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Physics Condensed Matter Materials Science

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Friday 22 May

Materials Science · Cross-listed from cond-mat.mes-hall

Momentum-Resolved Tunneling Modulation Induced Giant Multistate Resistance in Antiferroelectric Multiferroic Junction

Wei Yang, Yibo Xu, Shen Li et al. (+6 more)

Multiferroic tunnel junctions (MFTJs), integrating ferroelectric and ferromagnetic functionalities within a single nanoscale device, hold significant promise for non-volatile, multi-state memory and innovative computing paradigms. In conventional MFTJs, tunneling resistance modulation relies primarily on ferroelectric (FE) polarization switching, which alters interfacial electric fields and shifts the Fermi level of adjacent ferromagnetic electrodes. However, achieving high tunnelelectroresistance (TER) through this approach demands strong built-in electric fields, which simultaneously hinder FE polarization switching, creating an intrinsic trade-off between reliable data reading and efficient writing. Here, we propose a dual mechanism that combines antiferroelectric (AFE) phase-transition modulation of the evanescent decay states with interfacial spin filtering based on $Fe_3GaTe_2$/bilayer-$In_2Se_3$/$Fe_3GaTe_2$ heterostructure. Beyond altering the electrostatic potential as in AFE-FE switching, the transitions between head-type and tail-type AFE states preserve the centrosymmetric potential profile yet fundamentally modulate the momentum-resolved distribution of evanescent decay rates across the Brillouin zone. When integrated with perfect spin filtering at the $Fe_3GaTe_2$/$\alpha$-$In_2Se_3$ interface, this mechanism yields a giant TER (~$7.6\times10^3\%$), over 4 times that of conventional FE-based MFTJs, and a TMR exceeding $6.8\times10^5\%$, enhanced by two orders of magnitude over typical MFTJs. These mechanisms resolve the performance trade-off in MFTJs, enabling six distinct non-volatile resistance states at room temperature.
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All authors Wei Yang, Yibo Xu, Shen Li, Jiangchao Han, Jiayou Chen, Juan-Carlos Rojas-Sánchez, Stéphane Mangin, Xiaoyang Lin, Weisheng Zhao

Materials Science

Ambiguity in B-Site cation ordering: A Case study of the double perovskite Ca$_2$CoNbO$_6$

Svetlana A. Artiukova, Ivan V. Yatsyk, Ruslan G. Batulin et al. (+6 more)

The ordering of cations in the B sublattice remains a challenging issue in double perovskites. In this work, a combined experimental and theoretical approach was employed to investigate the Co/Nb distribution in Ca$_2$CoNbO$_6$ and its influence on magnetic and transport properties. The density functional theory, supported by magnetic susceptibility measurements, indicates that Co adopts a high-spin Co$^{3+}$ state. No long-range magnetic ordering was observed down to low temperatures; however, the presence of short-range correlations points to the partial disorder in the Co/Nb sublattice. This interpretation is further supported by electron paramagnetic resonance, which also reveals slight oxygen nonstoichiometry. Electrical transport follows a small-polaron hopping mechanism with an activation energy of 0.25 eV. The Seebeck coefficient reaches 0.4 mV/K at 600 K.
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All authors Svetlana A. Artiukova, Ivan V. Yatsyk, Ruslan G. Batulin, Yulia A. Deeva, Tatiana I. Chupakhina, Vladislav V. Bazhal, Alexandr I. Balitskiy, Rushana M. Eremina, Dina I. Fazlizhanova

Materials Science

Nonequilibrium dynamics of high energy transitions in monolayer WSe$_{2}$

Oleg Dogadov, Jorge Cervantes-Villanueva, Nicholas Olsen et al. (+6 more)

High-energy optical transitions in monolayer transition-metal dichalcogenides exhibit characteristics that are markedly distinct from those of lower-lying band-edge excitons. These differences arise from the involvement of electronic states located at regions of the Brillouin zone that are displaced from the $K$ valleys. In this work, we investigate the ultrafast dynamics of these high-energy excitations by employing broadband ultrafast transient absorption spectroscopy spanning the visible to ultraviolet spectral range. We observe that the formation and relaxation dynamics of one of the high energy transitions display a distinct behavior compared to the lower-energy excitonic resonances, developing on a significantly slower timescale. First-principles calculations of the excitonic landscape allow us to account for this delayed response and attribute it to the phonon-mediated formation of momentum-dark excitons.
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All authors Oleg Dogadov, Jorge Cervantes-Villanueva, Nicholas Olsen, Chiara Trovatello, Xiaoyang Zhu, Giulio Cerullo, Alejandro Molina-Sánchez, Davide Sangalli, Stefano Dal Conte

Materials Science

Intrinsic Point Defects and Frenkel Pair Formation in Photovoltaic Absorber Zn$_3$P$_2$: Regulating $p$-type Conductivity through Growth and Annealing Conditions

