Materials Science
Physically-Constrained Autoencoder-Assisted Bayesian Optimization for Refinement of High-Dimensional Defect-Sensitive Single Crystalline Structure
Joseph Oche Agada, Andrew McAninch, Haley Day et al. (+6 more)
Physical properties and functionalities of materials are dictated by global
crystal structures as well as local defects. To establish a structure-property relationship, not only the crystallographic
symmetry but also quantitative knowledge about defects are required. Here we present a hybrid Machine Learning framework that integrates a physically-constrained variational-autoencoder (pcVAE) with different
Bayesian Optimization (BO) methods to systematically accelerate and improve
crystal structure refinement with resolution of defects. We chose the pyrochlore structured Ho2Ti2O7 as a model system and employed the GSAS2 package for benchmarking crystallographic parameters from Rietveld refinement. However, the function space of these material systems is highly nonlinear, which limits optimizers like traditional Rietveld refinement, into trapping at local minima. Also, these naive methods don't provide an extensive learning about the overall function space, which is essential for large space, large time consuming explorations to identify various potential regions of interest. Thus, we present the approach of exploring the high Dimensional structure parameters of defect sensitive systems via pretrained pcVAE assisted BO and Sparse Axis Aligned BO. The pcVAE projects high-Dimensional diffraction data consisting of thousands of independently measured diffraction orders into a lowD latent space while enforcing scaling invariance and physical relevance. Then via BO methods, we aim to minimize the L2 norm based chisq errors in the real and latent spaces separately between experimental and simulated diffraction patterns, thereby steering the refinement towards potential optimum
crystal structure parameters. We investigated and compared the results among different pcVAE assisted BO, non pcVAE assisted BO, and Rietveld refinement.
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All authors Joseph Oche Agada, Andrew McAninch, Haley Day, Yasemin Tanyu, Ewan McCombs, Seyed M. Koohpayeh, Brian H. Toby, Yishu Wang, Arpan Biswas
Materials Science · Cross-listed from physics.soc-ph
The hidden sustainability bottleneck in high-entropy alloy design
Alexandre Nominé, Ayyoub El-Kasmi, Danielle Beaulieu et al. (+21 more)
Because of the enormous number of possible compositions, comparable to the number of
stars in the universe, high-
entropy alloys (HEAs) constitute a virtually inexhaustible materials space with highly versatile properties. Among these systems, HEAs are often proposed as potential substitutes for critical elements such as rare earths or platinum group metals. However, random or incremental exploration strategies are neither practical nor efficient at this scale. Targeted materials selection guided by sustainability considerations is therefore essential, yet identifying sustainable HEA compositions remains highly challenging. Here, we perform a comprehensive sustainability assessment of 30,201 equimolar HEA compositions and identify a resilient shortlist (approximately 5%) that consistently exhibits favorable sustainability profiles across multiple evaluation schemes. Our analysis integrates complementary criteria including carbon footprint, environmental, social and governance (ESG) risks, production compatibility, and resource availability. The resulting sustainability-based ranking provides a strategic roadmap for HEA research, enabling experimental efforts to be focused on compositions that are not only functionally promising but also scalable and resource-responsible. By aligning materials discovery with sustainability and supply constraints, this framework supports more efficient use of experimental resources while contributing to long-term industrial sustainability goals.
