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Physics Condensed Matter Materials Science

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Wednesday 21 January

Materials Science

Bending strain induced thermal conductivity suppression in freestanding BaTiO3 and SrTiO3 membranes

Ziyan Qian, Guangwu Zhang, Weikun Zhou et al. (+2 more)

Freestanding perovskite oxide membranes provide a novel platform for elastic strain engineering, enabling the manipulation of phonon transport free from substrate clamping. In this work, we investigate the thermal transport properties of strontium titanate (SrTiO3) and barium titanate (BaTiO3) membranes subjected to self-formed crease induced inhomogeneous strain. By integrating spatially resolved Frequency-Domain Thermoreflectance (FDTR) with micro-Raman spectroscopy, we observe a sharp, localized suppression of thermal conductivity (k) in high-curvature regions. Specifically, k is reduced from 4.43 to 3.62 W/(m K) in SrTiO3 and from 2.27 to 1.81 W/(m K) in BaTiO3 at the crease centers, directly correlating with the local strain distribution. First-principles calculations reveal that, unlike uniform strain, the symmetry breaking induced by strain gradients significantly broadens phonon dispersion and enhances scattering rates. These findings not only elucidate the microscopic mechanisms governing phonon-strain coupling but also demonstrate the potential of inhomogeneous strain fields as a potent tool for designing dynamic solid-state thermal switches and active thermal management devices.
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All authors Ziyan Qian, Guangwu Zhang, Weikun Zhou, Tsukasa Katayama, Qiye Zheng

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

Spin-orbit-driven quarter semimetals in rhombohedral graphene

Jing Ding, Hanxiao Xiang, Naitian Liu et al. (+7 more)

Semimetals exhibit intriguing characteristics attributed to the coexistence of both electrons and holes. In rhombohedral multilayer graphene, a strong trigonal warping effect gives rise to a semi-metallic state near the Fermi surface, offering unique opportunities to explore the interplay of semi-metallic properties with strong correlations and topologies. Here, the observation of quarter semimetals in rhombohedral multilayer graphene by introducing spin-orbit coupling (SOC) is reported. The semi-metallic characteristics of rhombohedral graphene manifest as nearly vanished Hall resistance and parabolic longitudinal resistance. The strong correlations arising from the surface flat band lead to spontaneous symmetry breaking. SOC proximitized by WSe2 further lifts the valley degeneracy, resulting in the spontaneous time-reversal symmetry breaking, as evidenced by the hysteretic anomalous Hall effect. The coexistence of fully polarized electrons and holes allows for the observation of a non-monotonic temperature dependence of the anomalous Hall resistance. Furthermore, the application of moderate magnetic fields induces a phase transition from quarter semimetals to Chern insulators. These findings establish rhombohedral multilayer graphene as an ideal platform for studying strong correlations and topologies in semimetals.
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All authors Jing Ding, Hanxiao Xiang, Naitian Liu, Wenqiang Zhou, Xinjie Fang, Zhangyuan Chen, Le Zhang, Kenji Watanabe, Takashi Taniguchi, Shuigang Xu

Materials Science

Modeling the Thermal Behavior of Photopolymers for In-Space Fabrication

Jonathan Ericson, Daniel Widerker, Eytan Stibbe et al. (+15 more)

Future long-duration space missions will require in-situ, on-demand manufacturing of tools and components. Photopolymer-based processes are attractive for this purpose due to their low energy requirements, volume efficiency, and precise control of curing. However, photopolymerization generates significant heat, which is difficult to regulate in microgravity where natural convection is absent, leading to defects such as surface blistering and deformation. In this work, we combine experimental studies and modeling to address these thermal challenges. We report results from International Space Station (ISS) experiments and a dedicated parabolic flight campaign, which confirm that suppressed convective heat transfer in microgravity exacerbates thermal buildup and defect formation. Building on these observations, we present a predictive thermal model that couples heat transfer, light absorption, and evolving material properties to simulate polymerization and temperature evolution under terrestrial and microgravity conditions. Laboratory validation demonstrates strong agreement between model predictions and measured temperature profiles. Applying the model to the ISS experiments, we show that the model accurately reproduces experimentally observed blistering in TJ-3704A, a commercial acrylate-based polymer resin, while also predicting defect-free outcomes for Norland optical adhesives. The model functions as a design tool for defect-free in-space manufacturing, enabling selection of polymer properties, exposure strategies, and environmental conditions that together inhibit excess thermal buildup, paving the way for scalable, reliable in-situ manufacturing during future missions.
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All authors Jonathan Ericson, Daniel Widerker, Eytan Stibbe, Mor Elgarisi, Yotam Katzman, Omer Luria, Khaled Gommed, Alexey Razin, Amos A. Hari, Israel Gabay, Valeri Frumkin, Hanan Abu Hamad, Ester Segal, Yaron Amouyal, Titus Szobody, Rachel Ticknor, Edward Balaban, Moran Bercovici

Materials Science

3D tomographic imaging of skyrmionic cocoons using HERALDO

Jhon J. Chiliquinga-Jacome, Matthieu Grelier, Riccardo Battistelli et al. (+11 more)

