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Physics Condensed Matter Materials Science

cond-mat.mtrl-sci Watch Materials Science

Monday 6 October

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

Surface Acoustic Wave Gas Sensors: Innovations in Functional Materials, Sensing Dynamics, and Signal Analysis

Suman Acharya, Balasubramanian Srinivasan, David Shanahan et al. (+3 more)

Surface Acoustic Wave gas sensors have garnered increasing attention as highly sensitive, miniaturized, and wireless compatible platforms for molecular detection. Their unique ability to convert surface perturbations into measurable acoustic shifts makes them ideal for gas sensing across diverse environments. This review synthesizes reported SAW platforms across substrates and modes Rayleigh, SH-SAW, Love links transduction pathways to material choice, and benchmarks performance for key analytes, e.g., NO2, NH3, VOCs, CO2, etc. We catalogue nanostructured oxides, polymers, carbon based films, and hybrid heterojunction coatings, highlighting attributes such as porosity, surface chemistry, and interfacial charge transfer that govern sensitivity and reversibility. We also highlight the emerging use of SAW devices to probe adsorption desorption dynamics, offering analyte specific interaction signatures beyond equilibrium, offering a new perspective into analyte specific interaction pathways. Additionally, the integration of machine learning is discussed as a transformative tool for signal decoding, environmental compensation, and adaptive calibration. We also identify key challenges, cross sensitivity, signal drift, material degradation, and deployment at the edge and review recent strategies to address them. Looking ahead, we envision the evolution of SAW platforms into intelligent, autonomous sensing systems with applications in environmental monitoring, industrial process control, and healthcare diagnostics.
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All authors Suman Acharya, Balasubramanian Srinivasan, David Shanahan, Utz Roedig, Alan O Riordan, Veda Sandeep Nagaraja

Materials Science

Atomistic Insights into the Degradation of Metal Phthalocyanine Catalysts during Oxygen Reduction Reaction

Huanhuan Yang and Guangfu Luo

Oxygen reduction catalysts frequently suffer from degradation under harsh operating conditions, and the limited understanding of the underlying mechanisms hampers the development of effective mitigation strategies. In this study, we integrate first-principles calculations with a time-dependent microkinetic model to investigate the deactivation pathways of six highly active metal phthalocyanines (MPc, M = Cr, Mn, Fe, Ru, Rh, and Ir) during the oxygen reduction reaction (ORR). We quantitatively assess the ORR processes, hydrogen peroxide generation, radical generation, and three primary degradation mechanisms, namely carbon oxidation, nitrogen protonation, and demetallation, through a reaction network involving 40 chemical species and 75 elementary reactions. Our findings reveal that the dominant degradation mechanism varies significantly across the MPcs. Under typical alkaline conditions, the primary byproducts arise from carbon oxidation, driven by .OH radical attack and structural reorganization of surface adsorbates, and from protonation at either the metal center or nitrogen sites. In the kinetics-controlled region, the ORR activity follows the order of RhPc > IrPc > FePc > MnPc > RuPc > CrPc. Notably, RhPc and IrPc demonstrate both higher ORR activity and greater stability than the widely studied FePc under elevated potentials.
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Materials Science

Investigating Anharmonicities in Polarization-Orientation Raman Spectra of Acene Crystals with Machine Learning

Paolo Lazzaroni, Shubham Sharma and Mariana Rossi

We present a first-principles machine-learning computational framework to investigate anharmonic effects in polarization-orientation (PO) Raman spectra of molecular crystals, focusing on anthracene and naphthalene. By combining machine learning models for interatomic potentials and polarizability tensors, we enable efficient, large-scale simulations that capture temperature-dependent vibrational dynamics beyond the harmonic approximation. Our approach reproduces key qualitative features observed experimentally. We show, systematically, what are the fingerprints of anharmonic lattice dynamics, thermal expansion, and Raman tensor symmetries on PO-Raman intensities. However, we find that the simulated polarization dependence of Raman intensities shows only subtle deviations from quasi-harmonic predictions, failing to capture the pronounced temperature-dependent changes that have been reported experimentally in anthracene. We propose that part of these inconsistencies stem from the impossibility to deconvolute certain vibrational peaks when only experimental data is available. This work therefore provides a foundation to improve the interpretation of PO-Raman experiments in complex molecular crystals with the aid of theoretical simulations.
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Materials Science · Cross-listed from cond-mat.str-el

Systematic cRPA study of two-dimensional MA$_2$Z$_4$ materials: From unconventional screening to correlation-driven instabilities

