Materials Science · Cross-listed from physics.optics
Propagation Maps, Maradona Exceptional Points, and Pele Singularities in Anisotropic, Tellegen, Chiral, Moving-Medium, Omega and Other Isotropy-Broken Materials
Maxim Durach
Anisotropic, Tellegen,
chiral, moving-medium-type, omega, gyrotropic,
hyperbolic, and multi-
hyperbolic materials form an important class of isotropy-broken photonic media in which wave propagation can no longer be characterized by the Fresnel wave surface alone. Here we show that Fresnel wave surfaces can be converted into propagation maps that organize positive- and negative-phase-velocity propagation together with attenuation and amplification. In Hermitian media, the boundary between forward and backward propagation forms the Michelangelo silhouette separatrix. This separatrix is also a continuous locus of Maradona exceptional points, where the index-of-refraction operator becomes defective even though the material medium remains Hermitian. In non-Hermitian media, the attenuation-amplification boundary forms the Caravaggio chiaroscuro separatrix. The associated Pele singularities occur where the handedness remains continuous while the gain-loss character changes sign. Their physical importance is revealed by the momentum-resolved density of states: at these points, the Lorentzian linewidth of the non-Hermitian momentum-resolved density of states (DOS) collapses, producing sharp DOS peaks whose sign reverses across the separatrix. Thus, Pele singularities are threshold-like gain-loss singularities of the Fresnel wave-surface propagation map, generated by non-Hermitian linewidth collapse. The result is a compact
geometric language for describing how handedness, degeneracy, loss, gain, and momentum-resolved DOS are organized in isotropy-broken electromagnetic materials.
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Materials Science · Cross-listed from cond-mat.soft
A Design Framework for Compositional Hierarchical Mechanical Metamaterials via a Qualitative Unit-Cell Library
Shammo Dutta, Girish Krishnan and Sree Kalyan Patiballa
Hierarchically designed mechanical metamaterials involve nested levels of structural organization, mimicking natural structures (such as bones, wood, and bird feathers) to create advanced functional materials. Compositional hierarchy, a specific type of hierarchical strategy that involves the methodical assembly of discrete building blocks, offers unique advantages in engineering design due to its modular nature. This involves proper selection and spatial arrangements of distinct microstructures, as a result of which the desired macro-scale mechanical behavior can be achieved. Towards the design of such compositional hierarchical metamaterials, this paper presents a two-step design framework. First, material optimization of the design domain is performed using a parameterized elasticity
matrix to obtain optimal conceptual designs. Second, building-block microstructure geometries are selected from a qualitative library and subjected to shape-size refinement to satisfy the desired kinematic or stiffness requirements. To construct the qualitative library, a novel parametrization scheme is initially introduced, which categorizes the planar orthotropic elasticity
matrix into four distinct classes. Utilizing a kinetostatic load flow visualization technique, the candidate microstructure geometries are then populated within these four classes. The framework is validated for the design of a cantilever beam with a specified lateral stiffness requirement and the design of planar sheets that exhibit specified target deformation patterns. Thus, the present work provides a systematic and physically intuitive methodology applicable to arbitrary kinematic deformation and stiffness requirements.
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Materials Science · Cross-listed from physics.plasm-ph
Capturing many-body effects in electrical conductivity of warm dense matter
Brian P. Robinson, Alina Kononov, Lucas J. Stanek et al. (+3 more)
Conductivity models for warm dense matter inform simulations of planetary structure and fusion experiments. State-of-the-art conductivity calculations based on density functional theory approximate many-body physics and neglect electron-electron scattering lifetimes. We introduce a many-body framework for electrical conductivity using the GW approximation of the electronic self-energy. For beryllium, improved transition energies yield a surprisingly large reduction in low-temperature DC conductivity, while electron-electron scattering primarily reduces high-temperature DC conductivity.
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All authors Brian P. Robinson, Alina Kononov, Lucas J. Stanek, Andrew D. Baczewski, André Schleife, Stephanie B. Hansen
Materials Science · Cross-listed from cond-mat.supr-con
First-principles real-space embedding theory of the superconducting proximity effect
Nicolas Baù, Mitra Dowlatabadi, Tommaso Chiarotti et al. (+2 more)
When a
superconductor is placed in contact with a normal material, Cooper pairs penetrate the latter and induce superconductivity via the proximity effect. Despite its central role in
quantum materials, superconducting devices and topological platforms, a predictive first-principles description of the proximity effect at realistic interfaces has remained computationally prohibitive so far. Here, we fill this gap by developing a Green's-function framework based on real-space dynamical
embedding that enables first-principles simulations of superconducting proximity in mesoscopic systems. We show that the proximity effect admits a transparent diagrammatic formulation in terms of normal and anomalous
embedding self-energies, which disentangle and quantify the distinct renormalization mechanisms generated by coupling to a superconducting bath. By combining this formalism with recursive schemes, we compute local spectral functions and proximity lengths extending over hundreds of nanometers into the bulk without resorting to thick interface slabs. We deploy the approach on tight-binding models (Qi-Hughes-Zhang and Fu-Kane-Mele), where we analyze mixed-parity superconductivity in topological
insulators proximitized by $s$-wave
superconductors, and on first-principles simulations of NbSe$_2$/CrBr$_3$
heterostructures based on density-functional theory and maximally-localized Wannier functions, the latter enabling direct comparison with scanning tunneling spectroscopy experiments. Our work provides a scalable and conceptually unified framework that bridges microscopic electronic structure and mesoscale proximity physics, enabling predictive atomistic simulations of superconducting interfaces.
