Materials Science
Quantifying Thermal, Photovoltage, and Defect Contributions to Transient Absorption of Ta$_{3}$N$_{5}$ Photoanodes
Johannes Dittloff, Lukas M. Wolz, Matthias U. Quintern et al. (+4 more)
Ta$_{3}$N$_{5}$ is among the most intensively studied photoanode materials for
solar-driven water oxidation, yet its performance often remains limited by short carrier lifetimes and defect mediated recombination. Although transient absorption spectroscopy is widely used to probe carrier dynamics in photoelectrodes, spectral assignments are frequently ambiguous due to overlapping contributions. Here, microsecond-to-second transient absorption of Ta$_{3}$N$_{5}$ thin films is combined with complementary optical spectroscopies to disentangle contributions from
lattice heating, electrostatics, and defect states. Photoreflectance reveals three critical points in the Ta$_{3}$N$_{5}$ band structure, including two anisotropic near-edge transitions at 2.14 eV and 2.27 eV and a higher-lying transition near 2.80 eV, all closely aligned with dominant transient absorption features. A previously unreported photo-induced absorption at 2.80 eV is attributed to pump-induced
lattice heating, while potential-dependent measurements reveal that near-edge bleach features arise from pump-induced band flattening and subsequent surface photovoltage relaxation. Fitting transient absorption spectra with independently measured thermal and electrostatic components enables quantification of both thermal and photovoltage dynamics, while the sub-bandgap response provides insight into the redistribution of defect charge states. Thus, this approach to quantifying thermal, electrostatic, and defect-mediated contributions to microsecond-to-second transient absorption provides broadly applicable insights into photoexcitation and relaxation mechanisms in functional
semiconductor photoelectrodes.
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All authors Johannes Dittloff, Lukas M. Wolz, Matthias U. Quintern, Laura I. Wagner, Matthias Kuhl, Johanna Eichhorn, Ian D. Sharp
Materials Science
Anharmonic phonon coupling enabled by local inversion symmetry breaking at domain walls in ferroelastics
Seyyed Jabbar Mousavi, Vivek Unikandanunni, Niccolo Sellati et al. (+7 more)
In ferroelastic materials, spontaneous
symmetry breaking leads to the formation of twin domains. Although the bulk
crystal typically remains centrosymmetric, inversion
symmetry can be locally broken at the domain walls, potentially changing
phonon selection rules and enabling local anharmonic
phonon coupling. Here we report direct evidence of such anharmonic coupling in ferroelastic LaAlO$_3$ using two-dimensional Raman-terahertz spectroscopy. We attribute the cross-peaks observed in the two-dimensional spectra to both mechanical and electrical anharmonicity between the $A_{1g}$ Raman-active
phonon and the $E_g$
phonon, which acquires finite infrared activity through local inversion
symmetry breaking at ferroelastic domain walls. These findings provide new insight into the complex
lattice dynamics of ferroelastic materials and highlight the potential of two-dimensional Raman-terahertz spectroscopy to uncover subtle
symmetry breaking through the detection of intrinsically weak anharmonic signals.
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All authors Seyyed Jabbar Mousavi, Vivek Unikandanunni, Niccolo Sellati, Paolo Barone, Martina Basini, Steven L. Johnson, Andrey Shalit, Peter Hamm, Mattia Udina, Thomas Feurer
Materials Science · Cross-listed from cond-mat.str-el
Dimensionality-Driven Electronic and Orbital Transitions Mediating Interfacial Magnetism in LaNiO3/CaMnO3 Observed In Situ
B-A. Courchene, A. Hampel, S. Beck et al. (+16 more)
Emergent
magnetic states at
oxide interfaces arise from the interplay of charge transfer, orbital reconstruction, and dimensional confinement, offering a route to engineered correlated-electron behavior in nanoscale spintronic materials. Here, we combine in situ synthesis, polarization-dependent angle-resolved photoelectron spectroscopy, X-ray
magnetic circular dichroism, and first-principles electronic-structure calculations to investigate LaNiO3/CaMnO3 superlattices. We show that reducing the LaNiO3 thickness drives a metal-
insulator transition accompanied by loss of electronic coherence and an orbital-polarization crossover in the ultrathin limit. These changes weaken charge transfer across the interface and suppress the interfacial Mn
magnetic moment in CaMnO3, revealing that the emergent
ferromagnetic state is directly governed by electronic confinement in LaNiO3. The insulating state and orbital reconstruction are reproduced by density functional theory combined with dynamical mean-field theory. Together, these results establish a direct and tunable coupling among electronic, orbital, and
magnetic degrees of freedom in
oxide heterostructures.