Nico Kawashima and Silvana Botti

This study investigates the ground-state energetics and thermodynamics of intrinsic point defects in zinc phosphide Zn$_3$P$_2$ using ab initio density functional theory combined with an extensive potential energy landscape search. Our analysis reveals that the defect chemistry is dominated by zinc vacancies $V_\mathrm{Zn}$ and zinc interstitials Zn$_i$, with equilibrium concentrations significantly surpassing those of other intrinsic species. Notably, we find that phosphorus interstitials P$_i$, previously suggested to be significant, possess high formation energies and likely exist only in negligible quantities. The characteristic $p$-type conductivity of undoped Zn$_3$P$_2$ is shown to be a direct consequence of zinc vacancies, which act as shallow acceptors and pull the Fermi level toward the valence band. Furthermore, we identify a positive binding energy between $V_\mathrm{Zn}$ and Zn$_i$, leading to the formation of electrically benign Frenkel pairs that partially compensate the intrinsic p-type conductivity. Our results suggest that achieving $n$-type conductivity is fundamentally limited by these thermodynamic constraints. We conclude that hole densities can be optimized through phosphorus-rich growth conditions and high-temperature annealing, and suggest that future photovoltaic strategies should prioritize interface engineering over bulk $n$-type doping.
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Materials Science · Cross-listed from cs.LG

Non-normal spectral signatures of instability in neural network training dynamics

Souvik Ghosh

Training instabilities in deep networks - loss spikes, oscillatory convergence, and gradient pathologies - are empirically prevalent but lack a rigorous operator-theoretic explanation. We show that the linearized update operators for practically used optimizers are generically non-normal: for Adam, non-normality is controlled by the commutator [H, M] between the Hessian and the diagonal adaptive preconditioner, while for SGD with momentum it arises from the augmented state-space structure of the update map. Applying non-normal stability theory to these operators, we derive a conservative pseudospectral precursor bound in which \kappa(V) serves as an early-warning indicator of transient amplification even when the spectral radius remains below one, and we establish that exceptional points of the update operator appear as the \kappa(V) -> \infty limiting case of this framework. Numerical experiments on two-layer networks confirm that the spectral radius \rho(J) provides no separation between stable and unstable training phases while \kappa(V) separates them by approximately one order of magnitude, complementing the classical sharpness criterion with a continuous severity measure of non-normal amplification. These results establish non-Hermitian operator theory as a useful and underexplored framework for neural network optimization stability, offering a diagnostic language and proof-of-concept benchmark for understanding adaptive optimization stability.
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Materials Science

Self-organized formation of step-terrace structure in SrRuO3 thin films grown on mixed-terminated SrTiO3 (100) substrates

Ryotaro Arakawa, Sachio Komori, Kotaro Tomita et al. (+3 more)

Surface morphology of the substrate and bottom layers plays a critical role in the epitaxial growth of oxide thin films. Here, we report on the self-organized formation of a step-terrace structure in SrRuO3 (SRO) thin films grown using pulsed laser deposition on mixed-terminated SrTiO3 (100) substrates without any prior surface treatment. Atomic force microscopy observations reveal that SRO films initially grow in a three-dimensional island mode and subsequently undergo a transition to a step-flow growth mode through island coalescence as the film thickness increases, resulting in a well-defined step-terrace morphology with a step height consistent with the SRO unit-cell parameter. The average terrace width of the self-organized structure can be systematically tuned by varying the substrate temperature and the target-substrate distance, which we attribute to changes in the critical island radius that governs the nucleation behavior. To demonstrate the utility of this self-organized morphology, we show that BiFeO3 thin films grown on SRO films with such a step-terrace structure exhibit improved surface flatness and crystalline quality compared to those grown directly on bare SrTiO3 substrates. These findings provide a clear understanding of the mechanism of thickness-driven growth-mode transitions in perovskite oxide thin films under various growth conditions.
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All authors Ryotaro Arakawa, Sachio Komori, Kotaro Tomita, Shunsei Komori, Masaaki A. Tanaka, Tomoyasu Taniyama

Materials Science

Symmetry-protected nodal planes and accidental nodal surfaces in mixed odd-even wave spin-momentum locking of relativistic altermagnets

Xujia Gong, Amar Fakhredine, Sahar Izadi Vishkayi et al. (+1 more)

Non-relativistic spin–momentum locking in altermagnets exhibits an even number of nodal planes. In the relativistic limit, the number of nodal planes can be lowered by symmetry reduction due to the Néel vector and spin–orbit coupling in noncentrosymmetric systems. Therefore, an analysis of the evolution of the nodal planes in relativistic altermagnets is required. While $g$-wave spin–momentum locking is straightforward to realize in non-relativistic altermagnets, this $g$-wave does not necessarily survive in the relativistic case. In this work, we investigate the relativistic spin–momentum locking of the centrosymmetric CrSb and the noncentrosymmetric wurtzite MnTe. As a first result, we show that in both systems the dominant spin component retains its $g$-wave character in the relativistic regime only when the Néel vector is oriented along the $z$-axis, while the subdominant components exhibit $d$-wave symmetry in CrSb and $p$-wave symmetry in ferroelectric wurtzite MnTe. More generally, the $g$-wave character is preserved in the relativistic limit only when both the Néel vector and the electric field associated with inversion-symmetry breaking are oriented along the $z$-axis. As a second result, we show that relativistic spin–momentum locking of ferroelectric altermagnets can exhibit $p$-wave magnetism with one symmetry-protected nodal plane and an accidental nodal surface not protected by symmetry, or can have two accidental nodal surfaces. With the Néel vector aligned along the $x$-axis, selected bands of ferroelectric altermagnet wurtzite MnTe exhibit $p$-wave magnetism. Our results establish that altermagnets can host distinct spin components that realize a mixture of angular-momentum wave symmetries in momentum space in the relativistic limit.
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All authors Xujia Gong, Amar Fakhredine, Sahar Izadi Vishkayi, Carmine Autieri

Materials Science · Cross-listed from cond-mat.mes-hall

Selective Fermi-Level Pinning: A Design Strategy for Giant Rectification in Molecular Junctions

