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Physics Condensed Matter Materials Science

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Tuesday 14 July

Materials Science

The self-organized vacancy order in Pr$_9$Ge$_{16}$

Jayashani S. T. Wickramasinghe, Melissa G. Anderson, Kelci Graville et al. (+11 more)

In this work, we report the discovery of a new crystal structure on the Ge-rich side of the Pr-Ge binary phase diagram. Using a high-temperature flux technique, we grew single crystals of $Pr_9Ge_{16}$, which adopt a previously unreported orthorhombic $Fdd$2 structure type featuring ordered Ge vacancies. We present the anisotropic magnetic properties and identify the crystallographic $b$ axis perpendicular to the crystal plane as the magnetic easy axis. Temperature-dependent resistivity measurements reveal metallic behavior with a distinct anomaly at $T_{\mathrm{C}}$ = 14.3 K. Hall resistivity data indicate that electron-like carriers dominate, with a carrier concentration on the order of $10^{27}~\mathrm{m}^{-3}$. The magnetic order is readily suppressed by a magnetic field of approximately 0.4 T applied along the easy $b$ axis.
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All authors Jayashani S. T. Wickramasinghe, Melissa G. Anderson, Kelci Graville, Gregory T. McCandless, Zachary J. Morgan, Brianna R. Billingsley, Tai Kong, Hyunsoo Kim, Aleksandr V. Chernatynskiy, Simon G. Mitchell, Liang Wu, Julia Y. Chan, Feng Ye, Halyna Hodovanets

Materials Science

Antiphase boundary-driven large exchange bias and negative magnetoresistance in NiCo2O4

Biswanath Pramanik, Pushpesh Pathak and Binoy K. Hazra

Nickel-based spinel oxide has recently attracted significant attention due to its remarkable magneto-transport properties, promising various spintronic applications. In this study, we observe an exchange bias effect, where the magnetic hysteresis loop shifts both horizontally and vertically when single-phase nanocrystalline NiCo2O4 is cooled in the presence of an external magnetic field. The magnitude of the exchange bias field is approximately 375 Oe at 5 K, and this effect disappears around 300 K. Furthermore, NiCo2O4 shows semiconducting behaviour in longitudinal resistivity and demonstrates a substantial negative magnetoresistance of ~31.5% at 10 K. The magnetoresistance exhibits a butterfly-shaped behaviour at low magnetic fields and becomes almost linear at high magnetic fields. A detailed analysis of the negative magnetoresistance and exchange bias reveals that both phenomena originate from the formation of antiphase boundaries in the as-synthesized NiCo2O4 sample, as confirmed by the high-resolution transmission electron micrograph.
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Materials Science · Cross-listed from physics.ins-det

Influence of compaction pressure on the impedance of Gadolinium Doped Ceria electrolytes for IT-SOFCs

Renato A. N. de Oliveira, Celeste Z.A. Aquino, Magna Monteiro Schaerer et al. (+15 more)

Gadolinium doped ceria (GDC) is one promising oxygen-ion conducting ceramic electrolyte for intermediate-temperature solid oxide fuel cells (IT-SOFCs), due to its high ionic conductivity at reduced operating temperatures, favorable defect chemistry, and compatibility with a broad range of electrode materials [1,2]. Despite extensive understanding of its intrinsic ion transport mechanisms, the influence of ceramic processing parameters on the effective electrical behavior of polycrystalline GDC electrolytes remains an active topic for investigation [3-5]. In particular, processing steps that govern green body formation and sintering can strongly affect microstructural features such as density, grain size, grain boundary character, and residual porosity, which in turn determine the macroscopic conductivity [4-6]. In this work, dense GDC ceramic pellets were fabricated under systematically varied isostatic compaction pressures ranging from 49 to 140 MPa, followed by sintering at 1350 C for four hours under identical thermal conditions. Platinum electrodes were deposited on both sides of the pellets by electron-beam deposition, and the electrical properties were characterized by electrochemical impedance spectroscopy (EIS) over a wide temperature range. The results demonstrate a dependence of the impedance with respect to compaction pressure.
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All authors Renato A. N. de Oliveira, Celeste Z.A. Aquino, Magna Monteiro Schaerer, Rafael Galiza Yoshimura, Liliana Mogni, Maurico Arce, Diego G. Lamas, Lucia Toscani, Maria Belén Arcentales Vera, Esteban Elvis Asto Ramos, Marcia Carvalho de Abreu Fantini, Taofeeq Oladayo Bello, Tereza da Silva Martins, Danilo Waismann Losito, James Moraes de Almeida, Valeria Spolon Marangoni, Leopoldo Suescunand, Santiago Figueroa

Materials Science

Emergent exchange bias in ultra-thin La0.67Sr0.33MnO3 films driven by ferro-antiferromagnetic phase coexistence

Inés García-Manuz, Gabriel Caballero, Jose Luis F. Cunado et al. (+7 more)

Ultra-thin La0.67Sr0.33MnO3 (LSMO) films are generally regarded as single-phase ferromagnets, yet their reduced dimensionality enhances the impact of oxygen stoichiometry and local structural distortions. Here we demonstrate that LSMO layers with thicknesses between 3 and 17 nm develop a robust and thickness-independent exchange bias (EB) despite the absence of an engineered ferromagnetic-antiferromagnetic (FM-AFM) interface. Angular magneto-optical Kerr effect measurements reveal a reproducible hysteresis-loop shift that reverses sign upon 180-degree rotation, confirming its intrinsic origin. X-ray photoelectron spectroscopy shows a thickness-independent Mn2+ fraction (about 20 percent), indicating the presence of oxygen-deficient reduced regions embedded within the Mn3+/Mn4+ ferromagnetic matrix. A simple interfacial-exchange model suggests that only a sub-percent fraction of antiferromagnetic (or frustrated spin-ice-like) regions is required to generate the observed EB. These results demonstrate that ultra-thin LSMO can spontaneously host internal FM-AFM interfaces driven by oxygen deficiency, revealing an emergent route to exchange bias in nominally single-phase manganite films.
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All authors Inés García-Manuz, Gabriel Caballero, Jose Luis F. Cunado, Miguel Romera, Mariela Menghini, Carlos León, Jacobo Santamaría, Julio Camarero, Paolo Perna, Fernando Ajejas