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All authors B-A. Courchene, A. Hampel, S. Beck, J. R. Paudel, J. D. Grassi, L. A. Lapinski, A. M. Derrico, M. Terilli, M. Kareev, C. Klewe, A. Gloskovskii, C. Schlueter, S. K. Chaluvadi, F. Mazzola, I. Vobornik, P. Orgiani, J. Chakhalian, A. J. Millis, A. X. Gray
Materials Science · Cross-listed from cond-mat.other
A nanoionic diode: Equilibrium rectifying junction enabling large and stable resistance variations
Chuanlian Xiao and Joachim Maier
We report on a new type of rectifier which is in full contact
equilibrium and thus, if down-sized to the nanoscale, shows no drift even if exposed to elevated temperatures and/or extreme waiting times. This is in contrast to existing diodes which rely on frozen doping profiles and are hence non-
equilibrium devices. Our rectifiers are related to Schottky diodes but employ "dopants" whose mobilities are high enough to follow the electrical field quickly but low enough to not compete with the electrons in terms of conductivities. In order to realize such a device based on mixed conductors, we use nanosized TiO2 films on Ru as a substrate which can store Li at the interface according to a job-sharing mechanism (Li-ions on the TiO2 side, electrons on the Ru side). The excellent functionality of this nanoionic device is demonstrated (e.g., current on-off ratio can exceed 6-7 orders of magnitude) and the additional advantages stressed (such as ease of preparation and tuning the characteristics electrochemically).
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Materials Science
Fragment-Constrained Charge Equilibration for Charge-Aware Machine Learning Potentials at Electrochemical Interfaces
Akhil Reddy Peeketi, Blas P Uberuaga and Travis E Jones
Predictive simulation of electrochemical interfaces requires atomistic models that capture reactive bond rearrangements, long-range electrostatics, and charge distributions reflecting the electronic distinctness of electrode and electrolyte. Existing charge-aware machine-learned interatomic potentials (MLIPs) built on global charge equilibration (QEq) settle electrode and electrolyte at a common electrochemical potential, leaving no room for the interfacial gradient that the double layer requires and admitting spurious charge transfer between electronically disconnected regions. Per-fragment charge equilibration is the established remedy in classical molecular dynamics, but reliance on predefined molecular
topology has confined it to non-reactive systems. We lift this restriction by making fragment identification itself a differentiable function of atomic geometry, yielding soft fragment-constrained charge equilibration (Soft-FQEq) – a solver layer that restores fragment-resolved charge conservation in reactive MLIPs. The layer consumes four scalar MLP readouts from a shared atomic-feature network – per-atom electronegativity, source charge, short-range energy, and a soft bond connectivity – and returns equilibrated charges together with per-fragment chemical potentials. We implement Soft-FQEq as an extension of the hippynn framework on a HIP-NN feature network and train it on DFT energies, forces, and DDEC6 charges for IrO2/H2O/Na+/ClO4- interfaces. The trained model recovers a clear electrode-to-electrolyte gradient in the per-atom electrochemical potential. With the same trained weights but the fragment-constrained solver replaced by global QEq at inference, this gradient collapses to an essentially uniform profile, directly showing that the gradient cannot be sustained within global QEq while the fragment formulation recovers it.
