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Physics Condensed Matter Materials Science

cond-mat.mtrl-sci Watch Materials Science

Thursday 20 November

Materials Science · Cross-listed from quant-ph

Symmetry-Controlled Ultrastrong Phonon-Photon Coupling in a Terahertz Cavity

Dasom Kim, Maxime Dherbécourt, Sae R. Endo et al. (+8 more)

Optical cavities provide a powerful means to engineer light-matter hybrid states by coupling confined electromagnetic fields with matter excitations. Achieving in situ control of the coupling strength is essential for investigating how such hybridization evolves with the coupling strength. In this work, we use a symmetry-changing structural phase transition in lead halide perovskites to reversibly tune the phonon-photon coupling strength, leveraging the fact that their phonon frequencies and oscillator strengths are dictated by lattice symmetry. Terahertz time-domain spectroscopy of MAPbI3 embedded in nanoslot cavities reveals three polariton branches above the critical temperature Tc = 162.5 K, and the emergence of an additional branch below Tc, activated by a new phonon mode in the low-temperature phase. The full dispersion is accurately reproduced using a multimode Hopfield model, confirming that all normalized coupling strengths remain in the ultrastrong coupling regime. These results demonstrate symmetry-controlled tuning of ultrastrong coupling via phonon engineering in optical cavities.
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All authors Dasom Kim, Maxime Dherbécourt, Sae R. Endo, Geon Lee, Ayush Agrawal, Sunghwan Kim, Wen-Hua Wu, Aditya D. Mohite, Minah Seo, David Hagenmüller, Junichiro Kono

Materials Science

van der Waals epitaxy of lead monoxide - PbO and application as a virtual substrate for oxide membranes

Marcel S. Claro and Francisco Rivadulla

We report the successful growth of epitaxial layers of van der Waals (vdW) lead oxide (PbO) polymorphs on different substrates, via pulsed laser deposition. A thin layer of vdW ${\alpha}$-PbO (~5 nm) was subsequently used as a virtual substrate to grow thin films of perovskites BaTiO$_3$, SrTiO$_3$, and the spinel CoFe$_2$O$_4$ with a crystal quality comparable to direct epitaxy on single-crystal oxide substrates. Notably, films with the larger lattice parameter underwent spontaneous, controlled spalling within days of growth, remaining structurally intact, or were easily exfoliated and transferred using the tape method. This work establishes PbO as a promising virtual substrate technology for the fabrication of free-standing oxide membranes through vdW epitaxy, offering advantages over conventional sacrificial layer methods.
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Materials Science

Prediction of atomic H adsorption energies in metalloid doped MSSe (M = Mo/W) Janus layers: A combined DFT and machine learning study

G. Tejaswini, Anjana E Sudheer, Amrendra Kumar et al. (+9 more)

Janus derivatives of 2H MX2 (M = Mo/W; X = S/Se), namely MSSe, have already been experimentally realized and explored for applications in photocatalysis, photovoltaics, and optoelectronics. Focusing on the photocatalytic properties of these layers, we investigate the adsorption of atomic hydrogen on the MSSe layers in the presence of metalloid dopants B, Si, and Ge. The layers in their pristine form exhibit positive adsorption energies, indicating an endothermic nature. Substitution of a dopant in the pristine MSSe layers alters the local symmetry, bonding character, and charge distribution, thereby increasing the number of active sites for hosting H adsorption and reducing the adsorption energy. We select distinct sites, both atomic and interstitial, for the substitution of dopants. The energetics of the H atom at various sites is studied to find the most favorable active site on the MSSe Janus layers. Our results based on density functional theory calculations show that the adsorption process becomes spontaneous and less attractive in the presence of atomic site dopant substitution, whereas the interstitial site results in an endothermic behavior. Moreover, having the data from DFT, we develop a supervised machine learning model for predicting the hydrogen adsorption energy. For this purpose, we utilize 23 elemental features of the atoms involved in the structure, thereby eliminating the need for DFT calculations in feature design. The dimensionality reduction technique, principal component analysis, is employed to reduce the dimensionality of the feature space, yielding independent features that are mutually orthogonal. The model is implemented as a multi-layer perceptron regressor with two hidden layers. The data augmentation technique is employed to artificially expand the dataset size, thereby enhancing the accuracy of the neural network model by 0.90% on the testing data.
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All authors G. Tejaswini, Anjana E Sudheer, Amrendra Kumar, M. Vallinayagam, Pavan Kumar Perepu, Attila Cangi, Mani Lokamani, M. Posselt, M. Zschornak, C. Kamal, D. Amaranatha Reddy, D. Murali

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

Geometrical properties of strained and twisted moiré heterostructures

Federico Escudero, Francisco Guinea and Zhen Zhan

The experimental observations of many interaction-driven electronic phases in moiré superlattices have stimulated intense theoretical and experimental efforts to understand and engineer these correlated physics. Strain is a powerful tool for manipulating and controlling the geometrical and electronic structures of moiré superlattices. This review provides a comprehensive introduction to the geometry of strained moiré superlattices. First, starting from the linear elasticity theory, we briefly introduce the general formalism of small deformations in two-dimensional materials, and discuss the particular cases of uniaxial, shear and biaxial strain. Then, we apply the theory to twisted and strained moiré materials, mainly focusing on the hexagonal homobilayers, hexagonal heterobilayers and monoclinic lattices. Special moiré geometries, like the quasi-unidimensional patterns, square patterns and hexagonal, are theoretically predicted by manipulating the strain and twist. Finally, we review recently developed strain techniques and the special moiré geometries realized via these approaches. This review aims at equipping the reader with a robust understanding on the description and implementation of strain in moiré materials, as well as highlight some major breakthroughs in this active field.
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Materials Science

