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Physics Condensed Matter Materials Science

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Friday 6 February

Materials Science

Microscopic Origin of the Ultralow Lattice Thermal Conductivity in Vacancy-Ordered Halide Double Perovskites Cs$_2BX_6$ ($B$ = Zr, Pd, Sn, Te, Hf, and Pt; $X$= Cl, Br, and I)

Lingzhi Cao, Yateng Wang, Zhonghao Xia et al. (+1 more)

Vacancy-ordered halide double perovskites Cs$_2BX_6$ have recently attracted significant attention due to their intrinsically ultralow lattice thermal conductivity ($\kappa_{\mathrm{L}}$), which is highly desirable for thermal insulation and thermoelectric applications. In this work, we systematically investigate the anharmonic lattice dynamics and thermal transport properties of Cs$_2BX_6$ ($B$ = Zr, Pd, Sn, Te, Hf, and Pt; $X$ = Cl, Br, and I) using state-of-the-art first-principles calculations, based on a unified theory of thermal transport for crystals and glasses. All studied compounds are found to exhibit ultralow $\kappa_{\mathrm{L}}$ below 1.0~W\,m$^{-1}$\,K$^{-1}$ at room temperature and large derivation from the conventional $T^{-1}$ temperature dependence. Our analysis combining with machine-learning approach show that low sound velocities (1100 – 1600~m\,s$^{-1}$), which originates from the intrinsically weak chemical bonding, play a crucial role in suppressing heat transport of the most compounds, instead of the strong scattering of rattling phonon modes expected from the large void in the structure. Furthermore, the influence of $B$ and $X$-site elements on phonon dispersion, anharmonicity, and scattering phase space is clarified. Our results provide microscopic insights into the origin of ultralow $\kappa_{\mathrm{L}}$ in Cs$_2BX_6$ and offer guiding principles for the rational design of halide-based materials with tailored thermal transport properties.
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All authors Lingzhi Cao, Yateng Wang, Zhonghao Xia, Jiangang He

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

Wavefront-Dislocation Evolution via Quadratic Band Touching Annihilation

Rasoul Ghadimi, Jaehyeon Ahn and Sangmo Cheon

Wavefront dislocations (WDs) – phase singularities observed in quasiparticle interference (QPI) experiments – have been widely interpreted as the definitive real-space signatures of Berry phases in graphene-family systems. Here, we disentangle the roles of topological charge and pseudospin texture in WD experiments. By investigating various way of the annihilation of quadratic band touchings (QBTs) in bilayer graphene and magneto-spin-orbit graphene systems, we demonstrate that WD evolution is governed exclusively by changes in the underlying pseudospin winding, while remaining insensitive to the topological charge (i.e., vorticity) of the band touching itself. Our results imply that WD measures wavefunction pseudospin texture rather than a diagnostic of topological charge and provide solid-state platforms in which WD evolution can be engineered and observed.
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Materials Science

Epitaxial growth and magneto-transport properties of kagome metal FeGe thin films

Xiaoyue Song, Yanshen Chen, Yongcheng Deng et al. (+5 more)

Antiferromagnetic kagome metal FeGe has attracted tremendous attention in condensed matter physics due to the charge density wave (CDW) being well below its magnetic transition temperature. Up to now, numerous works on kagome FeGe have been based on single crystal bulk, but its thin film form has still not been reported. Here, we achieved epitaxial growth of FeGe thin films on Al2O3 substrates using molecular beam epitaxy. Structural characterization with x-ray diffraction, atomic force microscopy, and high-resolution scanning transmission electron microscopy reveals single phase with flat surface of kagome FeGe thin films. Moreover, a Néel temperature of 397 K and a rapid variation of Hall coefficient and magnetoresistance around 100 K, which might be related to the CDW, were revealed via transport measurements. The high quality kagome FeGe thin films are expected to provide a versatile platform to study the mechanism of CDW and explore the application of FeGe in antiferromagnetic spintronics.
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All authors Xiaoyue Song, Yanshen Chen, Yongcheng Deng, Tongao Sun, Fei Wang, Guodong Wei, Xionghua Liu, Kaiyou Wang

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

Shallow Trap States Control Electrical Performance of Amorphous Oxide Semiconductor Thin-Film Transistors

Måns J. Mattsson, Jinhan Lee, Christopher E. Malmberg et al. (+10 more)

The performance of n-type amorphous oxide semiconductor thin-film transistors (TFTs) is largely controlled by the density of states (DoS) near the conduction band mobility edge. Here, the full subgap DoS of amorphous InGaZnO (a-IGZO) TFTs, used in display panels and dynamic random-access memory (DRAM) development, is measured by ultrabroadband photoconduction (UP-DoS) microscopy to within 0.1 eV of the mobility edge. The measured subgap DoS for 25 TFT processing conditions accurately predicts each transfer curve, showing how shallow defect states are electron traps that rigidly tune subthreshold swing, threshold voltage and drift mobility. For a set of TFTs, the subthreshold transfer characteristics can be independently simulated from the experimental shallow defect DoS, with no adjustable parameters. The full transfer curve is simulated by introducing a single parameter: the conduction band tail energy. Additionally, the simulation reveals that the shallow trap density controlling subthreshold behavior can be directly extracted from transfer curves. Finally, a systematic In-enrichment study, combined with DFT+U DoS simulations, enables identification of vacancy cation coordination environments for all experimentally observed subgap peaks. The dominant trap controlling conventional a-IGZO TFT performance is centered at ~0.32 eV below the conduction band mobility edge and is assigned to a Ga-Ga-In oxygen vacancy defect.
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All authors Måns J. Mattsson, Jinhan Lee, Christopher E. Malmberg, Jared Parker, Kyle T. Vogt, Hyemi Kim, Minji Hong, Pilsang Yun, Daewon Ha, Taeyoon Lee, Paul H.-Y. Cheong, John F. Wager, Matt W. Graham

