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Physics Condensed Matter Materials Science

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Thursday 17 September

Materials Science

Ultralow-Tensile Strain Enables Exciton Funneling and Energy Transfer to Boost MoSe2 Photoluminescence Quantum Yield

Gayatri, Mehdi Arfaoui, Debashish Das et al. (+14 more)

Strain engineering is a powerful route for controlling the exciton dynamics in van der Waals (vdW) heterostructures (HSs). The interlayer energy transfer (ET) process is another key factor in controlling the photocarrier relaxation pathways in vdW HSs. In this work, we combine these two processes to achieve an 8-fold enhancement to the relative photoluminescence (PL) quantum yield (QY) in a HS formed from monolayers of ReS2 and MoSe2, separated by a thin hBN interlayer, placed onto an hBN bubble. We achieve this enhancement by applying only 0.1% biaxial tensile strain, which results in efficient exciton funneling and an increased transition dipole moment. Our experimental data are supported by first-principles density-functional theory and coherent transfer-matrix method calculations, ruling out optical interference as the dominant origin of the enhancement. This work provides an innovative route for enhancing the PL QY of vdW materials via interplay between the tensile strain and the ET process.
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All authors Gayatri, Mehdi Arfaoui, Debashish Das, Mateusz Raczyński, Marta Bilska, Piotr Tatarczak, Aleksandra Krystyna Dąbrowska, Tomasz Kazimierczuk, Takashi Taniguchi, Kenji Watanabe, Piotr Kossacki, Andrzej Wysmołek, Saroj Kumar Nayak, Adam Babiński, Johannes Binder, Maciej R. Molas, Arka Karmakar

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

Predictive Structure to Thermal Conductivity Modeling Framework for BEOL Interconnect Stacks in Advanced Technology Nodes Enabled by Extensive Layer Resolved Thermal Measurements

Zifeng Huang, Yiyang Sun, Tianyu Jia et al. (+2 more)

The increasing structural complexity of BEOL interconnect stacks in advanced integrated circuits demands a structure-aware thermal conductivity (ąppa}) modeling framework. However, generalizable models derived from layer-resolved thermal measurements that quantitatively capture the dependence of ąppa} on interconnect structures remain lacking, limiting predictive thermal analysis. Here, we establish an experimentally derived, structure-aware ąppa} modeling framework enabled by time-domain thermoreflectance measurements with ~100 nm depth resolution. Statistical analysis of a compiled dataset comprising over 40 experimentally measured layer-resolved ąppa} values across diverse BEOL layers reveals a generalizable empirical structure-to-ąppa} relationship, enabling predictive modeling based on interconnect structure. An intra-layer three-dimensional ąppa} distribution model based on effective medium theory and realistic layouts further resolves spatial ąppa} variations within practical interconnect layers. Together, these models establish an experimentally derived, structure-aware ąppa} modeling framework for predictive and generalizable thermal analysis of advanced interconnect stacks and 3D ICs.
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All authors Zifeng Huang, Yiyang Sun, Tianyu Jia, Runsheng Wang, Zhe Cheng

Materials Science

A Computational Method to Simulate Electrostatic Actuation in Polycatenated Architected Materials

Mateus Maciel Vivaldi and Alexandre F. Fonseca

A novel class of polycatenated architected materials (PAMs) has recently emerged, exhibiting distinctive mechanical properties such as stress-strain hysteresis and a geometry-driven transition between solid-like and fluid-like behavior. Although finite element models exist for conventional PAMs, none currently account for electrostatic effects. Here, we propose and qualitatively validate a computational framework to simulate the structure, dynamics, and mechanical response of electrostatically charged PAMs. Our approach successfully reproduces electrostatic actuation in close agreement with experiments, and reveals that electrostatic charges enable a reversible fluid-to-solid transition while also providing a means to estimate electrostatic stiffness. Notably, stiffness enhancements of up to sevenfold are achieved upon charging. This work expands the design space of PAMs and offers a predictive tool for actively controllable mechanical metamaterials.
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Materials Science

Divergent Solid-state Conversion Pathways in Evaporated All-perovskite Tandem Solar Cells

Huagui Lai, Amber Wright, Nick Huber et al. (+8 more)

Sequential thermal evaporation (sTE) is emerging as a solvent-free route to high-quality mid-bandgap perovskites, but its extension to mixed-halide wide-bandgap (WBG) and Sn-Pb narrow-bandgap (NBG) absorbers for all-perovskite tandem solar cells (TSCs) remains limited by an incomplete understanding of solid-state conversion. Here, using time-sliced ex situ analysis, we reveal divergent solid-state conversion mechanisms in sequentially evaporated WBG and NBG precursor stacks. In WBG stacks, formamidinium (FA)-containing species penetrate the inorganic template and Br/I redistribution precedes substantial three-dimensional perovskite formation. The photoactive phase then crystallizes from a chemically mixed reservoir, and absolute PbBr$_2$ thickness, rather than nominal PbBr$_2$/PbI$_2$ ratio, determines the final bandgap. In NBG stacks, by contrast, an early Pb-rich perovskite phase forms upon formamidinium iodide deposition, restricting further FA penetration into the buried SnI$_2$ precursor. Subsequent annealing promotes rapid lattice reorganization faster than Sn/Pb interdiffusion, leaving vertical compositional gradients. Guided by these insights, we develop sTE absorbers with bandgaps spanning 1.26-1.96 eV and demonstrate the first evaporated all-perovskite TSC, reaching a power conversion efficiency of 19.2%. Encapsulated tandems retain on average 80% of their initial efficiency after 1,200 h at 65 $^\circ$C (ISOS-D-2). These results establish bandgap-specific control of solid-state conversion as a design principle for sequentially evaporated perovskite tandem photovoltaics.
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All authors Huagui Lai, Amber Wright, Nick Huber, Niels Uythoven, Federico De Giorgi, Jincheng Luo, Tristan Sachsenweger, Sunil B. Shivarudraiah, Chih-Jen Shih, Wolfgang Tress, Fan Fu