Uncovering the rich and intricate characteristics of three-dimensional (3D) magnetic textures is essential for functional materials such as magnetic multilayers, where the delicate balance of various magnetic interactions leads to complex 3D spin arrangements. Among these textures, skyrmionic cocoons-tubular 3D magnetic structures characterized by a closed magnetization surface wrapping around a core-have emerged as particularly intriguing. Stabilized by competing magnetic interactions, these textures reside within a fraction of the thickness of the magnetic material and exhibit a typical lateral size of approximately 100 nm. Here, we present a vector tomographic reconstruction of the 3D magnetization in aperiodic Pt/Co/Al chiral multilayers, where skyrmionic cocoons have been recently reported. Using soft X-ray Holography with Extended Reference by Autocorrelation Linear Differential Operator (HERALDO), we acquire tomographic projections of the magnetic configuration and reconstruct the full 3D magnetization vector field with a spatial resolution of approximately 30 nm, as determined by Fourier shell correlation (FSC). This resolution allows us to observe critical features of the cocoons, such as their vertical misalignment and their overall chirality. Our findings demonstrate that HERALDO-based vector tomography is a powerful approach for revealing the internal structure and vertical extent of these nanoscale magnetic textures, offering new experimental insights into their intrinsic behavior.
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All authors Jhon J. Chiliquinga-Jacome, Matthieu Grelier, Riccardo Battistelli, William Bouckaert, Krishnanjana Puzhekadavil Joy, Sophie Collin, Florian Godel, Marisel Di Pietro Martínez, Claire Donnelly, Felix Büttner, Horia Popescu, Vincent Cros, Nicolas Reyren, Nicolas Jaouen

Materials Science

Role of Defects in the Paramagnetism of Fe-doped Cs$_{2}$AgBiBr$_{6}$ Double Perovskite

Volodymyr Vasylkovskyi, Olga Trukhina, Patrick Dörflinger et al. (+6 more)

Transition-metal doping enables the introduction of spin functionality into halide double perovskites, while simultaneously modifying optical properties. Here, we combine controlled single-crystal growth, optical characterization, comprehensive electron paramagnetic resonance (EPR) spectroscopy, and first-principles modeling to identify the microscopic nature of Fe-related centers in Fe-doped Cs$_{2}$AgBiBr$_{6}$. Single crystals with nominal Fe$^{3+}$ concentrations up to 15% in the precursor stage were grown using a controlled-cooling method, yielding reproducible Fe incorporation up to 0.1% w.r.t. Bi, without secondary phases. Despite this low concentration, Fe doping introduces electronic states that influence optical absorption and photoluminescence. EPR measurements reveal an S = 5/2 Fe$^{3+}$-related center whose anisotropy follows the cubic-to-tetragonal phase transition below 120 K. Angular-dependent EPR resolves two configurations of this nearly axial spin center, with principal axes rotated by 90$^\circ$ and aligned with the $a/b$ plane of the tetragonal lattice. Density-functional calculations attribute these centers to impurity-vacancy complexes, most likely Fe$_{\rm Bi}$-V$_{\rm Br}$, that stabilise in a basal configuration of the low-temperature phase. This approach resolves vacancy-coupled defect orientations, narrowing possible models to Fe$^{3+}$-vacancy complexes and establishing them as stable, orientation-sensitive spin probes of structural symmetry in halide double perovskites, while providing a microscopic basis for tuning their magnetic and optical responses.
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All authors Volodymyr Vasylkovskyi, Olga Trukhina, Patrick Dörflinger, Mykola Slipchenko, Wolf Gero Schmidt, Timur Biktagirov, Anastasiia Kultaeva, Yakov Kopelevich, Vladimir Dyakonov

Materials Science

Significance of the dispersion force for ferroelectric switching in ZnO and related materials

Lingyao Zhang, Musen Li, Nisha Metha et al. (+3 more)

Wurtzite-ZnO is a wide-bandgap polar material with a ferroelectric-switching barrier that is too high to utilize, but the barrier can be reduced and switching observed in substituted materials such as Zn0.5Mg0.5O. Here, we seek to understand atomic-scale features that control concerted polarization switching in these and related systems, focusing on the planar hexagonal structures h-ZnO and Zn0.5Mg0.5O that may act as metastable intermediate phases along the switching pathway. Consensus is obtained by considering a range of pure and dispersion-corrected density-functional theory (DFT) computational approaches, as well as ab initio Hartree-Fock (HF), Møller-Plesset perturbation-theory (MP2), and random-phase approximation (RPA) calculations. The perceived stability of h-ZnO is found to be strongly influenced by the dispersion correction, with the consensus being that dispersion interactions are insufficient to stabilize h-ZnO as a metastable phase in infinite crystals. In contrast, h-Zn0.5Mg0.5O is consistently predicted to be at least metastable, with some dispersion-corrected DFT approaches predicting it to be more stable than its wurtzite form; all DFT methods overestimate its stability compared to MP2 and RPA. Dispersion forces are found to be most significant for hypothetical planar hexagonal structures constrained to the lattice vectors of the wurtzite phases. In general, our results demonstrate that an accurate treatment of dispersion forces is essential when describing polarization switching and ferroelectric behavior in wurtzite-structured materials.
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All authors Lingyao Zhang, Musen Li, Nisha Metha, Carla Verdi, Wei Ren, Jeffrey R. Reimers

Materials Science

Non-monotonic hydration dependence of ionic transport in atomically-thin MnO2 sheets

Ata Utku Özkan, Talip Serkan Kas{\i}rga and Aykut Erbaş

The interlamellar space in van der Waals materials has served as a test bed for studies of strongly confined fluid and ion transport. Despite theoretical and experimental progress, it remains unclear how electrostatic correlations arising from interactions between the confined, hydrated ions and the substrate layer can be reconciled with non-equilibrium ionic transport and the substrate's morphological response. Here, we investigate field-driven ionic conduction in sodium- and water-intercalated layered manganese oxides ($\mathrm{MnO_2}$), a self-intercalated metal oxide nanofluidic system, as a model intrinsically-intercalated van der Waals solid, using nonequilibrium all-atom molecular dynamics simulations that explicitly capture ion-water correlations and layer morphology. We demonstrate that electric-field bias induces spontaneous nanoscale segregation of water within the interlayer space, producing coexisting hydrated and weakly hydrated ionic domains coupled to local lattice distortions. This feedback between hydration and morphology gives rise to heterogeneous transport pathways and leads to a non-monotonic dependence of ionic conductivity on water content. Suppressed conduction arises either from layer collapse at low water levels or from ionic hop blockage by excess water at high water levels. Concurrently, ionic transport exhibits a maximum at intermediate levels of intercalated water, where less hydrated ions can move at the boundaries of strongly hydrated ion clusters. While such non-monotonicity could explain the experimentally observed memristive response of $\mathrm{MnO_2}$ single crystals, these findings also provide a molecular-level mechanism linking intercalated water levels to ionic metal oxide nanofluidic systems, suggesting general design principles for robust, water-assisted ionic conductors.
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Materials Science