Materials Science

Structural Distortions and Ferroelectricity in Antiperovskite Oxides with Tetrel Elements

He Zhu and Turan Birol

Antiperovskites share the same structure as perovskites, but allow completely different chemistries and nominal charge states of anions to be stabilized. This gives rise to many interesting phenomena, including septet superconductivity and topological crystalline insulating phases in these systems. Despite this, the work on the crystal structural trends in these compounds is more limited compared to perovskites. In this study, we consider the family of antiperovskite oxides with tetrel elements (Si, Ge, Sn, Pb) and alkaline earth metals (Ca, Sr, Ba), and perform a detailed study of their crystal structures using first principles density functional theory. We show how tolerance factor arguments can be constructed to predict their structure in a way parallel to the perovskites, and furthermore, how heterostructuring (or cation-order) can be used to induce ferroelectricity in these systems which may provide an experimental knob to modify electronic structure. We conclude with a discussion of the electronic structure of antiperovskites, and show that they display interesting trends not observed in regular perovskites, including significant antibonding interactions between face-center ions, which might need to be taken into account when building effective electronic models of these compounds.
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Materials Science

Disorder-induced symmetry breaking in moiré bands of marginally twisted bilayer MoS$_2$

Pablo Reséndiz-Vázquez, Christophe de Beule, Thi-Hai-Yen Vu et al. (+8 more)

Twisted transition-metal dichalcogenides host highly tunable moiré potentials, flat bands, and correlated electronic phases, yet the role of disorder in shaping these emergent properties remains largely unresolved. Using scanning tunneling spectroscopy, we investigate the impact of electrostatic disorder on the electronic structure of marginally twisted ($\theta \approx 0.95^\circ$) bilayer MoS$_2$. Differences of 15 meV in the onset energies of the valence and conduction bands between MX- and XM-stacked regions are observed and are unexpected based on symmetry considerations. We further observe spatially correlated disorder in the band onset energy that is consistent with a background random charge density of a few $10^{11}\,\mathrm{cm}^{-2}$. Continuum model calculations for twisted MoS$_2$ reveal dramatic changes in the low-energy moiré bands in response to an electric displacement field, in quantitative agreement with experiment. Moreover, the calculated local density of states including disorder broadening reproduces the experimental observations only when structural relaxation is taken into account. These results highlight the critical role of electrostatic disorder in determining the electronic structure of moiré materials at the nanoscale.
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All authors Pablo Reséndiz-Vázquez, Christophe de Beule, Thi-Hai-Yen Vu, Kaijian Xing, Daniel McEwen, Daniel Bennett, Liangtao Peng, Héctor González-Herrero, Shaffique Adam, Mark T. Edmonds, Michael S. Fuhrer

Materials Science · Cross-listed from physics.optics

Direct laser writing of high aspect ratio nanochannels for nanofluidics

Stoffel D. Janssens, Meissha Ayu Ardini, David Vázquez-Cortés et al. (+2 more)

Nanochannels with high width-to-height aspect ratios are desirable for many applications, particularly those requiring optical access, but remain challenging to fabricate. In this work, the direct laser writing of such channels between diamond films and glass substrates is introduced. As previously reported, laser light can transform a portion of diamond film into a nanostrip. The strip induces delamination of the surrounding film, causing the formation of two nanochannels with triangular cross-sections. Here, it is demonstrated that nanochannels with rectangular cross-sections and width-to-height aspect ratios exceeding fifty can form between pairs of nanostrips. With atomic force microscopy, the maximum strip spacing that produces these nanochannels is investigated, and it is demonstrated that the reflectance of the channels can be measured by microspectrophotometry. The microstructure of the nanochannels, including nanostrips, and processes that occur during laser writing are inferred from transmission electron microscopy and electron energy loss spectroscopy. By fabricating a nanofluidic device and using microspectrophotometry, it is found that the nanochannels fill with water through capillary action, are resistant to clogging, and are mechanically stable against water filling. A versatile platform for producing high-aspect-ratio nanochannels that are optically accessible and fluidically functional is presented, thereby expanding opportunities for advanced applications.
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All authors Stoffel D. Janssens, Meissha Ayu Ardini, David Vázquez-Cortés, Cathal Cassidy, Eliot Fried

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

Superconductivity of 30.4 K and its Reemergence under Pressure in Fe1.11Se Synthesized via Ion-exchange and De-intercalation Reaction

Mingzhang Yang, Yuxin Ma, Qi Li et al. (+11 more)