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Materials Science
Generation of magnetic metal-organic frameworks
Alexander C. Tyner, Avinash Pathapati and Alexander V. Balatsky
The potential to utilize metal-organic frameworks as a replacement for rare earth materials as well as in technological applications has prompted increased interested in this material class. The simulation of organic materials, including metal-organic frameworks (MOFs), represents a computational challenge due to an increased average number of atoms in the unit
cell. Compounding this challenge, modern materials databases are generally limited to inorganic structures due to their utility in modern technologies such as batteries and integrated
circuits. Machine-learning tools appear ideally suited to study these systems. However, organic materials are generally underrepresented in the training sets of foundational models. In this work we leverage the the Organic Materials Database (OMDB) to create a training
dataset comprised of more than 15,000 single-point first-principles computations for finetuning machine learned interatomic potentials. Specifically, we fine tune CHGNet and implement a site substitution workflow to identify novel, highly
magnetic, MOFs from structural prototypes within the QMOF database.
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Materials Science
Theory for the mixed alkali effect in glasses
Justus Leiber, Quinn Emilia Fischer, Sven Lohmann et al. (+1 more)
The mixed alkali or mixed mobile ion effect in glasses manifests itself by strong nonlinear variations of ionic transport properties upon mixing of different types of mobile ions. We develop a theory for this effect based on thermally activated hopping transport in disordered site energy landscapes that consistently incorporates the statistical-mechanical and kinetic aspects of a mobile ion mixture. This includes a consideration of the joint probability density of site energy states, generalized
Fermi distributions for mean site occupations, and cross-terms in the current response described by nondiagonal Onsager coefficients. The theory shows that a mixed alkali effect can arise even when the two ion species share identical site energy distributions. It suffices that sites have distinct energies when occupied by ions of different type. Taking into account that a mismatch energy is needed for ions of one type to occupy sites adapted to the other type, the mixed alkali effect becomes stronger. Spatial correlations between site energies are needed for the mobility of the majority ion to decrease stronger than exponential upon replacement by the minority ion. The theory agrees well with kinetic Monte Carlo simulations. Application to mixed alkali phosphate glasses yields good agreement with measured conductivity activation energies.
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All authors Justus Leiber, Quinn Emilia Fischer, Sven Lohmann, Philipp Maass
Materials Science · Cross-listed from physics.optics
Second harmonic generation and third harmonic generation in topological insulator-based van der Waals metamaterials
Alessandra Di Gaspare, Sara Ghayeb, Craig Knox et al. (+3 more)
High-order harmonic generation (HHG) in solids - the frequency up-conversion of an optical signal - is governed by
symmetries. At terahertz (THz) frequencies, HHG is a key technology to access high frequency spectral windows that are usually difficult to cover using conventional solid state
laser technologies. This effect has been recently exploited in
graphene where HHG has been demonstrated, albeit only at odd multiples of the driving frequency owing to its inherent centro-
symmetry. In topological
insulators (TIs), the combination of
spin-orbit interaction and time-reversal
symmetry create an insulating bulk state with an inverted band order, inseparably connected with conducting surface states. TIs have been predicted to support unconventional high harmonic generation from the bulk and topological surface, which are usually difficult to be distinguished. However, no experimental results have been provided, so far. Here, we exploit the strong optical field amplification provided by an array of single or double split ring resonators, with embedded Bi2Se3 or (InxBi(1-x))2Se3/ Bi2Se3 van der Waals
heterostructures, to achieve up-conversion in the 6.4 (even) - 9.7 (odd) THz frequency range. This results from bulk centro-
symmetry (odd states) and
symmetry breaking in the topological surface states (odd and even).
