Materials Science · Cross-listed from cond-mat.str-el
High-pressure phase transitions in the quantum spin liquid candidate Na2Co2TeO6 probed by Raman spectroscopy
Ihsan Ahmed Kolasseri, Maria Mei Ravnebæk, Subhadip Das et al. (+5 more)
The quasi-2D magnet Na2Co2TeO6 (NCTO) is a candidate for a Kitaev
Quantum Spin Liquid (KQSL) state. Pressure-tuning in such materials is of interest as a potential method to tune the Kitaev exchange interactions, which are strongly dependent on bond geometry. Here we report a Raman spectroscopic study of NCTO inside a diamond anvil
cell (DAC) with pressure applied up to 16.3 GPa. Based on the changes in the Raman modes, this pressure range is divided into three regions. The appearance and disappearance of several modes and changes in the polarization dependence of the representative modes, most prominently above 13.8 GPa, point to pressure-induced phase transitions in this material.
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All authors Ihsan Ahmed Kolasseri, Maria Mei Ravnebæk, Subhadip Das, Carl Jonas Linnemann, Haidong Zhou, Christian Frydendahl, Martin Bremholm, Yong P. Chen
Materials Science
Predicting THz Generation Capability of Organic Crystals through Data Mining and Crystal Nonlinearity Models
Sin Hang Ho, Ashton Roma, Connor Barlow et al. (+5 more)
We report the use of DFT computation and mathematical models to predict the nonlinear dielectric polarization (P^{NL}) and nonlinear susceptibility coefficients (\chi^{(2)}_{IJK}) of organic materials based on their
crystal structures. We apply this computation approach on single-component
crystals, co-
crystals and ionic
crystals found through data mining the Cambridge Structural Database. We verify these computational results with experimental terahertz (THz) generation efficiencies for known THz generators, demonstrating consistency between the measurements and the computed values. Several mined structures show similar or larger PNL values compared to state-of-the-art THz generation
crystals DAST, OH1 and BNA, suggesting great potential for their use in nonlinear optical (NLO) applications. Importantly, we also compared the resulting model of nonlinear optical tensor components with commonly-used simplifications of nonlinearity, showing that the comprehensive approach should be the standard method to evaluate the nonlinear optical properties of single-crystalline materials.
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All authors Sin Hang Ho, Ashton Roma, Connor Barlow, Matthew Lutz, Natalie Green, Stacey Smith, David Michaelis, Jeremy A Johnson
Materials Science
AlV$_2$O$_4$ thin films via in-situ interfacial topotaxy
Jeong Rae Kim, Alexis Ashby, Sandra Glotzer et al. (+3 more)
Conventional oxide epitaxy approaches face challenges when the oxidation conditions of constituent elements differ significantly. Here we demonstrate that V–O thin films can serve as solid-phase precursors for epitaxial AlV$_2$O$_4$, comprising a pyrochlore V$^{2.5+}$ network coexisting with AlO$_4$ tetrahedra within the spinel structure. The epitaxial AlV$_2$O$_4$/Al$_2$O$_3$ (0001)
heterostructures are realized via interfacial topotactic transformation, involving V–O growth at a moderate temperature followed by in-situ ultra-high-temperature post-annealing to drive a reaction with the Al$_2$O$_3$ substrate. Transmission electron microscopy and temperature-dependent X-ray diffraction analyses reveal excellent structural characteristics that closely reproduce the known charge-ordering transition. This study presents a novel approach to realizing epitaxial structures with convoluted oxidation states where thermodynamic and kinetic barriers would otherwise limit synthesizability.
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All authors Jeong Rae Kim, Alexis Ashby, Sandra Glotzer, Darryl Shima, Ganesh Balakrishnan, Joseph Falson
Materials Science
Multiscale Modelling of Ferroelectrics using a Physics-Informed Neural Network Driven by Molecular Dynamics Data: Parameter Identification and Field Reconstruction
Xuejian Wang, Frank Wendler, Hikaru Auzuma et al. (+5 more)
In multiscale modeling of ferroelectrics, combining atomistic simulation with continuum-scale phase-field models (PFM) remains a fundamental challenge. A key difficulty lies in faithfully capturing discrete atomic-level information within a continuum modeling framework, while accurately representing material behavior at the mesoscale. In this paper, a Physics-Informed Neural Network (PINN) driven by molecular dynamics (MD) data is used. The loss function of the network consists of a supervised term that fits the discrete spatial polarization distributions obtained from MD simulations of systems containing domain walls, and a physics-based term that incorporates the residuals of partial differential equations (PDEs) of steady-state PFM. To ensure stable and balanced training among the different loss components, adaptive gradient normalization (GradNorm) is used to dynamically adjust the task weights. By minimizing the total loss, the model not only reconstructs the polarization field along with the associated strain, stress, and energy landscape at the continuum scale, but also identifies critical physical parameters of the phase-field model, including the characteristic energy density, characteristic length factor, gradient energy anisotropy factor, and Landau
polynomial coefficients. By using the PINN-predicted physical parameters in COMSOL Multiphysics to solve the corresponding PDEs within a finite element framework, we demonstrate that these parameters enable accurate reproduction of the ferroelectric domain structure and the associated material response, including stress/strain distributions and energy landscape. This framework provides an effective methodology for establishing multiscale connections between atomistic and continuum descriptions, and holds the potential to infer underlying physical properties directly from polarization distributions for a wide range of materials.
