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Physics Condensed Matter Materials Science

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Friday 10 October

Materials Science

A Room-Temperature Ferrotoroidic Material Exhibiting Magnetic Semiconductor Properties with Superhigh Hole Mobility

Jianyuan Qi, Shijie Xiong, Beining Ma et al. (+1 more)

The design and fabrication of room-temperature ferrotoroidic materials and magnetic semiconductors are recognized worldwide as a great challenge, and of both theoretical and practical importance in the field of condensed matter physics and information storage. Reported herein are ferrotoroidic crystal powder and film formed by supramolecular self-assembly based on uranyl and cyclodextrin, with the Curie temperature above room temperature. Experimental measurements and calculations demonstrate spontaneous vortex-like alignment of magnetic moments and furthermore a macroscopic long-range arrangement in the crystal, which breaks simultaneously space-inversion and time-reversal symmetries, exhibiting strong superexchange, spin-orbit coupling as well as anomalous Hall effect (AHE). The electrical measurements show the film with a superhigh carrier mobility of 3200 cm2*V-1*s-1 and a Hall resistivity as high as 0.32 mV*A-1*cm at room temperature. This work is expected to pave greatly the applied research on new-generation magnetoresistive random access memory (MRAM), especially as flexible material.
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All authors Jianyuan Qi, Shijie Xiong, Beining Ma, Xinghai Shen

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

Field-induced magnetic phases in the Kitaev candidate Na$_3$Co$_2$SbO$_6$

Kranthi Kumar Bestha, Manaswini Sahoo Niccolò Francini, Robert Kluge et al. (+13 more)

We report a rich anisotropic magnetic phase diagram of Na$_3$Co$_2$SbO$_6$, a previously proposed cobaltate Kitaev candidate, based on field- and temperature-dependent magnetization, specific heat, and magnetocaloric effect studies. At low temperatures, our experiments uncover a low-lying $j_{\textrm{eff}} = \frac{1}{2}$ state with an antiferromagnetic ground state and pronounced in-plane versus out-of-plane anisotropy. The experimentally identified magnetic phases are theoretically characterized through classical Monte Carlo simulations within an extended Kitaev-Heisenberg model with additional ring exchange interactions. The resulting phase diagram reveals a variety of exotic field-induced magnetic phases, including double-$\textbf{q}$, $\frac{1}{3}$-AFM, zigzag, and vortex phases.
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All authors Kranthi Kumar Bestha, Manaswini Sahoo Niccolò Francini, Robert Kluge, Ryan Morrow, Andrey Maljuk, Sabine Wurmehl, Sven Luther, Yurii Skourski, Hannes Kühne, Swarnamayee Mishra, Jochen Geck, Manuel Brando, Bernd Büchner, Laura T. Corredor, Lukas Janssen, Anja U. B. Wolter

Materials Science

Strained Donor-Bound Excitons in $^{28}$Si

David A. Vogl, Noah L. Braitsch, Başak Ç. Özcan et al. (+3 more)

We present a comprehensive experimental study of the neutral donor to donor-bound exciton transition (D$^0$$\rightarrow\,$D$^0$X) in isotopically enriched $^{28}$Si, focusing on the group-V donors P, As, and Sb under finely tuned uniaxial stress along the [100] and [110] crystal axes and magnetic fields from 3.5 mT to 1.7 T. From these measurements, donor-specific deformation potentials are extracted. The uniaxial electron deformation potential $\Xi_\mathrm{u}$ is found to be significantly larger than values reported for other states or transitions in silicon and shows a clear dependence on the donor species, indicating an increased sensitivity of the D$^0$X state to strain and central-cell effects. We also observe a magnetic field dependence of the hole shear deformation potential $d$, suggesting a more complex strain coupling mechanism than captured by standard theory. Diamagnetic shift parameters determined from Zeeman spectra show good agreement with earlier measurements. Our results provide a refined parameter set critical for the design of silicon quantum devices based on D$^0$X transitions.
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All authors David A. Vogl, Noah L. Braitsch, Başak Ç. Özcan, Niklas S. Vart, M. L. W. Thewalt, Martin S. Brandt

Materials Science

$\beta$-Ga$_2$O$_3$(001) surface reconstructions from first principles and experiment

Konstantin Lion, Piero Mazzolini, Kingsley Egbo et al. (+4 more)

We present a comprehensive investigation of reconstructions on $\beta$-Ga$_2$O$_3$(001) combining first-principles calculations with experimental observations. Using ab initio atomistic thermodynamics and replica-exchange grand-canonical molecular dynamics simulations, we explore the configurational space of possible reconstructions under varying chemical potentials of oxygen and gallium. Our calculations reveal several stable surface reconstructions, most notably a previously unreported 1$\times$2 reconstruction consisting of paired GaO$_4$ tetrahedra that exhibits remarkable stability across a wide range of experimental growth conditions. In this reconstruction, two Ga atoms share one oxygen bond and are separated by a distance of 2.64 Å along the [010] direction. High-angle annular dark-field scanning transmission electron microscopy imaging of homoepitaxially grown (001) layers is consistent with the predicted structure. Additional investigations of possible indium substitution at the surface sites, which can occur during metal-exchange catalysis growth, reveal a cooperative effect in In incorporation, with distinct stability regions for In-substituted structures under O-rich conditions. Our findings provide an understanding for controlling surface properties during epitaxial growth of $\beta$-Ga$_2$O$_3$(001).
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All authors Konstantin Lion, Piero Mazzolini, Kingsley Egbo, Toni Markurt, Oliver Bierwagen, Martin Albrecht, Claudia Draxl