F. Bagherpour, Y. Yekta, H. Hadipour et al. (+5 more)

Understanding the interplay between screening, electronic correlations, and collective excitations is essential for the design of two-dimensional quantum materials. Here, we present a comprehensive first-principles study of more than 60 MA$_2$Z$_4$ monolayers, encompassing semiconducting, metallic, cold-metallic, magnetic, and topological phases. Using the constrained random phase approximation (cRPA), we compute material-specific effective Coulomb interaction parameters $U$, $U'$, and $J$, including their spatial dependence across distinct correlated subspaces defined by local coordination and crystal symmetry. In semiconducting compounds, long-range nonlocal interactions persist, revealing unconventional screening and suggesting strong excitonic effects beyond simple dielectric models. In cold-metallic systems, sizable long-range Coulomb interactions remain despite the presence of free carriers, highlighting their atypical metallic screening. Among 33-valence-electron compounds, we find $U_{\mathrm{eff}} > W$ in the $\beta_2$ phase, indicating proximity to charge-density-wave or Mott instabilities. Several V- and Nb-based systems exhibit intermediate-to-strong correlation strength, with $U/W > 1 $ in multiple cases. Using cRPA-derived Stoner parameters, we identify magnetic instabilities in various V-, Nb-, Cr-, and Mn-based compounds. Finally, selected cold-metallic systems display plasmon dispersions that deviate from the conventional $\sqrt{q}$ behavior, revealing nearly non-dispersive low-energy modes. These results position MA$_2$Z$_4$ monolayers as a versatile platform for investigating correlation-driven instabilities and emergent collective behavior in two dimensions.
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All authors F. Bagherpour, Y. Yekta, H. Hadipour, E. Sasioglu, A. Khademi, S. A. Jafari, I. Mertig, S. Lounis

Materials Science

Electronic and thermal properties of the phase-change memory material, Ge2Sb2Te5, and results from spatially resolved transport calculations

Kishor Nepal, Aashish Gautam, Ridwan Hussein et al. (+4 more)

We report new insights into the electronic, structural, and transport (heat and charge) properties of the phase-change memory material Ge2Sb2Te5. Using realistic structural models of Konstantinou et. al. [Nat. Commun. 10, 3065 (2019)], we analyze the topology, electronic states, and lattice dynamics with density functional methods, including hybrid-functional calculations and machine-learned interatomic potentials. The Kohn-Sham orbitals near the Fermi level display a strong electron-phonon coupling, and exhibit large energy fluctuations at room temperature. The conduction tail states exhibit larger phonon-induced fluctuations than the valence tail states. To resolve transport at the atomic scale, we employ space-projected electronic conductivity and site-projected thermal conductivity methods. Local analysis of heat transport highlights the role of filamentary networks dominated by Te, with Sb and Ge making progressively smaller contributions.
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All authors Kishor Nepal, Aashish Gautam, Ridwan Hussein, Konstantinos Konstantinou, Stephen. R. Elliott, Chinonso Ugwumadu, David A. Drabold

Materials Science

Correlative Analysis of Iron-Driven Structural, Optical, and Magnetic Properties in Natural Biotite Crystals

Raphaela de Oliveira, Yara Galvão Gobato, Ronei C. de Oliveira et al. (+9 more)

Biotite crystals are phyllosilicate trioctahedral micas with the general chemical formula K(Mg,Fe)3AlSi3O10(OH)2 that form a solid-solution series with iron-poor phlogopite and iron-rich annite endmembers. With a wide band gap energy and a layered structure with free surface charges, biotite nanosheets can be readily obtained by cleavage methods and used as dielectrics in nanodevice fabrication for the next generation of electronics and energy harvesting. Here, a comprehensive study of biotite samples with different iron concentrations and oxidation states is presented. Structural, optical, magneto-optical, and magnetic characterizations were performed using several experimental techniques, including state-of-the-art synchrotron-based techniques, to correlate the iron chemistry (content and oxidation state) with the macroscopic properties of both minerals. The study reveals a nanoscale-homogeneous Fe distribution via synchrotron X-ray fluorescence mapping, defect-mediated optical transitions modulated by Fe3+/Fe2+ ratios, and temperature-dependent magnetic transitions from paramagnetism to competing ferro-/antiferromagnetic interactions. Furthermore, the use of these biotite crystals as substrates for ultrathin heterostructures incorporating monolayer (ML) MoSe2 is explored by magneto photoluminescence at cryogenic temperatures. The results show that the presence of iron impurities in different oxidation states significantly impacts the valley properties for ML-MoSe2. Overall, these findings offer a comprehensive interpretation of the physical properties of bulk biotites in a correlative approach, serving as a robust reference for future studies aiming to explore biotites in their ultrathin form.
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All authors Raphaela de Oliveira, Yara Galvão Gobato, Ronei C. de Oliveira, José R. de Toledo, Verônica C. Teixeira, Angelo Malachias, Cesar R. Rabahi, Chunwei Hsu, Adilson J. A. de Oliveira, Herre. S. J. van der Zant, Ingrid D. Barcelos, Alisson R. Cadore

Materials Science

AtomWorld: A Benchmark for Evaluating Spatial Reasoning in Large Language Models on Crystalline Materials

Taoyuze Lv, Alexander Chen, Fengyu Xie et al. (+7 more)

Large language models (LLMs) have shown promising potential in scientific research, enabling tasks ranging from knowledge retrieval to property prediction. Existing science benchmarks mainly focus on perceptual or knowledge-based tasks, largely ignoring the modelling tasks, a fundamental starting point for any real scientific research. For materials science, constructing and manipulating atomic structures is one of the most creative and least automated steps. In this work, we introduce AtomWorld, a benchmark designed to evaluate the abilities of LLMs on structure modifications. The benchmark includes ten fundamental actions under four widely used modelling categories, enabling verifiable evaluation metrics. We find that Claude Opus 4.6 generally performs the best. While the success rate decreases markedly with increasing modelling complexity, with particularly low success rates (below 12% for rotation) for operations involving complex spatial relations. Our results suggest that contemporary LLMs are better suited as copilots for materials structure modelling rather than fully unsupervised autonomous scientific agents. Beyond evaluation, AtomWorld also serves as a testbed and playground for developing future structure-aware models, including reinforcement learning and agentic approaches.
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All authors Taoyuze Lv, Alexander Chen, Fengyu Xie, Chu Wu, Jeffrey Meng, Dongzhan Zhou, Yingheng Wang, Bram Hoex, Zhicheng Zhong, Tong Xie