Junnan Guo, Wenhui Fang, Jian Huang et al. (+3 more)

Molecular rectifiers are key functional components of molecular-scale integrated circuits, yet achieving high rectification ratios remains a longstanding challenge due to the intrinsic symmetry of resonant tunneling and the complexity of interfacial energy-level alignment. Here, we propose a rectifier design strategy based on selective Fermi-level pinning that breaks transport symmetry via pinning interactions between molecular frontier orbitals and electrodes. This framework enforces tunneling transport to be predominantly governed by unoccupied molecular orbitals, while substantially suppressing contributions from occupied states, thereby establishing a simplified and highly controllable rectification mechanism. The resulting cyclo[n]carbon-based molecular junctions exhibit giant rectification ratios exceeding 103, while retaining exceptional structural robustness against variations in both donor chain length and carbon ring size. This work reveals the critical role of selective Fermi-level pinning in molecular junctions and provides a general design principle for engineering functional single-molecule electronic devices.
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All authors Junnan Guo, Wenhui Fang, Jian Huang, Weikang Wu, Hui Li, Lishu Zhang

Materials Science

A Metadynamics-Based Framework for Free Energy Surface Mapping of Multiparticle Diffusion in Crystals

Shunya Yamada and Kazuaki Toyoura

We propose two metadynamics (MetaD)-based methodologies for efficiently mapping free energy surfaces (FESs) of multiple interacting carriers diffusing in crystalline solids. Our approaches circumvent the challenges of high-dimensional collective variables (CVs) by employing replica state exchange MetaD and parallel bias MetaD, both of which decompose the high-dimensional CVs into multiple lower-dimensional ones. As a benchmark, we investigate two-dimensional lithium (Li) diffusion in LixTiS2 across a wide range of Li concentrations. The Li jump frequencies estimated from the obtained FESs exhibit concentration- and temperature-dependent trends consistent with previous kinetic Monte Carlo simulations and nuclear magnetic resonance measurements. Our approaches provide a promising framework for capturing the slow dynamics of diffusive carriers that are typically inaccessible to conventional molecular dynamics simulations.
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Materials Science

Cathodoluminescence Analysis of Defects and Grain Boundaries in Zn3P2 Thin Films Grown on Graphene by MOVPE and MBE

Thomas Hagger, Mohammadreza Hassanzadeh, Aidas Urbonavicius et al. (+12 more)

Zn3P2 is a promising earth-abundant absorber for thin-film photovoltaics, yet its development is hindered by the lack of lattice-matched substrates, its incompatible thermal expansion coefficient, and a complex defect landscape. Here, we demonstrate the quasi-van der Waals epitaxy of Zn3P2 on graphene by metal-organic vapour phase epitaxy (MOVPE) and directly link the density of antiphase boundaries to optical emission modulation using correlative electron microscopy and cathodoluminescence (CL). Moreover, it is observed through CL that grain boundaries act as non-radiative sinks for excited charge carriers. The effect extends several micrometres into the grains, making grain boundaries detrimental to the applicability of Zn3P2 in read devices. Further comparison with molecular beam epitaxy grown films reveals the suppression of strain-related sub-bandgap emission in MOVPE-grown material. Overall, quasi-van der Waals epitaxy of Zn3P2 by MOVPE resulted in larger grains and improved material quality. In addition, these results directly link extended defects to recombination pathways in Zn3P2 and highlight grain-size control as a key strategy for improving earth-abundant photovoltaic absorbers.
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All authors Thomas Hagger, Mohammadreza Hassanzadeh, Aidas Urbonavicius, Ahmed El Alouani, Victor Boureau, Gulnaz Ganeeva, Nico Kawashima, Raphael Lemerle, Kamil Arthur Wodzislawski, Sebastian Lehmann, Kimberly A. Dick, Silvana Botti, Adrien Michon, Anna Fontcuberta i Morral, Simon Escobar Steinvall

Materials Science

Studying Creep-Fatigue interaction of Nickel-Based Superalloys using Crystal Plasticity and Entropy-Based life prediction model

Santosh Kumar Shaw, Sabyasachi Chatterjee, Alankar Alankar et al. (+1 more)

Creep-fatigue interaction in single-crystal nickel superalloys is difficult to predict because the response depends on the combined effects of loading parameters, hold time, temperature, and the underlying deformation mechanisms. This is important for turbine blade applications, where components experience both fatigue and creep during service. In the present work, a crystal plasticity finite element (CPFE) framework is used to study the creep-fatigue response of a single-crystal nickel superalloy under a range of practically relevant thermo-mechanical loading conditions. In particular, the effects of strain amplitude, R-ratio, hold duration, and temperature on cyclic deformation, stress relaxation, damage evolution, and creep-fatigue life are examined. Particular attention is given to separate the roles of fatigue and creep damage, understanding their interaction, and identify the creep-dominated and fatigue-dominated regimes as a function of strain amplitude and hold time. The study brings together these effects within a single framework and shows that the predicted trends in cyclic response and life are in good agreement with experimental observations reported in the literature.
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All authors Santosh Kumar Shaw, Sabyasachi Chatterjee, Alankar Alankar, Ayan Bhowmik

Materials Science

Thermodynamic stability of twisted domains in AgCrSe$_{2}$ thin films grown on lattice-matched YSZ(111) substrate

Haruto Sato, Kota Mihara, Kenshin Inamura et al. (+4 more)