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

Plasmons in twisted bilayer graphene across dispersive and flat bands

Antonio Palamara, Michele Pisarra and Antonello Sindona

The dynamical dielectric response of twisted bilayer graphene is explored in large-angle dispersive-band and small-angle quasi-flat-band regimes using time-dependent density-functional theory within the random-phase approximation. At the reference bilayer-graphene interlayer distance, weak coupling in the largest-angle structures preserves Dirac dispersions and the intrinsic $\pi$ plasmon. Electron doping activates a two-dimensional Dirac plasmon with energies obeying approximate geometric twist-angle scaling, while acoustic-like branches remain embedded in the single-particle continuum. The prohibitively large first-magic-angle supercell is represented by a tractable cell with its interlayer separation reduced to the angle-dependent magic distance, where four quasi-flat bands emerge around the Fermi level. Their partial occupation produces a dispersive low-energy plasmon-like excitation without a clear dielectric zero at resonance. A distinct interband plasmon is instead identified, supported by transitions involving the quasi-flat manifold and neighboring high-density-of-states regions. Band-energy rescaling places its characteristic energy in the mid-infrared range of interband collective excitations measured near the magic angle.
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Materials Science

Coherent antiferromagnetic resonance in MnO driven by impulsive terahertz excitation

Yinchuan Lv, Martin J. Cross, Hari Paudyal et al. (+4 more)

Antiferromagnets (AFMs) offer a promising platform for ultrafast information processing owing to their intrinsically fast spin dynamics and vanishing net magnetization. Realizing this potential, however, requires understanding how terahertz fields excite coherent magnons and how the resulting spin motion is transduced into an optical signal. Here we report impulsive terahertz (THz) excitation and time-domain detection of the antiferromagnetic resonance (AFMR) in single-crystal manganese(II) oxide. Time-resolved birefringence measurements reveal long-lived coherent spin oscillations in the AFM phase. The resonance softens and becomes strongly damped upon warming toward the Néel temperature. Despite this similar excitation behavior, the detected birefringence in MnO is markedly weaker than in NiO. We associate this suppressed optical visibility with the weak spin–orbit-mediated magneto-optical coupling of orbital-singlet, high-spin Mn$^{2+}$. These results demonstrate that coherent magnon excitation and its optical detection are governed by distinct microscopic interactions.
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All authors Yinchuan Lv, Martin J. Cross, Hari Paudyal, Christopher T. Parzyck, A. H. M. Reid, Durga Paudyal, Matthias C. Hoffmann

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

Type-I superconductivity in a quasi-2D topologically nontrivial YbBi$_2$

Karolina Górnicka, Sudip Malick, Joanna Bławat et al. (+13 more)

Intrinsic superconductivity in stoichiometric materials with nontrivial electronic topology remains uncommon, limiting opportunities to investigate how these two phenomena coexist within a single electronic system. Here, we report bulk type-I superconductivity below $T_c$ $\sim$ 0.9~K in YbBi$_2$, a layered rare-earth compound with a nonsymmorphic crystal structure and a quasi-two-dimensional Fermi surface. Thermodynamic and transport measurements establish the superconducting ground state, while quantum oscillations reveal exceptionally light carriers, with a cyclotron mass as low as 0.07 $m_e$, and a nonzero Berry phase of approximately 0.82 $\pi$. The latter closely matches the calculated value of the corresponding Wilson phase 0.99 $\pi$ for the corresponding orbit near a symmetry-protected band degeneracy. ARPES measurements show good agreement with key features of the calculated electronic structure, providing complementary experimental constraints on the normal-state band structure. The combination of intrinsic type-I superconductivity, light quasi-two-dimensional carriers, and signatures of nontrivial electronic topology identifies YbBi$_2$ as a distinct stoichiometric platform for investigating superconductivity in a topologically nontrivial electronic environment.
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All authors Karolina Górnicka, Sudip Malick, Joanna Bławat, Michał Modrzejewski, Hanna Świątek, Michał J. Winiarski, Federico Mazzola, Ivana Vobornik, Chiara Bigi, Jacob Cook, Brenden R. Ortiz, Andrew F. May, Andrzej P. Kądzielawa, John Singleton, Bartlomiej Wiendlocha, Tomasz Klimczuk

Materials Science

Modeling Dynamic Magnetic Response of Spinel Soft Ferrites with the Steepest-Entropy-Ascent Quantum Thermodynamics Formalism

Deepak Dhariwal, William T. Reynolds Jr. and Michael R. von Spakovsky

Selecting soft ferrites for alternating-field applications requires balancing magnetic response, dissipation, nonlinearity, and heating. We use a field-driven steepest-entropy-ascent quantum thermodynamic (SEAQT) model to compare the electron, phonon, and magnon responses of Fe$_3$O$_4$, MnFe$2$O$4$, and (Mn${0.5}$Zn${0.5}$)Fe$_2$O$_4$ within a common first-principles framework. The model treats spatially uniform longitudinal relaxation and neglects domain-wall motion, transverse rotation, resonance, eddy-current effects, and heat removal. We use $\tau_e=0.05$ ps and $\tau_p=3$ ps for all three materials, with effective longitudinal magnon relaxation times of 500, 200, and 85 ps, respectively; these literature-motivated values are model inputs, not fits to measured losses. Within this framework, the Mn–Zn ferrite gives the largest peak longitudinal magnetization change, retains its response best at high frequency, and shows the largest work per cycle and peak-to-peak magnon temperature change, whereas MnFe$_2$O$_4$ gives the largest normalized $\chi''$ peak. Thus, no single ferrite ranks highest across all metrics: the preferred material depends on the property, frequency, field amplitude, and cation configuration of interest. The model therefore provides a spectrum- and kinetics-resolved screening tool for engineering comparison of ferrites rather than a prediction of total core loss.
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Materials Science