Near-Atomic-Scale Compositional Complexity in a 2D Transition Metal Oxide

Mathias Krämer, Bar Favelukis, J. Manoj Prabhakar et al. (+5 more)

2D materials hold transformative promise for next-generation nanoelectronics. However, successfully integrating these materials from laboratory-scale discoveries into real-world devices depends on precisely controlling their properties, which are fundamentally determined by their composition. Detailed characterisation using atom probe tomography of 2D Ti0.87O2, a candidate high-$\kappa$ dielectric, reveals deviations from its commonly assumed stoichiometry. Compositional analysis and comparison with the bulk K0.8[Ti1.73Li0.27]O4 precursor evidences an oxygen deficit indicative of oxygen vacancy formation in the 2D material, as well as the retention of low concentrations of alkali metals that were presumed to be removed during synthesis. Such deviations from stoichiometry indicate a reconstruction mechanism that mitigates the effect of the characteristic, negatively charged vacancies on the titanium sublattice, thereby influencing the local electronic structure and, consequently, functional properties. These findings underscore the importance of a detailed compositional analysis in both understanding and optimizing the extraordinary functional properties of 2D materials, opening pathways to tailored functionalities in next-generation nanoelectronics.
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All authors Mathias Krämer, Bar Favelukis, J. Manoj Prabhakar, Aleksander Albrecht, Brian A. Rosen, Noam Eliaz, Maxim Sokol, Baptiste Gault

Materials Science

Influence of Charge Density Waves on the Hall coefficient in NiTi

Adrian Braun, Henrik Dick, Timon Sieweke et al. (+4 more)

We present a mean-field charge density wave theory for NiTi using density functional theory bandstructure as a starting point. We calculate the Hall coefficient as a function of temperature and compare with recent experimental results. We analyze the contributions to the Hall coefficient from different parts of the Fermi surface and find that the Hall coefficient is dominated by certain “hot spots”. The analysis shows that these hot spots are mostly dominated by Ni d-orbitals. We demonstrate that the Hall coefficient is not well reproduced by Boltzmann transport theory within the constant relaxation time approximation without charge density waves. We consider both uniaxial and biaxial charge density waves and show that biaxial charge density waves can account well for the Hall coefficient, while uniaxial cannot. We also investigate the temperature dependence of the resistivity and the specific heat.
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All authors Adrian Braun, Henrik Dick, Timon Sieweke, Alexander Kunzmann, Klara Lünser, Gabi Schierning, Thomas Dahm

Materials Science

Langmuir and Langmuir-Blodgett films of a maleic anhydride derivative: effect of subphase divalent cations

Beatriz Martín-García, M. Mercedes Velázquez, Jose Antonio Pérez-Hernández et al. (+1 more)

We report the study of the equilibrium and dynamic properties of Langmuir monolayers of poly (styrene-co-maleic anhydride) partial 2-buthoxy ethyl ester cumene terminated polymer and the effect of the Mg(NO3)2 addition in the water subphase on the film properties. Results show that the polymer monolayer becomes more expanded when the electrolyte concentration in the subphase increases. Dense polymer films aggregate at the interface. The aggregates are transferred onto Silicon wafers using the Langmuir-Blodgett methodology and the morphology is observed by AFM. The structure of aggregates depends on the subphase composition of the Langmuir film transferred onto the silicon wafer.
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All authors Beatriz Martín-García, M. Mercedes Velázquez, Jose Antonio Pérez-Hernández, Juan Hernández-Toro

Materials Science

Functionalization of reduced graphite oxide sheets with a zwitterionic surfactant

Beatriz Martín-García, M. Mercedes Velázquez, Francesco Rossella et al. (+7 more)

Films of a few layers in thickness of reduced graphite oxide, RGO, sheets functionalized by the zwitterionic surfactant N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate, DDPS, are obtained by using the Langmuir-Blodgett method. The quality of the RGO sheets is checked by analyzing the degrees of reduction and defect repair by means of X-ray photoelectron spectroscopy, atomic force microscopy -AFM, field-emission scanning electron microscopy -SEM, micro-Raman spectroscopy, and electrical conductivity measurements. A modified Hummers method is used to obtain highly oxidized graphite oxide, GO, together with a centrifugation-based method to improve the quality of GO. The GO samples are reduced by hydrazine or vitamin C. Functionalization of RGO with the zwitterionic surfactant improves the degrees of reduction and defect repair of the two reducing agents and significantly increases the electrical conductivity of paperlike films compared with those prepared from unfunctionalized RGO.
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All authors Beatriz Martín-García, M. Mercedes Velázquez, Francesco Rossella, Vittorio Bellani, Enrique Diez, Jose Luis García Fierro, Jose Antonio Pérez-Hernández, Juan Hernández-Toro, Sergi Claramunt, Albert Cirera

Materials Science

Hydrogen Activation via Dihydride Formation on a Rh1/Fe3O4(001) Single-Atom Catalyst

Chunlei Wang, Panukorn Sombut, Lena Puntscher et al. (+10 more)

