Materials Science · Cross-listed from astro-ph.EP
Hydrogen-helium immiscibility boundary in gas-giant planetary interiors from machine-learning molecular dynamics
Xiaoyu Wang, Sebastien Hamel and Bingqing Cheng
The location of the hydrogen-helium (H/He) immiscibility boundary controls whether and where helium rain occurs in
giant planets, yet it remains uncertain because high-pressure experiments are challenging and ab initio simulations are limited in system size and simulation time. We map this boundary by computing composition-dependent chemical potentials from large-scale molecular dynamics driven by machine learning potentials trained on three density functional approximations (PBE, vdW-DF, and the hybrid HSE). Across the pressure range of 100-1000 GPa, the demixing temperatures are typically ~2000 K lower than previous ab initio simulations using small system sizes, and this discrepancy is larger than the estimated error from the chosen density functionals, machine learning potentials, neglect of
nuclear quantum effects, and statistical uncertainties. Fitting the H/He mixing free energy to a Redlich-Kister regular solution model rationalizes the thermodynamic driving force for phase separation and provides a predictive representation of the boundary. Comparing with current planetary interior profiles indicates that helium rain is plausible in Saturn but unlikely in the warmer interior of Jupiter. Our results narrow the uncertainty in the H/He immiscibility boundary and provide inputs for planetary models that couple demixing, heat transport, and composition gradients in gas
giants.
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Materials Science · Cross-listed from physics.chem-ph
Quantum Coherence Governs Macroscopic Polymorphism in Organic Semiconductors
Hai Wang, Tianhong Huang, Jiawei Chang et al. (+1 more)
Polymorphism in organic
semiconductors is conventionally framed as
equilibrium thermodynamic selection, yet atmospheric-pressure vapor deposition routinely produces metastable phases that defy classical nucleation theory. We develop a
symmetry-resolved open
quantum system formulation of
quantum dissipative assembly (QDA), in which the fundamental assembly unit is a vibronic wavepacket whose internal degrees of freedom are classified by the irreducible representations of the molecular point group. The carrier-gas environment acts as a structured dissipative bath with irrep-resolved spectral densities, and polymorph selection corresponds to relaxation into a
symmetry-resolved maximum-transmittance attractor (MTA) rooted in
quantum scattering theory and impedance matching. Guided by this theory, we tune the carrier-gas dissipative environment via reactor geometry, flow velocity, and precursor concentration to selectively synthesize a previously unreported polar polymorph of copper phthalocyanine, omega-CuPc, crystallizing in space group P2 with a dimerized
bilayer superstructure and an extreme Davydov splitting of 154 nm. Further structural refinement with a 4-molecule modulated supercell model resolves the majority of discrepancies between powder X-ray diffraction and energy minimization, revealing secondary layer orientation modulation as a higher-order dissipative optimization product. The framework consistently explains the formation windows of the eta, alpha, and beta polymorphs, their distinct morphologies, and the marked difference in crystalline order between open-shell CuPc and closed-shell NiPc. Our findings establish a
symmetry-guided, environment-controlled polymorph engineering strategy rooted in QDA, where the carrier-gas atmosphere serves as an active dissipative medium shaping the
symmetry-resolved dissipative landscape rather than acting as an inert thermal bath.
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All authors Hai Wang, Tianhong Huang, Jiawei Chang, Wenbo Zhang
Materials Science
Pentagonal PdTe2 Monolayer for Sustainable Solar-driven Hydrogen Production
Narender Kumar, Shambhu Bhandari, Dario Alfè et al. (+2 more)
This investigation demonstrates that the pentagonal PdTe$_2$ (penta-PdTe$_2$) monolayer is a highly tunable two-dimensional (2D) photocatalyst, characterized by a bandgap of 1.87~eV and high hole mobility. Using density functional theory (DFT) calculations with the HSE06 functional, we show that tensile strain engineering, particularly at $+2%$ and $+3%$, is essential for enabling spontaneous water splitting. At these strain values, the valence-band maximum (VBM) and conduction-band maximum (CBM) straddle the water redox potentials ($\mathrm{H^+/H_2}$ and $\mathrm{O_2/H_2O}$) under both acidic ($\mathrm{pH}=0$) and neutral ($\mathrm{pH}=7$) conditions. The monolayer's low hole effective mass facilitates rapid charge extraction, mitigating electron–hole recombination and promoting the oxygen
evolution reaction (OER) more effectively than many hexagonal and pentagonal counterparts. The Gibbs free energy ($\Delta G$) pathways indicate that the overpotentials for the hydrogen
evolution reaction (HER) and OER are highly sensitive to mechanical deformation, specifically biaxial strain. In particular, a tensile strain of $+3%$ yields an optimized balance of overpotentials, with $\eta_{\mathrm{HER}} = 0.70~\mathrm{V}$ at $\mathrm{pH}=0$ and $\eta_{\mathrm{OER}} = 0.72~\mathrm{V}$ at $\mathrm{pH}=7$. Finally, integrating optical absorption with thermodynamic driving forces results in a
solar-to-hydrogen (STH) efficiency of $20.40%$ at $\mathrm{pH}=7$. This performance exceeds that of several previously reported two-dimensional catalysts, positioning penta-PdTe$_2$ as a superior candidate for sustainable,
solar-driven hydrogen production.