Control of structural domains in epitaxial thin films of functional materials is a fundamental technique to utilize their intrinsic physical and chemical properties in solid-state devices. In this study, we report on suppression of twisted-domain formation in thin-film growth of polar magnetic semiconductor AgCrSe$_{2}$ using pulsed-laser deposition. In exploring concomitant optimized growth temperature and Ag/Cr composition ratio of supply, we find the critical growth temperature ($T\mathrm{_{sub}}$) for obtaining single 60$^{\circ}$ domain in c-axis oriented AgCrSe$_{2}$ thin film on a lattice-matched (111) plane of the yttria-stabilized zirconia substrate. At temperatures below and above the critical $T\mathrm{_{sub}}$, metastable 0$^{\circ}$ domain in addition to the 60$^{\circ}$ domain emerges, indicating delicate energy balance of thermodynamic stability for obtaining the single-domain structure. Surface structural analysis using time-of-flight low-energy atom scattering spectroscopy reveals the presence of two polar orientations along $+Z$ and $-Z$ directions. These findings provide valuable insights into the thin-film growth mechanisms for a family of two-dimensional compounds with rhombohedral lattices.
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All authors Haruto Sato, Kota Mihara, Kenshin Inamura, Yusuke Tajima, Kazutaka Kudo, Jobu Matsuno, Junichi Shiogai

Materials Science

In situ estimation of local acoustic pressure amplitude by force balancing with a ferrofluid droplet probe

Seiya Usui, Yoshiaki Uchida, Akira Nagakubo et al. (+1 more)

Acoustic tweezers enable non-contact manipulation of microscale objects, but quantitative in situ evaluation of the peak local pressure amplitude remains difficult in confined devices. Conventional hydrophone-based measurements are often limited at the microscale by probe size and installation constraints. Here, we present a force-balance method in which a trapped ferrofluid droplet serves as a local probe in a standing-wave acoustic field and an externally applied magnetic-field gradient is tuned so that the magnetic force balances the maximum primary acoustic radiation force on the droplet. From the magnetic force on the ferrofluid droplet, determined at the balance point, we estimate a peak local pressure amplitude of $2.2\times10^{5}$~Pa for 7.2~MHz operation at 10~V$_{\mathrm{pp}}$. This approach provides a practical route for quantitative in situ characterization of microscale acoustic fields and for setting operating conditions in compact acoustofluidic devices.
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All authors Seiya Usui, Yoshiaki Uchida, Akira Nagakubo, Norikazu Nishiyama

Materials Science

Non-adiabatic phonon renormalization in metallic versus insulating rutile oxides

Reshma Kumawat, Shubham Farswan, Simranjeet Kaur et al. (+1 more)

We present a comparative Raman scattering study of metallic rutile oxides (RuO$_2$ and IrO$_2$) and insulating rutiles (TiO$_2$ and SnO$_2$). Temperature-dependent Raman spectra reveal that the metallic compounds exhibit pronounced phonon frequency hardening, $\omega(11~\mathrm{K})-\omega(300~\mathrm{K})=\Delta\omega \approx 6$-$10~\mathrm{cm}^{-1}$, whereas the insulating rutiles show only modest hardening, $\Delta\omega \approx 1$-$3~\mathrm{cm}^{-1}$. In contrast, the linewidth changes, $\Delta\Gamma \approx 1$–$7~\mathrm{cm}^{-1}$, do not display a systematic metallic-insulating classification. Fits with the conventional Klemens anharmonic decay model reproduce the overall temperature trends but yield inconsistent anharmonic parameters for the metallic compounds when benchmarked against insulating rutile analogues. A modified Klemens framework, incorporating an additional $T^{2}$ correction to the phonon frequency arising from the electronic contribution to the phonon self-energy, quantitatively accounts for the enhanced renormalization observed in metallic systems. These results establish finite non-adiabatic electron-phonon coupling in metallic rutiles and demonstrate that phonon renormalization can be identified even in the absence of observable Fano asymmetry in the phonon line shapes.
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All authors Reshma Kumawat, Shubham Farswan, Simranjeet Kaur, Kaushik Sen

Materials Science · Cross-listed from quant-ph

Transition-state lattice modes and the breakdown of adiabatic tunneling for hydrogen and deuterium in bcc Nb

P. Graham Pritchard and James M. Rondinelli

Interstitial hydrogen and deuterium in body-centered-cubic metals constitute archetypal quantum tunneling systems. Their relevance has been renewed by the connection between hydrogenic tunneling in Nb and defect-induced decoherence in superconducting qubits, motivating a predictive microscopic theory. Existing theoretical treatments invoke an adiabatic separation between the light interstitial and the host lattice, an assumption whose validity has not been rigorously established for hydrogenic species. Here, we show that the experimentally measured tunnel splittings of O-trapped H and D in bcc Nb are quantitatively well-described within a five-dimensional (5D) Lattice-Renormalized Born-Oppenheimer (LRBO) framework. This approach treats three interstitial modes and two judiciously selected lattice modes, which includes a transition-state mode, on equal quantum footing. By recasting nested Born-Oppenheimer hierarchies within this same formalism and benchmarking against modern ab initio potential energy surfaces, we show that adiabatic separation of the light particle from lattice dynamics is satisfied only in the positive-muon ($\mu^{+}$) mass limit. In contrast, tunneling for H and D is fundamentally a collective, nonadiabatic process mediated by anharmonic lattice couplings. Finally, we show that the breakdown of adiabaticity can be anticipated from simple energy estimates involving the ground-state light-particle energy evaluated at a small number of fixed lattice configurations, providing a practical criterion for assessing the validity of adiabatic tunneling theories in other systems.
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Materials Science · Cross-listed from cond-mat.mes-hall