Materials Science

Quasiparticle effects and strong excitonic features in exfoliable 1D semiconducting materials

Simone Grillo, Chiara Cignarella, Friedhelm Bechstedt et al. (+5 more)

We report a comprehensive first-principles study of the electronic and optical properties of recently identified exfoliable one-dimensional semiconducting materials, focusing on chalcogenide-based atomic chains derived from van der Waals-bonded bulk crystals. Specifically, we investigate covalently bonded S3 and Te3 chains, and polar-bonded As2S3 and Bi2Te3 chains, using a fully first-principles approach that combines density-functional theory (DFT), density-functional perturbation theory (DFPT), and many-body perturbation theory within the GW approximation and Bethe-Salpeter equation (BSE). Our vibrational analysis shows that freestanding isolated wires remain dynamically stable, with the zone-center optical phonon modes leading to infrared activity. The main finding of this study is the presence of very strong exciton binding energies (1-3 eV), which make these novel 1D materials ideal platforms for room-temperature excitonic applications. Interestingly, the exciton character remains Wannier-Mott-like, as indicated by average electron-hole separations larger than the lattice constant. Notably, the optical gaps of these materials span a wide range - from infrared (0.8 eV, Bi2Te3), through visible spectrum (yellow: 2.17 eV, Te3; blue: 2.71 eV, As2S3), up to ultraviolet (4.07 eV, S3) - highlighting their versatility for broadband optoelectronic applications. Our results offer a detailed, many-body perspective on the optoelectronic behavior of these low-dimensional materials and underscore their potential for applications in next-generation nanoscale optoelectronic devices.
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All authors Simone Grillo, Chiara Cignarella, Friedhelm Bechstedt, Paola Gori, Maurizia Palummo, Davide Campi, Nicola Marzari, Olivia Pulci

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

Altermagnetism and Superconductivity: A Short Historical Review

Zhao Liu, Hui Hu and Xia-Ji Liu

This review is organized into three parts. In the first part, we explore the deep interconnections among three seemingly unrelated concepts in condensed matter physics: electronic liquid crystal phases, multipole expansions, and altermagnetism. At the heart of these phenomena lies a shared foundation: spin-momentum locking in the nonrelativistic regime. Originally proposed in the context of electronic liquid crystal phases, nonrelativistic spin-momentum locking was later elegantly incorporated into the formalism of multipole expansions. This framework can be further extended across multiple atomic sites, making it particularly effective for describing altermagnets, which host localized magnetic moments with anisotropic magnetization densities distributed over sublattices. In the second part, we examine superconducting phenomena associated with altermagnetism from three complementary perspectives. First, we investigate superconductivity associated with nonrelativistic spin-momentum locked Fermi surfaces, the unifying theme of the first part, highlighting a rich variety of unconventional superconducting states. These include finite-momentum pairing, $d$-wave and spin-triplet superconductivity, and topological Bogoliubov Fermi surfaces, among others. We then review superconductivity emerging from either static altermagnetic order or altermagnetic fluctuations. Finally, we discuss the possible competition and intertwining between altermagnetic order and superconductivity, illustrated using the repulsive Hubbard model. Additional related topics are addressed in the concluding part. Overall, this work offers both an accessible introduction to the newly identified magnetic order known as altermagnetism and a conceptual guide for researchers aiming to harness the ensuing unconventional superconductivity in the development of future quantum technologies.
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Materials Science · Cross-listed from physics.acc-ph

Influence of an external static magnetic field on prebreakdown electron emission and heating

Roni Koitermaa, Marzhan Toktaganova, Andreas Kyritsakis et al. (+4 more)

High magnetic fields can increase the occurrence of vacuum arcing, suggesting that both electric and magnetic fields can play a role in the vacuum arcing process. The mechanism of vacuum arcing in high magnetic fields is believed to involve both the cathode and the anode, with the cathode serving as the originator of field-emitting nanoprotrusions or tips, while the anode serves a secondary role. Significant heating of the anode surface can be achieved by magnetic focusing of the emitted electron beam, leading to increased heat flux due to greater current density. We simulated the emitted electron beam in different configurations of the electric and magnetic fields using the particle-in-cell (PIC) and finite element methods (FEM). The heating caused by the impacting electron beam was simulated for magnetic fields ranging from 0 T to 30 T. The directions of the electric and magnetic fields were found to play a major role in the focusing of the electron beam. We found that a sufficient temperature increase on the anode surface for evaporation can be reached at magnetic fields on the order of 10-30 T, suggesting the possibility of plasma initiation on the anode side.
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All authors Roni Koitermaa, Marzhan Toktaganova, Andreas Kyritsakis, Tauno Tiirats, Alexej Grudiev, Veronika Zadin, Flyura Djurabekova

Materials Science

Atomistic origin of low thermal conductivity in quaternary chalcogenides Cu(Cd, Zn)$_2$InTe$_4$