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All authors Alexandre Nominé, Ayyoub El-Kasmi, Danielle Beaulieu, Outhmane Ouahri, Nguyen Thuy Huong, Wassim Amzil, Aymane Droussi, Oleksandra Kuksa, Eirini Katsarou, Chahrazed Labba, Anne Boyer, Hani Henein, Thierry Belmonte, François Rousseau, Tuncay Gurbuz, Elena Mitrofanova, Agnès Samper, Pascal Bolz, Valentin Milichko, Olga Chernoburova, Alexandre Chagnes, Michel Cathelineau, Uroš Cvelbar, Janez Zavašnik
Materials Science · Cross-listed from physics.ins-det
Autonomous battery research: Principles of heuristic operando experimentation
Emily Lu, Gabriel Perez, Peter Baker et al. (+21 more)
Unravelling the complex processes governing battery degradation is critical to the energy transition, yet the efficacy of operando characterisation is severely constrained by a lack of Reliability, Representativeness, and Reproducibility (the 3Rs). Current methods rely on bespoke hardware and passive, pre-programmed methodologies that are ill-equipped to capture
stochastic failure events. Here, using the Rutherford Appleton Laboratory's multi-modal toolkit as a case study, we expose the systemic inability of conventional experiments to capture transient phenomena like dendrite initiation. To address this, we propose Heuristic Operando experiments: a framework where an AI pilot leverages physics-based digital twins to actively steer the beamline to predict and deterministically capture these rare events. Distinct from uncertainty-driven active learning, this proactive search anticipates failure precursors, redefining experimental efficiency via an
entropy-based metric that prioritises scientific insight per
photon,
neutron, or
muon. By focusing measurements only on mechanistically decisive moments, this framework simultaneously mitigates beam damage and drastically reduces data redundancy. When integrated with FAIR data principles, this approach serves as a blueprint for the trusted autonomous battery laboratories of the future.
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All authors Emily Lu, Gabriel Perez, Peter Baker, Daniel Irving, Santosh Kumar, Veronica Celorrio, Sylvia Britto, Thomas F. Headen, Miguel Gomez-Gonzalez, Connor Wright, Calum Green, Robert Scott Young, Oleg Kirichek, Ali Mortazavi, Sarah Day, Isabel Antony, Zoe Wright, Thomas Wood, Tim Snow, Jeyan Thiyagalingam, Paul Quinn, Martin Owen Jones, William David, James Le Houx
Materials Science · Cross-listed from cs.AI
CASCADE: Cumulative Agentic Skill Creation through Autonomous Development and Evolution
Xu Huang, Junwu Chen, Yuxing Fei et al. (+3 more)
Large
language model (LLM)
agents currently depend on predefined tools or early-stage tool generation, limiting their adaptability and scalability to complex scientific tasks. We introduce CASCADE, a self-evolving agentic framework representing an early instantiation of the transition from "LLM + tool use" to "LLM + skill acquisition". CASCADE enables
agents to master complex external tools and codify knowledge through two meta-skills: continuous learning via web search, code extraction, and
memory utilization; self-reflection via introspection, knowledge graph exploration, and others. We evaluate CASCADE on SciSkillBench, a
benchmark of 116 materials science and chemistry research tasks. CASCADE achieves a 93.3% success rate using GPT-5, compared to 35.4% without
evolution mechanisms. We further demonstrate real-world applications in computational analysis, autonomous laboratory experiments, and selective reproduction of published papers. Along with human-
agent collaboration and
memory consolidation, CASCADE accumulates executable skills that can be shared across
agents and scientists, moving toward scalable AI-assisted scientific research.
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All authors Xu Huang, Junwu Chen, Yuxing Fei, Zhuohan Li, Philippe Schwaller, Gerbrand Ceder
Materials Science · Cross-listed from physics.chem-ph
Surface adsorption at the thermodynamic limit using periodic DLPNO-MP2 theory: A study of CO on MgO at dilute and dense coverages
Andrew Zhu, Poramas Komonvasee, Arman Nejad et al. (+1 more)
We apply periodic domain-based local pair natural orbital second-order Møller–Plesset perturbation theory (DLPNO-MP2) to probe the adsorption energy of CO on MgO(001), the consensus toy model system for surface adsorption. A number of robust correlated wavefunction methods now achieve excellent agreement with experiment for the adsorption of a single CO molecule onto the MgO surface. However, studies probing denser coverage ratios are scarce because of the increased computational expense and the larger configuration space to optimize. We leverage the computational efficiency of periodic DLPNO-MP2 to perform simulations beyond a single unit
cell. By using large supercells, we highlight the importance of accurately representing the thermodynamic limit of the surface, and demonstrate in turn that different coverage ratios can be consistently probed. In the dilute regime, we show that adsorption energies obtained from periodic DLPNO-MP2 agree with existing
benchmarks. We then obtain adsorption energies at increasing densities approaching full monolayer coverage. Our results show a reduction in binding strength at full coverage, agreeing with experimental observations, which is explained by the increasing lateral repulsions between the COs. This study demonstrates the efficacy of periodic DLPNO-MP2 for probing increasingly sophisticated adsorption systems at the thermodynamic limit.