Anharmonic Phonon Renormalization and Defect Tolerance of the Thermoelectric Power Factor in Monolayer SnSe

Nguyen Tran Gia Bao, Thang Bach Phan, Vu Thi Hanh Thu et al. (+1 more)

Monolayer tin selenide (SnSe) exhibits phase-dependent anharmonic lattice dynamics, yet their consequences for the thermoelectric power factor (PF) and point-defect tolerance remain unresolved. We combine density functional theory, the stochastic self-consistent harmonic approximation (SSCHA), and Boltzmann transport calculations including electron-phonon and electron-defect scattering to investigate monolayer $\alpha$-SnSe (Pnma) and $\beta$-SnSe (Cmcm). In dynamically stable $\alpha$-SnSe, SSCHA renormalizes the finite-temperature phonons without changing the qualitative n-type transport picture. In $\beta$-SnSe, SSCHA removes the harmonic soft-mode instability of the Cmcm phase at 800-1000 K, and thereby enables high-temperature transport calculations; LO/TO-2 is the principal electron-scattering channel. In the lower-density window near $10^{12}$ cm$^{-2}$, the n-type PF reaches 15-19 $\mu\mathrm{W}/(\mathrm{K}^{2}\cdot\mathrm{cm})$ at 800-900 K and exceeds the p-type PF primarily because of the higher electrical conductivity. Se vacancies ($V_{\mathrm{Se}}$) produce weaker electron-defect scattering than Sn vacancies ($V_{\mathrm{Sn}}$), and p-type transport is less defect tolerant than n-type transport in both phases. We define an operational critical defect concentration, $C_{\mathrm{crit}}$, at which the PF decreases by 15% relative to the corresponding defect-free value. The lowest $C_{\mathrm{crit}}$ is $8.841\times10^{-5}$ (approximately 88 ppm) for p-type $\alpha$-SnSe with $V_{\mathrm{Sn}}$; for n-type $\beta$-SnSe with $V_{\mathrm{Se}}$, the 15% threshold is not reached up to $5\times10^{-3}$ (5000 ppm). These results distinguish finite-temperature phonon renormalization in stable $\alpha$-SnSe from anharmonic stabilization in $\beta$-SnSe and provide defect-concentration limits for preserving the PF.
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All authors Nguyen Tran Gia Bao, Thang Bach Phan, Vu Thi Hanh Thu, Nguyen Tuan Hung

Materials Science

Machine Learning for High-Entropy Catalysts: Methods and Applications

Hao Chen, Zongrui Pei and Xianglin Liu

High-entropy alloys (HEAs) exhibit exceptional catalytic performance in various reactions due to their high configurational entropy, synergistic elemental effects, tunable electronic structures, and excellent structural stability. However, the vast compositional space of HEA catalysts makes traditional experimental and theoretical design costly and inefficient. In recent years, data-driven machine learning (ML) methods have emerged as powerful tools for studying HEAs in catalysis. Through predictive models and ML surrogates, researchers can decipher the intricate composition-structure-performance relationships of these materials. In addition, by leveraging large language models (LLMs) for knowledge extraction, hypothesis generation and validation, and as a foundation to build integrated design workflows, ML approaches can significantly accelerate the design of novel HEA catalysts. This review systematically summarizes the latest methodological advances and applications of ML methods for HEAs in catalysis, discusses the challenges, and offers insights into future research directions to support the rational design and efficient development of catalysts.
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Materials Science

Orbital fingerprinting of magnetism across the Mn-Ni-Ga Heusler ternary

Felipe Hawthorne, Daniela A. Damasceno, Raphael M. Tromer et al. (+2 more)

In Mn–Ni–Ga Heusler alloys, the competing magnetic states are separated by differences of a few meV per atom, so the ground state has to be resolved from the electronic structure and cannot be read off the composition. Moving away from the stoichiometric compounds, where the established rules for Heusler magnetism fall short, increases this difficulty. To overcome it, we introduce orbital fingerprints taken from the projected density of states, and use them as the input to machine-learning models for classification of the magnetic ordering, for the magnetic moment amplitude, for the spin polarization at the Fermi level and for interpolation of the phase diagram. The models are trained on a dataset of 370 spin-polarized first-principles calculations of quasirandom structures covering the ternary, which show good agreement with the magnetic ground states and lattice parameters available in the literature. For magnetic ordering, the top-ranked descriptor is the same quantity found by first-principles calculations to distinguish the phases, suggesting that the fingerprints capture the underlying physics.
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All authors Felipe Hawthorne, Daniela A. Damasceno, Raphael M. Tromer, Ronaldo R. Pelá, Cristiano F. Woellner

Materials Science

The Roadmap of Inorganic Computational Materials Databases: Capabilities, Credibility, Coverage, and the Open Frontier

Miao Liu, Jianghao Jin, Tenglong Lu et al. (+4 more)