Nirmalya Jana, Amit Agarwal and Koushik Pal

Crystalline semiconductors with intrinsically low lattice thermal conductivity ($\mathcal{K}$) are vital for device applications such as barrier coatings and thermoelectrics. Quaternary chalcogenide semiconductors such as CuCd$_2$InTe$_4$ and CuZn$_2$InTe$_4$ are experimentally shown to exhibit low $\mathcal{K}$, yet its microscopic origin remains poorly understood. Here, we analyse their thermal transport mechanisms using a unified first-principles framework that captures both the Peierls (particle-like propagation, $\mathcal{K}_P$) and coherence (wave-like tunneling, $\mathcal{K}_C$) mechanisms of phonon transport. We show that extended antibonding states below the Fermi level lead to enhanced phonon anharmonicity and strong scattering of heat-carrying phonon modes, suppressing $\mathcal{K}$ in these chalcogenides. We show that $\mathcal{K}_P$ dominates the total thermal conductivity, while $\mathcal{K}_C$ remains negligible even under strong anharmonicity of the phonon modes. The heavier Cd ions in CuCd$_2$InTe$_4$ induce greater acoustic-optical phonon overlap and scattering compared to CuZn$_2$InTe$_4$, further lowering thermal conductivity of the former. Additionally, grain boundary scattering in realistic samples contributes to further suppression of thermal transport. Our findings establish the atomistic origins of low $\mathcal{K}$ in quaternary chalcogenides and offer guiding principles for designing low-thermal-conductivity semiconductors.
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Materials Science

X-ray imaging of antiferromagnetic octupole domains in Mn$_3$Sn

Max T. Birch, Sebastian Wintz, Yuhan Sun et al. (+4 more)

Novel antiferromagnets with broken time reversal symmetry (TRS) have launched a new direction in spintronics research, combining the advantageous dynamical properties of conventional antiferromagnets with the controllability typically associated with ferromagnets. However, antiferromagnetic domains are notoriously challenging to image in real-space. X-ray magnetic circular dichroism (XMCD) offers a route to overcome this difficulty: XMCD contrast may be finite in TRS-breaking antiferromagnets with an appropriate magnetic space group. Here, we exploit this to image the octupole domains in a focused ion beam-fabricated device of the non-collinear antiferromagnet Mn$_3$Sn. Using scanning transmission x-ray microscopy, we spatially resolve the weak pre-edge XMCD contrast (of 0.2%) that is sensitive to $T_z$, achieving a contrast resolution better than 0.02%. We observe hysteretic switching of the octupole order through both the XMCD contrast and the corresponding anomalous Hall effect within the same device. These results confirm the bulk nature of this contrast, and establish XMCD-based microscopy as a powerful real space imaging method for TRS-breaking antiferromagnets, including altermagnets, enabling future studies of their dynamics, switching, and symmetry-tunable phenomena.
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All authors Max T. Birch, Sebastian Wintz, Yuhan Sun, Akiko Kikkawa, Markus Weigand, Takahisa Arima, Yoshinori Tokura

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

Collective Variables Based on Multipole Expansion of Ewald Summation for Crystallization

YaoKun Lei, MaoDong Li and Yi Isaac Yang

Crystallization, a fundamental phase transition process governing material formation in natural and industrial contexts, involves the spontaneous emergence of long-range structural order from disordered phases. This long-range periodicity involves spatial and molecular orientation order. Molecular dynamics (MD) simulations of crystallization require collective variables (CVs) that accurately distinguish this long-\range periodicity. Existing CVs based on local descriptors (e.g., bond-orientational order) often lack transferability across crystal structures. To address this, we propose a unified CV framework derived from the multipole expansion of Ewald summation: a mathematical formalism bridging X-ray diffraction (XRD) principles and electrostatic energy computation in MD. By projecting atomic configurations onto a basis of spherical harmonics (complete for angular function representation), our CV achieves high-fidelity encoding of both translational and orientational order. Metadynamics simulations demonstrate that this CV drives efficient sampling of polymorphic pathways for known crystals and predicts stable phases even without crystal structures. This approach shows potential as a transferable platform for ab initio crystal structure prediction.
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Materials Science

High-Throughput Screening of Transition Metal-Based 2D Multilayer Kagome Materials via the "1 + 3" Design Strategy

Xing-Yu Wang, En-Qi Bao, Su-Yang Shen et al. (+2 more)

Two-dimensional (2D) kagome materials have drawn extensive research interest due to their unique electronic properties, like flat bands, magnetic frustration, and topological quantum states, which enable precise quantum state control and novel device innovation. Yet, simultaneously achieving high stability, tunability, and multifunctionality in 2D kagome systems remains a key material design challenge. In this study, we innovatively propose a new paradigm for constructing two-dimensional multi-kagome-layer materials based on the "1+3" design concept. By seamlessly integrating high-throughput screening techniques, we have successfully identified 6,379 novel 2D multilayer kagome candidates from a vast pool of candidates. These materials exhibit a rich diversity of types, encompassing 173 metals, 27 semimetals, 166 ferromagnetic semiconductors, and as many as 6,013 semiconductors. Furthermore, based on the 2D flat-band scoring criteria, we conducted a detailed analysis of the flat-band characteristics of the energy bands near the Fermi level in the predicted systems. Our findings reveal that approximately two-thirds of the systems meet the 2D flat-band scoring criteria, and notably, several systems exhibit nearly perfect flat-band characteristics. Our work provides an excellent paradigm for the design and research of 2D multilayer kagome materials
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All authors Xing-Yu Wang, En-Qi Bao, Su-Yang Shen, Jun-Hui Yuan, Jiafu Wang

