Materials Science · Cross-listed from physics.flu-dyn
Polymer extension at stagnation points governs flow thickening of polymer solutions in ordered porous media
Emily Y. Chen, Simon J. Haward, Amy Q. Shen et al. (+1 more)
Polymer solutions exhibit anomalous flow thickening – marked by an abrupt increase in the macroscopic flow resistance – above a threshold flow rate in a porous medium, but not in bulk solution. This phenomenon has evaded a mechanistic description for over half a century. Here, we develop a model that quantitatively links pore-scale flow fields and fluid rheology to macroscopic flow thickening, and validate it in experiments in two- and three-dimensional (2D and 3D) porous media. We find that flow thickening in ordered media is governed by
polymer extension at stagnation points – in contrast to disordered media, where viscous dissipation by unsteady flow fluctuations also contributes substantially. Our results provide a foundation to predict and control such flows in energy, environmental, industrial, and microfluidic applications.
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All authors Emily Y. Chen, Simon J. Haward, Amy Q. Shen, Sujit S. Datta
Materials Science · Cross-listed from physics.chem-ph
Bokang Hou, Johan E. Runeson, Samuel L. Rudge et al. (+4 more)
Hot-
exciton relaxation in
semiconductor nanocrystals (NCs) is often described using perturbative theories, but their accuracy is difficult to assess for realistic
exciton–
phonon Hamiltonians. Here, we
benchmark the perturbative
quantum master equation (QME) and several mixed
quantum–classical (MQC) methods against fully
quantum mechanical dynamics. Using atomistically parameterized models for CdSe core and CdSe/CdS core–shell NCs, we find that bare CdSe exhibits an ultrafast initial decay followed by slower cooling, whereas the core–shell system is dominated by the slower component. Analysis of reduced models shows that the ultrafast component arises from rapid diabatic state mixing driven by thermal fluctuations of low-frequency
phonons, rather than from
nuclear-assisted energy relaxation. The QME captures the initial fast decay but can fail for the slower relaxation in the diabatic representation, while the mapping approach to surface hopping (MASH) gives the most consistent agreement with both
benchmark dynamics and
equilibrium populations. These results establish a
benchmark for
exciton-cooling dynamics in NCs and clarify the physical regimes in which widely used approximate methods are reliable.
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All authors Bokang Hou, Johan E. Runeson, Samuel L. Rudge, Salvatore Gatto, Hans-Dieter Meyer, Michael Thoss, Eran Rabani
Materials Science
Relaxation-driven topological domains in moiré materials
Arjyama Bordoloi, Daniel Kaplan and Sobhit Singh
Spatial control of
topology is highly desirable for realizing tunable
quantum functionalities in materials. Moiré superlattices formed by twisting van der Waals
heterostructures provide a natural platform for spatially modulated electronic phases, yet the emergence of tunable topological domains in these systems remains largely unexplored. Here we show that structural relaxation in twisted
bilayer BiSb drives the formation of a distinct moiré topological phase, characterized by coexisting topologically nontrivial (Z$_2$ = 1) and trivial (Z$_2$ = 0) domains within a single moiré unit
cell. As the twist angle is reduced, relaxation-induced modulation of the interlayer separation stabilizes an expanding network of topological regions embedded within trivial backgrounds of the moiré unit
cell. The resulting internal domain boundaries host topologically-protected gapless 1D edge states that are directly visible in our simulated scanning-tunnelling microscopy maps.Furthermore, we demonstrate that the real-space topological domain structure and associated gapless edge states can be reversibly tuned by an out-of-plane electric field. Together, these results establish twisted BiSb as a promising platform for programmable topological domain patterning, where intrinsic networks of edge channels can be continuously tuned and electrically reconfigured in moiré materials.