Transfer of Freestanding Fluoropolymer Films for Advanced Semiconductor Devices

Mohammad Monish, Koki Hino, Yosuke Sasama et al. (+5 more)

High-quality dielectric films are essential for fabricating advanced electronic devices, but their direct deposition often degrades the films and their underlying interfaces, which compromises device performance, especially on sensitive or low-adhesion surfaces. To overcome these limitations, film transfer methods enable the integration of high-quality dielectric films onto such surfaces without damaging the underlying interfaces. However, existing transfer methods have predominantly focused on high-dielectric-constant (high-$\kappa$) materials, leaving a critical gap for transferable, high-quality low-$\kappa$ alternatives, which are required for enabling low-power and high-speed electronics. Herein, we address this need by demonstrating a method to integrate freestanding low-$\kappa$ fluoropolymer dielectric films with smooth surface morphology onto diverse substrates, including low-adhesion surfaces like hydrogen-terminated diamond. The transferred films revealed high breakdown fields of ${8.0}\pm{1.2}$ MV cm$^{-1}$, with leakage current density remaining typically below ${10}^{-7}$ A cm$^{-2}$ before the breakdown. The incorporation of these fluoropolymer films as gate dielectrics in p-channel hydrogen-terminated diamond field-effect transistors resulted in transfer and output characteristics with negligible hysteresis, high channel mobility (${\approx}400$ cm$^{2}$V$^{-1}$s$^{-1}$) and a low interface trap density (${\le}3{\times}10^{11}$ cm$^{-2}$eV$^{-1}$). These findings highlight the versatility of the transfer method and position freestanding fluoropolymers as a promising platform for forming high-quality dielectric/semiconductor interfaces for advanced electronics.
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All authors Mohammad Monish, Koki Hino, Yosuke Sasama, Masato Urakami, Takehiro Ota, Kenji Sakamoto, Kenichiro Takakura, Yamaguchi Takahide

Materials Science

Following the Long-Term Evolution of sp$^3$-type Defects in Tritiated Graphene using Raman Spectroscopy

Genrich Zeller, Magnus Schlösser and Helmut H. Telle

We report on the evolution of tritium-induced sp$^3$-defects in monolayer graphene on a Si/SiO$_2$ substrate, by comparing large-area Raman maps of the same two samples, acquired just after fabrication and twice thereafter, about 9-12 months apart. Inbetween measurements the samples were kept under standard laboratory conditions. Using a conservative classification of sp$^3$-type spectra, based on the D/D' peak intensity ratio, we observed almost complete depletion of sp$^3$-type defects over the investigation period of about two years. This by far exceeds the ~5.5% annual reduction expected from tritium decay alone (~3x larger). This change in the defect composition is accompanied by a recovery of the 2D-band of graphene and an overall decrease in defect-density, as determined via the D/G intensity ratio. Hydogenated graphene is reported to be reasonably stable over several months, when kept under vacuum, but suffers substantial hydrogen loss under laboratory air conditions. While the results shown here for tritiated graphene exhibit similarities with hydrogenated graphene, however, some distinct differences are observed.
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Materials Science

Synthesis and optical characterization of Mn-doped ZnS nanocrystalline thin films prepared via chemical bath deposition

Himal Pokhrel and Urvashi Verma

Manganese-doped zinc sulfide nanocrystalline thin films were synthesized using a low-temperature chemical bath deposition and deposited on glass substrates for controlled durations using triethanolamine (TEA) as a complexing and capping agent. After deposition, the films were annealed at 200 degrees Celsius and characterized by X-ray diffraction (XRD), SEM, and UV-Vis spectroscopy. XRD patterns confirmed the formation of nanocrystalline cubic zinc blende ZnS with an average crystallite size of approximately 37 nm. Optical measurements revealed strong transparency in the visible region, blue-shifted absorption edges due to quantum confinement, and a band gap value of approximately 3.70 eV. Mn incorporation resulted in a slight red shift of the absorption edge, attributed to particle growth and sp-d exchange interactions. These results demonstrate that Mn doping modifies the optical response of ZnS while preserving its crystalline structure, suggesting its suitability for optoelectronic and photovoltaic applications.
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Materials Science

Coherent high-velocity chiral magnons in the metallic altermagnet CrSb

Ashutosh K. Singh, Niclas Heinsdorf, Abraham A. Mancilla et al. (+8 more)

We report the collective magnetism of the metallic altermagnet CrSb. Magnetic susceptibility and polarized neutron diffraction measurements show that CrSb is a perfectly compensated Ising altermagnet below a Néel temperature of $T_N = 733(4)$ K. Inelastic neutron scattering experiments reveal coherent and highly-dispersive antiferromagnetic spin waves with large group velocities of 61(2) km s$^{-1}$ and 58(2) km s$^{-1}$ along the in-plane and out-of-plane directions, respectively. The observed magnon dispersions along high-symmetry directions of the Brillouin zone are well described by a minimal Heisenberg model up to third nearest neighbors of alternating antiferromagnetic and ferromagnetic character, $J_1 = 23(4)$ meV, $J_2 = -5.4(8)$ meV, $J_3 = 5.2(8)$ meV, and an Ising single-ion anisotropy term $D = 0.15(4)$ meV. We observe clear momentum space signatures of chiral spin splitting along the low-symmetry $\Gamma$-$L$ direction, characteristic of higher-order altermagnetic exchange interactions, the first such observation in a metallic altermagnet. These findings identify CrSb as a singular material: a metallic altermagnet in which coherent spin-split magnons persist well above room temperature, providing a compelling platform for spintronic applications.
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All authors Ashutosh K. Singh, Niclas Heinsdorf, Abraham A. Mancilla, Jörn Bannies, Avishek Maity, Alexander I. Kolesnikov, Masaaki Matsuda, Matthew B. Stone, Marcel Franz, Jonathan Gaudet, Alannah M. Hallas