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All authors Narender Kumar, Shambhu Bhandari, Dario Alfè, Nacir Tit, Ravindra Pandey
Materials Science · Cross-listed from cond-mat.mes-hall
Observation of Floquet-induced gap in graphene
Fei Wang, Xuanxi Cai, Xiao Tang et al. (+8 more)
Floquet engineering provides a powerful pathway for creating non-
equilibrium phases of matter with tailored electronic structures and properties through time-periodic driving. As the original theoretical prototype,
graphene established the framework in which the Floquet topological
insulator with light-induced anomalous
Hall effect was proposed. However, the defining spectroscopic signature of Floquet engineering in
graphene–light-induced hybridization (avoided-crossing) gap at Floquet band crossings, has remained experimentally elusive. Here, we report direct observation of Floquet-induced hybridization gap in monolayer
graphene under resonant driving by a strong light field. Time- and angle-resolved photoemission spectroscopy reveals gap opening at Floquet band crossings, accompanied by coherent Floquet sidebands. The gap exhibits pronounced momentum anisotropy, featuring two Dirac nodes protected by the spatiotemporal
symmetry and tunable by light polarization. These results provide long-sought experimental demonstration of Floquet band engineering in
graphene, opening up opportunities for light-field engineered
quantum phases in
graphene and related materials.
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All authors Fei Wang, Xuanxi Cai, Xiao Tang, Jinxi Lu, Wanying Chen, Tianshuang Sheng, Runfa Feng, Haoyuan Zhong, Hongyun Zhang, Pu Yu, Shuyun Zhou
Materials Science · Cross-listed from cond-mat.mes-hall
Robust Floquet-induced gap in irradiated graphite
Fei Wang, Xuanxi Cai, Wanying Chen et al. (+6 more)
Floquet engineering provides an emerging pathway for tailoring the electronic states of
quantum materials through time-periodic drive. A critical step along this direction is achieving light-induced modifications of the dynamical electronic structure, such as avoided-crossing gap at the Floquet Brillouin zone boundary, via efficient coupling of electrons with the coherent light-field. Here, we report robust Floquet-induced gap in bulk graphite that persists despite the presence of interlayer coupling and photo-excitation. Using time- and angle-resolved photoemission spectroscopy with intense mid-infrared pumping, we directly reveal Floquet-induced gaps at resonance points both in the valence and conduction bands, accompanied by coherent Floquet sidebands. The gap and sidebands coexist with photo-excited carriers, yet their distinct timescales allow us to disentangle their origins. Our demonstration of robust Floquet-induced gaps establishes graphite as a platform for coherent manipulation of Dirac
fermions and realization of light-engineered
quantum phases.
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All authors Fei Wang, Xuanxi Cai, Wanying Chen, Jinxi Lu, Tianshuang Sheng, Xiao Tang, Jiansong Li, Hongyun Zhang, Shuyun Zhou
Materials Science
Oxygen as a dual function regulator in MoS2 CVD synthesis: enhancing precursor evaporation while modulating reaction kinetics
Keerthana S Kumar, Abhijit Gogoi, Madhavan DK Nampoothiri et al. (+4 more)
Molybdenum disulfide (MoS2) is a promising 2D transition metal dichalcogenide (TMD) for optoelectronics and
quantum technology applications, but faces challenges in scalable synthesis and defect engineering. Oxygen-assisted chemical vapor deposition (O-CVD), which introduces in-situ oxygen during growth, shows excellent potential in resolving both issues at once. Although co-flowing oxygen shows improvement in growth, the underlying mechanistic role of oxygen remains unclear. In this work, a combination of oxygen dosing experiments, density functional theory (DFT) calculations, computational fluid dynamics (CFD) simulations, and ab initio molecular dynamics (AIMD) simulations, uncover the dual role of oxygen in O-CVD. Firstly, AIMD reveals that oxygen increases MoO3 sublimation and enhances Mo3O9 supply. Concomitantly, DFT reveals that sulphur
oxides, due to their bulkier nature than pure S2, limit the formation of reactive MoS6 intermediates. Subsequently, by experimentally varying the oxygen flow-interval, flow-rate, and flow-time, and correlating them with CFD simulations, we decouple oxygen's roles in source-poisoning prevention (i.e. MoO3 evaporation) and growth regulation. We find that maintaining a low sulphur-to-oxygen (S:O2) ratio at the MoO3 boat and substrate during nucleation, and a high S:O2 ratio at the substrate during growth is the key to obtaining large-area high-quality monolayer MoS2, confirmed by our optical measurements. Based on our understanding, we present a kinetic phase diagram for MoS2 synthesis, which will enable controlled oxygen dosing as a tuning parameter for scalable, defect-controlled monolayer MoS2 synthesis.