Hydrogen activation is a key elementary step in catalytic hydrogenation. In heterogeneous catalysis, it usually proceeds through dissociative adsorption on metal nanoparticles followed by surface diffusion or spillover, whereas homogeneous catalysts activate H2 through dihydride or dihydrogen intermediates at a single metal center. Here, we show that isolated Rh adatoms supported on Fe3O4(001) activate hydrogen through formation of a stable dihydride species without atomic H spillover. Temperature-programmed desorption, X-ray photoelectron spectroscopy, and scanning tunneling microscopy collectively reveal strong (approximately 1 eV) hydrogen adsorption exclusively at isolated Rh1 sites, while isotope-exchange experiments further demonstrate that hydrogen remains localized. Density-functional theory based calculations indicate a barrierless conversion from molecular H2 to the dihydride, and random-phase approximation calculations further confirm the relative stability of the dihydride. Together, these results show that single-atom Rh sites cleave and bind H2 through a dihydride pathway analogous to homogeneous complexes, establishing a mechanistic bridge between homogeneous and heterogeneous catalysis.
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All authors Chunlei Wang, Panukorn Sombut, Lena Puntscher, Nail Barama, Maosheng Hao, Florian Kraushofer, Jiri Pavelec, Matthias Meier, Florian Libisch, Michael Schmid, Ulrike Diebold, Cesare Franchini, Gareth S. Parkinson

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

Physicochemical properties of lunar regolith simulant for in situ oxygen production

Alyssa Ang De Guzman, Anish Mathai Varghese, Saif Alshalloudi et al. (+3 more)

Permanent lunar settlements will rely on in situ oxygen production from regolith for life support and propulsion. While oxygen is abundant in lunar materials, it is chemically bound within metal oxides whose extractability depends strongly on regolith composition and processing strategy. In this study, we validate and characterize high-fidelity lunar regolith simulants representative of the lunar highlands and south pole using scanning electron microscopy with energy-dispersive X-ray spectroscopy, X-ray diffraction, Brunauer-Emmett-Teller surface area and pore structure analysis, and hydrogen temperature-programmed reduction. The simulants exhibit strong mineralogical and compositional fidelity to returned Apollo and Chang'e samples, with ilmenite confirmed as the most readily reducible oxygen-bearing phase. However, despite low ilmenite abundance, bulk highland simulants display favorable reduction behavior arising from distributed Fe-bearing silicate and glassy phases, as well as surface and microstructural properties that influence gas-solid interactions. Adsorption experiments with gases (H2, CH4, and CO2) and water indicate that mineralogical heterogeneity and pore accessibility influence their uptake in simulants. These results indicate that oxygen extraction behavior in realistic lunar regolith is governed by whole-regolith response rather than ilmenite content alone, supporting the option of whole-regolith processing strategies for oxygen production in lunar in situ resource utilization architectures.
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All authors Alyssa Ang De Guzman, Anish Mathai Varghese, Saif Alshalloudi, Lance Kosca, Kyriaki Polychronopoulou, Marko Gacesa

Materials Science · Cross-listed from physics.optics

Transient Pauli blocking in a InN film as a mechanism for broadband ultrafast optical switching

Junjun Jia, Minseok Kim, Yuzo Shigesato et al. (+5 more)

The transient Pauli blocking effect offers a promising route for achieving ultrafast optical switching in semiconductors, enabling a rapid switching from an initially opaque state to a relatively transparent state upon photoexcitation. Herein, we demonstrate broadband ultrafast optical switching in degenerate InN thin films, spanning the visible to near-infrared spectral range, using pump-probe transient transmittance measurements. To elucidate the underlying physical mechanism, we perform probe-energy-resolved analysis for ultrafast dynamics, and develop a theoretical model based on a quasi-equilibrium Fermi-Dirac distribution. The model successfully captures the experimental transients and yields an electron-phonon coupling constant of $1.0\times10^{17}\,\mathrm{W\,m^{-3}\,K^{-1}}$, along with an electronic specific heat coefficient ranging from 1.52 to 2.02 $\mathrm{mJ\,mol^{-1}\,K^{-2}}$, which allow direct prediction of the spectral switching window. Notably, we demonstrate that the Pauli blocking effect can be induced solely by a laser-excitation driven rise in electronic temperature, without requiring significant carrier injection into the conduction band in degenerate semiconductors. These findings offer new insights for designing ultrafast optical modulators, shutters, and photonic devices for next-generation communication and computing technologies.
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All authors Junjun Jia, Minseok Kim, Yuzo Shigesato, Ryotaro Nakazawa, Keisuke Fukutani, Satoshi Kera, Toshiki Makimoto, Takashi Yagi

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

Dosimetry for Proton Therapy Using a \beta-Ga$_2$O$_3$ Metal-Semiconductor-Metal Detector with Low-Noise Amplification

Hunter D. Ellis, Ajayvarman Mallapillai, Jared Miller et al. (+4 more)

Intensity-modulated proton therapy (IMPT) employs proton radiation rather than conventional X-rays to treat cancerous tumors. This approach offers significant advantages by minimizing the radiation exposure of surrounding healthy tissue, leading to improved patient outcomes and reduced side effects compared to traditional X-ray therapy. To ensure patient safety, each treatment plan must be experimentally validated before clinical implementation. However, current dosimetry devices face limitations in performing angled beam measurements and obtaining multi-depth assessments, both of which are essential for verifying IMPT treatment plans. In this study, the performance of a \beta-Ga$_2$O$_3$-based metal-semiconductor-metal (MSM) detector with a low-noise amplifier is studied and evaluated under various proton radiation doses and energy levels delivered by a MEVION S250i proton accelerator. The detector performance is also compared with that of an ionization chamber. The \beta-Ga$_2$O$_3$ detector exhibits a linear response with proton dose for single-spot irradiations, and its response to varying proton energies closely matches both the ion chamber data and simulated dose distributions. These findings highlight the potential of \beta-Ga$_2$O$_3$-based detectors as robust dosimetry devices for IMPT applications.
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All authors Hunter D. Ellis, Ajayvarman Mallapillai, Jared Miller, Imteaz Rahaman, Botong Li, Vikren Sarkar, Kai Fu