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All authors Xuejian Wang, Frank Wendler, Hikaru Auzuma, Michael Zaiser, Shuji Ogata, Ryo Kobayashi, Lei Zeng, Xingchen Tan
Materials Science · Cross-listed from cond-mat.supr-con
Enhanced Superconductivity in Multilayer FeSe Films by Simplified Molecular Beam Epitaxy
Maria Hilse, Hemian Yi, Zhe Chen et al. (+7 more)
Multi-unit-
cell (UC) e̱ta}-FeSe films grown on SrTiO3(100) continue to attract
attention because of the significant enhancement in the superconducting transition temperature (Tc) compared to that in bulk FeSe. In prior reports of molecular beam epitaxy (MBE)-grown e̱ta}-FeSe/SrTiO3(100), elaborate growth protocols have been used to achieve enhanced Tc, leading to a general belief that careful pre-treatment of the SrTiO3 substrate and post-growth annealing in ultrahigh
vacuum (UHV) are essential. Here, we report a greatly simplified protocol for the MBE growth of superconducting multi-UC e̱ta}-FeSe films on SrTiO3(100), eliminating the need for careful substrate pre-treatment and post-growth UHV annealing while still achieving an enhanced Tc. With appropriate capping, epitaxial films with 14 UC thickness exhibit a zero-resistance transition temperature Tc ~ 20 K in ex situ electrical transport measurements. The MBE optimization process is guided by the growth-parameter dependencies of film morphology and structural properties, as characterized by reflection high-energy electron diffraction, X-ray diffraction, atomic force microscopy, and scanning transmission electron microscopy.
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All authors Maria Hilse, Hemian Yi, Zhe Chen, Jessica L. Thompson, Kalana D. Halanayake, Danielle Reifsnyder Hickey, Seong H. Kim, Cui-Zu Chang, Nitin Samarth, Roman Engel-Herbert
Materials Science · Cross-listed from physics.app-ph
Imaging atomic scale stochasticity: noise in the nearly commensurate charge density wave of 1T-TaS$_2$ for leaky-integrator-and-fire neuromorphic emulator
Michael Verhage
Low-dimensional materials are promising materials for emulating
neuron-like firing were understanding of the atomic-scale origin of this behavior can benefit integration into neurormophic hardware. Using scanning tunneling microscopy (STM) as a highly local probe of correlations in the charge density wave (CDW), we study atomic-scale CDW noise in $1T\text{-}\mathrm{TaS}_2$ at room temperature. We statistically analyze the tunnel-junction noise and, to explain the temporally correlated current bursts, introduce a mesoscopic model in which the fractal structure and reorganization of CDW discommensurations are related. This noise mimics leaky-integrate-and-fire
neuron dynamics. Our results show that atomic-scale current bursts in the tunnel junction can act as effective nonlinear elements in spiking neural emulators, enabling bioinspired processing through rich CDW dynamics at room temperature and supporting the miniaturization of spiking devices.
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Materials Science
Thermal diffuse scattering in TEM: complex absorptive potentials compared to the frozen phonon model
Martin Hájek and Ján Rusz
In transmission electron microscopy, electrons undergo inelastic scattering primarily through
phonon excitations, known as thermal diffuse scattering. To capture the inelastic scattering effects on the elastic scattering components, absorptive effects must be included in the modeling of electron propagation, accounting for the gradual depletion of the elastic channel of the electron beam. Several approaches to modeling this absorption exist. In this paper, we compare the widely used complex absorptive potentials method to the more elaborate frozen
phonon model, based on correlated atomic motion and on the Einstein model of atomic motion.