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All authors Keerthana S Kumar, Abhijit Gogoi, Madhavan DK Nampoothiri, Bhavesh Kumar Acharya, Manvi Verma, Ananth Govind Rajan, Akshay Singh
Materials Science
Stable Asymmetric Magnetization Reversal in Epitaxial Co(001)/CoO(001) Bilayer
Maik Gaerner, Judith Bünte, Finn Peters et al. (+8 more)
The exchange bias (EB) in
ferromagnetic/
antiferromagnetic (FM/AFM)
bilayer systems causes a shift of the
magnetic hysteresis curve after field cooling through the Néel temperature of the AFM. In some cases, this shift is accompanied by an asymmetry between ascending and descending branches. In the past, this asymmetric magnetization reversal has been studied in different
bilayer systems, including polycrystalline Co/CoO thin films. Here we investigate the asymmetric magnetization reversal in an epitaxial fcc-Co(001)/CoO(001) thin film grown on MgO(001) by molecular beam epitaxy at varying temperatures up to room temperature after field cooling along the easy and the hard axes. Room temperature measurements of the longitudinal and transverse magneto-optic Kerr effect show different magnetization reversal processes via stable intermediate states for different angles between external
magnetic field and
magnetic easy axes. Once the sample is cooled below the blocking temperature, a pronounced asymmetric magnetization reversal can be observed. We show that in contrast to polycrystalline
bilayers, the loop asymmetry stays constant after multiple training cycles and that the magnitude of the asymmetry is directly correlated with the magnitude of the EB.
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All authors Maik Gaerner, Judith Bünte, Finn Peters, Inga Ennen, Hermann Tetzlaff, Johannes Fiedler, Tomasz Blachowicz, Luana Caron, Andreas Hütten, Andrea Ehrmann, Martin Wortmann
Materials Science
Role of spatiotemporal nonuniformities in laser-induced magnetization precession damping
P. I. Gerevenkov, Ia. A. Filatov, L. A. Shelukhin et al. (+2 more)
Laser-induced magnetization precession measurements in ferromagnets often reveal an anomalous decrease in the damping time near a field-induced second-order
spin-orientation transition, a behavior that cannot be described by the linearized Landau-Lifshitz-Gilbert equation. Here we demonstrate that this anomaly is not a material property but results from interference of precessing local magnetizations within the inhomogeneously excited region. By combining pump-probe experiments, analytical modeling that accounts for the finite sizes of the pump and probe spots, and micromagnetic simulations, we show that the standard macrospin approach fails to capture the observed dynamics. The inhomogeneous relaxation of
magnetic parameters within the excitation area distorts the measured precession envelope, while dipole fields give rise to a temporally non-monotonic term in its frequency. Our results highlight the critical role of excitation locality in a vicinity of critical fields.
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All authors P. I. Gerevenkov, Ia. A. Filatov, L. A. Shelukhin, P. A. Dvortsova, A. M. Kalashnikova
Materials Science · Cross-listed from cs.RO
A Foldable and Agile Soft Electromagnetic Robot for Multimodal Navigation in Confined and Unstructured Environments
Zhihao Lv, Xiaoyong Zhang, Mengfan Zhang et al. (+5 more)
Multimodal locomotion is crucial for an animal's adaptability in unstructured wild environments. Similarly, in the human gastrointestinal tract, characterized by viscoelastic mucus, complex rugae, and narrow sphincters like the cardia,
multimodal locomotion is also essential for a small-scale soft
robot to conduct tasks. Here, we introduce a small-scale compact, foldable, and robust soft electromagnetic
robot (M-SEMR) with more than nine locomotion modes designed for such a scenario. Featuring a six-spoke elastomer body embedded with
liquid metal channels and driven by Laplace forces under a static
magnetic field, the M-SEMR is capable of rapid transitions (< 0.35 s) among different locomotion modes. It achieves exceptional agility, including high-speed rolling (818 mm/s, 26 BL/s), omnidirectional crawling, jumping, and swimming. Notably, the
robot can fold to reduce its volume by 79%, enabling it to traverse confined spaces. We further validate its
navigation capabilities on complex terrains, including discrete obstacles, viscoelastic gelatin surfaces, viscous fluids, and simulated biological tissues. This system offers a versatile strategy for developing high-mobility soft
robots for future biomedical applications.
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All authors Zhihao Lv, Xiaoyong Zhang, Mengfan Zhang, Xiaoyu Song, Xingyue Liu, Yide Liu, Shaoxing Qu, Guoyong Mao
Materials Science
Incipient magnetic instability in RuO$_2$ with random phase approximation
Diana Csontosová, Kyo-Hoon Ahn and Jan Kuneš
We study the instability in RuO$_2$ using the Hartree-Fock approximation followed by the random phase approximation. We employ a three-orbital Hubbard model without
spin-orbit coupling. An analysis of the
eigenvalues and eigenvectors of the static susceptibility in the non-
magnetic phase for various local interaction parameters $U$, $J_H$, and hole doping $n$ shows that the
spin susceptibility is the dominant response channel. In the stoichiometric system without
spin-orbit coupling, commensurate altermagnetic order is identified as the leading instability at sufficiently low temperatures, whereas at higher temperatures or finite hole doping, incommensurate wave vectors emerge. To elucidate the origin of the
magnetic instability, we analyze the band spitting by the staggered Weiss field and discuss the qualitative difference between altermagnets and antiferromagnets.
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Materials Science
Work-Function-Resolved Imaging of Relaxation Oscillations and Chemical Spillover in CO Oxidation over Platinum Surfaces
Karel Vařeka, Michal Potoček, Adam Očkovič et al. (+4 more)
Chemical waves of CO oxidation on platinum surfaces exhibit complex spatio-temporal self-oscillations, yet the local electronic mechanisms driving their propagation remain poorly understood under operando conditions. In this work, we combine operando scanning electron microscopy with frequency-modulated Kelvin probe force microscopy (FM-KPFM) to simultaneously map secondary electron contrast and local work-function variations during CO oxidation on Pt. By utilizing the KPFM tip as a localized sensor, we provide the first work-function-resolved imaging of reaction fronts, enabling an unambiguous physical assignment of CO- and oxygen-covered states. Our results demonstrate that the spillover process of chemical wave-the transition and expansion of adsorbate phases-is characterized by a pronounced temporal asymmetry and spatial heterogeneity transition thresholds. KPFM identifies a rapid onset of oxygen coverage followed by a gradual, diffuse relaxation back to the CO-covered state, indicative of relaxation-type oscillations even at low pressures (10^-2 Pa). Correlative reaction-
diffusion simulations reproduce this wave morphology, confirming that the high-resolution work-function signal provides unique insights into the internal structure and kinetic heterogeneity of the working catalyst surface.
