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Physics Condensed Matter Materials Science

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Tuesday 13 October · still arriving

Materials Science

Hydrogen partial pressure as a key parameter in the reactive sputtering of photochromic YHO thin films

E. Strods, M. Zubkins, E. Letko et al. (+3 more)

Oxygen-containing yttrium hydride (YHO) thin films were synthesised by reactive magnetron sputtering over a wide range of hydrogen-to-argon gas flow ratios (H2/Ar) during yttrium hydride deposition, followed by post-oxidation in air. The deposition process was monitored in situ by plasma optical emission spectroscopy. X-ray diffraction reveals a strong dependence of crystallographic texture on the H2/Ar ratio: highly <100>-textured films formed at low H2/Ar ratios progressively evolve into more polycrystalline microstructures with slightly reduced crystallite size as the hydrogen content increases. The optical transmittance gradually decreases from approximately 85 % to 70 % as the H2/Ar ratio is raised to 0.6, followed by a sharp drop at higher hydrogen contents. The photochromic contrast of 705-845 nm thick YHO films, measured after 10 min of illumination with a solar simulator, decreases from 14 % to 7 % as the H2/Ar ratio increases from 0.08 to 0.45. Thinner films (365-505 nm) exhibit a similar but less pronounced trend. Bleaching kinetics, analysed using a double-exponential model, indicate faster recovery, with effective bleaching time constants of approximately 1-15 min for films deposited at H2/Ar < 0.25. X-ray photoelectron spectroscopy reveals higher oxygen incorporation in films deposited at lower H2/Ar ratios, deviating from the commonly reported correlation between oxygen content and photochromic response in YHO. These results demonstrate that low hydrogen partial pressure during reactive sputtering is advantageous for achieving higher optical transparency, enhanced photochromic contrast, and faster bleaching kinetics in YHO thin films.
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All authors E. Strods, M. Zubkins, E. Letko, V. Vibornijs, A. Sarakovskis, J. Purans

Materials Science

Knowledge-Guided Construction of Machine Learning Potentials for Reactive Behavior in Energetic Materials

Guanchen Dong, Huanrong Liu, Lin Wang et al. (+12 more)

Energetic materials involve multiple interactions, making reliable machine-learning potentials (MLPs) difficult to construct without extensive datasets. Here, we enhance the ability of the neural network interatomic potential (NNAP) to describe molecular systems using sphmulti, a refined spherical-harmonic basis set that combines extra radial basis with element-specific chemical weighting in each radial channel. We further integrate hyperactive learning, farthest-point sampling, and iterative sufficiency criteria with knowledge-guided key events, enabling the NNAP to retain essential information for describing the material system. For 3,3'-diamino-4,4'-azoxyfurazan, the resulting NNAP is trained on only 13,680 configurations yet captures key properties, including the isomerization barrier, pyrolysis activation energy, and shock Hugoniot behavior. NNAP-driven molecular dynamics simulations further reveal that pyrolysis proceeds through furazan-ring opening followed by chain reactions, whereas shock loading induces anisotropic hydrogen transfer driven by interlayer structural deformations. This work enables efficient and reliable MLPs for energetic materials.
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All authors Guanchen Dong, Huanrong Liu, Lin Wang, Yuxuan Chen, Zhengyu Tu, Qing-an Li, Bin Xu, Jian Li, Shan Zhang, Xiaotong Li, Linghua Tan, Guangcheng Yang, Jun Kang, Rui Su, Pengfei Guan

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

Direct view of exciton and carrier dynamics in twisted bilayer graphene

Gianmarco Gatti, Anders S. Mortensen, Zhihao Jiang et al. (+12 more)