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All authors Nicolas Baù, Mitra Dowlatabadi, Tommaso Chiarotti, Massimo Capone, Antimo Marrazzo
Materials Science · Cross-listed from cond-mat.mes-hall
Zijian Zhou, Ran Jing, Heng Wang et al. (+3 more)
Hyperbolic polaritons are conventionally associated with structural anisotropy or phononic Reststrahlen bands. Here, we predict a new class of
hyperbolic polaritons arising from
magnetic-field-induced cyclotron motion of charge carriers. When a perpendicular
magnetic field is applied to high-mobility semimetals, the cyclotron response drives the in-plane dielectric function from metallic- to insulating-like below the cyclotron resonance frequency, while the out-of-plane response remains metallic. This anisotropy creates a
hyperbolic dielectric environment that supports field-tunable
hyperbolic polaritons. We develop a comprehensive theoretical framework incorporating coupling to other collective excitations and show that these modes can be directly visualized in real space via terahertz near-field nanoscopy. Our work identifies cyclotron motion as a new route to
hyperbolic polaritons and establishes a versatile platform for magnetically programmable nanophotonics.
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All authors Zijian Zhou, Ran Jing, Heng Wang, Lukas Wehmeier, Mengkun Liu, Bing Cheng
Materials Science
Symmetry Guided Band-Gap Opening via Periodic Topological Defects in Graphene
D. N. Garzon, Leonel Cabrera-Loor, Jacopo Gliozzi et al. (+3 more)
Graphene lacks an intrinsic band-gap, which limits its use in electronic applications. Here we demonstrate that periodic arrays of topological defects can open and control a band-gap in a predictable manner governed by defect spacing and
lattice symmetry. Using first-principles density functional theory calculations supported by tight-binding models, we investigate
graphene superlattices containing Stone-Wales and flower-like defects over a range of $N \times N$ periodicities, where $N$ determines the defect separation. We show that band-gap opening occurs only when translation
symmetry is reduced in a specific way: for supercells with $N$ a multiple of three, Brillouin-zone folding brings the Dirac cones at $K$ and $K'$ to the same momentum in the reduced Brillouin zone. In particular, flower-like defect superlattices produce larger and tunable band-gaps, whose magnitude decreases systematically with increasing defect separation and approaches zero in the dilute-defect limit. These results establish a predictive framework for band-gap engineering in defect-patterned
graphene and clarify the microscopic mechanism underlying gap formation in periodically reconstructed
lattices.
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All authors D. N. Garzon, Leonel Cabrera-Loor, Jacopo Gliozzi, Marco Fronzi, Catherine Stampfl, Henry P. Pinto
Materials Science
Synergistic doping of the grain interior and grain boundary alters deformation mechanisms and enables extreme strength in nanocrystalline Ni-Cr-Y alloys
Yi Liu, Jason R. Trelewicz and Timothy J. Rupert
Solid solution addition and grain boundary segregation have been independently shown to enhance the strength of nanocrystalline
alloys. In the present study, the interplay between these two effects is investigated in nanocrystalline Ni-Cr-Y sputtered films through systematic variation of alloying element contents with grain size kept constant. Cr is introduced into a solid solution and serves to strengthen the
lattice, while Y segregates to the grain boundaries to stabilize these features. Nanoindentation is used to probe hardness, with unexpected trends and very high values observed. Cr additions led to nanocrystalline solid solution strengthening, yet saturation was observed at higher concentrations due to the emergence of grain boundary dominated processes, as evidenced by pile-up morphologies containing slip steps and grain rotation. Y enrichment was observed at the examined grain boundaries, enhancing boundary-mediated strengthening by pinning the dislocations and suppressing dislocation emission, grain boundary sliding, and grain rotation processes. With increasing Y concentration, the nanocrystalline solid solution strengthening effect induced by Cr addition becomes weaker. This phenomenon can be attributed to a reduced dislocation bowing distance caused by dopant pinning. Most notably, the strongest ternary Ni-Cr-Y
alloy exhibited a hardness of 11.0 GPa, among the highest hardness values reported for single-phase nanocrystalline Ni
alloys with grain sizes near 20 nm. These findings highlight how tuning grain and grain boundary chemistry offers a viable strategy to control dislocation mechanics and improve the strength of nanocrystalline metals.
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Materials Science · Cross-listed from cond-mat.str-el
Contrasting structural reversibility and magnetic correlations in isostructural honeycomb magnets CrCl$_3$ and $\alpha$-RuCl$_3$
Zachary Morgan, Iris Ye, Jiasen Guo et al. (+2 more)
We report a comparative
neutron single
crystal diffraction study of the structural and
magnetic properties of layered halides CrCl$_3$ and $\alpha$-RuCl$_3$, which host a honeycomb arrangement of transition metal ions with distinct electronic configurations and undergo a first-order structural transition between high-temperature C2/m and low-temperature R$\bar{3}$. Both compounds show a step-like change in the $c$-
lattice, consistent with an expected stacking rearrangement. In contrast, the in-plane
lattice response is quite different: $\alpha$-RuCl$_3$ exhibits an abrupt hysteretic change across the transition accompanied by progressive crystalline degradation upon thermal cycling, whereas CrCl$_3$ shows a smooth in-plane
lattice evolution and remains structurally robust. Magnetically, CrCl$_3$ orders into an A-type
antiferromagnetic structure at T$_N$=14\,K and exhibits pronounced diffuse
magnetic scattering extending up to about 40\,K. $\alpha$-RuCl$_3$ shows no observable
magnetic diffuse scattering above its zig-zag
antiferromagnetic ordering temperature T$_N$=7.6\,K. These results suggest that the contrasting structural and
magnetic behaviors arise from an interplay between interlayer sliding energetics and the fundamentally different electronic configurations of the two compounds.
