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Physics Condensed Matter Materials Science

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Friday 26 December

Materials Science

Phonon-induced electronic degeneracy breaking: a SSAdNDP interpretation

Javiera Cabezas-Escares, Andrea Echeverri, Francisco Muñoz et al. (+1 more)

This work explores how phonon perturbations can induce the breaking of electronic degeneracies near the Fermi level and how this response can be interpreted from a chemical perspective through the SSAdNDP method. We apply this approach to a family of structurally similar yet electronically distinct hexagonal materials-MgB2, graphene, and hBN-to analyze how a single phonon mode simultaneously modifies the electronic structure (band dispersion) and the nature of chemical bonding (natural occupations and nodal patterns) in real space. Our results show that band splitting becomes physically relevant only when it is accompanied by an electronic redistribution, reflected in changes of the occupation numbers or bonding topology. Thus, SSAdNDP provides a direct bridge between reciprocal- and real-space representations, translating phenomena such as electron-phonon coupling into chemically intuitive reorganizations of multicenter bonds, and offering a unified framework to interpret vibrationally driven electronic effects in solids.
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All authors Javiera Cabezas-Escares, Andrea Echeverri, Francisco Muñoz, Anastassia N. Alexandrova

Materials Science

Thermal Equilibrium Vacancy Concentration in an Alloy with Chemical Short-Range Order

Hao Tang, Hoje Chun, Rafael Gomez-Bombarelli et al. (+2 more)

The equilibrium vacancy concentration in multi-principal element alloys remains a controversial and nontrivial subject, primarily because of chemical complexity and chemical short-range order (CSRO). Here we derive an exact expression that is amenable to atomistic calculations, using multiple perspectives. We applied this expression to equiatomic CrCoNi alloys in the face-centered cubic structure. The derived equilibrium vacancy concentration is used in our recent work, which predicts the chemical short-range order formation timescale consistent with experimental observation. The results demonstrate the practical utility of the approach for predicting equilibrium vacancy concentrations in compositionally complex alloys.
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All authors Hao Tang, Hoje Chun, Rafael Gomez-Bombarelli, Yuri Mishin, Ju Li

Materials Science

Interface Modeling of Perovskite Polymer Heterostructures for Enhanced Charge Transfer Efficiency in Hybrid Photovoltaic Materials

Somayyeh Alidoust and V. Ongun Özçelik

Perovskite solar cells (PSCs) based on methylammonium lead iodide (MAPbI3) exhibit remarkable photovoltaic performance, where interface engineering with hole transport layers (HTLs) is crucial for optimizing charge transfer and device efficiency. In this work, we present a density functional theory (DFT) study of the MAPbI3/poly(3-hexylthiophene) (P3HT) hybrid interface, focusing on the role of perovskite surface termination in determining interfacial stability and electronic structure. We model MAI- and PbI-terminated MAPbI3 surfaces interfaced with P3HT and compare their interfacial electronic properties. Electronic structure calculations reveal distinct differences in orbital hybridization and band alignment: the MAI/m-P3HT interface exhibits weak coupling, whereas the PbI/m-P3HT interface shows stronger hybridization and enhanced charge transfer. Band alignment confirms type-II, hole-selective character in both cases, with more pronounced valence band maximum adjustment for PbI. Charge difference maps, Bader analysis, and local density of states consistently indicate higher charge transfer and stronger electronic coupling for PbI termination. Electrostatic potential offsets and transport parameters further highlight termination-dependent differences, with lighter effective masses at PbI/m-P3HT and higher hole velocity at MAI/m-P3HT. These findings provide theoretical insight into interfacial charge transport mechanisms and offer guidelines for optimizing perovskite-organic hybrid solar cells.
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Materials Science

Keffer-like form of the symmetric Heisenberg exchange integral: Contribution to the Landau–Lifshitz–Gilbert equation and spin wave dispersion dependence

Pavel A. Andreev

The symmetric Heisenberg exchange interaction and antisymmetric Dzyaloshinskii-Moriya interaction are parts of the tensor potential describing effective spin-spin interaction caused by the superexchange interaction of magnetic ions via nonmagnetic ion. There is the Keffer form of the vector constant of the Dzyaloshinskii-Moriya interaction, which includes the shift of the nonmagnetic ion (ligand) from the line connecting two magnetic ions. It is suggested, in this paper, that the ligand shift can give contribution in the constant of the symmetric Heisenberg interaction in antiferromagnetic or ferrimagnetic materials. Hence, the constant of the Heisenberg interaction is composed minimum of two terms. One does not depend on the ligand shift an gives standard contribution in the energy density like term with no derivatives of the spin densities or term containing two spatial derivatives of the spin densities. It is demonstrated that additional term gives a term in the energy density containing one spatial derivative of the spin density. Corresponding contribution in the Landau–Lifshitz–Gilbert equation is found. Possibility of the noncollinear equilibrium order of spin under influence of new spin torque is discussed. Modification of the spin wave (normal modes) dispersion dependencies in the antiferromagnetic materials is found for the collinear order and for the cycloidal order of spins. Effective spin current is derived and applied for the spin-current model of the polarization origin in multiferroics.
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Materials Science

Unlocking klockmannite: formation of colloidal quasi-2D CuSe nanocrystals and photo-physical properties arising from crystal anisotropy

Urvi Parekh, Nadiia Didukh, Samira Dabelstein et al. (+8 more)