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All authors Karel Vařeka, Michal Potoček, Adam Očkovič, Tomáš Šikola, Zhu-Jun Wang, Petr Bábor, Miroslav Kolíbal
Materials Science
Quantitative Analysis of Light Induced Ion Segregation in Mixed-Halide Perovskites
Petr Machovec, Lukáš Horák, Milan Dopita et al. (+4 more)
Mixed-halide perovskites (MHPs) offer good band gap tunability by stoichiometry changes, which is an essential property for the creation of multijunction
solar cells. However, under illumination, halide ions in MHP segregate and create I- and Br-rich regions, which decreases the efficiency of potential
solar cells. In this work, a method for a detailed investigation of the distribution of halide ions within the MHP during and after illumination is introduced. Calculations of the strain field created by the halide segregation were performed, and the obtained local displacement of atoms was used to calculate the x-ray diffuse scattering. By fitting the experimental data measured on the thin polycrystalline layer of FA0.83Cs0.17Pb(I0.6Br0.4)3 the distribution of Br- and I- ions within an illuminated MHP was determined and the subsequent relaxation process of the segregation in the dark was tracked. Creation of highly Br-rich regions within slightly I-rich volume during the illumination was observed.
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All authors Petr Machovec, Lukáš Horák, Milan Dopita, Neda Neykova, Lucie Landová, Jakub Holovský, Václav Holý
Materials Science
Intrinsically ultralow thermal conductivity in all-inorganic superatomic bulk crystals
Mingzhang Yang, Yuxi Wang, Jun Deng et al. (+7 more)
Superatomic compounds, composed of atomic clusters interwoven by weak chemical bonds exhibit large anharmonicity vibrations, are excellent candidates for ultralow thermal conductivity (ąppa}) materials. However, growing bulk superatomic single
crystals is challenging due to complex chemical composition and chemical bonds, and studies on their intrinsic thermal property are scarce. Here, we grew high-quality superatomic single
crystals of Re6Se8Te7 and Re6Te15, both of which are narrow band gap
semiconductors that change into metals under external physical pressure. At room-temperature, the ąppa} are 0.32 W m-1 K-1 and 0.53 W m-1 K-1 in Re6Se8Te7 and Re6Te15, respectively, ranking among the lowest value reported in all-inorganic bulk
crystals. It is mainly attributed to the large Grüneisen parameter (1.93) and low average sound speed (< 1482 m/s), which are due to soft Te7 nets weakly embedded among the rigid Re6Se8 (Re6Te8) quasi-cubic clusters. The appearance of
boson peak, i. e., hump of C(T)/T3, verifies the existence of disordered
phonon transports. Besides, the temperature dependence of ąppa} can be described by classic Debye-Callaway model. Notably, above 350 K, the ąppa} values of Re6Se8Te7 and Re6Te15 are remarkably close to the upper limit derived from glassy-like
diffusion model. This finding sets the superatomic compounds as a promising family for searching ultralow-ąppa} and energy management materials.
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All authors Mingzhang Yang, Yuxi Wang, Jun Deng, Tianping Ying, Qinghua Zhang, Nianjie Liang, Xiaobing Liu, Bai Song, Jian-gang Guo, Xiaolong Chen
Materials Science
Grain boundary defects induced Tc increment in MnSi
Adrian Benedit-Cardenas, Tobias Fox, Stéphanie Bruyère et al. (+6 more)
The rapid advancement of digital technologies necessitates significant progress in functional materials, which are often derived from scarce elements and involve complex manufacturing processes. Additionally, the trend towards miniaturization in high-tech devices has heightened the demand for extremely small components with tailored functionalities. In the domains of
ferromagnetic materials, the
market is mostly dominated by rare-earth elements-based structures, which are also limited in abundance. In this work, we focus on the microstructure and properties of MnSi. It is a
ferromagnetic material with a relatively low Curie temperature (TC) of 30 K. However, our study demonstrates that Tc can be increased by a factor of 4 through careful control of the
crystal size. MnSi thin films were synthesized by combining two non-
equilibrium techniques: magnetron sputtering and
laser annealing.
Laser annealing provoked the crystallinity
evolution, by heat accumulation, of the barely crystallized films deposited by magnetron sputtering. The
laser beam scanning parameters were adjusted to achieve different fluence values, pulse numbers, and pulse frequencies at each point of the film. Films with a
crystal size of around 20 nm exhibited a TC of up to 120 K. These properties were obtained under conditions of low fluence and a high number pulse. Local
laser impacts were applied to as-deposited samples, enabling spatially controlled crystallization. The interface between the poorly and well-crystallized regions was showcased using high-resolution transmission electron microscopy (HR-TEM). A spatial resolution of approximately 100 {\mu}m was achieved. These results demonstrate the strong potential of
laser annealing as a versatile and promising approach for the fabrication of miniaturized devices.