Computational materials databases have become central infrastructure for data-driven discovery of inorganic materials, yet their growth remains strikingly uneven across property families. This perspective synthesizes a systematic survey of mainstream density functional theory (DFT) software, the computational cost and credibility of nineteen material-property families, and the coverage of existing computational databases, into a coherent picture of where the field stands and where it should go. We show that the ecosystem of first-principles codes is methodologically mature: for nearly every property of technological interest, at least one production-grade code can compute it.The binding constraint is no longer methodological capability but the economics of trust - which properties can be computed cheaply enough, and accurately enough, to be harvested at database scale. Mapping database coverage onto a Gartner-style readiness cycle reveals a sharp divide: ground-state structure, energetics, elasticity, and topology have reached routine production, while nine property families - including NMR/EPR parameters, core-level spectra, electron-phonon properties, thermal conductivity, and quantum transport - remain without any systematic computational database. We argue that these blank zones define the scientific opportunity of the next decade, and we propose a three-horizon roadmap: consolidating coverage and interoperability in the near term, industrializing mid-cost properties through surrogate-accelerated workflows in the medium term, and conquering the high-cost frontier through machine-learned interatomic potentials, autonomous computing infrastructure, and community governance in the long term.
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All authors Miao Liu, Jianghao Jin, Tenglong Lu, Jianguo Si, Yin Shi, Sheng Meng, Weihua Wang

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

Vortex-mediated spin current injection into two-dimensional superconductor NbSe2

Meng Yang, Xiaolong Yin, Jingjing Liu et al. (+7 more)

Injection of pure spin current into superconductors remains a major challenge in superconducting spintronics. Previous studies have primarily focused on spin-polarized quasiparticles and spin-triplet supercurrents, while vortices, ubiquitous topological defects in type-2 superconductors, have been theoretically proposed as alternative carriers of spin angular momentum, yet direct experimental evidence is still lacking. Here, we report the vortex-mediated spin current injection in NbSe2/LiAl2Fe3O8(LAFO) bilayer, an Ising superconductor/ferrimagnetic insulator heterostructure. Under an out-of-plane temperature gradient and an in-plane magnetic field, the NbSe2/LAFO bilayer shows a pronounced thermoelectric peak near the upper critical magnetic field, which has the opposite sign to the conventional vortex Nernst signal observed in a single-layer NbSe2. The sign reversal suggests that the vortex flow induced by spin current injection is opposite to the flow driven by the temperature gradient, which is consistent with theoretical mechanisms including spin-vorticity transmutation and the inverse vortex spin Hall effect. By mapping the field temperature phase diagram, we reveal that the spin current injection occurs exclusively in the vortex liquid phase of NbSe2. Absence of the thermoelectric signal above the superconducting transition temperature further rules out the quasiparticle contribution. Our results establish vortices as efficient carriers of spin information in superconductors, opening a new route towards vortex-mediated superconducting spintronic devices.
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All authors Meng Yang, Xiaolong Yin, Jingjing Liu, Yifeng Chen, Qinwu Gao, Hongxing Zhu, Danni Huang, Lang Chen, Shuo-Ying Yang, Junxue Li

Materials Science · Cross-listed from physics.optics

Voltage-Current-Emission Framework of Time-Resolved Electroluminescence

Hui Bao, Longjia Wu, Yanan Guo et al. (+3 more)

Time-resolved electroluminescence has been a central methodology in the development of LED devices. However, it still lacks quantitative theoretical descriptions of time-resolved electroluminescence. By combining the equivalent circuit models of semiconductor devices and the carrier dynamics of electroluminescence processes, we here developed a Voltage-Current-Emission framework to illustrate time-resolved electroluminescence from the pulse voltage as input, to injection current and finally generate electroluminescence as output. The features of different LED devices can be quantitatively interpreted, confirming the universal applicability across LEDs with different materials and device structures. Importantly, the curves of time-resolved electroluminescence can be simulated to quantitatively describe the dynamics of current and carriers of LED operation. In all, the Voltage-Current-Emission framework offers theoretical guidance for achieving advanced LED devices for AI technology.
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All authors Hui Bao, Longjia Wu, Yanan Guo, Jianchang Yan, Ying Wang, Haizheng Zhong

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

Observation of Dyakonov-Perel-type magnon spin relaxation in uniaxial antiferromagnetic insulators

Qinwu Gao, Andi Cong, Bokai Liang et al. (+6 more)

Long-distance transport of magnon spin currents in antiferromagnetic (AFM) insulators has attracted tremendous attention recently, however, the AFM magnon spin relaxation mechanisms remain elusive. Here, we report that the Dyakonov-Perel-type magnon spin relaxation mechanism governs the spin current transport along the easy axis in two prototypical uniaxial AFM insulators, Cr2O3 and alpha-Fe2O3. First, an over 450% enhancement of the first-harmonic nonlocal signal induced by a magnetic field is observed prior to the spin-flop transition, which can be well-interpreted by our model incorporating Dyakonov-Perel-type magnon spin relaxation. Secondly, we find that the magnon spin diffusion length in both crystals increases with magnetic field and saturates at fields above 0.8 T, consistent with our model. Finally, the temperature dependence of the zero-field magnon spin diffusion length in both AFM insulators can be qualitatively explained through our model. These findings are valuable for the development of low-dissipation antiferromagnetic spintronic devices.
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All authors Qinwu Gao, Andi Cong, Bokai Liang, Meng Yang, Jingjing Liu, Lang Chen, Shiwei Wu, Ka Shen, Junxue Li

Materials Science

Grain Boundary Distortion Reorients the Local 4f Easy Axis and Splits Light from Heavy Lanthanides in Nd2Fe14B

