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Physics Condensed Matter Materials Science

cond-mat.mtrl-sci Watch Materials Science

Monday 25 May

Materials Science

Dealloying by peritectic melting

Mingwang Zhong and Alain Karma

Peritectic melting of Ti–Ag has been shown experimentally to form bicontinuous structures, but the mechanism remains unclear. Here we use phase-field simulations to show that these structures arise from a morphological instability of liquid film migration in three dimensions: a Ti-rich solid growing through an Ag-rich liquid film develops a branched seaweed or dendritic structure whose side branches coalesce to form handles, generating a high-genus bicontinuous topology. A sharp-interface theory predicts a solidification-front velocity and an initial ligament width that are constant in time, in contrast to liquid metal dealloying; subsequent $t^{1/3}$ coarsening reproduces the experimentally observed final ligament width.
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Materials Science

kikuchipy: an open-source toolbox for analysis of EBSD patterns

Håkon W. Ånes, Phillip Crout, Lars Andreas Lervik et al. (+5 more)

We present kikuchipy, an open-source toolbox for analysis of electron backscatter diffraction patterns, written in Python. The software is capable of both Hough and dictionary indexing and orientation and/or projection center refinement of patterns stored in file formats from all major vendors. Indexing results can be validated using maps independent of indexing and by visually comparing experimental and simulated patterns. By leveraging scientific packages in the Python ecosystem, emphasis is put on making the indexing workflow flexible and improve results through fast iteration. The software's capabilities are demonstrated on three application examples: analysis of orientation relationships in a super duplex stainless steel, phase differentiation of aluminium and silicon in a cast modified Al-Si alloy, and phase differentiation of particles in an Al-Mn alloy as Al6Mn or alpha-AlMnSi. The diffraction patterns and analysis workflows are made publicly available. kikuchipy was created and is developed as a resource for the electron microscopy community, allowing anyone to improve the software or include it into their own analysis workflows or softwares.
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All authors Håkon W. Ånes, Phillip Crout, Lars Andreas Lervik, Ole Natlandsmyr, Tina Bergh, Jarle Hjelen, Antonius T. J. van Helvoort, Knut Marthinsen

Materials Science

Alignment-free ultra-broadband parametric frequency conversion in lead-halide perovskites

Abhishek Shiva Kumar, Dusan Lorenc, Ayan A. Zhumekenov et al. (+2 more)

Lead-halide perovskites were demonstrated to exhibit some of the largest known optical nonlinearities, yet their potential for frequency conversion remains largely untapped. Here we demonstrate ultra-broadband four-wave mixing of near- and mid-infrared femtosecond pulses in thick single-crystal LHPs, generating bright, coherent, and highly collimated emission across an exceptionally wide continuous tuning range without phase-matching engineering, angular alignment, or dispersion optimization. Time resolved measurements reveal that the emission originates near the crystal surfaces, where phase-matching constraints are relaxed, while the unusually large intrinsic $\chi^{(3)}$ response preserves efficient and directional frequency conversion despite the strongly localized interaction volume. These results position LHPs as a powerful bulk platform for ultra-broadband nonlinear photonics, opening a pathway toward compact, alignment-free architectures for ultrafast frequency conversion.
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All authors Abhishek Shiva Kumar, Dusan Lorenc, Ayan A. Zhumekenov, Osman M. Bakr, Zhanybek Alpichshev

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

Machine Learning Multiscale Interactions

Àlex Solé, Sergio Suárez-Dou, Albert Mosella-Montoro et al. (+4 more)

Realistic physical systems are characterised by emergent interactions across multiple length and time scales, posing a significant challenge for predictive machine learning (ML) models. Most scientific ML models focus on a narrow range of interactions. While machine learning force fields (MLFFs) offer near-quantum accuracy, the ubiquitous message-passing layers miss long-range many-body effects. Here we introduce the Multiscale Structural Ensemble (MuSE), a hierarchical model that uses Soft Coarse-Graining Pooling to construct coarse representations from smooth fractional assignments of atoms to coarse nodes, enabling MLFF modules to operate across multiple scales. MuSE is architecture-agnostic and coupled with SO3krates, MACE, and PaiNN MLFFs for both molecules and materials. We demonstrate the power of MuSE through Hessian-based benchmarks, folding trajectories for biomolecules, and energy profiles in molecule-graphene nanostructures, where MuSE accurately captures quantum-mechanical interactions at relevant scales – unlike other recent long-range ML models.
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All authors Àlex Solé, Sergio Suárez-Dou, Albert Mosella-Montoro, Silvia Gómez-Coca, Eliseo Ruiz, Alexandre Tkatchenko, Javier Ruiz-Hidalgo

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

Pulse magnet of 10 T for power laser experiments with x-ray free-electron laser diagnostics

Akihiko Ikeda, Kosuke Noda, Yutaro Yamanaka et al. (+22 more)