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All authors Alessandra Di Gaspare, Sara Ghayeb, Craig Knox, Edmund H. Linfield, Joshua Freeman, Miriam S. Vitiello
Materials Science · Cross-listed from cond-mat.str-el
Magnetic excitation spectrum and hierarchy of magnetic interactions in ErFeO3
Dnyaneshwar R. Bhosale, Piotr Fabrykiewicz, Devashibhai Adroja et al. (+3 more)
We report a comprehensive investigation of the excitation spectrum of ErFeO$_3$ orthoferrite by means of time-of-flight
neutron spectroscopy. The spectrum consists of two distinct components: strongly dispersive
spin wave excitations of the Fe$^{3+}$ sublattice spanning $\approx$~9 - 65 meV, and
crystal electric field (CEF) excitations of Er$^{3+}$ ions below 36 meV. The observed
spin wave dispersions and spectral weight are well captured within linear
spin wave theory, enabling extraction of the key Fe-Fe exchange parameters. Low-energy incident
neutrons with their enhanced energy resolution, further revealed the dispersive character and splitting of Kramers-degenerate CEF levels. We show that the dispersion is caused by the exchange coupling between Er$^{3+}$ ions, while the degeneracy is lifted by interactions between the Er$^{3+}$ and Fe$^{3+}$ sublattices. We further explore the influence of dipolar and antisymmetric exchange interactions with the focus on the
magnetic ground state of ErFeO$_3$, with particular
attention to the low-temperature
spin arrangement. Taken together, our results provide a detailed account of the
spin dynamics in ErFeO$_3$ and reveal a hierarchy of interactions scales characteristic for orthoferrites.
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All authors Dnyaneshwar R. Bhosale, Piotr Fabrykiewicz, Devashibhai Adroja, Martin Meven, Astrid Schneidewind, Michal Stekiel
Materials Science · Cross-listed from cond-mat.stat-mech
Geometric memory in incomplete phase transitions across dimensions
F. Tolea and M. Tolea
We model a direct solid-state phase transition through a nucleation-and-growth process in which plates have simple, regular shapes - squares, cubes, or square-faced lamellae - and grow homothetically (self-similarly) until they either reach a randomly assigned maximum size or are stopped by impingement with previously formed plates. The reverse transformation is represented by the preferential disappearance of smaller plates, while larger plates are retained during an incomplete reversion. A subsequent direct transformation therefore produces a modified plate-size distribution, a
memory effect that forms the main focus of this study. Building upon an earlier two-dimensional (2D) formulation, we extend the model to cubes (3D) and to lamellar plates (3DL) in order to examine how dimensionality affects transformation
memory. We introduce a quantitative descriptor of
memory, the size mass ratio, and find that
memory is robust in all geometries but overall stronger in 2D than in 3D or 3DL. We provide growth snapshots, arrest-regrowth cycles, size distributions, and differential scanning calorimetry simulations, and we compute the Shannon size-
entropy to quantify configurational diversity. Although motivated by the thermal
memory effect in shape-
memory alloys, the model more generally identifies a purely
geometric mechanism for
memory in first-order solid-solid transformations, highlighting the role of dimensionality and
geometric blocking in controlling the strength of transformation
memory.
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Materials Science
Data-Efficient Indentation Size Effect Mitigation in Steels Using Machine Learning and a Physics-Constrained Neural Network
Radmir Karamov and Tagir Karamov
Shallow nanoindentation, needed for thin layers, individual phases and small volumes, gives hardness inflated by the indentation size effect (ISE). The Nix-Gao correction reduces this inflation but requires deep indentations of the same material, often impossible in such samples. We present a data-efficient workflow, trained on a few reference materials, that predicts a high-load reference hardness from a single shallow indentation. Seven models were trained on 703 indentations (5-350 mN) on three steels (2.0-6.5 GPa): Ridge regression, Random Forest, XGBoost, two feed-forward networks, and two physics-constrained neural networks (PCNNs) that apply a bounded learned correction q as H_app/sqrt(1+q). Over five seeds, models were tested on an external steel not used for training and indented at different loads, in leave-one-steel-out folds, and on fused silica, an out-of-domain material where all models failed. Random Forest and XGBoost, the best on the development set, transferred worst over all loads (MAPE 10.2 and 8.5%), whereas Ridge regression, the PCNNs and one feed-forward network reached 3.7-4.8%; with three training steels, development-set accuracy thus did not indicate accuracy on an unseen steel. From only the external steel's 3-15 mN indentations, the smooth (non-tree) models estimated the reference within 0.03-0.19 GPa on average. The PCNNs matched the linear baseline within the training hardness range and were the most accurate for a held-out steel softer than the training steels, but failed for a harder one. The workflow uses no steel-specific inputs and can be retrained for other material classes.