Twisted bilayer graphene (tBLG) on H-intercalated SiC (twist angle of $9 \pm 1^{\circ}$) is studied by position\mbox{-}, time\mbox{-}, and angle-resolved photoemission spectroscopy (ARPES). Both graphene layers are hole doped, with a Dirac-point offset of 30~meV. Time-resolved ARPES near the van Hove singularity reveals a spectral feature within the hybridization-induced band gap, together with excited electrons in the conduction band. The in-gap feature exhibits distinct dynamics, and its binding energy and $k$-space location are consistent with the proposed non-hydrogenic “ghost-Fano” exciton arising from destructive interference between two nearly degenerate excitonic channels. Near the Dirac points, the twist separates the largely layer-polarized Dirac cones in momentum space, providing direct layer selectivity. We find markedly different peak electronic temperatures in the two graphene layers, even though their subsequent cooling dynamics are very similar. We attribute this to an ultrafast interlayer redistribution of electronic energy, driven by an interlayer electric field that arises from the different doping in the layers.
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All authors Gianmarco Gatti, Anders S. Mortensen, Zhihao Jiang, Alfred J. H. Jones, Gesa-R. Siemann, Thomas S. Nielsen, Søren Ulstrup, Naina Kushwaha, Bruce Weaver, Yu Zhang, Emma Springate, Charlotte E. Sanders, Vibha Reddy, Ulrich Starke, Philip Hofmann

Materials Science

Field-free Magnetoacoustic Nonreciprocity via Exchange-bias Engineering

Weiqi Du, Mingyuan Ma, Zheng Sun et al. (+8 more)

Nonreciprocal wave propagation is essential for directional signal processing and isolation. Magnetoacoustic systems are attractive platforms for nonreciprocal devices because of their small footprint and passive operation. However, pronounced nonreciprocity typically requires an external bias field,limiting device miniaturization and integration. Here, we demonstrate field-free magnetoacoustic nonreciprocity in an IrMn/FeGaB/Cu/FeGaB multilayer integrated with a shear-horizontal-wave delay line. Exchange-bias engineering stabilizes a remanent magnetic state with broken time-reversal and spatial-inversion symmetries while tailoring the spin-wave dispersion to achieve resonant magnetoacoustic coupling at zero field. The normalized isolation reaches 21 dB/mm, with its sign programmable by the field-sweep history. Further numerical optimization predicts a zero-field normalized isolation of 91 dB/mm by increasing the dynamical magnetic asymmetry. Our work establishes exchange-bias engineering as a promising route toward compact RF isolators and circulators and as a general strategy for realizing magnetoacoustic devices without external magnetic fields.
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All authors Weiqi Du, Mingyuan Ma, Zheng Sun, Chong Chen, Shixuan Liang, Deqi Tao, Shihai An, Yifan Fu, Sulei Fu, Feng Pan, Cheng Song

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

All-electron machine-learned orbital-free density functional for periodic CHNO systems

Marc K. Ickler, Andreas Hermann and Fred A. Hamprecht

We introduce STRUCTURES-P, a machine-learned all-electron orbital-free density functional for periodic CHNO systems. Electrostatics are evaluated analytically in an atom-centered Gaussian basis, while an equivariant network learns the kinetic and exchange-correlation energy from energies and gradients of non-ground-state densities. Minimizing the functional from a superposition of atomic densities recovers the Kohn-Sham ground state: on held-out crystals, the density error falls from 6.7% to 0.4% of the electron count and the energy error reaches 0.25 mHa per atom, half that of a same-architecture interatomic potential. A single functional can be trained for both isolated molecules and periodic systems, and the model generalizes to larger unit cells. It largely preserves relative-energy rankings and reproduces convex-hull vertices on the tested H-N composition slice. Together, these results demonstrate the viability of machine-learned all-electron orbital-free DFT for variational density optimization and energy evaluation in fixed-geometry solids.
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Materials Science

Thermal stability and structural ordering of thick Co$_2$MnAl films grown on MgO and Si substrates

David Robles-Cuenca, Juan Antonio González, Juan Pedro Andrés et al. (+3 more)

