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Physics Condensed Matter Materials Science

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Wednesday 8 April

Materials Science

Stability of Supported Pd-based Ethanol Oxidation Reaction Electrocatalysts in Alkaline Media

Tuani C. Gentil, Maria Minichova, Valentín Briega-Martos et al. (+7 more)

This study evaluates the dissolution of the supported electrocatalysts Pd/C, PdSn/C, PdNb/C, and PdFe3O4/C during ethanol oxidation reaction for ADLFC applications. A scanning flow cell (SFC) coupled to an inductively coupled mass spectrometry (online ICP-MS) is used to assess the dissolution stability in a broad potential window. Accelerated stress tests with and without ethanol are developed using a rotating disk electrode (RDE) with dissolution products analysis by ex-situ ICP-MS. Potential profiles simulating those experienced by the catalyst during regular fuel cell operation were used. Sn and Fe catalysts demonstrate improved activity and stability compared with the material with Pd alone. For these reasons, PdSn/C and PdFe3O4/C are suitable for ADLFC applications. Severe Nb dissolution destabilizes Pd, increasing its leaching. This work demonstrates that while additional metals and oxides can improve the alcohol oxidation kinetics of Pd, these additives' dissolution stability must already be considered at the catalyst design stage.
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All authors Tuani C. Gentil, Maria Minichova, Valentín Briega-Martos, Victor S. Pinheiro, Felipe M. Souza, João Paulo C. Moura, Júlio César M. Silva, Bruno L. Batista, Mauro C. Santos, Serhiy Cherevko

Materials Science · Cross-listed from cs.LG

Bayesian Optimization for Mixed-Variable Problems in the Natural Sciences

Yuhao Zhang, Ti John, Matthias Stosiek et al. (+1 more)

Optimizing expensive black-box objectives over mixed search spaces is a common challenge across the natural sciences. Bayesian optimization (BO) offers sample-efficient strategies through probabilistic surrogate models and acquisition functions. However, its effectiveness diminishes in mixed or high-cardinality discrete spaces, where gradients are unavailable and optimizing the acquisition function becomes computationally demanding. In this work, we generalize the probabilistic reparameterization (PR) approach of Daulton et al. to handle non-equidistant discrete variables, enabling gradient-based optimization in fully mixed-variable settings with Gaussian process (GP) surrogates. With real-world scientific optimization tasks in mind, we conduct systematic benchmarks on synthetic and experimental objectives to obtain an optimized kernel formulations and demonstrate the robustness of our generalized PR method. We additionally show that, when combined with a modified BO workflow, our approach can efficiently optimize highly discontinuous and discretized objective landscapes. This work establishes a practical BO framework for addressing fully mixed optimization problems in the natural sciences, and is particularly well suited to autonomous laboratory settings where noise, discretization, and limited data are inherent.
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All authors Yuhao Zhang, Ti John, Matthias Stosiek, Patrick Rinke

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

Classification of magnon thermal Hall systems based on U(1) to non-Abelian gauge fields

Masataka Kawano and Chisa Hotta

Magnon thermal Hall effect in insulating magnets is the manifestation of Berry curvature in magnon bands, which is formulated using the emergent gauge fields that act on magnons as a fictitious magnetic field. In ferromagnets, it is commonly accepted as the outcome of U(1) gauge fields generated by Dzyaloshinskii-Moriya interactions and spin textures, but this mechanism is often suppressed by symmetry-enforced cancellations in many lattice geometries, known as a no-go rule. As a result, antiferromagnetic insulators have long been considered as unfavorable platforms for the effect. We show that antiferromagnets with multiple magnetic sublattices naturally host non-Abelian SU(N) gauge fields in magnon band structures, providing a robust rule-to-go mechanism. The noncommutativity of these gauge fields prevents Berry-curvature cancellation and guarantees a nonvanishing thermal Hall response. As a minimal realization, we demonstrate that a coplanar 120$^{\circ}$ antiferromagnet with Dzyaloshinskii-Moriya interactions constitutes a canonical SU(3) platform for the magnon thermal Hall effect. We provide a table of so-far-known two-dimensional lattice geometries and variants of magnetic structures, along with the corresponding gauge fields, providing a unified guideline for identifying magnetic materials, including antiferromagnets and altermagnets, that host thermal Hall transport.
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Materials Science · Cross-listed from gr-qc

When waves meet rays: Seismic vibrations and cosmic showers to test gravity

Aneta Wojnar

We propose a novel laboratory test of gravity combining seismic-wave measurements with cosmic-ray muon detections. Quantum-gravity corrections to the anharmonic Debye model are derived, yielding a modified bulk modulus that encodes deviations from standard gravity. The usual dependence on density, a dominant source of uncertainty, is removed via muon tomography and seismic velocities measurement. We show that this setup can constrain gravity parameters at a level comparable to current laboratory experiments. Prospects for further improvements are briefly discussed.
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Materials Science · Cross-listed from physics.chem-ph

Explicit Electric Potential-Embedded Machine Learning Framework: A Unified Description from Atomic to Electronic Scales

Jingwen Zhou, Yawen Yu, Xuwei Liu et al. (+1 more)