Materials Science · Cross-listed from physics.ins-det

A critical consideration of X-ray detectors based on Ga2O3: excitation, carrier transport mechanisms and performance standardization

Alfred Moore, Daniel A Lamb, Lijie Li et al. (+11 more)

X-ray detection underpins a wide range of applications in medicine, security, industrial inspection, scientific research for non-destructive imaging and material analysis. The rapid development of Ga2O3-based X-ray detectors offers a promising pathway toward next-generation detectors with high sensitivity, low noise, and harsh environment applications, benefiting from its intrinsic material properties such as high density, wide band gap energy, and high thermal-chemical stability. However, the underlying device operating mechanisms, including both carrier excitation and transport processes, have not yet been adequately studied, largely due to the misuse of X-ray sources in previous studies. Besides, benchmarking of device characteristics has been problematic due to experimental or data analysis issues, as well as misunderstandings of the applied equations associated with parameter definitions. In this work, we have designed and performed an instructive research work based on epitaxial beta-Ga2O3:Si and its planar Schottky detectors, measured with energy-tuneable monochromatic X-ray beams on a synchrotron beamline, clarifying the device excitation and carrier transport mechanisms with properly benchmarked device performance. In the end, we propose a set of protocols for correctly measuring and analysing the device performance. The proposed protocols are broadly applicable and can be readily extended to other semiconductor X-ray detectors.
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All authors Alfred Moore, Daniel A Lamb, Lijie Li, Oliver Fox, Kawal Sawhney, Ciaran Llewelyn, Jon E Evans, Saqib Rafique, Tiantian Chai, John Harrington, Zabeada Aslam, Andrew P Brown, Rik Drummond-Brydson, Yaonan Hou

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

IceCAPA: patterning particles and microorganisms at a freezing front

Isabelle M. Feller, Jakob Paulsen, Muriel Scherer et al. (+2 more)

The ability to precisely pattern micro- and nano-scale objects on surfaces is important for a range of different applications. For example, colloidal patterning has been used to create plasmonic surfaces, light-emitting diodes or authentication marks, while microbial cell patterning can be applied to screening antibiotic response and cellular interactions over larger populations at the single-cell level. However, we still lack versatile techniques that can pattern a wide range of synthetic and biological objects on a spectrum of different substrate types. Here, we present a robust patterning technique based on the directional freezing of a particle or bacterial suspension over a patterned substrate. Growing ice pushes the desired objects into traps in the substrate, while sweeping away non-trapped ones, leaving behind high-fidelity patterns. We show that this method works for a range of different materials (both synthetic particles and microbial cells), and is unaffected by substrate wettability. Furthermore, patterned bacterial cells retain excellent post-assembly viability, highlighting the gentle nature of the assembly technique. Beyond patterning applications, our results also give insights into processes involving the freezing of particulate suspensions. In particular, we demonstrate the importance of the temperature gradient as a key control which determines how particles interact with freezing fronts. Finally, we highlight a tight analogy between particles interacting with a freezing front and with air-water interfaces, suggesting that results from capillarity may shed light on freezing phenomena.
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All authors Isabelle M. Feller, Jakob Paulsen, Muriel Scherer, Robert W. Style, Lucio Isa

Materials Science

Compatibility of Martensitic Microstructures in Polycrystals

John M. Ball and Myrto Galanopoulou

The paper studies martensitic microstructures in polycrystals, focussing on their compatibility across grain boundaries. After a reduction to the case of a planar grain boundary, the case when the grain boundary separates two constant gradients of zero energy is considered. It is shown that for cubic-to tetragonal transformations such a configuration can occur when the relative grain rotation is not in the cubic group. Then the case when the grain boundary separates two simple laminates of zero energy is considered, it being shown using a computer-assisted symbolic calculation that in the cubic-to-tetragonal case compatibility is only possible for a closed set of measure zero in the manifold of grain boundary normals and relative grain rotations, and that a similar slightly weaker result holds for cubic-to-orthorhombic transformations. The results suggest why higher-order laminates are often observed for such transformations. The Taylor set of deformation gradients is defined and studied, this set having the property that any deformation whose gradient belongs to it corresponds to a zero-energy microstructure for the polycrystal independent of grain geometry and grain rotations. New upper bounds for the Taylor set are proved for cubic-to-tetragonal and cubic-to-orthorhombic transformations, generalizing those of Bhattacharya and Kohn using the geometrically linearized theory. We give a simple proof of a related result of Peigney characterizing the positive diagonal matrices in the quasiconvex hull of the energy wells for cubic-to-tetragonal transformations.
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Materials Science · Cross-listed from cond-mat.str-el