The importance of investigating magnetized plasmas/solids in extreme conditions has grown over the last decades, particularly in the field of high energy density physics (HEDP), such as laboratory astrophysics and inertial confinement fusion. However, up to now, the unique capabilities of an X-ray free-electron laser (XFEL), such as high brilliance and low divergence have never been exploited for this type of research. In this paper, we present the first platform developed at SACLA, Japan, that combines a high-power optical laser for generating matter under extreme conditions of pressure and temperature, an XFEL probe, and an external magnetic field. The high current is produced using a 2 kV, 4.8 kJ pulsed power system giving a maximum current of 10 kA which is synchronized with the optical laser and XFEL in a vacuum environment. It flows through a split-pair coil to generate a high magnetic field (10 T at 6 kA) which has 1 cm access every 45$^{\circ}$ in the equatorial plane and 90$^{\circ}$ in the poloidal one. This platform offers new opportunities to study high-energy-density matter in strong magnetic fields, including shock propagation, instability growth, and turbulent plasma dynamics.
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All authors Akihiko Ikeda, Kosuke Noda, Yutaro Yamanaka, Yuma Urabe, Keiichiro Kawai, Yasuhiro H. Matsuda, Hirotaka Nakamura, Ryusuke Yamamoto, Yoshiki Naito, Yasuhiro Kuramitsu, Kai Taketoshi, Naoki Yamagata, Norimasa Ozaki, Tatiana Pikuz, Yoichi Sakawa, Takayoshi Sano, Ryosuke Kodama, Taichi Morita, Tomoya Ogawa, Kohei Miyanishi, Toshinori Yabuuchi, Rigon Gabriel, Bakandreas Stavros, Koenig Michel, Bruno Albertazzi

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

Near-Room-Temperature Antiferromagnetic Ordering in the Quadruple Perovskite Sr4NaRu3O12

Subham Naik, Biswajit Singh, Hiranmayee Senapati et al. (+10 more)

We report the synthesis, structure and magnetic properties of two 1:3 ordered quadruple perovskites Sr4MRu3O12 (M = Li and Na). Sr4NaRu3O12 crystallizes in the centrosymmetric space group R-3 and Sr4LiRu3O12 appears to be isostructural to the Na compound based on the PXRD data. In Sr4NaRu3O12, both Na and Ru are predominantly ordered at the B sites (here Na/Li and Ru) and the structure contains only corner-connected RuO6 and NaO6 octahedra. This atomic ordering also leads to a rather large unit cell with a = 11.25 Å and c = 27.6 Å compared to the basic 12R structure (a = 5.5 Å and c ~ 27 Å). Magnetic measurements reveal that Sr4NaRu3O12 undergoes a magnetic transition to an antiferromagnetic state below TN ~ 265 K which is confirmed by DSC and neutron diffraction. The Ru moments show a collinear antiferromagnetic spin alignment along the hexagonal c axis with a propagation vector k = (0, 0, 1.5). Interestingly, those Ru moments lying on the three-fold roto-inversion do not significantly contribute to the magnetic order, since they are located between antiferromagnetically coupled Ru atoms and are therefore probably highly frustrated. Band structure calculations on Sr4NaRu3O12 complement the observed magnetic ground state and a semiconducting behavior in the compound. Sr4LiRu3O12 shows a magnetic anomaly below 110 K, possibly associated with competing ferromagnetic and antiferromagnetic interactions.
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All authors Subham Naik, Biswajit Singh, Hiranmayee Senapati, Akshay K. U., Ramesh C. Nath, Soumyojit Chatterjee, Rahul Sharma, Thomas Doert, Walter Schnelle, Manfred Reehuis, Thomas C. Hansen, Michael Ruck, Gohil S. Thakur

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

Magnetoelastic effects in the metallic frustrated antiferromagnet CrB$_2$

Tadataka Watanabe, Mai Watanabe, Sakurako Suganuma et al. (+2 more)

Hexagonal chromium diboride CrB$_2$ is a metallic frustrated antiferromagnet with a Néel temperature $T_N \sim$ 88 K. In CrB$_2$, Cr 3$d$ electrons not only give rise to localized magnetic moments but also contribute to metallic conduction. We perform ultrasound velocity measurements on a single crystal of hexagonal CrB$_2$ to determine its elastic properties. The temperature dependence of the $ab$-plane shear elastic modulus exhibits Curie-type softening upon cooling from $\sim$120 K down to $T_N$. This behavior is interpreted as a precursor to a symmetry-lowering lattice distortion at $T_N$, indicating that magnetic frustration is relieved via transverse magnetoelastic coupling. In addition, the $a$-axis and $c$-axis compressive elastic moduli show unusual softness and their suppression upon cooling, which are naturally explained by Fermi-surface nesting and its suppression upon cooling. The present results suggest that, in CrB$_2$, longitudinal magnetoelastic coupling suppresses Fermi-surface nesting and enhances frustrated exchange interactions, while transverse magnetoelastic coupling plays a key role in relieving the frustration.
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All authors Tadataka Watanabe, Mai Watanabe, Sakurako Suganuma, Andreas Bauer, Christian Pfleiderer

Materials Science

Transition from Homogeneous to Domain-Wall-Mediated Polarization Switching in BaTiO3: A Machine-Learning Molecular Dynamics Study

Po-Yen Chen and Teruyasu Mizoguchi

Polarization switching in ferroelectric BaTiO3 can proceed through fundamentally different mechanisms - yet the conditions that determine which pathway is realized remain poorly understood. Using machine-learning potential-based molecular dynamics with the MACEField model, we systematically vary supercell size to reveal a clear transition from homogeneous polarization switching to domain-wall (DW)-mediated switching, accompanied by a coercive field increase of over 50%. Shannon entropy analysis demonstrates that this transition is driven by size-dependent polarization fluctuations that promote 180 degree DW nucleation - establishing a direct, quantitative link between local configurational disorder and macroscopic switching behavior. Additional analyses further suggest that the switching pathway is also influenced by the kinetic competition between DW nucleation/growth and annihilation. Furthermore, the switching pathway and hysteresis response are shown to depend critically on supercell geometry and the relative orientation of applied stress and electric field. These findings reveal that homogeneous and DW-mediated switching are distinct physical regimes in BaTiO3, and that atomistic simulations must account for system size to correctly capture the operative switching mechanism.
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Materials Science

Spin layer groups and their corepresentations

Zeying Zhang, Gui-Bin Liu, Mu Tian et al. (+3 more)