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All authors Andrew Zhu, Poramas Komonvasee, Arman Nejad, David P. Tew
Materials Science
Hydrogen localization under thermal gradients in hydride forming metals
K. A. Forsberg and A. R. Massih
Migration of hydrogen and hydride formation under thermal gradient leads to hydrogen redistribution in certain metals. These metals include zirconium, titanium, hafnium and their
alloys with tendency to form hydrides. A computational method for hydrogen localization in such metals is presented. The method utilizes the heat flux in a steady state to compute temperature distribution (as input), and hydrogen mass flux under temperature gradient to determine hydrogen distribution both in solid solution and in the hydride phase in a two-dimensional setting. Hydrogen precipitation to hydride is determined by a solid solubility relation with an exponential function of the enthalpy of mixing per a van 't Hoff relation. The enthalpy of mixing is treated here as a
stochastic variable subject to thermodynamic fluctuations. Henceforth, the Einstein-Boltzmann fluctuation theory is adapted to calculate the spatial distribution of hydrogen in solid solution and in the hydride phase. Hydrogen concentration gets localized in the colder region of the body (Soret effect). We apply the model to the case of a zirconium
alloy, Zircaloy-4, which is a material for fuel cladding utilized in pressurized water reactors. Cladding continuously picks up hydrogen due to Zr oxidation during reactor service, which we take into account. Our calculated results, hydrogen concentration profiles are comparable to experimental observations reported in the literature.
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Materials Science
Magnetically recoverable MgFe$_2$O$_4$ nanoparticles as efficient catalysts for rapid dye degradation in water
A. F. Cabrera, C. E. Rodríguez Torres, P. de la Presa et al. (+1 more)
Monophasic MgFe$_2$O$_4$ nanoparticles synthesized by a simple autocombustion method were assessed as magnetically recoverable catalysts for the degradation of methylene blue (MB) in water. The NPs exhibit a crystallite size of $\sim$ 9 nm, a band gap of $\sim$ 2.11 eV, and soft ferrimagnetic behavior, enabling efficient photocatalytic and Fenton-like activity. The effects of irradiation, H$_2$O$_2$ concentration, agitation mode, catalyst loading, and exposure time were systematically evaluated. Rapid and complete MB discoloration was achieved within minutes in the presence of H$_2$O$_2$, even without illumination, indicating that the process is dominated by a surface-mediated heterogeneous Fenton-like mechanism rather than photocatalysis. Kinetic analysis reveals pseudo-first-order behavior, with rate constants governed by the combined effects of catalyst concentration, oxidant dosage, and dye concentration. Structural stability and excellent recyclability confirm the robustness of the catalyst. These findings position MgFe$_2$O$_4$ nanoparticles as a low-cost, efficient, and reusable material for sustainable wastewater under operationally simple conditions.