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Materials Science
Interface-Controlled Defect Engineering in TiN/TaN Superlattices for Enhanced Hardness and Fracture Toughness
Zecui Gaoa, Qimin Wang, Julian Buchinger et al. (+6 more)
TiNTaN superlattice coatings were designed to investigate how atomic-scale interface chemistry and defect-stabilized TaN layers govern hardness and fracture toughness. Guided by first-principles predictions identifying TaN-based layers as more damage tolerant than TiN, coherent superlattices with a bilayer period of 6 nm were synthesized by reactive magnetron sputtering and interfacially doped with C, B, or Si. Structural and chemical analyses reveal coherent fcc architectures with well-defined interfaces. Si segregates preferentially to the interfaces while incorporating into both TiN and TaN, whereas C and B predominantly diffuse into the TaN layers, modifying coherency strain, bonding, and defect populations. Consequently, hardness increases from 34 GPa for the undoped superlattice to 41 GPa for the Si-doped architecture, whereas fracture toughness increases from 2.8 to 4.0 MPam0.5 for the B-doped superlattice. First-principles calculations show that vacancy-stabilized TaxNy enhances elastic compliance and elastic contrast rather than intrinsic toughness, while the additional toughening induced by B indicates localized defect-assisted energy dissipation at chemically engineered interfaces. Thus, Si maximizes interface strengthening, whereas B provides the most favourable hardness-toughness balance while preserving high hardness, 38 GPa. These findings establish interface chemistry as an additional design parameter for tailoring the mechanical performance of ceramic nitride superlattices.
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All authors Zecui Gaoa, Qimin Wang, Julian Buchinger, Nikola Koutna, Marcus Hans, Zaoli Zhang, Jochen Schneider, Daniel Primetzhofer, Paul Heinz Mayrhofer
Materials Science · Cross-listed from math.NA
Randomized Block Davidson Eigensolvers for Plane-Wave Density-Functional Theory
Moritz Gubler, Taejun Park, Augustin Bussy et al. (+4 more)
Iterative diagonalization is the dominant cost of plane-wave density-functional theory (DFT), with search-space orthogonalization scaling particularly quickly with problem size and the number of target states. We present a randomized block Davidson-type eigensolver that replaces Euclidean orthogonalization with randomized Gram-Schmidt in a sketched inner product, requiring only a single pass over the basis while keeping its conditioning bounded independently of the input vectors. This modification changes only the Rayleigh-Ritz step, which becomes a definite generalized Hermitian eigenproblem. Ritz extraction remains exact, preserving true Ritz pairs and the interlacing property that makes each band energy an upper bound on the true one. The method is implemented in mixed precision for CPUs and GPUs from a single Julia code, interfaces
matrix-free with DFTK, and is released in the open-source RandESC library. On sparse test problems with a fixed number of eigenpairs, the sketched solver overtakes its deterministic counterpart beyond
matrix dimensions of about $2\times 10^4$ and is $25\%$ faster at $5\times 10^5$. In full self-consistent field DFT calculations, however, both Davidson variants outperform the locally optimal block preconditioned conjugate gradient (LOBPCG) reference only by $5$ to $11\%$ in total time, while the additional benefit of sketching is limited. As the number of requested states grows with system size, orthogonalization savings are offset by the generalized eigenproblem. Therefore, the regime in which sketching pays off is set by how the number of wanted states scales with the problem dimension, not by the eigensolver as such.
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All authors Moritz Gubler, Taejun Park, Augustin Bussy, Johannes Laute, Nicola Marzari, Iurii Timrov, Laura Grigori
Materials Science
Structure-Agnostic Prediction of the Electronic Density of States with a Chemical Language Model
Ivan D. Rubtsov, Ivan V. Dudakov and Vadim V. Korolev
The electronic density of states (DOS) is conventionally computed from a relaxed
crystal structure, which is unavailable for compounds that have been neither synthesized nor cataloged. Here we introduce DOSSIER ($\textbf{D}$ensity $\textbf{o}$f $\textbf{S}$tates from $\textbf{S}$to$\textbf{i}$chiometry with $\textbf{E}$ncoder $\textbf{R}$epresentations), a chemical
language model that maps elemental composition directly to this spectrum. The encoder is pretrained by cross-modal knowledge
distillation from a universal machine-learning interatomic potential; the transfer lowers the error by 11% when only 1,000 training examples are available. On the Mat2Spec
benchmark, DOSSIER reaches a mean absolute error of 3.76 states eV$^{-1}$ against 3.64 for the best structure-aware model; on an extended Materials Project
dataset, the predicted spectra yield band gaps and $\textit{d}$-band descriptors with useful accuracy. Screening 11,977 binary and 13,251 five-component high-
entropy alloy compositions for a $\textit{d}$-projected DOS resembling that of NiPt$_{3}$ places known oxygen reduction electrocatalysts near the top of the ranking.