Spin layer groups are the crystallographic symmetry groups with a periodic plane, and their symmetry operations are inherited from three-dimensional (3D) spin space groups. However, the direct application of 3D symmetry groups to two-dimensional systems is often inadequate due to anisotropic axes and dimensional reduction. In this work, we systematically classify inequivalent spin layer groups and analytically derive their irreducible corepresentations. This classification establishes a foundational framework for investigating symmetry-protected properties and novel quantum states in low-dimensional magnetic materials.
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All authors Zeying Zhang, Gui-Bin Liu, Mu Tian, Run-Wu Zhang, Zhi-Ming Yu, Yugui Yao

Materials Science

Local Structural Signatures of Shear Bands in Metallic Glasses via Electron Nanodiffraction

Huyen T. Pham, Daniel East, Chunguang Tang et al. (+4 more)

Structural changes in a glass due to deformation are subtle and difficult to quantify using conventional imaging and diffraction techniques. Additionally, transmission electron microscopy (TEM) sample preparation using energetic ions often causes structural modifications that are challenging to detect in disordered materials. By preparing inverted cross-sectional transmission electron microscopy lamellae of shear bands formed during bending, and employing cryogenic ion polishing to minimize preparation artefacts, we preserve the intrinsic atomic structure. Using sensitive, new parameters derived from electron nano-diffraction, we directly probe the local nano-scale structure in the plastic zone beneath surface shear steps in metallic glasses. Mapping of local centrosymmetry and strain reveals nanoscale, stripe-like regions oriented at 45 degree to the applied strain where strain has localized. These regions exhibit a high density of local atomic structures that have transformed to configurations with reduced centrosymmetry and increased magnitudes of shear and normal strain. Our results demonstrate that plastic deformation in metallic glasses arises from coordinated nanoscale structural transformations, providing direct experimental insight into a long-standing problem.
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All authors Huyen T. Pham, Daniel East, Chunguang Tang, Matteo Baggioli, Alessio Zaccone, Timothy C. Petersen, Amelia C. Y. Liu

Materials Science

Disentangling the contributions of individual cations to magnetic order in a spinel high entropy oxide

Mario Ulises González-Rivas, Chun-Fu Chang, Martin Bluschke et al. (+8 more)

High entropy oxides (HEOs) can possess long-range ordered magnetic states despite their extreme chemical disorder. Very little is known about how the different chemical constituents in HEOs contribute to the emergence of these magnetic states. In this work, we leverage element-specific magnetometry attained via x-ray magnetic circular dichroism (XMCD) to understand how magnetic order is driven in two ferrimagnetic spinel-structured HEOs with compositions (Cr,Mn,Fe,Co,Ni)$_3$O$_4$ and (Cr,Mn,Fe,Co,Ni)$_{2.4}$Ga$_{0.6}$O$_4$. We find that while the magnetic transition is simultaneous for all chemical species, the rate at which their magnetic moments grow is strongly cation dependent. This behavior is explained by the varying $\textit{3d}$ crystal field level fillings of the magnetic cations, which in turn determine their ability to participate in the different magnetic exchange pathways available in the spinel structure. Dominant $A$-$B$ sublattice exchange enables some species to harden rapidly ($\textit{e.g.}$ tetrahedral Fe$^{3+}$ and octahedral Ni$^{2+}$) while others exhibit a sluggish transition due to frustration from competing interactions ($\textit{e.g.}$ octahedral Fe$^{3+}$ and Cr$^{3+}$). Non-magnetic substitution suppresses these differences, introducing broken magnetic linkages that relieve frustration. Tailoring the magnetism of HEO spinels therefore requires detailed knowledge of both their site selectivities and their exchange pathways.
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All authors Mario Ulises González-Rivas, Chun-Fu Chang, Martin Bluschke, Jessica Freese, Peter Bencok, Ronny Sutarto, Teak D. Boyko, Robert J. Green, George A. Sawatzky, Liu Hao Tjeng, Alannah M. Hallas

Materials Science

Experimental and computational diffusion analysis in Ni-X binary and Ni-Al-X (X = Cr, Mo, Ta, W, Re) ternary systems

Ankur Srivastava, Suman Sadhu, Satyam Kumar et al. (+5 more)

An extensive diffusion analysis is presented for binary Ni-X and ternary Ni-Al-X (X = Cr, Mo, Ta, W, Re) systems, which play a crucial role in microstructural evolution and phase stability in Ni-Al-based superalloys. Specifically, we highlight changes in the diffusion coefficients of X in the presence of Al and compare diffusional interactions across systems considered. First-principles calculations, combined with activation energies derived from temperature-dependent experiments, reveal consistent trends in Ni-X systems, with variations in activation energies largely attributed to differences in migration energies. In ternary systems, diffusion coefficients estimated from intersecting diffusion profiles show that the main interdiffusion coefficient of X is comparable to its binary counterpart, with similar activation energies. However, cross-diffusion coefficients are shown to significantly influence fluxes, either enhancing or reducing diffusion lengths depending on the relative directions of diffusing elements. For Ni-Al-Re, a single-profile method is employed to overcome uncertainties in estimating composition gradients at the near-end-member intersecting composition. The diffusion coefficients obtained correlate well with the nature of diffusion paths when represented on Gibbs triangles. To extend these findings, a physics-informed neural network (PINN) optimization method is applied to extract composition-dependent diffusion coefficients across the full composition range. The analysis demonstrates the necessity of incorporating experimentally estimated diffusion coefficients as equality constraints, without which optimization reliability is compromised. Overall, the results establish a robust framework for diffusion studies in Ni-Al-X systems, highlighting the critical role of cross-diffusion effects and constraint-enhanced numerical methods.
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All authors Ankur Srivastava, Suman Sadhu, Satyam Kumar, Ujjval Bansal, Raju Ravi, Saswata Bhattacharyya, Gopalakrishnan Sai Gautam, Aloke Paul