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All authors A. F. Cabrera, C. E. Rodríguez Torres, P. de la Presa, S. J. Stewart
Materials Science
Octahedral rotation instability in Ba$_2$IrO$_4$
Alaska Subedi
Ba$_2$IrO$_4$ has been refined in the tetragonal $I4/mmm$ phase without octahedral rotations, and its physical properties have been interpreted in this high-
symmetry structure. However, the dynamical stability of this undistorted phase has not previously been questioned. It is important to establish whether other lower-
symmetry structures are energetically more favorable because octahedral rotations control electronic bandwidths and constrain which
magnetic interactions are allowed by
symmetry. Here I compute first-principles
phonon dispersions of $I4/mmm$ Ba$_2$IrO$_4$ including
spin-orbit interaction. I find a nearly-flat nondegenerate unstable branch along the Brillouin-zone boundary segment $XP$ associated with inplane rotations of the IrO$_6$ octahedra. Using group-theoretical analysis, I enumerate the
symmetry-allowed distortions associated with the $X_2^+$ and $P_4$ instabilities and fully relax the resulting structures. Only five of the twelve possible distortions can be stabilized, and the energy gain scales with the number of layers that exhibit octahedral rotations: phases with rotations in every IrO$_6$ layer are lower by $-5.8$ meV/atom and are nearly degenerate with respect to the stacking phase. Electronic structure calculations show that these rotated phases host a narrow and well-separated half-filled $J_{\textrm{eff}} = 1/2$
manifold, whereas structures with rotations only in alternate layers have broader and more entangled bands. This motivates a reinvestigation of the
crystal structure of Ba$_2$IrO$_4$ and indicates that octahedral rotations should be considered in modeling its correlated electronic and
magnetic properties.
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Materials Science
The Fundamental Lemma of Altermagnetism: Emergence of Alterferrimagnetism
Chanchal K. Barman, Bishal Das, Alessio Filippetti et al. (+2 more)
Recent years have seen a proliferation in investigations on Altermagnetism due to its exciting prospects both from an applications perspective and theoretical standpoint. Traditionally, altermagnets are distinguished from collinear antiferromagnets using the central concept of halving subgroups within the
spin space group formalism. In this work, we propose the Fundamental Lemma of Altermagnetism (FLAM) deriving the exact conditions required for the existence of altermagnetic phase in a
magnetic material on the basis of site-
symmetry groups and halving subgroups for a given crystallographic space group. The
spin group formalism further clubs ferrimagnetism with ferromagnetism since the same-
spin and opposite-
spin sublattices lose their meaning in the presence of multiple
magnetic species. As a consequence of FLAM, we further propose a class of fully compensated ferrimagnets, termed as Alterferrimagnets (AFiMs), which can show alternating momentum-dependent
spin-polarized non-relativistic electronic bands within the first Brillouin zone. We show that alterferrimagnetism is a generalization of traditional collinear altermagnetism where multiple
magnetic species are allowed to coexist forming fully compensated
magnetic-sublattices, each with individual up-
spin and down-
spin sublattices.
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All authors Chanchal K. Barman, Bishal Das, Alessio Filippetti, Aftab Alam, Fabio Bernardini
Materials Science · Cross-listed from cond-mat.soft
Exploring phase transitions and thermal dynamics in nanoconfined liquid crystals using liquid-phase TEM
Olga Kaczmarczyk, Konrad Cyprych, Dominika Benkowska-Biernacka et al. (+6 more)
Nanoconfined
liquid crystals (LCs) and their nanocomposites are driving the next generation of photonic applications. Consequently, deepening our understanding of mesophase stability, defect
topology, and the dynamic response of LCs at the nanoscale requires the development of novel characterization approaches. This motivates us to perform in situ observations on model 4'-octyl-4-cyanobiphenyl (8CB) LC using
liquid-phase scanning transmission electron microscopy (LP-STEM). We find that the electron beam induced consecutive phase changes from smectic A to nematic (SmA-N) and from nematic to isotropic (N-I). The kinetic dependence of the phase transition on dose rate shows that the time between SmA-N and N-I shortens with increasing rate, revealing the hypothesis that a higher electron dose rate increases the energy dissipation rate, leading to substantial heat generation in the sample. We report on the spontaneous formation of disclinations, ordering effects, and complete process reversibility. Radiolytic effects of the electron beam are discussed in detail, and additional experiments with external heating indicate that the observed phenomena are mainly thermal in nature. The results are supported by calculations of heat
diffusion, suggesting the nanoconfined 8CB differs significantly in thermal properties compared to the bulk one. This is the first detailed study of LC phase transitions using LP-STEM, which paves the way for further studies of nanoconfined LCs and for the development of the technique for advanced LC materials research.