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Materials Science
Plastic Relaxation without Dislocations in $\beta$-Ga$_2$O$_3$ Heteroepitaxy: A Structural Peculiarity of Ga$_2$O$_3$ Polymorphs
I. Bertoni, A. Marzegalli, A. Ugolotti et al. (+3 more)
$\beta$-Ga$_2$O$_3$ grows as three-dimensional islands on mismatched c-plane sapphire, often on a thin $\alpha$-Ga$_2$O$_3$ wetting layer, and eventually forms a continuous film by island coalescence. Experiments reveal the $\beta$ film to be relaxed. Surprisingly no dislocations are observed, suggesting an unusual plastic relaxation path. We reveal the key mechanisms underlying this process by combining density functional theory, continuum nucleation theory, and molecular dynamics simulations. Plastic relaxation occurs through rearrangement of the Ga atoms beneath the oxygen plane shared by both the wetting layer and the (-201) $\beta$-Ga$_2$O$_3$ structures. A local $\alpha$ to $\beta$-relaxed phase transition is thus realized within a single plane, allowing full relaxation of the $\beta$-Ga$_2$O$_3$ island.
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All authors I. Bertoni, A. Marzegalli, A. Ugolotti, R. He, F. Djurabekova, L. Miglio
Materials Science · Cross-listed from cond-mat.str-el
Anomalous magnetocaloric effects in the quasi-one-dimensional antiferromagnet BaCo$_2$V$_2$O$_8$
Jiahao Yang, Chao Dong, Xinlong Shi et al. (+8 more)
We investigate the transverse-field thermodynamics of the quasi-one-dimensional Ising-like antiferromagnet BaCo$_2$V$_2$O$_8$, whose tilted screw-chain geometry and anisotropic Landé $g$ tensor generate spatially modulated Zeeman couplings. Angle-resolved magnetocaloric-effect (MCE) measurements reveal a high-field temperature minimum near the transverse-field Ising critical field for $H\parallel[110]$ that persists and shifts only weakly upon field rotation. Tensor-network calculations show that the rotation-induced staggered transverse field rapidly lowers the Ising critical field and that the
magnetic Grüneisen ratio changes sign near the high-field temperature minimum, consistent with experiment. Our results establish that a dominant MCE response can persist away from the Ising critical region, suggesting a route to
magnetic cooling by tailoring anisotropic Zeeman-coupling configurations in
quantum magnets.
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All authors Jiahao Yang, Chao Dong, Xinlong Shi, Zhuo Wang, Tiantian Li, Liusuo Wu, Junfeng Wang, Zhangzhen He, Liang Li, Yongkang Luo, Jianda Wu
Materials Science
A colossal dielectric response of Bi1-xSmxFeO3 nanopowders
Vladyslav O. Kolupaiev, Olexander S. Pylypchuk, Vladimir N. Poroshin et al. (+6 more)
The dielectric permittivity of the pressed powder samples of Bi1-xSmxFeO3, with Sm content "x" varying in the range 0 - 0.2, has been investigated in the temperature range from 20 to 400 C and the frequency range from 100 Hz to 100 kHz. We have shown that the Sm content impacts significantly the real and imaginary parts of effective dielectric permittivity, which have expanded diffuse maxima with a colossal magnitude up to 105 (for the real part) and up to 108 (for the imaginary one) at temperatures 300 - 400 K. Analysis of experimental data carried has shown that both the real and imaginary parts of effective dielectric permittivity may be comprehensively explained by considering a complex interplay of a diffuse ferroelectric-paraelectric phase transition and the Maxwell-Wagner-Sillars effects, which emerge from the formation of spatial charges at interfaces between nanograins and at the ferroelectric nanoparticle-air interface. Processing of experimental data for the real and imaginary parts of the effective dielectric permittivity within effective medium approach allows us to separate and analyze the colossal dielectric response of the nanoparticles itself. The main trends followed from experiments are supported by the theoretically simulated dependences, which reveal correlations between the temperature behavior of dielectric properties and phase state of the Bi1-xSmxFeO3 nanoparticles.