Copper selenide is an exceptional quasi-layered monolithic material that exhibits both semiconducting and metallic properties in adjacent visible and near-infrared (NIR) spectral ranges. Here we introduce a thiol-free colloidal synthesis for generating quasi-2D klockmannite copper selenide nanocrystals via hot injection method, achieving shape control by tuning the injection temperature and precursor concentrations without any additional ligands. This approach produces large klockmannite nanosheets with lateral sizes from 200 nm to several micrometres, as well as uniform triangular nanoplatelets with sizes of 12-25 nm that are monocrystalline and display strong NIR plasmonic absorption. The spectral features of the anisotropic klockmannite phase in the NIR have been analysed using complex-scaled discrete dipole approximation (CSDDA) calculations, which reveal pronounced optical anisotropy and the emergence of hyperbolic regime. The combined effect of propagating and evanescent fields is regarded as the underlying reason of such modes in the hyperbolic domain. Finally, the ultrafast photophysical behaviour of the material in klockmannite phase is examined, including hot-hole cooling, trapping, and coherent phonons generation. Our findings emphasize the important role of the intrinsic crystal anisotropy in governing the physical properties of nanoscale klockmannite.
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All authors Urvi Parekh, Nadiia Didukh, Samira Dabelstein, Ronja Piehler, Eugen Klein, Jivesh Kaushal, Tobias Korn, Stefan Lochbrunner, Christian Klinke, Stefan Scheel, Rostyslav Lesyuk

Materials Science

Spin Dynamics in the van der Waals Ferromagnet CrTe2 Engineered by Niobium Doping

Dhan Raj Lawati, Prem Bahadur Karki, Jitender Kumar et al. (+14 more)

Understanding and controlling spin dynamics in two-dimensional (2D) van der Waals (vdW) ferromagnets is essential for their application in magnonics and hybrid quantum platforms. Here, we investigate the spin dynamics of the vdW ferromagnet 1T-CrTe_{2} and demonstrate their systematic tunability via niobium (Nb) substitution in Cr_{1-x}Nb_{x}Te_{2}(x=0-0.2). Ferromagnetic resonance (FMR) spectroscopy reveals that Nb doping enables wide-band tuning of the resonance frequency from 40 GHz down to the few-GHz regime, accompanied by a moderate increase in the Gilbert damping constant from ~0.066 to ~0.14, while preserving robust room-temperature ferromagnetism. Complementary magnetometry shows a concurrent reduction of the Curie temperature and saturation magnetization with increasing Nb content. Density functional theory calculations attribute the observed spin-dynamic trends to Nb-induced modifications of magnetic anisotropy and magnetic exchange interactions. Furthermore, CrTe_{2} flakes (~80 nm thick) exhibit lower resonance frequencies and damping than bulk crystals, consistent with thickness, surface/interface, and shape-dependent magnetic anisotropy. These results establish Nb-doped CrTe_{2} as a tunable vdW ferromagnet with controllable spin dynamics, extending its functionality from spintronics to broadband magnonics and quantum magnonics.
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All authors Dhan Raj Lawati, Prem Bahadur Karki, Jitender Kumar, Karishma Prasad, Mohamed A. Elekhtiar, Kai Huang, Bibek Tiwari, Suvechhya Lamichhane, Rupak Timalsina, Zane Hubble, Ayodimeji E. Aregbesola, John Watt, Sy-Hwang Liou, Evgeny Y. Tsymbal, Jian Wang, Kapildeb Ambal, Abdelghani Laraoui

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

Non-reciprocal circular dichroism of axial phonons coupled to ferro-rotational order

H. Y. Huang, G. Channagowdra, D. Banerjee et al. (+10 more)

Circular dichroism (CD) in X-ray absorption, defined as the difference in absorption between opposite circular polarizations, is fundamentally enabled by the breaking of time-reversal symmetry or inversion symmetry. It is therefore sensitive to magnetism, chirality, and their interplay. In contrast, the sample symmetry alone is insufficient to determine whether CD in resonant inelastic X-ray scattering (RIXS) is allowed. Rather, RIXS-CD is governed by both the sample symmetry and the scattering geometry. Here, using RIXS, we identify circularly polarized phonons coupled to ferro-rotational order in MnTiO$_3$, which we refer to as ferro-axial phonons. Their excitations provide a direct demonstration of non-reciprocal RIXS-CD, in which the dichroic response changes upon reversing the propagation direction of the incident X-rays, although the system globally preserves both inversion and time-reversal symmetries. We propose that a condensate of these phonons, manifested as standing waves, underlies the ferro-rotational order in MnTiO$_3$. The observed non-reciprocal CD reflects the interplay among photon helicity, phonon polarization, and ferro-rotational order.
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All authors H. Y. Huang, G. Channagowdra, D. Banerjee, E. V. Komleva, J. Okamoto, C. T. Chen, M. Guennou, S. Johnston, S. V. Streltsov, C. Y. Mou, A. Fujimori, S-W. Cheong, D. J. Huang

Materials Science

Interplay between electronic and phononic energy dissipation channels in the adsorption of CO on Cu(110)

Carmen A. Tachino, Federio J. Gonzalez, Alberto S. Muzas et al. (+3 more)

In this work, we investigate the relative importance of electronic and phononic energy dissipation during the molecular adsorption of CO on Cu(110). Initial sticking probabilities as a function of impact energy for CO impinging at normal incidence at a surface temperature of 90 K were computed using classical trajectory simulations. To this aim, we use a full-dimensional potential energy surface constructed using an atomistic neural network trained on density functional theory data obtained with the nonlocal vdW-DF2 exchange-correlation functional. Two models are compared: one allowing only energy transfer and dissipation from the molecule to lattice vibrations, and the other also incorporating the effect of molecular energy loss due to the excitation of electron-hole pairs, modeled within the local-density friction approximation. Our results reveal, firstly, that the molecule mainly transfers energy to lattice vibrations, and this channel determines the adsorption probabilities, with electronic friction playing a minor role. Secondly, once the molecule is trapped near the surface (where electronic density is higher), electron-hole pair excitations accelerate energy dissipation, significantly promoting CO thermalization. Still, the faster energy dissipation when electron-hole pair excitations are accounted for accelerates the accommodation of the adsorbed molecules in the chemisorption well but does not significantly alter their lateral displacements over the surface.
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All authors Carmen A. Tachino, Federio J. Gonzalez, Alberto S. Muzas, J. Iñaki Juaristi, Maite Alducin, H. Fabio Busnengo