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All authors Olga Kaczmarczyk, Konrad Cyprych, Dominika Benkowska-Biernacka, Rafał Kowalczyk, Katarzyna Matczyszyn, Hanglong Wu, Frances M. Ross, Andrzej Miniewicz, Andrzej Żak
Materials Science
Phosphorus-based lubricant additives on iron with Machine Learning Interatomic Potentials
Paolo Restuccia, Enrico Pedretti, Francesca Benini et al. (+2 more)
Phosphorus-based lubricant additives are used for protecting metallic contacts under boundary lubrication by forming surface films that reduce wear and friction. Despite their importance, the molecular mechanisms driving their friction-reducing effects remain unclear, especially for phosphate esters, whose molecular structure critically impact tribological behavior. In this study, we use machine learning-based molecular dynamics simulations to investigate the tribological performance of three representative phosphorus-based additives, Dibutyl Hydrogen Phosphite (DBHP), Octyl Acid Phosphate (OAP), and Methyl Polyethylene Glycol Phosphate (mPEG-P), on iron surfaces. The mPEG-P family is further analyzed by varying esterification degree and chain length. DBHP exhibits the lowest friction and largest interfacial separation, resulting from steric hindrance and tribochemical reactivity, as indicated by P-O bond cleavage and enhanced O-Fe interactions. In contrast, OAP and mPEG-P monoesters produce higher friction due to limited steric protection and reduced resistance to shear, leading to partial loss of surface coverage under extreme conditions. Within the mPEG-P family, multi-ester and longer-chain molecules significantly lower friction by maintaining larger separations, demonstrating that steric effects can outweigh surface reactivity under severe confinement. Overall, these results provide atomistic insights into how molecular architecture controls additive performance and support the design of phosphorus-based lubricants combining reactive anchoring with optimized steric structures for durable, low-friction interfaces.
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All authors Paolo Restuccia, Enrico Pedretti, Francesca Benini, Sophie Loehlé, M. Clelia Righi
Materials Science · Cross-listed from physics.optics
Low loss switchable topological photonic crystal enabled by submicron-scale patterning and phase-change of Sb2Se3
Takahiro Uemura, Yuto Moritake, Eiichi Kuramochi et al. (+3 more)
Photonic topological
insulators (PTIs) offer robust platforms for light manipulation, but reconfigurable control of their topological properties without degrading performance remains a major challenge. While phase-change materials (PCMs) provide large refractive index modulation, widely used materials such as Ge2Sb2Te5 (GST) have been successfully deployed in commercial applications including optical data storage. However, they exhibit significant optical absorption in their crystalline state, which poses a challenge for transmissive photonic devices such as PTIs where high transparency is essential. Here, we overcome this fundamental limitation by integrating the ultra-low-loss PCM antimony triselenide (Sb2Se3) onto a silicon-based 2D PTI. We achieve submicron-scale selective patterning of Sb2Se3 on a photonic
crystal for the first time, and demonstrate a topological phase transition induced by the material phase change. Owing to the transparency of Sb2Se3 in both its amorphous and crystalline states, a high Q-factor on the order of 10^3 is preserved-representing nearly an order-of-magnitude improvement over previous GST-based devices. This work resolves the absorption-loss bottleneck in reconfigurable PTIs and paves the way for practical, low-loss, tunable topological photonic devices.