Binary stoichiometry FeSe (s-FeSe) is a well-known parent of high-temperature unconventional superconductors owing to its charge-neutral layer, highly tunable structure and electronic properties, and rich interplays among multiple electronic phases. Yet the s-FeSe synthesized via high-temperature equilibrium reactions bears the notorious interstitial Fe,where merely 3% of them is sufficient to kill the superconductivity. Here, we successfully synthesized a new non-stoichiometric Fe1.11Se single crystal with a superconducting onset temperature (Tconset) of 30.4 K through a hydrothermal ion-exchange and de-intercalation route. 11% interstitial Fe ions exceed the equilibrium phase diagram limit. Intriguingly, under physical pressure, the Tconset of exhibits a "V"-shaped evolution with a minimum at 2-2.6 GPa, and then upturning into a second superconducting region, reminiscent of the behaviors in FeSe intercalates. Furthermore, a pressure-induced possible magnetic order, previously only observed in pressurized s-FeSe, shows up. These results offer fresh insights into the role of interstitial Fe in governing superconducting and transport properties under non-equilibrium synthesis and tuning strategies.
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All authors Mingzhang Yang, Yuxin Ma, Qi Li, Ke Ma, Jiali Lu, Zhaolong Liu, Ruijin Sun, Tianping Ying, Mengdi Wang, Xin Chen, Changchun Zhao, Jian-gang Guo, Shifeng Jin, Xiaolong Chen

Materials Science

Observation of robust spin-phonon coupling and indication of hidden structural transition in the spin-driven ferroelectrics Mn4B_2O_9 (B= Nb, Ta)

Rajesh Jana, Alka Garg, Rekha Rao et al. (+1 more)

We report detailed Raman spectroscopic and magnetic susceptibility studies on the spin-driven ferroelectric compounds Mn4Nb2O9 (MNO) and Mn4Ta2O9 (MTO). Both systems exhibit strong spin-phonon coupling below the short-range magnetic ordering temperature (T(sro)=223 K), followed by further renormalization of several Raman modes at the long-range magnetic ordering temperatures (TN = 120 K for MNO and 110 K for MTO). Pronounced anomalies in Raman mode frequencies and linewidths, along with the emergence of octahedral modes between Tsro and TN, indicate a possible low-symmetry structural transition, more evident in MNO and closely linked to magnetic ordering in MTO. Distinct low-temperature evolutions of Raman mode shift, linewidth, and integrated intensity in MNO and MTO highlight the role of the nonmagnetic B-site cation in tuning spin-lattice coupling, driven by differences in spin-orbit coupling and orbital hybridization between Nb5+ (4d) and Ta5+ (5d). By combining Raman spectroscopy with nuclear magnetic resonance, and diffuse reflectance spectroscopy, we further show that Mn-based systems possess a more distorted local structure than their Co analogues, while their electronic structures differ despite comparable band gaps. These results provide a comprehensive understanding of spin-lattice coupling in Mn- and Co-based A4B2O9 magnetoelectric systems.
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All authors Rajesh Jana, Alka Garg, Rekha Rao, Thomas Meier

Thursday 5 February

Materials Science

Thin-Film Stabilization and Magnetism of {\eta}-Carbide Type Iron Nitrides

Baptiste Julien, Abrar Rauf, Liam A. V. Nagle-Cocco et al. (+4 more)

Transition-metal nitrides in {\eta}-carbide type structures exhibit unusual bonding motifs and proximity to magnetic instabilities. Yet they remain unexplored in thin-film form due to the difficulty of stabilizing nitrogen-poor ternaries among competing phases. Here, we report the thin-film synthesis and phase-stability mapping of the {\eta}-nitride systems Fe-W-N and Fe-Mo-N. Amorphous Fe-M-N (M = W, Mo) combinatorial libraries deposited by reactive co-sputtering crystallize upon rapid thermal annealing, enabling systematic identification of synthesis windows as a function of composition and annealing temperature. Using laboratory powder X-ray diffraction and synchrotron grazing incidence wide angle X-ray scattering, we establish that Fe3Mo3N-based {\eta}-carbide phases form over a substantially broader compositional and thermal range than W-based compositions, where {\eta} structures are stabilized only when the films are Fe-rich. These trends are rationalized using mixed chemical-potential vs. composition phase diagrams that capture the narrow nitrogen chemical-potential stability of {\eta}-nitrides. Magnetic measurements reveal that ferromagnetism is induced in Fe-rich Fe3.54Mo2.46N with a small exchange-bias-like response that is absent in Fe3W3N-based compositions, highlighting the sensitivity of magnetic behavior to modest deviations from stoichiometry. This work establishes practical thin-film synthesis routes for {\eta}-nitride materials and demonstrates how composition can be tuned to access emergent magnetic phenomena in these complex nitrides.
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All authors Baptiste Julien, Abrar Rauf, Liam A. V. Nagle-Cocco, Rebecca W. Smaha, Wenhao Sun, Andriy Zakutayev, Sage R. Bauers

Materials Science

Enhanced Elevated-Temperature Strength in Refractory Complex Concentrated Alloys via Temperature-Induced Transition from Screw-to-Edge Dislocation Control

Tamanna Zakia, Ayeman Nahin, Dunji Yu et al. (+8 more)

Refractory complex concentrated alloys (RCCAs) show promise for high-temperature applications but often lose strength due to screw-dislocation-controlled plasticity. We demonstrate a temperature-driven transition from screw- to edge-dislocation-controlled deformation in a single-phase NbTaTiV RCCA. Tensile tests from 298-1573 K reveal a pronounced intermediate-temperature strength plateau and yield strengths surpassing other ductile RCCAs and the Ni-based superalloy CMSX-4 above 1273 K. In-situ neutron diffraction, TEM, and molecular dynamics identify a crossover near ~900 K, where edge dislocation glide stabilized by V-induced lattice distortion dominates, enabling enhanced strength retention and a clear design strategy for ultrahigh-temperature applications.
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All authors Tamanna Zakia, Ayeman Nahin, Dunji Yu, Jacob Pustelnik, Juntan Li, Mason Kincheloe, Lia Amalia, Yan Chen, Peter K. Liaw, Haixuan Xu, Mingwei Zhang