Materials Science

Hidden integer quantum ferroelectricity in chiral Tellurium

Wei Luo, Sihan Deng, Muting Xie et al. (+3 more)

Ferroelectricity is a cornerstone of functional materials research, enabling diverse technologies from non-volatile memory to optoelectronics. Recently, type-I integer quantum ferroelectricity (IQFE), unconstrained by symmetry, has been proposed and experimentally demonstrated; however, as it arises from ionic displacements of an integer lattice vector, the initial and final states are macroscopically indistinguishable, rendering the physical properties unchanged. Here, we propose for the first time the nontrivial counterpart (i.e., type-II IQFE) where the polarization difference between the initial and final states is quantized but the macroscopical properties differ. We further demonstrate the existence of type-II IQFE in bulk chiral tellurium. In few-layer tellurium, the total polarization remains nearly quantized, composed of a bulk-inherited quantum component and a small surface-induced contribution. Molecular dynamics simulations reveal surface-initiated, layer-by-layer switching driven by reduced energy barriers, explaining why ferroelectricity was observed experimentally in few-layer tellurium, but not in bulk tellurium yet. Interestingly, the chirality of the initial and final states in bulk tellurium is opposite, suggesting a novel way to control structural chirality with electric field in chiral photonics and nonvolatile ferroelectric memory devices.
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All authors Wei Luo, Sihan Deng, Muting Xie, Junyi Ji, Hongjun Xiang, Laurent Bellaiche

Materials Science

Modeling changes in atomic structure around a vacancy with increasing temperature and calculation of temperature dependences of vacancy characteristics in bcc iron

M. Boboqambarova and A.V. Nazarov

We have created an innovative natural thermostat algorithm to mimic the direct impact of temperature on interatomic distances in both a perfect crystal and a system containing a vacancy. Unlike previous research, our findings demonstrate that in a system with a defect, the radii of the initial ten coordination spheres increase almost linearly with temperature. However, the coefficients defining these dependencies, unlike those for interatomic distances farther from the vacancy, do not equal the linear thermal expansion coefficient of the perfect crystal. Knowing the atomic coordinates and their temperature evolution, we calculated the temperature dependence of the vacancy formation energy and the vacancy relaxation volume in bcc iron. The significance of these effects for the accurate calculation of atomic diffusion coefficients is analyzed and discussed.
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Thursday 9 October

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

Control of Neuronal Survival and Development Using Conductive Diamond

Samira Falahatdoost, Yair D. J. Prawer, Danli Peng et al. (+13 more)

This study demonstrates the control of neuronal survival and development using nitrogen-doped ultrananocrystalline diamond (N-UNCD). We highlight the role of N-UNCD in regulating neuronal activity via near-infrared illumination, demonstrating the generation of stable photocurrents that enhance neuronal survival and neurite outgrowth and foster a more active, synchronized neuronal network. Whole transcriptome RNA sequencing reveals that diamond substrates improve cellular-substrate interaction by upregulating extracellular matrix and gap junction-related genes. Our findings underscore the potential of conductive diamond as a robust and biocompatible platform for noninvasive and effective neural tissue engineering.
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All authors Samira Falahatdoost, Yair D. J. Prawer, Danli Peng, Andre Chambers, Hualin Zhan, Leon Pope, Alastair Stacey, Arman Ahnood, Hassan N. Al Hashem, Sorel E. De Leon, David J. Garrett, Kate Fox, Michael B. Clark, Michael R. Ibbotson, Steven Prawer, Wei Tong

Materials Science · Cross-listed from cs.ET

Wireless Sensing of Temperature, Strain and Crack Growth in 3D-Printed Metal Structures via Magneto-Responsive Inclusions

Connor G. McMahan, Chia-Ming Chang, Raymond Nguyen et al. (+4 more)

This study demonstrates the first realization of wireless strain, temperature and crack growth sensing within 3D-printed metallic structures using standard electromagnetic inspection hardware. This establishes a path toward need-based maintenance for parts operating in harsh environments driven by accurate, real-time damage assessments instead of relying on regularly scheduled maintenance teardowns. To this end, we encapsulate and embed magnetoelastic and thermomagnetic materials during additive manufacturing. Mechanical and thermal stimuli affect the magnetic permeability of the embedded materials, which modulates the flux through a coil placed on or near the part's surface. We demonstrate strain sensing accurate to +/-27x10-6 and temperature sensing accurate to +/-0.75 oC. We highlight these sensors' capabilities by detecting the onset of plasticity and fatigue-driven crack growth thousands of cycles before critical failure.
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All authors Connor G. McMahan, Chia-Ming Chang, Raymond Nguyen, Souren Soukiazian, David A. Smith, Tobias Schaedler, David Shahan

Materials Science

Ensemble-Based Data Assimilation for Material Model Characterization in High-Velocity Impact