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

Spin injection of exciton-polaritons with halide perovskites at room temperature

Elena Sendarrubias Arias-Camisón, Maksim Lednev, Jorge Cuadra et al. (+6 more)

Exciton-polaritons, hybrid photon-exciton quasiparticles, constitute a useful platform for the study of light-matter interaction and nonlinear photonic applications. In this work, we realize a monolithic Tamm-plasmon microcavity embedding a thin film of two-dimensional halide perovskites with a tunable polymer spacer that controls the exciton-photon detuning. Angle-resolved optical spectroscopy at room temperature reveals the lower polariton branch dispersions in the linear regime for several detunings. Under circularly polarized, quasi-resonant laser excitation, the spin injection of excitons and their relaxation towards the lower polariton branch demonstrates its preservation. The observed spin-polarized emission is consistent with the fast decay of polaritons: spin relaxation mechanisms become inefficient during this short time. Our results provide promising insights into the spin control of polaritonic devices, including chiral lasers and switches.
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All authors Elena Sendarrubias Arias-Camisón, Maksim Lednev, Jorge Cuadra, Raúl Gago, Luis Viña, Francisco José García Vidal, Johannes Feist, Ferry Prins, Carlos Antón Solanas

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

Pronounced orbital-selective electron-electron correlation and electron-phonon coupling in V2Se2O

Mingzhe Hu, Ziyin Song, Jingwen Cheng et al. (+16 more)

Orbital-selective many-body effects, in which electrons occupying different orbitals experience distinct interaction strengths, play a crucial role in correlated multiorbital materials. However, these effects usually manifest in a complex manner, obscuring their microscopic origins. Here, by combining angle-resolved photoemission spectroscopy measurements with theoretical calculations, we reveal pronounced orbital selectivity in both electron-electron correlation and electron-phonon coupling in the van der Waals material V2Se2O. Electron correlation induces distinct bandwidth renormalization exclusively in the V d_xy-derived band, while the bands mainly composed of the other d orbitals remain essentially unrenormalized. Orbital-resolved analyses identify that the filling number and the bandwidth are decisive factors governing orbital-dependent correlation. Simultaneously, the d_(xz/yz)-derived band exhibits a sharp kink anomaly, arising from enhanced coupling to high-energy phonon modes dominated by oxygen vibrations. Such pronounced orbital selectivity positions V2Se2O as a rare and prototypical platform for unravelling the microscopic mechanisms of orbital-selective electron-electron and electron-phonon interactions, and offers guiding principles for the design of correlated multiorbital materials.
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All authors Mingzhe Hu, Ziyin Song, Jingwen Cheng, Gexing Qu, Zhanghuan Li, Yu Huang, Jundong Zhu, Guangyu Zhang, Dacheng Tian, Lan Chen, Zhijun Tu, Hechang Lei, Xiaoping Ma, Huaixin Yang, Zhongxu Wei, Genfu Chen, Hongming Weng, Tian Qian, Hang Li

Materials Science

Investigating into mechanisms of high temperature strength of refractory high-entropy alloys

Sai Anandhi Seetharaman, Soumyadipta Maiti, Ambesh Gupta et al. (+1 more)

The yield strength plateau of two BCC refractory high entropy alloys (RHEAs) - MoNbTaVW and MoNbTaW was examined through hybrid Monte Carlo and molecular dynamics (MC/MD) simulations. By analyzing atomic diffusivities derived from vacancy formation and migration energies around the edge dislocation cores, the number of critical atomic swaps were calculated at different temperatures. Using hybrid MC/MD simulations of these critical swaps, we demonstrate that above 1400K, the stress required to move the dislocations gets saturated, indicating the effect of Dynamic Strain Ageing (DSA) via cross core motion. Further simulations on random solid solutions (0 MC swaps) revealed a similar plateau effect at the intermediate temperatures. This was attributed to the additional athermal stress arising from lattice distortions due to solid solution strengthening. Our findings suggest that the yield strength plateau results from an interplay between the DSA-driven diffusion process and athermal stress. Specifically, the plateau emerges from DSA mechanisms in the presence of atomic diffusion, whereas in the absence of diffusion, it is governed by athermal statistical lattice distortions. This dual mechanism framework provides a comprehensive explanation for the experimentally observed Yield strength behavior in RHEAs at intermediate temperatures.
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All authors Sai Anandhi Seetharaman, Soumyadipta Maiti, Ambesh Gupta, Beena Rai

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

Dynamic Landau-Lifshitz-Bloch-Slonczewski equations for spintronics

Pascal Thibaudeau, Mouad Fattouhi and Liliana D. Buda-Prejbeanu

The atomistic Landau-Lifshitz-Gilbert equation is challenged when modeling spintronic devices where Joule heating is significant, due to its core assumption of a constant magnetization magnitude. Based on a statistical framework that treats the magnetization magnitude as a dynamic variable coupled to a thermal bath, we derive a dynamic Landau-Lifshitz-Bloch-Slonczewski set of equations for torques, that captures the transient, heating-induced demagnetization that occurs during high-current operation. Integrating these dynamic equations and comparing them to their stochastic equivalents reveals that both the energy landscape and switching dynamics in high-anisotropy systems are similarly modified. This approach yields accurate and accelerated predictions of critical currents and switching times.
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Materials Science · Cross-listed from cond-mat.supr-con