Materials Science

Engineering altermagnetic symmetry to enable anomalous Hall response in Cr$_{1-x}$Mn$_x$Sb

Miriam G. Fischer, Lukas Odenbreit, Olena Gomonay et al. (+10 more)

Altermagnets are a promising class of materials for spintronic applications. However, compounds that simultaneously combine the symmetry required to support an anomalous Hall effect with good metallic conductivity and magnetic ordering temperatures well above room temperature have remained elusive. Here, we demonstrate that partial substitution of Cr by Mn in epitaxial CrSb(100) thin films provides a viable route to engineer the combined structural and magnetic symmetry necessary to enable an otherwise symmetry-forbidden anomalous Hall effect. We systematically explore the magnetic phase diagram of Cr(Mn)Sb thin films including neutron diffraction. This allows us to identify a magnetic symmetry that supports the anomalous Hall effect, which we demonstrate in Cr(0.75)Mn(0.25)Sb. Guided by Landau theory, we model the field-driven reorientation of the Neel vector and the resulting anomalous Hall response, achieving good qualitative agreement with the experimental observations.
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All authors Miriam G. Fischer, Lukas Odenbreit, Olena Gomonay, Sonka Reimers, Pascal Manuel, Dmitry Khalyavin, Sean Langridge, Jairo Sinova, Thibaud Denneulin, Joseph V. Vaz, Rafal E. Dunin-Borkowski, Mathias Kläui, Martin Jourdan

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

Robust flat bands of the honeycomb wire network

Chunxiao Liu, Benoît Douçot and Jérôme Cayssol

We show that periodic honeycomb networks of ballistic conducting channels generically host exact flat bands spanning the entire Brillouin zone. These flat bands are independent of microscopic vertex scattering, persist for any number of transverse modes, and occur in a universal $1\colon 2$ ratio with dispersive bands. Their existence is enforced by local $D_3$ vertex symmetry and lattice translations. We construct compact localized states obeying a Bohr-Sommerfeld-type quantization condition and demonstrate that flat bands survive in realistic antidot lattices, establishing honeycomb wire networks as a robust flat band platform relevant to gated high-mobility 2D electron gases and molecule-patterned metallic surfaces.
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Materials Science

Novel non-thermal Ablation Mechanics in the Laser Ablation of Silicon

Dominic Klein, Simon Kümmel and Johannes Roth

We investigate the non-thermal material dynamics of strongly excited silicon during ultra-fast laser ablation. In contrast to metals, silicon shows strongly excitation-dependent interatomic bonding strengths, which gives rise to a number of unique material dynamics like non-thermal melting, Coulomb explosions and altered carrier heat conduction due to charge carrier confinement. In this study, we report novel non-thermal ablation mechanisms in the ultra-fast single shot laser ablation of silicon and perform large scale massive multi-parallel simulations on experimentally achievable length scales with atomistic resolution. For this, we model the ultra-fast carrier dynamics utilizing the Thermal-Spike-Model coupled to Molecular Dynamics simulations and include the accompanied excitation-dependent nonthermal bonding strength manipulation by application of the excitation-dependent modified Tersoff Potential. Further, we present first results on the systematic construction of the excitation-dependent phase diagram of silicon by thermodynamic integration.
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Materials Science

Platform and Framework for Time-Resolved Nanoscale Thermal Transport Measurements in STEM

Mairi McCauley, Joel Martis, Ondrej L. Krivanek et al. (+12 more)

Understanding heat transport at the nanometer scale is critical for semiconductor devices, quantum materials, and thermal management of nanostructures, yet direct local measurements of thermal conductivity and heat capacity remain scarce. We developed a laser-excitation system integrated into a scanning transmission electron microscope (STEM) for nanoscale thermal transport measurements using ultra-high-resolution electron energy-loss spectroscopy (EELS). A fiber-coupled laser is introduced via a modified aperture mechanism, enabling flexible holder geometries and large tilt angles without optical elements in the polepiece gap. Synchronization of pulsed laser excitation with an externally gated direct electron detector provides temporal resolution about 50 ns at <10 meV energy resolution. Local temperatures are determined via the principle of detailed balance, and thermal transport parameters are extracted by fitting a forward-time central-space heat diffusion model including radiative losses. For amorphous carbon films, we obtain a thermal conductivity of 1.24 $\frac{W}{m\cdot K}$ and a heat capacity of 821 $\frac{J}{kg\cdot K}$, consistent with literature. This framework enables time-resolved nanoscale measurements of thermal transport in materials and devices.
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All authors Mairi McCauley, Joel Martis, Ondrej L. Krivanek, Ben Plotkin-Swing, Andreas Mittelberger, Tolga Wagner, Hüseyin Çelik, Grigory Kornilov, Meng Zhao, Matthias Meffert, Luca Piazza, Tracy C. Lovejoy, Guillaume Radtke, Christoph Koch, Benedikt Haas

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

Topological piezomagnetic effect in two-dimensional Dirac quadrupole altermagnets