Avik Mahataa and M. Zakotnik

The coercivity of rare-earth permanent magnets originates from the crystal field acting on localized 4f electrons, yet how grain boundaries modify the underlying single-ion Hamiltonian remains largely unknown. Here we determine the 4f crystal field in bulk and grain-boundary environments of Nd2Fe14B using embedded multireference electronic-structure calculations for the substitutional series Ce, Pr, Nd, Gd, Tb, and Dy. Grain-boundary distortion produces a crossover across the lanthanide series: the crystal field weakens by up to 36% for light lanthanides but strengthens by about 25% for heavy lanthanides, with Nd marking the crossover. The ground-state doublet of Dy becomes more isolated and axial, whereas Ce exhibits a collapsed low-lying excitation that suppresses anisotropy. The local easy axis rotates from the bulk c axis toward the basal plane, identifying grain-boundary sites as favorable nucleation centers for magnetization reversal. These results provide an atomistic basis for heavy rare-earth grain-boundary diffusion and furnish transferable single-ion parameters for spin-lattice simulations.
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Materials Science

Hydrogen Reorganization in Hot Dense Ammonia

Yu Tao, Jingyi Liu and Li Lei

Hydrogen is known to form stable compounds with a variety of simple molecular solids under high pressure. However, ammonia hydride has not been experimentally observed. Here, through a series of laser-heating diamond anvil cell (LHDAC) experiments, we observed the transparent ionic ammonia transformed into an opaque phase above 150 GPa and 1000 K. Raman spectroscopy reveals a pronounced structural change, accompanied by the emergence of two distinct asymmetric H$_2$ vibrons ($\nu_1$ and $\nu_2$). A new ionic phase produced following thermally induced dissociation and hydrogen reorganization in hot dense ammonia. The combination of experimental results and first-principles calculations identify this phase as the theoretically predicted ionic $P4_12_12$ phase of NH$_7$ , which consists of NH$_4^+$, H$^-$and H$_2$ units. Our results suggest that the formation of ammonia hydrides follows a thermodynamic pathway distinct from those of other H$_2$-containing mixtures, revealing a new ionic transformation pathway driven by hydrogen reorganization in hot dense ammonia.
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Materials Science · Cross-listed from cond-mat.str-el

Universality of the $1/9$ Magnetization Plateau and Quantum-Disordered States in the Kagome Family $\mathrm{Cs_8AB_3Ti_{12}F_{48}}$ ($A=\mathrm{Rb},\mathrm{Li}$; $B=\mathrm{K},\mathrm{Na}$)

Prena Chaudhary, Asiri Thennakoon, Tommy Park et al. (+17 more)

The microscopic origin of the low-field $1/9$ magnetization plateau in spin-$1/2$ kagome antiferromagnets remains unresolved. Here, we show that chemical pressure reshapes the hierarchy of fractional magnetization plateaus in the titanium-based kagome family $\mathrm{Cs_8AB_3Ti_{12}F_{48}}$ ($A=\mathrm{Rb},\mathrm{Li}$; $B=\mathrm{K},\mathrm{Na}$). High-field magnetization measurements up to 60 T reveal a robust $1/9$ plateau-like phase in the expanded $\mathrm{Cs_8RbK_3Ti_{12}F_{48}}$ and $\mathrm{Cs_8LiK_3Ti_{12}F_{48}}$ compounds, despite the absence of the conventionally more robust $1/3$ plateau. In contrast, compressed $\mathrm{Cs_8LiNa_3Ti_{12}F_{48}}$ exhibits neither the $1/9$ plateau-like phase nor a quantum-disordered ground state. Specific-heat measurements and first-principles calculations show that lattice expansion preserves a frustrated, fully connected kagome exchange network and gapless quantum-disordered ground states, whereas compression reorganizes the exchange network into weakly coupled quasi-one-dimensional subsystems and induces successive magnetic transitions. These results demonstrate that the $1/9$ and $1/3$ plateaus need not share a common microscopic origin and suggest that the $1/9$ plateau may represent a more universal feature of frustrated spin-$1/2$ kagome magnetism.
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All authors Prena Chaudhary, Asiri Thennakoon, Tommy Park, Hanru Wang, Leshan Zhao, Laurel Winter, Neil Herrison, Christina Hoffmann, Junghong H. He, Harald O. Jeschke, Hiroyuki Nojiri, Akira Matsuo, Koichi Kindo, Miwako Takahashi, Yukio Noda, Taku J. Sato, Shiyan Li, Hiroaki Ueda, Gia-Wei Chern, Seung-Hun Lee

Materials Science

Portfolio-Based Constrained Multi-Objective Bayesian Optimization for Materials Design

Sushant Sinha, Christofer Hardcastle, Robert Robinson et al. (+4 more)

Materials discovery and design campaigns can be formulated as constrained multi-objective Bayesian optimization (CMOBO) problems, within which each experimental decision negotiates between two coupled but competing goals: discovering feasible candidates and refining the underlying Pareto front. Here we recast acquisition-function choice as an adaptive policy-selection problem over a portfolio of conventional and feasibility-focused acquisition functions. This was done using two controllers: UCB-Bandit, a modified UCB multi-armed bandit, and Agentic-Switch, a multi-agent decision system driven by a large language model (LLM). Both were evaluated against fixed-policy baselines in silico across five synthetic benchmark functions and two materials design case studies. The adaptive policies performed competitively in terms of both cumulative feasibility count and feasible hypervolume improvement, while each individual acquisition function performed well for only one metric, suggesting that adaptive policies are better suited for constrained materials science problems.
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All authors Sushant Sinha, Christofer Hardcastle, Robert Robinson, Shakti Prasad Padhy, Brent Vela, Douglas Allaire, Raymundo Arroyave

Wednesday 16 September

Materials Science

NTBuilder: Commensurate Construction of Nanotubes from Arbitrary Two-Dimensional Crystals and a Catalog of 20 Million Structures