Optical Transmission of 2D Material with Quantum Anomalous Hall Effect

Nathan Pravda, Oleg L. Berman and Klaus Ziegler

We study the optical properties of gapped two-dimensional materials which are subject to the quantum anomalous Hall effect. At sufficiently low temperatures the transmission, reflection and absorption coefficients are found to have a universal behavior that depends only on the ratio of the photonic energy and the gap energy. There is a singular behavior with total reflection when these energies are equal. In the limit of a vanishing gap we recover results for graphene, where the optical coefficients depend only on the fine-structure constant. The observed optical properties provide an accurate measurement of the bandgap.
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Materials Science

Pulsed thermal annealing enables switching of chiral antiferromagnetic order with a sub-millitesla field in Mn$_3$Sn

Xiaokang Li, Jing Zhang, Xiaodong Guo et al. (+1 more)

The manipulation of antiferromagnetic (AFM) order is a central theme in modern spintronics. In this work, we achieve reliable switching of the chiral AFM state in the Weyl antiferromagnet Mn$_3$Sn using a heat pulse combined with a very small magnetic field as small as 0.1 mT. By systematically measuring the anomalous Hall effect (AHE) in high-quality single crystals, we show that the field needed for switching decreases as the temperature approaches the Néel temperature $T_N$, and vanishes at $T_N$. Pulsed thermal annealing above $T_N$ followed by cooling in a tiny external field enables full and reproducible switching of the magnetic octupole order. Our results show that thermal softening (heating above $T_N$ to temporarily remove the magnetic anisotropy) is a key step that lowers the energy barrier to nearly zero. This allows an extremely weak directional field (like the effective field from spin-orbit torque in thin-film devices) to set the final magnetic state during cooling. We also provide a simple model to estimate the temperature rise in nanoscale devices under current pulses, giving practical guidance for device design. This work highlights that thermal effects are not a side issue but an important partner to spin torques, and suggests that future work should take both into account.
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All authors Xiaokang Li, Jing Zhang, Xiaodong Guo, Zengwei Zhu

Thursday 21 May

Materials Science · Cross-listed from cond-mat.mes-hall

Surface States in Strain-Induced Nodal-Line Topological Semiconductors

Vitaly N. Golovach and Alexander Khaetskii

This work explores the topological phase diagram of inverted-band-gap semiconductors under strain and spin-orbit coupling. Using a minimalistic Luttinger Hamiltonian model, we follow the transitions between a 3D topological insulator, a Dirac semimetal, a nodal-line semimetal, and a Weyl semimetal. Analytical and exact solutions for surface states are derived for high-symmetry directions as well as in several limiting cases. We demonstrate the continuous evolution of these surface states across phase boundaries, providing a unified picture that synthesizes previous literature. Specifically, we detail the progression from a Dirac to a nodal-line and then to a Weyl semimetal as spin-orbit coupling originating from bulk inversion asymmetry is introduced. A hierarchy of energy scales is established, defining the criteria for realizing these phases. Finally, we reveal a non-analyticity in the surface-state dispersion at the projected nodal line, originating from distinct, terminating patches of surface states with unique spin textures in momentum space.
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Materials Science

A new method to probe conducting filaments in MoS$_2$-based memristors

Pierre Trousset, Lucie Le Van-Jodin, Bruno Reig et al. (+7 more)

Two-dimensional (2D) transition metal dichalcogenides (TMDs), such as molybdenum disulfide (MoS$_2$), are emerging as promising materials for next-generation electronic devices. They have proved to be serious candidates for integration with memristors in non-volatile memory and radio frequency (RF) applications. However, the physical mechanisms behind their resistive switching, particularly the formation and resorption of conducting filaments, remain unclear. In this study, we present a novel mechanical exfoliation technique that selectively removes the top metallic electrode from MoS$_2$-based memristors by exploiting the weak van der Waals interaction between MoS$_2$ and the top electrode. This method enables direct and multi-scale characterization of the MoS$_2$ surface in different states (initial, ON and OFF) using Kelvin Probe Force Microscopy (KPFM) and Raman spectroscopy mapping. To complete this study, cross-sectional Transmission Electron Microscopy (TEM) was also performed in different conductive states. Our results reveal that the conducting filament is formed by metallic atom migration from the top electrode into the MoS$_2$ layer. Additionally, we demonstrate that the choice of metallic electrodes (gold vs. nickel) significantly impacts the switching behavior due to differences in adsorption and diffusion energies. This work not only clarifies the filament formation mechanism and introduces a reproducible approach for in-operando characterization but also represents a real progress in the understanding and optimization of 2D material-based memristors.
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All authors Pierre Trousset, Lucie Le Van-Jodin, Bruno Reig, Clotilde Ligaud, Thomas Jalabert, Hanako Okuno, Le Van-Hoan, Paul Brunet, Stéphane Cadot, Matthieu Jamet

Materials Science

Thickness-Dependent Spintronic Terahertz Emission in MBE-Grown PtTe$_2$: From Semiconductor to Type-II Dirac Semimetal

Rahul Sharma, Sylvain Massabeau, Armando Pezo et al. (+13 more)