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Materials Science
The second altermagnet candidate in organic conductors: $\kappa$-(BEDT-TTF)$_2$$m$-HOOCC$_6$H$_4$SO$_3$
Kohei Tokura, Takato Masuta, Kazushi Aoyama et al. (+3 more)
We have developed a novel BEDT-TTF-based organic conductor, $\kappa$-(BEDT-TTF)$_2 m$-HOOCC$_6$H$_4$SO$_3$ ($\kappa$-$m$-SBA), and propose it as a candidate for altermagnet. Tight-binding band calculations of $\kappa$-$m$-SBA provide a $t'/t$ of 1.01 at 100 K, indicating that the
spin structure is closely aligned to an equilateral triangle ($t'/t= 1$). While most $\kappa$-type BEDT-TTF-based salts become
spin liquids due to the
spin frustration caused by the triangular
lattice, $\kappa$-$m$-SBA surprisingly shows a weak
ferromagnetic transition at $T_\mathrm{N} = 14$ K due to a canted
antiferromagnetic (AFM)
spin structure. Until recently, $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Cl ($\kappa$-Cl) was the only $\kappa$-type organic conductor known to exhibit this order, and it is also recognized as the first candidate for altermagnetism in organic conductors. This was theoretically predicted by Naka et al. in 2019, who demonstrated that $\kappa$-type organic conductors can be candidates for altermagnetism if they display such order. Consequently, $\kappa$-$m$-SBA can be considered the second candidate for altermagnetism in organic conductors. Furthermore, numerical calculations demonstrate a characteristic of altermagnets in $\kappa$-$m$-SBA, namely
spin splitting of energy bands.
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All authors Kohei Tokura, Takato Masuta, Kazushi Aoyama, Hiroki Akutsu, Yasuhiro Nakazawa, Scott S. Turner
Materials Science
On the proposed concept of mechanical phasons in Ni-Mn-Ga modulated martensite
Petr Sedlák, Tomáš Grabec and Hanuš Seiner
We discuss modulation phasons as a possible source of unusual elastic behavior of five-layer modulated (10\,M) martensite of the Ni-Mn-Ga shape
memory alloy. This material exhibits anomalous macroscopic shear compliance along specific planes perpendicular to the modulation vector, and this compliance disappears when the modulations become incommensurate. Using a simple mechanical model, we show that modulation phasons in Ni-Mn-Ga can have macroscopic mechanical manifestations, and that the resulting 'mechanical phasons' can relax external shear loadings for commensurate and weakly incommensurate modulations, but not for strongly incommensurate modulations. The model merges ideas from the adaptive martensite theory and electronic-structure considerations, and enables straightforward explanations of several properties of the 10\,M
lattice, such as spontaneous monoclinic distortion or easy formation and propagation of $a/b$ twins.
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Materials Science
Emergent electric fields driven by phonon-coupled skyrmion resonances
Seno Aji
We develop a coarse-grained theoretical description of the macroscopic emergent electric field generated by
phonon-coupled
lattice deformations in the breathing and rotational dynamics of a skyrmion
lattice under microwave excitation. The analysis identifies the
symmetry and dynamical conditions that yield rectified (dc) and oscillating (ac) electric fields, even in the absence of net translational motion of the skyrmion
lattice, particularly in the dilute-
lattice limit. Using experimentally measurable skyrmion profile parameters such as the
equilibrium radius, domain-wall width, and dynamical resonance frequency of skyrmion
lattice, the model further enables identification of harmonic components contributing to the observed macroscopic electrodynamic response in the long-wavelength
phonon limit ($q \to 0$) and at finite
phonon frequency, providing a unified framework for
phonon-driven
spin-charge-
lattice coupling in topological magnets.