Materials Science

CO$_2$ Dissociative Sticking on Cu(110)

Federico J. Gonzalez, Carmen A. Tachino and H. Fabio Busnengo

In this work we investigate the dissociation of CO$_2$ on Cu(110) by performing density functional theory calculations using the vdW-DF2 exchange-correlation functional, with a potential energy surface parameterized using artificial neural networks. We computed quasi-classical trajectory calculations of molecular and dissociative adsorption probabilities as a function of the initial impact energy of the molecules and surface temperature, by comparing our results with available supersonic molecular beam experimental data for normal incidence. Concerning the general dependence of the molecular and dissociative adsorption probabilities on the initial translational energy of the molecules, our theoretical results agree with experiments. Also in agreement with experiments, we have found that dissociative adsorption is not affected by surface temperature between 50 and 400 K, for impact energies for which the dissociation probability is larger than $\sim 10^{-3}$. We have investigated the influence of impact energy and surface temperature on the final state of the dissociation products by extending the time integration of the reactive trajectories up to 10 ps. We have found that above $\sim 2.5$ eV and close to or above room temperature, CO$_2$ dissociation induces strong surface distortions including final structures involving Cu adatoms. The creation of Cu vacancy-adatom pairs is stimulated by the presence of both CO$_{ads}$ and O$_{ads}$ which interact strongly with the Cu adatoms and even give rise to unexpected (O-Cu-CO)$_{ads}$ linear moieties anchored to the surface by the dissociated O atom and involving a Cu adatom almost detached from the surface. These surface distortions produced by dissociation products of high energy CO$_2$ molecules at and above room temperature might explain recent experiments that have found a greater saturation oxygen coverage for high energy molecules.
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Materials Science

Tunable Magnetic and Topological Phases in EuMnXBi$_2$ (X=Mn, Fe, Co, Zn) Pnictides

Deep Sagar, Abhishek Sharma and Arti Kashyap

We present a comprehensive density functional theory (DFT) study of the electronic, magnetic, and topological properties of the layered pnictides EuMnXBi2 (X = Mn, Fe, Co, Zn), focusing in particular on the relatively unexplored Bi-based member of the EuMn2X2 family. Unlike the well-studied As-, Sb-, and P–based analogues, we show that EuMn2Bi2 stabilizes in a C-type antiferromagnetic ground state with a narrow-gap semiconducting character. Inclusion of spin-orbit coupling (SOC) drives a transition from this trivial antiferromagnetic semiconductor to a Weyl semimetal hosting four symmetry-related Weyl points and robust Fermi arc states. Systematic substitution of Mn with Fe, Co, and Zn further reveals a tunable sequence of magnetic ground states: Fe and Co induce ferrimagnetism with semimetallic behavior, while Zn stabilizes a ferromagnetic semimetal with a large net moment. These findings establish Bi-based EuMnXBi2 pnictides as a versatile platform where magnetic exchange interactions and band topology can be engineered through SOC and chemical substitution. The complex interplay of magnetic interactions and topological effects in the proposed bulk and doped pnictides opens a promising avenue to explore a wide range of electronic and magnetic phenomena. In particular, this study demonstrates that EuMn2Bi2 hosts tunable magnetic and topological phases driven by electron correlations, chemical substitution, and spin-orbit coupling.
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Materials Science · Cross-listed from cond-mat.mes-hall

Topological constraints on the electronic band structure of hexagonal lattice in a magnetic field

Qi Gao and Wei Chen

The impact of projective lattice symmetry on electronic band structures has attracted significant attention in recent years, particularly in light of growing experimental studies of two-dimensional hexagonal materials in magnetic fields. Yet, most theoretical work to date has focused on the square lattice due to its relative simplicity. In this work, we investigate the role of projective lattice symmetry in a hexagonal lattice with rational magnetic flux, emphasizing the resulting topological constraints on the electronic band structure. We show that, at pi flux, the symmetry in the hexagonal lattice enforces novel Dirac band touchings at E not equal to zero, and for general rational flux it constrains the number of Dirac points at E = 0. We further analyze the symmetry-imposed constraints on the Chern numbers of both isolated gapped bands and band multiplets connected by Dirac-point touchings. Our results demonstrate that these constraints in the hexagonal lattice differ substantially from those in the square lattice.
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Materials Science

Revealing the Partially Coherent Nature of Transport in IGZO

Ying Zhao, Michiel J. van Setten, Anastasiia Kruv et al. (+10 more)

Thin-film transistors based on amorphous oxide semiconductors (AOS) are promising candidates for enabling further DRAM scaling and 3D integration, which are critical for advanced computing. Despite extensive research, the charge transport mechanism in these disordered semiconductors remains poorly understood. In this work, we investigate charge transport in the archetypical AOS material, indium gallium zinc oxide (IGZO), across a range of compositions and temperatures using thin-film transistors and Hall bar structures. Our results show that the electrons involved in transport exhibit partially spatial coherence and non-degenerate conduction. Under these conditions, transport is dominated by electron transfer across insulating gaps between locally coherent regions, rather than by degenerate percolative transport above a mobility edge, or by localized-state hopping, both of which are widely assumed in the literature. While fluctuation-induced tunnelling has previously been invoked to describe low-temperature transport in oxide transistors, we show that such behavior originates from partially coherent electronic states and develop a field-effect-aware fluctuation-induced tunnelling (FEAFIT) framework that explicitly accounts for gate modulation of the tunneling landscape. The FEAFIT model accurately predicts experimental data across all compositions, temperatures, and gate voltages, enabling extraction of fundamental transport parameters. These tunnelling parameters are then correlated with electron coherence dimensions and the degree of energetic disorder obtained from first-principles calculations. Our findings advance the fundamental understanding of charge transport in AOS-based transistors and provide a foundation for further performance improvements
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All authors Ying Zhao, Michiel J. van Setten, Anastasiia Kruv, Pietro Rinaudo, Harold Dekkers, Jacopo Franco, Ben Kaczer, Mischa Thesberg, Gerhard Rzepa, Franz Schanovsky, Attilio Belmonte, Gouri Sankar Kar, Adrian Chasin