To further develop accurate and large-scale simulations of electrochemical interfaces, we propose a unified explicit electric potential framework to simultaneously predict atomic forces and electron density distributions. The framework consists of three components: data generation, model training, and application. The data generation component, implemented in Hy-DFT, efficiently regulates the potential during constant-potential ab initio molecular dynamics (CP-AIMD), reducing the number of single-point calculations required for convergence. The model training component includes two modules: Potential-Embedded MACE (PE-MACE) and Potential-Embedded Electron Density Prediction (PE-EDP). PE-MACE implements an explicit electric potential machine learning force field (EEP-MLFF) based on the MACE architecture. We develop PE-EDP to overcome the limitation of EEP-MLFF in describing atom forces. PE-EDP, also based on equivariant graph neural networks, predicts electron density distributions under arbitrary potentials. Using the Pt(111)/water interface as a model system, both PE-MACE and PE-EDP show high accuracy on training and test sets. Radial distribution functions from CP-MLMD agree well with CP-AIMD, and long-timescale simulations reveal potential-induced reorganization of interfacial water. Planar-integrated charge profiles and Bader analysis from PE-EDP are consistent with DFT results. These results demonstrate that the framework can simultaneously describe atomic dynamics and electron density distributions under arbitrary potentials, providing a useful tool for studying electrochemical interfaces.
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All authors Jingwen Zhou, Yawen Yu, Xuwei Liu, Chungen Liu

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

Topological Magneto-Optical Switching in Even-Layered MnBi$_2$Te$_4$

Shahid Sattar, Roman Stepanov and C. M. Canali

MnBi$_2$Te$_4$ (MBT) thin films provide a unique material platform in which magnetism, topology, and magneto-optical (MO) response can be tuned through layer-thickness and relative spin alignments. In this work, using a low-energy coupled Dirac cone model together with Wannier-based tight-binding Hamiltonian derived from ab-initio calculations, we investigate topological MO switching in even-layered MBT films. We argue that the relative spin alignment of the outermost septuple-layers (SL) mainly controls the total Chern number, optical conductibility, and consequently, the MO response. For a 6-SL MBT thin film, we found that reversing the outermost-SL alignments from antiparallel to parallel switches the system from axion insulating state with $C=0$ and vanishing Faraday rotation to a Chern insulating state with $C=1$ and a quantized MO response, irrespective of $PT$-symmetry and net magnetization. Increasing thickness reveals an additional regime: while 8-SL MBT hosts only $C=0$ and $1$ states, a 12-SL MBT film supports a higher Chern number phase with $C=2$ with a doubled low-frequency Faraday rotation. Our results provide a thickness-dependent route to multilevel MO switching and establish MO spectroscopy as a direct probe of surface magnetism and topological order in MBT thin films.
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Materials Science · Cross-listed from cs.CE

Improved Implementation of Approximate Full Mass Matrix Inverse Methods into Material Point Method Simulations

John A. Nairn

Approximate full mass matrix methods for the material point method, known as FMPM(k) of order k, can improve the calculation of grid velocities from grid momentum. It can be implemented in any MPM code by inserting a new calculation task whenever grid velocities are needed. The implementation recommended in this paper only needs these calculations once per time step just before when updating particle positions and velocities. FMPM implementation issues arise, however, when its methods are mixed with other MPM feature that rely on lumped mass calculations. Some common lumped-mass MPM features are grid-based, velocity boundary condition, multimaterial contact calculations, crack contact calculations, and imperfect interfaces. This paper first derives a revised FMPM(k) implementation that both simplifies and clarifies the "FMPM Loop" that can be added to MPM codes. Next, that loop is modified to allow FMPM(k) to work well even in simulations that need other MPM features that previously caused conflicts. Two other FMPM(k) issues are apparent loss of stability at very higher order k and inherent computational cost. These issues are discussed in an analysis of temporal stability as a function of order k and in consideration of options to improve efficiency.
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Materials Science

Symmetry-protected four double-Weyl fermions and their topological phase transitions in nonmagnetic crystals

Yun-Yun Bai, Ke-Xin Pang and Yan Gao

Realizing Weyl semimetals (WSMs) with the minimal number of Weyl points (WPs) fundamentally simplifies extracting intrinsic topological responses. While a minimum of four conventional ($|C|=1$) WPs in nonmagnetic crystals is well-established, the exact symmetry requirements and material realization for the unique configuration of four unconventional double-Weyl points (DWPs, $|C|=2$) remain unresolved. Here, we establish rigorous crystalline symmetry constraints restricting the existence of exactly four symmetry-protected DWPs to merely 28 space groups in both nonmagnetic spinless and spinful systems. Guided by this classification, we identify an $sp$$^2$–$sp$$^3$ hybridized chiral carbon allotrope, THRLN-C$_{32}$, as an ideal candidate hosting precisely this four-DWP configuration near the Fermi level. These $C_4$-protected DWPs project extended or closed-loop Fermi arcs onto the surface Brillouin zone, providing unambiguous spectroscopic signatures. Furthermore, external strain drives profound topological phase transitions encapsulated in a unified evolution landscape: the pristine four-DWP state dissociates into two exotic three-terminal Weyl complexes, degenerates into eight conventional $|C|=1$ WPs, or collapses into a trivial insulator. This work provides a definitive theoretical framework for minimal double-WSMs in nonmagnetic spinful systems and introduces an optimal material platform for investigating strain-tunable topological quantum phenomena.
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Materials Science

Physics-Informed 3D Atomic Reconstruction and Dynamics of Free-Standing Graphene from Single Low-Dose TEM Images

Xiaojun Zhang, Shih-Wei Hung, Yawei Wu et al. (+4 more)