Materials Science

Three-Dimensional Atomic-Scale Structural Transformation in a SrTiO3 Grain Boundary

Xiaoyue Gao, Jiake Wei, Bo Han et al. (+9 more)

Grain boundaries (GBs) in complex oxides play critical roles in governing their functional properties, which are intrinsically linked to their three-dimensional (3D) atomic configurations and local chemical environments that can deviate markedly from those of the bulk. However, the 3D atomic structures of GBs remain poorly understood due to the projection limitations of conventional (S)TEM. Here, using multislice electron ptychography, we resolve the 3D atomic structure of a {\Sigma}13(510)/[001] tilt GB in SrTiO3 with simultaneous visualization of both cation and oxygen columns. Depth-resolved reconstruction reveals pronounced structural inhomogeneity along the GB, uncovering a transition from the canonical symmetric configuration (STR1) to an asymmetric configuration (STR2) that is hidden in conventional projection imaging. Quantitative analysis of atomic-column intensities demonstrates that these two GB configurations possess distinct local chemical and vacancy distributions. By further mapping the atomic displacement fields, we reveal that the transformation between STR1 and STR2 proceeds via local atomic shuffling at the GB core and collective shear displacement in the adjoining grains, mediated by the step and dislocation character of the junction, respectively. Moreover, analysis of oxygen octahedral rotations reveals a strong dependence on the local atomic structure with pronounced asymmetry around the STR2 region. These findings establish a direct link among the 3D atomic structure, local chemical composition, and lattice order parameters at the GB, underscoring the critical importance of depth-resolved characterization in understanding and engineering GB-mediated functionalities in complex oxides.
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All authors Xiaoyue Gao, Jiake Wei, Bo Han, Junpin Luo, Ruilin Mao, Xiaowen Zhang, Xiaomei Li, Ryo Ishikawa, Bin Feng, Naoya Shibata, Yuichi Ikuhara, Peng Gao

Materials Science

Effects of Band Symmetry on Spin-Dependent Transport in Noncollinear Antiferromagnetic Tunnel Junctions

Mohamed Elekhtiar, Ding-Fu Shao and Evgeny Y. Tsymbal

Antiferromagnetic tunnel junctions (AFMTJs) can exhibit large tunneling magnetoresistance (TMR), making them promising candidates for ultrafast and field-robust spintronic devices. Here, we elucidate the role of band symmetry in governing spin-dependent transport in AFMTJs. Using first-principles density-functional theory combined with quantum-transport calculations, we investigate Mn3NiN/LaAlO3/Mn3NiN (001) junctions based on the noncollinear $\Gamma_{4g}$ antiferromagnetic phase of Mn3NiN. Although Mn3NiN exhibits a large momentum-dependent spin polarization due to broken $PT$ symmetry, we show that the tunneling conductance is critically controlled by band symmetry of the electrode Bloch states and their symmetry-selective coupling to evanescent states in the LaAlO3 barrier. Orbital-symmetry selection rules suppress interband transmission in the parallel configuration, whereas the antiparallel configuration enables symmetry-compatible interband tunneling along the diagonal directions of the two-dimensional Brillouin zone. These additional transmission channels enhance the antiparallel conductance and reduce the TMR relative to predictions based solely on spin polarization. Nevertheless, the TMR remains exceptionally large, exceeding 2000%, while band symmetry controls the attainable magnitude of TMR in AFMTJs. Our results establish band-symmetry filtering as an essential ingredient of spin-dependent tunneling in AFMTJs.
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Materials Science

Anomalous Subsurface Vacancy Stabilization Dictated by Geometry-Electronic Decoupling on Metal Surfaces

Yiming Tan and Pai Li

Vacancy formation energetics fundamentally govern the structural integrity and catalytic behavior of metal surfaces. Contrary to conventional coordination-dependent broken-bond models, we identify an anomalous thermodynamic inversion on close-packed surfaces across Ir, Pt, Au with face-centered cubic (FCC) lattice and Be, Zn, Cd with hexagonal close-packed (HCP) lattice, where subsurface vacancies are intrinsically more stable than surface ones. Using high-throughput DFT calculations and machine learning force fields, we demonstrate that the physical origins of this anomaly are fundamentally decoupled between the two crystal systems. For the FCC trio, pronounced surface relaxation and a profound real-space electronic localization induce directional, covalent-like intralayer bonding, materializing a "geometry-electronic decoupling" mechanism. Crucially, this unconventional thermodynamic hierarchy enables a dynamic "self-healing" mechanism on Pt(111) that preserves an intact topmost layer and prevents catalytic degradation during hydrogen evolution and oxygen reduction reactions. It also successfully decoding the critical defect threshold (~8%) for lifting the Au(100) surface reconstruction. Our work challenges classical scalar defect models and provides a fresh paradigm for engineering catalyst surface integrity.
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Sunday 24 May

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

Structural gradients and strain partitioning across the mouse Achilles tendon enthesis revealed by in situ X-ray scattering

Isabella Silva Barreto, Moritz L. Stammer, Moritz P.K. Frewein et al. (+8 more)