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All authors Zachary Morgan, Iris Ye, Jiasen Guo, Michael A McGuire, Jiaqiang Yan
Materials Science
Application of the exact-factorization density-functional perturbation approach to pentacene crystal and monolayer MoS2
Rachel Steinitz-Eliyahu, Galit Cohen, Guy Vosco et al. (+3 more)
Non-adiabatic effects arising from electron-
phonon interactions are often neglected within the Born-Oppenheimer (BO) approximation, which assumes that electronic states adjust instantaneously to
nuclear motion. The exact factorization (EF) formalism provides a rigorous framework for treating such effects beyond the adiabatic regime and has recently been adapted to density functional theory (DFT) in the harmonic limit. Building on these foundations, we previously introduced an EF-based perturbative scheme, the EF density-functional perturbation theory (EF-DFPT), that enables the computation of
phonon-driven non-adiabatic (NA) corrections to Kohn-Sham (KS) electronic states, up to second order in
nuclear displacements. Here, we present the first implementation and application of EF-DFPT to extended periodic materials, focusing on its impact on experimentally relevant observables. Using the pentacene molecular
crystal and monolayer MoS2 as representative soft- and stiff-mode systems, respectively, we demonstrate how NA electron-
phonon interactions modify the static dielectric response. We show that these modifications originate from the combined effect of NA
phonon-dressed electronic wavefunctions and second-order NA energy renormalizations. The resulting behavior is strongly material dependent: NA effects are negligible in monolayer MoS2, whereas in pentacene they lead to pronounced long-range screening effects associated with soft vibrational modes and enhanced electron-
phonon coupling.
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All authors Rachel Steinitz-Eliyahu, Galit Cohen, Guy Vosco, E.K.U. Gross, Ryan Requist, Sivan Refaely-Abramson
Materials Science
Adamantane plasma polymers: fluorine-free vacuum-processable triboelectric thin films for all-triboelectric nanogenerator configurations
Gloria P. Moreno-Martinez, Fernando Nunez-Galvez, Hari Krishna Mishra et al. (+12 more)
Triboelectric nanogenerators (TENGs) are major drivers in on-site power generation for smart devices, enable self-powered sensors, and introduce novel catalytic processes. Here, we present the advantages of adamantane
plasma layers as bivalently triboelectric surfaces capable of exhibiting both tribopositive and tribonegative character through simple modification of the synthesis conditions without the need for additives or functionalization. Fabrication facing or backfacing the
plasma yields thin film
polymers with different dielectric constants, Young's
moduli, and secondary electron emission. The conformality, stability, and processability of the
polymers enable direct implementation across solid-solid, solid-
liquid, and hybrid piezo-triboelectric configurations. Additional texturization by buckling is shown to provide voltage and current outputs as high as 90 V cm2 and 0.6 uA for a 2.8 um (tribonegative) vs. 400 nm (tribopositive) combination. A maximum power density of 2.1 uW cm-2 is generated from salty droplets in a switch-electrode drop-TENG configuration employing a 500 nm-thick tribopositive adamantane
polymer as the triboelectric surface. These layers have demonstrated outstanding durability, enabling more than 10^5 cycles in solid-solid nanogenerators and 10^4 droplet impacts in solid-
liquid configurations. The synthetic method is environmentally friendly and industrially scalable, making the adamantane
plasma polymer a reliable and competitive solution for thin film triboelectric materials.
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All authors Gloria P. Moreno-Martinez, Fernando Nunez-Galvez, Hari Krishna Mishra, Triana Czermak, Xabier Garcia-Casas, Vanda Cristina Godinho, Bernd Wicklein, Juan Carlos Sanchez-Lopez, Javier Ferrer, Isabel Montero, Juan Ramon Sanchez-Valencia, Andris Sutka, Francisco Aparicio, Angel Barranco, Ana Borras
Materials Science
Optical selection rules in hexagonal Ge polytypes and their lifting by symmetry perturbation
Martin Keller, Haichen Wang, Friedhelm Bechstedt et al. (+2 more)
Hexagonal germanium polytypes have emerged as promising direct-gap
semiconductors for silicon-integrated optoelectronics, yet their optical properties remain largely unexplored beyond the well-studied 2H phase. We present a comprehensive theoretical study of optical properties of hexagonal 2H-, 4H-, and 6H-Ge polytypes through ab initio calculations of quasiparticle band structures, dipole transition
matrix elements, and solution of the Bethe-Salpeter equation. While all three polytypes exhibit direct band gaps of increasing size from 2H to 6H, we reveal that the fundamental optical transition in 4H-Ge is parity-forbidden due to matching band parities at the valence and conduction band edges. This selection rule results in a radiative lifetime seven orders of magnitude longer than in 2H- and 6H-Ge, severely limiting light emission capabilities. To demonstrate that the selection rule can be lifted, we introduce controlled
symmetry perturbations by substituting single Ge atoms with Si in each unit
cell, breaking the
crystal symmetry. This perturbation increases the optical
matrix elements by up to two orders of magnitude and reduces radiative lifetimes for all perturbed polytypes. We also compute absorption coefficients and frequency-dependent dielectric tensors for both light polarizations, including excitonic effects up to 5 eV, providing complete optical characterization of ideal and
symmetry-perturbed hexagonal Ge systems relevant for optoelectronic applications.