Resolving the three-dimensional (3D) atomic geometry of free-standing graphene in real time is essential for understanding how intrinsic rippling governs its electronic properties. However, the low electron doses required to mitigate radiation damage impose severe signal-to-noise constraints that limit conventional reconstruction methods. Here, we present a physics-informed computational framework that reconstructs 3D atomic coordinates of single-layer graphene from individual low-dose transmission electron microscopy (TEM) frames (8x10^3 e-/Ang^2, 1 ms temporal resolution). The approach combines simulated annealing optimisation with molecular dynamics regularisation, achieving sub-angstrom out-of-plane accuracy (sigma_z < 0.45 Ang), validated against ground-truth simulations. A Kullback-Leibler divergence-based calibration aligns the forward model with experimental image statistics, reducing systematic bias. Applied to high-speed time-series data, the framework enables simultaneous extraction of real-time ripple dynamics, strain tensors, surface curvature, bond-length distributions, and density functional theory (DFT)-derived electron localisation functions (ELF). We establish quantitative relationships linking local geometry, strain, and bond-length variations to electron localisation, demonstrating that sub-angstrom structural fluctuations drive spatially localised, millisecond-scale electronic modulation. A critical dose threshold is identified below which structural information becomes irrecoverable, providing practical guidance for experimental design. The framework is broadly applicable to beam-sensitive two-dimensional materials.
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All authors Xiaojun Zhang, Shih-Wei Hung, Yawei Wu, Jyh-Pin Chou, Angus I. Kirkland, Roar Kilaas, Fu-Rong Chen

Materials Science

Determining the Free-Carrier Fraction in 2D Perovskites using Power Dependent Photoluminescence

Antonella Cutrupi, Marc Melendez Schofield, Raquel Utrera-Melero et al. (+4 more)

Determining the nature of the optical excited state (excitons or free carriers) in nanostructured materials is crucial for device design, as optoelectronic and photovoltaic technologies require different considerations regarding the optimized excited state dynamics. Power-dependent photoluminescence is widely used to distinguish between excitons and free carriers, but the classical power-law analysis oversimplifies the underlying physics when the exponent lies between the linear (pure excitons) and quadratic (pure free carriers) limits. In this work, we present a complete study enabling a direct and quantitative analysis of the free-carrier fraction based on power-dependent peak photoluminescence and placing its analysis in the context of the Saha-equation. We study Ruddlesden-Popper perovskites with varying thickness as a model system, as they cover a wide range of exciton binding energies and the full range of free carrier fractions. Our results agree with previously reported values for the exciton binding energies in these materials, confirming the reliability of this approach and providing a simple and effective tool for probing the nature of optically excited states in semiconductors with intermediate exciton binding energies. We demonstrate that our method allows probing spatial variations in the fraction of free charges near grain boundaries or edges at micrometer spatial resolution. Finally, our results highlight the importance of performing optical characterization under excitation densities relevant to realistic operating conditions, as higher fluences can artificially enhance exciton formation and distort excited-state interpretation under solar-fluence conditions.
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All authors Antonella Cutrupi, Marc Melendez Schofield, Raquel Utrera-Melero, Michel Frising, Enrique Arevalo Rodriguez, Upasana Das, Ferry Prins

Materials Science

Programmable Photocatalysis via Symmetry-Defined Periodic Potentials

Qun Yang, Di Luo and Prineha Narang

Photocatalysis in atomically thin semiconductors is often limited by rapid electron-hole recombination, making it difficult to translate favorable band structures into efficient chemical function. Here we propose symmetry-defined periodic potentials as a strategy for photocatalysis: instead of modifying the chemistry of the active layer, one engineers a long-wavelength electrostatic landscape that spatially separates photoexcited electrons and holes. Applied to monolayer InSe, we show that experimentally accessible moiré patterns, such as those generated by twisted hBN, produce miniband formation, band-gap renormalization, and robust carrier separation. Using commensurate BN/InSe local registries, we further show that the moiré control layer transfers a measurable electrostatic modulation to InSe, providing the microscopic link between continuum potential engineering and the local surface environment. The key result is that the periodic potential strongly reorganizes carrier distribution while only weakly perturbing adsorption trends, thereby identifying a practically useful regime in which charge separation can be engineered without demanding major changes to the underlying surface chemistry. These results position periodic potentials as a broadly applicable design principle for photocatalysis and other light-driven interfacial phenomena in two-dimensional materials.
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Materials Science · Cross-listed from cond-mat.mes-hall

Atomic-Scale Detection of Néel Vector Switching in the Single-Layer A-type Antiferromagnet Cr2S3-2D

Affan Safeer, Calisa Dias, Mahdi Ghorbani-Asl et al. (+9 more)

The detection of Néel vector switching in a single-layer A-type antiferromagnet marks an important step toward functional two-dimensional spintronics. Here, Cr$_2$S$_3$-2D, grown on graphene on Ir(110), is established as a first single-layer A-type antiferromagnet. Spin-polarized scanning tunneling microscopy reveals hysteresis loops with a large switching field and a pronounced dependence on island size. X-ray magnetic circular dichroism at the Cr L$_{2,3}$ edges exhibits a tiny signal with a linear magnetic field dependence, consistent with a nearly compensated antiferromagnetic ground state and a Néel temperature of about 160 K. Quantitative analysis of the island-size dependence of the switching field, together with first principles calculations, indicates a slight imbalance between the magnetic moments of the two Cr planes of Cr$_2$S$_3$-2D when supported on a substrate. This imbalance results in a net magnetization for the A-type antiferromagnet, which enables the 180$^\circ$ rotation of the Néel vector. Moreover, Cr$_2$S$_3$-2D retains its magnetic properties after several days of exposure to air.
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All authors Affan Safeer, Calisa Dias, Mahdi Ghorbani-Asl, Abdallah Karaka, Pradyumna Bawankule, Weibin Li, Pierluigi Gargiani, Wouter Jolie, Arkady V. Krasheninnikov, Amilcar Bedoya-Pinto, Thomas Michely, Jeison Fischer

Materials Science

Magnetoelastic Transport-Path Reconstruction and Giant Magnetotransport Responses in a Two-Dimensional Antiferromagnet