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Materials Science
Atomic-Scale Observation of Symmetry Breaking in Altermagnetic MnTe
Guodong Ren, Jonathan M. DeStefano, Xiao-Wei Zhang et al. (+20 more)
The recent discovery of altermagnetism has sparked growing interest in compensated
magnetic systems as promising platforms for highly scalable spintronics. Altermagnetism is a distinct
magnetic order where opposite
spin sublattices are connected by rotation, yielding zero net magnetization but momentum-dependent
spin splitting. To date, experimental verification of altermagnetic order has been achieved predominantly through bulk-sensitive techniques, including
spin-dependent electronic spectra and transport responses. However, direct atomic-scale evidence that explicitly correlates
crystal symmetry, local structural distortions, and
magnetic ordering has remained unexplored. Here, we report the direct atomic-scale observation of coexisting polar distortions and altermagnetic order in MnTe, combining atomic resolution scanning transmission electron microscopy (STEM) imaging with electron
magnetic chiral dichroism (EMCD) measurements. We reveal that MnTe is not an ideal uniform P63/mmc g-wave altermagnet at the atomic scale. Instead, it hosts ubiquitous inversion-
symmetry-breaking distortions that lower the
spin-space-group (SSG)
symmetry, admits d-wave altermagnetic components, and in lower-
symmetry regimes, even allow s-wave
spin splitting (net magnetization). The coexistence of ferroelectric signatures and altermagnetic order establishes local
lattice symmetry in MnTe as a control knob for altermagnetic
spin splitting,
spin current generation, and multiferroic
memory applications.
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All authors Guodong Ren, Jonathan M. DeStefano, Xiao-Wei Zhang, Arashdeep S. Thind, Rajiv Giridharagopal, Jose Angel Castellanos-Reyes, Paul M. Zeiger, Noah Kamm, Sijie Xu, Zhaoyu Liu, Yaofeng Xie, Filip Krizek, Jan Michalicka, Richard Campion, Pengcheng Dai, Peter Wadley, David S. Ginger, Tomas Jungwirth, Robert F. Klie, Jan Rusz, Di Xiao, Jiun-Haw Chu, Juan Carlos Idrobo
Materials Science
Microstructure-Aware Deep Learning Bridges Atomistics to Macroscale for Shock-to-Detonation Prediction
Simon Gonzalez-Zapata, Aidan Pantoya, Chunyu Li et al. (+2 more)
The shock-to-detonation transition in energetic materials is governed by coupled processes spanning Angstroms to millimeters and femtoseconds to microseconds, where traditional multiscale models fail due to the lack of scale separation. We address this grand challenge by directly bridging large-scale molecular dynamics (MD) simulations with continuum finite-element (FE) models using MISTnetX, a convolutional deep neural network. Trained on MD simulations of shock propagation through complex microstructures, MISTnetX captures shock-microstructure interactions, hotspot formation, and the transition to deflagration, supplying critical sub-grid information to FE simulations of mechanics, shocks, thermal transport, and chemistry. Applied to a synthetic but realistic nanostructured plastic-bonded RDX composite, MISTnetX enables parameter-free prediction of the full run-to-detonation transition.
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All authors Simon Gonzalez-Zapata, Aidan Pantoya, Chunyu Li, Marisol Koslowski, Alejandro Strachan
Materials Science
Atomically precise mechanosynthesis of carbon structures on hydrogenated Si(100) by inverted-mode STM
Megan Cowie, Chris Deimert, Ryan Groome et al. (+44 more)
The ability to build atomically precise structures on surfaces with complete control over both atomic placement and chemical bonding remains a central challenge in nanoscale fabrication. Here, we demonstrate simultaneous spatial and chemical control over the mechanosynthetic fabrication of carbon structures. Using inverted-mode STM, C$_2$ units are donated from surface-deposited molecules to pre-patterned reactive sites on a hydrogen-passivated Si(100) surface. We demonstrate single-site C$_2$ donation, spatially patterned multi-site C$_2$ donation, and the stepwise assembly of polyyne structures through successive C-C bond formation. Together, these results establish controlled mechanosynthetic donation as a foundational capability for programmable atomically precise fabrication.