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All authors Adrian Benedit-Cardenas, Tobias Fox, Stéphanie Bruyère, Christoph Pauly, Flavio Soldera, Sylvie Migot, Frank Mücklich, David Horwat, Alexandre Nominé
Materials Science · Cross-listed from cond-mat.str-el
Tomonaga-Luttinger liquid and charge-density wave in a quasi-one-dimensional material
Jing Li, Guo-Wei Yang, Bai-Zhuo Li et al. (+18 more)
In one-dimensional (1D) electron systems, the
Fermi liquid state breaks down due either to electron interactions, which results in a Tomonaga-Luttinger
liquid (TLL) state, or to Peierls instability, which leads to an insulating charge-density-wave (CDW) phase. In general, these two phenomena are mutually exclusive, and their coexistence remains elusive in real materials. Here, we report the discovery of a new quasi-1D material, Cs$_{1-\delta}$Cr$_3$S$_3$, which unexpectedly exhibits coexistence of the antithetical CDW and TLL states. The CDW state is evidenced by the intra-unit-
cell dimerization, and the opening of an optical band gap of $\sim$250 meV. Meanwhile, TLL behaviour is unambiguously demonstrated by the measurements of electrical transport and angle-resolved photoemission spectroscopy, which reveal a power-law scaling with temperature, bias voltage and electron energy. Band structure calculations reveal isolated, linearly dispersive, 1D bands around the
Fermi level. For the dimerized CDW phase, the 1D
Fermi-surface sheets located at the boundary of the Brillouin zone are gapped from intra-unit-
cell bond
symmetry breaking. Experimentally, subtle Cs vacancies shift the
Fermi level into the linearly dispersive valence band, enabling the observation of TLL behaviour without interrupting the CDW order. This work establishes Cs$_{1-\delta}$Cr$_3$S$_3$ as a rare material platform in which the antagonistic
Fermi-
liquid instabilities coexist and intertwine, opening new avenues for studying emergent
quantum phenomena in 1D systems.
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All authors Jing Li, Guo-Wei Yang, Bai-Zhuo Li, Yi Liu, Si-Qi Wu, Ji-Yong Liu, Jin-Ke Bao, Xiaoxian Yan, Hua-Xun Li, Jia-Xin Li, Jia-Lu Wang, Yun-Lei Sun, Yi-Ming Lu, Jia-Yi Lu, Yi-Qiang Lin, Hui Xing, Chao Cao, Hao Jiang, Yang Liu, Guang-Han Cao, Hai-Qing Lin
Materials Science
Maskless Electron Beam-Induced Etching of Diamond in Air: A Secondary Electron-Driven Mechanism
Duc-Duy Tran, Cedric Mannequin, Fabrice Donatini et al. (+2 more)
We report a direct, maskless electron beam-induced etching (EBIE) process for diamond in air, enabling high-precision patterning without lithography or
plasma processing. Through a comprehensive analysis of electron-gas, electron-diamond, and gas-surface interactions in the SEM environment, we demonstrate that etching is predominantly governed by low-energy secondary electrons, which drive gas dissociation and radical generation. The resulting oxygen- and nitrogen-based radicals chemisorb on the diamond surface, form volatile carbon-containing species, and desorb under continued electron irradiation, enabling controlled material removal. The process exhibits two distinct regimes: a molecule-limited regime governed by gas flux and an electron-limited regime controlled by current density. Etch depths up to 212 nm and lateral resolution down to 200 nm are achieved. Time-dependent anisotropy is observed, with (100) surfaces transitioning to (111)-faceted morphologies, enhancing etch yield. These results establish a general secondary electron-driven mechanism for EBIE in gas environments, providing a maskless, damage-free nanofabrication route for diamond
semiconductor and other chemically inert materials.
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All authors Duc-Duy Tran, Cedric Mannequin, Fabrice Donatini, Masahiro Sasaki, Etienne Gheeraert
Materials Science
Hematite Thin Films Grown on Z-Cut and Y-Cut Lithium Niobate Piezoelectric Substrates by Pulsed Laser Deposition
Maximilian Mihm, Stephan Glamsch, Christian Holzmann et al. (+3 more)
Altermagnets are a newly identified class of materials that combine advantageous characteristics of both ferro- and antiferromagnets, making them highly promising for spintronic applications. Hematite has recently been identified as an altermagnetic material and exhibits several noteworthy properties, including a high Néel temperature, a temperature dependent
spin reorientation transition (SRT) at the Morin temperature ($T_\mathrm{M}$), and low
magnetic damping. In this work, we demonstrate the epitaxial growth of hematite thin films on y- and z-cut lithium niobate (LiNbO$_3$) substrates using pulsed
laser deposition (PLD). LiNbO$_3$ as piezoelectric substrate is of particular interest as it enables the efficient excitation of surface acoustic waves (SAWs) with interdigital transducers. The different substrate cuts allow for different orientations of the Néel vector. Films grown on y-cut LiNbO3 are single-crystalline and single-phase, while those deposited on z-cut LiNbO$_3$ exhibit two distinct in-plane (ip) domains rotated 60{\deg} relative to each other. On both substrates, the hematite thin films exhibit a temperature dependent SRT which allows the
antiferromagnetic Néel vector to be controlled. This study paves the way for the development of high-quality piezoelectric/altermagnetic hyprids for magnonics and spintronics.