Rong Jin, Guangyao Wang and Xingsheng Sun

High-fidelity simulations are essential for predicting material behavior under high-velocity impact (HVI), but their accuracy depends on material models and parameters that are often calibrated by manual fitting to multiple costly experiments. In this study, we develop an ensemble-based data assimilation framework for automatic calibration of selected plasticity, fracture, and equation-of-state (EOS) parameters in HVI simulations using data from a single HVI test. The framework combines Smoothed Particle Hydrodynamics, the ensemble Kalman filter (EnKF), and adaptive covariance inflation to mitigate variance collapse. A simple benchmark first shows that the framework is at least one order of magnitude more computationally efficient than Markov chain Monte Carlo at comparable identification accuracy. We then use synthetic back-face deflection data for an AZ31B magnesium plate to identify representative parameters in the Johnson-Cook plasticity and fracture models and the Mie-Gruneisen EOS. Results under under-biased, over-biased, and limited-observation cases show that parameters to which the data are sufficiently sensitive can be accurately recovered within about five iterations, with convergent ensemble standard deviations. In contrast, insensitive parameters tend to converge to incorrect values and retain large ensemble spreads. With fewer observations, convergence is still achieved for sensitive parameters but requires more iterations. Under extreme prior bias, a parameter rejuvenation strategy drives sensitive parameters toward the true values even when the truth lies outside the initial ensemble spread. These results show that ensemble standard deviation provides a practical diagnostic for parameter sensitivity, identifiability, and potential non-uniqueness. Overall, the proposed framework offers an efficient and robust approach for material model characterization in HVI problems.
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Materials Science

Work Function Mapping Across a-In2Se3 to {\alpha}-In2Se3 to {\gamma}-InSe in RF-Sputtered Thin Films

Marius O. Eji, Md. Sakauat Hasan Sakib and Joseph P. Corbett

Indium selenide is a phase-change chalcogenide whose polymorphism enables a variety of physical properties to be tuned. Here we directly quantify the evolution of the surface work function across the amorphous-to-crystalline transition in RF-sputtered In2Se3 thin films grown on c-plane Al2O3 (001). By varying deposition temperature (100-500 {\deg}C) and film thickness, we establish processing windows for a- In2Se3, {\alpha}-In2Se3, and {\gamma}-InSe, and correlate structure with electronic and morphological properties. X-ray diffraction shows films deposited at 100 to 200 {\deg}C are amorphous, 300 to 400 {\deg}C yields {\alpha}-In2Se3, whereas 500 {\deg}C yields {\gamma}-InSe. Kelvin probe force microscopy (KPFM) maps the surface potential and yields spatially averaged work functions spanning 5.26 to 6.64 eV across the amorphous-crystalline transformation; pronounced intra-film heterogeneity is observed, with select {\alpha}-phase and {\gamma}-InSe grains exhibiting work functions exceeding the local mean. Topographical distinctions are found between phases with hexagonally faceted grains in the crystalline state, whereas homogeneous nano-mounds are found in amorphous films. Analysis of Tauc plots revealed optical bandgaps in the range of 2.50 to 1.55 eV across the observed phases. X-ray fluorescence (XRF) measurements further indicated that the indium to selenium concentration ratio varied between 0.70 {\pm} 0.1 to 1.01 {\pm} 0.1 as the deposition temperature increased from 100 to 500 {\deg}C. These measurements provide direct, spatially resolved quantification of work-function evolution through the phase change, supplying parameters essential for contact engineering and device integration of In2Se3 and InSe.
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Materials Science

Accelerated prediction of dielectric functions in solar cell materials with graph neural networks

Caden Ginter, Kamal Choudhary and Subhasish Mandal

We present an atomistic line graph neural network (ALIGNN) model for predicting dielectric functions directly from crystal structures. Trained on $\sim$7000 dielectric functions from the JARVIS-DFT database computed with a meta-GGA exchange-correlation functional, the model accurately reproduces spectral features, including peak intensities and overall line shapes, while enabling efficient high-throughput screening. Applied to the recently developed Alexandria materials database, containing over four hundred thousand insulating materials, we uncover a clear elemental trend, with vanadium emerging as a strong indicator of materials with high-spectroscopic limited maximum efficiency (SLME). In particular, vanadium-based perovskite materials show a substantially higher fraction of high-SLME compounds compared to the database average, underscoring their promise for optoelectronic applications.
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Materials Science

Theory of non-resonant Raman scattering from electrons in nodal and flat bands

Predrag Nikolić

Raman scattering is emerging as a surprising probe of electron topology in quantum materials. It has been used recently to detect and characterize a topological phase transition that accompanies the magnetic transition in Nd$_2$Ir$_2$O$_7$. Here we present a theory of Raman scattering from nodal electrons with Weyl and quadratic band touching spectra, which has to reach beyond the standard effective mass approximation. After reviewing and providing the details of our previous theory development, we discuss several new results. We show that the light-polarization dependence of Raman scattering is universal in the case of Weyl electrons and given by an analytic expression, while it contains symmetry-protected features in the case of quadratic band-touching nodes. We also analyze modifications of the Raman signal due to the ubiquitous tilting of the Weyl spectrum, and argue that universality is lost only in a finite frequency range that springs out of the threshold frequency for untilted nodes. Finally, we explore the frequency dependence of Raman scattering for the case of Dirac electrons coexisting with a flat band in the same region of the first Brillouin zone, which is inspired by the material V$_{1/3}$NbS$_2$.
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Materials Science · Cross-listed from eess.IV

Interlaced dynamic XCT reconstruction with spatio-temporal implicit neural representations

