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Physics Condensed Matter Materials Science

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Wednesday 22 July

Materials Science

Thickness-dependent crack suppression and wrinkle formation in freestanding BaTiO3 membranes

Rajesh Mandal, Ajay Kumar and Nini Pryds

Freestanding ferroelectric oxide membranes offer a promising route toward silicon-integrated low-power electronic and memory devices, but reproducible membrane quality remains challenging, particularly in ultrathin BaTiO3 (BTO), where cracking and wrinkling hinder integration. Here, we show that crack and wrinkle formation in released BaTiO3 membranes can be tuned by jointly controlling oxide thickness and polymer support thickness. Reducing the BaTiO3 thickness from 15 nm to 5 nm suppresses large-area, optically visible cracking across the analysed regions, but promotes dense nanoscale wrinkling, revealing a trade-off between fracture mitigation and morphological instability. In 10 nm BaTiO3 membranes, polymer support thickness further modulates wrinkling, with an intermediate thickness reducing wrinkle density. Piezoresponse force microscopy reveals time-dependent evolution of written contrast after release, indicating that apparent ferroelectric response is influenced by polarization switching, post-release morphology, interfacial contact, charge screening, and strain relaxation. These results establish processing guidelines for integrating freestanding ferroelectrics into future devices.
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Materials Science

Exploring Multifunctionality in MgO-Based Magnetic Tunnel Junctions with Coexisting Magnetoresistance and Memristive Properties

Alejandro Schulman, Elvira Paz, Tim Böhnert et al. (+2 more)

Magnetic tunnel junctions (MTJs) and memristors are two key emerging nanotechnologies that attracted significant interest for potential applications at the forefront of the digital revolution, including sensing, data storage, and non-conventional computation. The co-integration of these phenomena into a single multifunctional device is an important step toward harnessing the re-programmability of memristive systems with the high yield and varied functionality of MTJs. This study demonstrates the co-existence of magnetoresistance and memristive properties on MgO-based MTJs. These devices show a magnetoresistance with a linear response as a function of a magnetic field and no hysteresis, which are the requirements for good magnetic field sensors, as well as demonstrating a non-volatile and quasi-analogue memristive behavior as a function of an applied electrical field down to nanosecond pulses. Furthermore, by doping the oxide barrier, the memristive power consumption is lowered by 20% giving the multi-functionality of the devices a promising scalability potential. This study also shows that, memristive switching can be reversibly used to completely suppress and recover the spintronic functionalities. These results can pave the way for a seamless co-integration of memristors and spintronic devices in complex reprogrammable circuits addressing applications such as reprogrammable multifunctional field sensor arrays and neuromorphic computing.
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All authors Alejandro Schulman, Elvira Paz, Tim Böhnert, Alex Steven Jenkins, Ricardo Ferreira

Materials Science

Huge hole injection in tungsten dichalcogenide heterostructures without electric gating: a DFT study

Dawid Ciszewski and Wojciech Grochala

Van der Waals heterostructures based on transition metal dichalcogenides, TMDs, provide a versatile platform for tailoring electronic properties through interlayer charge transfer, CT. Precise control of CT is essential because it directly determines the electronic structure and carrier concentration in atomically thin materials. Recently, the concept of a chemical capacitor has been proposed as a route to achieving exceptionally high carrier densities through CT across insulating separator layers. Here, we extend this concept to van der Waals heterostructures by investigating TMD hBN OX, oxidizer, systems using density functional theory, DFT. Following the screening of candidate TMDs and electron acceptors, XeF2 and KrF2 were identified as suitable acceptors exhibiting type III broken gap band alignment with WS2 and WSe2, respectively. Periodic DFT calculations of large supercells reveal CT corresponding to hole concentrations of up to 0.23 h+ and 0.35 h+ per W atom in WS2 hBN XeF2 and WSe2 hBN KrF2 heterostructures, respectively. The resulting charge redistribution demonstrates that noble gas fluorides provide an efficient route for noncontact engineering of carrier density in TMD heterostructures, offering a new strategy for tuning correlated electronic phases in two dimensional materials.
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Materials Science

Interface-resolved structural properties of epitaxial Y$_3$Fe$_5$O$_{12}$/ Gd$_3$Fe$_5$O$_{12}$ bilayers grown on GGG(111) by pulsed laser deposition

Kshitij Singh Rathore, Abhisek Mishra, Swayang Priya Mahanta et al. (+10 more)

Epitaxial Y$_3$Fe$_5$O$_{12}$ (YIG) and Gd$_3$Fe$_5$O$_{12}$ (GdIG) thin films, along with their bilayer heterostructures, were grown on Gd$_3$Ga$_5$O$_{12}$(GGG)(111) substrates using pulsed laser deposition. Structural properties were investigated using X-ray diffraction, reciprocal space mapping, and cross-sectional transmission electron microscopy. The results confirm high crystalline quality and coherent epitaxial growth, with RSM revealing a coexistence of strained and partially relaxed regions governed by layer sequence. TEM analysis shows sharp interfaces, columnar microstructures, and antiphase boundaries that facilitate strain relaxation. A comparative study indicates that the GGG/YIG/GdIG stacking sequence exhibits improved structural quality with reduced defect density, attributed to the superior epitaxial growth of YIG on the substrate. These findings highlight the critical role of growth sequence in controlling strain and interfacial structure in garnet heterostructures.
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All authors Kshitij Singh Rathore, Abhisek Mishra, Swayang Priya Mahanta, Shubhransu Sahoo, Anupama Swain, Pushpendra Gupta, Kapil Gupta, Jose M. Caicedo-Roque, Jessica Padilla-Pantoja, Francisco J. Belarre, Belen Ballesteros, Jose Santiso, Subhankar Bedanta

Materials Science

Symmetry-Based Design Rules for Second-Harmonic Generation in Stacked and Twisted MoS2 Bilayers