Materials Science

Zeeman-like coupling to valley degree of freedom in Si-based spin qubits

S Akbar Jafari, Hendrik J Bluhm and David P Divincenzo

Increasing the valley splitting in Si-based heterostructures is critical for improving the performance of semiconductor qubits. This paper demonstrates that the two low-energy conduction band valleys are not independent parabolic bands. Instead, they originate from the X-point of the Brillouin zone, where they are interconnected by a degeneracy protected by the non-symmorphic symmetry of the diamond lattice. This semi-Dirac-node degeneracy gives rise to the $\Delta_1$ and $\Delta_{2'}$ bands, which constitute the valley degrees of freedom. By explicitly computing the two-component Bloch functions $X_1^\pm$, using the wave vector group at the X-point, we determine the transformation properties of the object $(X_1^+,X_1^-)$. We demonstrate that these properties are fundamentally different from those of a spinor. Consequently, we introduce the term "valleyor" to emphasize this fundamental distinction. The transformation properties of valleyors induce corresponding transformations of the Pauli matrices $\tau_1,\tau_2$ and $\tau_3$ in the valley space. Determining these transformations allows us to classify possible external perturbations that couple to each valley Pauli matrix, thereby identifying candidates for valley-magnetic fields, ${\mathsf B}$. These fields are defined by a Zeeman-like coupling ${\mathsf B}\cdot\vec\tau$ to the valley degree of freedom. In this way, we identify scenarios where an applied magnetic field $\vec B$ can leverage other background fields, such as strain, to generate a valley-magnetic field ${\mathsf B}$. This analysis suggests that beyond the well-known mechanism of potential scattering from Ge impurities, there exist additional channels (mediated by combinations of magnetic and strain-induced vector potentials) to control the valley degree of freedom
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Materials Science

Raman Spectroscopic Investigation of Ferroaxial Order in Na2BaNi(PO4)2 Single Crystals

Ryunosuke Takahashi, Hayato Seno, Marin Takahashi et al. (+9 more)

Ferroaxial order is characterized by the breaking of mirror symmetry parallel to the crystallographic principal axis, which often originates from spontaneous rotational distortions of the crystal lattice. Such rotational distortions are, by symmetry, allowed to couple to specific phonon modes. However, Raman-active phonons associated with these rotational distortions have not yet been clearly identified on a symmetry-consistent basis. Here, we perform polarization-resolved Raman spectroscopy on the ferroaxial phase of Na2BaNi(PO4)2 single crystals and combine the measurements with first-principles lattice-dynamics calculations. This symmetry-guided analysis enables a comprehensive assignment of Raman-active modes in the ferroaxial phase. Several low-frequency Ag modes exhibit finite linewidth broadening, suggesting that these phonons may be weakly affected by the underlying rotational distortion. These results establish a symmetry-based spectroscopic framework for analyzing phonons associated with rotational distortions in ferroaxial materials and provide a basis for future studies of ferroaxial order in complex oxides.
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All authors Ryunosuke Takahashi, Hayato Seno, Marin Takahashi, Shigetoshi Tomita, Reo Fukunaga, Suguru Nakata, Takayuki Nagai, Shigetada Yamagishi, Yoichi Kajita, Tsuyoshi Kimura, Masami Kanzaki, Hiroki Wadati

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

Electrically induced ferromagnetism in an irradiated complex oxide

Nareg Ghazikhanian, Pavel Salev, Dayne Sasaki et al. (+2 more)

In metal-insulator transition materials, a small perturbation can shift the delicate balance between competing or coexisting electronic phases, leading to dramatic changes of the material's properties. Using La0.7Sr0.3MnO3, a prototypical metal-insulator transition manganite, we show that local low-dose focused ion beam irradiation increases resistivity by several orders of magnitude, converting the ferromagnetic-metal ground state into a paramagnetic-insulator. Surprisingly, we found that applying electric stimuli to the irradiated material induces a non-thermal insulator-to-metal transition, which results in low-power, repeatable volatile resistive switching. Magnetotransport measurements revealed that this voltage-induced metallic phase in the irradiated material is ferromagnetic, exhibiting clear anisotropic magnetoresistance. This work, thus, reports the discovery of a unique material in which the electrical triggering of the electronic phase transition results in the onset of magnetism, in stark contrast to the magnetic order suppression commonly observed in other metal-insulator transition switching materials. We demonstrate that local focused ion beam irradiation provides new and exciting opportunities to engineer electronic and magnetic functionalities that can find practical applications ranging from spintronics to neuromorphic hardware.
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All authors Nareg Ghazikhanian, Pavel Salev, Dayne Sasaki, Yayoi Takamura, Ivan K. Schuller

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

Chirality-selective topological magnon phase transition induced by interplay of anisotropic exchange interactions in honeycomb ferromagnet

Jin-Yu Ni, Xia-Ming Zheng, Peng-Tao Wei et al. (+2 more)