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Materials Science
Conditional Generative Models Enable Targeted Exploration of MAX Phase Design Space
Jamie Swain, Cyprien Bone, Matthew T. Darby et al. (+2 more)
MAX phases (M$_{n+1}$AX$_n$), precursors to MXenes, span a vast compositional space, motivating efficient computational screening for synthesisable candidates. We employ CrystaLLM$-\pi$, a large
language model fine-tuned on 6,179 double transition-metal MAX phases, and demonstrate its ability to generate out-of-sample structures consistent with known experimental trends. Using a conditioning vector with two dimensions (a statistically derived MXene derivative count and a surrogate for A-site binding energy), the model was able to target MXene-favourable regions of phase space for generation. Specific condition vectors double novel stable structure generation rates versus unconditioned baselines. Of ten compositionally novel candidates, five exhibit DFT-validated stability ($E_{hull} < 0.050$ eV/atom). This work showcases the potential for autoregressive generative models to explore targeted materials' spaces, offering a scalable framework for accelerated discovery in compositionally complex systems.
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All authors Jamie Swain, Cyprien Bone, Matthew T. Darby, Ewan Galloway, Keith T. Butler
Materials Science
VibroML: an automated toolkit for high-throughput vibrational analysis and dynamic instability remediation of crystalline materials using machine-learned potentials
Rogério Almeida Gouvêa and Gian-Marco Rignanese
While machine-learned interatomic potentials (MLIPs) accelerate
phonon dispersion calculations, merely identifying dynamical instabilities in computationally predicted materials is insufficient; automated pathways to resolve them are required. We introduce VibroML, an open-source Python toolkit driven by foundational MLIPs that shifts the paradigm from stability verification to automated structural remediation. VibroML employs an energy-guided genetic
algorithm that vastly outperforms traditional soft-mode following, efficiently navigating the potential energy surface to uncover diverse, dynamically stable polymorphs. As 0 K harmonic stability does not guarantee macroscopic viability, an automated molecular dynamics workflow evaluates finite-temperature structural retention. VibroML also couples with ProtoCSP, our combinatorial structure prediction engine, to stabilize frustrated
crystal topologies via targeted alloying, successfully rescuing functional perovskite networks like Cs$_2$KInI$_6$ and KTaSe$_3$. Demonstrating broader applicability, we mined the Alexandria database – where ~50% of quaternary and 99.5% of quinary elemental combinations lack any structural entries – to identify thousands of abandoned, high-
symmetry stoichiometries. Deploying ProtoCSP's "cold start"
retrieval and VibroML's evolutionary search on a sample, we successfully identified dynamically stable low-
symmetry candidates. Through integrated structural remediation, thermal validation, and systematic compositional exploration, VibroML enables a comprehensive deep-screening approach, yielding physically sound structural propositions that far surpass standard high-throughput workflows.
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Materials Science · Cross-listed from cond-mat.mes-hall
Curvature-induced nonlinear anomalous Hall effect in thin magnetic shells
Maria Teresa Mercaldo, Mario Cuoco and Carmine Ortix
Optoelectronic and nonlinear transport experiments probe the
quantum geometric tensor of Bloch states, whose real and imaginary components – the
quantum metric and the Berry
curvature – are typically constrained by
symmetry. Here, we show that
geometric bending provides a route to engineer such responses in centrosymmetric ferromagnets.
Curvature-induced strain gradients across the shell thickness break inversion
symmetry and activate an orbital Rashba coupling. In the presence of in-plane magnetization and
spin-orbit coupling, this generates
spin textures with a nontrivial
quantum geometry, leading to an intrinsic nonlinear anomalous
Hall effect (NAHE) governed by the
quantum metric and maximized when the magnetization aligns with the applied electric field. When
geometric deformations further break twofold rotational
symmetry around the out-of-plane axis, an additional NAHE emerges, maximal for magnetization perpendicular to the driving electric field and governed by the Berry
curvature dipole, thus giving access to the imaginary component of the
quantum geometric tensor. These results establish curved
ferromagnetic shells as a platform for engineering anisotropic nonlinear transport and for selectively probing both components of the
quantum geometric tensor.