The enthesis is the insertion site of tendon into bone and exhibits a high mechanical durability despite the large mismatch in material properties between the two tissues. This durability stems from gradients in composition, structure and organization on multiple hierarchical length scales. Despite extensive research on enthesis structure and mechanics, the local deformation mechanisms are poorly understood. Synchrotron scanning small- and wide-angle X-ray scattering was combined with in situ tensile testing of the mouse Achilles tendon enthesis to extensively map the mechanical response of the collagen fibrils and molecules as well as the hydroxyapatite mineral particles and crystals. Gradients in nano- and molecular scale structure and a stronger and more immediate deformation response towards the interface compared to further away were observed in both the soft and mineralized tissue. The strain decreased progressively across hierarchical levels; with an applied tissue strain of 20% the nanoscale fibrils were strained by ~1-2%, the collagen molecules by ~0.5% and the hydroxyapatite crystals by ~0.05%, thus following an approximate ratio of 1 : 0.1 : 0.01 : 0.001. These results show that load transfer across the enthesis is both spatially heterogeneous and hierarchy-dependent. This indicates that the graded attachment accommodates deformation through region-specific load sharing and hierarchical strain partitioning, consistent with a contribution from dissipation within the non-collagenous matrix. In doing so, the enthesis can mitigate stress concentrations and maintain mechanical integrity across the tendon-to-bone transition.
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All authors Isabella Silva Barreto, Moritz L. Stammer, Moritz P.K. Frewein, Claire Camy, Juraj Todt, Michael Meindlhumer, Jozef Keckes, Stefano Checchia, Sandrine Roffino, Martine Pithioux, Tilman A. Grünewald

Materials Science

Thermal PBE in warm dense matter: Does it matter and is it accurate?

Kushal Ramakrishna, Mani Lokamani, Zhandos A. Moldabekov et al. (+3 more)

Conditional probability density functional theory has recently been used to derive the temperature dependence of the Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA) for the exchange-correlation (XC) free energy. We implement and systematically benchmark thermal PBE within Kohn-Sham density functional theory calculations of warm dense matter. Comparisons with the local density approximation (LDA) and PBE functionals, as well as thermal LDA, show that thermal PBE significantly improves the description of warm dense matter properties, including energies, forces, pressures, and electronic charge densities. In particular, thermal PBE exhibits close agreement with path integral Monte Carlo (PIMC) reference data at negligible additional computational cost. This work demonstrates the practical utility of thermal PBE as an accurate semilocal functional for simulations in the warm dense regime.
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All authors Kushal Ramakrishna, Mani Lokamani, Zhandos A. Moldabekov, Tobias Dornheim, Kieron Burke, Attila Cangi

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

Non-local low energy neutral excitations in a strongly disordered triangular Mott magnet Cr$_3$Se$_2$Br$_5$

Wenhao Liu, Dechen Zhang, Yuanqi Lyu et al. (+13 more)

Understanding if low-energy excitations can remain itinerant in the presence of strong disorder remains a central challenge in frustrated quantum magnets, where disorder is generally expected to localize excitations through Anderson-like mechanisms. Here we report the emergence of charge-neutral itinerant excitations in a van der Waals compound Cr$_3$Se$_2$Br$_5$, a strongly disordered $S = 3/2$ Mott insulator with a frustrated triangular lattice. Structural analysis reveals substantial intrinsic disorder arising from Cr-site deficiency and Se/Br-site mixing, which appear to be fixed and cannot be readily tuned. No long-range magnetic order or conventional glassy behavior is observed. In addition to its highly insulating nature, the magnetic specific heat C_mag/T and thermal conductivity ąppa}_xx/T both exhibit linear temperature dependencies with substantial finite intercepts. In particular, a sizeable field-independent residual term $\kappa/T \approx 0.03~\mathrm{W\,m^{-1}\,K^{-2}}$ is observed, providing compelling evidence of itinerant low-energy excitations that carry entropy without charge. These findings conceptually advance our understanding of quantum matter by demonstrating a rare regime where the interplay of disorder, frustration, and electronic correlations actively reshapes the nature of low-energy excitations, allowing itinerant neutral excitations to coexist with strong intrinsic disorder.
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All authors Wenhao Liu, Dechen Zhang, Yuanqi Lyu, Lebing Chen, Lifang Hu, Keith M. Teddei, Yuting Zhang, Steve Shelton, Moon Kim, Xiqu Wang, Michael A. Susner, James G. Analytis, Dung-Hai Lee, Lu Li, Bing Lv, Robert J. Birgeneau

Materials Science

Imaging Surface Magnetization in Altermagnetic MnTe Films

Ling-Jie Zhou, Senlei Li, Zi-Jie Yan et al. (+15 more)

Altermagnets with pronounced spin-splitting band structure, unconventional magnetic and crystal symmetries, and exotic magneto-transport properties have received immense interest in cutting-edge spintronics, materials science, and condensed matter physics research. Microscopic imaging of spontaneous magnetic domains and phases in altermagnets constitutes an important step for investigating their underlying material properties, mechanisms, and spin behaviors. Taking advantage of scanning-probe quantum microscopy, here we report nanoscale quantum sensing of a prototypical altermagnet candidate $\alpha$-MnTe. We visualize evanescent magnetization and the associated magnetic domains in epitaxial MnTe films, which allows external magnetic fields to control the intrinsic altermagnetic order and configurations. By evaluating a series of MnTe films with different thicknesses down to the atomic scale, we further present evidence for the interfacial origin of the observed weak magnetization and show its correlation with the anomalous Hall effect in MnTe film. Our results advance the current understanding of emergent altermagnetism, providing insights into future material design of altermagnet-integrated spintronic devices.
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All authors Ling-Jie Zhou, Senlei Li, Zi-Jie Yan, Yufei Zhao, Hongtao Rong, Zelong Xiong, Yiran Zhao, Zhaorong Gu, Pu Xiao, Ke Wang, Lok Kan Lai, Hyeonhu Bae, Haoyu Liu, Chao-Xing Liu, Binghai Yan, Cui-Zu Chang, Hailong Wang, Chunhui Rita Du