A variety of distinct anisotropic exchange interactions commonly exist in one magnetic material due to complex crystal, magnetic and orbital symmetries. Here we investigate the effects of multiple anisotropic exchange interactions on topological magnon in a honeycomb ferromagnet, and find a chirality-selective topological magnon phase transition induced by a complicated interplay of Dzyaloshinsky-Moriya interaction (DMI) and pseudo-dipolar interaction (PDI), accompanied by the bulk gap close and reopen with chiral inversion. Moreover, this novel topological phase transition involves band inversion at high symmetry points $K$ and $K'$, which can be regarded as a pseudo-orbital reversal, i.e. magnon valley degree of freedom, implying a new manipulation corresponding to a sign change of the magnon thermal Hall conductivity. Indeed, it can be realized in 4$d$ or 5$d$ correlated materials with both spin-orbit coupling and orbital localized states, such as iridates and ruthenates, etc. This novel regulation may have potential applications on magnon devices and topological magnonics.
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All authors Jin-Yu Ni, Xia-Ming Zheng, Peng-Tao Wei, Da-Yong Liu, Liang-Jian Zou

Materials Science

Spectroscopic Characterization of Metallocene Single Crystals Grown by Physical Vapor Transport Method

Ian B. Logue, Sandaruka Jayasooriya Arachchilage, Lance M. Griswold et al. (+8 more)

High-quality metallocene single crystals with a low density of impurities and high homogeneity were prepared using the physical vapor transport method. These crystals were then characterized using various spectroscopic tools and X-ray diffraction. Laser-induced breakdown spectroscopy confirmed the presence of metal ions in each freshly grown sample despite all these crystals undergoing physical deformation with different lifetimes. X-ray diffraction analysis confirmed that all our metallocene single crystals retained a monoclinic structure at room temperature. The vibrational properties of our metallocene crystals were examined using Raman and Fourier-transform infrared spectroscopy. The inter- and intra-ring vibrational modes, along with additional modes associated with the crystalline form, were identified as inherent vibrational properties of our metallocene single crystals. Given the increasingly important role of metallocene in organic solar cells, organic light-emitting displays and molecular quantum systems, this research will enhance our understanding of the intrinsic physical properties of cleaner, more crystalline metallocene single crystals.
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All authors Ian B. Logue, Sandaruka Jayasooriya Arachchilage, Lance M. Griswold, Moses B. Gaither-Ganim, Lincoln W. Weber, Robyn Cook, Stephen Hofer, Praveena Satkunam, Dipanjan Mazumdar, Poopalasingam Sivakumar, Bumsu Lee

Materials Science

Universal thermodynamic framework for quasi-van der Waals epitaxy

Renhong Liang, Mao Ye, Renkui Zheng et al. (+4 more)

van der Waals (vdW) epitaxy is conventionally regarded as a rotation-free and strain-free growth mode driven by weak, isotropic interactions, yet many interfaces paradoxically exhibit strictly locked orientations that defy standard surface-energy models. We resolve this inconsistency by establishing a unified quantitative framework for 2D-3D systems, in which strong electrostatic and chemical interactions compete with entropic forces. We introduce a two-tier descriptor set-the predictive index (I_pre) and the thermodynamic locking criterion (I_lock)-to quantify the energetic sufficiency for locked epitaxy. Our theory accurately predicted the competitive interactions at the interface within the 2D-3D system, precisely characterized whether the epitaxial layer underwent free growth or was constrained in a locked growth mode, demonstrating robust consistency with diverse experimental observations. This framework unifies orientation selection in 3D-on-2D films and rotational locking in 2D-on-3D layers within a single-phase diagram. Our work provides a generalizable, predictive route to controlling epitaxial orientation across a broad spectrum of layered heterostructure
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All authors Renhong Liang, Mao Ye, Renkui Zheng, Jianhua Hao, Haitao Huang, Longlong Shu, Shanming Ke

Materials Science

Physics-informed Neural Network (PINN) to Predict Vibrational Stability of Inorganic Semiconductors

M. H. Zeb and M. Z. Kabir

We tackle the challenge of predicting vibrational stability in inorganic semiconductors for high-throughput screening, an essential attribute for evaluating synthesizability alongside thermodynamic stability, frequently missing in prominent materials databases. We create a physics-informed neural network (PINN) that incorporates the Born stability requirements directly into its loss function. This integration is a key learning constraint since it only allows the model to make predictions that do not violate fundamental physics. The model shows consistent and improved performance, having been trained on a dataset of 2112 inorganic materials with validated phonon spectra, and getting an F1-score of 0.83 for both stable and unstable classes. The model shows an AUC-ROC of 0.82 on a benchmark dataset of 1296 materials. Our PINN surpasses the best models in comparative tests, especially when it comes to accurately identifying unstable materials, which is crucial for a stability filter. This work offers a comprehensive screening tool for identifying materials and a methodology for incorporating domain knowledge to enhance predictive accuracy in materials informatics.
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Materials Science

Toward Generalizable Surrogate Models for Molecular Dynamics via Graph Neural Networks

Judah Immanuel, Avik Mahata and Aniruddha Maiti

We present a graph neural network (GNN) based surrogate framework for molecular dynamics simulations that directly predicts atomic displacements and learns the underlying evolution operator of an atomistic system. Unlike conventional molecular dynamics, which relies on repeated force evaluations and numerical time integration, the proposed surrogate model propagates atomic configurations forward in time without explicit force computation. The approach represents atomic environments as graphs and combines message-passing layers with attention mechanisms to capture local coordination and many-body interactions in metallic systems. Trained on classical molecular dynamics trajectories of bulk aluminum, the surrogate achieves sub angstrom level accuracy within the training horizon and exhibits stable behavior during short- to mid-horizon temporal extrapolation. Structural and dynamical fidelity are validated through agreement with reference radial distribution functions and mean squared displacement trends, demonstrating that the model preserves key physical signatures beyond pointwise coordinate accuracy. These results establish GNN-based surrogate integrators as a promising and computationally efficient complement to traditional molecular dynamics for accelerated atomistic simulations within a validated regime.
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Materials Science