Research on topological materials using ultrafast spectroscopy

Hao Liu and Jian-Qiao Meng

Topological materials, characterized by symmetry-protected nontrivial band structures such as Dirac cones and Weyl nodes, host diverse quantum phenomena, with potential applications in quantum transport, spintronics, and nonlinear optics. Ultrafast pump-probe spectroscopy has emerged as a powerful tool for exploring nonequilibrium dynamics in these systems. Its femtosecond resolution allows charge, spin, orbital, and lattice interactions to be tracked on their intrinsic timescales, thereby revealing key coupling mechanisms in topological phases. This review summarizes progress in ultrafast spectroscopic studies of topological insulators, topological semimetals, and magnetic topological materials. We first discuss the relaxation pathways of photoexcited surface and bulk electronic states, emphasizing electron-phonon scattering, surface-bulk charge transfer, and ultrafast spin conversion. We then examine population inversion in Dirac and Weyl semimetals, spin-polarization dynamics associated with tilted Weyl bands, and the effects of magnetic order on topological states, including coherent phonon and magnon excitations, magnetically driven topological transitions, and terahertz emission. We further review photoinduced topological phase transitions driven by electronic correlations, lattice distortions, and magnetic order under intense optical excitation, highlighting routes toward nonthermal control of quantum phases. Finally, we outline future directions that combine multidimensional ultrafast spectroscopy with temporal, energy, momentum, and spin resolution and advanced theoretical modeling to establish a unified picture of nonequilibrium topological states. This review aims to provide a useful reference for ultrafast studies of topological quantum materials and to advance their applications in high-speed, low-power information processing, spintronics, and quantum technologies.
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Materials Science

Many-body interactions in the dielectric theory of stopping power of solids for classical and quantum projectiles

Vladimir U. Nazarov and Vyacheslav M. Silkin

We take account of the many-body dynamic interactions by including the wave-vector and frequency-dependent exchange and correlations (xc) kernel $f_{xc}(\qv,\omega)$ in the framework of the dielectric theory of the stopping power of crystals for moving charges. The cases of classical and quantum projectiles are considered. We find that (I) the role of the xc effects in slowing of charges in crystalline solids is more pronounced than it is within the jellium model and (II) For velocities below the stopping maximum, inclusion of xc leads to the improvement of the comparison of the theory with experiment over a range of solid targets. On the other hand, in the high-velocity regime, the role of the dynamic xc proves negligible, which does not contradict experiment, and which we substantiate analytically within the jellium model of the target. The input to our theory is the microscopic dielectric matrix $\epsilon_{\Gv \Gv'}(\qv,\omega)$ within the random phase approximation (i.e., with neglect of $f_{xc}$), the calculation of which matrix is implemented in the existing solid-state codes.
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Materials Science

Quantized Photocurrents in Gapless Topological Matter

Byunghoon Kim, Tenzin Norden, Mohammad Yahyavi et al. (+17 more)

The quantum Hall effect establishes that topology can fix a material response to integer multiples of fundamental constants when an energy gap isolates the relevant symmetry-protected electronic states. Whether such universal quantization can also emerge in gapless matter, where topological bands coexist with a continuum of metallic excitations, has remained a fundamental question in the field of quantum materials. Chiral topological semimetals provide a unique setting in which to explore this principle; when optical transitions are confined to a single chiral node, the resulting circular photogalvanic effect is predicted to be quantized by the topological charge of the node. In real materials, however, this nonlinear optical phenomenon has remained experimentally elusive, obscured by trivial band transitions, insufficient energy separation between node pairs, and their relative positions with respect to the Fermi level. Here we observe a quantized circular photogalvanic effect in the chiral topological semimetal Rh0.95Ni0.05Si. Band engineering via Ni substitution opens a photon-energy window dominated by interband optical transitions at the {\Gamma}-point multifold node. This allows circularly polarized near- to mid-infrared pulses to drive a helicity-odd terahertz response that manifests three hallmarks of quantization: a sharp onset, a photon-energy-independent plateau governed by the magnitude of the monopole charge, and an abrupt long-wavelength cutoff imposed by Pauli blocking. Our work thus establishes an all-optical analogue of the quantum Hall effect and a new paradigm to realize topological quantization in gapless matter.
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All authors Byunghoon Kim, Tenzin Norden, Mohammad Yahyavi, Kaustuv Manna, Tyler A. Cochran, Zi-Jia Cheng, Xian P. Yang, Yu-Xiao Jiang, Xiangyu Luo, Payman Kazemikhah, Areeq Hasan, Vladimir N. Strocov, Sergey Shilov, Ilya Belopolski, Claudia Felser, Md Shafayat Hossain, Rohit P. Prasankumar, Guoqing Chang, M. Zahid Hasan, Prashant Padmanabhan

Materials Science

Magnetic Contributions to Phase Stability in the Co-Ni Binary: A First-Principles CALPHAD Study

Prajna Jalagam, Zhigang Wu, John Lawson et al. (+1 more)

We propose a simple method to employ ab-initio calculations to determine magnetic contributions to free energy of alloys. Validation on the Co-Ni binary demonstrates that this ab initio approach reproduces experimental FCC-HCP phase equilibria while providing physically transparent, structure-dependent magnetic parameters suitable for multicomponent extrapolation. Our results demonstrate that physically grounded magnetic parametrization, derived directly from electronic structure calculations, enables predictive phase diagram modeling for magnetic alloy systems.
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All authors Prajna Jalagam, Zhigang Wu, John Lawson, Axel van de Walle

Materials Science · Cross-listed from physics.optics

Peak-Decomposition-Free Inverse Metrology of Hyperspectral Moiré Photoluminescence