While most studies on Co$2$MnAl (CMA) focus on the ultrathin regime for spintronic junctions, applications scaling with magnetic volume–such as Anomalous Nernst Effect (ANE) energy harvesters, integrated power inductors, and magnetic MEMS–require significantly thicker films. In this work, 750 nm thick CMA films were grown on MgO (001) and Si substrates to establish a roadmap for their thermal integration. Structural analysis reveals that B2 ordering is optimized on MgO at 600 $^\circ$C, which is important for functional devices. In contrast, samples on Si substrates exhibit a disorder transition toward the A2 phase, closely linked to diffusion-driven degradation processes. Above 500 $^\circ$C, Si interdiffusion into the Heusler layer triggers the formation of interfacial silicides and Kirkendall voids, leading to microcracking, electrical resistivity jumps, and a final fully disordered A2 phase. Despite the Mn-poor stoichiometry (Co$2.25$Mn$0.7$Al$1.05$), the films maintain a robust saturation magnetization of ~600 emu/cm$^3$, consistent with recent theoretical models for off-stoichiometric Heusler alloys. These results define the processing windows for integrating thick CMA layers into multifunctional CMOS-compatible platforms.
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All authors David Robles-Cuenca, Juan Antonio González, Juan Pedro Andrés, Miguel Ángel Arranz, Mohamed Salaheldeen, Ricardo López Antón

Materials Science · Cross-listed from physics.optics

On-chip optical isolators

Abanoub Mikhail, Nikolay Solodovchenko, Sen Xu et al. (+5 more)

Optical isolation suppresses back-reflections that destabilize lasers, produce parasitic interference, and increase crosstalk in photonic systems. Although free-space and fibre-optic isolators are mature, no broadly adopted on-chip technology yet combines strong isolation, low insertion loss, broad bandwidth, compact footprint, and scalable fabrication. The difficulty is twofold: Lorentz reciprocity must be broken, and every relevant backward spatial, spectral, and polarization channel must be suppressed. This Review develops a common framework for assessing proposed solutions. We first distinguish pairwise nonreciprocity from intensity-asymmetric transmission and from true isolation using the constitutive relations and scattering matrix. We then compare three physical routes to an integrated nonreciprocal response: magneto-optical bias, externally driven space-time modulation, and nonlinear optical processes. Topological and non-Hermitian systems are treated as cross-cutting design frameworks that can route, protect, or enhance an underlying nonreciprocal response, but do not by themselves break reciprocity. Finally, we examine fundamental constraints and practical trade-offs among isolation, insertion loss, bandwidth, footprint, modal coverage, drive requirements, and fabrication compatibility. The resulting mechanism-based perspective connects reciprocity-breaking physics to the requirements of scalable photonic integrated circuits.
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All authors Abanoub Mikhail, Nikolay Solodovchenko, Sen Xu, Fan Zhang, Maksim Tumashov, Mingzhao Song, Sergei Makarov, Andrey Bogdanov

Materials Science

ARES: a plane-wave density-functional-theory code in Rust

Zehan Liu, Qian Wang, Xiangyu Zheng et al. (+7 more)

ARES (Ab initio atomic mateRial modEling Software at JLU) is a plane-wave density-functional-theory (DFT) code written entirely in Rust as a layered 22-crate workspace. It supports norm-conserving pseudopotentials and the projector augmented-wave (PAW) method with the local-density approximation (LDA) and Perdew-Burke-Ernzerhof (PBE) functionals, {\Gamma}-only and explicit k-point sampling, smearing and mixing, forces, stress, relaxation, band structures, densities of states (DOS) and dispersion corrections; execution is serial or MPI-distributed over k-point pools, band groups and plane-wave rows. The code's distinguishing engineering choice is its validation apparatus. Every numerical component is checked against Quantum ESPRESSO driven from the same physical and numerical inputs, and the comparison is organized as numbered fixtures kept in the repository whose reference data is regenerated by a script instead of being captured once. On six materials spanning a semiconductor, three metals and two two-dimensional systems, band energies within one electronvolt of the Fermi level agree with the reference to a median below 0.4 meV. The pseudopotentials the code reads are validated against all-electron reference data as well: the median delta factor is 0.696 meV/atom over all 65 paired elements. Restrictions are stated with the capabilities: two functionals, no spin-polarized or spin-orbit path, a stress implementation with an explicit scope, and no accelerator execution.
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All authors Zehan Liu, Qian Wang, Xiangyu Zheng, Jiaxiang Li, Chuntian Wu, Yongshuo Chen, Xuecheng Shao, Qiang Xu, Yu Xie, Yanming Ma

Materials Science

Colossal Spin–Lattice–Charge Coupling in Noncollinear Ferrimagnetic NbMnX$_2$ ($X$ = S, Se, Te) Monolayers