Materials Science

Elucidating the High-Pressure Phases of MAPbBr3 Using a Machine Learning Force Field

Rashid Rafeek V Valappil, Sayan Maity and Varadharajan Srinivasan

High-pressure phases of the hybrid perovskite MAPbBr3 have been investigated in detail using a novel machine learning force field (MLFF). MLFF simulations successfully reproduce the sequence of pressure-induced phase transitions from the $\alpha$ ($Pm\bar{3}m$) to the $\beta$ ($Im\bar{3}$) and finally the $\gamma$ ($Pnma$/$Pmn2_1$) phase. In the $\alpha$ phase, the simulations confirm the triple-well character of the potential energy surface for octahedral tilting shedding light into the local dynamic distortions. In the $\beta$ phase, our simulations reveal MA sublattice doubling yielding both orientationally disordered and ordered MA ions mirroring experimental observation. This mixed-order phase results from locally frustrated host-guest couplings arising from the in-phase octahedral tilt system ($a^+a^+a^+$). In the high-pressure $\gamma$ phase, we confirm the formation of polar and anti-polar domains, with the latter have higher lifetimes and persist for over 50 ps at pressures above 1.5 GPa. By elucidating the behavior of various phases of MAPbBr3, this work provides a fundamental understanding of how host-guest interactions and octahedral tilting govern the material's properties. Further, the importance of time scales and length scales in characterizing these phases is emphasized.
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Materials Science · Cross-listed from cond-mat.mes-hall

Nature of 2D XY antiferromagnetism in a van der Waals monolayer

Cheol-Yeon Cheon, Volodymyr Multian, Kenji Watanabe et al. (+3 more)

Two-dimensional antiferromagnetism has long attracted significant interest in many areas of condensed matter physics, but only recently has experimental exploration become feasible due to the isolation of van der Waals antiferromagnetic monolayers. Probing the magnetic phase diagram of these monolayers remains however challenging because established experimental techniques often lack the required sensitivity. Here, we investigate antiferromagnetism in atomically thin van der Waals magnet NiPS3 using magnetotransport measurements in field-effect transistor devices. Temperature-dependent conductance and magnetoresistance data reveal a distinct magnetic behavior in monolayers as compared to thicker samples. While bilayer and multilayer NiPS3 exhibit a single magnetic phase transition into a zig-zag antiferromagnetic state driven by uniaxial anisotropy, monolayer NiPS3 undergoes two magnetic transitions, with a low-temperature phase governed by in-plane hexagonal magnetic anisotropy. The experimentally constructed phase diagram for monolayer NiPS3 matches theoretical predictions from the six-state clock and 2D-XY models incorporating hexagonal anisotropy.
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All authors Cheol-Yeon Cheon, Volodymyr Multian, Kenji Watanabe, Takashi Taniguchi, Alberto F. Morpurgo, Dmitry Lebedev

Materials Science

Extending SLUSCHI for Automated Diffusion Calculations

Qi-Jun Hong, Qing Chen, Ligen Wang et al. (+6 more)

We present an extension of the SLUSCHI package (Solid and Liquid in Ultra Small Coexistence with Hovering Interfaces) to enable automated diffusion calculations from first-principles molecular dynamics. While the original SLUSCHI workflow was designed for melting temperature estimation via solid-liquid coexistence, we adapt its input and output handling to isolate the volume search stage and generate one production trajectory suitable for diffusion analysis. Post-processing tools parse VASP outputs, compute mean-square displacements (MSD), and extract tracer diffusivities using the Einstein relation with robust error estimates through block averaging. Diagnostic plots, including MSD curves, running slopes, and velocity autocorrelations, are produced automatically to help identify diffusive regimes. The method has been validated through representative case studies: self- and inter-diffusion in Al-Cu liquid alloys, sublattice melting in Li_7La_3Zr_2O_12 and Er_2O_3, interstitial oxygen transport in bcc and fcc Fe, and oxygen diffusivity in Fe-O liquids with variable Si and Al contents. Viscosity and diffusivity are linked through the Stokes-Einstein relation, with composition dependence assessed via simple linear mixing. This capability broadens SLUSCHI from melting-point predictions to transport property evaluation, enabling high-throughput, fully first-principles datasets of diffusion coefficients and viscosities across metals and oxides.
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All authors Qi-Jun Hong, Qing Chen, Ligen Wang, Dallin Fisher, Audrey CampBell, Si-Da Xue, Linqin Mu, Noemi Leick, Seetharaman Sridhar

Materials Science

High-Throughput Exploration of Refractory High-Entropy Alloys for Strength and Plasticity

Stephen A. Giles, Hugh Shortt, Peter K. Liaw et al. (+1 more)