Systematic evolution of superconducting pairing strength and Seebeck coefficients in correlated infinite-layer La$_{1-x}$Sr$_x$NiO$_2$

Motoki Osada, Shusaku Imajo, Yuji Seki et al. (+6 more)

The recently discovered superconducting infinite-layer nickelates offer a novel platform to explore an exotic pairing mechanism in multi-band systems towards high-temperature superconductivity and associated rich quantum phases, contrasting with cuprates. Here, we show that infinite-layer (La,Sr)NiO$_2$ exhibits strong-coupling superconductivity, resilient to in-plane magnetic fields exceeding 47 T at optimal doping - more than twice the Pauli limit for conventional BCS superconductors. This violation becomes pronounced towards the underdoped regime, implying an intriguing evolution of pairing glue. The unexpected observation of positive Seebeck coefficients in this regime indicates the presence of nontrivial electron correlations. Furthermore, our comprehensive investigation across the superconducting dome reveals that the evolution of (thermo)electric normal-state properties - specifically, the sign changes of the Hall and Seebeck coefficients - coincide with the evolution of superconducting anisotropy and pairing strength. This demonstrates a definitive link between electron correlations and strong-coupling superconductivity in (La,Sr)NiO$_2$, contributing to a unified framework for understanding unconventional superconductivity.
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All authors Motoki Osada, Shusaku Imajo, Yuji Seki, Kousuke Ishida, Tsutomu Nojima, Kohei Fujiwara, Koichi Kindo, Yusuke Nomura, Atsushi Tsukazaki

Sunday 5 October

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

Dynamic micromagnetism a la Ericksen-Leslie, and the constrained polar continuum mechanics of hard magnetic soft materials

Amit Acharya and Siladitya Pal

A model of dissipative micromagnetics coupled to (visco-)elasticity is explored, following the procedures of the Ericksen-Leslie theory of nematic liquid crystals allowing for angular momentum due to magnetization. An outcome is the Landau-Lifshitz-Gilbert theory coupled to material spin. A further power-less augmentation to the angular momentum of the theory with classical kinetic energy density is also considered, with a preliminary exploration of its potential in representing the Einstein-de Haas and Barnett effects within continuum mechanics. A treatment of the continuum mechanics of hard magnetic soft materials as a constrained polar material is presented. The models of DeSimone and James (2002) and Zhao et al. (2019) are discussed as two different, namely energetically and kinematically, constrained models of magnetoelasticity encompassed within the overall framework.
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Materials Science

On the Origin of Carrier Loss in Mg-Doped N-Polar GaN

Masahiro Kamiyama, Shashwat Rathkanthiwar, Cristyan Quiñones-García et al. (+6 more)

The neutral $(V_N-3Mg_{Ga})^0$ complex was found to be the primary compensator in Mg-doped, N-polar GaN. The experimental data showed a sharp drop in hole concentration once [Mg] exceeded ~$10^{19} cm^{-3}$. Temperature-dependent Hall measurements, in conjunction with a charge balance model, revealed that the carrier loss was due to a drastic reduction in acceptor concentration ($N_A$), suggesting that a significant fraction of Mg atoms incorporated in an electrically neutral configuration. A quantitative semi-empirical model based on the grand canonical formalism pointed to the formation of $(V_N-3Mg_{Ga})^0$ complexes as the primary cause for the observed carrier loss.
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All authors Masahiro Kamiyama, Shashwat Rathkanthiwar, Cristyan Quiñones-García, Seiji Mita, Dolar Khachariya, Pramod Reddy, Ronny Kirste, Ramón Collazo, Zlatko Sitar

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

Defects in hexagonal boron nitride for quantum technologies

Tobias Vogl, Viktor Ivády, Isaac J. Luxmoore et al. (+1 more)

Atomic defects in solid-state materials are building blocks for future quantum technologies, such as quantum communication networks, computers, and sensors. Until recently, a handful of defects in a small selection of host materials have been possible candidates. Recent developments have revealed that hexagonal boron nitride, a wide-bandgap two-dimensional material, hosts single-photon-emitting atomic defects with access to optically addressable electronic and nuclear spins at room temperature. Now, atomically thin quantum devices that operate at ambient conditions are a possibility. In this perspective, we discuss the recent progress, and challenges, in understanding the fundamental photophysics of defects in hBN, as well as specific opportunities they present for the development of quantum technologies.
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All authors Tobias Vogl, Viktor Ivády, Isaac J. Luxmoore, Hannah L. Stern

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

Novel family of near-room-temperature compensated itinerant pyrochlore ferrimagnets, $R{\mathrm{In}}{\mathrm{Co}}_{4}$ ($R=$ Dy-Tm)