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All authors Martin Keller, Haichen Wang, Friedhelm Bechstedt, Jürgen Furthmüller, Silvana Botti
Materials Science · Cross-listed from cond-mat.supr-con
Transverse Magnetic Response from Orbitally Polarized Cooper Pairs in Elemental Superconductors
Gabor Csire, Maria Teresa Mercaldo, Balazs Ujfalussy et al. (+2 more)
We demonstrate how crystalline
symmetry lowering, as for instance through strain, allows elemental
superconductors such as vanadium and niobium to realize
spin-singlet orbitally polarized Cooper pairs composed of electrons with identical orbital moments. Using superconducting density functional theory, we show that lowering of trigonal
symmetry to $C_s$, thus keeping only a single mirror plane, activates interorbital pairing in bulk and (111) surfaces, with a pronounced surface enhancement. In a
magnetic field, the resulting orbitally polarized superconducting state leads to a novel transverse
magnetic response. For in–plane field orientations that break the remaining mirror
symmetry, a sizable orbital magnetization emerges perpendicular to the applied field. We show that this effect is a direct consequence of equal–orbital-moment Cooper pairing, providing an experimentally accessible signature of this state. Our results establish strained elemental
superconductors as a minimal material platform for superconducting orbitronics.
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All authors Gabor Csire, Maria Teresa Mercaldo, Balazs Ujfalussy, Carmine Ortix, Mario Cuoco
Materials Science · Cross-listed from cond-mat.supr-con
Ginzburg-Landau Theory for Confined Thin-Film Superconductors
Giovanni A. Ummarino and Alessio Zaccone
We develop a Ginzburg-Landau theory for superconducting thin films under
quantum confinement. Starting from the Ginzburg-Landau theory and the recently developed confinement theory of metallic thin films, explicit analytical expressions are derived for the coherence length, penetration depth, electronic mean free path, and Ginzburg-Landau parameter in confined geometries. The central result is that
quantum confinement directly renormalizes the intrinsic superconducting coherence length through confinement-induced modifications of the electronic density of states and
Fermi energy. This effect is absent in conventional thin-film transport theories based solely on surface scattering. As a consequence, confinement simultaneously suppresses the coherence length and enhances the penetration depth, thereby driving
superconductors toward stronger type-II behavior over a broad thickness range. The interplay between
quantum confinement and disorder can also produce reentrant type-I/type-II crossovers. The theory predicts a crossover regime in which confinement-induced renormalization of superconducting length scales and transport scattering become strongly intertwined. Comparison with recent penetration-depth measurements in Al thin films shows that the observed enhancement of the penetration depth originates from the interplay between confinement-induced renormalization of the coherence length and suppression of the effective mean free path by surface and disorder scattering. The results establish a direct connection between
quantum confinement and superconducting electrodynamics in confined metallic films.
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Materials Science
Micro-environment of the Eu interstitial in $\beta$-SiAlON:Eu$^{2+}$ green phosphor
Julien Bouquiaux, Samuel Poncé, Yongchao Jia et al. (+2 more)
The precise atomic-scale structure around Eu$^{2+}$ activators in the $\beta$-Si$_{6-z}$Al$_z$O$_z$N$_{8-z}$:Eu$^{2+}$ commercial green phosphor remains elusive. We use the first-principles $\Delta$SCF excited-state method,
embedding of the interatomic force constants for supercells up to 3501 atoms, and Huang-Rhys theory to clarify this issue. Monte Carlo exploration is used to identify representative low-energy structural models spanning different levels of Al/O concentration $z$. For the lowest-energy structure at low $z$, our computed photoluminescence spectrum reproduces the experimental vibronic peaks at 6~K with excellent agreement in peak positions and intensities, validating the Eu-N$_9$ coordination model with Al, O, and Eu confined to the same crystallographic plane. Analysis of the low-energy structures reveals that the electron-
phonon coupling is weak ($S \approx 2.15$) with a robust characteristic
phonon signature across different Al/O arrangements, explaining the surprising persistence of resolved
phonon replicas with increasing $z$. We explain the experimentally observed red-shift of emission with increasing $z$ through systematic trends in zero-
phonon line energies, modest increases in Huang-Rhys factors, and larger configurational diversity at higher compositions.
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All authors Julien Bouquiaux, Samuel Poncé, Yongchao Jia, Masayoshi Mikami, Xavier Gonze
Materials Science
Oxygen vacancies beyond the dilute limit in doped CaMnO3 perovskites and implications for screening materials in thermochemical applications
Harender S. Dhattarwal, Colin M. Hylton-Farrington, Ian G. McKendry et al. (+2 more)
Thermochemical energy storage (TCES) in
oxide perovskites relies on reversible oxygen vacancy formation, and computational high-throughput screening of candidate materials has predominantly used the single oxygen vacancy formation energy (OVFE) as the key descriptor. We demonstrate that the OVFE is insufficient for screening cubic CaMnO3 perovskites, because the stoichiometric compound is not the minimum energy reference state; vacancies are inherently present at operating temperatures. Materials with negative single OVFEs are routinely excluded from screening
datasets as unsuitable, but this reflects a mischoice of reference state rather than a genuine materials limitation, and risks discarding promising TCES candidates. We address this by computing OVFEs as a function of vacancy concentration using ab initio density functional theory, establishing the
equilibrium vacancy concentration as the correct reference point. OVFE curves referenced to this minimum align with experimentally measured reduction enthalpies, providing a framework directly comparable to experiments. We further show that A-site and B-site doping modify the vacancy formation landscape through distinct mechanisms. A-site dopants act primarily through strain relaxation and
symmetry breaking, while B-site dopants reshape the local redox environment and introduce strong configurational dependence. Finally, we develop a thermodynamic model incorporating configurational
entropy that accurately predicts
equilibrium oxygen stoichiometry as a function of temperature and oxygen partial pressure and reveals that selective reduction of Mn4+ versus B-site dopant ions can tune the onset temperature for vacancy formation. These results establish a screening framework for perovskite TCES materials and provide practical guidance for extending high-throughput workflows beyond the single-vacancy paradigm.