Sumanti Patra and Caterina Cocchi

Understanding how stacking controls the nonlinear optical response of two-dimensional materials is key to designing van der Waals heterostructures with tailored functionalities. Here, we establish a comprehensive symmetry-based framework mapping the structural configuration of MoS2 bilayers across four point groups (D3h, D3d, C3v, C3) to their second-order susceptibility tensor chi^(2). Using group-theory arguments benchmarked against first-principles response-function calculations, we demonstrate how symmetry breaking controls the activation and suppression of individual tensor elements in these systems. We show that the emergence of the in-plane component chi_xxx in twisted configurations (C3 group) induces a rigid azimuthal rotation of the second-harmonic generation polar lobes, which remains frequency-independent across the entire optical spectrum, locking to half of the structural twist angle. Our findings establish a direct, wavelength-independent optical route for twist-angle determination and provide a clear roadmap for engineering nonlinear optical responses in two-dimensional materials.
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Materials Science

The origin of ferroelectricity, polarization and high resistivity in Aurivillius CaBi2B2O9 (B = Ta, Nb)

Fengzhang Tang, Shi Wei, Qi Hu et al. (+4 more)

Aurivillius layered oxides are important candidates for high-temperature ferroelectric and piezoelectric application. In this work, we combine group theoretic analysis with first-principles calculations to systematically investigate the origin of ferroelectric phase transition, polarization, piezoelectric response, and intrinsic electrical insulation of the two-layer Aurivillius ferroelectrics CaBi$_{2}$B$_{2}$O$_{9}$ (B = Ta, Nb). The results show that the \textit{A2$_1$am} ferroelectric phase arises from the cooperative condensation of a polar mode and nonpolar oxygen octahedral rotation/tilting modes, whose the $\Gamma_5^-$X$_2^+$X$_3^-$ trilinear coupling substantially lowers the total energy and deepens the ferroelectric potential well. The spontaneous polarization and anisotropic piezoelectric response are governed primarily by the cooperative displacements of the Bi$_{2}$O$_{2}$ layers and Ta/NbO$_{6}$ octahedra, with Bi ions providing an indispensable contribution to both responses. More importantly, the polar distortion can be traced to the relative in-plane displacement between adjacent the Bi$_{2}$O$_{2}$ layer and the perovskite-like block. Because this displacement is intrinsic to the alternating Bi$_{2}$O$_{2}$/perovskite-block stacking topology and is independent of the number of perovskite layers, we identify interlayer sliding as a general, layer-number-independent structural mechanism for ferroelectricity in Aurivillius oxides. Our findings establish a unified microscopic picture linking structural distortions, ferroelectric polarization, piezoelectric response, and electronic insulation in CaBi$_{2}$B$_{2}$O$_{9}$ (B = Ta, Nb), and provide theoretical guidance for designing layered ferroelectric oxides with high Curie temperatures and robust insulating behavior.
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All authors Fengzhang Tang, Shi Wei, Qi Hu, Jie Xing, Jianguo Zhu, Zhi Tan, Qiang Chen

Materials Science

Numerical exploration on unveiling the photovoltaic potential of MgXS3(X = Ti, Zr, Hf) chalcogenide perovskites

Md. Mizanur Rahman, Md. Nahid Hasan, Tanvir Ahmed et al. (+1 more)

Lead-free chalcogenide perovskites offer a nontoxic and thermally robust path beyond Pb-based perovskite solar cells (PSCs), but their device-level behavior in realistic three-dimensional geometries remains insufficiently characterized. In this work, we investigate ZnSe/MgXS3(X = Ti, Zr, Hf)/Sb2S3 solar cell architecture where MgXS3 absorbers from the II-IV-VI chalcogenide perovskite family is employed as the absorber layer. The device is analyzed using 3D finite-element simulations in COMSOL Multiphysics that self-consistently couple optical generation, drift-diffusion carrier transport, and heat transfer under AM 1.5G 1-sun illumination, following a fully coupled opto-electro-thermal framework. For each absorber composition, the impacts of absorber thickness, doping, and defect density are systematically investigated, and the contribution of an Sb2S3 back-surface-field (BSF) layer to carrier collection and spectral response is quantified. Under optimized conditions, MgZrS3, MgTiS3, MgHfS3-based devices achieves a simulated power conversion efficiency (PCE) of 28.18%, 26.72%, and 28.16%, respectively. The corresponding open-circuit voltage (VOC) values are 0.94 V, 0.74 V, and, 1.07 V while the short-circuit current density (JSC) values are 34.46 mA/cm2, 42.69 mA/cm2, and 29.89 mA/cm2, with fill factor (FF) values of 86.99%, 84.58%, and 88.02%, respectively. Coupled electro-thermal simulations further reveal a small spatially non-uniform steady-state temperature rise across the ultrathin cell stack, mainly governed by non-radiative recombination and Joule dissipation within the active layers. Overall, these results confirm MgXS3(X = Ti, Zr, Hf) chalcogenide perovskites as promising lead-free absorber materials and offer practical design guidance for achieving high-efficiency, thermally stable three-dimensional device architectures.
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All authors Md. Mizanur Rahman, Md. Nahid Hasan, Tanvir Ahmed, Jaker Hossain

Materials Science

A Thermodynamic-Limit Pinning Criterion for Two-Dimensional Structural Superlubricity