Refractory high-entropy alloys (RHEAs) are compositionally complex materials which have been demonstrated to have the potential for exceptional strength at high operating temperatures. However, their composition space is vast, and other property requirements, such as acceptable plasticity at room-temperature, must be met. Here, we leverage recently published, state-of-the-art deep learning models to predict compressive yield strength at 1,000 {\deg}C and room-temperature plasticity of >100,000 RHEAs. Multiple candidate materials were identified which exhibited exceptional balance between strength and plasticity. Upon experimental synthesis, multiple candidates were proven to outperform any previously reported RHEAs for simultaneous strength and plasticity. Our work demonstrates the power of data-driven approaches for rapid materials design, and enables continued multi-property optimization and materials discovery.
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All authors Stephen A. Giles, Hugh Shortt, Peter K. Liaw, Debasis Sengupta

Materials Science

Quantum Reorientational Excitations in the Raman Spectrum of Hydrogen

Philip Dalladay-Simpson, Eric Edmund, Huixin Hu et al. (+2 more)

Low-frequency Raman peaks, below 250 cm-1, are observed in hydrogen between 2-174 GPa and 13-300 K. The origin of these features is attributed to reorientational transitions (DeltaJ = 0; Q0-branch), which shift from the Rayleigh line as anisotropic intermolecular interactions lift the mJ degeneracy. This family of excitations closely follows the behavior of the S0-branches, sharing their dependence on pressure, temperature, and ortho-H2 concentration. Above 65 K, spectra corrected by the Bose-Einstein population factor reveal a broad continuum arising from populated higher J-states and increased ortho-para disorder. Upon entering phase III, where molecular rotation is inhibited, this continuum is quenched, giving way to well-established optical phonons. Below 25 K, equilibrated samples demonstrate a fine structure from isolated and pair excitations from impurity ortho-H2 molecules in a parahydrogen lattice, the latter a sensitive probe to anisotropic intermolecular interactions relevant to the quantum modeling of solid H2.
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All authors Philip Dalladay-Simpson, Eric Edmund, Huixin Hu, Mario Santoro, Federico Aiace Gorelli

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

Skyrmionium meta-matter: a topologically heterogeneous magnetic crystal with emergent hybrid dynamics

Andrey O. Leonov and Kaito Nakamura

We introduce and systematically investigate a new class of topological magnetic textures, skyrmionium meta-matter, composed of skyrmioniums (Skm, $Q=0$) and skyrmions (Sk, $Q=-1$) arranged in periodic lattices mimicking the richness of atomic materials. Pure skyrmionium lattices are unstable against elongation distortions and relax into the spiral phase, but even a small fraction of skyrmions acts as topological "pins" that stabilize diverse mixed Skm–Sk crystals. We classify these states by topological stoichiometry (Skm$_n$Sk$_m$) and show that each composition hosts multiple metastable polymorphs with distinct plane-group symmetries. Structural transformations between polymorphs are achieved by varying the lattice spacing, suggesting experimental control via pressure or strain. The collective spin dynamics is explored for both in-plane and out-of-plane AC magnetic fields. The resulting absorption spectra show resonant modes beyond the two rotational and one breathing mode of conventional skyrmion lattices. We identify hybrid excitations unique to Skm–Sk crystals, including (i) deformation-assisted rotations, where skyrmions acquire polygonal shapes and rotate, and (ii) orbital modes, where breathing skyrmioniums induce circular motion of confined skyrmions without changing their size. Mode frequencies span sub-GHz to above 10 GHz, consistent with exchange and DMI energy scales. Our results establish skyrmionium-based meta-matter as a versatile platform for tunable, topologically heterogeneous magnetic lattices with rich structural and dynamical properties, paving the way for reconfigurable magnonic and spintronic applications.
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Materials Science · Cross-listed from cond-mat.supr-con

Conventional superconductivity in single-crystalline BiPt

S. Sharma, M. Pula, Sajilesh K. P. et al. (+10 more)

Binary Bi-Pd/Pt systems have attracted a lot of interest because of their topologically non-trivial nature along with superconductivity. We report the structural and superconducting properties of high-quality single-crystalline BiPt using a comprehensive range of experimental techniques, including X-ray diffraction, electron microscopy, muon spin rotation/relaxation ({\mu}SR), magnetization, resistivity, and heat capacity. Our findings establish that BiPt is a weak type-II superconductor with a transition temperature (Tc) of 1.2 K which exhibits pronounced anisotropic superconducting characteristics attributed to its hexagonal crystal structure. Magnetization and electronic transport studies reveal that BiPt lies within the dirty limit, while {\mu}SR and heat capacity data indicate conventional s-wave superconductivity that maintains time-reversal symmetry. This work provides valuable insights into the pairing symmetry and superconducting mechanism of topologically trivial BiPt, a sound comparison system for other Bi-based topologically nontrivial superconductors.
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All authors S. Sharma, M. Pula, Sajilesh K. P., J. Gautreau, B. S. Agboola, J. P. Clancy, J. E. Sonier, A. Ghara, S. R. Dunsiger, M. Greven, M. J. Lagos, A. Kanigel, G. M. Luke

Wednesday 19 November

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

Effects of high-order Van Hove singularities on exciton and trion energy dispersions