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All authors Harender S. Dhattarwal, Colin M. Hylton-Farrington, Ian G. McKendry, Christopher Abram, Richard C. Remsing
Materials Science
Thermodynamics and dynamics of non-compact prismatic dislocation loops simulated using a machine-learning model
Sho Hayakawa, Sergei L. Dudarev and Max Boleininger
We explore how the thermodynamic properties and dynamics of a self-interstitial prismatic dislocation loop are affected by microscopic-scale variations in its
geometric configuration, an aspect that rarely received
attention in literature. We develop a machine-learning (ML) model to predict the formation energy of an arbitrary geometrically complex configuration of a self-interstitial atom dislocation loop. Trained on atomistic simulation data, the ML model achieves high predictive accuracy across a broad range of configurations, with a typical error in the 1% range. From the ML model, we evaluate the density of configurational microstates as a function of loop's formation energy and derive analytical expressions valid in tractable limiting cases. Using statistical mechanics, we derive the configurational free energy, the average energy, and the thermodynamic
entropy of a dislocation loop as a function of temperature. We also simulate the dynamics of self-climb of dislocation loops with various geometries and evaluate their
diffusion coefficients and effective activation energy for
diffusion. Analysis shows that there is a single universal parameter describing the morphological irregularity of the loop configuration in its ground state. This parameter determines the thermodynamic properties of a loop as well as its dynamics, and simulations illustrate how the properties and mobility of a configurationally complex loop vary as functions of the irregularity parameter.
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Materials Science
Data-driven body-centered cubic phase prediction in cobalt free high-entropy alloys
Xuliang Luo, Yulin Li, Tero Mäkinen et al. (+3 more)
High-
entropy alloys (HEAs) are known for superb combination of performance attributes, making them ideal for advanced applications, e.g.,
nuclear engineering. The concept of cobalt-free HEAs aims to mitigate concerns about cobalt's radioactivity, however, predicting their phase formation remains challenging due to their complex compositions. In this work, we integrate six semiempirical parameters, i.e., mixing
entropy ({\Delta}Smix), mixing enthalpy ({\Delta}Hmix), atomic size difference ({\delta}), valence electron concentration (VEC), d-orbital energy level (Md), and the {\Omega} parameter, along with machine learning (ML) to predict the body-centered cubic phase stability in Co free HEAs. To address the limitations of experimental data, generative
adversarial networks were used to augment the
dataset, thus improving the accuracy of the Gaussian process classification model used for phase prediction. After dimensionality reduction to five principal components, the model achieved an accuracy of 84%, with {\Delta}Hmix and {\delta} identified as the key descriptors influencing phase formation. This approach highlights the synergy of ML and data augmentation in accelerating the design of HEAs for advanced applications.
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All authors Xuliang Luo, Yulin Li, Tero Mäkinen, Silvia Bonfanti, Wenyi Huo, Mikko J. Alava
Materials Science
Rare-Earth-Tuned Evolution from d- to f-Orbital Dominance and Giant Anomalous Hall Effect in Topological RGaGe (R = Ce, Pr, Nd) Semimetals
Zhian Xu, Jian Yuan, Ze Yan et al. (+4 more)
The family of noncentrosymmetric rare-earth germanides RGaGe (R = Ce, Pr, Nd) provides a rich materials platform to explore the intertwined physics of strong magnetism, electronic correlations, and topological band structures. Through a combination of
crystal growth, characterization, and first-principles calculations, we reveal that these compounds exhibit a pronounced uniaxial
magnetic anisotropy, leading to distinct ground states: RGaGe orders ferromagnetically with moments along the crystallographic c-axis, and shows an
antiferromagnetic-like structure in the ab-plane. A key finding is a significantly enhanced intrinsic anomalous
Hall conductivity (AHC) compared to their well-known RAlGe counterparts, which even reaches as high as 948 {\Omega}-1 cm-1 at 2 K in PrGaGe. Our theoretical analysis predicts that this AHC originates from a robust Weyl semimetallic state driven by inversion
symmetry breaking, where Weyl points near the
Fermi level couple strongly to the
magnetic order. Importantly, this topological state persists above the
magnetic ordering temperature, confirming its intrinsic electronic origin. Our calculation also reveals that, while the near-
Fermi-level states in CeGaGe and PrGaGe are dominated by d-orbital contributions, NdGaGe exhibits significant f-orbital involvement, signaling a progressive
evolution from d- to f-orbital dominated
topology. These results establish the RGaGe system as a tunable platform for systematically extending the RAlGe-related family, showcasing a large anomalous
Hall response and orbital
evolution near the
Fermi level, and advancing the understanding of the interplay between
topology and magnetism in
quantum materials.
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All authors Zhian Xu, Jian Yuan, Ze Yan, Xia Wang, Na Yu, Shihao Zhang, Yanfeng Guo
Materials Science
Laser-induced demagnetization in a MAX phase (Cr0.5Mn0.5)2GaC
Iaroslav Mogunov, Artyom Gorshkov, Mikhail Rautskii et al. (+2 more)
Magnetic MAX phases are nanolaminated metals that combine ceramic-like thermal and mechanical stability with peculiar
magnetic ordering, making them attractive for thin-film optoelectronics and spintronics. However,
laser-induced magnetization dynamics in MAX phases remains largely unexplored. Here, we investigate
laser-induced ultrafast demagnetization in a 40-nm-thick epitaxial film of the
magnetic MAX phase (Cr0.5Mn0.5)2GaC, which magnetically orders below ~220 K, using time-resolved magneto-optical Kerr effect spectroscopy. We reveal, that the demagnetization transients exhibit a two-step type-II demagnetization, common for two-dimensional
magnetic systems. The second stage dominates the process and has a characteristic time of approximately 100 ps. Applying the three-temperature model, we extract the electron-
lattice,
spin-
lattice, and electron-
spin coupling constants. The reconstructed
spin heat capacity exhibits a weak temperature dependence, accounting for only a subtle slowing down of demagnetization at elevated temperatures and fluences.