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

Reducing non-linear effects in Kelvin Probe Force Microscopy of back-gated 2D semiconductors

Zander Scholl, Ezra Frohlich, Natalie Rogers et al. (+6 more)

In 2D field effect transistors the gate electrostatically dopes the 2D semiconductor (2DSC) channel, tuning the Fermi level. In principle, Kelvin probe force microscopy (KPFM) can detect the Fermi level, and its dependence on gate bias as well as position, potentially directly yielding band gaps, contact barriers, spatial nonuniformities, and sub-gap densities of states in such devices. However, KPFM relies on an oscillating probe voltage which itself electrostatically dopes the 2DSC, potentially creating a nonlinear response. Here, we show that when a suitably thin hBN back-gate dielectric is used, the KPFM signal agrees well with expectations, as explained by a quasistatic charge-balance model. Corresponding experimental results are consistent with the literature values of the bandgaps of monolayer and trilayer WSe2. With this approach, the widely available technique of KPFM should find improved utility and new uses in the study of 2D devices.
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All authors Zander Scholl, Ezra Frohlich, Natalie Rogers, Paul Nguyen, Baker Hase, Joseph Tatsuro Murphy, Joel Toledo-Urena, David Cobden, Jennifer T. Heath

Materials Science

Above Room Temperature Ferroelectricity in Epitaxially Strained KTaO3

Tobias Schwaigert, Salva Salmani-Rezaie, Sankalpa Hazra et al. (+9 more)

Epitaxial strain is a powerful means to engineer emergent phenomena in thin films and heterostructures. Here, we demonstrate that KTaO3, a cubic perovskite in bulk form, can be epitaxially strained into a highly tunable ferroelectric. KTaO3 films grown commensurate to SrTiO3 (001) substrates experience an in-plane strain of -2.1 % that transforms the cubic structure into a tetragonal polar phase with transition temperature of 475 K, consistent with our thermodynamic calculations. We show that the Curie temperature and the spontaneous electric polarization can be system- atically controlled with epitaxial strain. Scanning transmission electron microscopy reveals cooperative polar displacements of the potassium columns with respect to the neighboring tantalum columns at room temperature. Optical second-harmonic generation results are described by a tetragonal polar point group (4mm), indicating the emergence of a global polar ground state. We observe a ferroelectric hysteresis response, using metal-insulator-metal capacitor test structures. The results demon- strate a robust intrinsic ferroelectric state in epitaxially strained KTaO3 thin films.
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All authors Tobias Schwaigert, Salva Salmani-Rezaie, Sankalpa Hazra, Utkarsh Saha, Maya Ramesh, Aiden Ross, Betul Pamuk, Long-Qing Chen, David A. Muller, Darrell G. Schlom, Venkatraman Gopalan, Kaveh Ahadi

Materials Science

Nanodroplet-Confined Electroplating Enables Submicron Printing of Metals and Oxide Ceramics

Mirco Nydegger, Rebecca A. Gallivan, Arthur Barras et al. (+2 more)

The fabrication of functional micro- and nano-electronic devices requires the deposition of high-quality materials of different electronic material classes, such as conductors, semiconductors and insulators. To establish ultra-high-resolution additive manufacturing as a viable addition to existing fabrication methods requires the combinatorial additive deposition of different electronic material classes. However, current techniques do not provide such a capability. Here, we demonstrate that droplet confined electroplating, an ultra-high-resolution AM technique initially developed for metals as electrohydrodynamic redox printing (EHD-RP), allows not only the direct deposition of many metals, but also of metal-oxides. Particularly, we demonstrate that applying fundamental electrochemical principles in combination with on-the-fly switching of the deposited material allows for the direct co-deposition of metals, metal-hydroxides and -oxides. Our results exemplify the feasibility of leveraging simple water-based electrochemical concepts to produce intricate and multi-material structures at the nanoscale.
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All authors Mirco Nydegger, Rebecca A. Gallivan, Arthur Barras, Henning Galinski, Ralph Spolenak

Materials Science

Heterogeneous Transfer of Thin Film BaTiO$_3$ onto Silicon for Device Fabrication

Temazulu S. Zulu, Larissa B. Little, Aaron M. Day et al. (+9 more)

Thin film BaTiO$_3$ has one of the highest known Pockels coefficients (>1200 pm/V), making it an attractive material for use in electro-optic devices. It is advantageous to integrate BaTiO$_3$ on silicon to enable complementary metal-oxide-semiconductor (CMOS) compatible processing. However, synthesis of high-quality BaTiO$_3$ directly on silicon remains a challenge. Here, we synthesize BaTiO$_3$ using hybrid metal-organic molecular beam epitaxy (hMBE) and demonstrate its transfer onto silicon using thermocompression bonding and chemical lift-off. Hybrid metal-organic MBE enables self-regulated synthesis of highly stoichiometric thin films at high growth rates (>100nm/hr). Our transfer method results in millimeter-scale areas of atomically flat, crack-free BaTiO$_3$ making it a potentially scalable method. Finally, we demonstrate the applicability of our process to device fabrication through characterization of lithographically-patterned and etch-transferred sub-micron features.
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All authors Temazulu S. Zulu, Larissa B. Little, Aaron M. Day, Chaoshen Zhang, Keith Powell, Kyeong-Yoon Baek, Benazir Fazlioglu-Yalcin, Neil Sinclair, Charles M. Brooks, David R. Barton, Marko Loncar, Julia A. Mundy