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All authors Takahiro Uemura, Yuto Moritake, Eiichi Kuramochi, Masaaki Ono, Hisashi Sumikura, Masaya Notomi
Materials Science
Operando study of the evolution of peritectic structures in metal solidification by quasi-simultaneous synchrotron X-ray diffraction and tomography
Kang Xiang, Yueyuan Wang, Shi Huang et al. (+8 more)
Using quasi-simultaneous synchrotron X-ray diffraction and
tomography techniques, we have studied in-situ and in real-time the nucleation and co-growth dynamics of the peritectic structures in an Al-Mn
alloy during solidification. We collected ~30 TB 4D
datasets which allow us to elucidate the phases' co-growth dynamics and their spatial, crystallographic and compositional relationship. The primary Al4Mn hexagonal prisms nucleate and grow with high kinetic anisotropy -70 times faster in the axial direction than the radial direction. In all cases, a ~5 um Mn-rich
diffusion layer forms at the
liquid-solid interface, creating a sharp local solute gradient that governs subsequent phase transformation. The peritectic Al6Mn phases nucleate epitaxially within this
diffusion zone, initially forming a thin shell surrounding the Al4Mn with an orientation relationship of {10-10}HCP // {110}O, [0001]HCP // [001]O. Such ~5 um Mn-rich
diffusion layers also cause solute depletion at the
liquid side of the
liquid-solid interface, limiting further epitaxial phase growth, but prompting phase re-nucleation and branching at
crystal edges, resulting tetragonal prism structures that no longer follow the initial orientation relationship. The anisotropic
diffusion also led to the formation of core defects at the centre of both phases. Furthermore, increasing cooling rate from 0.17 to 20 {\deg}C/s can disrupt the stability of the solute
diffusion zone, effectively suppressing the formation of the core defects and forcing a transition from faceted to non-faceted morphologies. Our work establishes a new theoretical framework for how to tailor and control the peritectic structures in metallic
alloys through solidification processes.
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All authors Kang Xiang, Yueyuan Wang, Shi Huang, Hongyuan Song, Alberto Leonardi, Peter Garland, Sharif Ahmed, Michał M. Kłosowski, Hongmei Yang, Mengnie Li, Jiawei Mi
Materials Science
Universal and Experiment-calibrated Prediction of XANES through Crystal Graph Neural Network and Transfer Learning Strategy
Zichang Lin, Wenjie Chen, Yitao Lin et al. (+2 more)
Theoretical simulation is helpful for accurate interpretation of experimental X-ray absorption near-edge structure (XANES) spectra that contain rich atomic and electronic structure information of materials. However, current simulation methods are usually too complex to give the needed accuracy and timeliness when a large amount of data need to be analyzed, such as for in-situ characterization of battery materials. To address these problems, artificial intelligence (AI) models have been developed for XANES prediction. However, instead of using experimental XANES data, the existing models are trained using simulated data, resulting in significant discrepancies between the predicted and experimental spectra. Also, the universality across different elements has not been well studied for such models. In this work, we firstly establish a
crystal graph neural network, pre-trained on simulated XANES data covering 48 elements, to achieve universal XANES prediction with a low average relative square error of 0.020223; and then utilize transfer learning to calibrate the model using a small experimental XANES
dataset. After calibration, the edge energy misalignment error of the predicted S, Ti and Fe K edge XANES is significantly reduced by about 80%. The method demonstrated in this work opens up a new way to achieve fast, universal, and experiment-calibrated XANES prediction.