Temperature and Pressure Dependent Vibrational Properties of Pristine and Doped Vacancy-Ordered Double Perovskite

Aalok Tiwari, Karamjyoti Panigrahi, Mrinmay Sahu et al. (+2 more)

Understanding lattice dynamics and structural transitions in vacancy-ordered double perovskites is crucial for developing lead-free optoelectronic materials, yet the role of dopants in modulatingthese properties remains poorly understood. We investigate the vibrational and optical properties of pristine and Antimony(Sb)-doped Cs$_2$TiCl$_6$ vacancy-ordered double perovskite through temperature-dependent Raman spectroscopy (4-273 K), high-pressure studies (0- \~30 GPa), ambient powder XRD, and photoluminescence measurements. Sb doping improves phase purity, reducing impurity-related Raman modes present in pristine samples. Most notably, Sb-doped samples exhibit an anomalous Raman mode M$_1$ appearing exclusively below 100 K at 314-319 cm$^{-1}$, accompanied by changes in the temperature coefficient $\chi$ and anharmonic constant $A$ across this threshold. This behavior is absent in pristine Cs$_2$TiCl$_6$. While these observations suggest possible structural changes at low temperature, the origin of the M$_1$ mode remains unclear and may arise from disorder-activated vibrations, symmetry breaking, or dopant-induced local distortions. Low-temperature structural characterization is needed to confirm the nature of this transition. Photoluminescence shows broad self-trapped exciton emission at 448 nm with broader FWHM in Sb-doped samples (164.73 nm) compared to Bi-doped samples (138.2 nm), consistent with enhanced structural disorder. High-pressure Raman measurements reveal continuous mode hardening to 30 GPa with no phase transitions. These results demonstrate that Sb doping modulates the vibrational properties of Cs$_2$TiCl$_6$, though further investigation is required to establish the underlying mechanisms.
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All authors Aalok Tiwari, Karamjyoti Panigrahi, Mrinmay Sahu, Sayan Bhattacharyya, Goutam Dev Mukherjee

Thursday 25 December

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

A novel large-strain kinematic framework for fiber-reinforced laminated composites and its application in the characterization of damage

Shivam and Sandipan Paul

In this paper, a novel kinematic framework for fiber-reinforced composite materials is presented. For this purpose, we use the multiple natural configurations in conjunction with the multi-continuum theory of Bedford and Stern~(1972). Keeping the underlying physics of the proposed kinematics consistent. The proposed kinematics results in a three-term decomposition of the deformation gradient i.e. $\mathbf{F}=\mathbf{F}^e\mathbf{F}^r_\alpha\mathbf{F}^d_\alpha$, where $\alpha$ represents either the matrix or the fiber. After discussing the kinematic framework in detail, we use this new kinematic framework to characterize the damage contents associated with four damage mechanisms. These damage mechanisms are matrix cracking, fiber breakage, interfacial slip or debonding, and delamination. While the first two are derived by measuring the incompatibility of the pertinent configuration occupied by individual constituents, the latter two involve a relative displacement between either the constituents or the laminæ. The geometric interpretation corresponding to these damage mechanisms is also presented using tools from differential geometry. The derived damage contents can be used in developing an appropriate constitutive model for laminated composites undergoing damage.
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Materials Science

Thermodynamic Phase Stability, Structural, Mechanical, Optoelectronic, and Thermoelectric Properties of the III-V Semiconductor AlSb for Energy Conversion Applications

Iskandar Raufov, Dilshod Nematov, Saidjafar Murodzoda et al. (+2 more)

This study presents a first principles investigation of the structural, thermodynamic, electronic, optical and thermoelectric properties of aluminum antimonide (AlSb) in its cubic (F-43m) and hexagonal (P63mc) phases. Both structures are dynamically and mechanically stable, as confirmed by phonon calculations and the Born Huang criteria. The lattice constants obtained using the SCAN and PBEsol functionals show good agreement with experimental data. The cubic phase exhibits a direct band gap of 1.66 to 1.78 eV, while the hexagonal phase shows a band gap of 1.48 to 1.59 eV, as confirmed by mBJ and HSE06 calculations. Under external pressure, the band gap decreases in the cubic phase and increases in the hexagonal phase due to different s p orbital hybridization mechanisms. The optical absorption coefficient reaches 1e6 cm-1, which is comparable to or higher than values reported for other III V semiconductors. The Seebeck coefficient exceeds 1500 microV per K under intrinsic conditions, and the thermoelectric performance improves above 600 K due to enhanced phonon scattering and lattice anharmonicity. The calculated formation energies (-1.316 eV for F-43m and -1.258 eV for P63mc) confirm that the cubic phase is thermodynamically more stable. The hexagonal phase exhibits higher anisotropy and lower lattice stiffness, which is favorable for thermoelectric applications. These results demonstrate the strong interplay between crystal symmetry, phonon behavior and charge transport, and provide useful guidance for the design of AlSb based materials for optoelectronic and energy conversion technologies.
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All authors Iskandar Raufov, Dilshod Nematov, Saidjafar Murodzoda, Sakhidod Sattorzoda, Anushervon Ashurov

Materials Science

Unveiling the Thermoelectric Properties of Group III-Nitride Biphenylene Networks