Peng Wang, Nanshu Liu, Hongkuan Yuan et al. (+2 more)

Noncollinear ferrimagnets combine a net magnetic moment with internal spin degrees of freedom, creating opportunities to couple magnetic, mechanical, thermal, and electronic responses. Using first-principles calculations, symmetry analysis, atomistic spin simulations, and transport modeling, we identify stable NbMnX$_2$ ($X$ = S, Se, Te) monolayers with noncollinear ferrimagnetic ground states and Curie temperatures up to 313 K. Competing exchange, Dzyaloshinskii–Moriya interactions, and magnetic anisotropy produce field-responsive canted states that couple strongly to the lattice and charge carriers. Magnetization rotation changes the elastic modulus of NbMnTe$_2$ by up to 36%, while lattice entropy substantially enhances the near-room-temperature magnetocaloric response of NbMnS$_2$ and NbMnSe$_2$. In NbMnSe$_2$, rotation restores a mirror-protected crossing, continuously closes an approximately 0.13 eV gap, and produces a conductivity-based anisotropic magnetoresistance ratio up to 10,000% at 100 K. The combined results establish noncollinear NbMnX$_2$ monolayers as a multifunctional platform for two-dimensional spintronics, nanomechanics, and thermal control.
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All authors Peng Wang, Nanshu Liu, Hongkuan Yuan, Xue Jiang, Jijun Zhao

Materials Science

Orthoexciton-to-paraexciton conversion rate in Cu$_2$O at sub-Kelvin temperatures

Yusuke Morita, Yukihiro Akiyama, Yuki Haita et al. (+2 more)

We measured the conversion rate from 1s orthoexcitons to 1s paraexcitons in Cu$_2$O below 1 K using a dilution refrigerator. To determine the conversion rate, we performed time-resolved luminescence spectroscopy of 1s orthoexcitons and 1s paraexcitons confined in a strain-induced trap potential. From the time evolution of the ${\Gamma}$$_3$$^{-}$-LO-phonon-assisted luminescence of 1s orthoexcitons, we obtained a conversion rate of $5.9 \times 10^7$ s${^{-1}}$ at a mixing-chamber temperature of 64 mK under an applied stress of 2.2 kbar. Between 64 and 800 mK, the conversion rate depends only weakly on temperature, and it is less than half of the value of $1.6 \times 10^8$ s${^{-1}}$ reported at 2 K under the same applied stress. The conversion rate therefore continues to decrease below 2 K, contrary to the previous expectation that it becomes nearly constant below about 1.7 K. We discuss these results in terms of the momentum distribution of orthoexcitons, which does not reach equilibrium with the lattice during the conversion at these low temperatures. These findings call for a theory that accounts for both the small magnitude of the conversion rate and its weak temperature dependence below 1 K.
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All authors Yusuke Morita, Yukihiro Akiyama, Yuki Haita, Yuga Hirota, Kosuke Yoshioka

Materials Science

Polarons in Lead-Halide Perovskites: What the Structural Evidence Does and Does Not Establish

Inhee Maeng, Young Mi Lee, Jinwoo Park et al. (+2 more)

Photogenerated carriers in lead-halide perovskites are widely believed to move as large polarons, whose accompanying lattice distortion is thought to screen the carrier from charged defects. Three recent findings call this picture into question. First, time-resolved structural probes now measure the photoinduced lattice distortion directly; yet calculations show that thermal motion of the lattice alone can raise the carrier effective mass-an increase previously taken as evidence for polarons-so that it remains unclear whether the lattice distortion around a carrier is really a polaron. Second, the carrier-lattice coupling strengthens as the material is thinned from 3D to 2D. In the two main families of 2D perovskites, Ruddlesden-Popper and Dion-Jacobson, the carrier couples to the lattice through different channels, and their contrasting behavior is set by which coupling channel dominates rather than by lattice stiffness alone. Third, the first direct test of defect tolerance does not point to polaron screening; its authors attribute defect tolerance to the soft, disordered lattice. New transport data show that polaron screening reduces scattering between carriers, but not that it protects carriers from defects. Polaron formation, even if established, therefore does not establish polarons as the cause of defect tolerance. Separating the two requires measuring the lattice distortion as a function of excitation density, over a range spanning the low densities of a solar cell under sunlight and the much higher densities used in most ultrafast experiments. On this basis, we propose experiments that can clarify how photogenerated carriers behave in lead-halide perovskites.
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All authors Inhee Maeng, Young Mi Lee, Jinwoo Park, Seung Jae Oh, Min-Cherl Jung