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All authors Vladyslav O. Kolupaiev, Olexander S. Pylypchuk, Vladimir N. Poroshin, Denis O. Stetsenko, Ihor V. Fesych, Lesya D. Demchenko, Eugene A. Eliseev, Victor V. Vainberg, Anna N. Morozovska
Materials Science
A hidden low-temperature transformation pathway in compositionally complex materials
Yujiao Li, Elaheh Akbarnejad, Quentin Bizot et al. (+9 more)
Most compositionally complex materials (CCMs, frequently referred to as high
entropy alloys) are metastable and their attractive properties often belong to kinetically trapped states. However, pathways towards lower-free-energy phase states governing long-term stability, can remain hidden because
diffusion-controlled atomic redistribution is too slow to be revealed at experimentally accessible timescales. This blind spot is acute in CCM design: enormous compositional spaces are screened for performance, yet the low-temperature kinetics and the associated transformation pathways determining whether that performance persists are rarely considered in material selection. Here we use defect-rich nanoscale volumes coupled with atom-probe
tomography to access and reconstruct the hidden phase-
evolution pathway in a metastable Ag24Au20Pd50Pt6 electrocatalyst, without relying on elevated temperatures to accelerate the transformation. By varying microstructural starting state, annealing temperature and time, we reveal precipitation of a Pt-rich phase within the fcc
matrix, its coarsening and re-homogenization. The Pt-rich phase recurs after homogenization with delayed kinetic accessibility, while prolonged annealing extends the pathway to 300{\deg}C. Atomistic simulations independently predict the same Pt-rich phase selection. The transformation is accompanied by a 3.7-fold loss of catalytic activity for hydrogen
evolution. These results establish hidden phase-
evolution pathways as a materials-design variable: resolving them can guide the selection of metastable CCMs not only for their as-synthesized properties, but also for the phase states and associated functionalities they may access over time.
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All authors Yujiao Li, Elaheh Akbarnejad, Quentin Bizot, Aleksander Kostka, Natalia Pukhareva, Ridha Zerdoumi, Alan Savan, Matous Mrovec, Ralf Drautz, Baptiste Gault, Dierk Raabe, Alfred Ludwig
Materials Science · Cross-listed from physics.optics
Nonlocal-nonlinear phonon polaritons
Gonzalo Álvarez-Pérez, Line Jelver, Alexander Paarmann et al. (+1 more)
When light is confined to deeply subwavelength volumes, optical responses can become nonlocal. Yet how nonlocality reshapes nonlinear light-matter interactions remains largely unexplored, particularly in dielectrics. Here, we theoretically uncover a new class of nonlinear optical processes arising from the interplay between optical-
phonon propagation and ionic-bond anharmonicity in polar
crystals. We show that, unlike conventional local nonlinearities, this mechanism enables second-order responses even in centrosymmetric materials. Moreover, these responses can be tuned by the confinement and resonantly enhanced near optical-
phonon resonances. Using density functional perturbation theory, we quantify the resulting nonlocal second-order susceptibilities in representative polar
crystals. Our results establish nonlocal-nonlinear optics as a new regime of light-matter interaction in dielectrics.
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All authors Gonzalo Álvarez-Pérez, Line Jelver, Alexander Paarmann, Simone De Liberato
Materials Science · Cross-listed from cond-mat.supr-con
Polireddi Naveen and Abhiram Soori
We theoretically investigate
equilibrium currents in a one-dimensional Josephson junction incorporating a bipolar
magnetic semiconductor (BMS). We show that the intrinsic exchange splitting of the
spin-resolved bands enables purely electrical control of the $0$–$\pi$ transition through gate-tunable modulation of the BMS chemical potential, eliminating the need for an external
magnetic field. This provides a viable route toward electrically tunable $\pi$-junction behavior and highlights the potential of BMS-based Josephson devices for phase-controllable superconducting electronics. Furthermore, in the presence of Rashba
spin–orbit coupling, we find an anomalous Josephson effect characterized by a finite
equilibrium supercurrent at zero phase difference. This behavior originates from the intrinsic breaking of time-reversal
symmetry associated with the
spin-polarized electronic structure of the BMS. Interestingly, despite the simultaneous breaking of time-reversal and inversion
symmetries—conditions often associated with nonreciprocal superconducting transport—we do not observe a Josephson diode effect. Our results therefore highlight an important distinction between anomalous Josephson transport and superconducting nonreciprocity: the former does not necessarily imply a finite critical-current asymmetry between opposite current directions.