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All authors Maximilian Mihm, Stephan Glamsch, Christian Holzmann, Matthias Küß, Helmut Karl, Manfred Albrecht
Materials Science · Cross-listed from cs.CE
An efficient open-source framework for high-fidelity 3D surface topography and roughness prediction in milling
Hadi Bakhshan, Sima Farshbaf, Adrián Travieso-Disotuar et al. (+3 more)
With the emergence of data-driven approaches in science, there is growing interest in their application to manufacturing, particularly in surface precision engineering. However, generating large
datasets required for model training is often impractical experimentally due to high costs and the time-intensive nature of measurements. High-fidelity synthetic
datasets offer a viable alternative if they can be generated both efficiently and accurately. To address this challenge, this paper presents an efficient framework for generating accurate 3D surface topographies and roughness indicators in milling operations using numerical methods. First, a conventional topography prediction model is developed based on the forward solution method (FSM). Building on this, an optimized computational
algorithm is proposed to establish an efficient FSM with significantly improved performance. The model is validated against two independent sets of experimental results, assessing both prediction accuracy and computational efficiency. The results demonstrate acceptable prediction errors and an average computational speedup of 42.2x. The proposed open-source model provides a generalizable framework for large-scale analysis, enabling the generation of extensive
datasets for data-driven surrogate modeling.
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All authors Hadi Bakhshan, Sima Farshbaf, Adrián Travieso-Disotuar, Luciano Mijaíl Villarreal, Fernando Rastellini Canela, Josep Maria Carbonell
Materials Science · Cross-listed from cond-mat.str-el
Tilted and Twisted Magnetic Moments in the Kitaev Magnet $\alpha$-RuCl$_3$
Xiao Wang, Fengfeng Zhu, Markus Braden et al. (+8 more)
The layered honeycomb magnet $\alpha$-RuCl$_3$ has attracted intense scrutiny as a
prime candidate for realizing the Kitaev
quantum spin liquid, yet a consensus on its microscopic
Hamiltonian remains elusive due to the material's extreme sensitivity to structural details. Here, we report a comprehensive reexamination of the low-temperature crystallographic and
magnetic structures of high-quality $\alpha$-RuCl$_3$ single
crystals using unpolarized and polarized
neutron diffraction. We confirm a sharp, first-order structural phase transition to the rhombohedral $R\bar{3}$ space group with a pronounced thermal hysteresis. Crucially, using both spherical and longitudinal
neutron polarization analysis, we determine the 3D orientation of the ordered
magnetic moment without the ambiguity typically arising from domain distributions. We find that the Ru$^{3+}$
magnetic moments in the zigzag phase are tilted by $15.7^\circ$ out of the hexagonal plane and, remarkably, exhibit an additional in-plane twist of $-13.8^\circ$. This "tilted and twisted" geometry differentiates the ground state from the previously reported models based on unpolarized
neutron diffraction or resonant elastic X-ray scattering (REXS) analysis.
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All authors Xiao Wang, Fengfeng Zhu, Markus Braden, Karin Schmalzl, Wolfgang Schmidt, Martin Meven, Erxi Feng, Yinghao Zhu, Alexandre Bertin, Paul Steffens, Yixi Su
Materials Science
Quantification of magnetic interactions in van der Waals heterostructures using Lorentz transmission electron microscopy and electron holography
Joachim Dahl Thomsen, Qianqian Lan, Nikolai S. Kiselev et al. (+4 more)
Magnetic van der Waals (vdW) materials are promising for
memory and logic applications because of their highly tunable
magnetic properties and compatibility with vdW
heterostructure devices. However, coupling between
magnetic textures in stacked layers is difficult to resolve in conventional plan-view measurements because the
magnetic signal is integrated over the sample thickness. Here, these interactions are quantified in Fe$_3$GeTe$_2$ (FGT)/graphite/FGT
heterostructures using cross-sectional Lorentz transmission electron microscopy and off-axis electron holography, enabling reconstruction of the local
magnetic induction within and between the layers. Domain
alignment weakens with increasing FGT separation, yielding a stray-field coupling length scale of $\lambda = 37 \pm 7$ nm for the cross-sectional geometry studied here, corresponding to the average separation at which domain misalignment first emerges. This length scale corresponds to an approximately 23% reduction in the interlayer
magnetic induction relative to bulk FGT. Surface effects result in a reduced
magnetic induction compared to bulk FGT up to $\sim$100 nm from a surface. Comparisons of experimental data with model-based iterative reconstructions of the magnetization and micromagnetic simulations shows that the reduction in induction near surfaces is due to demagnetizing and stray fields. These results quantify the
magnetic induction in stacked vdW magnets and guide the design of devices that require controllable coupling between
magnetic textures.