T. Shiotani, T. Waki, Y. Tabata et al. (+2 more)

We successfully synthesized single crystals of a series of C15b Laves phase compounds, $R{\mathrm{In}}{\mathrm{Co}}_{4}$ ($R=$ Dy-Tm), with Co-pyrochlore and $R$-fcc sublattices, and systematically studied their magnetic properties via magnetometry measurements. These itinerant cubic compounds, with Curie temperatures above room temperature, show compensated ferrimagnetism featuring an antiferromagnetic coupling between the two sublattices. From this series, ${\mathrm{DyInCo}}_{4}$ exhibits the highest $T_{\rm C}$ (= 368 K) and a near-room-temperature compensation point $T_{\rm cp}$ (= 295 K). $T_{\rm C}$ does not change drastically with the $R$ atom, whereas $T_{\rm cp}$ depends on the de Gennes factor of $R^{3+}$. Another magnetization anomaly is observed in all the compounds at low temperatures, which may be indicative of changes in the lattice or magnetic structure. The easy axis the ferrimagnetic moment of ${\mathrm{DyInCo}}_{4}$, ${\mathrm{ErInCo}}_{4}$, and ${\mathrm{TmInCo}}_{4}$ is found at $T =$ 5 K to be along the [001], [111] and [110] directions, respectively. However, the simple easy-axis or easy-plane ferrimagnetic picture cannot be applied to ${\mathrm{HoInCo}}_{4}$. These observations suggest that the $R$ sublattice determines magnetic anisotropy and compensation, while the Co sublattice plays a role in strong magnetic ordering. The high Curie temperature, together with the magnetization compensation point near room temperature, renders these itinerant pyrochlore magnets interesting for spintronic applications.
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All authors T. Shiotani, T. Waki, Y. Tabata, I. Kézsmárki, H. Nakamura

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

Optical conductivity and band gap in the double-Weyl candidate SrSi2 at ambient pressure

L. Z. Maulana, A. A. Tsirlin, E. Uykur et al. (+4 more)

We probe the possible double-Weyl state in cubic SrSi2 using optical spectroscopy. The complex optical conductivity was measured in a frequency range from 70 to 22 000 cm-1 at temperatures down to 10 K at ambient pressure. The optical response of SrSi2 can be well separated into the intraband (free carriers) and interband contributions. Additionally, four infrared-active phonons are detected. As follows from the optical spectra, the free-carrier density decreases with decreasing temperature, consistent with an activation behaviour. Experimental interband conductivity juxtaposed with ab initio calculations shows that conventional density-functional theory fails to describe the electronic structure of SrSi2 in the vicinity of the Fermi level. A semi-local exchange-correlation potential allows a much better agreement with the experiment, resulting in the trivial (gapped) band structure of SrSi2. The direct gap estimated from the measurements is approximately 40 meV.
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All authors L. Z. Maulana, A. A. Tsirlin, E. Uykur, Y. Saito, M. Dressel, M. Imai, A. V. Pronin

Materials Science · Cross-listed from physics.optics

Quantum Emission in Monolayer WSe2 Transferred onto InP Nanowires

Palwinder Singh, Jasleen Kaur Jagde, Megha Jain et al. (+10 more)

Localized quantum emitters in transition-metal dichalcogenides (TMDs) have recently emerged as solid-state candidates for on-demand sources of single photons. Due to the role of strain in the site-selective creation of TMD emitters, their hybrid integration into photonic structures such as cavities and waveguides is possible using pick-and-place methods. Here we investigate quantum emission from a hybrid structure consisting of a monolayer of WSe2 interfaced with horizontally aligned InP nanowires (NWs). Our experiments reveal multiple narrow and bright emission peaks in the 715-785 nm spectral range and g(2)(0) as low as 0.049, indicating strong antibunching and good single photon purity. The faceted nature of III-V NWs provides unique opportunities for strain engineering, including the potential for placement of emitters on the top surface for optimal coupling. Our findings pave the way for realizing hybrid quantum light sources for integrated quantum photonics that could combine III-V quantum dots with TMD emitters into a single platform.
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All authors Palwinder Singh, Jasleen Kaur Jagde, Megha Jain, Edith Yeung, David B. Northeast, Simona Moisa, Seid J. Mohammed, Jean Lapointe, Una Rajnis, Annika Kienast, Philip J. Poole, Dan Dalacu, Kimberley C. Hall

Materials Science

Longitudinal transport spin polarization of spin degenerate antiferromagnets

Meng Zhu, Jianting Dong, Xinlu Li et al. (+4 more)

A vital goal in spintronics is the efficient electrical generation of spin currents, a pursuit that has recently focused on using antiferromagnets (AFMs) as spin current sources. It has been demonstrated that antiferromagnets with broken PT symmetry (parity + time reversal) can efficiently generate longitudinal and transverse spin currents. At the same time, it has been generally thought that antiferromagnets with PT symmetry (PT-AFMs) forbid the longitudinal spin polarization due to their spin-degenerate band structure. Here, in contrast to this common expectation, we show, using theoretical analysis based on magnetic point group symmetry, that most PT-AFMs can generate longitudinal spin currents due to spin-orbit coupling. Using density-functional theory, we calculate the longitudinal spin conductivity of representative PT-AFMs, L10-MnPt and Mn2Au, and show that its magnitude is comparable to that of their PT-broken counterparts. Our symmetry-enabled classification of antiferromagnets and theoretical results for the longitudinal spin conductivity in representative PT-AFMs expands our understanding of spin transport and shows the possibility of robust spin-current generation in a broad range of spin-degenerate antiferromagnets.
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All authors Meng Zhu, Jianting Dong, Xinlu Li, Jiahao Shentu, Yizhuo Song, Evgeny Y. Tsymbal, Jia Zhang