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Materials Science
Off-stoichiometric variable doping for exceptional power factors in L2$_1$ Fe$_2$VAlM$_x$ (M=Ti, W) epitaxial thin films
Jose María Domínguez-Vázquez, Miguel Angel Tenaguillo, Ketan Lohani et al. (+7 more)
We show that the addition of Ti or W to stoichiometric L2$_1$ Fe$_2$VAl thin films by sputter codeposition produces off stoichiometric thin film
alloys with superior thermoelectric properties than their stoichiometric counterparts. Ti incorporation induces p-type semiconducting behavior, while W incorporation shifts the material toward n-type, hereby enabling simultaneous tuning of both carrier types within a single parent (Fe$_2$VAl) material system, making it highly desirable for thermoelectric devices. The introduction of both Ti and W partly substitutes V in the stoichiometric compound. The partial substitution of V in the stoichiometric
alloy allows fine-tuning the band structure of the system and transport properties. With this approach we obtain exceptional maximum power factor values for p and n-type films of 1300 $\mu$W/m$\cdot$K$^2$ and 2100 $\mu$W/m$\cdot$K$^2$ ,respectively, yielding maximum figures of merit zT of 0.07 and 0.14, respectively.
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All authors Jose María Domínguez-Vázquez, Miguel Angel Tenaguillo, Ketan Lohani, Jose J. Plata, Antonio M. Marquez, Olga Caballero-Calero, Alfonso Cebollada, Andrés Conca, Ernst Bauer, Marisol Martín-González
Materials Science
Influence of Interface Energy Anisotropy on the Solid-state Instability in Ni-based Superalloy: A Multiscale Study
Sourav Ghosh, Christian Brandl and Rajdip Mukherjee
The microstructural stability of nickel-based superalloys critically depends on the morphology and
evolution of $\gamma'$-precipitates, which is governed by elastic and interfacial anisotropies at the atomic scale. Here, we present a novel quantitative multiscale framework that, for the first time, directly incorporates atomistically computed interface energy anisotropy into mesoscale phase-field simulations to elucidate morphological selection and instability in the Ni–Al system. We employ density functional theory (DFT) to accurately predict the orientation-dependent $\gamma/\gamma'$ interface energies for key crystallographic planes. A rigorous analytic mapping is then developed to systematically reduce the three-dimensional (3D) interface anisotropy landscape to the two-dimensional (2D) simulation plane. This enables quantitative transfer of DFT-informed anisotropy parameters into a continuum phase-field model that also accounts for elastic inhomogeneity and eigenstrain. Our simulations demonstrate that the explicit inclusion of DFT-based interface energy anisotropy fundamentally alters precipitate morphological
evolution, robustly suppressing instability and faceting phenomena otherwise promoted by supersaturation and elastic effects. The framework bridges atomic- to mesoscale modeling, enabling predictive control of precipitate shapes and providing new insights into the interplay of elastic and interfacial contributions in Ni-based superalloys. This approach paves the way for quantitative microstructural design in advanced high-temperature
alloys via first-principles-guided multiscale simulation.
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Materials Science
Two Microscopic Mechanisms of Piezomagnetism in CoF$_2$ from First-Principles Calculations
Hiroshi Katsumoto, Tamio Oguchi and Kunihiko Yamauchi
Rutile-structured CoF$_2$ has long been recognized as a prototypical piezomagnetic material. Recently, it has attracted renewed interest as an altermagnet, exhibiting
spin-split electronic bands even in the absence of
spin-orbit coupling. Although the piezomagnetic response of CoF$_2$ has been extensively discussed from the viewpoint of
magnetic symmetry, its microscopic origin has remained elusive. First-principles calculations reveal two distinct microscopic mechanisms of piezomagnetism in CoF$_2$. Under $xy$ shear strain, the local volumes of the CoF$_6$ octahedra surrounding the two Co sites become different, leading to unequal
magnetic moments on the two sublattices and hence a net magnetization. In contrast, under $yz$ shear strain, the piezomagnetic response originates from
spin canting induced by the Dzyaloshinskii–Moriya interaction through
spin-orbit coupling. The presence of two distinct microscopic mechanisms may be a general feature of piezomagnetic antiferromagnets.
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Materials Science
Excitonic fingerprints of magnetic configurations and switching in multilayer CrSBr
Lukas Krelle, Ryan Tan, Jakob Conradi et al. (+6 more)
The coupling between electronic states and magnetism provides a route towards optical readout and control of
magnetic information. In the
magnetic semiconductor CrSBr,
excitons are coupled to
magnetic order, making their optical response sensitive to the underlying magnetization. Here, we show that the energy and oscillator strength of bulk and surface
excitons provide distinct spectroscopic fingerprints of
magnetic configurations and switching pathways. We distinguish domain-wall-mediated magnetization reversal, manifested by continuous spectral
evolution as a domain wall traverses the optical spot, from abrupt, large-area magnetization reversal. Using the resulting excitonic fingerprints, we reconstruct successive
magnetic configurations in 4- and 5-layer CrSBr during the transition from ferromagnetic to antiferromagnetic order. We further find that the sensitivity to
magnetic order is strongly
exciton-dependent: low-energy
excitons resolve intermediate and surface-related configurations, whereas a higher-energy
exciton predominantly exhibits a transfer of oscillator strength between ferromagnetic and antiferromagnetic resonances. These results establish excitonic spectroscopy as a sensitive probe of layer-dependent
magnetic configurations and their switching pathways in layered
magnetic semiconductors.
