and selection rules (including, for example, the case of exciton-phonon coupling). Complementary to the symmetry classification, we introduce the concept of total crystal angular momentum for excitons in the presence of rotational symmetries, allowing the derivation of conservation laws. Furthermore, we demonstrate how these symmetry properties can be exploited to greatly enhance the computational efficiency of exciton calculations with the BSE. We apply our methodology to three prototypical systems to understand the role of symmetries in different contexts: (i) For LiF, we present the symmetry analysis of the entire excitonic dispersion and examine the selection rules for optical absorption. (ii) In the calculation of resonant Raman spectra of monolayer MoSe2, we demonstrate how the conservation of total crystal angular momentum governs exciton-phonon interactions, leading to the observed resonant enhancement. (iii) In bulk hBN, we analyze the role of symmetries for the coupling of finite-momentum excitons to finite-momentum phonons and their manifestation in the phonon-assisted luminescence spectra. This work establishes a general and robust framework for understanding the symmetry properties of excitons in crystals, providing a foundation for future studies.
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All authors Muralidhar Nalabothula, Davide Sangalli, Fulvio Paleari, Sven Reichardt, Ludger Wirtz