Marcelo Lopes Pereira Junior

Although tens of thousands of two-dimensional (2D) crystals are now known, the nanotubes they can form are usually constructed for individual materials, using procedures derived for graphene. Here, we develop a general theory of the rolling construction for arbitrary 2D crystals and implement it in \ntbuilder{} (NanoTube Builder), an open-source library with desktop and web interfaces. We express the size of the exact unit cell of a tube through the reduced form of a single rational number built from the lattice metric, which explains why triangular and square lattices always close with small cells, whereas rectangular, centered rectangular, and oblique lattices can require cells larger by many orders of magnitude. The optimal approximate cells are the best rational approximations of a second number, so that a cell small enough for first-principles calculations can be chosen with a controlled departure from periodicity, as in the biphenylene network, where such a cell is eight orders of magnitude smaller than the exact one. The 17 plane groups reduce to seven distinct chirality maps once rolling is taken into account, the rolling sense of layers without a horizontal mirror defines distinct tubes, and bonds formed or broken by curvature are detected pair by pair against the flat layer. The same framework assembles multiwalled tubes whose walls share one axial period, periodic bundles, and strained or twisted structures that remain periodic. Applied to 46,403 distinct 2D systems, it produced a public catalog of 20,334,372 nanotubes, of which 57.1% show no bond change upon rolling. All structures can be exported directly as input for first-principles and molecular dynamics codes.
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Materials Science · Cross-listed from quant-ph

Engineering and Probing a One-Dimensional Dipolar Spin Ensemble in Diamond

Lingjie Chen, Shreyas Parthasarathy, Simon A. Meynell et al. (+11 more)

Dimensionality plays a central role in determining the collective behavior of interacting quantum systems. Engineering strongly interacting ensembles of solid-state spin defects in reduced dimensions remains a significant challenge at the interface between the applied and fundamental sciences. Here, we create and characterize a positionally disordered, quasi-one-dimensional spin chain in diamond, consisting of optically dark substitutional nitrogen defects (P1 centers) and optically addressable probe nitrogen-vacancy (NV) centers. Our approach exploits the preferential incorporation of nitrogen along step bunches formed during chemical vapor deposition to achieve both lateral and vertical confinement. Combining spatially resolved materials characterization with nanoscale quantum sensing, we establish the one-dimensional character of the optically dark, unpolarized P1 spin ensemble. We then use correlation spectroscopy to probe local spin autocorrelations and investigate infinite-temperature dipolar spin transport. Our results establish a materials-based route for engineering low-dimensional quantum spin systems.
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All authors Lingjie Chen, Shreyas Parthasarathy, Simon A. Meynell, Lillian B. Hughes Wyatt, Eveline Postelnicu, Haopu Yang, Zilin Wang, Weijie Wu, Winston V. Peloso, Casey K. Kim, Chris R. Laumann, Kunal Mukherjee, Norman Y. Yao, Ania C. Bleszynski Jayich

Materials Science

ALIGNN 2.0: A Unified Line-Graph Neural Network Framework for Materials Screening, Force Fields, Inverse Design, Spectroscopy, and Microscopy

Jaehyung Lee, Charles Rhys Campbell, Akshaya Ajith et al. (+3 more)

Graph neural networks are central to materials property prediction and machine-learning interatomic potentials, yet their reliance on specialized graph libraries hampers portability and reproducibility, and property and force-field models have historically required separate graph pipelines. We present ALIGNN 2.0, a dependency-free, pure-PyTorch reimplementation of the Atomistic Line Graph Neural Network, with the line graph and its batching built from scratch, running on current-generation accelerators and unifying scalar, spectral, tensorial, per-atom, and force-field prediction behind a single graph, a combination that to our knowledge no existing framework provides. Comparing radius and k-nearest-neighbor (kNN) graphs, the wider kNN graph is more accurate for properties while the smoothly varying radius graph is required for energy-conserving molecular dynamics. On the JARVIS-Leaderboard, ALIGNN 2.0 leads on 26 of 30 single-property benchmarks against the original ALIGNN, with large gains for piezoelectric and dielectric maxima, exfoliation energy, moduli, and superconducting Tc. The LAMMPS- and OpenMM-compatible ALIGNN-FF matches leading universal potentials on the Matbench-Discovery and CHIPS-FF benchmarks at a small fraction of their parameters while scaling to hundred-thousand-atom cells. We further use ALIGNN 2.0 as the denoiser in a conditional crystal-diffusion model, where explicit line-graph message passing consistently lowers structural denoising loss. We also show, as work in progress, that an independently diffused, redundant bond-angle state is learnable but does not uniformly improve reconstruction or combine additively with the line graph. Finally, from a single relaxed structure the same framework reconstructs infrared, Raman, optical-dielectric, and neutron spectra in agreement with experiment and DFT, and drives frozen-phonon electron-microscopy image simulation.
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All authors Jaehyung Lee, Charles Rhys Campbell, Akshaya Ajith, Sergei V. Kalinin, Christopher Wolverton, Kamal Choudhary

Materials Science

The Origin of the Observed Raman Peaks in {\alpha}-MnTe

Nurul Azam, Syed Mohammad Shahed, Liam T. Schmidt et al. (+21 more)