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All authors Zichang Lin, Wenjie Chen, Yitao Lin, Xinxin Zhang, Yuegang Zhang
Materials Science
Order-disorder duality of high entropy alloys extends non-linear optics
Valentin A. Milichko, Ekaterina Gunina, Nikita Kulachenkov et al. (+21 more)
Order versus disorder in the structure of materials plays a key role in the theoretical prediction of their properties. However, this structural description appears to be ineffective for new families of materials such as high
entropy alloys (HEAs), which combine crystallographic order with chemical disorder. Here, we demonstrate for five-element HEAs as pure solid solutions that the chemical disorder of the elements decorating their cubic structure underlies the generation of second optical harmonics, overcoming the theoretical limit imposed on centrosymmetric
crystals. Moreover, we discover that this disorder, inherent to HEAs, sets a threshold for non-linear light emission from the 4th to the 26th order. As a consequence of the 0.5 eV broadening of the energy levels of the five elements of the HEA, the emission spectrum covers broad visible (400-650 nm) and infrared (800-1600 nm) ranges. In addition to the challenge of theoretically predicting non-linear effects in unconventional materials, the duality of structural order and chemical disorder in HEAs offers the opportunity to design sustainable alternatives to urgently needed optical materials.
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All authors Valentin A. Milichko, Ekaterina Gunina, Nikita Kulachenkov, Maxime Vergès, Luis Casillas Trujillo, Maria Timofeeva, Jaafar Ghanbaja, Stéphanie Bruyère, Andrey Krasilin, Mikhail Petrov, Michael Feuerbacher, Yann Battie, Rachel Grange, Jean-Pascal Borra, Jean-François Pierson, Vincent Fournée, Thierry Belmonte, Janez Zavašnik, Björn Alling, Joseph Kioseoglou, Ivan Iorsh, Julian Ledieu, Uroš Cvelbar, Alexandre Nominé
Materials Science · Cross-listed from cond-mat.mes-hall
Atomic-scale spin sensing of a 2D $d$-wave altermagnet via helical tunneling
Zhuying Wang, Shuikang Yu, Xingkai Cheng et al. (+17 more)
Altermagnetism simultaneously possesses nonrelativistic
spin responses and zero net magnetization, thus combining advantages of ferromagnetism and antiferromagnetism. This superiority originates from its unique dual feature, i.e., opposite-
magnetic sublattices in real space and alternating
spin polarization in momentum space enforced by the same
crystal symmetry. Therefore, the determination of an altermagnetic order and its unique
spin response inherently necessitates atomic-scale
spin-resolved measurements in real and momentum spaces, an experimental milestone yet to be achieved. Here, via utilizing the helical edge (hinge) modes of a higher order topological
insulator as the
spin sensor, we realize
spin-resolved scanning tunneling microscopy which enables us to pin down the dual-space feature of a layered $d$-wave altermagnet, KV$_2$Se$_2$O. In real space, atomic-registered mapping demonstrates the checkerboard
antiferromagnetic order together with density-wave
lattice modulation, and in momentum space,
spin-resolved spectroscopic imaging provides a direct visualization of d-wave
spin splitting of the band structure. Critically, using this new
topology-guaranteed
spin filter we directly reveal the unidirectional,
spin-polarized quasiparticle excitations originating from the
crystal symmetry-paired X and Y valleys around opposite
magnetic sublattices simultaneously –the unique
spin response for $d$-wave altermagnetism. Our experiments establish a solid basis for the exploration and utilization of altermagnetism in layered materials and further facilitate access to atomic-scale
spin sensing and manipulating of 2D
quantum materials.