Li Wang and Yunjie Ye

Incommensurability and elastic reconstruction do not by themselves define a structurally superlubric phase. We define fully sliding and pinned zero-temperature phases by $\limsup_{A\to\infty}\tau_{\rm dep}^{\max}(A)=0$ and $\liminf_{A\to\infty}\tau_{\rm dep}^{\min}(A)>0$, respectively; $\Lambda_n=|V_n|G_{n,i}[D_{\rm rel}^{-1}(\mathbf q_n)]_{ij}G_{n,j}$ measures only reconstruction susceptibility. Translational covariance then proves that a clean, smooth, infinite moiré continuum can reconstruct without acquiring a bulk sliding barrier. We restore atomic sampling in a two-dimensional discrete model of graphene/hBN and test both a diffusion quantum Monte Carlo first-star potential and a 15-harmonic Leven potential across three rational approximants and five directions. No physical-coupling equilibrium or metastable barrier is resolved. The Leven spectrum raises the largest tested $\Lambda$ from $0.142$ to $0.212$, while artificial scaling through $\Lambda=1$ reaches uncontrolled strain before a size-independent threshold appears. The tested zero-temperature in-plane models are therefore consistent with an elastically relaxed sliding regime; $\Lambda=1$ is a reconstruction scale, not a static phase criterion.
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Materials Science

Analytical Retrieval of Material Parameters in Monolayer Transition-Metal Dichalcogenides Based on a Solvable Exciton Model

Duy-Nhat Ly, Thanh-Son Nguyen, Ngoc-Tram D. Hoang et al. (+1 more)

We develop an analytical procedure to retrieve fundamental material parameters of monolayer transition-metal dichalcogenides from optical and magneto-optical exciton spectra, based on the solvable modified Kratzer model. The proposed retrieval procedure naturally consists of two complementary stages. In the first stage, explicit inversion formulas determine the quasiparticle bandgap, effective screening parameter, and energy scaling factor directly from the experimentally measured energies of the three lowest excitonic states, from which the screening length is subsequently obtained. In the second stage, an analytical expression for the magnetic-field dependence of the exciton energies independently yields the reduced exciton mass, from which the surrounding dielectric constant is then calculated. Once the complete set of material parameters has been retrieved, the framework analytically predicts the diamagnetic coefficients, exciton radii, and complete magnetoexciton spectra without introducing additional fitting parameters or matrix diagonalization. The method is applied to a broad range of experimental samples for WSe$_2$, WS$_2$, MoS$_2$, MoSe$_2$, and MoTe$_2$ monolayers embedded in different dielectric environments. The retrieved material parameters are in good agreement with independent experimental measurements and previous Rytova–Keldysh (RK) calculations, while the predicted excitonic properties accurately reproduce available magneto-optical observations. The proposed analytical theory provides an efficient, physically transparent alternative to conventional numerical fitting procedures and offers an effective tool for the rapid characterization of two-dimensional semiconductors via excitonic spectroscopy.
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All authors Duy-Nhat Ly, Thanh-Son Nguyen, Ngoc-Tram D. Hoang, Van-Hoang Le

Materials Science

Synergistic Interface Stability and High Room-Temperature Ionic Conductivity for Wide-Temperature All-Solid-State Batteries Based on Li6+xSixSb1-xS5I Electrolytes

Liang Ming, Qizhiran Sun, Guanping Xu et al. (+7 more)

Solid-state lithium-ion batteries (LIBs) are increasingly recognized for their exceptional energy density and safety. However, their widespread adoption is challenged by persistent issues such as thermal and electrochemical instability, dendrite formation, and limited compatibility with high-voltage cathodes. Sulfide-based solid electrolytes (SEs), particularly iodide argyrodites, offer outstanding ionic conductivity and stability; however, their practical application is constrained by the formation of space-charge layers, slow ion transport, and susceptibility to dendrite penetration. To address these challenges, we synthesized a novel Li6.6Si0.6Sb0.4S5I argyrodite electrolyte via ball milling and heat treatment, achieving a remarkable room-temperature ionic conductivity of 9.9 mS cm^-1. The electrolyte was integrated with a LiNbO3-coated LiNi0.7Co0.1Mn0.2O2 cathode to form an all-solid-state battery, which demonstrated an initial discharge capacity of 171.2 mAh g^-1, retained 84.2% of its capacity after 200 cycles at 0.5C, and maintained stable cycling across a broad temperature range from -20 degrees C to 60 degrees C. Our study shows that tailored electrolyte composition and a composite cathode configuration significantly enhance cycling stability and improve interfacial protection. These findings highlight the potential of Si-doped antimony-type iodide argyrodites for next-generation high-performance all-solid-state batteries, offering durable operation under diverse thermal conditions.
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All authors Liang Ming, Qizhiran Sun, Guanping Xu, Muqing Su, Enyan Zhao, Wenzhe Gu, Weng-Fu Io, Kwun Nam Hui, Chuang Yu, Hai-Feng Li

Materials Science

Raman spectroscopy of van der Waals topological magnet GdGaI

Nan Jiang, Yijin Zhang, Yujie Xia et al. (+9 more)

We report polarization-resolved Raman spectroscopy of a van der Waals compound GdGaI that is a candidate for excitonic insulators. By combining the symmetry analysis with density functional theory calculations, we identify six Raman-active phonons. The spectra exhibit only the expected anharmonic hardening down to 4 K: no additional peaks, no soft modes, and no signatures of zone folding are observed. This result indicates that any lattice distortion is below our experimental sensitivity, supporting an electronically driven origin for the band reconstruction reported by angle-resolved photoemission spectroscopy rather than an electron-phonon-driven mechanism. Moreover, we observe a pronounced circular dichroism of the $A_{1g}$ modes under an out-of-plane magnetic field. Based on symmetry considerations, we attribute this dichroic response to chiral $A_{1g}$ phonons with opposite angular momenta generated by spin-phonon coupling in the time-reversal-broken state. The temperature evolution of the degree of circular polarization further suggests that circularly polarized Raman spectroscopy detects the emergence of short-range antiferromagnetic correlations. Our results highlight GdGaI as a promising platform in which excitonic order, magnetism, and circularly polarized phonons can be intertwined, and demonstrate that circular-polarization Raman provides a sensitive probe of spin-phonon coupling in excitonic systems.
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All authors Nan Jiang, Yijin Zhang, Yujie Xia, Tomo Higashihara, Ryutaro Okuma, Jun-ichi Yamaura, Yoshinori Okada, Kenji Watanabe, Takashi Taniguchi, Tiantian Zhang, Tomoki Machida, Yasuhiro Niimi

