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Physics Condensed Matter Materials Science

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Friday 17 April

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

Host-guest co-amorphous structure revealed by the suppression of the first sharp diffraction peak in isotactic poly(4-methyl-1-pentene)

Tomoki Ogihara, Yusuke Hiejima and Ayano Chiba

While host-guest co-crystals are well established, and co-amorphous solids are recognized in materials science, the concept of a host-guest co-amorphous structure remains largely unexplored. A potential analogue is seen in SiO2 glass under high pressure with helium as a pressure medium; the drop in compressibility in this system is ascribed to helium atoms occupying internal voids. In this study, we investigated a semicrystalline polymer, isotactic poly(4-methyl-pentene-1) (P4MP1), which shares key characteristics with SiO2 glass, particularly regarding the first sharp diffraction peak (FSDP). The FSDP in P4MP1 is attributed to internal voids, as evidenced by its suppression under pressure and recovery upon decompression for molten P4MP1. Notably, the response to helium as a pressure medium is also known to parallel the behavior observed in SiO2 glass. Here, we analyzed two-dimensional X-ray diffraction (2D-XRD) patterns of stretched P4MP1 and found a suppression of FSDP when P4MP1 is immersed in decane. The use of stretched samples enabled the clear isolation of the amorphous FSDP from overlapping crystalline diffractions. Our findings reveal the existence of a host-guest co-amorphous system at room temperature and atmospheric pressure, in which decane molecules occupy the amorphous host matrix of P4MP1. Unlike conventional co-amorphous mixtures, this structure is defined by the specific accommodation of guests within the host's inherent voids. Intriguingly, the signature of this structure in diffraction measurements, manifested as changes in the FSDP intensity ratio, may be regarded to parallel the variations in Bragg peak intensity ratios in host-guest co-crystals. Since selective sorption and guest exchange are well-known in co-crystals, hosts capable of forming co-amorphous structures will be promising materials for molecular sieves, or more generally, liquid-phase molecular sieves.
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Materials Science

Direct Orientation Contrast Imaging of Anti-Phase Domains on III-V Materials Using Scanning Electron Microscopy

Brieg Le Corre, Clothilde Grenèche, Rozenn Bernard et al. (+16 more)

Direct orientation contrast imaging of zinc-blende III-V materials is studied using scanning electron microscopy. A quantitative approach is taken using a 3 {\mu}m thick orientation-patterned GaP grown on GaAs sample, studying the anti-phase domain contrast with respect to the electron beam energy and the tilt angle. A qualitative approach is taken for III-V grown on non-polar materials with and without chemical mechanical polishing. Finally, a processing of the acquired image for GaP on Si reveals in plane preferential anti-phase boundaries.
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All authors Brieg Le Corre, Clothilde Grenèche, Rozenn Bernard, Tony Rohel, Antoine Létoublon, Wijden Khelifi, Julie Le Pouliquen, Arnaud Grisard, Sylvain Combrié, Bruno Gérard, Abdelmounaim Harouri, Luc Le Gratiet, Grégoire Beaudoin, Konstantinos Pantzas, Isabelle Sagnes, Oliver Skibitzki, Gilles Patriarche, Charles Cornet, Yoan Léger

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

Persistence of large and gate-tunable anisotropic magnetoresistance in an atomically thin antiferromagnet

Cheol-Yeon Cheon, Kenji Watanabe, Takashi Taniguchi et al. (+2 more)

Anisotropic magnetoresistance (AMR) offers a robust electrical readout of antiferromagnetic (AFM) states, playing a central role in the rapidly advancing field of AFM spintronics. Despite its great versatility, electrical probing of the Néel vector via AMR remains challenging in the ultrathin limit due to interface disorder and reduced dimensionality. Here, we demonstrate electrical readout of the Néel vector down to 1.3 nm (two layers) in the two-dimensional van der Waals (vdW) AFM semiconductor NiPS3. Leveraging spin-flop-mediated rotation of the Néel vector and using both transistor and tunnel-junction device geometries, we identify two distinct AMR contributions in NiPS3, that dominate at low and high charge densities, respectively. We achieve full gate control over these contributions, enabling tunability of both the magnitude and sign of magnetoresistance. Our results establish semiconducting vdW antiferromagnets as a rich platform for studying AMR in the ultrathin limit, opening new avenues for multifunctional AFM spintronic devices.
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All authors Cheol-Yeon Cheon, Kenji Watanabe, Takashi Taniguchi, Alberto F. Morpurgo, Dmitry Lebedev

Materials Science

Flat-band energy filtering in interacting systems: conditions for improving thermoelectric performances

F. Cosco, R. Tuovinen, F. Plastina et al. (+1 more)

Motivated by recent theoretical and experimental studies on the role of flatbands in the thermoelectric properties of Ni$_3$In$_{1-x}$Sn$_x$ compounds, we investigate electron transport in two minimal one-dimensional flatband models, the sawtooth and diamond chains, which differ in a crucial aspect: the flatband is separated from the dispersive band by a finite gap in the former, while the two bands touch in the latter. Using a non-equilibrium Green function framework with interactions treated at the Hartree-Fock and GW levels, we compute the full set of thermoelectric coefficients and the figure of merit $zT$ as functions of gate voltage and temperature. We show that, contrary to naive expectation, a perfectly isolated flat-band is a physically ill-founded thermoelectric: the electrical conductivity vanishes as the chemical potential enters the flat-band, rendering the large Seebeck coefficient and the apparent violation of the Wiedemann-Franz law physically meaningless. Optimal thermoelectric performance is instead achieved just below the flat-band edge, where the transmission function varies most rapidly with energy, consistent with the Mahan-Sofo picture, and requires a finite broadening of the flat-band through hybridization with dispersive states. We further show that electron-electron interactions renormalize the flat-band structure itself, inducing an interaction-driven narrowing of the bandwidth and, in the diamond chain, a correlation-induced opening of a gap between the flat-band and the dispersive band near half-filling. Mean-field treatments are found to systematically overestimate \(zT\), highlighting the importance of beyond-mean-field correlations for quantitatively reliable predictions in flat-band thermoelectrics.
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All authors F. Cosco, R. Tuovinen, F. Plastina, N. Lo Gullo