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All authors Zhuying Wang, Shuikang Yu, Xingkai Cheng, Xiaoyu Xiao, Wanru Ma, Feixiong Quan, Hongxi Song, Kunming Zhang, Yunmei Zhang, Yitian Ma, Wenhao Liu, Priti Yadav, Xiangbiao Shi, Zhijun Wang, Qian Niu, Yang Gao, Bin Xiang, Junwei Liu, Zhenyu Wang, Xianhui Chen
Materials Science · Cross-listed from cond-mat.mes-hall
Observation of robust one-dimensional edge channels in a three-dimensional quantum spin Hall insulator
Shuikang Yu, Junze Deng, Wenhao Liu et al. (+13 more)
Topologically protected edge channels show prospects for
quantum devices. They have been found experimentally in two-dimensional (2D)
quantum spin Hall insulators (QSHIs), weak topological
insulators and higher-order topological
insulators (HOTIs), but the number of materials realizing these
topologies is still quite limited. Here, we provide evidence for topological edge states within a novel
topology named three-dimensional (3D) QSHIs. Its
topology originates solely from a nonzero $S_z$
spin Chern number for each $k_z$ plane of the
crystal and is realized in bulk $\alpha$-Bi$_4$I$_4$ with trivial
symmetry indicators, as we show by density functional theory calculations. We experimentally observe the related edge states at each type of monolayer and
bilayer step of this material by scanning tunneling microscopy. Consistently, the edge states are neither interrupted, nor backscattered by defects at the step edges corroborating their helical character as expected from the nontrivial
topology. Furthermore, two individual edge channels are directly observed at
bilayer steps without visible interaction gap opening, demonstrating the robustness of these edge modes against vertical stacking. Our results establish $\alpha$-Bi$_4$I$_4$ as the first material realization of a 3D QSHI whose definition goes beyond the scope of topological
symmetry indicators, and provide a pathway for realizing nearly-quantized
spin Hall conductivity per unit
cell in a bulk
crystal.
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All authors Shuikang Yu, Junze Deng, Wenhao Liu, Yunmei Zhang, Yiming Sun, Nikhil Dhale, Sheng Li, Wanru Ma, Zhuying Wang, Ping Wu, Zuowei Liang, Xuecheng Zhang, Bing Lv, Zhijun Wang, Zhenyu Wang, Xianhui Chen
Materials Science
A First-Principles Investigation of Goldene for Enhanced Hydrogen Evolution Reaction
Ashutosh Krishna Amaram, Aaditya Roy and Raghavan Ranganathan
The recent synthesis of Goldene, a 2D sheet of gold exfoliated from $Ti_3AuC_2$, offers high specific surface area (~260 $m^2g^{-1}$), roughly twice that of fine nanodots (~100 $m^2g^{-1}$), and unique electronic properties due to its dense d-orbital. In this work, we investigate the adsorption of single atom catalyst (SAC) of hydrogen atom on pristine goldene (pG), monovacant goldene (vG), and sulfur-functionalized variants (thiol-pG and thiol-vG) using ab initio calculations. The adsorption energy of a single H atom and $\Delta G_H$, determines the efficiency of the Volmer step of the hydrogen
evolution reaction (HER) and is a key descriptor for HER activity. We explore various potential sites for H adsorption and its impact on descriptors such as Bader charges, d-band shift and the exchange current density.
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Materials Science
Nanoscale determination of the metal-insulator transition in intercalated bulk VSe$_{2}$
Wanru Ma, Ye Yang, Zuowei Liang et al. (+4 more)
Two-dimensional (2D) materials provide unique opportunities to realize emergent phenomena by reducing dimensionality. Using scanning tunneling microscopy combined with first-principles calculations, we determine an intriguing case of a metal-
insulator transition (MIT) in a bulk compound, (TBA)$_{0.3}$VSe$_2$. Atomic-scale imaging reveals that the initial $4a_0 \times 4a_0$ charge density wave (CDW) order in 1T-VSe$_2$ transforms to $\sqrt{7}a_0 \times \sqrt{3}a_0$ ordering upon intercalation, which is associated with an insulating gap with a magnitude of up to approximately 115 meV. Our calculations reveal that this energy gap is highly tunable through electron doping introduced by the intercalant. Moreover, the robustness of the $\sqrt{7}a_0 \times \sqrt{3}a_0$ CDW order against the Lifshitz transition points to the key role of electron-
phonon interactions in stabilizing the CDW state. Our work clarifies a rare example of a CDW-driven MIT in quasi-2D materials and establishes cation intercalation as an effective pathway for tuning both the dimensionality and the carrier concentration without inducing strain or disorder.
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All authors Wanru Ma, Ye Yang, Zuowei Liang, Ping Wu, Fanbao Meng, Zhenyu Wang, Xianhui Chen