Materials Science

Nitrospinics as a platform from orbital-torque memory to artificial intelligence

Prabhat Kumar, Gaurav Kumar Shukla, Yoshio Miura et al. (+1 more)

The exploration of energy-efficient and functional spintronics has attracted considerable attention. Orbital transport has opened new pathways for current-induced torque generation beyond the conventional spin transport based on the spin Hall effect. In addition, artificial intelligence computing has been demonstrated using spintronic devices. Further progress of these devices can be anticipated through the development of unique and functional materials beyond the existing heavy metals and topological systems with strong spin-orbit coupling. The nitride materials exhibit unique chemical, magnetic, and structural versatility, including antiferromagnetism, high thermal stability, and compatibility with diverse device architectures. Here, we propose Nitrospinics as a conceptual and functional framework that exploits nitride materials for applications ranging from orbital-torque-based spintronic devices to artificial intelligence hardware. Using Cr2N, a two-dimensional nitride MXene with an atomic layered structure, as a prototype system, we discuss how nitrogen contributes to the structural stability, the orbital torque generation, and the interfacial orbital and spin conversion. We further outline key challenges and opportunities toward establishing nitride-based materials for next-generation computing technologies.
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All authors Prabhat Kumar, Gaurav Kumar Shukla, Yoshio Miura, Shinji Isogami

Tuesday 21 July

Materials Science

Macroscopic Polarization and Magnetization from Cavity Vacuum Fluctuations

Jingkai Quan, Chongxiao Fan, Benshu Fan et al. (+3 more)

Cavity light-matter interaction has recently emerged as a new avenue for manipulating material properties without driving fields. Here, we demonstrate that cavity vacuum fluctuations can induce macroscopic polarization (magnetization), even in materials that lack spontaneous polarization (net magnetization) in free space. Starting from the effective photon-free quantum-electrodynamics Hamiltonian, we identify all crystallographic (magnetic) point groups that allow such cavity-induced responses. We derive the form of the corresponding response tensors based on symmetry analysis, whose elements can be obtained by quantum electrodynamical density functional theory (QEDFT) calculations. As representative examples, we show that the cavity-induced polarization in $\alpha$-quartz can be continuously controlled by rotating the cavity. For antiferromagnetic Mn$_3$Sn, we demonstrate that cavity-induced symmetry breaking generates an out-of-plane magnetization, accompanied by an anomalous Hall conductivity component that is forbidden outside the cavity. Our work establishes symmetry as a guiding principle for cavity materials engineering and provides a route for controlling polarization and magnetization through quantum vacuum fluctuations, i.e., cavity materials engineering.
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All authors Jingkai Quan, Chongxiao Fan, Benshu Fan, I-Te Lu, Dante M. Kennes, Angel Rubio

Materials Science

Assessing the Influence of d-Orbital Radius on the Formation of Localized Photogenerated States in Corundum Metal Oxides

Erica P. Craddock and Kathryn E. Knowles

Photogenerated polarons are fundamental to the photophysics of transition metal oxide semiconductors. It is therefore imperative to understand the mechanisms by which polarons form upon photoexcitation of transition metal oxides to realize their potential in photoapplications. Hematite ({\alpha}-Fe2O3) is known to form photoexcited small polarons, which limit its performance as a photoelectrocatalyst for water oxidation. Here, we report a systematic comparison of the electronic, optical and vibrational properties of hematite to those other metal oxides in the corundum crystal family that elucidates the impact of d-orbital radius on carrier-phonon coupling. Three corundum metal oxides are analyzed: {\alpha}-Al2O3 (no d-electrons), {\alpha}-Fe2O3 (3d), and {\alpha}-Rh2O3 (4d) with a combined approach of resonance Raman spectroscopy, thermal difference optical spectroscopy, and computational modeling of electronic and vibrational states. We find that the Raman spectrum of {\alpha}-Al2O3 does not change as the Raman excitation is varied across the visible region, as there is no optical absorption. In contrast, both {\alpha}-Fe2O3 and {\alpha}-Rh2O3 exhibit strong coupling of phonons to optical transitions at the onset of absorption, which is evidence of excitation into a polaronic state. Closely comparing the optical polaronic properties of {\alpha}-Fe2O3 and {\alpha}-Rh2O3, we establish that increased lattice covalency in {\alpha}-Rh2O3 arising from the increased radial extension of the 4d orbitals influences which phonon modes mediate photogenerated polaron formation.
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Materials Science

Photoemission from Semi-Infinite Crystals: First-Principles Scattering States and Quantitative Ag(111) Validation

Tyler Wu, Truman Idso, Siddharth Karkare et al. (+1 more)

Photoemission is an escape problem, yet first-principles calculations usually trap the electron in a periodic box. We present a parameter-free ab initio framework that removes this artificial boundary by constructing open scattering states for semi-infinite crystal-vacuum interfaces from Wannier Hamiltonians and Green-function embedding. The method gives continuum-normalized time-reversed LEED final states with microscopic quasiparticle attenuation. For Ag(111), it predicts absolute quantum efficiency, vectorial photoemission, and mean transverse energy on the experimental scale.
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All authors Tyler Wu, Truman Idso, Siddharth Karkare, Tomás Arias

Materials Science

Electron-Induced Formation of C$_{2}$ on Si(100) from Acetylene and Ethylene

Oliver MacLean, Marc Savoie, Damian G. Allis et al. (+12 more)