Liu Yang, Ming Li, Shui-Sen Zhang et al. (+8 more)

Nonvolatile magnetotransport responses in a single magnetic material have generally not been expected to exhibit a large ON/OFF ratio, because they are usually tied to spin-orbit coupling and therefore remain relatively weak. Here we show, contrary to this expectation, that giant nonvolatile magnetotransport can arise in a single magnetic material through magnetoelastic reconstruction of nonrelativistic real-space transport paths. Using the two-dimensional antiferromagnet FePS$_{3}$ as a representative system, first-principles quantum transport calculations reveal that charge transport is strongly tied to its quasi-one-dimensional zigzag sublattice chains and, under suitable doping, can even become confined to them. Moreover, strain lifts the degeneracy among symmetry-related zigzag variants and thus reorients these transport paths through magnetoelastic coupling. As a result, both the longitudinal and transverse conductivities change dramatically, yielding a giant magnetoelastic magnetoresistance of up to $10^{4}$% and an energy-independent Hall ratio that far exceeds the spontaneous Hall ratios found in conventional magnets. These results establish a route to exploiting symmetry-related magnetic variants and their associated transport paths for reconfigurable, high-performance spintronic devices with large nonvolatile readout contrast.
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All authors Liu Yang, Ming Li, Shui-Sen Zhang, Hang Zhou, Yi-Dong Liu, Xiao-Yan Guo, Wen-Jian Lu, Yu-Ping Sun, Evgeny Y. Tsymbal, Kaiyou Wang, Ding-Fu Shao

Materials Science

Fe3O4 nano-octahedra and SnO2 nanorods modifying low-Pd amount electrocatalysts for alkaline direct ethanol fuel cells

Tuani C. Gentil, Lanna E.B. Lucchetti, João Paulo C. Moura et al. (+7 more)

This work describes the ethanol oxidation reaction (EOR) in alkaline medium using low-palladium nanoparticle electrocatalysts modified by Fe3O4 nano-octahedra and SnO2 nanorods. Operation studies on an alkaline direct ethanol fuel cell (ADEFC) were conducted using the developed electrocatalysts, and stability studies were performed using the advanced scanning flow cell (SFC) technique coupled to inductively coupled plasma mass spectrometry (online SFC-ICP-MS). The EOR was catalyzed by single (Pd/C and commercial Pd/C Alfa Aesar) and by synthesized binary and ternary electrocatalysts, in which Fe3O4 and SnO2 nanostructures partially replaced the high-cost noble metal. The PdFe3O4/C was identified as the most promising synthesized material in the electrochemical studies, exhibiting the highest mass activity (1426 mA mg-1 Pd) by cyclic voltammetry (CV), followed by the binary PdSnO2/C (1135 mA mg-1 Pd), and by the ternary (1074 mA mg-1 Pd). This enhancement was attributed to the bifunctional mechanism enabled by Fe3O4 and SnO2, therefore reducing poisoning and improving the EOR. Moreover, the operating results revealed that PdFe3O4/C showed the highest power density among the synthesized materials (31 mW cm-2 at 70 C), even with an approximately 45 percent reduction in Pd content compared to the commercial catalyst. XPS results showed that the Pd 3d5/2 and 3d3/2 peaks for PdFe3O4/C, PdSnO2/C, and PdFe3O4SnO2/C were shifted by approximately 0.5 eV to higher binding energies compared to Pd/C, indicating a loss of electron density in Pd due to strong metal-oxide interactions.
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All authors Tuani C. Gentil, Lanna E.B. Lucchetti, João Paulo C. Moura, Júlio César M. Silva, Maria Minichov, Valentín Briega-Martos, Aline B. Trench, Bruno L. Batista, Serhiy Cherevko, Mauro C. Santos

Materials Science

Photo-Assisted Pd-Nb2O5/Carbon Nanocomposites for Enhanced Ethanol Electro-Oxidation Kinetics and CO Tolerance in Alkaline Media

João V. T. Neves, Stephanie S. Aristides-Barros, Aline B. Trench et al. (+4 more)

Pd-based anodes for alcohol oxidation suffer from surface poisoning and sluggish kinetics. Here, we developed Pd-Nb2O5/C nanocomposites to improve ethanol electrooxidation kinetics and CO tolerance in alkaline media. Orthorhombic Nb2O5 prepared by the Pechini route was combined with fcc Pd nanoparticles via polyol reduction, yielding Pd(x)-Nb2O5(y)/C nanocomposites with x:y = 100:0, 70:30, 50:50, 30:70, 0:100. Rietveld-refined X-ray diffraction confirmed phase purity and showed similar Pd crystallite sizes (4.46 nm for Pd/C and 4.92-5.08 nm for Nb2O5-containing catalysts). Transmission and scanning electron microscopies coupled with energy-dispersive X-ray spectroscopy reveal uniformly dispersed Pd nanoparticles on Nb2O5 and carbon. UV-Vis diffuse reflectance indicated a band gap of 3.10 eV, and chopped-light photocurrent measurements confirm the strong ultraviolet responsiveness of Nb2O5. X-ray photoelectron spectroscopy reveals that Pd(0.5)Nb2O5(0.5)/C had the highest Pd0 content (58.99%). Electrochemical testing demonstrates that, relative to Pd/C, optimized Pd(0.5)Nb2O5(0.5)/C reduces the ethanol oxidation onset potential by up to 160 mV, increases poisoning tolerance by a factor of five at a fixed potential, and raises the current density from 1.59 to 1.76 mA cm-2. Under light irradiation, the current density increases from 1.07 to 2.10 mA cm-2, accompanied by improved stability and extended durability, attributed to light-induced electron-hole generation and enhanced OH- adsorption. These results highlight the synergistic contribution of oxide-metal interactions and photoactivation to ethanol oxidation and provide insights for designing efficient catalysts for alkaline fuel cells. s
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All authors João V. T. Neves, Stephanie S. Aristides-Barros, Aline B. Trench, Ivani M. Costa, Mauro C. Santos, Giancarlo R. Salazar-Banda, Katlin I. B. Eguiluz

Materials Science

Excitons in WSe2 time-resolved ARPES: particle or oscillation?