H. Radhakrishnan, B. Bell, C. Ortix et al. (+1 more)

Altermagnets provide a natural platform for studying and exploiting piezomagnetism. In this paper, we introduce a class of insulating altermagnets in two dimensions (2D) referred to as Dirac quadrupole altermagnets, and show based on microscopic minimal models that the orbital piezomagnetic polarizability of such altermagnets has a topological contribution described by topological response theory. The essential low-energy electronic structure of Dirac quadrupole altermagnets can be understood from a gapless parent phase (i.e., the Dirac quadrupole semimetal), which has important implications for their response to external fields. Focusing on the strain-induced response, here we demonstrate that the topological piezomagnetic effect is a consequence of the way in which strain affects the Dirac points forming a quadrupole. We consider two microscopic models: a spinless two-band model describing a band inversion of $s$ and $d$ states, and a Lieb lattice model with collinear Néel order. The latter is a prototypical minimal model for altermagnetism in 2D and is realized in a number of recently proposed material compounds, which are discussed.
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All authors H. Radhakrishnan, B. Bell, C. Ortix, J. W. F. Venderbos

Materials Science

Low-temperature spin dynamics in LAFO thin films: from cubic anisotropy to TLS-limited coherence

Srishti Pal, Guanxiong Qu, Hervé M. Carruzzo et al. (+6 more)

We investigate the low-temperature spin dynamics of epitaxial lithium aluminum ferrite (LAFO) thin films using broadband ferromagnetic resonance (FMR) spectroscopy from 0.44 K to 68 K. The results reveal a crossover from conventional cubic anisotropy-dominated behavior at higher temperatures to pronounced linewidth broadening and higher-order anisotropy contributions at cryogenic temperatures. With the magnetic field oriented along the [100] crystallographic direction, the resonance is well-captured by four-fold in-plane and out-of-plane uniaxial anisotropies. In contrast, measurements with the field along the [110] direction reveal the presence of an unusually large sixth-order cubic anisotropy term that is symmetry-suppressed for [100] but becomes apparent under this field orientation at ultralow temperatures, indicating a substantial modification of the anisotropy landscape. Independent linewidth analysis shows a pronounced peak near 8 K and a subtle monotonic enhancement with decreasing temperatures below 2 K, features consistent with dissipation mediated by a bath of two-level systems (TLS) arising from antisite defects and localized Fe$^{3+}$ moments. Comparison with TLS-based models demonstrates that both exchange-coupled impurities and nearly free paramagnetic centers contribute to the observed damping. Our results establish LAFO as a model ferrite system where disorder-induced TLS limit spin coherence at ultralow temperatures, providing new insights into anisotropy engineering, magnetic relaxation, and the design of ferrimagnetic insulators for coherent magnonics. These findings offer a framework for future optimization of growth conditions.
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All authors Srishti Pal, Guanxiong Qu, Hervé M. Carruzzo, Katya Mikhailova, Lerato Takana, Qin Xu, Yuri Suzuki, Clare C. Yu, Gregory D. Fuchs

Materials Science

AMDAT: An Open-Source Molecular Dynamics Analysis Toolkit for Supercooled Liquids, Glass-Forming Materials, and Complex Fluids

Pierre Kawak, William F. Drayer, Bao Khanh Ma et al. (+1 more)

AMDAT (Amorphous Molecular Dynamics Analysis Toolkit) is an open-source C++ toolkit for post-processing molecular dynamics trajectories, focused on high-performance static and dynamic analyses of amorphous, glassy, and polymer materials, including supercooled liquids and complex fluids. In this paper, we describe AMDAT's design for efficient long-timescale analysis via in-memory trajectory handling and exponential time sampling, and we demonstrate representative workflows for widely used observables such as radial distribution functions, structure factors, intermediate scattering functions, and neighbor correlations.
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All authors Pierre Kawak, William F. Drayer, Bao Khanh Ma, David S. Simmons

Materials Science

Questioning van der Waals Epitaxy of Non-Layered Materials on Mica: The Case of ScN

Susmita Chowdhury, Faezeh Alijan Farzad Lahiji, Mikael Ottoson et al. (+6 more)

Growing stress-free epitaxial films by van der Waals epitaxy (vdWE) is of interest for realizing flexible optoelectronics and energy devices from freestanding epilayers. However, vdWE of non-layered materials is often presumed or claimed on layered substrates such as mica with inadequate experimental evidence. Here, we demonstrate that the growth of single-domain rocksalt ScN(111) films by sputter deposition on fluorophlogopite mica(001) occurs by conventional epitaxy. X-ray diffraction and electron microscopy reveal the film/substrate epitaxial relationship to be [-101](111)ScN||[010](001)mica. Our results indicating strain buildup seen from the dependence of (111) interplanar spacings, and strain relaxation by dislocation generation, question prior claims of vdWE of non-layered metal nitrides on mica. Our findings show that conventional epitaxy should be the default assumption for non-layered materials unless conditions for vdWE are explicitly established.
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All authors Susmita Chowdhury, Faezeh Alijan Farzad Lahiji, Mikael Ottoson, Olivier Donzel-Gargand, Robert J. W. Frost, Martin Magnuson, Ganpati Ramanath, Arnaud le Febvrier, Per Eklund

Materials Science

Broken neural scaling laws in materials science

Max Großmann, Malte Grunert and Erich Runge

In materials science, data are scarce and expensive to generate, whether computationally or experimentally. Therefore, it is crucial to identify how model performance scales with dataset size and model capacity to distinguish between data- and model-limited regimes. Neural scaling laws provide a framework for quantifying this behavior and guide the design of materials datasets and machine learning architectures. Here, we investigate neural scaling laws for a paradigmatic materials science task: predicting the dielectric function of metals, a high-dimensional response that governs how solids interact with light. Using over 200,000 dielectric functions from high-throughput ab initio calculations, we study two multi-objective graph neural networks trained to predict the frequency-dependent complex interband dielectric function and the Drude frequency. We observe broken neural scaling laws with respect to dataset size, whereas scaling with the number of model parameters follows a simple power law that rapidly saturates.
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Materials Science