Lewis J. Burke, Mark T. Greenaway and Joseph J. Betouras

We investigate the effects of Van Hove singularities in the electronic band structure of two-dimensional semiconductors on the energy dispersion of excitons and positive trions. In particular, we study valence band energy dispersions which possess (i) a typical logarithmic Van Hove singularity, (ii) a line high-order Van Hove singularity (HOVHS) from a Mexican-hat dispersion or (iii) a point HOVHS such as a monkey saddle. We find that the density of states (DOS) of excitons and trions containing such singularities is dramatically enhanced and shows, in general, how the HOVHS in the valence band can strongly affect and be mirrored in the DOS of excitons and trions. This leads to new states that govern the optical properties of the system. In addition, we study a set of materials, InSe, GaSe and $\alpha$-SnAs, from a class of materials in which the topmost valence band has an inverted Mexican-hat shape. The most favourable exciton occurs when the singularity is at the $\Gamma$-point, as in the example of monolayer $\alpha$-SnAs, which hosts a HOVHS. Our work thus provides a pathway to engineer specific bound states in two-dimensional materials that host such singularities, thereby opening new avenues for potential applications.
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Materials Science

Electrical Modulation and Probing of Antiferromagnetism in Hybrid Multiferroic Heterostructures

Yuhan Liang, Huiping Han, Hetian Chen et al. (+11 more)

The unique features of ultrafast spin dynamics and the absence of macroscopic magnetization in antiferromagnetic (AFM) materials provide a distinct route towards high-speed magnetic storage devices with low energy consumption and high integration density. However, these advantages also introduce challenges in probing and controlling AFM order, thereby restricting their practical applications. In this study, we demonstrate an all-electric control and probing of the AFM order in heavy metal (HM)/AFM insulator (AFMI) heterostructures on a ferroelectric substrate at room temperature (RT). The AFM order was detected by the anomalous Hall effect (AHE) and manipulated by the ferroelectric field effect as well as the piezoelectric effect in heterostructures of Pt/NiO/0.7Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_{3}$–0.3PbTiO$_{3}$ (PMN–PT). The non-volatile control of AFM order gives rise to a 33% modulation of AHE, which is further evidenced by synchrotron-based X-ray magnetic linear dichroism (XMLD). Combined with the $in$-$situ$ piezoelectric response of AHE, we demonstrate that ferroelectric polarization contributes mainly to the control of the AFM order. Our results are expected to have broader implications for efficient spintronic devices.
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All authors Yuhan Liang, Huiping Han, Hetian Chen, Yujun Zhang, Yi Zhang, Chao Li, Shun Lan, Fangyuan Zhu, Ji Ma, Di Yi, Jing Ma, Liang Wu, Tianxiang Nan, Yuan-Hua Lin

Materials Science · Cross-listed from cond-mat.supr-con

Interplay of spin-orbit coupling and trigonal crystal field enhances superconductivity in $LaAlO_3/KTaO_3$ (111)

Long Cheng, Jia Liu, Tongying Liu et al. (+10 more)

In conventional superconductors, bulk physical properties typically degrade as the film thickness approaches the two-dimensional (2D) limit. Here in the (111) oriented LaAlO3/KTaO3 (LAO/KTO) heterostructure, we demonstrate experimental evidence that reducing the conducting layer thickness at the interface significantly enhances superconducting transition temperature Tc, in direct contrast to conventional wisdom. From the sum frequency generation (SFG) spectroscopy and superconducting upper-critical field measurements, both the trigonal symmetry and spin orbit scattering are enhanced with the increased Tc. We attribute the enhanced superconductivity (SC) to the synergic interplay between spin-orbit coupling (SOC) and trigonal crystal field, resulting in an enhanced electron-phonon coupling. Furthermore, we show the existence of unconventional SC: the approaching linear temperature dependence of normal state resistance with increasing Tc and the existence of a quantum critical point (QCP) near the superconducting phase. Our findings provide important insight into the underlying mechanism of the strong orientation-dependent KTO interface SC.
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All authors Long Cheng, Jia Liu, Tongying Liu, Pan Chen, Mingyue Zhang, Jiashi Li, Shiyu Zhang, Fei Ye, Qing Wang, Weitao Liu, Jian Kang, Jiandi Zhang, Xiaofang Zhai

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

Long-Range Magnetic Order in Structurally Embedded Mesospin Metamaterials

Christina Vantaraki, Oier Bikondoa, Matías P. Grassi et al. (+11 more)

Engineered assemblies of interacting magnetic elements-magnetic metamaterials-provide a powerful route to tailor collective magnetic order and dynamics. By structuring matter at the mesoscale, they bridge atomic magnetism and macroscopic functionality, enabling emergent behaviour inaccessible in conventional materials. However, realizing large-area metamaterials that combine high morphological uniformity with intrinsic long-range order has remained challenging, largely due to the structural disorder inherent to lithographic fabrication. Here we demonstrate a scalable route to structurally and magnetically coherent metamaterials by embedding iron-ions to form mesospins within a non-magnetic thin film palladium host matrix. Using controlled implantation, we realize morphologically uniform arrays that spontaneously develop extended antiferromagnetic order in the as-fabricated state - without the need of external annealing or field cycling. Resonant X-ray scattering and microscopy reveal sharp magnetic Bragg peaks modulated by the mesospin form factor, evidencing long-range antiferromagnetic order coupled to structural coherence. This embedded architecture establishes a platform for exploring coherent spin-photon interactions and functional X-ray scattering in magnetic metamaterials free from lithographic topography and disorder.
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All authors Christina Vantaraki, Oier Bikondoa, Matías P. Grassi, Brindaban Ojha, Alkaios Stamatelatos, Natalia Kwiatek-Maroszek, Miguel Angel Niño Orti, Michael Foerster, Thomas Saerbeck, Daniel Primetzhofer, Max Wolff, Nicolas Jaouen, Thomas P. A. Hase, Vassilios Kapaklis