Katsunori Wakabayashi

Hyperspectral photoluminescence (PL) of moiré transition-metal dichalcogenide heterobilayers encodes spatially varying exciton landscapes, but extracting that information is hampered by the ambiguity of multi-peak spectral decomposition. Here we develop a peak-decomposition-free inverse framework for quantitative optical metrology of effective disorder coordinates. From the raw cube $I(x,y,E)$ we construct physically motivated descriptor maps – centroid energy, dominant emission energy, spectral width, low/high spectral-weight ratio, and dominant–centroid offset. Their spatial autocorrelation hierarchy and covariance structure form a robust descriptor fingerprint of multi-scale disorder-sensitive spectral statistics. By matching these descriptor summary statistics to a minimal smooth-plus-trap generative model through a grid-Bayesian inverse, we infer effective disorder coordinates $\Thetaeff=\{\Ws,\xis,\Wt,\nt\}$ with explicit uncertainties. Using synthetic PL cubes generated from controlled multi-scale landscapes, we recover the well-identified smooth-disorder coordinates and constrain the trap sector up to a strength–density degeneracy. We report this degeneracy explicitly as an intrinsic identifiability limit rather than a deficiency of the method, and map four canonical disorder regimes onto a disorder-coordinate diagram. The descriptor statistics are stable against shot noise and pixel pitch, and behave predictably under optical-resolution and energy-window changes. The same pipeline ingests experimental cubes without modification, making it directly applicable to two-dimensional and moiré materials. Our results establish descriptor-based hyperspectral PL as a practical, minimal-assumption route to optical disorder diagnostics and provide the validated core of a reusable analysis workflow (HyperPL-Diag) for moiré exciton systems.
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Materials Science

Emergent toroidal induction in a polar Weyl ferromagnet

Yuuri Suzuki, Yukako Fujishiro, Masataka Mogi et al. (+12 more)

Spin-orbit coupling (SOC) underpins modern spintronics by enabling the electrical generation of spin torques. Its reciprocal counterpart, in which magnetization dynamics produce electromotive forces through a spin-dependent Berry phase, is known as emergent electromagnetic induction (EEMI). However, this effect has previously been observed only in magnetic textures with spatial gradients, such as domain walls, helices, and skyrmions. Here, we demonstrate that even a spatially uniform ferromagnet can host EEMI through a previously unrecognized Berry-phase mechanism inherent to noncentrosymmetric conductors. In the polar Weyl ferromagnet PrAlGe, an applied alternating current generates spin-orbit torques that drive collective magnetization dynamics. The resulting emergent toroidal moment (T = P \times M), where (P) is the crystal's polar axis and (M) is the net magnetization, acts as a gauge potential whose time derivative (dT/dt) induces a Hall voltage. This contribution appears specifically in the out-of-phase component of the AC Hall response and scales linearly with frequency, providing direct evidence for EEMI. First-principles calculations further reveal that this toroidal vector encodes the collective motion of Weyl nodes in momentum space. These findings establish "emergent toroidal induction" as a new manifestation of spin-orbit entanglement, unifying Berry phase, topology, and spin dynamics while opening a pathway toward intrinsic and energy-efficient spin-charge interconversion.
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All authors Yuuri Suzuki, Yukako Fujishiro, Masataka Mogi, Juba Bouaziz, Takahiro Anan, Akiko Kikkawa, Daiki Yamaguchi, Max T. Birch, Yuto Kiyonaga, Minoru Kawamura, Yasujiro Taguchi, Takahiro Morimoto, Naoto Nagaosa, Ryotaro Arita, Yoshinori Tokura

Monday 13 July

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

Cooper Pairing Mediated by Onsite Exchange: A Possible Mechanism For High-Temperature Superconductivity

Jacques R. Eone

Among the various mechanisms proposed for unconventional superconductivity, this paper focuses on the Coulomb interaction responsible for $d$-wave and $s_\pm$-wave pairing symmetries in cuprates and iron pnictides. Although the effective multi-orbital interaction $U_\text{eff}=U-J$ is predominantly repulsive, an attractive component driven by the Hund's coupling parameter $J$ is sufficient to bind fractional charges. This attractive multi-orbital contribution and a repulsive term of the single-band Hubbard model yield the superconducting pairing gap $\Delta_0$ and an estimate of the transition temperature $T_c$. Given the complex electronic structure and vast compositional space of these materials, the model focuses exclusively on the doped superconducting plane hosting these fractional charges. Through this approach, an analytical expression mainly dependent on the Hubbard $U$ and Hund $J$ parameters that reproduces the empirical superconducting dome is derived. Furthermore, the model addresses the characteristic electron and hole doping asymmetry observed in cuprates by accounting for Hund's coupling parameters. Finally, while the model describes strange metal behavior, it currently provides only a qualitative explanation for the pseudogap phase and the underdoped isotope effect.
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Materials Science

Phase-Controlled Epitaxy and Anisotropic Antiferromagnetism of Polar Wurtzite MnTe

Janusz Sadowski, Jaroslaw Z. Domagala, Piotr Dziawa et al. (+8 more)

Altermagnetic spintronics requires materials in which compensated magnetic order, symmetry-controlled electronic responses, and epitaxial tunability can be combined in experimentally accessible thin films. MnTe is a key material in this context, but experimental studies have focused mainly on the stable NiAs-type polymorph, whereas the polar wurtzite phase remains largely unexplored. Here we demonstrate molecular-beam epitaxy growth and investigate properties of nearly phase-pure wurtzite MnTe deposited directly on GaAs(111)B, and show that small changes in the growth conditions strongly modify the phase composition, from a multiphase state with endotaxial NiAs-type inclusions embedded in wurtzite MnTe matrix to an almost single-phase polar wurtzite layer.
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All authors Janusz Sadowski, Jaroslaw Z. Domagala, Piotr Dziawa, Anna Kaleta, Sania Dad, Maciej Wójcik, Dorota Janaszko, Sławomir Kret, Oleksii Liubchenko, Maciej Sawicki, Katarzyna Gas

Materials Science

Hyperfine Structure and Exchange Coupling of Vacancy-Induced Ce$^{3+}$ Spin Centers in Nuclear-Spin-Dilute CeO$_2$