Observation of large perpendicular magnetic anisotropy and excessive polar magneto-optical effect in Pt/CoFeB/Ru tri-layer system

Md Rejaul Karim, Aman Agrahari, Arun Singh Dev et al. (+7 more)

Heterostructures comprising ferromagnet (FM) and heavy metals (HM) with perpendicular magnetic anisotropy (PMA) and interfacial Dzyaloshinskii-Moriya interaction (iDMI) can host chiral domain walls and topological spin textures, making them highly promising for various spintronics applications. In this paper, we have investigated the magneto-optical properties, the anomalous Hall effect (AHE), and PMA of Pt/CoFeB/Ru multilayers engineered to possess significant iDMI. We utilized the Anomalous Hall effect (AHE), and the polar magneto-optical Kerr effect (p-MOKE), Hall response and the domain wall motion in Pt/CoFeB/Ru-systems. Both MOKE and AHE measurements confirm that the films maintain strong perpendicular magnetization for CoFeB thicknesses below 1.2 nm. The effective magnetic anisotropy K_{\mathrm{eff}} of 0.88 \times 10^6 erg/cm^3 has been achieved without any post-annealing, highlighting the high-quality interface in this multilayer design. The angular dependence of the switching field deviates from the conventional Kondorsky model and is well described using a modified Kondorsky formalism, capturing the role of field-induced domain-wall softening and pinning effects in the reversal process. Furthermore, p-MOKE microscopy imaging during the magnetization reversal process provides detailed insight into domain nucleation and subsequent domain-wall propagation. The observations reveal well-defined, stable magnetic domains that evolve coherently under the applied magnetic field. Such a behavior is expected in the system where interfacial DMI, PMA interact to stabilize the chiral Néel-type domain walls, which are essential for fast, low-power domain-wall motion driven by spin-orbit torques.
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All authors Md Rejaul Karim, Aman Agrahari, Arun Singh Dev, Rakhul Raj, Mohd S Sabir, Arun Jacob Mathew, Joseph Vimal Vas, Yasuhiro Fukuma, V. Raghavendra Reddy, Rohit Medwal

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

Electronic Structure and Superconducting Gap of HgBa$_2$Ca$_2$Cu$_3$O$_{8+\delta}$ Revealed by Laser-Based Angle-Resolved Photoemission Spectroscopy

Taimin Miao, Wenshan Hong, Qinghong Wang et al. (+18 more)

The spatially-resolved laser-based high resolution angle resolved photoemission spectroscopy (ARPES) measurements have been performed on the optimally-doped HgBa$_2$Ca$_2$Cu$_3$O$_{8+\delta}$ (Hg1223) superconductor with a $T_c$ at 133 K. Two distinct regions are identified on the cleaved surface: the single Fermi surface region where only one Fermi surface is observed, and the double Fermi surface region where two Fermi surface sheets are resolved coming from both the inner (IP) and outer (OP) CuO$_2$ planes. The electronic structure and superconducting gap are measured on both of these two regions. In both cases, the observed electronic states are mainly concentrated near the nodal region. The momentum dependence of superconducting gap deviates from the standard d-wave form. These results indicate that the surface electronic structure of Hg1223 behaves more like that of underdoped cuprates.
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All authors Taimin Miao, Wenshan Hong, Qinghong Wang, Shanshan Zhang, Bo Liang, Wenpei Zhu, Neng Cai, Mingkai Xu, Shenjin Zhang, Fengfeng Zhang, Feng Yang, Zhimin Wang, Qinjun Peng, Zuyan Xu, Hanqing Mao, Zhihai Zhu, Xintong Li, Guodong Liu, Lin Zhao, Yuan Li, X. J. Zhou

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

Direct observation of propagating spin waves in a spin-Hall nano-oscillator

Victor H. González, Frank Schulz, Nilamani Behera et al. (+10 more)

Constriction-based spin Hall nano-oscillators (SHNOs) show great promise for application as highly tunable microwave sources with straightforward scalability toward large coupled networks. However, details of the magnetization dynamics within SHNOs have thus far not been addressed experimentally, due to the minute time and length scales involved. In this work, we present direct imaging of the magnetization dynamics within a single CoFeB-based SHNO using time-resolved scanning transmission X-ray microscopy (STXM). Our measurements reveal that the magnon amplitude is the strongest at the two constriction edges, with a pronounced assymetry favoring one edge, and that emitted spin waves exhibit strongly anisotropic propagation. Micromagnetic simulations suggest that grain boundaries and the Dzyaloshinskii-Moriya interaction (DMI) play a key role in both effects. Furthermore, the magnetodynamics changed during the measurement, indicating that the CoFeB/MgO interface may be more susceptible to X-ray induced modifications than previously recognized, challenging its presumed radiation hardness.
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All authors Victor H. González, Frank Schulz, Nilamani Behera, Martina Ahlberg, Akash Kumar, Andreas Frisk, Felix Groß, Sven Erik Ilse, Steffen Wittrock, Markus Weigand, Gisela Schütz, Johan Åkerman, Sebastian Wintz

Materials Science · Cross-listed from cond-mat.soft

Probing Anharmonic and Heterogeneous Carrier Dynamics Across Sublattice Melting in a Minimal Model Superionic Conductor

Sucharita Niyogi, Takenobu Nakamura, Genki Kobayashi et al. (+2 more)