Kai Wu, Michele Puppin and Andrea Marini

The time-resolved angle-resolved photoemission spectra of WSe$_2$, a paradigmatic transition metal dichalcogenide, are dominated by a transient signal that, after being initially observed in the gap at the K valley, scatters, on an ultra-fast time scale of $\sim$ 30 fs, to the $\Sigma$ valley. In this work we question the common interpretation of the experimental dynamics in terms of a massive bound electron-hole exciton that scatters with phonons and behaves as a quasi-particle. By using a combined theoretical and experimental investigation, we demonstrate that the observed dynamics can be interpreted as the photo-induced transition from direct to indirect excitonic-insulating order. The features that appear in the experimental spectrum correspond to single-particle levels renormalized by the excitonic spontaneous polarization.
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Materials Science

Machine learning Hamiltonian enables scalable and accurate defect calculations: The case of oxygen vacancies in amorphous SiO$_2$

Zhenxing Dai, Zhong Yang, Mingjue Ni et al. (+4 more)

Point defects critically influence the properties of materials and devices, yet density functional theory (DFT) remains computationally demanding for defect supercell calculations. Machine learning interatomic potentials (MLIPs) offer high efficiency but require extensive datasets. MLIPs trained only on defect configurations in small supercells exhibit systematic energy errors in larger supercells, demonstrating limited transferability. Here, we present a machine learning Hamiltonian (MLH) model-based method for calculating total energies and atomic forces in defect supercells with linear-scaling computational cost, enabling efficient structural relaxation and accurate formation energy predictions. We take oxygen vacancies in amorphous SiO$_2$ as an example and train the MLH model on defect configurations in 95-atom supercells, with the training data derived from 120 self-consistent field calculations and 12 structural relaxations. The MLH model enables efficient structural relaxations for host (defect-free) and defect systems in larger supercells, avoiding the systematic energy errors observed in MLIPs. The cancellation of energy errors between host and defect systems yields accurate formation energy predictions, with deviations from DFT below 50 meV. The proposed method holds significant potential for defect simulations in complex materials.
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All authors Zhenxing Dai, Zhong Yang, Mingjue Ni, Menglin Huang, Hongjun Xiang, Xin-Gao Gong, Shiyou Chen

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

Revisiting quadratic band crossing: from interaction-driven instability to intrinsic topology

Yadong Jiang, Linghao Huang, Zhaochen Liu et al. (+2 more)

The realization of robust quantum anomalous Hall (QAH) phases at elevated temperatures remains a central challenge in condensed matter physics. While quadratic band crossing points (QBCP) provide a promising route towards QAH states, existing proposals are largely confined to idealized models or hindered by interaction-driven competing orders. Here, we demonstrate that these limitations are not intrinsic to QBCP but arise from their specific implementation. We propose a general mechanism where band inversion between a symmetry-protected orbital doublet (e.g. $d_{xz},d_{yz}$) and an isolated orbital (e.g. $d_{z^2}$)-generically generates a QBCP with opposite curvature. This crossing is directly gapped at the single-particle level by intrinsic atomic spin-orbit coupling, while the underlying band inversion naturally shields the resulting topological gap against other interaction-driven instabilities. We further suggest monolayer compounds $MNX_2$ ($M$= Ni, Pd, Pt; $N$= Nb, Ta; $X$= S, Se, Te) as a realistic material class that intrinsically realizes this mechanism. These findings provide a concrete pathway toward robust QAH phases in correlated materials.
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All authors Yadong Jiang, Linghao Huang, Zhaochen Liu, Huan Wang, Jing Wang

Materials Science

Photoexcited Hole States at the SrTiO3(001) Surface Imaged with Noncontact AFM

Igor Sokolovic, Florian Ellinger, Aji Alexander et al. (+8 more)

The behaviour of excess charges in ionic lattices, such as the formation of polarons and charge trapping at defect sites, influences the physical and chemical properties of materials and translates into applications in electronics, optics, photovoltaics, and catalysis. Here we show that the bulk-terminated SrTiO3(001) surface accumulates photoexcited charges and keeps the associated photovoltage for many days at cryogenic temperatures. A combination of scanning tunneling microscopy, atomic force microscopy (STM/AFM) and Kelvin probe force microscopy (KPFM) was used to measure this photovoltage and to localize the photoexcited charges with atomic precision down to the single-quasiparticle limit. Density functional theory (DFT) shows that holes favor localization at oxygen 2p orbitals adjacent to Sr vacancies, creating long-lived trapped states. The methodology presented here provides guidelines for imaging of charges trapped in the crystal lattice using noncontact AFM.
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All authors Igor Sokolovic, Florian Ellinger, Aji Alexander, Dominik Wrana, Llorenc Albons, Sreehari Sreekumar, Michael Schmid, Ulrike Diebold, Michele Reticcioli, Cesare Franchini, Martin Setvin

Materials Science

Ferroelectric Band Twinning from Pair-State Symmetry

Shibo Fang, Jianhua Wang, Zhenzhou Guo et al. (+7 more)