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All authors Megan Cowie, Chris Deimert, Ryan Groome, Alex Inayeh, Robert J. Kirby, Cameron J. Mackie, Jonathan Myall, Sam Rohe, Luis Sandoval, Khalil Sayed-Akhmad, Bheeshmon Thanabalasingam, Reid Wotton, Rafik Addou, Aly Asani, Brandon Blue, Adam Bottomley, Kareem A. Clarcia, Tyler Enright, James Zhangming Fan, Robert A. Freitas Jr., Alan T.K. Godfrey, Si Yue Guo, Aru Hill, Taleana Huff, Mark Jobes, Hadiya Ma, Adam C. Maahs, Oliver MacLean, Steven M. Maley, Michael Marshall, Terry McCallum, Ralph C. Merkle, Mathieu Morin, Ryan Plumadore, Henry Rodriguez, Marc Savoie, Benjamin Scheffel, Janice L. Wong, Damian G. Allis, Jeremy Barton, Michael Drew, Matthew R. Kennedy, Tait Takatani, Marco Taucer, Dusan Vobornik, Ryan Yamachika, Mathieu Durand
Materials Science
At-Scale Discovery of High-Voltage Cathode Active Materials for Na-Ion Batteries via Charged-Only Structures and Committee Divide-and-Conquer Learning
Suchona Akter, Md Omar Faruque and Mohammad R. Momeni
Current cathode-active materials (CAMs) designed for sodium-ion batteries (SIBs) suffer from low operating voltages, limited energy density, irreversible phase transitions, and low intrinsic electrical conductivity. In this work, we have developed generalizable and transferable descriptor-based machine learning (ML) models, along with composition- and graph-based neural networks (NNs) and transfer learning (TL) models, all trained on a
dataset of charged-only structures rather than charged/discharged pairs. Using a committee of the top four trained ML models, the mean voltage is predicted as the target property. More importantly, we show that uncertainty quantification can improve the reliability of candidate selection for training subsequent higher-generation models by including structures for which the current committee models are least confident. In this way, the new expensive ab initio data provide information in regions where the original models most need improvement. We have showcased the transferability of the final generation models by applying them to a large database of Na-free inorganic host structures curated from Materials Project, AFLOW, OQMD, and GNoME for at-scale SIB CAM discovery. A comprehensive interactive online database, NaCAM-Atlas, was also developed, containing 14,843 CAMs with voltages greater than 3 V for SIBs. Finally, a subset of the top-ranked candidate CAMs was validated through explicit, high-throughput first-principles calculations of voltage profiles, structural robustness upon sodiation/desodiation, and electronic properties. Together, this integrated data curation, ML training and improvement, and first-principles validation strategy establishes a scalable and transferable framework for accelerating the discovery of stable, high-voltage CAMs for SIBs and beyond.
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Materials Science
Disordered vs. Short-Range-Ordered Complexions: Consequences for Grain-Boundary-Mediated Plasticity in Nanocrystalline Al-Ni Alloys
Frederic Sansoz and Eve-Audrey Picard
Disordered grain-boundary (GB) complexions in thermally stable nanocrystalline
alloys are commonly assumed to be structurally uniform, yet their amorphous nature makes them susceptible to local short-range ordering (SRO). The influence of local SRO on GB-mediated plasticity mechanisms in such complexions remains poorly understood. This article employs large-scale Monte Carlo and molecular dynamics simulation to address this gap through simulations of nanocrystalline Al-Ni
alloys at two Ni concentrations, 2 at.% and 4 at.%. Annealing at 913 K produces thick uniform disordered intergranular film complexions, while annealing at 378 K produces semi-disordered complexions containing FCC-type and BCC-type SRO. These two complexion states produce fundamentally different mechanical responses. Amorphous complexions act as dislocation sinks, promoting homogeneous plasticity through shear transformation zones, but at the cost of intense shear localization and lower strength. SRO complexions generate reduced shear localization and higher strength but also promote heterogeneous stress concentrations across the GB network, regardless of Ni concentration. This contrast reflects a fundamental shift in governing mechanism, from shear-transformation-zone-controlled behavior in disordered complexion
alloys to GB-stress-heterogeneity-controlled behavior in SRO complexion
alloys. These findings highlight the potential of complexion engineering to tailor the mechanical properties of nanocrystalline materials.
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Materials Science · Cross-listed from cond-mat.mes-hall
High-mobility inertial domain walls driven by spin-transfer torque in a ferrimagnetic spinel oxide
Mingxing Wu, Shilei Ding, Laura van Schie et al. (+8 more)
Efficient electrical manipulation of domain walls is key to developing
magnetic devices with fast switching capabilities and low energy consumption. Here we demonstrate Bloch-type domain wall velocities exceeding 1 km s$^{-1}$ in the single-layer ferrimagnetic spinel
oxide NiCo$_2$O$_4$ induced by
spin-transfer torque at a current density of $2 \times 10^{11}$ A m$^{-2}$. This exceptional domain wall mobility is attributed to the combination of
giant nonadiabatic
spin-transfer torque, low magnetization, and high
spin polarization. Additionally, we report a pronounced domain wall inertia effect in this ferrimagnet due to the large nonadiabaticity of the torque. The characteristic time for domain wall acceleration and deceleration is $\sim 1$ ns, shorter than that reported for typical ferromagnets. Our findings highlight the potential of spinel
oxides as a promising platform for engineering high-performance domain wall devices that take advantage of ultrafast ferrimagnetic dynamics.