Hydrogen Desorption Lithography on Si(100) demonstrates the ability of the Scanning Tunneling Microscope (STM) to create functional atomic-scale structures and devices. The dehydrogenation of adsorbed molecules represents a potential complementary technique that has received little attention. For example, formation of C$_{2}$ could introduce local strain or act as centers for subsequent reactions and would inform positionally controlled mechanosynthesis, an approach with vast potential in surface patterning and functionalization. Here, we studied the electron-induced dehydrogenation of acetylene and ethylene on Si(100) at 4 K using STM. Excitation of acetylene at +3.2 V and above induces configurational switching, including to a new sublayer-bonded geometry previously predicted to be an adsorption precursor, as well as migration and desorption. Excitation also induces dehydrogenation to C$_{2}$. Switching between three observed C$_{2}$ configurations can be induced by excitation at +4.2 V and above. Simulations using density functional theory reproduced the experimental images based on choice of functional, dimer-buckling averaging, and inclusion of diffuse basis functions. In addition, dehydrogenation could be induced using field-emitted electrons on a scale ranging from a single molecule to a radius of >10 nm. These observations highlight the potential of carbon dehydrogenation as an additional tool in Atomically Precise Fabrication (APF).
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All authors Oliver MacLean, Marc Savoie, Damian G. Allis, Rafik Addou, Ryan Groome, Si Yue Guo, Aru Joy Hill, Alex Inayeh, Hadiya Ma, Cameron J. Mackie, Sheena Ou, Marco Taucer, Denis A. B. Therien, Finley Van Barr, Ryan Yamachika

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

Optical Magnetic Switching in Odd-Parity Magnets with Spin-Orbit Coupling

SangEun Han and Qiang Li

$p$-wave magnets exhibit odd-parity spin polarization in momentum space, with spin splitting that reverses under $\vec{k}\rightarrow -\vec{k}$, while preserving zero net magnetization. Here we show that, in odd-parity magnets with spin-orbit coupling, elliptically polarized light generates a momentum-independent spin-dependent term that dynamically switches a zero-net-magnetization $p$-wave state into a finite spin-polarized state. The Floquet-engineered bands also acquire a nonzero Chern number whose sign is controlled by the light polarization. For $f$-wave magnets, circularly polarized light induces a net out-of-plane magnetization, offering a direct experimental signature. Our results establish light as an efficient means of controlling magnetic states, with potential applications in spintronics and quantum information.
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Materials Science

Complete Hierarchy of Nonrelativistic Odd-Parity Spin Splitting in Collinear Magnets

Yichen Liu, Junxi Yu, Pu Zhang et al. (+1 more)

Momentum-dependent nonrelativistic spin splitting provides a symmetry fingerprint of collinear magnets and can govern unconventional electronic, magnonic, and transport phenomena. Whereas even-parity $s$-, $d$-, $g$-, and $i$-wave splittings in collinear magnets have been extensively studied, odd-parity counterparts remain unexplored beyond the $p$-wave and $f$-wave classes. Here, using group theory, we establish the complete classification of odd-parity spin splitting in collinear magnets. We show that, in addition to the $p$-wave and $f$-wave forms, $h$- and $k$-wave splittings with $\ell=5$ and $7$ are allowed, while $m$-wave splitting with $\ell=9$ constitutes the upper bound. We derive a complete mapping from crystallographic point-group irreducible representations to the lowest-order odd-parity basis functions and formulate the coupling rule between a symmetry-breaking axial field and the parent Néel order that selects the induced odd-parity class. We further construct minimal lattice models that realize $h$-, $k$-, and $m$-wave splitting. Guided by this classification, we screen the MAGNDATA database and show that circularly polarized light can drive the $\mathcal{PT}$-symmetric antiferromagnets Fe$_2$TeO$_6$ and MgFe$_6$Ge$_6$ into $h$-wave and $k$-wave phases, respectively, exhibiting the hallmark spin splittings in both electronic bands and magnon spectra. Symmetry analysis and Berry-curvature calculations show that collinear odd-parity magnets of both $h$- and $k$-wave allow an anomalous Hall response, whereas the $m$-wave class forbids it. Together, these results complete the partial-wave hierarchy of odd-parity spin splitting in collinear magnets and establish symmetry criteria for anomalous transport in the high-partial-wave classes.
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All authors Yichen Liu, Junxi Yu, Pu Zhang, Cheng-Cheng Liu

Materials Science

Time-resolved ARPES in pumped excitonic systems: Floquet physics induced by excitonic fields

Amir Eskandari-asl and Adolfo Avella

We develop a theoretical framework based on the Dynamical Projective Operatorial Approach (DPOA) to study the time- and angle-resolved photoemission spectroscopy (TR-ARPES) of pumped excitonic systems. Including Coulomb electron-electron interactions at the Hartree-Fock (HF) level, our formalism captures the formation of excitonic bound states under the application of pump pulses. Considering a prototypical two-dimensional two-band semiconductor, we analyze the equilibrium phase diagram, which shows the expected transition from a semiconducting to an excitonic-insulator phase as the Coulomb interaction strength or its range increase. Out of equilibrium, we find that when the pump frequency is resonant with an excitonic mode, coherent oscillations of the excitonic order parameter persist after the pump pulse subsides and give rise to clear Floquet sidebands in the TR-ARPES spectrum. These exciton-field-induced sidebands are distinct from those originating from the pump laser field. We also identify band-resonance-induced sidebands arising from residual coherences at momenta where the band gap is resonant with the pump frequency. Finally, we analyze the local Coulomb interaction limit. Our results corroborate recent experimental observations of exciton-field-induced Floquet-like sidebands and establish DPOA as an efficient and accurate method for simulating ultrafast phenomena in interacting electron systems.
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Materials Science

Spatially resolved in-situ characterisation of competing martensitic transformation pathways during nanoscratch in 316H Stainless Steel