Materials Science

Learning from fracture: Teaching language models to design through physical experience

Bo Ni

How can language models learn to optimize physical designs through experience? We introduce FracLearn, an experience-abstraction-generalization framework evaluated through a deliberately unfamiliar, Tetris-inspired fracture game. The model first proposes architectures and examines simulated fracture responses to accumulate experience. Abstraction organizes these observations into conditional principles linking reinforcement placement, crack evolution and load transfer, with supporting evidence and failure conditions. Generalization challenges the model to combine these principles in new architectures and surpass the earlier design record. Campaigns demonstrate progressive refinement and recovery through hypothesis revision, yielding a best composite score 11.0% above the experience-stage record. Fracture analyses connect improved performance to crack deflection and temporary trapping as toughening mechanisms, alongside greater residual load capacity. Learning occurs through external knowledge and decisions, without model-weight updates. This approach offers a route to scientific agents that turn computational experiments into reusable physical knowledge for materials discovery and engineering design.
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Materials Science

Hydrogen-Bond Density Drives the Order-Disorder Phase Transition from Orientational to Vibrational in Polyalcohol Plastic Crystals

Ares Sanuy, Carlos Escorihuela-Sayalero, Pol Lloveras et al. (+5 more)

Neopentane-derived plastic crystals undergo order-disorder phase transitions accompanied by exceptionally large entropy changes, making them attractive solid-state materials for thermal energy storage and barocaloric cooling. Yet it remains unclear whether the microscopic nature of this entropy change is an intrinsic property of each compound or can be understood generally and tuned systematically through molecular design. Here, we use classical molecular dynamics simulations of neopentyl glycol (NPG), pentaglycerine (PG), and pentaerythritol (PE), validated against experimental transition temperatures, enthalpy changes, and phase diagrams, to show that the character of the phase-transition entropy undergoes a complete inversion as the hydrogen-bond connectivity of the molecule increases: orientational disordering dominates in NPG, whereas lattice vibrations dominate in PE and become remarkably robust against pressure. PG occupies a balanced intermediate regime in which the orientational, conformational, and vibrational channels contribute comparably. This inversion reflects two coupled microscopic mechanisms: a growing low-frequency gap between the phonon densities of states of the ordered and disordered phases as the hydrogen-bond network densifies, and an increase in molecular symmetry that progressively suppresses orientational disordering. Because the vibrational channel is substantially less sensitive to pressure than the orientational one, hydrogen-bond density emerges as a practical, chemically accessible design handle for tailoring not only the magnitude but also the pressure robustness of the caloric response in hydrogen-bonded plastic crystals, with direct implications for the search for new barocaloric and thermal-energy-storage materials.
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All authors Ares Sanuy, Carlos Escorihuela-Sayalero, Pol Lloveras, María del Barrio, Hassan Allouchi, Josep-Lluís Tamarit, Luis Carlos Pardo, Claudio Cazorla

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

Observation of stacked vortex structure with number controllable Majorana zero mode in Bi$_{2}$Te$_{3}$/FeTe heterojunction

Xinru Han, Zhaokun Luo, Quanxin Hu et al. (+16 more)