Spintronic terahertz (THz) emitters have established themselves as among the most practical broadband THz sources available, yet their performance remains fundamentally limited by the spin Hall conductivity of the nonmagnetic conversion layer - a quantity that is fixed once the material is chosen. Here, we demonstrate that in PtTe$_2$, a type-II Dirac semimetal within the transition metal dichalcogenide family, this limitation can be circumvented by exploiting the dramatic thickness-driven electronic phase evolution of the material itself. Using molecular beam epitaxy to grow PtTe$_2$ films with single-monolayer precision from 1 to 20 ML, we show that the spintronic THz emission tracks the underlying electronic phase diagram directly: it is absent in the single-layer semiconducting phase, turns on sharply at the semimetal transition near 2 ML, and reaches a peak amplitude six times that of an equivalent Pt reference at 10 ML, before declining at larger thicknesses due to THz reabsorption in the increasingly metallic film. This non-monotonic behavior is inconsistent with a bulk inverse spin Hall mechanism and instead reflects a multi-channel spin-to-charge conversion process in which spin-momentum-locked topological surface states and a thickness-dependent interfacial Rashba splitting both contribute and strengthen as the type-II Dirac band structure develops. First-principles calculations of the interfacial spin accumulation reproduce the experimental trend quantitatively, confirming this physical picture. These findings introduce thickness engineering of van der Waals semimetals as a new and accessible route to optimizing spintronic THz emitters and spin-orbit torques in magnetic memories (SOT-MRAMs), with direct implications for the broader class of dimensionally tunable topological materials.
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All authors Rahul Sharma, Sylvain Massabeau, Armando Pezo, Ekta Yadav, Viliam Vretenár, Ravi K. Biroju, Fatima Ibrahim, Sukhdeep Dhillon, Alain Marty, Isabelle Gomes de Moraes, Adrien Michon, Jing Li, Mairbek Chshiev, Henri Jaffrès, Jean-Marie George, Matthieu Jamet

Materials Science

Genome-Guided Interpretable Screening of Phase-Stable, Lead-Free Double Perovskite Absorbers for All-Inorganic Semiconductors, Sensors, and Photovoltaics with DFT-Validated Design Rules

Nafis Ahtasu, Sohanur Rahman Sohan, Md. Mostaq Ahmed Himel et al. (+6 more)

The discovery of stable, lead-free halide perovskites for optoelectronic applications is constrained by vast compositional space and limited interpretability of conventional screening approaches. We present a genome-guided, physics-informed framework that decodes thermodynamic stability and optoelectronic behavior through four physically interpretable descriptor families: packing, bonding, polarization, and electronic identity. Trained on 1,221 DFT-calculated A2BB'X6 compounds, machine-learning surrogates achieve robust predictive performance, with a recall-optimized stability classifier (ROC-AUC = 0.92) and an XGBoost regressor for band-gap prediction (R2 = 0.93 on held-out data). Applying a staged inverse-design constraint stack to 13,088 charge-balanced, lead-free compositions reduces the search space to five DFT-validated, phase-stable semiconductors: Rb2SnMnBr6, Cs2CdSnBr6, Cs2CdSnI6, Cs2KGaI6, and Cs2AgAlBr6. These candidates lie on the convex hull (E_hull <= 0 meV/atom), preserve ordered double-perovskite structures, and exhibit strong optical absorption (alpha peak ~1e5 cm^-1). Genotype-phenotype coupling analysis reveals a hierarchical control mechanism: packing genes define structural formability, bonding genes govern near-edge optical transitions and conductivity, and optoelectronic response genes regulate dielectric response and exciton screening (epsilon0 = 4.6-8.2). This work establishes a generalizable paradigm for interpretable inverse design, linking descriptor-level genomics to experimentally relevant optoelectronic phenotypes and providing design rules for discovering stable, lead-free double perovskites for photovoltaics, sensing, and transparent electronic applications.
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All authors Nafis Ahtasu, Sohanur Rahman Sohan, Md. Mostaq Ahmed Himel, Md. Zahid Hassan, Muhammad Harussani Moklis, Masud Rana Rashel, Hasan Jamil, AKM Kamrul Islam, Mouhaydine Tlemcani

Materials Science · Cross-listed from cond-mat.soft

Hollow Needle Puncture Mechanics for Biopsy Sampling

Yiting Wu, Frederic Lechenault, Matteo Ciccotti et al. (+1 more)

Biopsy sampling relies on hollow needles that puncture soft tissues by propagating and opening a cylindrical crack, yet the mechanics governing this coring process remain only partially understood. Motivated by this gap, we develop a simple, energy based model for puncture by blunt hollow needles, grounded in brittle fracture mechanics and extended to include frictional interactions at the needle tissue interface. The model describes puncture as the competition between the fracture energy and the elastic energy. This energetic balance is controlled by the interplay among needle geometry (radius and wall thickness), material properties (toughness and elastic modulus), and interfacial parameters (adhesion and friction). This model provides semi-analytical predictions for five key quantities: core size, frictionless force, frictional force slope, critical insertion depth, and critical insertion force. Model predictions are validated against experiments, demonstrating that friction significantly improves force estimation and alters the puncture regime. These results offer quantitative insight into the mechanics of tissue coring and force generation during biopsy, providing a predictive foundation for needle design, sampling performance, and real time control in robotic biopsy and needle insertion systems.
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All authors Yiting Wu, Frederic Lechenault, Matteo Ciccotti, Mattia Bacca

Materials Science

Atomic scale demonstration of ferromagnetism in a single layer FeCl2 on Au(111)

Adriana E. Candia, Eliecer Peláez-Sifonte, Amitayush Jha Thakur et al. (+8 more)