Materials Science

Fully compensated and uncompensated ferrimagnetic ferrovalley semiconductors

Weifeng Xie, Libo Wang, Yunliang Yue et al. (+3 more)

Altermagnets (AMs) and fully compensated ferrimagnets (FC-FIMs) are emerging classes of magnetic materials that combine the advantages of antiferromagnets and ferromagnets. Here, we elucidate the mechanism behind the uniaxial strain-driven transformation from AM to FC-FIM and find that the accompanying non-relativistic valley polarization is positively correlated with the net magnetic moment between magnetic atoms in opposite spin sublattices. We then propose an uncompensated ferrimagnetic monolayer VCrSeTeO to achieve large intrinsic valley polarization. Spin-orbit coupling (SOC) is shown to further increase the valley polarization to over 400 meV under uniaxial strains and the reason is explained in terms of SOC perturbation theorem. Furthermore, we reveal a distinctive anomalous valley Hall effect in which the valley Hall voltage is reversed within the same valley in ferrimagnet VCrSeTeO. This work proposes a strategy for realizing giant valley polarization and provides theoretical guidance for the application of ferrimagnetic ferrovalley semiconductors derived from altermagnets in valleytronics.
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All authors Weifeng Xie, Libo Wang, Yunliang Yue, Xiong Xu, Huayan Xia, Hui Wang

Materials Science

Facet-dependent Chemical Kinetics Governed Growth of Twisted Graphene Layers with Pre-designed Angles

Chaowu Xue, Mengzhao Sun, Zixuan Zhou et al. (+8 more)

Twisted graphene layers (TGLs) provide a powerful platform for investigating multiple quantum phenomena, yet their scalable deployment is hindered by the lack of reliable synthesis with precise angle. Here, benefited from a deeper understanding of the interplay between grain index and graphene growth kinetics, we report a scalable strategy to grow TGLs with pre-designed twist angles on platinum (Pt) via chemical vapor deposition (CVD), Through a combination of complementary in situ methods, we identified the activity sequence of different Pt grains and attributed it to the area ratio of exposed (110) facets during graphene-induced surface reconstruction. Moreover, we revealed that CVD-grown graphene orientation is determined by the grain-orientation-dependent surface morphology. By leveraging the so-established correlations between grain index with both graphene growth priority and its orientation, we achieve controlled folding and tearing of graphene overlayer using a pair of adjacent grains with dramatically different catalytical activity and kink-free atomic steps. We reveal that overlayer-induced step bunching and terrace reconfiguration critically govern the domain morphology and folding direction. Building on this mechanistic insight, we demonstrate a substrate-engineering framework where specific platinum grains are rationally selected to yield TGLs with pre-designed twist angles, including magic angle with flat band dispersion. This work not only highlights fundamental kinetics of Pt catalyzed graphene CVD growth, but also offers a generalizable methodology for manipulating foldable two-dimensional materials via dynamic substrate reconstruction, exampled by programmable growth of high-quality TGLs on open surfaces.
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All authors Chaowu Xue, Mengzhao Sun, Zixuan Zhou, Zhuoran Yao, Li-Qun Shen, Xiao Kong, Honglong Zhao, Feng Ding, Marc Willinger, Zhongkai Liu, Zhu-Jun Wang

Thursday 16 April

Materials Science

Hole concentrations in doped gray {\alpha}-Sn on InSb and CdTe measured with infrared ellipsometry

Jaden R. Love, Carlos A. Armenta, Atlantis K. Moses et al. (+5 more)

Gray tin ({\alpha}-Sn) layers with 30 nm thickness were grown on InSb (001) substrates using molecular beam epitaxy. The surface preparation of the substrates was adjusted to achieve either n-type or p-type doping in the {\alpha}-Sn layer. Fourier-transform infrared ellipsometry was used to find the temperature-dependent dielectric function of the {\alpha}-Sn layers from 0.03 to 0.8 eV and from 10 to 300 K. Because of the inverted band structure of {\alpha}-Sn, the spectra show a strong absorption peak at 0.45 eV due to transitions from the inverted {\Gamma_-^7} "electron" valence band to the {\Gamma_+^8} heavy hole valence band. Applying the Thomas-Reiche-Kuhn f-sum rule, the integrated oscillator strength of this peak was used to calculate the heavy hole concentration as a function of temperature. For a nearly intrinsic {\alpha}-Sn layer, the heavy hole concentration agrees well with predictions based on degenerate Fermi-Dirac statistics. Deviations from the intrinsic {\alpha}-Sn carrier concentrations are attributed to substrate surface preparation leading to the diffusion of donor or acceptor ions into the {\alpha}-Sn layer causing n-type or p-type doping.
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All authors Jaden R. Love, Carlos A. Armenta, Atlantis K. Moses, Haley B. Woolf, Jan Hrabovsky, Stefan Zollner, Aaron N. Engel, Christopher J. Palmstrøm

Materials Science

Electronic Signature of Melting Onset in Polycrystalline Copper at Extreme Conditions

Edna R. Toro, Tobias Held, Armin Bergermann et al. (+8 more)