Materials Science

Composition-Driven High-Entropy Alloys with Enhanced Magnetocaloric Properties

Nishant Tiwari, Juan Rafael Gomez Quispe, Noorbasha Bhavani Sai et al. (+4 more)

High entropy alloys (HEAs) are promising magnetocaloric materials with tunable operating temperature conditions using compositional modifications. Here, we combine experiments and first principles based spin modelling to engineer magnetocaloric response in single-phase cubic HEAs consisting of earth-abundant elements such as Fe, Ni, Co, Cr, and Cu. An equiatomic (Fe20Ni20Co20Cr20Cu20) and a Fe and Co rich non equiatomic (Fe34Ni17.7Co24.8Cr15.2Cu8.3) show a continuous ferromagnetic to paramagnetic transition with Curie temperature (TC)=110 K and TC=420 K for non equiatomic. Under the 1.6 Tesla magnetic field, the investigated alloy shows the entropy change =1.24 J per kgK with relative cooling power (RCP) =92 J per kg due to a broader effective cooling span. Density functional theory simulations reveal that reducing Cu enhances the spin-polarized Fe, Co, or Ni, 3d weight near fermi level, consistent with stronger ferromagnetic exchange. Exchange couplings mapped onto a classical Heisenberg model and solved by atomistic Monte Carlo to theoretically predict TC of both the investigated alloys. A controlled theoretical Cu sweep in equimolar further confirms that increasing Cu monotonically dilutes the magnetic sublattice and lowers TC, providing a quantitative design guideline to tune magnetocaloric operating temperatures in transition metal HEAs.
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All authors Nishant Tiwari, Juan Rafael Gomez Quispe, Noorbasha Bhavani Sai, Saikat Talapatra, Pedro Alves Da Silva Autreto, Varun Chaudhary, Chandra Sekhar Tiwary

Materials Science

Exploring Multi-Transition-Metal NASICON Frameworks as High-Performance Cathodes for Sodium-Ion Batteries

Santosh Behara, Achinthya Krishna Bheemaguli and Gopalakrishnan Sai Gautam

The search for sustainable, high-performance cathodes has driven a growing interest in sodium superionic conductor (NASICON)-type phosphates for sodium-ion batteries (SIBs). To identify promising NASICONs containing earth-abundant transition metals (TMs) and to systematically examine the role of multiple TMs in influencing the various properties of NASICON cathodes, we employ density functional theory calculations to investigate nine NASICON compositions containing Mn, Cr, and/or Fe, and spanning unary, binary, and ternary combinations. Our calculations reveal that unary systems, in terms of their Na intercalation phase behavior, exhibit well-defined stabilization at intermediate Na contents ($x$ in Na$_x$TM$_2$(PO$_4$)$_3$), while binary and ternary systems display more complex phase behavior, with some systems showing a shift of thermodynamic minima from $x$ = 3 to 2. Intercalation voltages highlight the dominant role of Fe$^{4+}$/Fe$^{3+}$ redox activity in elevating average voltages ($\sim$4.0 V), while Mn and Cr introduce intermediate-to-low voltage redox activity. Electronic structure data demonstrate non-systematic changes in the band gap, especially in systems containing multiple TMs. Na$^+$ mobility results identify mixed-TM frameworks as favorable, achieving Na$^+$ migration barriers in the 0.3-0.4 eV range. Importantly, we identify Na$_x$MnFe$_{0.5}$Cr$_{0.5}$(PO$_4$)$_3$ to be a promising ternary composition for subsequent experimental validation, offering an optimal intersection of phase stability, voltages, thermodynamic (meta)stability, and Na$^+$ migration barriers. Together, our study provides fundamental insights into the interplay between compositional complexity, thermodynamic stability, electronic structure, and ionic transport in NASICONs, and offers actionable design principles for utilising multi-TM NASICONs as high performance SIB cathodes.
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Materials Science

First-principles finite-size correction schemes for point defects of Cu$_3$N

Abdul M. Reyes, Sebastian E. Reyes-Lillo and Eduardo Menéndez-Proupin

Point defects play a key role in determining semiconductor properties, such as electrical conductivity and photoluminescence, and often enable functional behavior. Accurate first-principles supercell simulations of point defects require reliable finite-size corrections. In this study, we combine PBE+U structural relaxations with HSE hybrid-functional calculations to determine defect formation energies and thermodynamic transition levels of Cu$_3$N. Finite-size trends are quantified using $\Gamma$-point calculations in supercells containing 256, 864, and 2048 atoms. We assess and extend the Makov-Payne and Lany-Zunger correction schemes by introducing additional $1/L^n$ terms, together with core-level potential alignment and defect-specific scaling models. Using Cu$_3$N as a case study, we show that charged vacancies with strongly localized defect states are accurately described by the Makov-Payne-type scaling ($1/L + 1/L^{3}$), whereas interstitial defects with shallow or weakly localized electronic character are better captured by a hydrogenic impurity model that accounts for defect-band dispersion. Residual trends for neutral or weakly localized defects are described by higher-order polynomial fits in $1/L^{3}$ and $1/L^{4}$. Hybrid-functional energetics corrected using PBE+U-based finite-size trends confirm the intrinsic $p$-type character of Cu$_3$N under the conditions considered and demonstrate that no single finite-size correction can be transferred across all defect types.
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Materials Science

Harmonic Hierarchy of Altermagnetic Spin Splitting from Symmetry-Adapted Wavefunctions

Yixuan Che, Peibo Xu, Haifeng Lv et al. (+2 more)