T.M. Chithresh and Rudra Banerjee

Oxygen vacancies in ceria CeO$_2$ donate electrons that localize as Ce$^{3+}$ ($4f^1$, $S=1/2$) small polarons, creating rare-earth spin centers through native defect chemistry rather than implantation or extrinsic doping. We investigate the magnetic environment of these centers using first-principles PBE$+U$ calculations with a linear-response Hubbard parameter ($U=5.8382$ eV), hyperfine tensors from the all-electron reconstruction of the projector-augmented-wave method, and Korringa-Kohn-Rostoker exchange calculations within the coherent-potential approximation. Four vacancy configurations spanning concentrations from $3.125\%$ to $12.5\%$ are considered. A distinctive feature of the host follows from cerium isotopics: all naturally occurring cerium isotopes possess nuclear spin $I=0$, eliminating on-site hyperfine interactions at the Ce$^{3+}$ center and leaving the nuclear-spin bath entirely on the oxygen sublattice, whose sole magnetic isotope, $^{17}O$ ($I=5/2$), occurs at $0.038\%$ natural abundance. The resulting $^{17}O$ hyperfine landscape consists of a small number of strongly coupled, nearly axial first-shell nuclei with contact couplings reaching $6$~MHz, surrounded by a weakly coupled and strongly anisotropic outer shell. These tensors define experimentally accessible signatures for $^{17}O$ ESEEM and HYSCORE measurements and provide the microscopic hyperfine parameters required for cluster-correlation-expansion calculations of spin coherence. Exchange interactions between neighboring polarons are weak and oxygen-mediated, leaving the vacancy-generated spins largely independent over the concentration range considered. Together, these results establish oxygen-deficient CeO$_2$ as a chemically generated and intrinsically nuclear-spin-dilute host for rare-earth spin centers, and provide the first-principles magnetic parameters needed to assess their coherence properties.
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Materials Science

Coordination-Resolved Surrogate Models for Thermodynamic Stability, Band Gaps, and Magnetic Moments of Spinel Oxides, Sulfides, and Selenides

Keltoum Khallouq, Ayoub El Maazouzi and Rachid Masrour

We curated 320 cubic ($Fd\bar{3}m$) spinel entries from the Materials Project-nitrides, oxides, sulfides, and selenides, including single-cation mixed-valence $A_3X_4$ compounds-and trained tree-ensemble surrogates for the formation energy, energy above the convex hull, band gap, total magnetization, and metallicity. Cations were assigned to tetrahedral-like and octahedral-like groups from CrystalNN coordination numbers rather than from element identity, and the evaluation was group-aware throughout: splits were grouped by reduced formula, every transform was fit on training folds only, and champion models were selected on cross-validated scores before the holdout was examined. Over twenty repeated grouped holdouts (single-seed refits that reuse the tuned hyperparameters, and are therefore mildly optimistic) the champions reach mean absolute errors of $0.121\pm0.030$ eV/atom for the formation energy, $0.048\pm0.013$ eV/atom for the hull distance, and $1.27\pm0.19$~\muBfu{} for the magnetization, with a metallicity accuracy of $0.85\pm0.06$. Band-gap regression does not beat a trivial baseline on the 19-member non-metal holdout under paired bootstrap testing, we report this negative result and trace it to sample scarcity and to the semi-local DFT labels. On the identical grouped split, the tabular champion is more accurate than an untuned MEGNet trained from scratch (0.087 versus 0.209 eV/atom formation-energy MAE on the primary holdout), a comparison that bounds, rather than settles, the descriptor-versus-graph question at this data scale. SHAP attribution ties the magnetization model to octahedral $d$-occupancy and the formation-energy and band-gap models to electronegativity descriptors, and grouped conformal intervals, permutation nulls, and leave-one-chemistry-out tests bound a domain of applicability that is uneven across anions and cations.
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Materials Science

Direct writing of individual quantum dots

Weikun Zhu, Natalie Ngoh, Shelly Ben-David et al. (+9 more)

Quantum light sources capable of generating single photons are fundamental building blocks for photonic quantum technologies. In the ongoing search for an ideal quantum emitter, inorganic halide perovskite nanocrystals have emerged as a promising source of single photons. Their unique optical response, with an unmatched ease of synthetic tunability, stands out amongst the competing platforms. However, their stochastic dispersion in solution challenges the deterministic and stable integration of individual emitters with photonic structures that is required for practical technologies. Notably, resolution and material compatibility constraints make conventional top-down fabrication processes insufficient for such heterogeneous integration. Here, we report direct writing of perovskite quantum dots (QDs) with individual-emitter resolution. By inducing a nanoscale-confined formation volume using a thermal scanning probe method, we achieve site-selective synthesis down to a single atomic-scale QD with spectral tunability and < 25 nm spatial control. As a result, we demonstrate high-yield arrays of CsPbI3 single-photon emitters with narrow linewidths and high single-photon purity up to 98% at room temperature, performance consistent with that of their state-of-the-art colloidal counterparts. Through such deterministic control, we uniquely realize the precise, on-demand coupling of these emitters to photonic cavities, as evidenced by a measured enhancement in the spontaneous emission rate. This represents a key advancement toward addressing the longstanding integration obstacles of these materials. Overall, by combining the atomic-scale tunability of chemical synthesis with the spatial control of additive manufacturing, our work opens new emitter engineering strategies to realize the untapped potential of colloidal materials for next-generation quantum technologies.
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All authors Weikun Zhu, Natalie Ngoh, Shelly Ben-David, Maxwell Conte, Teddy Hsieh, Sarah O. Spector, Tara Sverko, Patricia Jastrzebska-Perfect, Will Jack, Jinwoo Sim, Peter F. Satterthwaite, Farnaz Niroui

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

Universal scalings and switching entropy in yield-stress fluids

Rajam Elancheliyan, Jean Marc Fromental, Edouard Chauveau et al. (+1 more)