Gözde Özbal Sargin, Kai Gong and V. Ongun Özçelik

After the synthesis of the carbon biphenylene network (C-BPN), research has increasingly focused on adapting elements from other groups of the periodic table to this lattice structure. In this study, the direction-dependent electronic, thermal, and thermoelectric (TE) properties of semiconducting group-III (group-III = B, Al, Ga, In) nitride biphenylene networks are investigated using the non-equilibrium Green's function formalism in combination with first-principles calculations. Phonon spectra and force field molecular dynamics (MD) simulations were used to asses the dynamically and thermally stable structures. At room temperature, the lowest phonon thermal conductance values are obtained for InN-BPN, with $\kappa_{\mathrm{ph}}$ = 0.12 nW/K/nm and $\kappa_{\mathrm{ph}}$ = 0.21 nW/K/nm along the armchair and zigzag directions, respectively. The nearly dispersionless valence-band region between the $\Gamma$–$X$ symmetry points causes a sharp increase in the $p$-type electronic transmission, which significantly enhances the $p$-type thermoelectric figure of merit, $zT$. Among the investigated group-III nitride BPNs, InN-BPN exhibits the best performance, with a $p$-type $zT$ value of 2.33 in the zigzag direction at 800 K.
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Materials Science · Cross-listed from cs.AI

Accelerating Scientific Discovery with Autonomous Goal-evolving Agents

Yuanqi Du, Botao Yu, Tianyu Liu et al. (+25 more)

There has been unprecedented interest in developing agents that expand the boundary of scientific discovery, primarily by optimizing quantitative objective functions specified by scientists. However, for grand challenges in science, these objectives may only be imperfect proxies. We argue that automating objective function design is a central, yet unmet need for scientific discovery agents. In this work, we introduce the Scientific Autonomous Goal-evolving Agent (SAGA) to address this challenge. SAGA employs a bi-level architecture in which an outer loop of LLM agents analyzes optimization outcomes, proposes new objectives, and converts them into computable scoring functions, while an inner loop performs solution optimization under the current objectives. This bi-level design enables systematic exploration of the space of objectives and their trade-offs, rather than treating them as fixed inputs. We demonstrate the framework through a wide range of design applications, including antibiotics, nanobodies, functional DNA sequences, inorganic materials, and chemical processes. Notably, our experimental validation identifies a structurally novel hit with promising potency and safety profiles for E. coli in the antibiotic design task, and three de novo PD-L1 binders in the nanobody design task. These results suggest that automating objective formulation can substantially improve the effectiveness of scientific discovery agents.
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All authors Yuanqi Du, Botao Yu, Tianyu Liu, Tony Shen, Junwu Chen, Jan G. Rittig, Kunyang Sun, Yikun Zhang, Aarti Krishnan, Yu Zhang, Daniel Rosen, Rosali Pirone, Zhangde Song, Bo Zhou, Cassandra Masschelein, Yingze Wang, Haorui Wang, Haojun Jia, Chao Zhang, Hongyu Zhao, Martin Ester, Nir Hacohen, Teresa Head-Gordon, Carla P. Gomes, Huan Sun, Chenru Duan, Philippe Schwaller, Wengong Jin

Materials Science · Cross-listed from cond-mat.stat-mech

Memory of topologically constrained disorder in Shakti artificial spin ice

Priyanka Priyanka, Cristiano Nisoli and Yair Shokef

Complex behaviors often sit at a critical threshold between order and disorder. But not all disorder is created equal. Disorder can be trivial or constrained, and correlated disorder can even be topological. Crucially, constrained disorder can harbor memory, leading to non-trivial, sequence-dependent responses to external manipulations. And yet the fascinating subject of "memory of disorder" remains poorly explored, as memory is often associated to the retention of metastable order. In recent years artificial frustrated materials – in particular arrays of frustrated nanomagnets known as artificial spin ices – have been employed to study complex disorders and its wealth of exotic behaviors, yet their memory properties have received much less attention. Here, we investigate both analytically and numerically the sequence-dependent responses of two somehow opposite yet related artificial spin ices: the Landau-ordered square spin ice and the disordered but topologically-ordered Shakti spin ice. We find that Shakti exhibits a pronounced sequence-dependent response, whereas in the square lattice, such path dependence is absent. Within Shakti, even the minimal periodic supercell demonstrates both deterministic and stochastic forms of sequence memory, depending on the interaction strength. Extending our study to cyclic driving, we find that retracing the same input path leads to enhanced memory retention. These results open new perspectives on how topological constraints and correlated disorder generate robust memory effects in frustrated artificial materials, hitherto examined mainly in terms of their ground-state kinetics and thermodynamics.
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Materials Science

Concentration-Dependent Tungsten Effects on Chemical Short-Range Order and Deformation Behavior in Ni-W alloys

Shaozun Liu, Zehao Li, Hantong Chen et al. (+8 more)

Ni-W based medium heavy alloys offer a promising pathway to bridge the density-strength gap between tungsten heavy alloys and ultrahigh-strength steels. In this study, the effects of W concentration on chemical short-range order (CSRO), deformation behavior, and grain boundary chemistry of Ni-xW alloys in the range x = 0 to 38 wt% were systematically investigated using a suite of advanced characterization and modeling techniques, including synchrotron X-ray diffraction, transmission electron microscopy, atom probe tomography, and first-principles thermodynamic simulations. Our study reveals that strong CSRO emerges when W content exceeds about 30 wt%, producing distinct diffuse scattering and significantly enhancing strain-hardening capacity. During deformation, the presence of SRO promotes planar slip and twin formation, leading to strong dislocation interactions and elevated flow stress. Hall-Petch analysis demonstrates an exceptionally high grain boundary strengthening coefficient (ky about 1100 MPa micrometer^(1/2)) in Ni-38W, underscoring the intrinsic strengthening effect associated with CSRO. First-principles cluster expansion coupled with Monte Carlo simulations reveals that increasing W content enhances SRO tendency through the stabilization of Ni4W-type local configurations. These findings establish a mechanistic link between W concentration, CSRO evolution, and mechanical response, providing new insights for designing high-density, high-strength Ni-W based alloys with optimized performance.
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All authors Shaozun Liu, Zehao Li, Hantong Chen, Xingyuan San, Bi-Cheng Zhou, Dieter Isheim, Tiejun Wang, Nie Zhao, Yu Liu, Yong Gan, Xiaobing Hu