Ultrafast melting is fundamentally a structural transition of the ionic lattice, but this rearrangement also reshapes the electronic properties by changing the energy landscape and scattering mechanisms. Although the electrons react almost instantaneously, it is not a priori clear how much lattice disorder is required for a significant response. Here, we show that the onset of melting already produces a clear electronic signature in polycrystalline copper. Using single-shot terahertz time-domain spectroscopy on thin films excited over a wide range of laser fluences, we infer the transient conductivity during the first picoseconds after excitation. The data, supported by two-temperature molecular-dynamics simulations, show that before melting, electron transport is substantially limited by grain-boundary scattering and that melting strongly suppresses this channel. As melting begins at these interfaces, we observe a transient increase in the conductivity that directly marks the onset of the phase transition. More broadly, these results show that ionic and electronic relaxation stages are closely coupled in nonequilibrium laser-driven matter and that optical measurements can resolve distinct stages of melting.
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All authors Edna R. Toro, Tobias Held, Armin Bergermann, Megan Ikeya, Maximilian Maigler, Eric R. Sung, Dirk O. Gericke, Mianzhen Mo, Baerbel Rethfeld, Siegfried H. Glenzer, Benjamin K. Ofori-Okai

Materials Science

Device-area selection of memristive transport regimes in epitaxial $Hf_{0.5}Zr_{0.5}O_{2}$-based ferroelectric devices

Priscila A. Tapia Presas, Lautaro Galarregui, Wilson Román Acevedo et al. (+5 more)

Ferroelectric memristive devices based on hafnia are promising systems for neuromorphic electronics, yet the interplay between polarization-modulated resistive changes and defect-mediated transport often leads to complex and debated switching mechanisms. Here, we investigate this competition in epitaxial Hf$_{0.5}$Zr$_{0.5}$O$_2$/La$_{0.67}$Sr$_{0.33}$MnO$_3$ heterostructures with Pt top electrodes, by combining structural, ferroelectric, and memristive characterization with a statistical analysis across a broad range of device areas spanning three orders of magnitude. We identify two distinct memristive regimes with opposite resistance-voltage chiralities. Small devices exhibit a low-resistance state that scales inversely with area, consistent with area-distributed electronic transport, while larger devices display an area-independent resistance indicative of localized conductive channels. A statistical nucleation model quantitatively captures this behavior and yields a crossover characteristic area $A^* \approx 10^3\,\mu\mathrm{m}^2$. This crossover also correlates with the onset of ferroelectric wake-up for the larger devices, linking conductive-channel nucleation and oxygen-vacancy redistribution within a unified physical picture. These results establish lateral device size as a key parameter controlling the dominant transport mechanism in epitaxial hafnia-based devices.
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All authors Priscila A. Tapia Presas, Lautaro Galarregui, Wilson Román Acevedo, Myriam H. Aguirre, José Santiso, Sylvia Matzen, Beatriz Noheda, Diego Rubi

Materials Science · Cross-listed from physics.ins-det

SHIELD: A Reference Gas-Driven Permeation Platform for Hydrogen Permeation Studies

James Dark, Colin Weaver, Remi Delaporte-Mathurin et al. (+2 more)

A gas-driven permeation (GDP) platform, SHIELD (Salt-compatible Hydrogen barrier Investigation and EvaLuation for fusion Devices), has been developed to measure hydrogen transport properties in structural materials under controlled thermal and pressure conditions. The system is designed to minimise experimental uncertainties associated with leaks, temperature instability, and pressure measurement, while providing reproducible conditions for permeation experiments. The rig operates in a static GDP configuration with independent upstream and downstream volumes, enabling precise control of driving pressure and accurate measurement of downstream pressure rise. An openly documented data acquisition and processing framework is implemented to ensure data traceability and reproducibility. The platform's performance is demonstrated by hydrogen permeation measurements on 316 stainless steel and AISI 1018 low-carbon steel over the temperature range of \SIrange{100}{600}{\degreeCelsius}. Steady-state permeation fluxes are extracted from linear downstream pressure rise and used to determine permeability. The measured permeability exhibits Arrhenius behaviour and agrees well with published literature data for both materials. Permeability measurements are shown to be robust and reproducible. These results demonstrate that SHIELD provides a reliable reference platform for hydrogen permeation measurements and is well-suited to evaluating permeation barrier coatings and advanced materials for fusion applications.
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All authors James Dark, Colin Weaver, Remi Delaporte-Mathurin, Sara Ferry, Kevin B. Woller

Materials Science

Atomic-scale order enables high thermal boundary conductance at $\beta$-Ga$_2$O$_3$/4H-SiC interfaces

Hongao Yang, Yongtao Yang, Yuanbin Liu et al. (+8 more)

Thermal boundary conductance (TBC) at dissimilar interfaces imposes a fundamental limit on electronic device performance, yet predicting and understanding heat transport across realistic, disordered boundaries remains elusive. Here, we develop a computational framework that combines machine-learned interatomic potentials with lattice dynamics to address the long-standing problem of how interfacial structure, from disordered to atomically sharp, affects thermal transport in the technologically important $\beta$-Ga$_2$O$_3$/4H-SiC heterostructure. By explicitly accounting for phonon wave-particle duality, we show that interfacial disorder introduces additional interfacial phonon modes that facilitate vibrational impedance matching between the two highly dissimilar crystals, yet it simultaneously disrupts interfacial phonon coherence and limits the potential heat-transport benefit. Our atomistic simulations further indicate that restoring atomic-scale order preserves coherence and yields markedly higher conductance. These insights motivate the controlled epitaxial growth of $\beta$-Ga$_2$O$_3$/4H-SiC heterostructures with systematically tuned interfacial order. Experimental measurements validate our predictions, achieving a record-high TBC of 231 MW m$^{-2}$ K$^{-1}$ at atomically sharp junctions. Beyond the immediate implications for $\beta$-Ga$_2$O$_3$-based power electronics, our results establish the preservation of interfacial phonon coherence as an effective strategy for mitigating thermal bottlenecks in mismatched systems.
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All authors Hongao Yang, Yongtao Yang, Yuanbin Liu, Tao Ding, Yang Shen, Jiawei Huang, Weigang Ma, Linfeng Fei, Zhenping Wu, Gábor Csányi, Bingyang Cao