Mathias Boulanger and Ericmoore Jossou

In this work, we investigate the use of spatio-temporalImplicit Neural Representations (INRs) for dynamic X-ray computed tomography (XCT) reconstruction under interlaced acquisition schemes. The proposed approach combines ADMM-based optimization with INCODE, a conditioning framework incorporating prior knowledge, to enable efficient convergence. We evaluate our method under diverse acquisition scenarios, varying the severity of global undersampling, spatial complexity (quantified via spatial information), and noise levels. Across all settings, our model achieves strong performance and outperforms Time-Interlaced Model-Based Iterative Reconstruction (TIMBIR), a state-of-the-art model-based iterative method. In particular, we show that the inductive bias of the INR provides good robustness to moderate noise levels, and that introducing explicit noise modeling through a weighted least squares data fidelity term significantly improves performance in more challenging regimes. The final part of this work explores extensions toward a practical reconstruction framework. We demonstrate the modularity of our approach by explicitly modeling detector non-idealities, incorporating ring artifact correction directly within the reconstruction process. Additionally, we present a proof-of-concept 4D volumetric reconstruction by jointly optimizing over batched axial slices, an approach which opens up the possibilities for massive parallelization, a critical feature for processing large-scale datasets.
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Materials Science · Cross-listed from cond-mat.mes-hall

Classical to Quantum Diffusive Transport in Atomically Thin Semiconductors Capped with High-k Dielectric

Jaroslaw Pawlowski, Dickson Thian, Repaka Maheswar et al. (+5 more)

The dielectric environment surrounding semiconductors plays a crucial role in determining device performance, a role that becomes especially pronounced in atomically thin semiconductors where charge carriers are confined within a few atomic layers and strongly interact with their surroundings. High-k dielectrics, such as hafnium oxide (HfO2), have been shown to enhance the performance of two-dimensional (2D) materials by suppressing scattering from charged impurities and phonons, but most studies to date have focused on room-temperature transistor operation. Their influence on quantum transport properties at low temperatures remains largely unexplored. In this work, we investigate how capping monolayer molybdenum disulfide (MoS2) with HfO2 modifies its electronic behavior in the quantum regime. By comparing devices with and without HfO2 capping, we find that uncapped devices exhibit transport dominated by classical diffusive scattering, whereas capped devices show clear Fabry-Perot interference patterns, providing direct evidence of phase-coherent quantum transport enabled by dielectric screening. To gain further insight, we develop a tight-binding interferometer model that captures the effect of dielectric screening on conductive modes and reproduces the experimental trends. Our findings demonstrate that dielectric engineering provides a powerful route to control transport regimes in TMD devices.
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All authors Jaroslaw Pawlowski, Dickson Thian, Repaka Maheswar, Chai Jian Wei, Pawan Kumar, Sudhiranjan Tripathy, Hugh O.H. Churchill, Dharmraj Kotekar Patil

Materials Science

Trajectory-Dependent Electronic Energy Losses in Ion Range Simulations

Glen P. Kiely, Bruno Semião, Evgeniia Ponomareva et al. (+3 more)

The energy losses of energetic ions in materials depend on both nuclear and electronic interactions. In channeling geometries, the stopping effect of these interactions can be highly reduced, resulting in deeper ion penetration. Comprehensive, trajectory-dependent models for ion-material interactions are therefore crucial for the accurate prediction of ion range profiles. We present the implementation of a recent electron density-dependent energy-loss model in the efficient molecular dynamics-based MDRANGE code. The model captures ab initio electron dynamics using a parametrized ion energy loss function, based on calculations for explicit trajectories using real-time time-dependent density functional theory. We demonstrate the efficient simulation of trajectory-dependent ion range profiles with this comprehensive model for electronic energy losses. Our results indicate that accurate trajectory-dependent ion range profiles can be simulated using well-fitted parametrizations of this model. This method offers a unique tool for validation of the fitted energy-loss functions using energetic ion ranges, which can be measured experimentally but are beyond the capability of full MD simulations due to the computational expense.
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All authors Glen P. Kiely, Bruno Semião, Evgeniia Ponomareva, Rafael Nuñez-Palacio, Unna Arpiainen, Andrea E. Sand

Materials Science

Microstructure and phase stability within the AlMoNbTiZr system: design tools and compositional boundaries for a high-entropy alloy

Mariano Casas-Luna, Dalibor Preisler, Jiří Kozlík et al. (+1 more)

This study explores Ti-containing complex concentrated alloys (CCAs) within the AlMoNbTiZr system, focusing on compositions located in regions of the Bo-Md diagram characterized by low bond order (Bo) and d-orbital energy level (Md). Four alloys were designed near the line predicting stress-induced martensite formation in conventional Ti alloys, then cast, annealed at 1200{\deg}C, and water quenched. Their microstructures and phases were analyzed and compared against phase prediction tools commonly applied to high-entropy alloys (HEAs), namely empirical parameters and CALPHAD simulations. Results highlight the strong influence of chemical affinity, particularly the roles of Al, Mo, and Zr concentrations, on solid-solution stability. A maximum Al content of 10 at% was identified as the threshold for achieving a single-phase microstructure, observed in the 10Al15Mo10Nb35Ti30Zr alloy. This alloy exhibited a bcc/b2 structure with high compressive strength (above 1300 MPa), low Young's modulus (28 GPa), and limited strain (<6%), but lacked the transformation-induced strengthening mechanisms expected for Ti alloys with comparable Bo-Md values.
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All authors Mariano Casas-Luna, Dalibor Preisler, Jiří Kozlík, Josef Stráský

Materials Science

Enhanced thermal stability of inverted perovskite solar cells by bulky passivation with pyridine-functionalized triphenylamine