A. Kareer, R.W. Kerr, D. Craven et al. (+3 more)

Localised surface deformation beneath frictional contacts generates a tribolayer whose microstructure and properties differ from the bulk. In austenitic stainless steels, this tribolayer forms through two competing martensitic transformation pathways. Here, these pathways are isolated in 316H stainless steel using in-situ synchrotron X-ray nanodiffractometry combined with nanoscratch testing, which together yield spatial maps of the evolving strain field beneath a single sliding asperity. Finite element modelling interprets the resulting distribution of martensitic phases, revealing a pressure driven pathway selection where hydrostatic compression ahead of the contact suppresses $\alpha'$ formation and favours the $\gamma \rightarrow \varepsilon$ transformation, while lateral sliding relieves this constraint and introduces a shear strain driving $\varepsilon \rightarrow \alpha'$ , producing an overall sequential $\gamma \rightarrow \varepsilon \rightarrow \alpha'$ pathway in the tribolayer. Where hydrostatic constraint persists, $\varepsilon$-martensite is retained; where material piles up and is unconstrained above the surface, the transformation proceeds directly to $\alpha'$. The $\gamma$-austenite adjacent to $\alpha'$- martensite shows elevated dislocation density, indicating that $\alpha'$ formation is accommodated by plastic deformation in the surrounding matrix. This distinction could explain differences in galling performance among iron-based and cobalt-based hardfacing alloys, where the $\varepsilon$-martensite forming cobalt alloys offer superior galling resistance. The methodology presented resolves transient microstructural states inaccessible to static measurements of macroscale, multiple asperity contacts, establishing a route to mechanistic insight across tribological phenomena more broadly.
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All authors A. Kareer, R.W. Kerr, D. Craven, A.V. Davydok, C. Krywka, D.M. Collins

Materials Science

ATLAS: A Foundation Neural Sampler for Amorphous Materials

Mouyang Cheng, Denis Blessing, Botao Yu et al. (+4 more)

Amorphous materials exhibit exceptional mechanical and functional properties, yet their rugged energy landscapes are notoriously difficult to sample. Below the glass-transition temperature, conventional molecular dynamics and Monte Carlo become inefficient because equilibration relies on rare barrier-crossing events, while data-driven generative models are constrained by scarce and biased reference ensembles. Here, we introduce ATLAS, an efficient sampler that learns a diffusion process to generate Boltzmann-distributed amorphous structures directly from a target energy function. Parameterized by an equivariant graph neural network, ATLAS generalizes across system size, temperature, and composition. By exploiting the time reversal of the diffusion process, it enables efficient estimation of thermodynamic quantities and steering toward target observables. In two-dimensional Kob-Andersen systems, ATLAS reproduces parallel tempering Markov chain Monte Carlo structural distributions, free energies and entropies, achieving below 0.2% free energy error in the low-temperature glass regime with over 500-fold fewer energy evaluations. In Cu-Zr and Cr-Co-Ni metallic glasses, ATLAS recovers experimentally observed short-range-order trends and steers structures toward prescribed order parameters and optimized bulk moduli. Moreover, composition-amortized pretraining outperforms composition-specific training from scratch, reduces inverse-design costs by several hundred-fold, and enables sampling with expensive universal machine learning interatomic potentials. Coupled to a large language model agent, ATLAS searches an eight-element space for high-entropy metallic glasses balancing stiffness and ductility, identifying a converged Pareto frontier within 480 oracle evaluations. Together, these results establish ATLAS as a foundation model for sampling, steering and designing amorphous materials.
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All authors Mouyang Cheng, Denis Blessing, Botao Yu, Gerhard Neumann, Mingda Li, Carles Domingo-Enrich, Yuanqi Du

Materials Science

Thermal and electrical conductivity of a refractory high-entropy alloy after high-pressure torsion: Electron versus phonon contributions

Jacqueline Hidalgo-Jimenez, Payam Edalati, Md Amirul Islam et al. (+3 more)

The equiatomic refractory high-entropy alloy TiZrHfNbTa was processed by high-pressure torsion (HPT) to investigate the effect of nanostructuring and defect engineering on thermal and electrical transport properties. Severe plastic deformation (SPD) via the HPT treatment induces substantial accumulation of dislocations, grain refinement to the nanometer level (average: 40 nm), and partial transformation from the BCC phase to the omega phase. While hardness increases to a steady state with processing, the specific heat capacity exhibits a non-monotonic behavior: it decreases at low strains due to the suppression of low-frequency vibrational modes by dislocations, then partially recovers at high strains due to anharmonic vibrations at newly formed high-angle grain boundaries. Thermal conductivity decreases at low strains but shows a slight recovery at high strains, whereas electrical conductivity decreases monotonically to a steady state without recovery. Analysis using the Wiedemann-Franz law reveals that the electronic contribution dominates thermal transport, while the phononic contribution (limited by the scattering of phonons on defects) is only 11 to 23%, depending on the degree of straining. The contrasting evolution of thermal and electrical conductivity is ascribed to the transition from dislocation-dominated vibrations at low strains to grain boundary-dominated vibrations at high strains, which affects phonons and electrons with different efficiencies.
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All authors Jacqueline Hidalgo-Jimenez, Payam Edalati, Md Amirul Islam, Makoto Arita, Bidyut Baran Saha, Kaveh Edalati

Materials Science

Doping tunable charge density waves in misfit layer compounds

Hugo Le Du, Ludovica Zullo, Justine Cordiez et al. (+14 more)

The ability to tune charge density waves (CDWs) through external control knobs, such as doping, pressure or strain is crucial for exploring the phase diagram of two dimensional (2D) or quasi-2D materials. Yet, controlling CDWs critical temperature and ordering vector remains a challenge for current experimental techniques. In this work, we establish misfit layer compound heterostructures as a reliable platform to manipulate CDWs in transition metal dichalcogenides. By combining ab initio calculations with low-temperature scanning tunneling microscopy, we show how to achieve doping tunable control over NbSe2 CDW by chemically alloying in the rocksalt subunit. Crucially, we prove that tuning the La Pb ratio in the misfit family (LaxPb1xSe)1.14(NbSe2)2 enables stabilization of different CDW orders, such as 2x2 or 3x3 patterns, and even coexisting phases. This work paves the way for engineering transition metal dichalcogenides with tailored charge density waves within misfit heterostructures.
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All authors Hugo Le Du, Ludovica Zullo, Justine Cordiez, Robin Salvatore, Daniel Schmieg, Arindam Mukherjee, Giovanni Marini, Dominik Volavka, Francois Debontridder, Marie Herve, Tomas Samuely, Shunsuke Sasaki, Florent Pawula, Etienne Janod, Matteo Calandra, Laurent Cario, Tristan Cren