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All authors Iaroslav Mogunov, Artyom Gorshkov, Mikhail Rautskii, Tatyana Andryushchenko, Alexandra Kalashnikova
Materials Science · Cross-listed from cs.LG
QT-Net: Rethinking Evaluation of AI Models in Atomic Chemical Space
Pablo Martínez Crespo, Stefano Ribes, Martin Rahm et al. (+6 more)
Atomic properties such as partial charges or multipoles encode chemically meaningful information that can inform downstream molecular property prediction, but their evaluation as machine learning targets has been complicated by the absence of a principled out-of-distribution evaluation protocol at the atomic level. In this work, we propose a held-out evaluation protocol that clusters atomic environments by SOAP descriptors and computes metrics accounting only for cluster labels unseen during training. Following this procedure, we use 5$\times$5 cross-validation and Tukey's HSD to run a statistically rigorous comparison of E(3)-equivariant against non-equivariant, rotationally augmented models for predicting electron populations and multipoles of H, C, N, and O atoms. Building on our results, we introduce the
Quantum Topological Neural Network (QT-Net), a rotationally augmented, non-equivariant graph neural network. We show that QT-Net can be used to infer properties of atoms in molecules from QM9 outside our training set, and that these inferred properties can yield improvement when used as input features for downstream molecular property prediction. To further validate the framework, molecular dipole moments computed from QT-Net's per-atom outputs recover the ground-truth values reported in QM9. We release all code and data, including a JAX implementation of QT-Net, to support the broader use of learned QTA properties as inductive biases for atomic-scale molecular machine learning.
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All authors Pablo Martínez Crespo, Stefano Ribes, Martin Rahm, Richard Beckmann, Robert S. Jordan, Marisa Gliege, Santiago Miret, Vijay Kris Narasimhan, Rocío Mercado
Materials Science
Effective dynamic constants for nonequilibrium third-principles simulations
Mauro Pulzone, Iñigo Robredo-Magro and Jorge Íñiguez-González
Computational studies of the thermodynamic properties of materials at the mesoscopic and macroscopic scales – involving lengths and times of at least $\mu$m and $\mu$s, respectively – rely on a coarse-graining approximation such that only a few relevant collective variables are treated explicitly. Those variables typically take the form of fields defined everywhere in space or macroscopic quantities when spatial inhomogeneities can be treated implicitly. The free energy is usually expressed as a Landau-like potential whose temperature-dependent minima track stable states, characteristic
equilibrium fluctuations being implicitly accounted for. Further, the response of the system to external perturbations, and its relaxation toward thermal
equilibrium, are described in terms of simple equations of motion governed by effective inertial and viscous-damping constants. There is considerable literature on the problem of deriving Landau free energy potentials, from either experiment or predictive atomistic simulations, including recent efforts to develop systematic machine-learning approaches that we denote “third principles”. Much less
attention has received the calculation of the effective constants controlling the nonequilibrium macroscopic or mesoscopic dynamics. Here we tackle that problem, describing a protocol that allows us to compute the temperature-dependent inertial and damping coefficients associated to the electric polarization in representative soft-mode ferroelectric PbTiO$_{3}$. Our scheme lends itself to a widespread application, although the non-trivial behaviors found in PbTiO$_{3}$ suggest that more case studies will be needed to finetune a general and robust calculation protocol. Our results also allow us to comment on common assumptions in the literature of effective dynamic treatments of ferroelectrics and related materials.
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Materials Science
Strain-Enhanced Coherence in Curved hBN Quantum Emitters
Eyal Shoham, Sukanta Nandi, Ayelet Teitelboim et al. (+4 more)
Hexagonal boron nitride (hBN) hosts robust room-temperature single-
photon emitters, yet their coherence is typically limited by
phonon induced dephasing and spectral broadening. Here, we show that thermally induced
curvature in bulk like hBN flakes provides a strain enabled route to suppress defect
phonon coupling under ambient conditions. Nanoscale bubbles formed by thermal processing generate strong through thickness strain gradients, which we directly probe by infrared nano spectroscopy. These measurements reveal strain induced splitting of in-plane
phonon modes, evidencing a substantial local modification of the
phonon density of states.
Quantum emitters localized within these curved regions exhibit markedly enhanced room temperature spectral purity, with Debye Waller factors of 0.91 and narrower line widths than emitters in flat regions.
Photon correlation measurements confirm high-purity single
photon emission at room temperature. Supported by first-principles calculations, we attribute this behavior to strain driven
phonon redistribution, which depletes
phonons in tensile regions and accumulates them in compressive regions, thereby creating locally
phonon suppressed environments for defect emitters. These results establish strain engineering as an effective route for
phonon control in hBN and open a pathway toward high coherence, room-temperature
quantum light sources for integrated nano photonic platforms.