Creation and control of Majorana zero modes (MZMs) in solid devices is the key challenge in realizing fault tolerant topological quantum computation. Here we report spectroscopic evidence for number controllable MZMs in stoichiometric one quintuple layer Bi$_{2}$Te$_{3}$/FeTe heterostructures, together with the observation of stacked vortices in superconducting islands. Low temperature scanning tunnelling microscopy and spectroscopy reveal a long period moire superlattice arising from lattice mismatch, which induces nanoscale modulation of superconducting gaps. Magnetic vortices exhibit a stacked core structure with two layer derived bound state branches: a dispersive branch evolving into an X-shaped pattern away from the vortex center, and a quantum-limit branch characterized by a spatially unsplit MZMs together with discrete integer quantized Caroli-de Gennes-Matricon states. Notably, MZMs are observed in all measured vortices, and their number can be tuned by magnetic field and the size of Bi$_{2}$Te$_{3}$ islands. These results establish stoichiometric 1QL-Bi$_{2}$Te$_{3}$/FeTe as a patterned superconducting interface that enables isolated and few-vortex Majorana configurations, providing a controlled platform for fuse MZMs.
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All authors Xinru Han, Zhaokun Luo, Quanxin Hu, Xingjian Zhu, Xiaohong Pan, Dan Mu, Xinyu Liang, Zulin Xu, Qiheng Wang, Ryan L. Kong, Mingyuan Li, Zhengliang Zhang, Fazhi Yang, Mojun Pan, Xin Liu, Qiangqiang Gu, Baiqing Lv, Gan Wang, Hong Ding

Materials Science

A Parameter-Free Reference Hamiltonian for Tight-Binding Vanadium Oxides

Jun-qiang Lu

Density-functional tight binding has yet to yield a transferable description of vanadium oxides; for the standard second-order construction and the tested cells, we demonstrate that two of its approximations deviate by electron volts and offset each other in the band-index centroid separation between the V 3d and O 2p bands: the neutral two-centre reference omits electron-volt environmental terms, whereas the Mulliken charge model assigns the compact V 3d shell less than half of the self-consistent potential shift observed in PBE. Instead, we build a tight-binding model where the reference Hamiltonian derives from superposed neutral atoms, the full PBE exchange-correlation potential and frozen-core orthogonality, without adjustable terms, and where the orbital-resolved charge response employs three screening factors obtained from PBE linear response and an unscreened Coulomb range. Relative to the primary PAW reference, the plane-wave Harris-Foulkes energy differences of VO2 are matched by the reference energy at fixed geometry and the stated numerical settings, with signed deviations of +2.6 meV/atom for M1 relative to rutile (an energy difference of 92 meV/atom) and +6.2 and -5.2 meV/atom under 3% volume compression and expansion; the charge response reproduces 14 of 22 fitted and 5 of 8 held-out matched PBE responses within one tolerance, and 28 of the 30 within twice the tolerance, and in rutile the fitted first-radial V 3d shift is -4.39 eV, 0.47 eV more negative than PBE's -3.92 eV, whereas the tested DFTB2 charge model gives -1.53 eV. The convexity of the spinless model at fixed geometry prevents it from spontaneously differentiating symmetry-equivalent sites under the symmetries respected by its implemented operators and sampling, so within this model such differentiation in the Magnéli phases must arise from electron-lattice coupling or an explicit spin or correlation term.
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Materials Science

A grand potential for molten-salt electrochemical phase diagrams from first principles

Lorenzo Bastonero, Lucio Colombi Ciacchi and Nicola Marzari

The clean-energy transition is driving unprecedented demand for metals, whose extraction is among the largest industrial sources of carbon emissions. Molten-salt electrolysis offers a path toward carbon-free production of aluminum, iron, titanium, and critical metals. However, corrosive operating conditions and complex, multicomponent feedstocks make experimental development costly and challenging. Here, we introduce a first-principles framework for constructing electrochemical phase diagrams of multicomponent systems to accelerate electrode and electrolyte design and guide the selection of compatible materials and operating conditions for electrochemical cells. We derive a grand potential that generates these diagrams at scale, and supply free energies using first-principles data obtained through a fine-tuned foundation machine-learned interatomic potential. For silicon in molten CaCl$_2$, the construction not only recovers the experimental diagram when using the first-principles framework on a limited set of structures, but also predicts further domains when extended to a larger set of crystal structures. Finally, we demonstrate the predictive power of the approach by screening ten alkali and alkaline-earth halide electrolytes for aluminum electrowinning. Three thermodynamic criteria identify NaF, where cryolite emerges as the stable aluminum-bearing phase, recovering the chemistry that Hall and Héroult established empirically in 1886. The framework provides a first-principles basis for selecting compatible electrodes, electrolytes, and operating conditions toward low-carbon metal production from complex source materials.
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Materials Science · Cross-listed from astro-ph.EP