The Raman spectrum of the room-temperature altermagnet $\alpha$-MnTe is reported to contain unassigned peaks at 120(3) cm$^{-1}$ and 140(3) cm$^{-1}$, absent from the predicted phonon spectrum of the material. This has generated considerable debate within the altermagnet community and necessitates urgent resolution. This work establishes that these peaks, together with the 90(5) cm$^{-1}$ peak that matches a theoretically predicted mode, are all extrinsic, originating from elemental tellurium formed during air exposure of the surface. Raman spectra of MBE-grown thin films show that these peaks closely match those of elemental tellurium, emerge soon after air exposure, and are absent in AlO$_x$-capped films. X-ray photoelectron spectroscopy shows that air exposure breaks Mn–Te bonds and oxidizes Mn within a minute. Cross-sectional scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy reveals that the oxidation leads to Mn out-diffusion, forming a few-nanometer-thick oxide layer above a buried, Te-enriched region, likely responsible for the anomalous Raman peaks. Additionally, no sample exhibited the 175 cm$^{-1}$ Raman peak which is commonly attributed to MnTe$_2$. The aggressive surface oxidation and associated elemental Te formation in MnTe have important implications for any surface-sensitive and optical characterization of MnTe (and other similar Te-containing materials) involving even the briefest air exposure.
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All authors Nurul Azam, Syed Mohammad Shahed, Liam T. Schmidt, Sara Bey, Oksana Yastrubchak, Maria F. Munoz, Riccardo Torsi, Thi Hai Yen Pham, Dushyanthini Balasundaram, Resham Babu Regami, Wentao Liang, Imrankhan Mulani, Matthew Matzelle, Vineet Kumar Sharma, Sougata Mardanya, Sugata Chowdhury, Nirmal Ghimire, Patrick M. Vora, Angela R. Hight Walker, Xinyu Liu, Badih A. Assaf, Arun Bansil, Alberto De la Torre, Swastik Kar

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

Multiferroic Quantum Dot in an Artificial van der Waals Heterostructure

Antti Karjasilta, Mohammad Amini, Liwei Jing et al. (+5 more)

Quantum dots (QDs) provide a versatile platform for engineering quantum-confined electronic states with functionalities relevant for optoelectronics, spintronics, and quantum technologies. While substantial progress has been achieved in coupling confined states to spin, valley, topological, or ferroelectric degrees of freedom, the realization of a multiferroic QD in which quantum confinement simultaneously intertwines with magnetism and ferroelectricity remains elusive. Here, we engineer a multiferroic QD in an artificial van der Waals heterostructure grown by molecular beam epitaxy under ultra-high-vacuum conditions. The heterostructure consists of ferroelectric SnTe nanoislands deposited on the layered magnet CrBr$_2$ supported on highly oriented pyrolytic graphite. Combining scanning tunneling microscopy and spectroscopy with ab initio calculations and low-energy tight-binding models, we demonstrate the emergence of spin-polarized discretized electronic states confined within the SnTe islands. Remarkably, the spectroscopic response of the QD strongly depends on the ferroelectric domain configuration of the SnTe nanoislands, demonstrating an interplay between quantum confinement, magnetic exchange, and ferroelectric order at the atomic scale. Our results establish engineered van der Waals heterostructures as a platform for multiferroic quantum confinement and open new routes toward electrically tunable quantum spintronic devices.
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All authors Antti Karjasilta, Mohammad Amini, Liwei Jing, Robert Drost, Jose Lado, Shawulienu Kezilebieke, Peter Liljeroth, Adolfo O. Fumega

Materials Science

Pressure-dependent melting and crystallization of B2-NiAl from neural-network molecular dynamics

A. S. Onegin, P. R. Levashov and N. M. Chtchelkatchev

We investigate pressure-dependent melting of ordered B2-NiAl using neural-network molecular dynamics with a Deep Potential interatomic model. Melting temperatures are determined from two-phase solid-liquid coexistence simulations over a broad pressure range, yielding the melting curve $T_m(P)$. Relative to available experimental and previous molecular-dynamics results, the present calculations predict a stronger increase of the melting temperature with pressure at elevated compression. To assess the thermodynamic consistency of the calculated melting line, we evaluate the enthalpy and volume changes upon melting and compare the Clapeyron slope with the derivative of the fitted $T_m(P)$ curve. The two estimates are in good agreement over most of the investigated pressure range, supporting the internal consistency of the coexistence results. To probe the character of melting, we perform a layer-resolved composition analysis of the coexistence configurations and find that the coexisting liquid remains essentially equiatomic at all studied pressures, with deviations of the aluminum fraction from the stoichiometric value not exceeding $5\times10^{-3}$. This provides direct atomistic evidence that melting of B2-NiAl remains congruent within the present model. Together, these results establish a thermodynamically consistent pressure-dependent melting description of B2-NiAl and clarify the character of its melting under compression.
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Materials Science

Electron $g$-value comparison of paramagnetic muonium and hydrogen centers in rutile TiO$_2$

T. U. Ito, W. Higemoto and A. Koda

Muonium (Mu) is widely used as a model for isolated hydrogen in condensed matter, but whether its electronic state is truly equivalent to that of hydrogen remains to be tested with an orbital-sensitive probe. Here we use double electron-muon resonance to determine the electron $g$ values of the paramagnetic Mu center in rutile TiO$_2$ for magnetic fields along [100], [110], and [001] directions. The obtained $g$ values closely follow those of the H-related Ti$^{3+}$ paramagnetic center in both magnitude and anisotropy. This agreement tests the Mu–H correspondence at the level of the Ti $3d$ electronic state and supports a common localized Ti$^{3+}$ electronic core for the two centers.
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Materials Science · Cross-listed from physics.app-ph

Si- and C-Induced Modifications in Vapor-Solid-Grown InGaAs Nanowires: From Crystal Structure to Carrier Dynamics

Hamidreza Esmaielpour, Leopold Rothmayer, Thomas Trinkl et al. (+5 more)