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All authors Eyal Shoham, Sukanta Nandi, Ayelet Teitelboim, Jeny Jose, Gil Atar, Ashwin Ramasubramaniam Tomer Lewi, Doron Naveh
Materials Science
Saddle-node bifurcation in interfacial morphology selects battery degradation phase
Changdeuck Bae
We propose a minimal nonlinear closure ODE for the dynamic active-area factor of a battery interface and show that it exhibits a saddle-node bifurcation when the smoothing rate saturates with surface roughness. The closure is the simplest physically motivated extension of a recently introduced single-fixed-point closure [C. Bae, in preparation (2026)]: u = K - u/(1 + alphau^2), where u = xi - 1 is the dimensionless excess active area, K the dimensionless drive, and alpha a single saturation parameter. The bifurcation occurs at K_c = 1/(2sqrt(alpha)), separating a smooth passivating phase from a morphologically unstable phase. Mapping four canonical anode configurations – graphite, silicon composite, lithium metal, and anode-free Li/Cu – onto the closure via end-of-cycling steady-state xi extracted from publicly available long-cycle data populates the stable branch with monotonically increasing K/K_c ratios: graphite (~0.01), silicon composite (~0.24), lithium metal (~0.73), and anode-free (~0.95). The anode-free configuration sits within 5% of the saddle-node threshold, predicting a vanishingly small operational stability window in current density, temperature, and electrolyte composition. We test three falsifiable predictions of the framework – a critical current density, a critical temperature shift, and a mean-field critical-slowing-down exponent – and find them broadly consistent with publicly available data. We argue that this near-critical position is universal to nucleation-controlled deposition on non-passivating substrates.
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Materials Science
Parabolic-growth universality and its nucleation-driven breakdown across lithium-battery anode chemistries
Changdeuck Bae
Solid-electrolyte interphase (SEI) growth is widely modeled
cell-by-
cell with chemistry-specific closures, yet its underlying kinetic scaling is rarely tested across chemistries. By compiling cycle-resolved data from public long-cycle
datasets covering four anode configurations – graphite, silicon composite, lithium metal, and anode-free – we show that the cumulative interphase-loss index Lambda_int obeys the
parabolic law Lambda_int = A_chem * sqrt(1 - Theta_Li) in three of the four chemistries, with an exponent indistinguishable from alpha = 1/2 within experimental uncertainty. The chemistry-specific prefactor A_chem spans an order of magnitude, but the
diffusion-limited
parabolic kinetics is preserved. The fourth chemistry, anode-free configurations, deviates with a super-
parabolic exponent alpha approx 0.77, consistent with a nucleation-controlled growth regime. We rationalize the result using the Tammann-Deal-Grove
parabolic-growth framework adapted to interphase formation and identify the conditions under which universality is recovered. The observed regularity reduces SEI modeling complexity to a single rate constant per chemistry and provides a sharp falsifiable test for next-generation
cell formats.
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Materials Science
Non-homogeneous structure of complex concentrated alloys: Effect of intrinsic strain
Vaclav Paidar, Pavel Lejcek and Andrea Skolakova
Even if the atoms of a multicomponent
alloy occupy a common
lattice, their distribution is not homogeneous, and regions with different compositions can be detected. Three representative examples will be discussed: a Cantor-type system containing transition-metal elements (Cr, Mn, Fe, Ni, and Co), a refractory high-
entropy alloy (Ti, Zr, Nb, Ta, and Mo), and a multicomponent system combining transition and refractory metals (Cu, Ni, Ti, Zr, and Hf). Using a combination of theoretical analysis and experimental observations, we demonstrate that the formation of locally segregated regions can lead to a reduction in the overall energy of the system. This stabilization arises from the compensation of tensile and compressive strain fields associated with atoms of different sizes, highlighting the key role of local chemical and structural heterogeneity in determining the thermodynamic stability of multicomponent
alloys.
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Materials Science
Ytterbium charge state and stabilization in the Ba(Ca)F$_2$ host by electron paramagnetic resonance and infrared photoluminescence
David John, Shelja Sharma, Marius Stef et al. (+18 more)
Lanthanide-doped fluorides are promising materials for advanced photonic and
quantum applications due to their wide bandgap, low
phonon energy, and chemical stability. In this work, we present a systematic comparative study of ytterbium incorporation at low doping levels (0.05–0.2 mol%) in BaF$_2$ and CaF$_2$ single
crystals, focusing on the interplay between host
lattice properties, charge-state stabilization, and defect formation mechanisms. Using a combination of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), transmittance, and infrared photoluminescence (IR PL), we explore how host
lattice properties affect the stabilization of Yb$^{3+}$ and Yb$^{2+}$ ions. XRD confirmed cubic phase purity and
lattice parameter stability in both hosts, while XPS revealed surface chemical composition variations associated with charge-compensating defects and trace
impurities. EPR spectra indicated that BaF$_2$ favored perturbed Yb$^{3+}$ environments with increasing dopant levels, while CaF$_2$ maintained predominantly unperturbed sites, suggesting a more favorable ionic match for Yb$^{2+}$. Photothermal deflection spectroscopy (PDS) and IR PL results showed host-specific optical responses, with CaF$_2$ exhibiting
crystal-field splitting and broader local field effects. These results reveal a clear decoupling between long-range structural stability and local
lattice perturbations, and demonstrate that host cation identity governs the balance between Yb$^{2+}$ and Yb$^{3+}$ stabilization as well as defect-driven optical behavior. This offers valuable insights for optimizing rare-earth-doped fluoride
crystals in
laser, scintillator, and
quantum device applications.