Materials Science

Dynamic breaking of axial symmetry of acoustic waves in crystals as the origin of nonlinear elasticity and chaos: Analytical model and MD simulations

Zbigniew Kozioł

A Chain of Springs and Masses (CSM) model is used in the interpretation of molecular dynamics (MD) simulations of movement of atoms in orientated FCC crystals. A force of dynamic origin is found that is perpendicular to the direction of the external shear pressure. It is proportional to the square of the applied pressure; It causes breaking of axial symmetry for propagation of transverse acoustic waves. It leads to a non-linear elastic response of crystals and to chaotic patterns in the motion of atoms. We provide an analytical derivation of an effective atomistic 3D potential for interaction between crystallographic layers. The potential is found to possess a component that has an anharmonic threefold axial symmetry around one direction. It reduces to the Hénon-Heinen potential in a 2D cross-section, leading to mathematically rich, complex dynamic features. Results of simulation predict displacements of atoms that are inconsistent with the static theory of elasticity that may have been overlooked in experiments.
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Materials Science

Direct observation of band structure modifications from monolayer WSe2 to Janus WSSe

Masato Sakano, Shunsuke Akatsuka, Takato Yamamoto et al. (+19 more)

Janus monolayer transition metal dichalcogenides (TMDs), created by post-growth substitution of the top chalcogen layer, represent a new direction for engineering 2D crystal properties. However, their rapid ambient degradation and the difficulty of obtaining large-area monolayer samples have limited the available experimental probes, leaving their detailed electronic structure near the Fermi level largely unexplored. In this work, by performing micro-focused angle-resolved photoemission spectroscopy ({\mu}-ARPES) on an identical sample transformed from monolayer WSe2 to Janus WSSe via a H2 plasma-assisted chalcogen-exchange method, we reveal the evolution of its electronic band structure. We observe ARPES signature consistent with the Rashba-type spin splitting due to broken horizontal mirror symmetry, and a significant upward shift of the highest valence band at the {\Gamma}-point by approximately 160 meV. These direct observations clarify the key electronic modifications that govern the material's properties and provide a pathway for band engineering in Janus TMDs.
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All authors Masato Sakano, Shunsuke Akatsuka, Takato Yamamoto, Tianyishan Sun, Dingkun Bi, Hiroto Ogura, Naoya Yamaguchi, Fumiyuki Ishii, Natsuki Mitsuishi, Kenji Watanabe, Takashi Taniguchi, Miho Kitamura, Koji Horiba, Kenichi Ozawa, Katsuaki Sugawara, Seigo Souma, Takafumi Sato, Yuta Seo, Satoru Masubuchi, Tomoki Machida, Toshiaki Kato, Kyoko Ishizaka

Materials Science · Cross-listed from physics.optics

Binary $ZnSe:Fe^{2+}$ and ternary $ZnMgSe:Fe^{2+}$ optical crystals for mid-IR applications

Sergei V. Naydenov, Oleksii K. Kapustnyk, Igor M. Pritula et al. (+3 more)

In this study, binary $ZnSe:Fe^{2+}$ crystals and ternary $Zn_{1-x}Mg_{x}Se:Fe^{2+}$ crystals $(0 < x < 0.6)$ were grown by the vertical Bridgman method in graphite crucibles under high argon pressure. A comparative characterization of the structural, energetic, and optical parameters of the obtained crystals was performed. Theoretical explanations of the observed experimental characteristics, including the specific red-shift of the absorption and emission spectra with increasing solid-solution concentration, are provided. These results could be useful for the development of new laser media based on simple and complex AIIBVI crystals and for targeted engineering of their optical properties.
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All authors Sergei V. Naydenov, Oleksii K. Kapustnyk, Igor M. Pritula, Dmitro S. Sofronov, Igor S. Terzin, Nazar O. Kovalenko

Materials Science

Atomistic Machine Learning with Irreducible Cartesian Natural Tensors

Qun Chen, A. S. L. Subrahmanyam Pattamatta, Boyu Wang et al. (+2 more)

Atomistic machine learning is a powerful tool for accurate and efficient investigation of material behavior at the atomic scale. While attempts have been made to construct models directly within Cartesian space, they face challenges in providing a systematic framework based on irreducible representations - a core feature of widely used spherical models. Here we propose Cartesian Natural Tensor Networks to overcome these limitations and thus offer a general, symmetry-preserving framework for atomistic machine learning. We present a theory of irreducible representations using Cartesian natural tensors, comprising their construction, their products, and a systematic scheme to decompose and reconstruct high-rank physical tensors. Leveraging this machinery, we develop equivariant machine learning interatomic potentials for materials and molecular systems with performance on par with leading spherical models. It further captures accurate structure-property relationships for tensorial quantities ranging from low-rank dipole moments to high-rank tensors with complex symmetries, such as the elastic constant tensor.
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All authors Qun Chen, A. S. L. Subrahmanyam Pattamatta, Boyu Wang, David J. Srolovitz, Mingjian Wen

Saturday 4 October

Materials Science

A van der Waals material exhibiting room temperature broken inversion symmetry with ferroelectricity

Fabia F. Athena, Cooper A. Voigt, Mengkun Tian et al. (+8 more)