Tuesday 20 January

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

Majorana Fermions in spin up and down electronic complexes in spin-orbit coupled array of semiconductor quantum dots in proximity to $s$-type superconductor and in magnetic field

Mijanur Islam, Mahan Mohseni, Ibsal Assi et al. (+2 more)

Semiconductor-s-type superconductor nanowires host spinful fermions and cannot be reduced to a single spinless Kitaev chain hosting single Majorana zero mode. Instead, such systems can be converted into two coupled p-wave Kitaev-like chains associated with different spin sectors. Using the bond Fermion transformation and exact diagonalization, we analyze parity resolved spectra and local spectral functions, demonstrating that zero-energy modes strongly localized at the system boundaries emerge only in one effective chain. Inter-chain coupling lifts parity degeneracy and redistributes the low-energy spectral weight, providing a controlled framework to assess the stability of Majorana-like modes in the finite spinful nanowires.
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All authors Mijanur Islam, Mahan Mohseni, Ibsal Assi, Daniel Miravet, Pawel Hawrylak

Materials Science

3D bulk-resolved $g$-wave altermagnetic order parameter in CrSb

Mengmeng Long, Theodore I. Weinberger, Zheyu Wu et al. (+7 more)

Electronic phases of matter, such as magnetism and superconductivity, are defined and distinguished by their order parameters quantifying the spontaneous symmetry breaking underlying each phase. Simple cases include the uniform magnetisation of ferromagnets and isotropic gap function of conventional superconductors. Unconventional superconductors often have a nodal gap function, where the gap changes sign at nodes on the Fermi surface. This concept of unconventional/nodal order parameter symmetry has recently been extended to numerous magnetic systems, including altermagnets, in which up- and down-spin species are non-degenerate around the Fermi surface. Here we demonstrate that magnetic quantum oscillation measurements can provide a high resolution, bulk-sensitive, 3D mapping of the order parameter in an unconventional magnet. By rotating a magnetic field through high- and low-symmetry directions of the CrSb Brillouin zone, we show that this material's altermagnetic band structure leads to a reduction of symmetry for each spin-split Fermi sheet away from nodal orientations. In momentum space, the exchange splitting between up and down spins follows the profile of the $\mathcal{Y}_{4}^{-3}=zy(3x^2-y^2)$ real spherical harmonic - analogous to a $g$-orbital of the hydrogen atom. While notoriously difficult to resolve in unconventional superconductors, our work demonstrates that the order parameter symmetry of unconventional magnets can be precisely mapped via quantum-oscillatory quasiparticle spectroscopy, establishing CrSb as a prototypical $g$-wave metallic altermagnet.
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All authors Mengmeng Long, Theodore I. Weinberger, Zheyu Wu, Mads F. Hansen, Ran Tao, Mridul Shrestha, Dave Graf, Yurii Skourski, F. Malte Grosche, Alexander G. Eaton

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

Strain-tunable magnetic correlations in spin liquid candidate Nb$_3$Cl$_8$

Tharindu Fernando and Ting Cao

Recent research suggests the possibility of the two-dimensional breathing-Kagome magnet Nb$_3$Cl$_8$ hosting a quantum spin liquid state, warranting further study into its magnetic properties. Using ab initio calculations, we show that monolayer Nb$_3$Cl$_8$ has short-range antiferromagnetic correlations among Nb$_3$ trimers with S = 1/2, and becomes magnetically frustrated due to the underlying effective triangular lattice geometry, and is evidenced by a frustration index of f > 1. The high-temperature susceptibility shows a negative Weiss temperature from Monte Carlo calculations. Considering spin-orbit coupling, we investigate the magnetic anisotropy, including anisotropic exchange, single-ion anisotropy and the Dzyaloshinskii-Moriya interaction using the four-state energy mapping formalism. Although the elements have relatively small atomic numbers, the Dzyaloshinskii-Moriya interaction is comparable in magnitude to the anisotropic exchange. Additionally, we show that biaxial strain tunes the short-range correlations between antiferromagnetic, paramagnetic and ferromagnetic. These findings strengthen our understanding of Nb$_3$Cl$_8$ and advance its applications in current condensed matter physics and materials science research, including nanoscale mechanical and spintronics applications.
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Materials Science · Cross-listed from cond-mat.supr-con

A Modulated Electron Lattice (MEL) Criterion for Metallic Superconductivity

Jaehwahn Kim, Davis A. Rens, Waqas Khalid et al. (+1 more)

A central unresolved question in the theory of superconductivity is why only a small subset of metallic elements exhibit a superconducting state, whereas many others remain strictly normal. Neither the conventional Bardeen Cooper Schrieffer (BCS) framework nor its extensions involving charge density wave (CDW) or pair density wave (PDW) order provide a predictive or material-selective criterion capable of distinguishing superconducting metals from non-superconducting ones. In particular, the persistent absence of superconductivity in simple noble metals with well-defined Fermi surfaces poses a challenge for all traditional approaches. Here we address this problem using the Modulated Electron Lattice (MEL) Ginzburg Landau (GL) framework introduced in our previous work. In this formulation, a coarse-grained MEL charge field $\rho_{\mathrm{MEL}}(\mathbf{r})$ with momentum dependent stiffness $\alpha(q)$ is coupled to the superconducting (SC) order parameter $\psi(\mathbf{r})$. We show that metallic superconductivity emerges only when the system satisfies a specific “MEL enhancement window,” characterized by a negative minimum of $\alpha(q)$ at either a finite modulation wave vector $q^{\ast}$ or at $q=0$, together with sufficiently strong coupling between $\rho_{\mathrm{MEL}}$ and $\psi$. This unified criterion naturally partitions metallic elements into three universal classes: (i) MEL-enhanced superconductors with a finite-$q^{\ast}$ charge mode, (ii) conventional BCS superconductors as the homogeneous $q^{\ast}=0$ limit of the MEL framework, and (iii) metals for which $\alpha(q)$ remains positive for all $q$, suppressing all MEL modes and preventing any superconducting instability. By applying this criterion to simple metallic elements, we identify why some metals develop superconductivity while others do not, possibly resolving a selection problem long open within the BCS paradigm.
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All authors Jaehwahn Kim, Davis A. Rens, Waqas Khalid, Hyunchul Kim