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All authors David John, Shelja Sharma, Marius Stef, Gabriel Buse, Zdeněk Remeš, Anna Artemenko, Sergii Chertopalov, Vineet Sikarwar, Alan Mašláni, Jafar Fathi, Jakub Pilař, Tomáš Hostinský, Jan Zich, Tomáš Mates, Brenda Natalia Lopez Nino, Michal Hlína, Karol Bartosiewicz, Marina Konuhova, Anatoli Popov, Ján Lančok, Maksym Buryi
Materials Science
Chiral Porphyrin Monolayers on Ferromagnetic Thin Films: Ultrafast Spectroscopy of Hybrid Interfaces
Karol Hauza, Anna Lewandowska-Andralojc, Ruslan Salikhov et al. (+5 more)
Hybrid
ferromagnetic metal/organic interfaces (spinterfaces) exhibit unique properties, including
spin filtering. In parallel,
chiral organic molecules can themselves induce efficient
spin filtering, leading to unexpectedly high
spin polarizations. Here, we investigate how the proximity of gold-capped Co/Ni
ferromagnetic multilayers influences the spectroscopic properties and photoinduced electron dynamics of
chiral oligopeptides bearing a porphyrin chromophore. The molecules are covalently attached to the gold cap via a
chiral linker, forming a self-assembled monolayer. The porphyrin macrocycles adopt an orientation parallel to the surface, resulting in the formation of J-like aggregates. Photoinduced dynamics are probed using femtosecond pump-probe transient absorption spectroscopy. Despite excitation of only a single molecular layer, a clear transient absorption signal of the porphyrin singlet excited state is observed. Adsorption on the metal surface leads to a pronounced reduction of the excited-state lifetime. However, no signatures of long-lived photoinduced charge-transfer products are detected. Furthermore, no dependence of the excited-state dynamics on either the magnetization direction of the
ferromagnetic layer or the molecular chirality is observed.
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All authors Karol Hauza, Anna Lewandowska-Andralojc, Ruslan Salikhov, Jürgen Lindner, Gotard Burdzinski, Marcin Kwit, Bronislaw Marciniak, Aleksandra Lindner
Materials Science · Cross-listed from cs.LG
Generating Symmetric Materials using Latent Flow Matching
Anmar Karmush, Cedric Mathieu Brandenburg, Soheil Ershadrad et al. (+3 more)
Tackling the task of materials generation, we aim to enhance the previously proposed All-atom
Diffusion Transformer (ADiT) by introducing SymADiT, a
symmetry-aware variant. To do so, we use a representation of materials based on Wyckoff positions. We follow ADiT and perform generative modelling in latent space, adapted to our
symmetry-aware representation. By forcing the output of the generative model to adhere to the
symmetry restrictions imposed by the generated
crystal's space group and each atom's Wyckoff-position, the generated materials exhibit more realistic
symmetry properties. We
benchmark our method against both
symmetry-aware and
symmetry-agnostic models for materials generation and show competitive performance, generating stable, symmetric materials with a simple
Transformer architecture.
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All authors Anmar Karmush, Cedric Mathieu Brandenburg, Soheil Ershadrad, Johanna Rosén, Michael Felsberg, Filip Ekström Kelvinius
Materials Science · Cross-listed from cond-mat.soft
A molecular perspective on coordination, screening, and emergent length scales in lithium electrolytes
Amaury Coste, Eva Zunzunegui-Bru, Ambroise van Roekeghem et al. (+2 more)
Lithium electrolytes are commonly described using separate conceptual frameworks for local coordination chemistry, electrostatic screening, and ionic transport. This separation is effective in dilute conditions but breaks down at higher concentration, where coordination, ion pairing, clustering, and collective dynamics become intrinsically coupled. In this Perspective, we develop a unified multiscale framework that links local coordination motifs, mesoscopic ionic organization, and macroscopic transport within a single physical picture. Guided primarily by representative examples from molecular simulations, spanning carbonate
liquids,
polymer electrolytes, concentrated systems, and confinement, we show that increasing concentration drives a systematic
evolution from solvent-dominated Li$^+$ coordination to ion pairing, clustering, and correlated ionic domains. In this regime, screening and transport are not independent phenomena but emerge from the same underlying correlated structures. The resulting hierarchy provides both a conceptual framework for interpreting electrolyte behavior and a foundation for multiscale computational approaches, including machine-learning-assisted coarse-graining, that connect atomistic chemistry to macroscopic electrochemical performance. This Perspective implies that rational electrolyte design must simultaneously control short-range coordination, mesoscale organization, and collective electrostatic response.
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All authors Amaury Coste, Eva Zunzunegui-Bru, Ambroise van Roekeghem, Ioannis Skarmoutsos, Stefano Mossa
Materials Science · Cross-listed from quant-ph
A Single-Molecule Spin-Photon Interface
Simon Roggors, Thomas Unden, Anna Aubele et al. (+12 more)
Optical interfaces that connect long-lived
spin qubits to
photons are a central requirement for
quantum networking and distributed
quantum information processing. Currently, solid-state atomic defects are leading candidates due to their inherent
spin and optical coherence. Building on these advancements, synthetically tailored molecular systems represent a fundamental change in the field, utilizing precise atomic control and consistent bottom-up assembly. However, the lack of a robust
spin-
photon interface combining bright fluorescence, high spectral stability, and the persistent
spin lifetimes inherent to ground-state systems has prohibited the detection of individual molecular
qubits. Here we show that a triplet ground state carbene molecule, embedded within a structurally matched host
crystal, functions as a robust
spin-
photon interface with single-molecule addressability. The system exhibits narrow zero-
phonon lines, spectral stability over more than an hour,
spin-selective optical transitions and single-molecule optically detected
magnetic resonance. Coherent control yields millisecond-scale dynamical-decoupling coherence and tens-of-milliseconds
spin relaxation at a temperature of 4.5 K. These results establish molecular
qubits as a viable platform for single-emitter
quantum optics while preserving the advantages of bottom-up chemical design and processable materials.
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All authors Simon Roggors, Thomas Unden, Anna Aubele, Paul Mentzel, Gregor Bayer, Alon Salhov, Jochen Scharpf, Martin B. Plenio, Alex Retzker, Fedor Jelezko, Tim R. Eichhorn, Tobias A. Schaub, Matthias Pfender, Philipp Neumann, Ilai Schwartz