Since the initial synthesis of van der Waals two-dimensional indium selenide was first documented in 1957, five distinct polymorphs and their corresponding polytypes have been identified. In this study, we report a unique phase of indium selenide via Scanning Transmission Electron Microscopy (STEM) analysis in the synthesized large-area films – which we have named the $\beta^\text{p}$ phase. The quintuple layers of the $\beta^\text{p}$ phase, characterized by a unique zigzag atomic configuration with unequal indium-selenium bond lengths from the middle selenium atom, are distinct from any other previously reported phase of indium selenide. Cross-sectional STEM analysis has revealed that the $\beta^\text{p}$ layers exhibit intralayer shifting. We found that indium selenide films with $\beta^\text{p}$ layers display electric-field-induced switchable polarization characteristic of ferroelectric materials, suggesting the breaking of the inversion symmetry. Experimental observations of nonlinear optical phenomena – Second Harmonic Generation (SHG) responses further support this conclusion. This study reports a $\beta^\text{p}$ phase of indium selenide showing ferroelectricity over large areas at room temperature in a low-dimensional limit.
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All authors Fabia F. Athena, Cooper A. Voigt, Mengkun Tian, Anjan Goswami, Emily Toph, Moses Nnaji, Fanuel Mammo, Brent K. Wagner, Sungho Jeon, Wenshan Cai, Eric M. Vogel

Materials Science

Finding the temperature window for atomic layer deposition of ruthenium metal via efficient phonon calculations

Alexandr Fonari, Simon D. Elliott, Casey N. Brock et al. (+3 more)

We investigate the use of first principles thermodynamics based on periodic density functional theory (DFT) to examine the gas-surface chemistry of an oxidized ruthenium surface reacting with hydrogen gas. This reaction system features in the growth of ultrathin Ru films by atomic layer deposition (ALD). We reproduce and rationalize the experimental observation that ALD of the metal from RuO4 and H2 occurs only in a narrow temperature window above 100{\deg}C, and this validates the approach. Specifically, the temperature-dependent reaction free energies are computed for the competing potential reactions of the H2 reagent, and show that surface oxide is reduced to water, which is predicted to desorb thermally above 113{\deg}C, exposing bare Ru that can further react to surface hydride, and hence deposit Ru metal. The saturating coverages give a predicted growth rate of 0.7 Å/cycle of Ru. At lower temperatures, free energies indicate that water is retained at the surface and reacts with the RuO4 precursor to form an oxide film, also in agreement with experiment. The temperature dependence is obtained with the required accuracy by computing Gibbs free energy corrections from phonon calculations within the harmonic approximation. Surface phonons are computed rapidly and efficiently by parallelization on a cloud architecture within the Schrödinger Materials Science Suite. We also show that rotational and translational entropy of gases dominate the free energies, permitting an alternative approach without phonon calculations, which would be suitable for rapid pre-screening of gas-surface chemistries.
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All authors Alexandr Fonari, Simon D. Elliott, Casey N. Brock, Yan Li, Jacob Gavartin, Mathew D. Halls

Materials Science

Adsorption-induced surface magnetism

Miloš Baljozović, Shiladitya Karmakar, André L. Fernandes Cauduro et al. (+8 more)

We report the emergence of adsorption-induced magnetism from heterohelicene molecules on a non-magnetic Cu(100) surface. Spin-polarized low-energy electron microscopy (SP-LEEM) measurements reveal spin-dependent electron reflectivity for enantiopure 7,12,17-trioxa[11]helicene (TO[11]H) monolayers, indicating the formation of a spin-polarized state localized in the topmost copper layer. Control experiments on clean Cu(100) and TO[11]H on highly oriented pyrolytic graphite show no such effect, excluding artifacts and chirality-induced spin selectivity as origins. Spin-polarized density functional theory calculations with hybrid functionals attribute the magnetism to strong chemisorption, which induces hybridization between the molecular HOMO and copper s- and d-states, driving asymmetric spin-polarized charge redistribution at the interface. An extended Newns-Anderson-Grimley model incorporating on-site Coulomb repulsion in Cu d-orbitals reproduces the emergence of interfacial spin polarization above a threshold interaction strength, highlighting the key roles of hybridization parameters and Coulomb correlation. These findings reveal a mechanism for inducing magnetism at molecule-metal interfaces without inherently magnetic components, offering avenues for engineering spin-polarized states in organic-inorganic hybrid systems.
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All authors Miloš Baljozović, Shiladitya Karmakar, André L. Fernandes Cauduro, Mothuku Shyam Sundar, Marco Lozano, Manish Kumar, Diego Soler Polo, Andreas K. Schmid, Ashutosh V. Bedekar, Pavel Jelinek, Karl-Heinz Ernst

Materials Science

Hybrid MBE Route to Adsorption-Controlled Growth of BaTiO3 Membranes with Robust Polarization Switching