Ekaterina A. Ilicheva, Irina A. Chuyko, Lev O. Luchnikov et al. (+11 more)

Despite competitive efficiency compared to Si solar cells and relevant stability at near room temperatures the rapid degradation at elevated temperatures remains the critical obstacle for exploitation of perovskite photovoltaics. In this work, a 4-(pyridin-4-yl)triphenylamine (TPA-Py) with pyridine anchor group was employed for inter-grain bulk modification of double-cation CsCH(NH2)2PbI3 perovskite absorbers to enhance thermal stability. Through coordination and dipole-dipole interactions, nitrogen-containing fragments (diphenylamine and pyridine) of TPA-Py passivate uncoordinated cations and improve the phase resilience of perovskite films against segregation. This resulted in a power conversion efficiency of 21.3% with a high open-circuit voltage of 1.14 V. Notable impact of self-assembled monolayer incorporated into the bulk of perovskite film manifested in huge improvement of thermal stability at 85{\deg}C (ISOS-D-2). TPA-Py modification improved extended the T80 lifetime to ~600 h compared to only 200 h for the reference under harsh heating stress in ambient conditions. In-depth analysis using photoinduced voltage transients and admittance spectroscopy after different stress periods revealed the screening of ion migration (0.45 eV) for devices with TPA-Py. This work offers an important understanding of the bulk modification of microcrystalline perovskite absorbers and guide for robust design of bulk and buried interfaces in highly efficient perovskite solar cells.
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All authors Ekaterina A. Ilicheva, Irina A. Chuyko, Lev O. Luchnikov, Polina K. Sukhorukova, Nikita S. Saratovsky, Anton A. Vasilev, Luiza Alexanyan, Anna A. Zarudnyaya, Dmitri Yu. Dorofeev, Sergey S. Kozlov, Andrey P. Morozov, Dmitry S. Muratov, Yuriy N. Luponosov, Danila S. Saranin

Materials Science

Ultraviolet optical conductivity, exciton fine-structure and dispersion of freestanding monolayer h-BN

Jinhua Hong, Alberto Guandalini, Weibin Wu et al. (+7 more)

Excitons govern the light-matter interaction in 2D gapped materials with intrinsically large binding energies. In spite of plentiful optical measurements in the visible for semiconducting transition-metal dichalcogenides, we still lack optical-absorption studies of the exciton structure of insulating 2D materials that requires UV light. Moreover, measurements of the momentum dispersion of excitons in the vicinity of optical limit are rare owing to low resolutions but hold the key to reveal quasiparticle interactions. To close this gap, we employ high momentum resolution electron energy loss spectroscopy ($q$-EELS) to explore exciton dispersions of mono- and few-layer hexagonal boron nitride. Surprisingly, we reveal a fine structure of the first bright exciton dispersion band composed by two features (A and A$'$), visible only at small momentum, not predicted by Bethe-Salpeter calculations. Introducing an optical conductivity approximation (OCA), we extract from the experimental $q$-EELS spectra the ultraviolet (UV) optical conductivity at zero momentum, $\sigma(\omega)$, and discuss the exciton fine structure in $\sigma(\omega)$, consistent with previous photoluminescence observations. Our findings establish a general methodology to probe the fine structure of exciton dispersions, providing new insights into exciton-phonon sidebands and eventually polarons in low-dimensional materials.
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All authors Jinhua Hong, Alberto Guandalini, Weibin Wu, Haiming Sun, Fuwei Wu, Shulin Chen, Chao Ma, Kazu Suenaga, Thomas Pichler, Francesco Mauri

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

Topological surface magnon-polariton in an insulating canted antiferromagnet

Weixin Li, Rundong Yuan, Fenglin Zhong et al. (+3 more)

Excitation and control of antiferromagnetic magnon modes lie at the heart of coherent antiferromagnetic spintronics. Here, we propose a topological surface magnon-polariton as a new approach in the prototypical magnonic material hematite. We show that in an insulating canted antiferromagnet, where strong-coupled magnon-photon modes can be achieved using electrical on-chip layouts, a surface magnon-polariton mode exists in the gap of the bulk magnon-photon bands. The emergence of surface magnon-polariton mode is further attributed to the nontrivial topology of bulk magnon-photon bands. Magnon-photon coupling enhances the Berry curvature near the anticrossing points, leading to a topological bulk Chern band associated with the surface magnon-polaritons. Our work provides a general principle for the utilization of optomagnetic properties in antiferromagnets, with an illustration of its experimental feasibility and wide generality as manifested in hematite.
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All authors Weixin Li, Rundong Yuan, Fenglin Zhong, Bo Peng, Jean-Philippe Ansermet, Haiming Yu

Materials Science

Observation of electromagnons in a monolayer multiferroic

Mohammad Amini, Tiago V. C. Antão, Liwei Jing et al. (+7 more)