Yield-stress fluids transition from solid-like to liquid-like behavior at a critical stress threshold, governing phenomena from industrial processing to geological flows. While predominantly investigated under steady shear, large-amplitude oscillatory tests force these materials to cyclically navigate between arrested and fluidized states. Here, we discover a hidden universal behavior where the dynamic viscoelastic moduli of yield-stress fluids collapse onto master curves, revealing that these materials rearrange almost instantaneously to maintain a constant intra-cycle stress state. We fully capture this behavior using a novel theoretical framework based on the minimization of a governing function that exhibits symmetry breaking. Our findings reveal that recoverable elastic energy, yielding abruptness, and entropy production during stress inversion are fundamentally intertwined. This connection provides a unified physical picture for the dynamic yield stress, offering a novel thermomechanical foundation to define and predict this threshold across soft matter physics and materials science.
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All authors Rajam Elancheliyan, Jean Marc Fromental, Edouard Chauveau, Domenico Truzzolillo

Materials Science

Strain-controlled crystalline–amorphous transition and flat-band tuning in buckled silicon kagome

Chenhaoyue Wang and Amartya S. Banerjee

Electronic flat bands in an elemental two-dimensional material provide an attractive setting for electron interactions competing with suppressed kinetic energy. Here we propose a buckled silicon kagome lattice (SiKL), an unfunctionalized six-atom monolayer of bond-linked Si$_3$ triangles and dodecagonal pores. Its planar parent hosts a dispersionless Kohn–Sham band near the Fermi level but is unstable to out-of-plane distortions. Following three soft zone-centre phonons and relaxing displaced structures yields two nearly degenerate buckled forms. The high-buckling form retains a partially flat kagome-derived band near the Fermi level. Biaxial tension controls lattice dynamics and electronic dispersion: at 10% strain, the bandwidth decreases significantly, the density-of-states peak approaches the Fermi level, and the softest phonon hardens. At 315 K, $6\times6$ ab initio MD shows the unstrained network disordering while the strained network remains ordered, indicating finite-temperature metastability. Fifty-nanosecond classical MD of $36\times36$ sheets reveals a strain-controlled crystalline–amorphous transition and local-bonding crossover near 2% strain. Low-strain trajectories show gradual, two-stage disordering; higher strains undergo an abrupt, first-order-like collapse, with the transition temperature reaching approximately 600 K at 10% strain. An exploratory Ag(111) substrate model suggests epitaxial mismatch could supply comparable tension, retain a narrow SiKL band, and preserve crystalline order above room temperature. Unlike passivated or hybrid-lattice silicon kagome proposals aimed mainly at conventional semiconductors, SiKL is elemental and uses strain alone to couple thermal metastability, bond rearrangement, and near-Fermi flat-band tuning. Buckled SiKL is a candidate platform for strain-controlled flat-band and electronic correlation physics.
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Materials Science

Exchange topology and criticality in ferrite and chromium spinels: a unified Monte Carlo analysis

Keltoum Khallouq, Ayoub El Maazouzi, Kamal Boumhara et al. (+1 more)

We report a unified analysis of Metropolis Monte Carlo results for two families of magnetic spinels: inverse ferrites Fe$^{3+}_{A}$[M$^{2+}$Fe$^{3+}$]$_{B}$O$_4$ (M = Co, Cu, Fe, Ni), where superexchange couples two chemically distinct sublattices, and chromium spinels $A$Cr$_2X_4$ ($A$ = Zn, Cd, Hg, $X$ = S, Se) together with the breathing-lattice chromates Li$M$Cr$_4$O$_8$ ($M$ = Ga, In), where a single Cr$^{3+}$ species occupies a corner-sharing tetrahedral network. Placing the exchange constants, transition temperatures, critical exponents, hysteresis, and magnetocaloric responses of these systems on a common footing, we introduce two reduced quantities not previously reported: the ratio $\theta_{\mathrm{CW}}/T_C$ for the ferrites and the normalized ordering scale $t^{*}=k_BT_C/[J_1S(S+1)]$ for the chromium compounds. The ferrites cluster in the range $\theta_{\mathrm{CW}}/T_C = 0.94$-$1.19$, close to the mean-field expectation of unity and the signature of dominant, unfrustrated A-B superexchange, and their exponents ($\beta = 0.20$-$0.26$, $\gamma = 1.23$-$1.27$, $\delta = 4.76$-$4.78$) follow the three-dimensional Ising class. The chromium systems split into three regimes: $t^{*} \approx 1.4$-$1.9$ for Ising-treated sulfides, $t^{*} \approx 0.99$ for Heisenberg-treated selenides, and $t^{*} \approx 0.24$-$0.25$ for the antiferromagnetic breathing chromates, quantifying the combined suppression of $T_C$ by continuous spin symmetry and by geometric frustration. Finite-thickness simulations of Fe$_3$O$_4$ resolve a dimensionality crossover between two and four unit cells. We identify the Ising-versus-Heisenberg dependence of the predicted universality class in frustrated chromites as the principal open problem.
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All authors Keltoum Khallouq, Ayoub El Maazouzi, Kamal Boumhara, Rachid Masrour

Materials Science · Cross-listed from cs.LG

CatRetriever: Contrastive Representation Learning for Slab-to-Bulk Retrieval in Generative Catalyst Discovery

Jungho Oh, Woosung Kim, Dong Hyeon Mok et al. (+2 more)