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All authors Mingxing Wu, Shilei Ding, Laura van Schie, Shenghao Cai, Yuhao Qiu, Ao Du, Alexander E. Kossak, Rui Wu, Christian L. Degen, Xuegang Chen, Pietro Gambardella
Materials Science
Rapid estimation of synthesizability windows of inorganic materials from first principles
Finja Tadge, Javier Sanz Rodrigo and Andrea Crovetto
Fast prediction of the synthesizability conditions of materials remains challenging, even assuming synthesis under thermodynamic
equilibrium. We combine density functional theory (DFT) with machine-learned interatomic potentials to enable high-throughput generation of phase predominance diagrams as a function of temperature and partial pressures of the gaseous reactants. These diagrams can immediately be used by experimentalists to translate computational predictions into real synthesis parameters in the lab. Predominance diagrams are generated for a diverse set of binary compounds and for 48 more complex ternary metal phosphosulfide systems, but the method is in principle scalable to any inorganic material class. The calculated predominance diagrams generally show good agreement with the experimental synthesis literature, with a drastic reduction in computational cost compared to a full DFT approach. We find several examples of compounds that appear as metastable in a zero-temperature stability hull picture, but that become thermodynamically stable under well-defined synthesis windows.
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Materials Science
Simplified criteria for identifying and designing altermagnetic crystals in real space
Ying Chen, Qiushi Huang, Yu Wu et al. (+2 more)
Altermagnetism is a compensated
magnetic phase characterized by zero net magnetization and exchange-driven
spin splitting. However, identifying altermagnets among collinear antiferromagnets usually requires full
magnetic-space-group or
spin-group analysis, which is not always intuitive. Here we formulate a simple real-space criterion based on how the crystallographic operations of the host nonmagnetic structure permute the two opposite-
spin sublattices. We show that altermagnets usually exist on collinear compensated antiferromagnets whose
magnetic primitive
cell coincides with the host nonmagnetic crystallographic primitive
cell. In this case, altermagnetic
spin splitting is generally allowed unless an inversion-type operation exists that exchanges the two opposite-
spin sublattices. Using chemically ordered Mn2SSe prototypes derived from zinc-blende or rocksalt parent structures, we demonstrate that these criteria can be easily used to construct the three
symmetry classes by controlling chemical ordering and
magnetic-sublattice permutation. Similar rules can also be applied to low-dimensional
crystals or quasicrystals. Our work reduces the identification of altermagnets to a transparent real-space
symmetry test and provides a practical route for designing altermagnetic
crystals.
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All authors Ying Chen, Qiushi Huang, Yu Wu, Xiaolan Yan, Su-Huai Wei
Materials Science
Light-induced Faraday effect from dynamical breakdown of Kleinman symmetry
Niccolò Sellati, Jacopo Fiore and Lara Benfatto
The observation of anomalously large polarization rotations in pump-probe experiments with circularly polarized light has recently challenged the conventional understanding of the inverse Faraday effect. The striking magnitude of these responses implies the generation of effective
magnetic fields orders of magnitude larger than theoretical expectations, raising fundamental questions about the nature of light-induced time-reversal
symmetry breaking. In this work we demonstrate that a static polarization rotation can originate entirely from the antisymmetric component of the third-order optical susceptibility, without generating a macroscopic magnetization of the material. We show that this light-induced Faraday effect is inherently dynamical, emerging when Kleinman
symmetry breaks down. Using a minimal $sp$ tight-binding model on a square
lattice, we demonstrate that the light-induced Faraday response can be sizable even far from dissipative resonances. While the effect emerges at a purely electronic level, we show that resonant coupling with
phonons can significantly enhance the pump-probe response. Finally, we demonstrate that the light-induced Faraday contribution can strongly exceed the magneto-optical response mediated by the inverse Faraday effect in nonmagnetic materials.