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All authors Joachim Dahl Thomsen, Qianqian Lan, Nikolai S. Kiselev, Eva Duft, Arslan Rehmat, Zdeněk Sofer, Rafal E. Dunin-Borkowski
Materials Science · Cross-listed from physics.optics
Dirac branch-cut modes with relativistic transport
Bofeng Zhu, Chengzhi Ma, Qiang Wang et al. (+6 more)
Emergent Dirac fields, exhibiting effective relativistic physics, are most commonly associated with bulk and surface states in materials such as
graphene and topological
insulators. Here we show that phase dislocations of a complex Dirac mass support a distinct family of propagating Dirac defect states that behave as massless or massive one-dimensional relativistic modes. These traveling-wave defect states, termed Dirac branch-cut (DBC) modes, are governed by a one-dimensional Dirac
Hamiltonian whose effective mass is controlled by the phase difference across the branch cut. Using acoustic metamaterials, we experimentally demonstrate a range of relativistic phenomena exhibited by DBC modes, including relativistic dispersion, energy-independent confinement, Klein tunnelling, and transport along freeform (e.g., spiral) trajectories. Our results establish branch-cut defects as a distinct platform for realizing propagating Dirac defect states, extending relativistic Dirac physics from bulk and surface states to propagating modes confined to defect boundaries.
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All authors Bofeng Zhu, Chengzhi Ma, Qiang Wang, Gui-Geng Liu, Xiuhai Zhang, Zheyu Cheng, Qi Jie Wang, Baile Zhang, Y. D. Chong
Materials Science
Unconventional views on orbitronics supported by experimental results
Melissa Yactayo, A. Pezo, J. L. Ampuero et al. (+9 more)
Emerging orbitronics assumes long-range orbital current transport, analogous to
spin currents. However, recent theory and experiments challenge this view, showing rather local characters for orbital polarization and orbit-
spin conversions. We study angular momentum generated by
ferromagnetic resonance and thermal gradients in Ni/(Pt)Ti/Au
heterostructures. The observed charge current produced is independent of Ti thickness up to 60 nm, incompatible with orbital transport in Ti. Instead, its magnitude depends on both Ti interfaces, evidencing
spin-mediated transport in between after and before local orbit-
spin interconversions.
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All authors Melissa Yactayo, A. Pezo, J. L. Ampuero, M. Tian, L. Badie, J. Quispe-Marcatoma, C. V. Landauro, Y. Xu, Sébastien Petit-Watelot, Michel Hehn, A. Fert, J.-C. Rojas-Sánchez
Materials Science · Cross-listed from cond-mat.mes-hall
Dynamical diffraction formalism for imaging time-dependent diffuse scattering from coherent phonons with Dark-Field X-ray Microscopy
Darshan Chalise, Brinthan Kanesalingam, Dorian P. Luccioni et al. (+3 more)
Coherent acoustic
phonons, whose damping sets the upper bound of quality factors in acoustic resonators, play a critical role in advanced telecommunication and
quantum information technologies. Yet, probing their decay in the GHz regime remains challenging using conventional surface-based techniques. Dark-field X-ray microscopy (DFXM) offers a solution by enabling through-depth, non-destructive and full-field imaging of strain fields and dislocations inside bulk materials with high spatial and angular resolution. We previously used kinematic diffraction theory to describe DFXM signals based on how the Bragg peak shifts due to the strain wave, allowing us to reconstruct the frequency spectrum of coherent
phonons as a function of depth through the sample. The approach of tracking the Bragg peak shifts to study
phonon dynamics, however, places an upper-bound to the highest
phonon frequency that can be studied, determined by the spatial resolution of the measurement. In this work, we discuss how coherent
phonon dynamics can be studied with DFXM from time-dependent intensity oscillation sidebands. This approach simultaneously allows studying coherent
phonon dynamics in real and reciprocal space, overcoming frequency resolution limits imposed by the real-space resolution of Bragg-peak tracking. Using Takagi-Taupin dynamical diffraction formalism, we establish the spatial and reciprocal space resolution achievable for studying the coherent
phonon dynamics and evaluate conditions for observing long-lived intensity oscillations. We close by proposing experimental strategies to optimize excitation bandwidths and reciprocal-space selectivity. The formalism in the paper enables the design of DFXM experiments for quantitative, frequency-resolved measurements of acoustic
phonon decay and
phonon-defect interactions in bulk crystalline materials.
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All authors Darshan Chalise, Brinthan Kanesalingam, Dorian P. Luccioni, Daniel Schick, Aaron M. Lindenberg, Leora Dresselhaus-Marais
Materials Science
Fractional Modeling of Thermoelastic Fracture Behavior in a Cracked PZT-4 Strip under Transient Thermal Loading
Diksha, Soniya Chaudhary and Pawan Kumar Sharma
This paper investigates the thermoelastic fracture response of a transversely isotropic piezoelectric strip containing a vertical insulated crack under transient thermal shock loading and pre-existing stress fields. The analysis is conducted within the framework of generalized fractional heat conduction using the Ezzat model, which incorporates thermal relaxation and
memory-dependent effects. The problem is formulated as a mixed boundary value problem governed by fractional thermoelastic equations. The Laplace transform technique is employed to obtain temperature and coupled fields in the transform domain. The resulting system of singular integral equations is solved using the Lobatto-Chebyshev collocation method to determine the displacement discontinuity and the associated thermal stress intensity factors at the crack tips. The transient response in the time domain is recovered through numerical inversion of the Laplace transform using the Stehfest
algorithm. Numerical results for PZT-4 are presented to examine the influence of fractional order, thermal relaxation time, pre-existing stresses, and
geometric parameters on temperature distribution, thermoelastic stress fields, and stress intensity factors. The results demonstrate significant deviations from classical
Fourier predictions, revealing wave-like thermal behavior and inherent
memory effects associated with fractional heat conduction. The present formulation establishes a unified framework for the analysis of thermoelastic fracture in piezoelectric ceramics and provides insights into the design and reliability of smart structures operating under severe thermal conditions.