Materials Science

Pressure-Tuned Metamagnetism and Emergent Three-Body Interactions in CsFeCl$_3$

K. Nihongi, T. Kida, Y. Narumi et al. (+11 more)

We present a combined experimental and theoretical study of the triangular-lattice quantum antiferromagnet CsFeCl$_3$ under high magnetic fields and high pressure. Pulsed-field magnetization for the magnetic field along the symmetric $c$ direction at ambient pressure reveals a magnetization process from a nonmagnetic singlet ground state with a nearly linear increase between 3.7 and 10.7 T, a plateau-like region, and then a sharp stepwise metamagnetic transition near 32 T. Wide frequency–field range electron spin resonance indicates that the low-field regime originates from the $J = 1$ manifold, while the high-field metamagnetic transition suggests a level crossing between the $J = 1$ and $J = 2$ lowest states. Pulsed-field magnetic susceptibilities measured with a proximity detector oscillator under high pressure show that the low-field nonmagnetic singlet phase is gradually suppressed, while the high-field metamagnetic transition evolves into an increasingly rich pattern of fractional steps. While the observations at low to intermediate fields can be understood within the established spin-1 description, the high-field regime requires a new perspective, which we provide through a projected spin-1/2 framework built from Zeeman-selected crystal-field states not related by time reversal. This construction naturally allows emergent three-body interactions on triangular plaquettes and explains the asymmetric evolution of the fractional steps in the magnetization. Our findings reveal that high-field effective spin models in quantum magnets with separated yet accessible crystal-field multiplets are not constrained to even-body couplings, but can naturally host odd-body terms, opening a broader avenue for realizing field-asymmetric magnetization processes and exotic phases beyond conventional even-body physics.
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All authors K. Nihongi, T. Kida, Y. Narumi, Y. Etoh, D. Yamamoto, M. Matsumoto, N. Kurita, H. Tanaka, K. Yu. Povarov, S. A. Zvyagin, J. Wosnitza, K. Kindo, Y. Uwatoko, M. Hagiwara

Materials Science

Defect Engineering for Stabilizing Magnetic and Topological Properties in Mn(Bi1-xSbx)2Te4

Haonan Chen, Jiayu Wang, Huayao Li et al. (+18 more)

MnBi2Te4 is a versatile platform for exploring diverse topological quantum states, yet its potential is hampered by intrinsic antisite defects. While Sb substitution has been employed to tune the Fermi level towards the charge neutral point, it exacerbates the formation of Mn-Sb antisite defects. Here, we address this challenge by combining first-principles calculations with strategic synthesis to systematically investigate and control antisite defects in Mn(Bi1-xSbx)2Te4. Our calculations reveal that increasing antisite defect density progressively destroys the field-forced magnetic Weyl state, eventually driving the system into a trivial magnetic insulator. Motivated by these findings, we develop an optimized chemical vapor transport method, yielding high-quality Mn(Bi1-xSbx)2Te4 crystals with significantly reduced antisite defect density. The emergence of strong Shubnikov-de Haas oscillations in the forced ferromagnetic state and a pronounced anomalous Hall effect near charge neutrality, with opposite signs for n- and p-type samples, confirms the type-II Weyl semimetal nature. These findings underscore the critical role of antisite defects in determining the magnetic and topological properties of Mn(Bi1-xSbx)2Te4 and establish defect engineering via optimized synthesis as a crucial strategy for realizing its exotic magnetic topological states.
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All authors Haonan Chen, Jiayu Wang, Huayao Li, Xunkai Duan, Yuxiang Wang, Zixuan Xu, Yingchao Xia, Wenhao He, Zehao Jia, Xiangyu Cao, Yicheng Mou, Xiangyu Jiang, Jiaming Gu, Pengliang Leng, Fengfeng Zhu, Changlin Zheng, Xiang Yuan, Faxian Xiu, Tong Zhou, Lin Miao, Cheng Zhang

Materials Science

Symmetry-guided prediction of magnetic-ordered ground states

Yuhui Li, Sike Zeng, Yutong Yu et al. (+4 more)

Given the scarcity of experimentally confirmed magnetic structures, the prediction of magnetic ground states is crucial yet remains a long-sought challenge due to the complex potential energy landscape. Here, we propose a symmetry-guided framework that systematically generates magnetic configurations without requiring any experimental input or prior assumptions. Within a symmetry-breaking scenario, we incorporate the recently developed oriented spin space group formalism, which captures symmetry-breaking induced by both magnetic ordering and spin-orbit coupling. By performing nonrelativistic and relativistic first-principles calculations, we establish the energy ladder of the generated magnetic configurations. Exemplified by three prominent unconventional magnets, we demonstrate that only a few dozen calculations are sufficient to identify the ground-state magnetic structure. To demonstrate the universality and robustness of our approach, we conduct large-scale benchmark tests on the MAGNDATA database. Our framework successfully reproduces experimentally reported magnetic geometries for 78% of the surveyed materials, among which 93% have their spin orientations successfully generated when considering SOC. Furthermore, in a large-scale first-principles benchmark involving 305 compounds, 82% of experimentally reported magnetic structures are accurately captured within an energy tolerance of 5 meV per magnetic atom. Beyond reproducing known magnetic configurations, our framework further predicts a variety of low-energy metastable phases, including altermagnets, spin-orbit magnets, and noncollinear antiferromagnets with spin splitting or geometric Hall effect. Our work establishes a general and efficient route toward large-scale prediction of magnetic structures and unconventional magnets, and offers insight into the origins of magnetic interactions across diverse material systems.
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All authors Yuhui Li, Sike Zeng, Yutong Yu, Renzheng Xiong, Yu-Jun Zhao, Xiaobing Chen, Qihang Liu