Materials Science

Mode selectivity in electron promoted vibrational relaxation of chemisorbed hydrogen on molybdenum and tungsten surfaces

Nils Hertl, Connor L. Box and Reinhard J. Maurer

Electron-phonon coupling in atoms and molecules adsorbed at metal surfaces gives rise to finite vibrational linewidths in infrared or electron energy loss spectra. When it is the dominant contribution to the vibrational lifetime, it manifests itself in the form of a Fano line shape. Here, we report the linewidths of vibrational modes of chemisorbed hydrogen on the (100) and (110) surfaces of molybdenum and tungsten calculated from first-order time-dependent perturbation theory. For those modes with a Fano line shape, our results are in good agreement with the experiment. We further observe that the coupling strength between vibrations and electrons depends on the nature of the mode: for Lorentzian-shaped peaks, the experimental linewidths are always larger than those predicted based on pure electron-phonon coupling. The calculated linewidths exhibit a strong coverage dependence, decreasing towards higher coverages. This finding has important implications for nonadiabatic energy dissipation in hydrogen dynamics at metal surfaces. While electron-hole pair excitation is the dominant energy-transfer mechanism between hydrogen and pristine metal surfaces, other channels for energy dissipation, such as adsorbate-adsorbate interactions, may become more significant on metal surfaces densely covered with hydrogen.
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Materials Science

Mechanistic study of mixed lithium halides solid state electrolytes

Davide Tisi, Sergey Pozdnyakov and Michele Ceriotti

Lithium halides with the general formula Li$_x$M$_y$X$_6$, where M indicates metal ions and X halide anions are very actively studied as solid-state electrolytes, because of relatively low cost, high stability and Li conductivity. The structure and properties of these halide-based solid electrolytes (HSE) can be tuned by alloying, e.g. using different halides and/or transition metals simultaneously. The large chemical space is difficult to sample by experiments, making simulations based on broadly applicable machine-learning interatomic potentials (MLIPs) a promising approach to elucidate structure-property relations, and facilitate the design of better-performing compositions. Here we focus on the Li$_3$YCl$_{6x}$Br$_{6(1-x)}$ system, for which reliable experimental data exists, and use the recently-developed PET-MAD universal MLIP to investigate the structure of the alloy, the interplay of crystalline lattice, volume and chemical composition, and its effect on Li conductivity. We find that the distribution of Cl and Br atoms is only weakly correlated, and that the primary effect of alloying is to modulate the lattice parameter – although it can also trigger transition between different lattice symmetries. By comparing constant-volume and constant-pressure simulations, we disentangle the effect of lattice parameter and chemical composition on the conductivity, finding that the two effects compensate each other, reducing the overall dependency of conductivity on alloy composition. An extended study of the effect of metal substitution, enabled by the semi-quantitative accuracy of the universal model, shows that the effect of metal-site alloying is small, indicating that alloying can be used to optimize cost, or electrochemical stability, without major impact on bulk conductivity.
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Materials Science

Photoluminescence Mapping of Mobile and Fixed Defects in Halide Perovskite Films

Sarah C. Gillespie, Jérome Gautier, Linde M. van de Ven et al. (+6 more)

Metal halide perovskites exhibit coupled electronic and ionic properties that determine their photovoltaic performance and operational stability. Understanding and quantifying ionic transport are therefore essential for advancing perovskite optoelectronics. Conventional electrical methods such as impedance spectroscopy require fully integrated devices, and their interpretation is often complicated by interfacial and contact effects, limiting the ability to isolate intrinsic ionic behavior. Here, a localized adaptation of intensity-modulated photoluminescence spectroscopy (IMPLS) is utilized to optically probe lateral ionic transport in perovskite films. The frequency-dependent photoluminescence response is measured under controlled carrier injection levels and correlated with the photoluminescence quantum yield (PLQY). The proposed diffusion model indicates that mobile ionic defects laterally migrate from high light intensity regions, giving rise to characteristic photoluminescence modulations. Ionic diffusion coefficients extracted from IMPLS agree well with literature values obtained from electrical measurements. Importantly, IMPLS mapping separates mobile and immobile defect contributions through a defect contrast coefficient (DCC), which quantifies the normalized difference between the area-averaged photoluminescence intensity and phase data. This work ultimately demonstrates that localized IMPLS provides a contact-free means to extract lateral ion diffusion coefficients while spatially distinguishing defect types across the sample.
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All authors Sarah C. Gillespie, Jérome Gautier, Linde M. van de Ven, Agustin O. Alvarez, Bruno Ehrler, L.J. Geerligs, Veronique S. Gevaerts, Gianluca Coletti, Erik C. Garnett

Materials Science

Ab initio calculations of the thermoelectric figure of merit, within the relaxation time approximation

Laurent Chaput, Henrique Miranda, Atsushi Togo et al. (+4 more)