Ferroelectric switching provides a nonvolatile way to control electronic structures, but a general symmetry rule connecting the full Bloch bands of two switchable polarization states is still lacking. Here, we introduce ferroelectric band twinning, a pair-state relation in which the bands of two opposite-polarization states are mapped onto each other by a non-inversion state-exchange symmetry. Using dichromatic groups, we derive the band-twinning rule and identify 11 ferroelectric band-twinning point-group classes. Screening the Ferroelectric Materials Database yields 16 candidate compounds, of which the two lattice-metric-preserving candidates, bulk gamma-Ag3SI and BaAl2O4, are selected for first-principles validation. For gamma-Ag3SI, we further show that the same pair-state symmetry controls the transformation of shift-current tensor components under polarization reversal. These results establish ferroelectric band twinning as a general symmetry framework for nonvolatile control of momentum-dependent electronic structures in ferroelectrics.
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All authors Shibo Fang, Jianhua Wang, Zhenzhou Guo, Jialing Gong, Haiyu Meng, Ruixiang Fei, Wenhong Wang, Xiaotian Wang, Zhenxiang Cheng, Yee Sin Ang

2604.07112 · 8 Apr · This replacement retains the central concept of the previous version but substantially revises the examples, theoretical framework, and overall presentation. Although the new version differs considerably from the earlier manuscript, we chose to replace the arXiv submission to avoid confusion regarding the concept and its terminology

Abstract PDF $SYMM ×5$LATT ×1

Materials Science

Towards viable H$_2$ storage in Ca decorated low-dimensional materials with insights from reference quantum Monte Carlo

Yasmine S. Al-Hamdani, Dario Alfè and Andrea Zen

Hydrogen technology is set to be a key energy alternative for mitigating pollution and reducing CO$_2$ emissions. However, the current storage mechanism of hydrogen molecules in carbon fibre tanks detracts from the fuel economy of hydrogen in mobile applications, necessitating the development of alternative storage mechanisms. Adsorbing hydrogen in its molecular form (H$_2$) at typical operating conditions of proton exchange membranes can potentially meet storage requirements. However, H$_2$ is the smallest molecule with only two electrons and therefore it has very limited propensity to physisorb in a material within the binding energy window of $-0.2$ to $-0.4$ eV that is suitable for storage. Calcium atom decorators on graphene have previously shown promise for tunable H$_2$ binding, but the system is thermodynamically unstable toward the formation of calcium hydride. Moreover, the absolute adsorption of H$_2$ is challenging to predict accurately and is typically overestimated with van der Waals inclusive density functional approximations. In this work, we perform state-of-the-art fixed-node diffusion Monte Carlo alongside a selection of density functional approximations for two strategies of anchoring Ca: (i) Ca on boron doped graphene and (ii) Ca inside carbon nanotubes. We predict reliable Ca and H$_2$ binding energies, and establish that Ca is anchored inside carbon nanotubes and on boron doped graphene, while boosting the H$_2$ adsorption energy. Importantly, the H$_2$ adsorption energy is found to be improved by the anchoring strategies, with the energy inside a Ca decorated carbon nanotube reaching the viable storage window. The reference DMC binding energies provide much-needed benchmarks for developing data-driven methods and guiding experiment in the systematic design of hydrogen storage materials.
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Materials Science

Unveiling Mechanisms of SEI Formation and Sodium Loss in Sodium Batteries via Interface Reactor Sampling

Zhoulin Liu, Ziliang Wang, Zherui Chen et al. (+4 more)

The solid electrolyte interphase SEI critically dictates the cyclability and Coulombic efficiency of sodium-metal batteries, yet its dynamic formation mechanisms and atomic-scale evolution during electrochemical cycling remain elusive due to the spatiotemporal limitations of existing techniques. Here, an "Interface Reactor" sampling strategy is proposed to construct a charge-aware neuroevolution potential (qNEP). This approach overcomes the instability bottlenecks of conventional machine learning potentials, enabling stable, first-principles-accurate molecular dynamics simulations of complex electrode-electrolyte interfaces on the hundred-nanosecond scale. Fundamentally distinct SEI formation mechanisms are revealed during the early stage: carbonate-based electrolytes form heterogeneous organic-inorganic matrices via "mixed co-formation," whereas ether-based electrolytes generate dense, self-limiting inorganic barriers through "surface-energy-controlled" NaF crystallization. Metadynamics simulations further elucidate that these compositional disparities govern sodium-ion storage dynamics: NaF-rich SEIs facilitate efficient metallic deposition, while carbonate-dominated interphases induce irreversible sodium trapping and continuous electrolyte decomposition. These findings establish a comprehensive atomic-scale framework linking solvation structure, interfacial reaction networks, and electrochemical performance, providing mechanistic guidelines for rational SEI engineering in next-generation alkali-metal batteries. Crucially, a general and robust computational framework is established for simulating complex interfacial reactions in electrochemical systems.
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All authors Zhoulin Liu, Ziliang Wang, Zherui Chen, Jianchun Sha, Fengzijun Pan, Pingyang Zhang, Yinghe Zhang

Materials Science

Topological Defects in Amorphous Solids

Matteo Baggioli, Michael L. Falk and Walter Kob

Topological defects (TDs) are crucial for understanding important physical properties of crystalline materials including mechanical failure, ion transport, and two-dimensional melting. This concept has not translated to disordered materials like glasses because these solids have no obvious reference structure that can be used to define TDs. As a result, key properties related to those listed above have typically been modeled using purely phenomenological approaches. Recent studies have demonstrated that certain observables commonly associated with TDs can also be identified in disordered solids indicating that topological concepts may be as crucial in amorphous solids as in crystals. This hints that TDs may offer a first-principles framework for understanding their mechanical response and complex spatiotemporal dynamics. In this Perspective, we review recent theoretical, numerical, and experimental studies that have exploited topological concepts to rationalize mechanical properties of amorphous solids. We also highlight pressing open questions and some promising directions for future research in the field.
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Materials Science · Cross-listed from physics.chem-ph