Materials Science

Deciphering Mechanoluminescence: How the Nature of Mechanical Stress and Structural Dimensionality Shape Mechanisms and Responses

Alexis Duval, Xavier Rocquefelte, Yann Gueguen et al. (+2 more)

Mechanoluminescent materials exhibit a broad spectrum of controllable light-emission responses to mechanical stimuli of varying types and magnitudes. Yet progress toward high-performance systems remains constrained by an incomplete and often contradictory mechanistic understanding. Here, density functional theory (DFT) calculations optimized for the quantitative treatment of point defects are used to systematically investigate the interplay between stress type (hydrostatic vs. shear) and active-phase dimensionality (1D vs. 3D), using $SrAl_2O_4:Eu^{2+}, Dy^{3+}$ and $Ba_4Si_6O_{16}:Eu^{2+}, Ho^{3+}$ as representative model systems. Two distinct emission-driving mechanisms are identified: a piezoelectric contribution, and a second, apparently universal, mechanism arising from stress-induced structural reorganization at point defects sites. These results establish a design framework for mechanoluminescent materials in which crystal dimensionality, stress type and stress-sensitive point defects are deliberately matched to tune emission behavior and overall performance.
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All authors Alexis Duval, Xavier Rocquefelte, Yann Gueguen, Patrick Houizot, Tanguy Rouxel

Materials Science

Mag4: An Automated First-Principles Workflow to Extract Magnetic Interactions, from Pair Couplings to Four-Spin Ring Exchange

Xavier Rocquefelte and Peter Blaha

Chemists design magnetic materials through structure-property relationships built on pairwise exchange couplings $J$, now extracted routinely from first principles by energy-mapping. But wherever four magnetic centers close a loop—as in cuprate CuO$_2$ planes or infinite-layer square nets—a four-spin ring exchange $J_{\rm ring}$ can arise, reshaping ground states and corrupting the $J$ values used for design. Missing has been a direct, local way to obtain it, as the four-state method gives for $J$. We supply it by generalizing that method to a sixteen-state ($2^4$) scheme, automated in the open Mag4 package—the first automatic implementation of the four-state family. From one cif file, Mag4 proposes the supercell that isolates the couplings, generates DFT inputs in the user's chosen functional, extracts $J$ and $J_{\rm ring}$ with band-gap and local-moment diagnostics, and predicts the magnetic order, propagation vector, and critical temperature. Symmetry reduces the sixteen configurations to six or eight inequivalent energies, so the cost is modest. The derivation shows the conventional four-state $J$ is itself ring-renormalized by $\mp 2 J_{\rm ring} S^2$, the sign set by the reference state. T-La$_2$CuO$_4$ confirms this: three routes agree on $J_{\rm ring}$ to $0.2\%$, giving $J_{\rm ring}/J_1=0.25$, and a four-state $J_1$ quoted without its reference is wrong by $12\%$. The sixteen-state value is itself reference-dependent—Néel and ferromagnetic baths bracket it—a fourth-order fingerprint of interactions beyond the pair-plus-ring model, resolved by further baths into a bare $J_{\rm ring}$ and a converging tower of six- and eight-spin loop couplings. SrFeO$_2$ ($S=2$), same plaquette yet $J_{\rm ring}/J=0.006$, is the negative control: a plaquette is necessary for ring exchange but far from sufficient—a structure-property criterion for screening new multi-spin magnets.
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Materials Science

Stereochemical Vacuum Gap Explains Out-of-Plane Thermal Insulation in MXenes

O. Mateos-Lopez, J. G. Vilhena, Isaac Armstrong et al. (+3 more)

Two-dimensional MXenes are promising materials for thermal management and spectral camouflage, combining low out-of-plane thermal conductivity with low infrared emissivity and mechanical robustness. Yet the near-order-of-magnitude spread in experimental out-of-plane thermal conductivity measurements (0.14-0.8 W/mK) and the systematic overestimation by simulations point to a fundamental gap in our understanding of heat transport in these materials. Here, we argue these differences originate in the overlooked role of heterogeneous surface terminations. Using Non-Equilibrium Molecular Dynamics simulations of Ti3C2Tx, we show that this discrepancy arises from a stereochemically induced vacuum gap between adjacent layers, formed when surface terminations of different sizes coexist. Even minor deviations from homogeneous terminations drastically suppress out-of-plane thermal conductivity, bringing simulated values into quantitative agreement with experiment. We also show that thermal conductivity scales strongly with the atomic density, and that introducing bulky surface terminations, including residual water, reduces the thermal conductivity to 0.3 W/mK, an order of magnitude below homogeneous termination values and below the minimum thermal conductivity limit predicted for disordered solids. Thus, we propose a chemistry-driven route to engineer thermal transport in MXenes.
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All authors O. Mateos-Lopez, J. G. Vilhena, Isaac Armstrong, Hendrik Heinz, Miguel Muñoz Rojo, Juan Carlos Cuevas

Materials Science

Phase equilibria in MnSb2Te4-GeSb2Te4 system and magnetic properties of Mn1-xGexSb2Te4 solid solutions

Fuad Safarov, Dunya Babanly, Jerome Robert et al. (+4 more)