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

3D Finite Element-Based Multiphysics Simulation of a Shape Memory Alloy Hybrid Composite Module

Lukas Handl, Max Kaiser, Miro Duhovic et al. (+1 more)

Shape adaptive shape memory alloy hybrid composites (SMAHCs) are composites that incorporate shape memory alloys (SMAs) to realize shape transformation. Despite the availability of numerous analytical and finite element models for predicting the transient response of SMAHCs, many approaches exhibit limitations with respect to the thermomechanical coupling and comprehensive experimental validation. Therefore, this paper presents a coupled, multiphysics, 3D finite element approach for the simulation of a SMAHC actuator, integrating mechanical, thermal and electromagnetic solvers in the Finite Element Code ANSYS LS-DYNA. The proposed approach employs a micromechanical constitutive model implemented in ANSYS LS-DYNA, to accurately capture the complex thermomechanical phase transformation of SMAs. A key feature of the model is the ability to prescribe a defined martensitic pre-strain through a preceding simulation step, in which an initially scaled SMA wire is mechanically loaded and stretched to its nominal length. This procedure enables partial detwinning of the martensitic microstructure and provides a physically motivated initialization of the material state. Joule heating of the SMA wires, as well as varying mechanical loads and ambient temperature conditions, are explicitly considered. The simulation results are validated against experimental data and a fully coupled transient staggered scheme model to assess the predictive capability of the 3D approach. The results show good qualitative agreement, reproducing the characteristic hysteresis of actuator deflection as a function of temperature. Quantitatively, the predicted deflections are of the correct order of magnitude, although marginally outside the 95 % experimental confidence interval. Overall, a consistent trend between simulation and experiment is observed, giving rise to possibility of simulating more complex SMAHC systems.
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All authors Lukas Handl, Max Kaiser, Miro Duhovic, Martin Gurka

Materials Science

Optimal spin-qubit hallmarks of sulfur-vacancy defects in 4H-SiC: Design from first principles

Marisol Alcántara Ortigoza and Sergey Stolbov

By applying our methodology, we propose a defect in 4H-SiC which combines a Si vacancy and a C atom substituted with S (VSiSC) to have a spin-triplet ground state with the spin qubit functionality. Our calculations confirm that all configurations of the defect have a dynamically and thermodynamically stable triplet ground state and higher energy singlet states, essential for the spin-qubit polarization cycle. From GW calculations, we found that the electronic states associated with the defect form sharp and isolated peaks within the band gap for both triplet and singlet states. Further Bethe-Salpeter-equation calculations show that all considered configurations have intense optical excitations in the near infrared spectrum range. Analysis of the excitation energies and rates indicate that the VSiSC defect can be an excellent optically controlled spin qubit. Crucially, the host elements and the dopant have high-abundance isotopes with zero nuclear spin ensuring high spin-coherence time of the qubit.
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Materials Science

Lattice dynamics and complete polarization analysis of Raman-active modes in LaInO$_3$

Jonas Rose, Hai Nguyen, Moritz Meißner et al. (+7 more)

In this study, we present a comprehensive analysis of the Raman active phonon modes in orthorhombic LaInO$_3$ based on a combination of polarization-angle resolved Raman spectroscopy and density functional theory calculations. By using backscattering from multiple crystallographic surface orientations and employing a full symmetry analysis, we identify and assign most of the Raman-active $\Gamma$-point phonons to their irreducible representations of the D$_{\rm{2h}}$ point group. A multidimensional hyperspectral fitting procedure allows us to extract the relative Raman tensor elements from the angular dependence of the scattering intensities, even for strongly overlapping modes. First-principles calculations yield the phonon dispersion along high-symmetry directions, the phonon densities of states, and atomic displacement patterns, which are found to be in good agreement with the experimental mode frequencies.
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All authors Jonas Rose, Hai Nguyen, Moritz Meißner, Zbigniew Galazka, Roland Gillen, Georg Hoffmann, Oliver Brandt, Manfred Ramsteiner, Markus R. Wagner, Hans Tornatzky

Materials Science

Fully Atomic-Layer-Deposited Vertical Complementary FeRAM with Ultra-High 2Pr > 100 uC/cm2 and High Endurance > 1E10 cycles

Renhao Xue, Ruizhan Yan, Mansun Chan et al. (+1 more)