Elastic Properties of Titan Tholins Revealed by Brillouin Spectroscopy

Mark A. Johnson, Jose R. Montes-Bojorquez, Adis Husic et al. (+7 more)

Titan tholins, complex organic solids synthesized by simulating photochemical processes in Titan's atmosphere, are widely regarded as analogs to organic surface materials on the Saturnian moon. Their mechanical properties are critical for understanding the evolution of Titan's geomorphology and crustal layering. Here, we report the first direct experimental determination of the complete set of aggregate elastic moduli of tholins using Brillouin spectroscopy. Measurements conducted on tholin powder samples yielded a bulk modulus of 11 (2) GPa and a shear modulus of 3.9 (5) GPa, from which we derive a Young's modulus of 11 (1) GPa and a Poisson's ratio of 0.34 (3). Thin-film samples synthesized under slightly different conditions yield similar results. Compared with other Titan-relevant organic compounds, tholins exhibit the highest elastic stiffness measured to date, exceeding even H2O ice in both bulk and shear moduli. These results support the interpretation that complex organics contribute significantly to the mechanical stability and transport behavior of Titan's equatorial dune sediments. Measurements following prolonged atmospheric exposure reveal persistent stiffening of tholins that remains after vacuum treatment, likely associated with minor chemical modification and physical reorganization, highlighting the importance of controlled sample handling for future laboratory investigations. This study also demonstrates the effectiveness of Brillouin spectroscopy as a non-destructive technique for characterizing planetary organic materials and provides new constraints relevant to future seismic and geophysical investigations by the Dragonfly mission.
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All authors Mark A. Johnson, Jose R. Montes-Bojorquez, Adis Husic, Xinting Yu, Wenhao Su, Jia-Sen Hu, Ryan Blase, Edward Patrick, Alan Whittington, Jin S. Zhang

Materials Science

Electric Edelstein Effect in Two Dimensions

Xiaoran Lv, Tianqi Zhang, Chaoxi Cui et al. (+6 more)

We propose the electric Edelstein effect (EEE) in two-dimensional (2D) systems, where crossed out-of-plane and in-plane electric fields generate a spin polarization. The gate field converts the Edelstein response from a material-fixed spin polarization into a gate-tunable one. A symmetry analysis over all magnetic layer point groups shows that the EEE is allowed in a wide range of magnetic and nonmagnetic systems, including ferromagnets, antiferromagnets, and altermagnets. The EEE plays two distinct roles: it generates spin polarization where the linear Edelstein effect (EE) is symmetry-forbidden, as in centrosymmetric materials; and it adds a new gate-tunable component, generally non-collinear with the conventional one, where the EE is already present. The two fields then act as independent knobs and raise the control dimension of the spin polarization from one to two. Our microscopic theory traces the EEE to band-geometric quantities, which are strongly enhanced near small band gaps. First-principles calculations predict pronounced effects in the centrosymmetric ferromagnet MnSe2, where the linear EE is symmetry-forbidden. In the nonmagnet SnSe where the EE and EEE coexist, the two knobs cover an extended region of the spin sphere. Our findings establish the EEE as a symmetry-guided, gate-tunable route to electrical spin manipulation and open avenues for multistate spintronics in 2D quantum materials.
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All authors Xiaoran Lv, Tianqi Zhang, Chaoxi Cui, Yaling Li, Yi Shen, Weikang Wu, Ziming Zhu, Yugui Yao, Zhi-Ming Yu

Monday 12 October

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

Supercell-enabled inverse design of boundary-state dispersion for group-velocity control in pseudomagnetic phononic crystals

Hua-Yang Chen, Nan-Xin Yu, Zhen-Hui Qin et al. (+2 more)