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Materials Science · Cross-listed from physics.ins-det
A Levitated Random Telegraph Noise Spectrometer
Molly Message, Bianca C. J. Uy, Katie O'Flynn et al. (+7 more)
Random Telegraph Noise is a ubiquitous process manifesting across technology and the natural world. It is characterized by random jumps between two distinct states with Poissonian waiting times, and is the origin of 1/f noise. Understanding and characterizing this noise is critical for the reliable operation of micro-, nano- and
quantum-technologies. In this work we probe random telegraph noise using a levitated microparticle sensor whose dynamics are driven almost entirely by this non-white source of noise. We observe a startling resonant behaviour, characterized by a thousand-fold increase in the underdamped sensor's position fluctuations, enabling us to measure the spectral properties of the noise over six decades of timescale. This work not only provides a unique way to probe random telegraph noise, but also demonstrates a platform for studying non-
equilibrium stochastic dynamics in the presence of realistic non-white noise, with applications from biology to social behaviour.
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All authors Molly Message, Bianca C. J. Uy, Katie O'Flynn, Yugang Ren, Muddassar Rashid, Jonathan D. Pritchett, Qiongyuan Wu, Hyukjoon Kwon, Benjamin A. Stickler, James Millen
Materials Science
Origin of the Temperature-Induced Gap Bowing of Formamidinium-Methylammonium Lead Iodide Perovskites: Role of Cationic Rattlers
Kai Xu, Adrián Francisco-López, Bethan L. Charles et al. (+4 more)
A thorough understanding of the temperature dependence of
semiconductor band gaps is essential for optimizing optoelectronic devices. In this respect, the origin of the pronounced temperature-induced gap bowing observed in low-temperature phases of formamidinium-methylammonium (FA-MA) lead iodide perovskites has remained elusive until now. By combining temperature and pressure-dependent photoluminescence measurements on a series of FA$_x$MA$_{1-x}$PbI$_3$ mixed-cation single
crystals with $x\in[0,1]$, we unravel the origin of this bowing. Both thermal expansion as well as electron-
phonon interaction effects are responsible. However, the latter is the leading term, driven by the activation of an anomalous electron-
phonon coupling mechanism linked to mixed vibrational modes, which combine inorganic-cage
phonons involving octahedral tilting with low-frequency FA librations, i.e., FA rattler modes. This occurs in the orthorhombic and (pseudo)tetragonal low-temperature phases, presumably featuring stripe domains with alternating octahedral tilt-axis patterns for FA concentrations between 20% and 90%. In this way, we have shed light on an intriguing behavior of lead halide perovskites that directly affects their optoelectronic properties.
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All authors Kai Xu, Adrián Francisco-López, Bethan L. Charles, M. Isabel Alonso, Miquel Garriga, Mark T. Weller, Alejandro R. Goñi
Materials Science
Electron Polaron at Neutral 180$^\circ$ Domain Wall in PbTiO$_3$: Stability, Trapping Energies, and Transverse Polarization
Mohammad Amirabbasi, Jochen Rohrer and Karsten Albe
We use density-functional theory with a Hubbard correction to investigate Ti-centered electron polarons at neutral PbO-centered $180^\circ$ domain walls in tetragonal PbTiO$_{3}$. The Hubbard parameter for Ti $3d$ states is determined using the finite-size-corrected polaronic energy-level
alignment procedure, yielding stable electron-polaron formation in bulk PbTiO$_3$ with a trapping energy of $-$0.06 eV. In the domain-wall supercell, the excess electron localizes on Ti and forms a Ti$^{3+}$ center with an occupied $d_{xy}$ orbital in-gap state. Comparison of bulk-like and near-wall Ti sites shows that their trapping energies differ by only about 0.01 eV, indicating that this neutral domain wall does not provide a significant thermodynamic driving force for electron-polaron segregation. While the Ising-like reversal of the out-of-plane polarization is preserved, the localized electron induces a finite transverse polarization component normal to the wall, enhancing a local Néel-like distortion that is strongest when the polaron is located at the wall. These results show that neutral $180^\circ$ domain walls in PbTiO$_3$ do not substantially alter the stability of Ti-centered electron polarons, but they can couple to the polaron-induced
lattice distortion through a localized transverse polarization response.