A limited remanent polarization (Pr) in HfO2-based FeRAM remains a key obstacle to density scaling and reliability, while material and process optimizations offer only incremental improvements. This limitation fundamentally originates from the thickness-constrained switchable polarization and the intrinsic polarization ceiling of HfO2-based ferroelectrics. Here, we propose an all-ALD-grown vertical complementary FeRAM (VCF) architecture, in which the top and bottom stacked FeRAM cells maintain complementary polarization. This complementary dipole configuration converts the readout from a single-layer polarization response into a differential polarization summation, thereby amplifying the effective charge window without increasing the switching field of each individual layer or incurring area overhead. Viewed from top to bottom, an "up-down" polarization pair stores logic '1', whereas a "down-up" pair stores logic '0'. Using a complementary polarization write-read scheme, the VCF achieves an effective differential polarization above 100 uC/cm^2 and retains above 90 uC/cm^2 after 1e10 switching cycles without electrical breakdown. Robust retention (longer than 1e4 s at 85 degC) and strong disturb immunity are demonstrated, with an effective differential polarization above 80 uC/cm^2 under a V/3 scheme after 1e6 disturb pulses. Array-level operation is validated in a 5 x 5 selector-free crosspoint array. The performance enhancement of the VCF arises from the co-optimization of the all-ALD-grown process, device architecture, and operation scheme, enabling high density, a wide memory window, and strong reliability for scalable FeRAM integration.
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All authors Renhao Xue, Ruizhan Yan, Mansun Chan, Xiwen Liu

Materials Science

Disentangling the ferroelectric phases of epitaxial hafnia

Johanna van Gent Gonzalez, Ewout van der Veer, Yulei Li et al. (+2 more)

Since its discovery, ferroelectric hafnia has been extensively studied due to its CMOS-compatibility and ability to remain polarized at sub-10 nm thicknesses. The ferroelectric behaviour is generally attributed to a polar orthorhombic (OIII) phase. However, a second polar phase with rhombohedral symmetry (R-phase) has also been reported in epitaxial films. The nature of the R-phase remains disputed due to the subtle differences with the OIII-phase when probed by standard thin film characterisation techniques. Given the functional properties of ferroelectrics are crucially determined by the crystal symmetry, resolving this matter is imperative. In this work, we settle the controversy through extensive 3D reciprocal space surveys made possible via synchrotron-based grazing incidence diffraction from epitaxial films of both phases. These experiments, together with direct comparison of their temperature dependence and electrical responses, conclusively establish them as two distinct phases and provide insight into their key characteristics.
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All authors Johanna van Gent Gonzalez, Ewout van der Veer, Yulei Li, Daniel A. Chaney, Beatriz Noheda

Materials Science · Cross-listed from physics.optics

Optical Modulation Due to Energy Exchange Between Photonic and Exciton Modes in the Intermediate Coupling Regime

Evripidis Michail, Sander A. Mann, Kamyar Rashidi et al. (+4 more)

Actively tunable photonic devices are vital for next-generation optoelectronics requiring rapid switching and high bandwidth. Although organic optoelectronic devices have found wide application, their use as optical modulators has been limited by low absorption in the critical near-infrared (NIR) region, slow response time, and weak nonlinearities. To address these limitations, we developed a scheme based on intermediate exciton-photon coupling in a NIR absorbing squaraine-dye based photonic structure. Using energy-momentum resolved pump-probe spectroscopy, we show that the sign and magnitude of the optical response of our system depends strongly on the energy detuning between the excitonic and photonic modes. These data are analyzed using temporal coupled-mode theory to show that near resonance, a distinct energy exchange process emerges in the cross-over regime between strong and weak light-matter coupling. This effect enables dynamical control over the photoinduced response, providing a pathway for broadband optical signal modulation extending into the NIR spectral region.
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All authors Evripidis Michail, Sander A. Mann, Kamyar Rashidi, Christoph Lambert, Vinod M. Menon, Andrea Alu, Matthew Y. Sfeir

Materials Science

Controllable highly oriented skyrmion track array in Fe3GaTe2

Yunhao Wang, Shiyu Zhu, Chensong Hua et al. (+15 more)

Magnetic skyrmions are emerging as promising candidates for next-generation information technologies, while the realization of scalable skyrmion lattices with tailored configurations is essential for advancing fundamental skyrmion physics and developing future applications. Here we achieved the controllable generation and regulation of a large-area, highly oriented skyrmion track array (STA) in ferromagnetic Fe3GaTe2 using a vector magnetic field manipulation technique. The orientation and ordering of STA, along with the types and density of skyrmions, are precisely controlled by modulating parameters during the manipulation. The critical roles of in-plane magnetic fields and Dzyaloshinskii-Moriya interaction in STA generation is further confirmed by micromagnetic simulation. Our findings develop a strategy for engineering large-area and highly-oriented skyrmion configurations, offering a new pathway for the future application of next-generation spintronic and information technologies.
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All authors Yunhao Wang, Shiyu Zhu, Chensong Hua, Guojing Hu, Linxuan Li, Senhao Lv, Jianfeng Guo, Jiawei Hu, Runnong Zhou, Zizhao Gong, Chengmin Shen, Zhihai Cheng, Jinan Shi, Wu Zhou, Haitao Yang, Weichao Yu, Jiang Xiao, Hong-Jun Gao

Materials Science · Cross-listed from physics.optics

Toroidal Plasmonic Nanodimers for Enhanced Near-Infrared Emission in Heterostructured InP Quantum Dots

Arda Gulucu and Emre Ozan Polat

Near-infrared (NIR) emitters operating in the 650-900 nm range are highly attractive for imaging and sensing in turbid media; however, cadmium-free InP-based quantum dots (QDs) often suffer from limited brightness due to nonradiative pathways and inefficient photon outcoupling. In particular, heterostructured InP QDs can exhibit band alignments that induce partial spatial separation of charge carriers, leading to reduced electron-hole wavefunction overlap. This modifies intrinsic recombination dynamics and enhances the sensitivity of their emission to the surrounding photonic environment. Here, we investigate silver toroidal plasmonic nanoantenna dimers (Ag TPNDs) through finite-difference-time-domain (FDTD) simulations as a geometry-tunable platform for enhancing NIR emission of heterostructured InP-based QDs. The coupled toroidal geometry supports strongly confined bonding modes that generate intense nanogap hotspots, while its resonance can be systematically tuned through the toroid aspect ratio. By spectrally aligning the antenna response with QD emission bands (675-845 nm), we achieve large Purcell enhancements together with high quantum efficiencies, demonstrating efficient conversion of enhanced decay rates into radiative emission. We further show that nanometer-scale variations in emitter-antenna separation strongly modulate the radiative rates and spectral response. These results establish toroidal plasmonic nanodimers as a topology-driven platform for controlling emission in NIR quantum emitters and for advancing NIR nanophotonic applications.
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Materials Science · Cross-listed from physics.app-ph