Altermagnets combine magnetic compensation with spin-momentum-locked splitting in the absence of spin-orbit coupling, yet existing descriptions, formulated primarily in terms of spin symmetry and lattice geometry, provide limited insight into the electric-structure perspective of its angular harmonic form. Here, we identify a wavefunction-level framework for altermagnetism in two-dimensional square lattices. Using symmetry-adapted polynomial wavefunctions, we show that the harmonic structure of momentum-space spin splitting is inherited from the geometry of the electronic wavefunctions which can be selected by crystal fields. Identical orbital sectors preserve conventional antiferromagnetic degeneracy, whereas intertwined linear and quadratic wavefunctions generate d-wave and g-wave altermagnetic anisotropies, respectively. Tight-binding analysis connects this hierarchy to inequivalent same-spin hopping channels. First-principles calculations on the g-wave mcm-type reticular material platforms confirm high-symmetry-linear degeneracy together with finite generic-k splitting. Our results establish a hierarchy linking wavefunction geometry, orbital realization, microscopic hopping anisotropy, and altermagnetic electronic structure.
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All authors Yixuan Che, Peibo Xu, Haifeng Lv, Xiaojun Wu, Jinlong Yang

Materials Science · Cross-listed from cs.LG

Reinforcement Learning with a Bilevel World-Model Architecture for Scan-Order Optimisation in Laser Directed Energy Deposition

Xian Wu, Haoran Li, Yuanqi Chu et al. (+2 more)

Scan-order design in laser directed energy deposition (LDED) is a delayed, path-dependent thermo-mechanical decision problem, because sequence quality becomes observable only after the complete deposition and cooling cycle. This work formulates LDED scan-order optimisation as a finite-horizon, permutation-constrained reinforcement-learning problem and develops a bilevel finite-element-teacher-labelled AI workflow. A surrogate-assisted teacher-guided optimisation loop learns the Abaqus-labelled response landscape and provides a tractable terminal-reward environment for policy training. A frozen Maskable Proximal Policy Optimization (MaskablePPO) policy is then used to generate legal scan-order candidates, which are independently validated through Abaqus thermo-mechanical simulations. The results show bounded, N-dependent policy-generation value rather than record-level dominance over the mature surrogate-assisted optimiser. The strongest scan orders are obtained by the teacher-guided surrogate loop, whereas PPO autonomously reaches competitive regions of the native response landscape, with stronger rank concentration at smaller track counts and a clear reliability boundary at longer horizons. The teacher-labelled landscape further supports a physically gated lexicographic reward hierarchy in which warpage admissibility is the primary constraint, plastic strain acts as a safety filter and residual-stress-related improvement is pursued conditionally within the admissible region. Validated sequences also reveal an interpretable scale-separated ordering tendency that combines global spatial dispersion with local structured grouping. This workflow provides a route from fixed scan-rule selection toward finite-element-teacher-validated policy generation, while preserving independent finite-element validation as the final physical gate.
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All authors Xian Wu, Haoran Li, Yuanqi Chu, Dongbin Zhao, Bin Wang

Materials Science

Assessment of a GW-BSE approximation scheme on an asymmetric two-dimensional interacting electron system in a perpendicular magnetic field

Xiaoguang Wu

A GW-BSE approximation scheme is assessed by applying it to a model of asymmetric two-dimensional (2D) interacting electron system. The model is assumed to have a parabolic band characterized by two independent effective mass parameters. A perpendicular magnetic field is applied to the asymmetric 2D electron system, and the well-known Kohn's theorem is still valid, i.e., the cyclotron resonance is not affected by the electron-electron interaction. This theorem imposes a constraint on the approximation scheme employed in the treatment of electron-electron interaction. In the present study, the Green's function is calculated in the self-consistent Hartree-Fock approximation. The electron density correlation function is calculated by solving a Bethe-Salpeter equation (BSE) in the ladder diagram approximation. It is found that, the excitation frequency near the cyclotron resonance frequency approaches a value that is lower than the cyclotron resonance frequency at small wave vectors, when two effective masses are different. When two effective masses are the same, the excitation frequency approaches the cyclotron resonance frequency at small wave vectors as required. Our findings suggest that the approximation scheme used in this theoretical investigation fails to satisfy the requirement due to the Kohn's theorem, and one should go beyond this approximation scheme.
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Materials Science

Target-Distribution-Guided Cross-Functional Fine-Tuning of Machine-Learning Interatomic Potentials

Yuki Nagai, Bo Thomsen and Motoyuki Shiga

Cross-functional fine-tuning of machine-learning interatomic potentials (MLIPs) is often treated as a relabeling problem, where configurations generated at one density-functional level are relabeled using a higher-fidelity target functional. However, the resulting training data may be drawn from the wrong equilibrium distribution, because the statistical weights of configurations change across exchange–correlation functionals. Here we address this distribution mismatch using a target-distribution-guided workflow based on self-learning hybrid Monte Carlo (SLHMC), in which trial configurations are proposed by a machine-learning potential and accepted or rejected using target-functional density-functional-theory energies. Using rutile TiO$_2$ as a test system, we fine-tune the MACE-MP-0 foundation potential toward PBE, r$^2$SCAN, and HSE06 target functionals. The resulting adapted potentials reproduce target-anchored nearest-neighbor Ti–O distributions, radial distribution functions, and the NPT cell metrics examined here more accurately than the foundation-model and off-target relabeling controls considered in this work. In particular, HSE06-guided fine-tuning improves structural and thermodynamic properties that are difficult to access with direct hybrid-functional molecular dynamics because of the computational cost of exact exchange. These results indicate that target-distribution coverage is an essential component of cross-functional MLIP transfer, and that accurate target-level labels alone may be insufficient when the configurational distribution is mismatched.
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Materials Science