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Materials Science
Molecular Dynamics Study of Defect Evolution Mechanisms in 3C-SiC for Quantum Technologies
Irslan Ullah Ashraf, Gaetano Calogero, Ioannis Deretzis et al. (+4 more)
The migration of point defects and formation of
spin defects in 3C-SiC were investigated using molecular dynamics simulations, with migration barriers obtained from Nudged Elastic Band (NEB) calculations and finite temperature diffusivities evaluated using both mean square displacement (MSD) and jump frequency approaches. While both methods reproduce Arrhenius behavior, the jump frequency formulation exhibits improved statistical stability. Activation energies of 2.12~eV for carbon vacancies and 0.88~eV for carbon interstitials are obtained, consistent with literature. The resulting mobility hierarchy governs defect
evolution and complex formations. Interstitial vacancy recombination competes with vacancy aggregation into divacancies, influencing the stabilization of
spin active defect centers. The study also provides a consistent framework for
diffusion analysis in atomistic simulations.
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All authors Irslan Ullah Ashraf, Gaetano Calogero, Ioannis Deretzis, Giorgio Lo Presti, Damiano Ricciarelli, Elisabetta Paladino, Antonino La Magna
Materials Science
Individual Characterization of Fast-Responding Trap States at the NO-Annealed SiO$_2$/4H-SiC Interface
Takahiro Ono, Mizuki Ohashi, Tomohiro Shigeno et al. (+4 more)
Fast-responding trap states introduced by NO-annealing are suspected to limit the channel mobility of 4H-SiC MOSFETs, yet their microscopic characterization remains challenging because conventional electrical methods are spatially averaged and do not readily isolate such fast processes. Here, we visualize and analyze individual fast-responding trap states at the NO-annealed SiO$_2$/4H-SiC interface using the energy dissipation signal in frequency-modulation atomic force microscopy (FM-AFM), which selectively probes charge-exchange dynamics on sub-$\mu$s time scales. Ring-shaped dissipation patterns were observed in the NO-annealed sample but not in the control sample without NO-annealing, indicating that the detected states are associated with nitridation. Spectroscopic measurements were also performed to determine the dependence of energy dissipation on the tip bias and the tip-sample distance. Combined with finite-element electrostatic calculations, this analysis allowed us to determine trap energies relative to the
Fermi level, $E_t - E_f$, and revealed that the trap-energy distribution extends toward the interfacial conduction-band edge. These results provide microscopic evidence that NO-annealing generates fast-responding trap states near the SiO$_2$/4H-SiC interface.
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All authors Takahiro Ono, Mizuki Ohashi, Tomohiro Shigeno, Yutaro Uchida, Yuuki Yasui, Koji Kita, Yoshiaki Sugimoto
Materials Science · Cross-listed from physics.optics
Probing the Effects of Heat Treatment Atmosphere on the Structural and Electrical Properties of NBT via Eu Photoluminescence
Zongxue Wang, Duanting Yan and Hancheng Zhu
The effect of oxygen partial pressure during the pre-calcination step (high
vacuum, air, nitrogen, and oxygen) on the
crystal structure, microstructure, and electrical properties of Na0.5Bi0.465Sr0.02Eu0.005TiO3
oxide-ion-conducting ceramics was systematically investigated. Dense ceramic samples were prepared by a conventional solid-state reaction route under different atmospheres. The results show that the oxygen partial pressure strongly affects Bi volatilization, grain growth, and oxygen-vacancy concentration. The largest average grain size was obtained for the nitrogentreated sample, whereas the oxygen-treated sample exhibited the finest grains but the highest grain-boundary conductivity. X-ray diffraction and Raman spectroscopy indicate that low oxygen partial pressure enhances structural disorder, while high oxygen partial pressure stabilizes the
lattice and promotes charge-transfer transitions. Eu3+ photoluminescence further reveals the correlation between local structural
evolution and oxygen-vacancy concentration. These findings clarify how oxygen partial pressure regulates grain-boundary behavior and ion-transport mechanisms in NBT-based
oxide-ion conductors, providing guidance for optimizing the total conductivity of polycrystalline electrolytes.