Spontaneous Emission, Work Potential and Relaxation-Limited Processes in Setting Limits on Solar Energy Conversion Efficiency

Sumanta Mukherjee

Understanding the thermodynamics of radiation and the quantum-mechanical interactions between light and matter is important both for theoretical purposes and for technological advances, such as determining the limits of key processes like light-to-usable-energy conversion efficiencies. In this report, we discuss the physics of these two aspects, considering spontaneous emission as a pathway, and highlight the limitations of such descriptions in assessing energy-harvesting efficiency. In view of these limitations, we adopt a simplified approach to evaluate the exergy and work potential of radiation, providing a framework for assessing various aspects of light-to-usable-energy conversion efficiency. Our approach allows a theoretical estimate of the thermodynamic maximum limit for light-to-usable-energy conversion, which is approximately 76%. We validate these exergy and work potential estimates by modeling and accurately reproducing the Shockley-Queisser limit (~ 33.3%), which imposes a practical constraint on solar-to-usable-energy conversion efficiency. Beyond exergy considerations, our model incorporates processes such as spontaneous emission, nonradiative thermal losses, and photon upconversion, allowing us to evaluate their roles. The model further suggests that, under certain conditions, the maximum conversion efficiency can reach approximately 48%, for example with multijunction solar cells or via photon upconversion. These findings further suggest that the true thermodynamic limit for light-to-usable-energy conversion may be much higher (approximately 76%). However, accurately estimating this limit requires a more complete understanding of the thermodynamics of light, light-matter interactions, and the connection between them.
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Materials Science

Towards Non-van der Waals 2D Topological Insulators

Mani Lokamani, Gustav Bihlmayer, Gregor Michalicek et al. (+3 more)

Non-van der Waals two-dimensional (2D) materials derived from strongly bonded non-layered crystals have recently emerged as a novel and rising platform for nanoscale research. While uncovering and tuning their (opto-)electronic, catalytic, and magnetic properties has been the focus of intense research, the impact of spin-orbit coupling (SOC) onto their electronic structure has not yet been explored in detail. Studying these effects is, however, particularly relevant due to their surface cation termination and the presence of heavy elements in several representative compounds. Here, we investigate the effect of SOC onto the electronic structure of 2D AgBiO3, NaBiO3, and SbTlO3. While the first two systems show negligible band renormalization upon inclusion of relativistic effects around the band gap, SbTlO3 showcases a large SOC induced splitting (229meV) for the lowest conduction bands associated with a band inversion. Substitution of Tl with Pb forming SbPbO3 brings the band-inverted feature to the Fermi level. Analysis of topological invariants and investigation of edge states of zig-zag and armchair ribbons within the 200meV gap confirms the topological nature of the band splitting. Our work thus establishes a foundation for the systematic study of robust non-van der Waals 2D topological insulators.
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All authors Mani Lokamani, Gustav Bihlmayer, Gregor Michalicek, Daniel Wortmann, Stefan Blügel, Rico Friedrich

Materials Science

Magneto-optical imaging of macroscopic altermagnetic domains in MnTe

Gakuto Watanabe, Soichiro Yamane, Ryotaro Maki et al. (+6 more)

Altermagnets are a new class of magnets accompanying global time-reversal symmetry breaking (TRSB) without net magnetization. The TRSB results in formation of novel altermagnetic domains. Features of altermagnetic domains, in particular their responses to external stimuli, are essentially important but yet unexplored. Here, we report visualization of bulk altermagnetic domains in MnTe based on scanning magneto-optical Kerr-effect microscopy using telecom infrared wavelength. We found two distinct TRSB domains with large Kerr rotations that do not scale with its tiny bulk magnetization. We also revealed controllability and stability of domains against magnetic or thermal perturbations. Our first observation of altermagnetic domains using a laboratory-scale simple optical technique showing their movable nature provide firm bases for future fundamental and application studies of altermagnets.
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All authors Gakuto Watanabe, Soichiro Yamane, Ryotaro Maki, Atsutoshi Ikeda, Akimitsu Kirikoshi, Junya Otsuki, Takuya Aoyama, Kenya Ohgushi, Shingo Yonezawa

Materials Science · Cross-listed from physics.optics

Light-propelled microparticles based on symmetry-broken refractive index profiles

Julian Jeggle, Matthias Rüschenbaum, Adrian Paskert et al. (+7 more)