Processing-Controlled Structural Uniformity and Oxide-Ion Conduction in Na0.52Bi0.47TiO3 Ceramics Probed by Eu3+ Photoluminescence

Zhouyang He, Xiaoou Sun, Xinyue Wang et al. (+1 more)

Sodium bismuth titanate (NBT) is a promising oxide-ion conductor,but its electrical conductivity is highly sensitive to small changes in A-site stoichiometry and processing history.This sensitivity can reduce sample-to-sample reproducibility.Here we examine how precursor mixing controls structural uniformity and ionic transport in Na0.52Bi0.47TiO3 ceramics.Dry grinding,wet grinding with ethanol,and ball milling were compared by X-ray diffraction,electron microscopy,energy-dispersive spectroscopy,Eu3+ photoluminescence excitation spectroscopy,and electrochemical impedance spectroscopy.All processed powders and ceramics form the perovskite NBT phase within the detection limit of XRD.However,the microstructure,surface A-site cation ratio,Eu3+ excitation spectra,and electrical response change strongly with the mixing route.Continuous monitoring of Eu3+ excitation spectra at different emission wavelengths reveals different distributions of local Eu3+ environments.Larger spectral-shape variations are consistent with lower structural uniformity and stronger local distortion.Dry-ground samples show higher bulk conductivity than wet-ground samples,whereas wet-ground samples show much lower grain-boundary resistance.At 600 \u2103,the dry-60 min sample reaches a bulk conductivity of 13.54 mS cm-1,while wet-30 min shows the highest grain-boundary conductivity of 13.72 mS cm-1.These results suggest a processing-driven trade-off between bulk defect generation and grain-boundary blocking.Based on this processing understanding,Ca was introduced at the A site in Na0.52Bi0.47-xCaxTiO3.The x=0.04 sample reaches 8.35 mS cm-1 at 500 \u2103 and 18.98 mS cm-1 at 600 \u2103.
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All authors Zhouyang He, Xiaoou Sun, Xinyue Wang, Duanting Yan

Materials Science

Efficient cooling by ferroelectric or ferromagnetic hysteresis loops

M. Apostol

An efficient cooling effect is put forward, by means of external electric or magnetic fields along hysteresis loops. A simplified model of hysteresis is used for numerical illustration. The model is based upon a second-order expansion of the energy in powers of polarization and external field. The electrocaloric effect along hysteresis loops is discussed for comparison.
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Materials Science

Edge Dislocation Mediated Anomalous Charge Transfer in Face Centered Cubic High Entropy Alloys

Gautam Anand, Swarnava Ghosh, Suman Chabri et al. (+1 more)

Charge transfer in concentrated alloys governs their structural stability and functional response, and can be strongly perturbed by lattice defects. In high-entropy alloys, the interaction between edge dislocations and volume misfit plays a central role in solid-solution strengthening models; however, the influence of dislocations on the local charge transfer has not been explicitly investigated. In this work, large-scale ab initio calculations are employed to examine the dislocation-mediated charge transfer in CoNi, CoCrNi and CoCrFeMnNi alloys. The calculations reveal an anomalous charge redistribution near edge dislocation cores, including deviation from the conventional electronegativity trend. The observed behavior is shown to originate from collective electronegativity equalisation effects rather than simple pairwise atomic interactions. Furthermore, the asymmetric atomic volume response within the compressive and tensile regions of the dislocation field is rationalised in terms of anomalous magneto-volume fluctuations. These results establish a direct coupling between dislocation-induced electronic redistribution and local volumetric response in chemically complex alloys. The demonstrated coupling between dislocation-mediated charge transfer and atomic volume fluctuations provides a pathway toward electronically informed solid-solutions strengthening models and defect-aware alloy design strategy for chemically complex alloys. These findings further suggest that local electronic redistribution near dislocation-cores can play a critical role in governing the deformation behavior and defect-enbergetics in high-entropy alloys.
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All authors Gautam Anand, Swarnava Ghosh, Suman Chabri, Markus Eisenbach

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

Machine-learned atomistic simulations reveal the basis of hydrogen-induced crack-plane transition in alpha-Fe

Jiaqin Xu, Zhiqiang Zhao, Kazuma Ito et al. (+4 more)

Hydrogen-related fracture in body-centered cubic Fe and ferritic steels often appears as transgranular quasi-cleavage rather than purely intergranular failure, especially at low to moderate hydrogen contents. Fractography has suggested that hydrogen may change the dominant cleavage faceting from {100} toward {110}, but atomic-scale evidence for this possible crack-plane transition remains unclear. Here we construct an efficient neural-network potential for {\alpha}-Fe/H and combine large-scale, three-dimensional molecular dynamics with grand-canonical Monte Carlo (GCMC), allowing the near-tip crack-surface region and crack tip within a defined GCMC domain to exchange hydrogen with a reservoir at fixed chemical potential. A comparison of four crack systems identifies the controlling response: (100)[010], (100)[011], and (110)[001] remain cleavage-dominated, whereas the (110)[1-10] crack changes from dislocation emission in pure Fe to cleavage under hydrogen charging. The energetic origin is twofold. Hydrogen lowers the Griffith cleavage threshold of the {110} cleavage-plane family more strongly than that of {100}, and, for the controlling crack, a Rice-type energetic descriptor indicates that the surface-energy-controlled cleavage resistance decreases faster than the unstable-stacking-fault-controlled emission resistance, consistent with a weakened dislocation-emission shield. These results provide a thermodynamically consistent atomistic basis for a hydrogen-induced transgranular crack-plane transition in Fe.
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All authors Jiaqin Xu, Zhiqiang Zhao, Kazuma Ito, Shuhei Shinzato, Fanshun Meng, Shihao Zhang, Shigenobu Ogata