Inverse design is an emerging data-driven paradigm for efficiently navigating vast chemical spaces to discover new materials with targeted properties, and in the context of heterogeneous catalysis, surface generative models have recently advanced this goal by directly generating catalyst surface-adsorbate structures. However, these models typically operate at the slab level and do not provide the corresponding parent bulk structure, making it difficult to assess bulk-dependent properties such as formation energy, surface energy, crystallographic symmetry, and synthesizability. Here, we address this missing slab-to-bulk connection as a retrieval problem and introduce CatRetriever, a contrastive representation learning model that aligns slab and bulk crystal representations in a shared latent space. From a slab query, CatRetriever accurately retrieves plausible parent bulk candidates with R@1 > 91% and R@3 > 98% on both the in-distribution and holdout evaluation sets. We further extend the CatRetriever framework into an adsorption energy targeted bulk discovery pipeline that combines bulk retrieval, generative search space expansion, and adsorption energy distribution analysis. This workflow evaluates candidates by both structural compatibility with the query slab and their ability to access the target adsorption energy range across diverse surface environments. CatRetriever therefore provides a scalable route for connecting catalyst generative models with physically plausible and adsorption energy compatible bulk catalyst discovery.
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All authors Jungho Oh, Woosung Kim, Dong Hyeon Mok, Jonggeol Na, Seoin Back

Materials Science · Cross-listed from physics.chem-ph

Correlation-consistent Gaussian basis sets for copper solids from material-constrained atomic optimization

Jincheng Yu, Xiaoyu Zhang, Min-Ye Zhang et al. (+3 more)

Correlation-consistent Gaussian basis sets are central to systematic molecular quantum chemistry, but their direct use in periodic solids is often limited by severe linear dependence from diffuse atomically optimized primitives. This problem is particularly acute for metallic and metal-containing systems, where reliable complete-basis-set (CBS) extrapolation is needed for correlated-wavefunction benchmarks. We introduce material-constrained atomic optimization (MCAO), a basis-set optimization framework that preserves the atomic and correlation-consistent character of Gaussian basis sets while penalizing large overlap-matrix condition numbers in representative solids. As a proof of concept, we generate Dunning-style MCAO-cc-pVXZ basis sets (X = D, T, Q) for Cu with all-electron, scalar-relativistic all-electron, effective core potential (ECP), and pseudopotential treatments. The resulting basis sets remain numerically stable for Cu solids and surfaces while reproducing molecular Cu dimer energetics and plane-wave reference properties of bulk Cu. CBS-extrapolated random-phase approximation calculations further enable a controlled assessment of pseudopotential, relativistic, and basis-set errors in bulk Cu and CO adsorption on Cu(111), providing scalar-relativistic all-electron Gaussian-basis benchmarks for the CO adsorption puzzle.
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All authors Jincheng Yu, Xiaoyu Zhang, Min-Ye Zhang, Yu Cao, Qiming Sun, Hong-Zhou Ye

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

How do 3M Command strips work? A fracture mechanics approach

Xue-Ling Luo, Nikolaos Bouklas and Chung-Yuen Hui

Removable adhesive systems such as 3M Command strips are designed to support substantial loads while allowing clean, damage-free removal from the substrate. These systems rely on a highly extensible adhesive strip that bonds strongly during use but releases when stretched, causing the adhesive layer to elongate and progressively debond from the surfaces. A central challenge in the design of stretch-release adhesives is therefore to maximize load-bearing capacity while minimizing the force required for removal. This study investigates the finite-deformation mechanics governing both load support and tape release in a hyperelastic stretch-release adhesive system, with particular focus on the 3M Command tape geometry. Explicit analytical expressions are derived for the energy release rate of interfacial cracks under both load-bearing and release conditions and are validated against $J$-integral evaluations from finite element simulations. The results show that the ratio of maximum supported load to release force scales linearly with the ratio of bonded length to adhesive thickness, which is typically very large. We also investigate geometry-driven alternating crack propagation between the backing and substrate interfaces, governing tape removal, by analytical solutions and simulations. Parametric studies of competing interfacial fracture toughnesses produce failure envelopes that provide a predictive framework for estimating release forces and unstable crack propagation in multilayer stretch-release adhesive systems.
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Materials Science · Cross-listed from cond-mat.soft

Photoconductive nonpolar liquids based on azobenzene

Carlo Rigoni, Promeet K. Saha, Jinyu Sheng et al. (+1 more)

The weakly conductive properties of mixtures of organic surfactants in nonpolar liquids are fundamental to many electrohydrodynamic phenomena and underpin several cutting-edge technologies, particularly the development of electrophoretic displays. To date, tuning the electrical properties of these systems has involved modifying their composition, including surfactant type and concentration, water content, and the carrier liquid, all of which influence their behavior. Here, we use photoresponsive molecules to control electric phenomena in nonpolar liquids externally with light irradiation, thereby rendering them photoconductive. In particular, we examine azobenzene solutions in toluene, whose conductivity can be adjusted by two colors of light: UV induces trans to cis isomerization, leading to an increase in conductivity, while blue light triggers cis to trans back-isomerization, decreasing conductivity. The findings of this study suggest new ways to expand the applications of weakly conductive organic fluids, such as in self-regulating devices that respond to sunlight or in externally programmable displays.
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All authors Carlo Rigoni, Promeet K. Saha, Jinyu Sheng, Rafal Klajn

Materials Science

Synthesis of Ti2B2Clx MBenes in molten salts from theoretical and experimental perspectives

Rodrigo M. Ronchi, Emile Defoy, Andrejs Petruhins et al. (+11 more)