Active colloidal microparticles require reliable actuation to sustain directed motion. Light-based propulsion is particularly attractive as it provides persistent energy supply and enables direct spatiotemporal control. Here, we introduce 3D-printable particles with symmetry-broken refractive index profiles (SBRIP particles) that achieve propulsion through direct momentum transfer from asymmetric light refraction. Internal refractive-index gradients provide optical symmetry breaking independent of external shape, fundamentally decoupling propulsion from particle geometry. Geometrically symmetry-broken particles with a homogeneous refractive index are another special case, where propulsion originates from refractive contrast at the boundary instead of within the particle. Unlike conventional systems relying on absorption or reflection, this transparency-based mechanism minimizes heating and mitigates shadowing in bulk suspensions. We present a theoretical framework for refractive propulsion as well as numerical simulations of the SBRIP particles using raytracing and the finite volume method. This is complemented by experiments, validating the momentum transfer mechanism using particles with geometric symmetry breaking. The high transparency of our particles ensures deep light penetration, enabling the realization of volumetric active matter. This opens pathways toward adaptive nonlinear optical materials where light-driven particle reorganization modulates the local refractive index, establishing a dynamic feedback loop between the optical field and the material structure.
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All authors Julian Jeggle, Matthias Rüschenbaum, Adrian Paskert, Ivan Kalthoff, Elena Vinnemeier, Jesco Schönfelder, Jörg Imbrock, Cornelia Denz, Marcel Rey, Raphael Wittkowski

Materials Science

Reversable phase transitions in ferroic two-dimensional Nb2O2I4 through optically excited coherent phonons

Chuanlin Liu, Dan Liu, Jie Guan et al. (+1 more)

We investigate optically induced phase transitions in the two-dimensional (2D) ferroelectric (FE) material Nb2O2I4 using real-time time-dependent density functional theory (rt-TDDFT). Our results demonstrate that tailored laser pulses can activate specific coherent phonon modes. Specifically, the anharmonic atomic distortions of the A1-1 and A1-2 modes at the {\Gamma}-point facilitate the reversal of in-plane polarization. By fine-tuning laser parameters, additional phonon modes at both the Y and {\Gamma} points are excited. The resulting nonequilibrium atomic dynamics enable the formation of previously unreported ferroic phases, including three antiferroelectric (AFE) phases and one ferrielectric (FiE) phase. Notably, these optically induced phases can be reverted to the initial FE state using appropriate techniques. This controllable reversibility among multiple ferroic phases positions 2D Nb2O2I4 as a highly promising candidate for next-generation electronic storage applications.
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All authors Chuanlin Liu, Dan Liu, Jie Guan, Chao Lian

Materials Science

Propagation of laser-generated GHz surface acoustic wavepackets in FeRh/MgO(001) below and above the antiferromagnetic-ferromagnetic phase transition

Ia. A. Mogunov, A. Yu. Klokov, N. Yu. Frolov et al. (+6 more)

Magnetoacoustic devices that harness the strong coupling between acoustic waves and magnons have emerged as a promising platform for energy-efficient spintronics. Laser-generated pulsed surface acoustic waves (SAWs) are particularly attractive for such applications, offering broadband frequency content up to the gigahertz (GHz) range, remote excitation without lithographic patterning, and surface localization for efficient on-chip integration. In this work, we present a comprehensive experimental study of laser-generated SAW pulses in the Fe49Rh51/MgO(001) system. A thin film of the near-equiatomic FeRh alloy serves both as an opto-acoustic transducer and as a mechanical load that modulates SAW propagation. The antiferromagnetic to ferromagnetic phase transition in FeRh, occurring slightly above room temperature, is accompanied by abrupt changes in its elastic properties, enabling controlled modification of the SAW excitation efficiency and dispersion characteristics by tuning the sample temperature and laser fluence. Using 160 fs laser pulses for excitation and time-resolved Sagnac interferometry for detection, we evaluated key SAW parameters, including amplitude, spectral content, phase and group velocities, and their in-plane anisotropy. Particular emphasis is placed on the dispersion relation and its anisotropy, which govern the coherent interaction between phonons and magnons and are determined primarily by the FeRh film.
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All authors Ia. A. Mogunov, A. Yu. Klokov, N. Yu. Frolov, A. I. Sharkov, A. V. Protasov, G. E. Zhezlyaev, D. I. Devyaterikov, V. I. Zverev, A. M. Kalashnikova

Materials Science

Discovering structural, electronic and excitonic properties of bulk, nanostructured and doped C3N4 in diamond- and graphitic-like phases

Da Chen, Pietro Andreozzi, Giulia Frigerio et al. (+3 more)

In this systematic density functional theory study, we compare a standard gradient corrected functional (PBE) with a long-range hybrid functional (HSE06), with and without correction for the dispersion forces, relative to their ability to correctly reproduce structural and electronic properties of different bulk 3D C3N4 phases, encompassing diamond- and graphitic-like models. Corrugation is found to provide further stabilization to the layered structures with all methods. We observe that HSE06-D3 method provides results in good agreement with experimental data and with more sophisticated G0W0 calculations. Based on that, we exploited the method to investigate the nature of the bulk triplet excitons in these C3N4 structures to evaluate the S0-T1 energy difference, the self-trapping triplet exciton energy and the photoluminescence emission energy since this is a promising visible-light photocatalyst. Nanostructuring (0D and 2D) is another relevant aspect of these materials in practical applications; therefore, we have considered the effect of single- or multilayer exfoliation or space confinement in nanoparticles. Finally, we also discuss how the introduction of extrinsic dopants (e.g., S atoms) in the nanostructures modifies the atomic and electronic structure.
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All authors Da Chen, Pietro Andreozzi, Giulia Frigerio, Daniele Perilli, Paulo Siani, Cristiana Di Valentin

Materials Science

Pattern formation during melting of lamellar eutectics

Rahul Nellissery Rajan, Rajesh Kumari Rajendran, Guillaume Boussinot et al. (+3 more)