In this paper, we propose a computational framework, based on the VASP and phono3py computer codes, to obtain the thermoelectric figure of merit from the electron-phonon and phonon-phonon interactions using finite displacements in supercells. Several numerical techniques are developed for efficiency. The method is applied to several thermoelectric materials. We found different behaviors for the lifetimes of the electrons in PbTe, PbSe, and in compounds of the half-Heusler and magnesium silicide family. This is traced back to the different frequencies of the phonons involved in the scattering around the Fermi level. They have a lower frequency in PbTe and PbSe. The magnitude of the thermoelectric figures of merit we computed compare well with experiments, but the agreement is far from perfect. The role of the defects, not explicitly considered in our calculations, but abundant in thermoelectric materials, is discussed as a possible explanation. It is also shown that the choice of the exchange-correlation functional can strongly impact the results.
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All authors Laurent Chaput, Henrique Miranda, Atsushi Togo, Manuel Engel, Martin Schlipf, Martijn Marsman, Georg Kresse

Materials Science

Dynamic precipitation during high-pressure torsion of a magnesium-manganese alloy

Julian M. Rosalie and Anton Hohenwarter

An ultrafine grained magnesium alloy has been produced through room temperature high-pressure torsion (HPT) of solutionised Mg-1.35 wt.%Mn. Dynamic precipitation of nanometer-scale Mn particles occurred during deformation. These particles populated the grain boundaries, acting as pinning sites which allowed the alloy to develop a grain size of 140 nm after 0.5 rotations. Further HPT deformation resulted in a gradual increase in grain size with no increase in precipitate size. Despite the extensive deformation applied, the alloy did not develop a bimodal grain structure and retained a grain size of 230 nm after 10 complete rotations, demonstrating the stability and effectiveness of these pinning particles.
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Materials Science

Why Physics Still Matters: Improving Machine Learning Prediction of Material Properties with Phonon-Informed Datasets

Pol Benítez, Cibrán López, Edgardo Saucedo et al. (+2 more)

Machine learning (ML) methods have become powerful tools for predicting material properties with near first-principles accuracy and vastly reduced computational cost. However, the performance of ML models critically depends on the quality, size, and diversity of the training dataset. In materials science, this dependence is particularly important for learning from low-symmetry atomistic configurations that capture thermal excitations, structural defects, and chemical disorder, features that are ubiquitous in real materials but underrepresented in most datasets. The absence of systematic strategies for generating representative training data may therefore limit the predictive power of ML models in technologically critical fields such as energy conversion and photonics. In this work, we assess the effectiveness of graph neural network (GNN) models trained on two fundamentally different types of datasets: one composed of randomly generated atomic configurations and another constructed using physically informed sampling based on lattice vibrations. As a case study, we address the challenging task of predicting electronic and mechanical properties of a prototypical family of optoelectronic materials under realistic finite-temperature conditions. We find that the phonons-informed model consistently outperforms the randomly trained counterpart, despite relying on fewer data points. Explainability analyses further reveal that high-performing models assign greater weight to chemically meaningful bonds that control property variations, underscoring the importance of physically guided data generation. Overall, this work demonstrates that larger datasets do not necessarily yield better GNN predictive models and introduces a simple and general strategy for efficiently constructing high-quality training data in materials informatics.
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All authors Pol Benítez, Cibrán López, Edgardo Saucedo, Teruyasu Mizoguchi, Claudio Cazorla

Materials Science

Wannier based analysis of the direct-indirect bandgap transition by stacking MoS$_2$ layers

Shunsuke Hirai, Ibuki Terada and Michi-To Suzuki

Molybdenum disulfide (MoS$_2$), a layered van der Waals material, has attracted considerable attention as a promising alternative to graphene for applications in field-effect transistors and nanophotonic devices because of its sizable band gap, high carrier mobility, large on/off ratio, and strong photoluminescence efficiency. A particularly intriguing property of MoS$_2$ is the transition of its band gap character with layer thickness: while the monolayer exhibits a direct gap, the band gap becomes indirect in multilayer and bulk forms.In this study, we clarify the microscopic mechanism underlying this transition. Focusing on the roles of atomic orbitals and interlayer interactions, we perform an analysis combining first-principles calculations with a Wannier-based model. Although interlayer $p_z$–$p_z$ coupling between neighboring sulfur atoms has been recognized as a key factor in this transition, we find that a complete quantitative description additionally requires interlayer $p_z$–$p_x$ and $p_z$–$p_y$ couplings between neighboring sulfur atoms. These findings highlight the importance of both out-of-plane and in-plane orbital contributions in governing the electronic structure of layered MoS$_2$, providing deeper insight into its band gap engineering for future device applications.
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Materials Science

Effects of Interactions and Defect Motion on Ramp Reversal Memory in Locally Phase Separated Materials

Y. Sun, M. Alzate Banguero, P. Salev et al. (+4 more)

The ramp-reversal memory (RRM) effect in metal-insulator transition metal oxides (TMOs), a non-volatile resistance change induced by repeated temperature cycling, has attracted considerable interest in neuromorphic computing and non-volatile memory devices. Our previously introduced defect motion model successfully explained RRM in vanadium dioxide (VO$_2$), capturing observed critical temperature shifts and memory accumulation throughout the sample. However, this approach lacked interactions between metallic and insulating domains, whereas the RRM only appears when TMOs are brought into the metal-insulator coexistence regime. Here, we extend our model by combining the Random Field Ising Model with defect diffusion-segregation, thereby enabling accurate hysteresis modeling while predicting the relationship between RRM and domain interactions. Our simulations demonstrate that maximum RRM occurs when the turnaround temperature approaches the warming branch inflection point, consistent with experimental observations on VO$_2$. Most significantly, we find that increasing nearest-neighbor interactions enhances the maximum memory effect, thus providing a clear mechanism for optimizing RRM performance. Since our model employs minimal assumptions, we predict that RRM should be a widespread phenomenon in materials exhibiting patterned phase coexistence of electronic domains. This work not only advances fundamental understanding of memory behavior in TMOs but also establishes a much-needed theoretical framework for optimizing device applications.
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All authors Y. Sun, M. Alzate Banguero, P. Salev, Ivan K. Schuller, L. Aigouy, A. Zimmers, E. W. Carlson