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

Fracture initiation in silicate glasses via a universal shear localization mechanism

Matthieu Bourguignon, Gustavo Alberto Rosales-Sosa, Yoshinari Kato et al. (+6 more)

Shear bands lie at the root of fracture initiation in bulk metallic glasses and amorphous polymers. For silicate glasses, in contrast, studies have largely emphasized permanent volumetric strain, commonly referred to as densification. Here we systematically investigate indentation-induced fracture in two distinct families of aluminoborosilicate glasses. The results demonstrate that plastic shear flow plays a decisive role in governing fracture initiation. In addition, molecular dynamics simulations reveal a pronounced composition dependence of softening associated with plastic shear flow, closely mirroring the experimentally observed propensity for strain localization. We conclude that silicate glasses conform to a universal pattern of rupture initiation governed by localization of shear-deformation, aligning with a broad range of amorphous materials, including bulk metallic glasses and glassy polymers.
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All authors Matthieu Bourguignon, Gustavo Alberto Rosales-Sosa, Yoshinari Kato, Bruno Bresson, Hikaru Ikeda, Shingo Nakane, Gergely Molnár, Hiroki Yamazaki, Etienne Barthel

Materials Science

Nanoscopy of Excitons in Atomically Thin In-Plane Heterostructures with Nanointerfaces

Mahdi Ghafariasl, Tianyi Zhang, Sampath Gamage et al. (+8 more)

Atomically sharp 2D in-plane heterostructures with nanoscale interfaces provide a powerful platform for tailoring optical and electrical properties at the nanoscale, enabling novel device engineering and the exploration of new physical phenomena. However, direct experimental correlation between local dielectric response and excitonic properties across such interfaces has remained elusive. Here, we probed the nanoscale complex dielectric function and the corresponding localized photoluminescence (PL) modulations in heterostructure domains of lateral monolayer MoxW1-xS2 - WxMo1-xS2, synthesized using a liquid-phase precursor-assisted approach. Near-field nano imaging across the visible-near-infrared range enables real space mapping of sharp amplitude and phase changes at the heterointerface, resolving the local complex dielectric function with nano-meter scale spatial resolution. Excitation energy-dependent nano spectroscopy reveals a reversal of dielectric contrast between Mo-rich and W-rich domains at their respective excitonic resonances, consistent with Lorentz-oscillator fits. Complementary hyperspectral nano-PL mapping resolves the evolution of excitonic emission across the lateral heterointerface, with neutral-exciton intensities varying continuously from W-rich to Mo-rich regions. Effective-medium theory modeling of the imaginary part of the effective dielectric function of the heterostructure as a function of photon energy and Mo filling fraction reproduces the observed excitonic trends, linking the PL evolution to a composition-dependent dielectric response. Together, these results provide direct nanoscale correlation between dielectric and excitonic boundaries in laterally stitched monolayer heterostructures and establish a multimodal near-field spectroscopy framework for probing excitonic phenomena at the nanoscale.
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All authors Mahdi Ghafariasl, Tianyi Zhang, Sampath Gamage, Da Zhou, Muhammad Asjad, Sarabpreet Singh, Antonio Gomez-Rodriguez, Diego M. Solis, Venkataraman Swaminathan, Mauricio Terrones, Yohannes Abate

Materials Science

Photoluminescence Dynamics of CdSe QD/polymer Langmuir-Blodgett Thin Films: Morphology Effects

Beatriz Martín-García, Pedro M. R. Paulo, Sílvia M. B. Costa et al. (+1 more)

Thin films of colloidal semiconductor CdSe quantum-dots (QDs) and a styrene/maleic anhydride copolymer were prepared by the Langmuir-Blodgett technique and their photoluminescence was characterized by confocal fluorescence lifetime microscopy. In the films, the photoluminescence dynamics are strongly affected by excitation energy migration between close-packed quantum-dots and energy trapping by surface-defective QDs or small clusters of aggregated QDs. Polymer/QD films with more aggregated QD regions exhibit lower PL intensities and faster decays, which we attribute primarily to the increased ability of excitations to find trap sites among more close-packed QD regions. This was confirmed by comparing films from bilayer or co-spreading deposition, and varying the QD-to-polymer composition or the surface pressure at deposition. The more regular films, such as those obtained by bilayer deposition at high-pressure, display more surface emission intensity and decays with less accentuated curvature. Decay analysis was performed with a model that accounts for excitation energy migration and trapping in the films. In the framework of the model, the photoluminescence dynamics are related to film morphology through the density of energy traps. A lower amount of QD clustering in the films reflects on a lower density of energy traps and, thus, on more emissive QD films, as inferred here for bilayer films relatively to co-spreading films.
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All authors Beatriz Martín-García, Pedro M. R. Paulo, Sílvia M. B. Costa, M. Mercedes Velázquez