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Materials Science
Temperature dependence of the magnon-phonon coupling in yttrium iron garnet/gadolinium gallium garnet high overtone bulk acoustic resonators
Johannes Weber, Manuel Müller, Mikhail Cherkasskii et al. (+7 more)
Weexperimentally study the temperature dependence of the
magnon-
phonon coupling in a yttrium iron garnet (YIG)/gadolinium gallium garnet (GGG)
heterostructure. More specifically, we use broadband
ferromagnetic resonance to investigate the magneto-elastic coupling between the Kittel mode of a YIG thin film and the transverse acoustic
phonon modes of the YIG/GGG high overtone bulk acoustic wave acoustic resonator for in and out-of-plane field directions in the temperature range between T = 5K and 300K. We find that for a
magnetic field applied normal to the film surface, magneto-elastic coupling decreases with decreasing temperature, whereas it increases for the in-plane
magnetic field configuration. The observed temperature dependence differs from earlier observations on bulk YIG samples, which might be due to the temperature dependent stress imposed by the GGG substrate.
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All authors Johannes Weber, Manuel Müller, Mikhail Cherkasskii, Stephan Geprägs, Rudolf Gross, Christian H. Back, Sebastian T. B. Goennenwein, Silvia Viola Kusminskiy, Matthias Althammer, Hans Huebl
Materials Science
Active learning of collinear magnetic Moment Tensor Potentials using the spin-MLIP package from soft-constrained spin-polarized DFT calculations: a case study of Fe-Pd
Arseniy Burov, Alexey S. Kotykhov, Dmitry A. Aksyonov et al. (+2 more)
Explicit incorporation of
magnetic degrees of freedom in machine-learning interatomic potentials (
magnetic MLIPs) plays a crucial role in the correct description of
magnetic materials and their properties. An important ingredient for fitting of
magnetic MLIPs is
spin-polarized density functional theory (DFT) calculations with non-
equilibrium magnetic moments, i.e. DFT calculations with constraints on
magnetic moments. In this study, we present a workflow for active learning of
magnetic Moment Tensor Potential (mMTP) during molecular dynamics (MD) simulations.
Magnetic MTP and its active learning
algorithm were implemented in the open-source
spin-MLIP code, DFT soft-constrained
spin-polarized calculations were performed with the VASP code, and MD simulations were conducted in the open-source LAMMPS code. We test our workflow on the Fe-Pd
crystal. The dependencies of magnetization and
phonon density of states (DOSs) on the volume of a supercell (or, pressure) are in good agreement with those calculated with DFT. Furthermore, the calculated DOSs correspond to the experimental ones.
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All authors Arseniy Burov, Alexey S. Kotykhov, Dmitry A. Aksyonov, Ivan S. Novikov, Vladimir V. Ladygin
Materials Science · Cross-listed from cond-mat.mes-hall
Orbital Magnetization from Uniform and Periodic Magnetic Fields
Chunli Huang
Magnetization is thermodynamically defined as the derivative of the grand potential with respect to a uniform
magnetic field. However, a uniform
magnetic field makes the kinetic momentum operators noncommuting and Landau-quantizes the electron motion. This changes the zero-field momentum-space to Landau-levels and raises a fundamental question: how can the thermodynamic response to a uniform field be reproduced by a linear-response calculation carried out in the momentum space of the zero-field problem? We address this question analytically in a
quantum Hall ferromagnet that allows the orbital magnetization $M$ to be computed in a closed form. We first compute $M$ from the local Hartree–Fock projector response to a periodic
magnetic field with zero net flux. We then compute $M$ from the derivative of the grand potential with respect to a uniform
magnetic field along the Středa line. The two approaches give the same result, even though the first keeps the
Hilbert space fixed while the second changes the Landau-level degeneracy. Their agreement suggests that we should view orbital magnetization as the energy associated with the spectral flow that gives rise to the Středa formula. Our work provides a tutorial introduction to orbital magnetization and its relation to the Středa formula.
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Materials Science
Ultrafast signatures of Dirac / flat-band hybrid states from time-resolved ARPES
Maria-Elisabeth Federl, Johannes Gradl, Franziska Bergmeier et al. (+12 more)
Hybridization of highly itinerant Dirac electrons with localized flat-band states is predicted to yield emergent phenomena such as exotic heavy-
fermion behaviour. Epitaxial
graphene on two-dimensional adsorbate structures on SiC(0001), which host flat bands, offers a promising platform to explore these effects. However, direct experimental evidence of interlayer hybridization in such systems has so far been lacking. Here, we address this gap using time- and angle-resolved photoemission spectroscopy (trARPES) where interlayer hybridization manifests in three key observations: (1) accelerated Dirac-carrier relaxation arising from additional electronic and phononic decay channels provided by the flat-band subsystem, (2) transient charging of the Dirac cone enabled by direct optical excitation from the flat bands, and (3) ultrafast back-transfer of charge into the flat bands on timescales governed by the interlayer coupling strength. We further demonstrate that the degree of hybridization can be tuned via the atomic number of the atoms intercalated at the
graphene-SiC interface, establishing a controllable platform for investigating exotic correlated ground states.