Materials Science

Deep Learning Assisted Prediction of Electrochemical Lithiation State in Spinel Lithium Titanium Oxide Thin Films

Devin Chugh, Bhagath Sreenarayanan, Steven Suwito et al. (+4 more)

Machine Learning (ML) and Deep Learning (DL) based framework have evolved rapidly and generated considerable interests for predicting the properties of materials. In this work, we utilize ML-DL framework to predict the electrochemical lithiation state and associated electrical conductivity of spinel Li4Ti5O12 (LTO) thin films using Raman spectroscopy data. Raman spectroscopy, with its rapid, non-destructive, and high-resolution capabilities, is leveraged to monitor dynamic electrochemical changes in LTO films. A comprehensive dataset of 3,272 Raman spectra, representing lithiation states from 0% to 100%, was collected and preprocessed using advanced techniques including cosmic ray removal, smoothing, baseline correction, normalization, and data augmentation. Classical machine learning models such as Support Vector Machine (SVM), Linear Discriminant Analysis (LDA), and Random Forest (RF) were evaluated alongside a Convolutional Neural Network (CNN). While traditional models achieved moderate to high accuracy, they struggled with generalization and noise sensitivity. In contrast, the CNN demonstrated superior performance, achieving over 99.5% accuracy and robust predictions on unseen samples. The CNN model effectively captured non-linear spectral features and showed resilience to experimental variability. This pipeline not only enables accurate lithiation state classification but also facilitates conductivity estimation, offering a scalable approach for real-time battery material characterization and potential extension to other spectroscopic datasets.
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All authors Devin Chugh, Bhagath Sreenarayanan, Steven Suwito, Ganesh Raghavendran, Bing Joe Hwang, Ying Shirley Meng, Weinien Su

Materials Science

Anisotropic in-plane lattice thermal conductivity in bilayer ReS2

Ashutosh Srivastava, Nikhilesh Maiity and Abhishek Kumar Singh

The significantly weak interlayer coupling strength and puckered structure provide the novel layer-tolerant and anisotropic features in two-dimensional (2D) ReS2. These unique features offer an opportunity to modulate the optoelectronic, vibrational, and transport properties along different lattice directions in ReS2. Here, using first-principles density functional theory (DFT), we investigated the thermal transport properties of ReS2 in AA and AB stacking orders. The anisotopic ratios for lattice thermal conductivities (ąppa}) are found to be 1.08 and 1.12 for AA and AB stacking, respectively. This anisotropic nature remains intact even at higher temperatures up to 1000K, demonstrating anisotropic robustness. Lower symmetry in AB stacking leads to higher phonon scattering, which results in lower group velocity, smaller phonon lifetime, and thereby lower ąppa} along both directions as compared to AA stacking. The strong breathing and shear Raman modes in AB stacking indicate stronger layer coupling, further confirming the dominant contribution of acoustic modes towards thermal transport. The findings underscore that the stacking-order-driven preferential heat flow in ReS2 and opens up a new dimension for optimizing device performance.
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Materials Science

Integrating Atomic Scale Catalyst Design with Transport Engineering for Stable and Efficient CO2 Electrolysis to CO in a Membrane Electrode Assembly

Zahra Teimouri, Mahtab Masouminia, Ashkan Irannezhad et al. (+8 more)

Electrochemical CO2 reduction (CO2R) offers a promising approach to decarbonize chemical manufacturing through production of carbon-neutral fuels. However, insufficient performance and instability of membrane electrode assembly (MEA) reactors limit commercial viability, with both metrics directly impacted by the CO2R catalysts. Here we develop an atomically dispersed nickel-nitrogen-carbon (Ni-NC) catalyst through a scalable synthesis approach using two different carbon supports. When using carbon nanotubes as the support, the resulting Ni-NCNT electrode achieves a partial current density toward CO of 558 mA cm-2 with 92 percent Faradaic efficiency toward CO at a cell voltage of 3.2 V and an energy efficiency of 39 percent toward CO at a total current density of 607 mA cm-2. The MEA demonstrates stable operation at 100 mA cm-2 over 210 hours, outperforming previously reported Ni-NC catalysts. Focused ion beam scanning electron microscopy (FIB-SEM) tomography illustrates the key role of catalyst support on the performance of the electrode. COMSOL Multiphysics simulations using 3D reconstructed images of the catalyst layers from FIB-SEM tomography demonstrate that the higher CO2R performance of the Ni-NCNT electrode is due to improved CO2 diffusion and a more uniform current-density distribution compared to the Ni-NCB electrode prepared with carbon black as the support. The stability and performance of the Ni-NCNT compare favorably to state-of-the-art Ag-based catalysts, while bottom-up cost analysis estimates the purchase cost of the Ni-NCNT catalyst to be about 589 USD per kg, substantially lower than the 1900 USD per kg estimated for Ag-based catalysts.
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All authors Zahra Teimouri, Mahtab Masouminia, Ashkan Irannezhad, Reza Eslami, Joseph Deering, Navid Noor, Shunquan Tan, Amirhossein Foroozan, Shayan Angizi, Sung-Fu Hun, Drew Higgins