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All authors Lukas Krelle, Ryan Tan, Jakob Conradi, Priyanka Mondal, Wenze Lan, Kseniia Mosina, Regine von Klitzing, Zdenek Sofer, Bernhard Urbaszek
Materials Science
Depth-Resolved Evolution of Buried Polar Topologies in a PbTiO3/SrTiO3 Superlattice
Xinxin Hu, Penghan Lu, Noa Varela-Dominguez et al. (+9 more)
Polar
topologies in complex oxides gives rise to a rich spectrum of emergent functionalities and are fundamentally governed by three-dimensional (3D) atomic structures. However, direct experimental determination of buried 3D polar configurations remains a longstanding challenge because conventional (scanning) transmission electron microscopy ((S)TEM) provides primarily projected structural information with limited depth sensitivity. Here, we combine depth-sectioning low-angle annular dark-field (LAADF) STEM, high-angle ADF (HAADF) STEM, and multislice electron ptychography (MEP) to directly visualize the depth-dependent atomic structure and polarization
topology in a PTO/STO superlattice. Depth-sectioning STEM reveals pronounced focal-depth-dependent contrast variations and apparent splitting of Pb atomic columns, indicating significant structural heterogeneity along the beam direction. MEP reconstruction simultaneously resolves the Pb, Ti, and O sublattices with nanometer-scale depth resolution, enabling quantitative mapping of atomic displacements throughout the reconstructed volume. The resulting 3D atomic model reveals substantial depth-dependent displacements of Pb, Ti, and O atoms and a corresponding
evolution of the polarization
topology.
Vortex-like polarization structures are observed near the specimen surfaces but become strongly suppressed within the interior, where distinct polarization configurations emerge. These findings show that polarization patterns observed in conventional projection images can arise from the superposition of multiple depth-dependent polar states and may obscure the underlying 3D polarization texture. Our findings establish a direct experimental link between local atomic displacements and depth-dependent polarization
topology, opening new opportunities for investigating and engineering buried functional states in complex oxide nanostructures.
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All authors Xinxin Hu, Penghan Lu, Noa Varela-Dominguez, Anthony Edgeton, Chang Beom-Eom, Francisco Rivadulla, José Santiso, Yingzhuo Lun, Zhihua Sun, Rafal Dunin-Borkowski, Gustau Catalan, Jordi Arbiol
Materials Science
Classification of Metal - Insulator Transitions: Relating characteristic Properties to Quantum Chemical Bonding Descriptors
Tim Bartsch, Carl-Friedrich Schön, Dasol Kim et al. (+3 more)
Pressure induced metal
insulator transitions (MIT) are classified by the
evolution of characteristic optoelectronic and vibrational properties calculated with density functional theory. Three classes emerge: ionic solids metallize continuously at band-gap closure with hardening
phonons; covalent solids show discontinuous changes in atomic arrangement and optical
phonon frequencies; a third class exhibits complete
lattice softening and drastically enhanced electron
phonon coupling. A one dimensional hydrogen chain reproduces this behavior and serves as a toy model of the underlying bonding mechanism, termed metavalent. Two
quantum-chemical descriptors capture the distinct bonding changes behind the three classes. In metavalent solids, competing electron localization and delocalization yield soft optical modes and Peierls distortions on the insulating side, superconductivity on the metallic side, and low
lattice thermal conductivity near the MIT.