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All authors Maria-Elisabeth Federl, Johannes Gradl, Franziska Bergmeier, Leonard Weigl, Lukas Bruckmeier, Theresa Glaser, Zamin Mamiyev, Christoph Tegenkamp, Niclas Tilgner, Thomas Seyller, Teresa Tschirner, Domenica Convertino, Stiven Forti, Camilla Coletti, Isabella Gierz
Materials Science
Electronic properties governing the phase stability and elastic anisotropy of C14 and C15 Cr-Hf-Nb Laves phases
M. Díaz-Choque, C. F. Schuch and L. T. F. Eleno
This study utilizes Density Functional Theory (DFT) to investigate the thermodynamic stability, elastic anisotropy, and electronic properties of C14 and C15 Laves phases within the Cr–Hf–Nb system. Both formation enthalpies and comprehensive elastic property analyses confirm the energetic and mechanical stability of the C14 (HfNb$_2$, HfCr$_2$, NbCr$_2$) and C15 (HfCr$_2$, NbCr$_2$) phases. Furthermore, the evaluation of elastic anisotropy reveals a descending order of HfCr$_2$ > NbCr$_2$ > HfNb$_2$ for the C14 phase, contrasting with NbCr$_2$ > HfNb$_2$ > HfCr$_2$ for the C15 phase. Finally, electronic structure and COHP analyses indicate that strong anti-bonding behavior near the
Fermi level within the XM$_2$ M–M bonds acts as a primary destabilization mechanism for both of these Laves phases.
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Materials Science
Radiative electronic bound states in the continuum from defects in semiconductors
Seong Yun Hong, Liang Z. Tan, Ki Hoon Lee et al. (+2 more)
Continuum-buried defect states in
semiconductors are generally expected to be optically inactive due to their strong coupling to continuum bands. Here, we show that such defects can instead host radiative electronic bound states in the continuum (BICs), using the silicon G-center as a prototypical example. Hybrid-functional first-principles calculations with a Hubbard $U$ correction reveal that a localized defect state, initially buried below the valence band maximum (VBM) in the ground state, undergoes exchange-driven energy-level reordering under optical excitation and shifts above the VBM. This exchange-induced transition suppresses nonradiative decay and enables robust radiative emission. By computing temperature-dependent nonradiative lifetimes and comparing them with experimental photoluminescence (PL) lifetimes, we quantitatively reproduce the observed temperature dependence of the emission. These results uncover a stabilization mechanism for continuum-embedded defect states and establish electronic BICs as a general paradigm for designing defect-based optical systems, including
quantum emitters and
qubits.
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All authors Seong Yun Hong, Liang Z. Tan, Ki Hoon Lee, Youngho Kang, Yeonghun Lee
Materials Science · Cross-listed from cond-mat.mes-hall
Terahertz spin-current transparency through rough interfaces
Jiří Jechumtál, Jakub Zázvorka, Ondřej Novák et al. (+8 more)
Spin transport across interfaces is critical for spintronic devices, yet remains difficult to probe on ultrafast timescales. We use terahertz emission spectroscopy on Co|Pt
heterostructures whose interface roughness is tuned through the thickness of an underlying Au buffer layer, while leaving other growth parameters unchanged. From the measured THz electric field, we extract the interface
spin-current transparency ts after correcting for the changes in sample impedance and optical absorption of the stack. Surprisingly, we find that ts decreases by only approximately 30% as the interface root-mean-square roughness and the lateral grain size both increase by a factor of three, with no measurable change in the THz spectrum. These results demonstrate that interfacial
spin transport is relatively robust against morphological variations on ultrafast timescales, establishing terahertz emission spectroscopy as a reliable probe of
spin dynamics across imperfect interfaces.
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All authors Jiří Jechumtál, Jakub Zázvorka, Ondřej Novák, Martin Rejhon, Peter Kubaščík, Lukáš Nowak, Petr Němec, Eva Schmoranzerová, Martin Veis, Lukáš Nádvorník, Zdeněk Kašpar