Despite decades of research, the microscopic origin of sublattice melting and fast ion transport in superionic conductors remains elusive. Here, we introduce a chemically neutral minimal binary model consisting of a rigid host lattice stabilized by short-range steric repulsion and a soft carrier sublattice interacting via long-range Wigner-type forces. This contrast naturally produces distinct melting temperatures and an intermediate sublattice-melting phase in which carriers become fluidlike while the host remains crystalline. Molecular dynamics simulations identify three dynamical regimes-crystalline, sublattice-melt, and fully molten-marked by sharp changes in diffusivity, structural correlations, and dynamical heterogeneity. Near sublattice melting, carrier motion is strongly anharmonic and spatially heterogeneous, beyond mean-field hopping descriptions. By tuning the density, we demonstrate that sublattice melting can be continuously controlled, establishing a direct link between lattice softness, anharmonicity, and collective ion transport. Comparison with conventional long-range Coulombic models confirms that our minimal model reproduces the key dynamical signatures of superionicity, providing a unified microscopic foundation for designing mechanically robust superionic conductors.
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All authors Sucharita Niyogi, Takenobu Nakamura, Genki Kobayashi, Yasunobu Ando, Takeshi Kawasaki

Materials Science

Crystallinity Evolution of MOCVD-Grown $\beta$-Ga$_2$O$_3$ Films Probed by In Situ HT-XRD under Different Reactor Heights

Imteaz Rahaman, Botong Li, Bobby G. Duersch et al. (+3 more)

The crystallinity of $\beta$-Ga$_2$O$_3$ thin films grown by metal-organic chemical vapor deposition (MOCVD) is strongly influenced by reactor design and the resulting growth environment. In this work, we investigate the role of reactor height on the crystallinity evolution of MOCVD-grown $\beta$-Ga$_2$O$_3$ films by directly comparing long- and short-chamber showerhead configurations. Structural evolution was probed by in situ high-temperature X-ray diffraction (HT-XRD) as the MOCVD-grown films were heated from 25~$^\circ$C to 1100~$^\circ$C. Temperature-dependent XRD reveals a consistent redshift of the $\beta$-Ga$_2$O$_3$~($-201$) reflection after HT-XRD heating and subsequent cooling to room temperature for both reactor geometries, indicating a similar thermally driven strain response. Quantitative rocking-curve analysis shows a non-monotonic temperature dependence of the ($-201$) full width at half maximum (FWHM), with minimum values of approximately 2.03$^\circ$ and 2.72$^\circ$ for the short- and long-chamber films, respectively, reflecting differences in mosaic alignment established during growth. Atomic force microscopy further shows that short-chamber-grown films exhibit smoother surfaces, with root-mean-square roughness values of approximately 7.7~nm before and 7.3~nm after HT-XRD heating, compared to 19.3~nm and 12.3~nm, respectively, for long-chamber-grown films. Overall, these results indicate that reactor height influences the initial crystalline and morphological templates of $\beta$-Ga$_2$O$_3$ films and modulates their elevated-temperature structural response, providing practical insights for optimizing MOCVD reactor design for high-quality $\beta$-Ga$_2$O$_3$ growth.
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All authors Imteaz Rahaman, Botong Li, Bobby G. Duersch, Hunter D. Ellis, Kathy Anderson, Kai Fu

Materials Science

Ferroelectricity in Atomically Thin Metallic TaNiTe$_5$ with Ultrahigh Carrier Density

Zhihua Liu, Shichong Song, Xunqing Yin et al. (+9 more)

Ferroelectric metals, characterized by the coexistence of ferroelectricity and metallic conductivity, present a fundamental challenge due to the screening effect of free charge carriers on the long-range electric dipole order. Existing strategies to circumvent this obstacle include employing two-dimensional (2D) crystals, where reduced dimensionality and low carrier densities suppress screening, or designing materials of van der Waals (vdW) superlattice with spatially separated and decoupled conductive and nearly insulating ferroelectric layers. Here, we report an alternative paradigm in TaNiTe5, where an ultrahigh carrier density coexists with an out-of-plane ferroelectric order within the same surface monolayer. Using piezoresponse force microscopy (PFM), we observed robust ferroelectric behavior in TaNiTe5 down to single-unit-cell thickness (~1.3 nm) at room temperature. Scanning transmission electron microscopy (STEM) gives structural evidence that the ferroelectricity might originate from the vertical displacement of outmost Te atoms on the surface, breaking the inversion symmetry. Concurrently, electrical transport measurements reveal a metallic state with a carrier density on the order of 10$^{15}$ cm$^{-2}$ (or 10$^{22}$ cm$^{-3}$) – comparable to that of Copper (Cu). Our findings establish a unique platform for exploring the interplay between ferroelectricity and an ultrahigh density of mobile carriers in the 2D limit.
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All authors Zhihua Liu, Shichong Song, Xunqing Yin, Chenhang Xu, Feng Liu, Guohua Wang, Peng Chen, Shengwei Jiang, Chunqiang Xu, Xiaofeng Xu, Weidong Luo, Dong Qian

Materials Science

CatFlow: Co-generation of Slab-Adsorbate Systems via Flow Matching

Minkyu Kim, Nayoung Kim, Honghui Kim et al. (+1 more)