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Materials Science
The optical phonoelectric effect
D. Choi, M. Först, M. Fechner et al. (+9 more)
Piezoelectricity is a technologically important property of certain
insulators in which mechanical strain induces an electrical polarization. However, the rate at which a piezoelectric response can be established over a macroscopic volume is limited by the sound velocity, constraining applications in high-bit-rate transduction and sensing. Furthermore, the strength of the piezoelectric effect is not readily tunable, as it depends on intrinsic anharmonic coupling between strain and intra-unit-
cell distortions in a given material. Lastly, the maximum amplitude of the effect is bounded by material fracture, which sets in already at percent level strain values. Here we overcome these limitations by realizing a strain-free, piezoelectric-like response driven solely by photo-excited optical
phonon distortions. We demonstrate such optical phonoelectricity in the weak piezoelectric BPO$_4$, in which we induce electrical polarization through
phonon rectification. This effect is established over macroscopic volumes with four orders of magnitude higher speed than piezoelectric responses, ultimately limited by the speed of light. The maximum induced polarization is estimated to be far in excess of that attainable through strain at the fracture limit. Ultrafast phonoelectricity opens up new opportunities for optical control in
quantum materials, but also for device applications.
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All authors D. Choi, M. Först, M. Fechner, M. Buzzi, X. Deng, Z. Zeng, K. H. Martens, D. Prabhakaran, C. Putzke, P. Moll, P.G. Radaelli, A. Cavalleri
Materials Science
Phase-Transition Induced Magnetic Domain Evolution and Magnetization Dynamics in FePt/FeRh Bilayers for Advanced Heat-Assisted Magnetic Recording
Saroj K. Mishra, Y. Sasaki, S. Isogami et al. (+4 more)
Achieving ultrahigh recording densities with low power consumption is a central challenge for next generation heat assisted
magnetic recording (HAMR), as conventional L10 FePt media require intense
laser heating due to their high coercivity (Hc) and high Curie temperature (700 K). Here, we address this issue using FePt/FeRh
bilayers, where the
antiferromagnetic to
ferromagnetic transition of FeRh near 350 K generates strong interfacial exchange coupling that assists magnetization switching in the FePt layer. Magnetometry measurements reveal a 40% reduction in Hc from 300 K to 400 K in the
bilayer, compared to only 8% in single layer FePt. Temperature dependent MFM directly captures phase transition induced domain
evolution, showing a 30% reduction in domain size and enhanced phase contrast. TR-MOKE measurements reveal only a minor (0.4 T) modification of the effective anisotropy field during phase transition, confirming that the intrinsic anisotropy of FePt remains largely preserved. These results demonstrate that the reduction in Hc in FePt/FeRh
bilayers is primarily governed by phase transition induced domain wall mobility coupled with interfacial
magnetic interactions, rather than by intrinsic anisotropy softening. This mechanism provides a pathway toward efficient magnetization switching under reduced thermal load, making FePt/FeRh
heterostructures promising candidates for advanced HAMR media.
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All authors Saroj K. Mishra, Y. Sasaki, S. Isogami, I. Suzuki, Keerthana P, J. Mohanty, Y. K. Takahashi
Materials Science
Ultrafast Sliding Ferroelectric Switching in Bilayer Hexagonal Boron Nitride Revealed by Deep Learning Molecular Dynamics
Yinan Wang, Poyen Chen and Teruyasu Mizoguchi
Sliding ferroelectricity in
bilayer hexagonal boron nitride (h-BN) offers compelling prospects for next-generation non-volatile
memory, yet the atomistic dynamics of electric-field-driven polarization switching remain poorly understood. Here, we present a fully data-driven, coupled atomistic framework that integrates a fine-tuned MACE machine learning potential (MLP) with an equivariant graph convolutional neural network (EGCNN) for real-time Born effective charge (BEC) prediction, enabling large-scale non-
equilibrium molecular dynamics simulations of AB-stacked
bilayer h-BN under applied electric fields. By implementing a rigorous real-space path-integral polarization formalism combined with a state-constrained Gaussian convolution background extraction procedure, we successfully isolate the intrinsic spontaneous polarization from the dominant dielectric background. Our simulations reveal that coherent single-domain rigid sliding, completing within 5 ps, constitutes a physically viable ultrafast switching mechanism, and reproduces clean ferroelectric hysteresis loops whose shape is qualitatively consistent with experimental observations.