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Materials Science
Self-Limiting Mechanism of Anti-Stokes Optical Cooling in Diamond NV Centers
Haruki Manaka and Yasuhiro Yamada
Anti-Stokes optical cooling in diamond nitrogen-vacancy (NV) centers is experimentally and numerically investigated. Photoluminescence-excitation spectroscopy reveals pronounced
phonon-assisted anti-Stokes emission under excitation below the zero-
phonon line (ZPL). However, the below-ZPL excitation drives photoinduced charge-state conversion between negatively-charged NV- and neutral NV0, thereby suppressing the NV- mediated cooling channel. Time-resolved photoluminescence (PL) measurements reveal an increase in the effective PL lifetime with excitation density, reflecting an increasing NV0 contribution. By fitting nanosecond and millisecond PL dynamics with a minimal rate-equation model, we extract effective optical pumping and charge-conversion rates, which enables us to quantitatively simulate the cooling performance. The simulations predict a self-limiting behavior of anti-Stokes cooling and clarify the excitation conditions under which net cooling can be sustained within this effective model. The estimated cooling power per NV center is comparable, on a microscopic basis, to values discussed for
semiconductor quantum dots and rare-earth optical coolers. These results identify charge-state conversion as a key bottleneck for defect-based optical refrigeration.
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Materials Science
A Comparative Study of Molecular Dynamics Approaches for Simulating Ionic Conductivity in Solid Lithium Electrolytes
Dounia Shaaban Kabakibo, Félix Therrien, Yoshua Bengio et al. (+4 more)
Accurate prediction of ionic conductivity is critical for the design of high-performance solid-state electrolytes in next-generation batteries. We
benchmark molecular dynamics (MD) approaches for computing ionic conductivity in 21 lithium solid electrolytes for which experimental ionic conductivity has been previously reported in the literature. In particular, we compare simulations driven by density functional theory (DFT) and by universal machine-learning interatomic potentials (uMLIPs), namely a MACE foundation model. We find comparable performance between DFT and MACE, despite MACE on one GPU more than 350 times faster than DFT on a 64-CPU node. The framework developed here is designed to enable systematic comparisons with additional uMLIPs and fine-tuned models in future work.
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All authors Dounia Shaaban Kabakibo, Félix Therrien, Yoshua Bengio, Michel Côté, Hongyu Guo, Homin Shin, Alex Hernandez-Garcia
Materials Science
Graphitic-C3N4/TiO2(B) S-scheme Heterojunctions for Efficient Photocatalytic H2 Production and Organic Pollution Degradation
Xiaoyi Zhou, Min Zhang, Qiushi Wang et al. (+3 more)
Achieving both broad
solar-spectrum absorption and strong redox capability is critical for
semiconductor photocatalysts in environmental remediation and energy conversion. Herein, an S-scheme heterojunction photocatalyst is constructed by coupling TiO2(B) nanorods with g-C3N4 nanosheets. Its well-matched band structure extends light absorption from the UV to the visible region and enables efficient charge separation. Under simulated sunlight irradiation, the 40 wt% g-C3N4/TiO2(B) heterojunction delivers a H2
evolution rate of 1.98 mmol g-1 h-1 for water reduction with methanol as the sacrificial
agent, which is 1.5 and 2.0 times higher than those of pure g-C3N4 and TiO2(B), respectively. When exposed to amoxicillin wastewater instead of methanol solution, the heterojunction degrades 98.2% of amoxicillin and produces 20.70 umol g-1 of H2 within 90 min. Moreover, the heterojunction shows excellent photodegradation activity toward various organic antibiotics and dyes, owing to the S-scheme charge separation mechanism. This work highlights the promising potential of S-scheme heterojunctions for photocatalytic H2 production coupled with organic wastewater treatment.
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All authors Xiaoyi Zhou, Min Zhang, Qiushi Wang, Shiwen Du, Xuedong Jing, Zhenyi Zhang
Materials Science
Raman and Terahertz Spectroscopy of Low-Frequency Chiral Phonons in Amino Acids
Rahul Rao, Won Jin Choi, Joseph M. Slocik et al. (+6 more)
Chiral phonons are mirror-symmetric vibrations that correspond to twisting and rotational motions of atoms. In
chiral biomolecules, they correspond to low-energy terahertz (THz)-range vibrations of the molecular segments involving dozens of atoms whose energies are sensitive to the chirality of the molecules and local atomic geometries. Here we present spectral signatures of
chiral phonons in circularly polarized low-frequency Raman and Raman optical activity (ROA) spectra from
crystals of several amino acids in different enantiomeric forms. Along with complementary THz circular dichroism (TCD) measurements, our ROA data reveal two sets of bisignate peaks in valine, alanine, tyrosine and proline between 1 and 4.5 THz that are more intense than the ROA peaks in the fingerprint region. Density functional theory (DFT) calculations on L-alanine attribute these modes to twisting and shearing molecular motions. The strong agreement between the ROA and TCD data demonstrates the power of these complementary vibrational spectroscopy techniques to identify
chiral phonons in biomolecules, and offers new insights into their vibrational properties and interactions with circularly polarized light.
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All authors Rahul Rao, Won Jin Choi, Joseph M. Slocik, Thuc T. Mai, Michael A. Susner, Kelsey A. Collins, Michael J. Newburger, Petr Bouř, Nicholas A. Kotov