As a sister compound of the antiferromagnetic topological insulator MnBi2Te4, MnSb2Te4 is also a candidate for exotic magnetic topological phases. On the other hand, the structurally analogous but nonmagnetic phase-change material GeSb2Te4 is also known to exhibit nontrivial band topology. Motivated by their shared crystal structure, the similar ionic radii of Mn2+ and Ge2+, and the opportunity to explore the interplay between magnetism and topology, here we investigate the effects of Ge substitution at Mn sites in MnSb2Te4. The Mn1-xGexSb2Te4 solid solutions were synthesized via high-temperature solid-state reaction and characterized for composition, structure, phase behavior, and magnetism using SEM-EDS, PXRD, DTA, and SQUID. Ge substitution was successful across the full composition range, producing homogeneous, single-phase samples that melt via peritectic reactions, as confirmed by the MnSb2Te4GeS-b2Te4 phase diagram. Ge substitution strengthens the sample's paramagnetism, but with ferrimagnetic ordering up to x = 0.75, with both effective moment and saturation magnetization decreasing with increasing Ge content. Two distinct magnetic transitions - high-temperature paramagnetic to ferrimagnetic and low-temperature ferrimagnetic to ferromagnetic - were identified, with a dome-like shape dependence of the low-temperature magnetic transition on Ge substitution. A negative magnetization was observed in the pristine MnSb2Te4 and x = 0.12 substituted samples, while two distinct spin-flop transitions appeared in the samples with x = 0.32 and x = 0.55 Ge substitutions as a result of competing magnetic orderings. These findings facilitate future selective single-crystal growth of homogeneous, magnetic phases, paving the way for magneto-transport and topological surface states investigations.
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All authors Fuad Safarov, Dunya Babanly, Jerome Robert, Elnur Orujlu, Elvin Ahmadov, Martin Bowen, Bohdan Kundys

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

Multiscale ensemble Monte Carlo of transport and gas sensing in monolayer MoS$_2$

Lado Filipovic

Two-dimensional semiconductors such as monolayer MoS2 combine a useful band gap with a thickness of three atomic planes, which places the entire carrier density within a few angstroms of the surface and makes them attractive both as transistor channels and as chemical sensors. We present a self-consistent ensemble Monte Carlo (EMC) framework for monolayer MoS2 built on the ViennaEMC solver, which couples a Boltzmann transport model to Poisson's equation with degenerate free-carrier statistics. The transport model carries wavevector-dependent acoustic, polar-optical, homopolar and intervalley phonon scattering within and between the K valleys and the spin-orbit-split Q valleys, together with remote substrate phonons, screened charged impurities and surface roughness. Every intrinsic input is computed from first principles for this work, the band structure of the K and Q valleys, the phonon energies, the sound velocities and the dielectric response, and the electron-phonon couplings from density-functional perturbation theory matrix elements at 34 explicit phonon wavevectors and from Wannier interpolation with the two-dimensional long-range treatment. The model therefore contains no fitted transport parameter and no coupling taken from another calculation, and its scattering rates are checked against an independent solution of the linearised Boltzmann equation with the same couplings. The EMC velocity-field characteristic feeds a one-dimensional channel model, verified against a direct self-consistent three-dimensional EMC simulation of the gated channel, which reproduces the measured transfer characteristics of a CVD monolayer device in air and vacuum. An ambient and adsorbate extension reproduces the measured conductivity response to oxygen partial pressure and the concentration dependence of NO2 and NH3 sensing of independent monolayer devices.
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Materials Science · Cross-listed from cond-mat.supr-con

Self-consistent GW theory for superconductivity in SrTiO3 models

Zhi-Hao Cui, John Sous, Andrew J. Millis et al. (+1 more)

Superconductivity in doped SrTiO$_3$ occurs over a wide range of carrier densities, including those for which the Fermi energy is below the polar longitudinal optical phonon scale. In this regime, the assumptions underpinning conventional implementations of Migdal-Eliashberg theory, including frequency cutoffs at the phonon scale and a Coulomb pseudopotential $\mu^\ast$, are not valid. We solve the finite-temperature $GW$ equations with full momentum and frequency dependence, without cutoffs or $\mu^\ast$, for polar one-band models of SrTiO$_3$, using effective masses and three-phonon dielectric functions parameterized from ab initio calculations. Comparing different self-consistency levels, namely $G_0W_0$, $GW_0$, and fully self-consistent $GW$, we find that the one-shot ($G_0W_0$) kernel overestimates the pairing-onset temperature by one to two orders of magnitude. The dominant suppression comes from replacing $G_0$ by $G$, thereby incorporating the phonon renormalization factor in the electron Green function. Using the self-consistently computed interaction $W$ further lowers and narrows the pairing-onset dome. In the dilute limit, our calculations identify the pairing channel as the Fröhlich phonon interaction screened by the incipient ferroelectricity of the material, with plasmonic and electronic screening effects negligible. The numerical solution of the full equations reveals a pairing-onset scale that remains non-zero as the density tends to zero, whereas Fermi-surface projection or Fermi-energy frequency truncation removes it. This work highlights the relevance of incipient ferroelectricity, the importance of self-consistency, and the need for a full momentum- and frequency-dependent treatment in modeling superconductivity in SrTiO$_3$-like doped polar semiconductors.
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All authors Zhi-Hao Cui, John Sous, Andrew J. Millis, David R. Reichman

Materials Science

Alchemical thermodynamic integration for ab initio free-energy calculations in solutions

Liangrui Wei, Feng Zhang, Renata M. Wentzcovitch et al. (+2 more)

We develop an alchemical thermodynamic integration scheme that couples ab initio force calculators on the fly during Monte Carlo and molecular dynamics simulations. The implementation is validated against existing hybrid-Hamiltonian approaches. The scheme yields ab initio free energies of high-pressure Fe-Ni and ambient Li-Na liquid solutions that agree with previous calculations and reproduce the experimentally observed Li-Na miscibility gap. The code has an efficiency comparable to standard ab initio molecular dynamics. These results establish this scheme as a practical alchemical-integration framework for first-principles free-energy calculations in solutions.
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All authors Liangrui Wei, Feng Zhang, Renata M. Wentzcovitch, Kai-Ming Ho, Yang Sun