FeCl2 is a promising single-layer material with sizeable magnetic susceptibility and insulating character that can be easily grown by molecular beam epitaxy on various surfaces. In order to include it into the select palette of van der Waals materials used to engineer functional heterostructures, it is necessary to confirm its magnetic and electronic ground states, and understand the influence of the supporting substrate. In this work, we unambiguously demonstrate ferromagnetic ordering in a single-layer FeCl2 on Au(111) by means of spin-polarized scanning tunnelling microscopy. The material features a relatively wide insulating gap of 3.3 eV and a strongly spin-polarized conduction band that emerges at 1.5 eV above the Fermi level. Atomic scale defects with triangular shape play a primary role in the electronic gap and spin density distribution. Specifically, in a region of 1.6 nm around each defect, the conduction band is locally suppressed and the tunnelling magneto-conductance is reduced a factor of four. By tracking the spin-dependent tunnelling conductance as a function of the applied magnetic field, we record atomically resolved hysteresis loops, revealing a soft ferromagnetic ground state with pronounced out-of-plane anisotropy and coercive fields in the range of 15-50 mT.
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All authors Adriana E. Candia, Eliecer Peláez-Sifonte, Amitayush Jha Thakur, Sebastien E. Hadjadj, Samuel Kerschbaumer, Aymeric Saunot, Martina Corso, Maxim Ilyn, Jorge Lobo-Checa, Celia Rogero, David Serrate

Materials Science · Cross-listed from cond-mat.soft

Topological cell-openness index for porous materials

Michał Bogdan and Paweł Dłotko

We propose a method of estimating and parametrising the proportion of open and closed cells in a porous material based on measuring Betti numbers on the structures. We define a cell-openness index {\tau} which can be used to complement the proportion of open-celled volume reported by gas pycnometry, which is the current gold standard for pore type characterization. We discuss in what types of structures mismatches between the two measures can occur and how such mismatches convey additional information about the structure. We demonstrate examples of significant correlations between {\tau} and measurable physical quantities in both numerical and experimental structures. We also discuss how Betti curves can be used to estimate characteristic feature sizes in porous structures.
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Materials Science · Cross-listed from cs.HC

Summarizing Time-Varying Digital Image Correlation Strain Fields Using Sankey Diagrams

Victor Persson, Christofer Boo, Mohit Sharma et al. (+1 more)

Digital Image Correlation (DIC) enables dense, time-resolved measurement of surface strain in deforming materials, providing insight into strain localization and failure mechanisms. However, the resulting strain fields are typically explored frame-by-frame through spatial visualizations, making global temporal patterns difficult to discern. We present a visual summarization approach that represents the evolution of high-strain regions as a single Sankey diagram constructed from superlevel sets of the von Mises equivalent strain field. By tracking connected components over time via spatial overlap, the diagram encodes the birth, persistence, merging, and disappearance of strain concentrations. Applied to four tensile test datasets with varying notch geometries, the approach compactly captures differences in deformation regimes and qualitative precursors to failure, complementing traditional spatial strain visualizations with a global temporal overview.
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All authors Victor Persson, Christofer Boo, Mohit Sharma, Ingrid Hotz

Materials Science

A Local Probe Mass Spectrometer for Localized and Sensitive Product Detection in Environmental Electron Microscopy

Saleh Firoozabadi, Timofei Ivanov, Frederik Stender et al. (+4 more)

Aberration-corrected environmental transmission electron microscopy (ETEM) enables atomic-resolution imaging of dynamic catalytic processes. Correlating atomic-scale structural changes with reaction products detected by mass spectrometry offers a powerful route to uncover catalytic mechanisms. However, current approaches face fundamental limitations: closed-cell ETEM setups suffer from diffuse scattering by SiN windows, degrading spatial resolution and sensitivity, while open-cell configurations enable high-resolution imaging and maintain high sensitivity but suffer from significant dilution of reaction products during transport to the mass spectrometer (MS). To overcome these challenges, we develop a Local Probe Mass Spectrometer (LPMS) integrated with aberration-corrected ETEM. The setup combines a DENSsolution Stream holder with a MS. To preserve spatial resolution, both top and bottom SiN membranes of the MEMS chip are removed, while the gas environment is maintained via the ETEM chamber. Reaction products are sampled locally via a micro-capillary positioned near the catalyst and connected to a holder gas line that delivers the gas to the MS. Initial validation in environmental SEM confirmed controlled gas delivery to the MS. Co3O4 nanoplates serve as a model catalyst due to their inherent electron transparency, enabling atomic-resolution imaging without FIB lamella preparation and associated ion-beam damage. A novel micro-shuttle transfer strategy enables controlled placement of a defined number of nanoplates at the reaction site with precise crystallographic orientation. This establishes the foundation for quantitative structure reactivity correlation by enabling simultaneous, spatially resolved detection of reaction products and atomic-scale structural dynamics.
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All authors Saleh Firoozabadi, Timofei Ivanov, Frederik Stender, Julian Grahl, Stephan Schulz, Christian Jooss, Tobias Meyer

Materials Science

Upscaling DFT-trained machine-learning interatomic potential toward Quantum Monte Carlo accuracy: Sulfur-vacancy migration in monolayer MoS$_2$ as a testbed

Adam Hložný, Ján Brndiar, Ye Luo et al. (+1 more)