Discovering heterogeneous catalysts tailored for specific reaction intermediates remains a fundamental bottleneck in materials science. While traditional trial-and-error methods and recent generative models have shown promise, they struggle to capture the intrinsic coupling between surface geometry and adsorbate interactions. To address this limitation, we propose CatFlow, a flow matching-based framework for de novo design and structure prediction of heterogeneous catalysts. Our model operates on a primitive cell-based factorized representation of the slab-adsorbate complex, reducing the number of learnable variables by an average of 9.2x while explicitly encoding the surface orientation of the slab-adsorbate interface. Experiments on the Open Catalyst 2020 dataset demonstrate that CatFlow significantly improves the structural fidelity of generated catalysts compared to autoregressive and sequential baselines. Further experiments show that the generated structures accurately capture the adsorption energy distributions of physically plausible interfaces and lie closer to thermodynamic local minima.
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All authors Minkyu Kim, Nayoung Kim, Honghui Kim, Sungsoo Ahn

Materials Science

Quasi-One-Dimensional Electronic Nature of Ta4SiTe4 Underlying the Giant Thermoelectric Performance

Hironari Isshiki, Ayumu Tabata, Masafumi Horio et al. (+13 more)

Ta4SiTe4 is a one-dimensional van der Waals material that exhibits an exceptionally large thermoelectric power factor below room temperature. However, since this material has been available only in the form of acicular microcrystals, experimental exploration of the electronic properties responsible for its giant thermoelectric performance has long been challenging. In this study, we quantitatively evaluated the one-dimensional electronic nature of Ta4SiTe4 by combining micro-spot angle-resolved photoemission spectroscopy and transport measurements on focused-ion-beam-processed samples. The angle-resolved photoemission spectroscopy measurements reveal anisotropic band dispersions along and perpendicular to the crystallographic c axis. Consistently, transport measurements demonstrate that the resistivity perpendicular to the c axis is approximately five times larger than that along the c axis at 200 K. These results provide direct experimental evidence for the quasi-one-dimensional electronic character of Ta4SiTe4, which underlies its giant thermoelectric response reported previously, and offer fundamental insights into the role of electronic dimensionality in enhancing thermoelectric performance.
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All authors Hironari Isshiki, Ayumu Tabata, Masafumi Horio, Motoi Kimata, Youichi Yamakawa, Ryutaro Okuma, Miki Imai, Fumiya Matsunaga, Koshi Takenaka, Kenichi Ozawa, YoshiChika Otani, Fumio Komori, Iwao Matsuda, Masahiro Hara, Yoshihiko Okamoto, Masayuki Hashisaka

Materials Science

First-principles study of photovoltaic and thermoelectric properties of AgBiSCl2

Sihang Wang, Menghan Chen and Liping Zhang

This work systematically investigates the potential of the hybrid anion semiconductor AgBiSCl2 for photovoltaic and thermoelectric applications, aiming to provide theoretical guidance for high-performance energy conversion devices. Structural analysis reveals favorable ductility and a relatively low Debye temperature. Analysis of interatomic interactions indicates that Ag-S and Ag-Cl bonds are relatively weak, resulting in local structural softness and enhanced lattice anharmonicity. These weak bonds facilitate phonon scattering and give rise to low-frequency localized rattling vibrations primarily associated with Ag atoms, contributing to reduced lattice thermal conductivity. In contrast, Bi-S bonds exhibit stronger, directional interactions, which help stabilize the overall structure. The coexistence of weak bonding and strong lattice coupling enables favorable modulation of thermal transport properties.Optically, AgBiSCl2 possesses a high static dielectric constant and exhibits strong absorption in the ultraviolet region. In terms of thermal transport, phonon spectrum exhibit mode hardening with temperature increasing. The localized Ag vibrations intensify the anharmonicity, reducing phonon lifetimes and group velocities.For electronic transport, the p-type material maintains a higher Seebeck coefficient than the n-type, while the latter shows greater electrical conductivity. At 700 K, the figure of merit reaches 0.77 for p-type and 0.69 for n-type AgBiSCl2, indicating promising high-temperature thermoelectric performance.In summary, AgBiSCl2 exhibits excellent potential for dual photovoltaic and thermoelectric applications. Its unique bonding features and lattice response mechanisms offer valuable insights into designing multifunctional energy conversion materials.
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Materials Science

Physics-informed acquisition weighting for stoichiometry-constrained Bayesian optimization of oxide thin-film growth

Yuki K. Wakabayashi, Takuma Otsuka, Yoshiharu Krockenberger et al. (+1 more)

We present a physics-informed Bayesian optimization (PIBO) with a concise modification to its acquisition function to incorporate the physical prior knowledge. Specifically, this method multiplies the expected improvement (EI) by a weight encoding prior crystal growth physics. When applied to LaAlO3 molecular-beam epitaxy, the weighting function defines a flat stoichiometric window and penalizes off-window proposals, thereby steering the optimization toward physically plausible regions while maintaining controlled exploration. In a closed-loop optimization, relative to the bare EI, which often proposes off-stoichiometric conditions, the weighted EI constrains the search toward stoichiometric regions while retaining sufficient flexibility to explore neighboring conditions, eventually identifying an optimum slightly beyond the stoichiometric window. Within only 15 growth runs, the lattice constant of the grown LaAlO3 film converged to the bulk value, evidencing efficient and rapid optimization for the ideal stoichiometric growth. Because physics knowledge is incorporated solely through the weighting function, the approach requires only minimal modification to standard BO workflows and is readily applicable to other material systems, offering a general and practical route to AI-driven materials synthesis.
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All authors Yuki K. Wakabayashi, Takuma Otsuka, Yoshiharu Krockenberger, Yoshitaka Taniyasu