We present a study of the melting dynamics of a two-phase eutectic solid. In situ, thin-sample experiments using a transparent eutectic alloy and two-dimensional phase field simulations calibrated for the very same alloy are combined to assess pattern formation during directional melting in a temperature gradient. Depending on the melting velocity $V_m$ and the spacing $\lambda$ of the pre-solidified lamellar microstructure, an unexpectedly rich diversity of melting patterns is observed, with good agreement between experiments and simulations. We unravel the different physical mechanisms leading to this diversity, and establish the scaling behaviors of (i) the penetration of the liquid along the solid-solid interface at large $V_m$, (ii) the thickening of the primary-phase fingers at low $V_m$, and (iii) a period-doubling instability for small $\lambda$ values. Our study provides a fundamental basis for further investigations of eutectic melting, including additive manufacturing during which melting/solidification cycles take place.
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All authors Rahul Nellissery Rajan, Rajesh Kumari Rajendran, Guillaume Boussinot, Kamal Sbargoud, Sabine Bottin-Rousseau, Silvère Akamatsu

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

Layer-dependent quantum transport in KV2Se2O-based altermagnetic tunnel junctions

Yue Zhao, Bin Xiao, Jiawei Liu et al. (+2 more)

Magnetic tunnel junction (MTJ) is the key component to enable information access and increasing number of MTJs is integrated to develop high-density spintronic devices. However, continuous miniaturization of the conventional MTJs is hindered by stray magnetic fields. Altermagnets, combining the advantages of both ferromagnets and antiferromagnets, provide a promising alternative to fabricate versatile MTJs with exotic properties, such as giant spin splitting, high intrinsic frequency, and absence of stray fields. Inspired by the altermagnetic metal candidate KV2Se2O reported recently, we design an altermagnetic tunnel junction (AMTJ) based on KV2Se2O/SrTiO3/KV2Se2O. Using density functional theory combined with non-equilibrium Green's function, we investigate the layer-dependent quantum transport properties and the tunneling magnetoresistance (TMR) of such AMTJ device. Our calculated results reveal that the transmission of the AMTJ device exhibits a pronounced oscillation behavior dependent on the number of layers of the SrTiO3 semiconductor, which is attributed to the interface configuration determined by parity of the layer number. In odd-layer devices, the electron-rich O-Se interface exhibits a smooth effective potential and enables transverse momentum (k||) transport channels, leading to enhanced transmission. In contrast, in even-layer devices, the Ti-Se interface presents a steeper effective potential, impeding quantum transport through transverse momentum (k||) channels. A giant TMR of 4.6*10^7% is predicted to be realized by using a 4-layer SrTiO3. Our findings not only provide physical understanding relevant to the quantum transport in AMTJs, but also unveil that the barrier interface engineering is a strategy to tune the magnetoelectric performance.
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All authors Yue Zhao, Bin Xiao, Jiawei Liu, Hui Zeng, Jun Zhao

Materials Science

Spin-Valley-Mismatched Altermagnet for Giant Tunneling Magnetoresistance

Kun Yan, Yizhi Hu, Wei-Hua Xiao et al. (+3 more)

Altermagnet-based heterojunctions have demonstrated magnetoresistive effects in experiments, however, a predictive theoretical model for non-ferromagnetic structures has remained elusive. In this work, we develop a tunneling-based spin-transport theory that explicitly incorporates the transverse-wavevector ($\bf{k}_\|$)-dependent spin polarization of an altermagnet's transport channels, enabling the prediction of giant tunneling magnetoresistance (TMR). Based on the theory, we predict that the altermagnet KV$_2$Se$_2$O can reach the extreme limit of magnetoresistance. By performing first-principles transport calculations, we verify that magnetic tunnel junctions using the metallic KV$_2$Se$_2$O as the electrodes and few-layer MgO as the spacer exhibit zero-bias magnetoresistance larger than $7.57\times10^7$%, which is robust against the bias and thickness of the spacer. Our research provides a quantitative design principle for next-generation spin-electronic devices and establishes KV$_2$Se$_2$O/MgO/KV$_2$Se$_2$O as a leading candidate material system for room-temperature ultra-high-density non-volatile memory.
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All authors Kun Yan, Yizhi Hu, Wei-Hua Xiao, Xiaolong Zou, Xiaobin Chen, Wenhui Duan

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

Thermal conductivity tuning of scalable nanopatterned silicon membranes measured with a three-probe method

Jose M. Sojo-Gordillo, Alex Rodriguez-Iglesias, Dominik M. Koch et al. (+5 more)

Phononic silicon structures have emerged as an integrable and scalable nanosystem for tailoring thermal transport. However, their widespread adoption has been limited by their complex fabrication pathways. Alongside, the reliable characterization of thermal properties in suspended nanostructured films remains challenging, as thermal contact resistances often hinder the accuracy of measurements. In this work, we demonstrate a clear and controllable reduction of thermal conductivity in nanopatterned silicon membranes. A block copolymer self-assembly approach is employed to fabricate nanoholed silicon films with a pitch of 63 nm and hole diameters of 35 nm. Additionally, we introduce an extension of the three-probe technique that enables robust, quantitative, and spatially resolved thermal conductivity measurements in complex thin-film systems, accounting for thermal contact artifacts. The method is validated through measurements on unpatterned 40 nm-thick silicon thin films between 30 and 350 K, yielding a room-temperature thermal conductivity of 46.5 W/m.K. Finally, we further show that controlled etching of the nanoholes provides a powerful means to tune thermal transport in the overall studied temperature range, establishing hole etch depth control as an effective parameter in phononic silicon. Specifically, a fivefold reduction in thermal conductivity is achieved, reaching 7.3 W/m.K for fully etched-through membranes at room temperature.
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All authors Jose M. Sojo-Gordillo, Alex Rodriguez-Iglesias, Dominik M. Koch, Arianna Nigro, Iñigo Martin-Fernandez, Marta Fernandez-Regulez, Marc Salleras, Ilaria Zardo