Van der Waals multiferroics have emerged as a promising platform to explore novel magnetoelectric phenomena. Recently, it has been shown that monolayer NiI$_2$ hosts robust type-II multiferroicity down to the two-dimensional limit, a giant dynamical magnetoelectric coupling at terahertz frequencies, and an electrically switchable spin polarization. These developments present the possibility of engineering ultrafast, low-energy-consumption, and electrically-tunable spintronic devices based on the collective excitations of the multiferroic order, electromagnons. However, the direct visualization of these bosonic modes in real space and within the monolayer limit remains elusive. Here, we report the atomic-scale observation of electromagnons in monolayer NiI$_2$ using low-temperature scanning tunneling microscopy. By tracking the thermal evolution of the multiferroic phase, we establish the energy scale and resolve coherent in-gap excitations of the symmetry-broken multiferroic state. Comparison with first-principles and spin-model calculations reveals that the low-energy modes originate from electromagnon excitations. Spatially resolved inelastic tunneling spectroscopy maps show a stripe-like modulation of the local spectral function at electromagnon energies, matching theoretical predictions. These results provide direct evidence of the internal structure of electromagnons and establish a methodology to probe these modes at the atomic scale, opening avenues for electrically tunable spintronics.
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All authors Mohammad Amini, Tiago V. C. Antão, Liwei Jing, Ziying Wang, Antti Karjasilta, Robert Drost, Shawulienu Kezilebieke, Jose L. Lado, Adolfo O. Fumega, Peter Liljeroth

Materials Science

Mechanical coupling of polar topologies and oxygen octahedra rotations in PbTiO$_3$/SrTiO$_3$ superlattices

Fernando Gómez-Ortiz, Louis Bastogne, Xu He et al. (+1 more)

PbTiO$_3$/SrTiO$_3$ artificial superlattices recently emerged as a prototypical platform for the emergence and study of polar topologies. While previous studies mainly focused on the polar textures inherent to the ferroelectric PbTiO$_3$ layers, the oxygen octahedra rotations inherent to the paraelectric SrTiO$_3$ layers have attracted much little attention. Here, we highlight a biunivocal relationship between distinct polar topologies – including $a_1/a_2$ domains, polar vortices, and skyrmions – within the PbTiO$_3$ layers and specific patterns of oxygen octahedra rotations in the SrTiO$_3$ layers. This relationship arises from a strain-mediated coupling between the two materials and is shown to be reciprocal. Through second-principles atomistic simulations, we demonstrate that each polar texture imposes a corresponding rotation pattern, while conversely, a frozen oxygen octahedra rotation dictates the emergence of the associated polar state. This confirms the strong coupling between oxygen octahedra rotations in SrTiO$_3$ and polarization in PbTiO$_3$, highlighting their cooperative role in stabilizing complex polar textures in related superlattices.
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All authors Fernando Gómez-Ortiz, Louis Bastogne, Xu He, Philippe Ghosez

Materials Science

Multimodal Topological Textures Arising from Coupled Structural Orders in SrTiO$_3$

Fernando Gómez-Ortiz, Louis Bastogne and Philippe Ghosez

Magnetic spin topological textures recently found their electrical counterparts in polar topologies emerging from the condensation of inhomogeneous polar atomic distortions. Here, we further extend the concept to other non-polar atomic degrees of freedom. Taking SrTiO$_3$ as a prototypical example, we investigate from second-principles atomistic simulations, the equilibrium domain structures and topological textures associated with the natural antiferrodistortive rotations of its oxygen octahedra. % Besides the common 90$^\circ$ antiferrodistortive domain walls (twin boundaries), we identify new metastable 180$^\circ$ domain walls oriented along the $\lbrace100\rbrace_\mathrm{pc}$ direction, when compressive epitaxial strain is applied. These domains exhibit complex antiferrodistortive Bloch- and Néel-like configurations with the later being the most favorable. We also stabilize antiferrodistortive vortex and antivortex structures which are accompanied by co-localized polarization vortices and a complex pattern of the local strain field, giving rise to a trimodal topological structures. Our results extends the concept of topological ordering to non-polar structural degrees of freedom and highlights the role of lattice-mediated couplings in stabilizing complex textures in perovskite oxides.
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Materials Science

Rare-Earth Engineering of NaAlO3 Perovskites Unlocks Unified Optoelectronic, Thermoelectric, and Spintronic Functionalities

Muhammad Imran, Sikander Azam, Qaiser Rafiq et al. (+1 more)

Perovskite oxides are promising for energy and quantum technologies, but wide-gap hosts such as NaAlO3 suffer from deep-UV absorption and limited carrier transport. Using first-principles GGA+U+SOC calculations, we investigate Eu3+-, Gd3+-, and Tb3+-doped NaAlO3 and evaluate their electronic, optical, elastic, and thermoelectric properties. Rare-earth substitution is thermodynamically favorable (formation energies 1.2-1.6 eV) and induces strong f-p hybridization, reducing the pristine band gap (about 6.2 eV) to about 3.1 eV for Tb. Spin-resolved band structures reveal Gd-driven half-metallicity, Eu-induced spin-selective metallicity, and Tb-stabilized p-type semiconducting behavior. The optical spectra show a red-shifted absorption edge (about 2.0-2.2 eV), a large static dielectric response (epsilon1(0) about 95 for Eu), and plasmonic resonances near 4 eV, enabling visible-light harvesting. Elastic analysis indicates mild lattice softening with preserved ductility (Pugh ratio B/G about 1.56-1.57). Thermoelectric performance is enhanced, with Seebeck coefficients greater than 210 uV/K for Eu and Tb and ZT about 0.45 at 500 K. These results identify rare-earth-doped NaAlO3 as a multifunctional perovskite platform for photovoltaics, photocatalysis, thermoelectrics, and spintronics.
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All authors Muhammad Imran, Sikander Azam, Qaiser Rafiq, Amin Ur Rahman