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

Altermagnetic phases and phase transitions in Lieb-$5$ Hubbard model

Sougata Biswas, Achintyaa and Paramita Dutta

The emergence of altermagnetism, the collinear magnetic phase characterized by momentum-dependent spin-split bands but zero net magnetization, has fundamentally reshaped the classification of magnetic order. We propose an altermagnetic (AM) order in a repulsive Hubbard model on the Lieb-$5$ lattice. Considering only nearest-neighbor hoppings within the lattice, we show a phase transition from the nonmagnetic to a unique AM isolated band metal phase (AMIM), allowing clear identification of spin-split states. Additionally, the AM metallic phase (AMM) is also shown to appear as an intermediate phase during the transition from the normal metal to the AMIM in the presence of the diagonal hopping within each unit cell of the Lieb-$5$ lattice. The manifestation of distinct AM phases and the phase transitions, driven by Hubbard interaction and hopping integrals, have been explored in terms of spin-resolved band structure, spectral function, and the behavior of the AM order parameter. The stability of these AM phases against the spin-orbit coupling and temperature is also established.
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Materials Science

Faster grain-boundary diffusion with a higher activation enthalpy than bulk diffusion in ionic space-charge layers

Timon F. Kielgas and Roger A. De Souza

Faster diffusion of cations along grain boundaries is reported in the literature for a variety of acceptor-doped $AB\mathrm{O}_{3}$ perovskite-type oxides. The ratio $r$ of the activation enthalpy of grain-boundary diffusion ($\Delta H^\mathrm{gb}$) to the activation enthalpy of bulk diffusion ($\Delta H^\mathrm{b}$) is seen experimentally to lie in the range $0.7 < r = \Delta H^\mathrm{gb} / \Delta H^\mathrm{b} < 1.3$, albeit with substantial errors. In a previous publication [Parras and De Souza, Acta Mater. 195 (2020) 383] it was shown through a set of continuum simulations that cation-vacancy accumulation within negative space-charge layers at grain boundaries in acceptor-doped perovskites will give rise to faster grain-boundary diffusion of cations, with the associated values of $r$ approaching but not exceeding unity. In the present study, we demonstrate by means of continuum simulations that $r > 1$ is possible for faster cation diffusion along grain boundaries in an acceptor-doped perovskite. The specific case we consider is cation diffusion occurring by two related mechanisms, by slower (charged) isolated cation vacancies and by faster (neutral) defect associates of cation and anion vacancies. Within the negative space-charge layers, the isolated cation vacancies are strongly accumulated, whereas the neutral associates are unaffected. We calculate diffusion profiles for a two-dimensional bicrystal geometry by solving, first, Poisson's equation, and subsequently, the diffusion equation. We find that, if a small concentration of faster defect associates is responsible for bulk diffusion, and a hugely enhanced concentration of slower isolated vacancies yields faster diffusion along space-charge layers, $r>1$ is obtained. The conditions under which $r > 1$ may be observed are described, and issues with experimental confirmation are discussed.
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Materials Science

Resolving Overlapping EBSD Patterns by Experiment – Simulation Residuals Analysis

Grzegorz Cios, Aimo Winkelmann, Tomasz Tokarski et al. (+2 more)

In the technique of Electron Backscatter Diffraction (EBSD), the accurate detection and identification of different phases existing in a sample is often limited by overlapping Kikuchi diffraction patterns originating from the extended probing volume of the individual EBSD map points measured in the scanning electron microscope (SEM). We present an iterative approach that uses simulated Kikuchi patterns to resolve several overlapping diffraction signals. For each measured EBSD pattern, our method first identifies the best-fit simulated Kikuchi pattern using dynamic template matching. This simulated, ideal reference pattern is then further processed to optimally match the experimental image, uncovering any underlying weaker signals after subtraction. Repeatedly utilizing dynamic template matching and pattern subtraction on residual signals of subsequent steps enables the identification of minor phases that might otherwise be missed from the probing volume of the EBSD map point. This method significantly improves phase detection in complex materials, addressing a key limitation of conventional EBSD analysis that conventionally assigns a single phase to each map point. The present method does not require a known orientation relationship between the phases of the overlapping patterns or close neighbor experimental patterns like previously published approaches.
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All authors Grzegorz Cios, Aimo Winkelmann, Tomasz Tokarski, Wiktor Bednarczyk, Piotr Bała

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

Efficient charge transfer in solution-processed PbS Quantum Dot-reduced graphene oxide hybrid materials

Beatriz Martín-García, Anatolii Polovitsyn, Mirko Prato et al. (+1 more)

Quantum dot - graphene hybrid materials have raised significant interest due to the unique synergy of the optical properties of colloidal quantum dots (QDs) and the transport properties of graphene. This stimulated the development of low-cost and up-scalable solution-processed strategies for hybrid materials with potential application in light harvesting and opto-electronic devices. Here we report a versatile covalent-linking based approach for the functionalization of reduced graphene oxide (rGO), to prepare a variety of QD-rGO hybrid dispersions with QDs of different size and composition (PbS, PbS/CdS and CdSe QDs), and shape (CdSe/CdS dot-in-rods). We achieved a well-controlled QD coverage of the rGO sheets by functionalizing the rGO surface with mercapto-silane linkers. A further spectroscopic investigation of near-infrared PbS QD-rGO materials demonstrates efficient electronic coupling between both materials. The QD photoluminescence emission quenching and exciton lifetime shortening up to 95%, together with subtle graphene Raman G-band shifts upon QD linking, supports electron transfer as the dominant relaxation pathway from the QD to the rGO. The use of core/shell PbS/CdS QDs allows tuning of the transfer efficiency from 94% for a 0.2 nm thin CdS shell, down to 30% for a 1.1 nm thick shell.
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All authors Beatriz Martín-García, Anatolii Polovitsyn, Mirko Prato, Iwan Moreels