Self-consistent Hessian-level meta-generalized gradient approximation

Pooria Dabbaghi, Juan Maria García Lastra and Piotr de Silva

The $\vartheta$-MGGA class of density functionals is formally reformulated as Hessian-level meta-generalized gradient approximations (HL-MGGAs). In contrast to standard meta-GGAs that rely on the orbital-dependent kinetic-energy density or the density Laplacian, HL-MGGAs utilize the full density Hessian. We introduce a simplified, non-empirical functional, $\vartheta$-PBE, and present a roadmap for its self-consistent implementation within the projector augmented-wave (PAW) method. By utilizing the complete set of spatial second-order density derivatives, the functional's underlying descriptor successfully distinguishes between distinct one-electron density limits, such as single-center atomic densities and two-center bonds, that standard iso-orbital indicators often conflate. Benchmarks across molecular and solid-state datasets reveal that while $\vartheta$-PBE delivers accurate chemisorption energies and molecular properties, challenges remain in predicting bulk lattice constants. Ultimately, this work demonstrates the physical utility and feasibility of designing orbital-independent, Hessian-based exchange-correlation functionals.
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Materials Science · Cross-listed from cond-mat.mes-hall

Microscopic evidence of spin-driven multiferroicity and topological spin textures in monolayer NiI2

Haitao Wang, Tianxing Jiang, Weiyi Pan et al. (+12 more)

In type II multiferroics, noncollinear spin textures are expected to induce electric polarization directly, leading to strong magnetoelectric coupling. Realizing such spin driven multiferroicity in two-dimensional systems, and elucidating the interplay between local spins and electric polarization, are of both fundamental and technological importance. Here, using vectorial spin polarized scanning tunneling microscopy, we investigated the spin-driven multiferroicity in monolayer NiI2 at atomic scale. We identify a canted spin-spiral state with fully determined spin rotation plane, accompanied by a 2Q charge modulation. At spin spiral domain walls, we discover topological spin textures that composed of meron/antimeron pairs. These textures are associated with distinct charge pattern and notable band shifts, indicating local bound charges induced by variations of ferroelectricity at domain wall. Our observations are well captured by a realistic spin model incorporating Kitaev interactions and generalized spin-current model of type II multiferroicity. The findings provide microscopic evidence of spin-driven multiferroicity in an extreme 2D system and establish a platform for low-dissipation, electric-field control of topological spin textures.
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All authors Haitao Wang, Tianxing Jiang, Weiyi Pan, Xu Wang, Hongyu Wang, Junchao Tian, Lianchuang Li, Dongming Zhao, Qingle Zhang, Chenxi Wang, Ying Yang, Hongjun Xiang, Changsong Xu, Donglai Feng, Tong Zhang

Materials Science

Microscopic contributions to the deviation from Amontons friction law

Suresh Ravisankar, Ravikant Kumar, Antonio Cammarata et al. (+2 more)

We investigate the nanoscale friction behaviour of MX2 monolayers (M = Mo, W; X = S, Se) on Au(111) and Ag(111) substrates with a silicon tip using classical molecular dynamics simulations with machine-learning-based force fields. This approach enables an accurate description of tip-surface interactions and friction mechanisms at the atomic scale. We observe a pronounced non-monotonic dependence of the friction force on the applied normal load, indicating a breakdown of Amontons's law at the nanoscale. Analysis of lateral force' signals and their spatial Fourier transforms reveals the coexistence of multiple sliding modes, including longitudinal sliding, lateral slip, and zig-zag motions. We show that the overall friction response is governed by the relative contributions of these motions. While the qualitative features of friction are largely substrate-independent, both the magnitude of friction and the balance between sliding modes depend sensitively on the substrate-monolayer combination. In particular, Au/MoSe2/Si exhibits significantly reduced friction due to suppression of lateral slip motion. Our results indicate that the method is broadly applicable for probing nanoscale friction in related heterostructures.
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All authors Suresh Ravisankar, Ravikant Kumar, Antonio Cammarata, Thilo Glatzel, Tomas Polcar