S. Choo, S. Varshney, J. Shah et al. (+5 more)

Freestanding ferroelectric membranes are promising for flexible electronics, nonvolatile memory, photonics, and spintronics, but their synthesis is challenged by the need for reproducibility with precise stoichiometric control. Here, we demonstrate the adsorption-controlled growth of single-crystalline, epitaxial BaTiO3 films by hybrid molecular beam epitaxy (MBE) on a binary oxide sacrificial layer. Using a simple water-droplet lift-off method, we obtained submillimeter- to millimeter-sized membranes that retained crystallinity, as confirmed by high-resolution X-ray diffraction, and exhibited robust tetragonal symmetry by Raman spectroscopy. Impedance spectroscopy confirmed a high dielectric constant of 1340, reflecting the robust dielectric response of the membranes. Ferroelectric functionality was revealed by piezoresponse force microscopy (PFM) and further verified by polarization-electric field (P-E) loop measurements with Positive-Up-Negative-Down (PUND). The P-E loops exhibited a remnant polarization of 5 microC cm-2 and a coercive field of 63 kV cm-1. These results were interpreted in relation to c- and a-domain configurations. These results establish hybrid MBE as a generalizable route for producing stoichiometry-controlled ferroelectric membranes, enabling their integration into next-generation flexible and multifunctional quantum oxide devices.
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All authors S. Choo, S. Varshney, J. Shah, A. K. Manjeshwar, D. K. Lee, K. A. Mkhoyan, R. D. James, B. Jalan

Materials Science

Observation of a Novel Charge Density Wave Superstructure in Monolayer 1T-$VS_{2}$ at Room Temperature and its Evolution in Multilayers

Samanta Pal, Kaustuv Chatterjee, Jyotirmoy Sau et al. (+4 more)

Spontaneous formation of charge density wave (CDW) superstructures in monolayers (MLs) of a two-dimensional (2D) crystal lattice is fundamental in understanding its complex quantum states. We report a successful top-down liquid phase exfoliation and stamp transfer process (LPESTP) to create ML VS\textsubscript{2}, undergoing a CDW transition at room temperature. Using high-resolution transmission electron microscopy (HRTEM) and electron diffraction (ED), we observed the coexistence of 1T and 2H polymorphic phases in VS\textsubscript{2} at room temperature, and only the 1T phase undergoes CDW transition. We discovered a novel incommensurate CDW superstructure ($\sqrt{7} \times \sqrt{7}$) R19.1\textsuperscript{o} in ML 1T-VS\textsubscript{2}. With an increase in the number of layers, the CDW order changes to a commensurate ($2 \times 2\times 1$) superstructure. Using angle-dependent photoelectron spectroscopy and TEM, we have shown that vanadium atoms self-intercalate as V\textsuperscript{3+} ions in multilayer VS\textsubscript{2} and are responsible for the evolution of the CDW superstructure from the incommensurate ($\sqrt{7} \times \sqrt{7}$) R 19.1\textsuperscript{o} to the commensurate ($2\times2\times1$) order. We also report the observation of novel Moiré superlattices in twisted bilayer 1T-VS\textsubscript{2} flakes with trapped CDW superstructure of the monolayer. The density functional theory (DFT) calculation performed on ML 1T-VS\textsubscript{2} show that the observed ($\sqrt{7} \times \sqrt{7}$) R 19.1\textsuperscript{o} CDW superstructure has lower energy compared to that of the pristine undistorted ML and the CDW instability is driven by formation of strong soft-phonon modes. Our findings provide an important platform for understanding the evolution of CDW superstructures in 1T-VS\textsubscript{2} with layer numbers and V self-intercalation.
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All authors Samanta Pal, Kaustuv Chatterjee, Jyotirmoy Sau, Biswarup Satpati, Manoranjan Kumar, A. K. Raychaudhuri, Prabir Pal

Materials Science

The magnon spectra of g-type altermagnet bulk CrSb

Murat Tas

We present a calculation of the magnon spectra and chiral lifetimes of altermagnons in bulk CrSb using the many-body perturbation theory. The spin-split band structure is evident in the magnon spectra. Altermagnons attain an energy of 275 meV at the K point of the Brillouin zone. Due to large spin splitting at a specific ${\bf q}$ point along the A - M direction, lifetime differences of chiral altermagnons reach approximately 20 fs.
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Materials Science · Cross-listed from cond-mat.str-el

Insulating transport in anisotropic metals: breakdown of Drude transport and the puzzling $c$-axis resistivity of Sr$_2$RuO$_4$ and other layered oxides

Sophie Beck, Matthew Shammami, Lorenzo Van Muñoz et al. (+3 more)

We reveal a mechanism that may explain the non-metallic out-of-plane resistivity in layered metals. By carefully examining how the Drude-Boltzmann expression for the $c$-axis conductivity emerges out of the Kubo formula, we find, besides the standard metallic term proportional to the carrier lifetime $\tau$, a non-Drude contribution proportional to $1/\tau$. The Drude behavior breaks down when $1/\tau > 2 \eta^*$, the crossover value $\eta^*$ being small (and hence observable) when the $c$-axis velocities vary rapidly with the distance from the Fermi surface. We consider the Hund metal Sr$_2$RuO$_4$ as a test case, which we study within a realistic dynamical mean-field theory approach. The non-Drude behavior observed experimentally in $c$-axis transport is reproduced and explained by our considerations, showing that earlier invoked extrinsic mechanisms that involve either impurities or phonons are unnecessary. We point out that the small value of $\eta^*$ is due to a peculiar accidental cancellation due to destructive interference characteristic of body-centered tetragonal lattices.
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All authors Sophie Beck, Matthew Shammami, Lorenzo Van Muñoz, Jason Kaye, Antoine Georges, Jernej Mravlje