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Materials Science · Cross-listed from cond-mat.str-el
Exotic Spin Excitation Continuum in a Weakly Coupled Quantum Chainsaw Antiferromagnet
Asiri Thennakoon, Prena Chaudhary, Sankha Subhra Bakshi et al. (+9 more)
Collective motions in strongly interacting magnets involve many
spins and are often described in terms of integer-
spin excitations. However, in certain cases, the collective motion can behave as if these integer excitations break apart into smaller, particle-like entities with unusual properties. Such fractionalized excitations in
quantum magnets are commonly associated either with topological order in two dimensions or with criticality in one dimension. It remains unclear how these distinct mechanisms are connected across a dimensional crossover. Here we investigate the Ti-based
quantum antiferromagnet, $Cs_{8}LiNa_{3}Ti_{12}F_{48}$, in which $Ti^{3+}$ ($3d^{1}$, $S=1/2$) ions interact antiferromagnetically within distorted kagome planes. Our inelastic
neutron scattering study on a single
crystal reveals a frustrated network of weakly coupled
spin-$1/2$ chainsaws, realizing a regime of dimensional frustration in which interchain couplings fail to establish coherent two-dimensional order. The
magnetic excitation spectrum exhibits a strong continuum spanning the full measured momentum and energy phase space. In addition, the dynamic
spin correlation function displays rod-like scattering in momentum space, indicating a quasi-one-dimensional nature of the
magnetic correlations. These results point to fractionalized excitations with intrinsically directional character, demonstrating that signatures of one-dimensional criticality can persist within a two-dimensional
lattice. Our findings establish anisotropic fractionalization as a distinct organizing principle for
quantum-disordered states.
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All authors Asiri Thennakoon, Prena Chaudhary, Sankha Subhra Bakshi, Tommy Park, Tristen Lowrey, Daniel Pajerowski, Christina Hoffmann, Junghong H. He, Hiroaki Ueda, Collin Broholm, Gia-Wei Chern, Seung-Hun Lee
Materials Science · Cross-listed from cond-mat.mes-hall
Electrically Tunable Terahertz Chirality from Quantum Geometry
Sobhan Subhra Mishra, Thomas CaiWei Tan, Faxian Xiu et al. (+1 more)
Quantum geometry encoded in the momentum space structure of electronic wavefunctions, governs charge dynamics through Berry
curvature, enabling unconventional transport and optical responses. In topological semimetals, this geometry is sampled over
Fermi pockets, suggesting electrical control by
Fermi surface tuning, yet such control has remained largely limited to DC transport. Here we show that electrostatic gating of the 3D Dirac semimetal Cd3As2 reshapes
Fermi pockets surrounding photoinduced Floquet Weyl nodes, enabling electrical control of terahertz (THz) emission chirality. Gate tuning selectively modulates the Berry
curvature driven linearly polarized THz component by up to 60% and 49% at positive and negative bias, respectively, while the orthogonal linearly polarized
photon-drag component remains unchanged. With the two orthogonal fields intrinsically phase-locked at \sfrac{\pi}{2} by the excitation geometry, the selective gate-tuned amplitude control enables the polarization tuning across the Poincaré sphere, achieving near-circular polarization (\chi\approx-42{\deg}) at +10 V. These results establish
Fermi surface tuning as a general route to programmable
quantum geometric control of
chiral terahertz emission.
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All authors Sobhan Subhra Mishra, Thomas CaiWei Tan, Faxian Xiu, Ranjan Singh