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All authors Tim Bartsch, Carl-Friedrich Schön, Dasol Kim, Raagya Arora, Umesh Waghmare, Matthias Wuttig
Materials Science · Cross-listed from cond-mat.supr-con
Phase-controlled perfect nonlocal spin and charge diode effects in a four-terminal Josephson junction with $p$-wave magnets
Lovy Sharma, Bimal Ghimire and Manisha Thakurathi
We theoretically investigate charge and
spin transport in a four-terminal Josephson junction with a normal-metal barrier. The top and bottom superconducting leads are equal-
spin triplet $p_y$-wave
superconductors, while the left and right leads are $p$-wave magnets with proximity-induced conventional $s$-wave superconductivity. When the transverse macroscopic phase difference between the top and bottom leads is set to zero, a longitudinal phase bias generates a pure transverse
spin current with perfect 100% nonreciprocity. Remarkably, a finite transverse phase difference preserves the perfect
spin-diode effect while simultaneously inducing a perfect charge-diode effect, enabling fully nonreciprocal
spin and charge transport. Moreover, the
spin-diode efficiency exhibits sharp, step-like switching as a function of both the gate voltage applied to the barrier and the crystallographic orientation of the $p$-wave magnet, providing independent and experimentally accessible knobs for controlling the diode polarity. The diode response remains robust against asymmetric interface couplings, nonmagnetic disorder, variations in the relative singlet and triplet pairing strengths, temperature, and junction dimensions, demonstrating that the effect is not a consequence of fine-tuned parameters. These findings establish the proposed four-terminal junction as a highly tunable and structurally robust platform for dissipationless, phase-controlled
spin and charge rectification, with potential applications in superconducting spintronics.
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Materials Science
Bonding Signatures of Incipient Electron Localization in Topological Chiral Semimetals Near the Metal-Insulator Transition
Felix Hoff, Jonathan Frank, Mohit Raghuwanshi et al. (+11 more)
How do electronic localization and delocalization compete in solids beyond the traditional limiting cases of metals and iono-covalent
insulators? Topological chiral semimetals (TCSMs), characterized by their unique
crystal symmetry, offer an intriguing platform to explore this question. Here, we systematically compare TCSMs with covalent compounds, ordinary metals, and metavalent solids (incipient metals), and show that TCSMs occupy a distinct region in a multidimensional property fingerprint. Atom probe
tomography reveals an unusual bond-rupture signature, consistent with a bonding regime intermediate between electron localization and delocalization. This interpretation is supported by measurements of optical properties showing a transfer of spectral weight from interband to intraband transitions. For highly conductive TCSMs, this transition is accompanied by the disappearance of the Born effective charge, a measure of chemical bond polarizability, while less conductive TCSMs retain a nonzero value. Together, these results identify a property based bonding perspective on TCSMs that distinguishes them from metals, covalent solids, and metavalent compounds. Although metavalent solids and TCSMs both lie near the metal-
insulator transition and exhibit distorted
crystal structures, ultrafast coherent
phonon spectroscopy reveals fundamentally different
lattice-dynamical responses: a
phonon-driven Peierls-like instability in metavalent solids versus a robust chiral B20 bonding motif in TCSMs.
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All authors Felix Hoff, Jonathan Frank, Mohit Raghuwanshi, Jan Köttgen, Vicky Hasse, Tim Bartsch, Navjot Bamrah, Elias Hildebrand, Carl-Friedrich Schön, Kaustuv Manna, Chandra Shekhar, Claudia Felser, Ricardo P. S. M. Lobo, Matthias Wuttig
Materials Science
Engineering Dirac interface states
Gabriele Domaine, Moritz M. Hirschmann and Andreas P. Schnyder
We develop a low-energy theory of interface states in anisotropic multivalley Dirac systems whose masses and kinetic parameters are allowed to vary across an interface. For sharp interfaces, current-conserving matching conditions yield analytic expressions for the existence, localization and dispersion of the bound states. We show that the interface velocity is determined by the weighted tangential kinetic terms on the two sides of the seam. Their cancellation can suppress the linear velocity and generate an interface band that is flat to leading order near the projected Dirac point. For the special antisymmetric configuration in which both the Dirac mass and the tangential kinetic coefficient reverse sign with unchanged magnitude, the transparent sharp-interface solution is exactly dispersionless for all conserved momenta within the linear Dirac theory, while the surrounding bulk bands remain dispersive. We extend the theory to smooth interfaces, where the modified bound-state envelope changes the linear interface velocity through a spatial average of the tangential kinetic coefficient. We also investigate quadratic corrections in the kinetic \(\sigma_x\) and \(\sigma_y\) channels. To first order in their coefficients and through linear order in the interface momentum, these terms shift the interface-state energy but produce no additional correction to the linear velocity. Finally, we combine continuum and
lattice models to show how interface modes from distinct valleys hybridize and how the resulting dispersions depend on microscopic interface properties. Our results establish design principles for controlling the dispersion, localization, and hybridization of Dirac interface states. We further examine two
graphene-based mass-domain-wall models as experimentally inspired examples of dispersive copropagating and counterpropagating interface states.
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