Materials Science

Volume Collapse Without a Structural Transition in Shock-Compressed FeO

C. Crépisson, T. Stevens, M. Fitzgerald et al. (+121 more)

We report x-ray diffraction and emission spectroscopy of FeO under laser-driven shock compression between 31-199 GPa. FeO retains the B1 (rocksalt) structure along the Hugoniot to the melt boundary at 191 GPa. While the phase and volume are broadly consistent with results from static compression, we observe an anomalous 7-10% volume collapse around 60 GPa absent in static experiments. We identify this as an isostructural high-spin to low-spin metallic transition in FeO. The low-spin state is directly evidenced by x-ray emission spectroscopy at 180 GPa.
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All authors C. Crépisson, T. Stevens, M. Fitzgerald, C. Camarda, P. G. Heighway, D. Peake, D. McGonegle, A. Descamps, A. Amouretti, D. A. Chin, K. K. Alaa El-Din, S. Azadi, E. Brambrink, K. Buakor, L. Pennacchioni, M. Sieber, A. Coutinho Dutra, J. Hernandez Gordillo, K. Yamamoto, J.-A. Hernandez, R. Torchio, T. Tschentscher, Y. Wang, H. Taylor, J. Pintor, O. S. Humphries, M. Andrzejewski, C. Baehtz, E. Barraud, A. B. Belonoshko, D. S. Bespalov, E. Boulard, R. Briggs, D. Cabaret, O. Castelnau, A. Chakraborti, J. Chantel, D. M. Cheshire, G. Collins, T. E. Cowan, Y. J. Deng, S. Di Dio Cafiso, L. Dresselhaus-Marais, X. Fang, A. Forte, S. Galitskiy, E. Galtier, T. Gawne, H. Ginestet, F. Hanby, A. Hari, N. J. Hartley, H. Höppner, N. Jaisle, J. Kim, Z. Konôpková, A. Krygier, J. Kuhlke, C. M. Lonsdale, S-N. Luo, J. Lütgert, M. Masruri, E. E. McBride, J. D. McHardy, M. I. McMahon, R. S. McWilliams, S. Merkel, T. Michelat, J-P. Naedler, B. Nagler, M. Nakatsutsumi, A-M. Norton, I. K. Ocampo, I. I. Oleynik, C. Otzen, N. Ozaki, C. A. J. Palmer, S. E. Parsons, A. Pelka, A. Phelipeau, C. Prescher, N. Pulver, C. Prestwood, C. Qu, D. Ranjan, R. Redmer, C. Sahle, A. A. Sanjuan Mora, S. Schumacher, J-P. Schwinkendorf, N. Sévelin-Radiguet, G. Shoulga, R. F. Smith, S. Singh, C. N. Somarathna, M. Stevenson, C. V. Storm, C. Strohm, T-A. Suer, M. X. Tang, A. Tipeev, M. Toncian, T. Toncian, U. Trdan, J. D. Tunacao, J. D. Umpleby-Thorp, L. Wang, M. Wilke, U. Zastrau, G. Gregori, D. Polsin, C. Sternemann, J. S. Wark, T. M. Hutchinson, C. McGuire, S. Pandolfi, A. Sollier, A. Higginbotham, T. R. Preston, D. Kraus, J. H. Eggert, K. Appel, M. Harmand, S. M. Vinko

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

Nonlinear phononics in LaFeAsO: Optical control of the crystal structure toward possible enhancement of superconductivity

Shu Kamiyama, Tatsuya Kaneko, Kazuhiko Kuroki et al. (+1 more)

Nonlinear phononics provides a route to control crystal structures through light-induced phonon excitation. In this study, we apply nonlinear phononics to an iron-based superconductor, LaFeAsO, with the aim of tuning its crystal structure toward the ideal one to enhance superconductivity. We simulate light-induced phonon dynamics on the anharmonic lattice potential determined by first-principles calculations. We find that the anion height $h$, a key structural parameter in iron-based superconductors, approaches its ideal value when an appropriate infrared-active phonon mode is selectively excited. This result suggests the possibility of controlling crystal structures and enhancing superconductivity in iron-based superconductors based on the concept of nonlinear phononics.
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All authors Shu Kamiyama, Tatsuya Kaneko, Kazuhiko Kuroki, Masayuki Ochi

Materials Science

Bond-Strength-Based Understanding of Oxygen Vacancy Migration Barriers in Rutile Oxides

Inseo Kim and Minseok Choi

We carry out bond-strength based analysis for the migration barrier ($E_{\rm B}$) of oxygen vacancies in rutile-type 3$d$ transition-metal dioxides by combining density-functional theory (DFT) and the bond-valence model. The covalent and ionic contributions to chemical bonding are explicitly decomposed and quantified by the sum of the integrated crystal orbital Hamilton population ($S_c$) and the Madelung energy ($S_i$), respectively. Both $S_c$ and $S_i$ exhibit strong correlations with the $E_{\rm B}$ from DFT ($E_{\rm B}^{\rm DFT}$), and their average $\bar{S}$ provides a reasonable estimate of $E_{\rm B}^{\rm DFT}$ across the oxide series. Inspired by the bond-valence model, two parameters are extracted by fitting to a large dataset of 3$d$ transition-metal dioxides. Our results show that using these parameters, $E_{\rm B}$ of oxygen vacancies can be efficiently estimated.
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Materials Science · Cross-listed from cond-mat.supr-con

Directional Andreev-Reflection Signatures of Inter-Orbital Pairing in Sr$_2$RuO$_4$

G. Csire, Y. Fukaya, M. Cuoco et al. (+6 more)

Unconventional superconductivity in quasi–two-dimensional systems is commonly identified through the emergence of Andreev bound states (ABS) at in-plane edges, while surfaces perpendicular to out-of-plane direction remain fully gapped due to weak interlayer coherence. This directional anisotropy has long served as a key paradigm for constraining pairing symmetries. Here, we show that Sr$_2$RuO$_4$ exhibits a striking reversal of this behavior. Using edge- and surface-sensitive spectroscopy, we observe pronounced in-gap ABS at surfaces perpendicular to the out-of-plane direction, whereas in-plane edges exhibit a reduced intensity of the in-gap spectral features. We show that this anomalous anisotropy can arise from the inter-orbital character of the superconducting pairing. Both even- and odd-parity inter-orbital pairing channels naturally generate robust surface ABS while suppressing planar edge modes and can also provide a mechanism for the appearance of a horizontal line node. Supported by ab initio and model calculations, including Sr$_2$RuO$_4$/Ag interface reconstructions, our results highlight the possible role of inter-orbital correlations in shaping the spectroscopic response and provide constraints on the structure of the superconducting order parameter in Sr$_2$RuO$_4$.
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All authors G. Csire, Y. Fukaya, M. Cuoco, Y. Tanaka, R.K. Kremer, A.S. Gibbs, G.A. Ummarino, D. Daghero, R.S. Gonnelli