The unique properties and application possibilities of two-dimensional (2D) materials motivates the exploration of different nanolaminated compounds. Here, by using a molten salt approach, we selectively etch Ti2InB2 with ZnCl2 to produce a multilayer (ml) Ti2B2Clx MBene. Scanning transmission electron microscopy, in combination with energy dispersive X-ray, and electron energy loss spectroscopies show that In atoms are completely removed from the precursor upon etching, being replaced by chlorine surface terminations with a coverage 1.1 < x < 1.4. Further, in situ X-ray diffraction indicates a direct biphasic transformation from Ti2InB2 to ml-MBene, with no signs of intermediate phase formation. A computational framework based on density functional theory further corroborates these experimental observations by showing a negative reaction free energy for the formation of ml-MBene, favourable over all competing processes. In addition, A-element substitution into to the 3D Ti2ZnB2 phase is predicted to be endergonic, consistent with the absence of experimental evidence for its formation. Initial Li-ion battery performance evaluation showed a stable discharge capacity similar or better than MAX phases and other borides. Altogether, the theoretical framework combined with materials synthesis and characterization provides a general approach for 2D materials development, for further expansion of the family of 2D materials.
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All authors Rodrigo M. Ronchi, Emile Defoy, Andrejs Petruhins, Justinas Palisaitis, Lianghao Yu, Lan Tang, Solenn Reguer, Dominique Thiaudière, Ningjun Chen, Durga Sankar Vavilapalli, David Portehault, Jonas Björk, Per O. Å. Persson, Johanna Rosen

Materials Science

Capturing the calendering U-shape in lithium-ion electrode thermal conductivity

Julius Störk

Calendering is a key manufacturing step in lithium-ion electrode production, increasing volumetric energy density by reducing electrode porosity. Its effect on through-plane effective thermal conductivity, however, can be non-monotonic: measurements of graphite-based anodes show an initial decrease in thermal conductivity during early calendering followed by recovery at higher compaction. Conventional porosity-based effective-medium closures cannot reproduce this U-shaped behaviour. We develop a calendering-aware extension of the Zehner–Bauer–Schlünder model that combines a Knudsen-corrected porous-medium baseline with a compression-indexed contact contribution. For graphite electrodes, the model represents the competing effects of increasing particle contact and calendering-induced reorientation of anisotropic graphite particles, which initially reduces favourable through-plane heat-transport pathways. For quasi-isotropic NMC cathodes, the observed response is instead captured through process-dependent contact-network evolution. Across 27 calendering states spanning thin and thick graphite anodes and NMC622 and NMC811 cathodes, the proposed closure reduces the mean absolute percentage error from 31.1% for the zero-fit reference model to 4.5%. The result shows that incorporating process-dependent microstructural evolution is necessary to capture the measured conductivity minimum. Validation across additional electrode formulations, thicknesses, and chemistries remains necessary to assess transferability.
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Materials Science

Amorphous materials as a frontier challenge for universal interatomic potentials

Natascia L. Fragapane and Volker L. Deringer

Pre-trained or 'foundational' machine-learned interatomic potentials (MLIPs) are now widely used in materials modelling. However, early pre-trained models and benchmarks have largely focused on ordered, crystalline structures, and their transferability to non-crystalline solids remains unclear. Here, we show that the amorphous state is indeed a central challenge for future universal MLIPs, based on a systematic evaluation of current mainstream models in this domain. We introduce a benchmarking framework built on a curated reference dataset of canonical amorphous systems, as well as validation for structures and properties. Our study identifies limitations in the transferability of many current pre-trained models and investigates fine-tuning strategies tailored to disordered phases. Together, our results can facilitate future applications of MLIPs in the fast-growing field of amorphous functional materials, and they provide guidance for designing next-generation training datasets and transferable atomistic models.
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Materials Science

PAC Studio Machine Learning: Human-in-the-Loop Analysis of TDPAC Spectra

Thien Thanh Dang, Juliana Heiniger-Schell and Doru Constantin Lupascu

Time-differential perturbed angular correlation (TDPAC or PAC) analysis is an ill-conditioned inverse problem in which site count, interaction type, correlated hyperfine parameters, damping, and initialization choices can produce competing numerical solutions. This software paper presents PAC Studio ML, a human-in-the-loop Python desktop environment for physics-informed inverse analysis of PAC spectra. The software integrates a Hamiltonian-based forward PAC model, user-defined synthetic training libraries, feature extraction, one-, two-, and three-site machine-learning predictors, direct parameter prediction, Auto sites model-family screening, ML-seeded nonlinear least-squares refinement, visualization, benchmarking, diagnostics, model-card reporting, and export tools. The ML component is designed to support, not replace, conventional fitting and expert interpretation by accelerating parameter exploration, suggesting plausible initialization regions, comparing site-count hypotheses, and improving reproducibility. Held-out synthetic tests demonstrate proof of operation and illustrate the unequal recoverability of PAC parameters in difficult inverse problems. Selected BiFeO3 examples demonstrate conventional, direct-ML, ML-seeded, and Auto sites workflows as software case studies, not as a complete experimental validation corpus. PAC Studio ML is therefore positioned as a supporting tool for expert PAC analysis: it improves workflow speed and diagnostic transparency while final model choice, physical constraints, and materials interpretation remain the responsibility of the researcher.
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Materials Science · Cross-listed from cond-mat.mes-hall

Tellurium Metasurface Beam Splitter with Pulse Laser-Controlled Anisotropy

Takuto Hiraoka, Mizuho Matoba, Yuta Kobayashi et al. (+5 more)

Laser-programmable optical anisotropy offers a new route to developing reconfigurable metasurfaces without conventional nanofabrication processes. Here, we demonstrate a lithography-free approach based on spatial control of the crystallographic $c$ axis orientation in tellurium (Te) using pulse laser irradiation. As a proof of concept, we demonstrate a Te metasurface beam splitter by laser-written optical-axis patterning and experimentally confirm that its optical response is in good agreement with theoretical predictions and numerical simulations. By directly programming the local optical anisotropy, this method enables a simple fabrication process while offering the possibility of rewriting and dynamically reconfiguring device functionality. These features make this approach a promising platform for non-resonant active metasurfaces and other reconfigurable flat-optics applications.
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All authors Takuto Hiraoka, Mizuho Matoba, Yuta Kobayashi, Arata Mitsuzuka, Masashi Kawaguchi, Haruyuki Sakurai, Kuniaki Konishi, Masamitsu Hayashi