Determining the impact of dopants in ternary III-V nanowires (NWs) is crucial for their applications in next-generation optoelectronic and photonic devices; however, the interplay between dopant species, crystal structure, and carrier dynamics in vapor-solid-grown InGaAs NWs remains underexplored. In this study, the influence of silicon (Si) and carbon (C) doping on the structural, morphological, and optical properties of catalyst-free, selective-area grown InGaAs NWs fabricated by molecular beam epitaxy is investigated. Scanning electron microscopy demonstrates that C-doping increases the NW aspect ratio by over 100% compared to undoped NWs, while a concomitant decrease in their planar defect density is observed, indicating enhanced crystal quality with C-doping. Despite their superior crystal quality, the C-doped NWs present a diminished photoluminescence intensity, due to point defects and increased surface recombination velocity. This is further supported by time-resolved photoluminescence spectroscopy, revealing accelerated non-radiative recombination in the C-doped NWs. Lastly, hot-carrier effects in these nanostructures are studied, and their signatures remain observable, albeit with reduced intensity in both Si- and C-doped NWs. This comprehensive study establishes clear correlations between doping parameters and the resulting material characteristics and carrier dynamics, offering valuable insights into the fundamental mechanisms governing the VS-grown InGaAs NWs and their implications in optoelectronic devices.
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All authors Hamidreza Esmaielpour, Leopold Rothmayer, Thomas Trinkl, Laura Niermann, Tore Niermann, Michael Lehmann, Jonathan J. Finley, Gregor Koblmüller

Materials Science

Melting of corundum ({\alpha}-Al2O3) under compression to 8 GPa: A decreasing melting slope

Vladimir L. Solozhenko

Melting of corundum ({\alpha}-Al2O3) has been studied under compression up to 8 GPa using in situ electrical resistivity measurements. The melting temperature increases monotonically from T0 = 2317 K at ambient pressure to 3090 K at 7.7 GPa. Fitting the experimental data to the Simon-Glatzel equation yields the empirical parameters a = 5.60 GPa and c = 0.334, providing a closed-form analytical description of the entire melting boundary. Most significantly, the melting slope, dTm/dp, decreases continuously from 138 K/GPa at ambient pressure down to 78 K/GPa at 7.7 GPa, confirming unequivocal negative curvature of the melting curve (d2Tm/dp2 < 0). This phenomenon is consistent with the preferential densification of the liquid phase and the associated reduction in the volume change upon melting as pressure rises. These findings provide a robust empirical standard for the high-pressure melting of corundum, establishing essential constraints for future ab initio simulations, thermodynamic databases, and geophysical models of alumina-rich deep-Earth systems.
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Materials Science · Cross-listed from physics.chem-ph

Floquet-Plasmon Enhanced Charge Transfer at Catalytic Interfaces

Annabelle Canestraight, Phillip Christopher and Vojtech Vlcek

Plasmonic excitation of metallic nanostructures can generate energetic carriers capable of transferring charge into nearby adsorbed molecules, providing a possible pathway for driving chemical transformations. In many theoretical descriptions of this process, charge transfer is determined by the equilibrium electronic structure of the molecule and substrate together with the time-dependent carrier distribution produced during plasmon decay. However, the intense transient electric fields associated with localized surface plasmons can also dynamically perturb the molecular electronic structure itself on ultrafast timescales. In this work, we investigate how these time-dependent fields modify the molecular spectral function and influence charge injection at catalytic interfaces. Using a model of CO2 adsorbed on Au(111), we compute the real-time molecular Green's function within a correlated frontier-orbital active space under plasmon-like driving. We find that the driving field rapidly produces transient Floquet-type replica bands in the molecular density of states, opening additional resonant pathways for hot-carrier injection that are absent without external driving or in time-local descriptions. Coupling the evolving molecular spectrum to a time-dependent hot-electron distribution described within a two-temperature Sommerfeld framework, we predict large enhancements in quasiparticle spectral overlap that governs injection during plasmon dephasing. These results suggest that dynamically generated non-equilibrium spectral structure may play an important role in plasmon-assisted catalysis and provide a framework for studying driven catalytic interfaces beyond static electronic structure descriptions.
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Materials Science

Divergence between long- and short-wavelength magnon damping in spinel ferrites

Christopher T. Parzyck, Octave Duros, Hari Paudyal et al. (+14 more)

The realization of practical, high-speed magnonic devices requires engineering magnetic materials with low dissipation over wide frequency ranges. While optical and microwave probes are used to infer the damping of low energy/long wavelength modes, the degree to which these $q\sim0$ properties translate into higher-energy, finite-momentum modes remains an important open question. Here, we utilize a combination of ferromagnetic resonance (FMR) and resonant inelastic x-ray scattering on spinel ferrites Li$_{0.5}$Al$_x$Fe$_{2.5-x}$O$_4$ to probe magnons in both the short- and long-wavelength limits. We observe that aluminum substitution both markedly reduces the magnon bandwidth and drastically shortens the high-$q$ magnon lifetimes, in sharp contrast to the ultralow magnon damping inferred from FMR. These findings demonstrate a disparity between how non-magnetic substituents impact magnon damping in the long- and short-wavelength limits, providing a new perspective for assessing candidate materials for magnonic devices.
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All authors Christopher T. Parzyck, Octave Duros, Hari Paudyal, Noah M. Edmiston, Katya Mikhailova, Lerato Takana, Daisy O'Mahoney, Sauviz P. Alaei, Daniel J. Foster, Hiroki Suga, Naoya Kurahashi, Jun Miyawaki, Michael E. Flatte, Georgi Dakovski, Yuri Suzuki, Durga Paudyal, Wei-Sheng Lee