We designed a procedure to train a machine learning interatomic potential (MLIP) at benchmark-quality quantum Monte Carlo (QMC) accuracy. To avoid the complexities of high-quality atomic force determination with the stochastic QMC methods, we use a multi-fidelity approach wherein high-level QMC energies are used alongside suitably processed low-level DFT atomic forces to train a QMC fine-tuned MLIP which significantly improves both the energetics and atomic forces over the baseline DFT-based MLIP. Fine-tuning is only applied to the readout layers of an equivariant message-passing MACE MLIP. We used sulfur mono- and multiple vacancies in monolayer MoS$_2$ as a testbed and demonstrate a near QMC accuracy of the model in a number of in- and out-of-domain tests. We show that a fairly limited dataset of QMC energies suffice to significantly improve the baseline DFT MLIP. The accuracy of our approach is demonstrated on energy and free energy migration barriers of mono- and multiple S-vacancy defects. The results open the window to large-scale near QMC quality simulations with large numbers of atoms and/or molecular dynamics configurations which would not be possible by a direct brute-force application of QMC methods.
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All authors Adam Hložný, Ján Brndiar, Ye Luo, Ivan Štich

Materials Science · Cross-listed from cond-mat.str-el

Competing incommensurability, electronic correlations, and superconductivity in a hybrid transition metal dichalcogenide

Jean C. Souza, Moshe Haim, Lorenzo Crippa et al. (+8 more)

The engineering of superlattices in two-dimensional van der Waals materials has enabled the realization of rich phase diagrams hosting topological and strongly correlated phases. While incommensurability is widespread in three-dimensional systems, the role of moiré potentials in bulk materials remains largely unexplored. Here, using scanning tunneling microscopy, we demonstrate that a bulk transition-metal dichalcogenide polytype, 4Hb-TaS$_2$, hosts an emergent incommensurate potential between its alternating 1T and 1H layers. Interplay with a concomitant incommensurate charge-density wave suppresses the long-range order of this potential, leading to intricate coupling with electronic correlations in the doped 1T surface layer. Combining density functional theory with dynamical mean-field theory, we show that the lattice mismatch locally modulates the interlayer distance, thereby tuning both hybridization and charge transfer between the correlated 1T and metallic 1H layers. This redistribution of charge drives the system towards a doped Mott regime, in which the remaining local moments become self-screened, giving rise to a zero-bias resonance. We further find that bulk superconductivity competes with both the underlying landscape and the associated charge transfer. Our results establish incommensurate potentials as a previously overlooked ingredient in hybrid transition-metal dichalcogenides, highlighting their central role in the interplay between electronic correlations, charge-density-wave order, and unconventional superconductivity.
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All authors Jean C. Souza, Moshe Haim, Lorenzo Crippa, Hyeonhu Bae, Edanel Fishbein, Jonathan Ruhman, Binghai Yan, Amit Kanigel, Roser Valentí, Nurit Avraham, Haim Beidenkopf

Materials Science

Observation of magnetically switchable quantum geometric photocurrents

Qi Tian, Zhuoliang Ni, Matthew Cothrine et al. (+6 more)

In non-centrosymmetric materials, light can be rectified into two types of DC photocurrents, known as injection and shift currents, through the bulk photovoltaic effect. Recent theory has uncovered their deep relation with the two-state quantum geometry of resonant transitions: In non-magnetic crystals, where these currents have been routinely observed, the injection current responds to circular light and probes the Berry curvature, while the shift current responds to linear light and probes the geometric connection. Magnetic crystals have been predicted to show a new set of hitherto unobserved magnetically switchable photocurrents, with the roles of linear and circular light interchanged: A linear injection current, which probes the quantum metric, and a circular shift current, which probes the geometric torsion. In this work, we demonstrate the existence of such currents for the first time, demonstrating the switching of the current by flipping the Néel vector in a van der Waals antiferromagnet. Furthermore, their specific frequency and temperature dependence confirm the assignment of circular shift and linear injection currents. Our work demonstrates a new way to control photocurrents in magnets that are directly tied to geometry and have promising applications in antiferromagnetic spintronics and light harvesting.
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All authors Qi Tian, Zhuoliang Ni, Matthew Cothrine, David G. Mandrus, Eugene J. Mele, Andrew M. Rappe, Charles L. Kane, Fernando de Juan, Liang Wu

Materials Science

A sulfonitride transparent conductive thin film with ultra-high refractive index

Eugène Bertin, Shima Kadkhodazadeh, José María Castillo-Robles et al. (+9 more)

With the rise of AI-assisted materials screening, extraordinary properties are now frequently predicted in experimentally uncharted material systems, highlighting the need to develop new synthesis methods for unconventional materials beyond the classic bulk powder form. Here, we establish the first thin-film growth route for any metal sulfonitride compound by realizing Zr2SN2 films with a rare and compelling combination of optical and electrical properties. Zr2SN2 is transparent across most of the visible range while exhibiting a very high average refractive index of 2.95 in the visible, exceeding expectations based on conventional refractive index-bandgap scaling. Importantly, the same Zr2SN2 film shows degenerate n-type conductivity with carrier density above 10^20 cm-3 and intragrain mobility above 8 cm2V-1s-1, approaching those of established transparent conductive oxides. Zr2SN2 thus demonstrates that strong light-matter interaction, optical transparency and electrical conductivity can be reconciled within a single material platform, revealing a new class of high-refractive-index transparent conductors.
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All authors Eugène Bertin, Shima Kadkhodazadeh, José María Castillo-Robles, Finja Tadge, Alba Pérez Millan, Anat Itzhak, Javier Sanz Rodrigo, Manuel Dillenz, Juan Maria García Lastra, Søren Raza, Ivano E. Castelli, Andrea Crovetto