Topological boundary states offer a platform for guided-wave propagation within bulk bandgaps. Their propagation characteristics depend not only on the spectral extent of the states but also on their dispersion within the operating bandwidth. A nearly linear dispersion provides more uniform group velocity across frequency, whereas a reduced slope enables slow-wave propagation. Despite advances in inverse design of prescribed dispersion relations and topological dispersion tuning, application-oriented characteristics of boundary-state dispersion have rarely been direct inverse-design objectives. Here, we develop a supercell-enabled, surrogate-assisted framework for the inverse design of boundary-state dispersion in a phononic crystal with pseudomagnetic fields. Area-preserving anisotropic deformation shifts the local Dirac points, and independent variation of 21 rows within the supercell provides distributed control over the boundary-state branch. A constraint-preserving surrogate reconstructs the dispersion from an initial frequency and positive frequency increments, with first- and second-difference losses introduced to capture local slope and curvature. Coupled with a genetic algorithm, the framework targets two prescribed-bandwidth design tasks: highly linear dispersion and slow-wave dispersion. For the former, finite-element verification yields relative group-velocity variations of 0.142% to 1.183% over prescribed bandwidths of 4% to 6%. For slow-wave designs, mean group velocities of 190 to 385 m/s are obtained over bandwidths of 0.2% to 0.8% while satisfying a 5% group-velocity variation limit. These results establish boundary-state dispersion as a direct inverse-design target beyond conventional bandwidth optimization, opening a flexible route to group-velocity control through application-oriented dispersion engineering over prescribed relative bandwidths.
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All authors Hua-Yang Chen, Nan-Xin Yu, Zhen-Hui Qin, Yan-Feng Chen, Si-Yuan Yu

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

Energy levels of screened charged impurity in gapped graphene under magnetic field

Andrii Iurov and V.P. Gusynin

In this paper, we present a comprehensive theoretical study of the electronic states in gapped graphene in the presence of a charge impurity and a magnetic field. We focus on the impurity-induced modification of the Landau levels under the influence of an attractive or repulsive Coulomb potential. Since analytical solutions of the Dirac equation are not available in this case, the study is carried out using the Wentzel-Kramers-Brillouin (WKB) approach. Special attention is paid to the screening of the Coulomb potential in the presence of metallic gates situated at some distances from graphene sheet, as is commonly the case in experiments. Using both analytical and numerical methods, we determine the energy spectrum as a function of the magnetic field strength and impurity charge for screened and unscreened Coulomb potentials. Our results reveal a significant dependence of the energy levels for sample-to-gate distances of less than 50 nm.
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Materials Science

Crystal growth, magnetic, and transport properties of the candidate altermagnet Cr$_{1/4}$NbS$_{2}$

Nashra Pistawala, Martin R. Lees, Geetha Balakrishnan et al. (+1 more)

Altermagnetism is a rapidly emerging paradigm in condensed matter magnetism, yet the number of experimentally confirmed altermagnetic candidates remains remarkably limited. Cr$_{1/4}$NbS$_2$ has long been theoretically proposed as a promising altermagnet, but there are no reports on experimental investigation of single crystals. Here, we report the successful growth of high-quality single crystals of Cr$_{1/4}$NbS$_2$ and present its structural, thermal, magnetic, and transport properties. Single-crystal X-ray diffraction provides detailed crystallographic information and confirms the hexagonal crystal symmetry ($P6_3/mmc$), one of the criteria for altermagnetic behavior. Resistivity and heat capacity measurements unambiguously identify an antiferromagnetic Néel temperature of $T_N \sim 141$ K. Magnetotransport measurements reveal non-trivial features in the magnetoresistance and Hall response, indicative of quantum diffusive transport phenomena. Cr$_{1/4}$NbS$_2$, synthesized here for the first time in single-crystalline form, therefore serves as an experimental platform for probing the rich spectrum of phenomena predicted for altermagnetic materials.
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All authors Nashra Pistawala, Martin R. Lees, Geetha Balakrishnan, Daniel A. Mayoh

Materials Science

High-Resolution X-ray Spectro-Microscopy of Au Microcrystallites

Chaitali Sow, Martin Seyrich, Johannes Hagemann et al. (+4 more)

Understanding the functionalities of heterogeneous catalysts requires high-resolution nanoscale chemical imaging tools. X-ray spectro-ptychography offers this capability by enabling X-ray absorption near edge structure (XANES) based amplitude and phase contrasts imaging with high spatial resolution. However, such demonstrations at energies above 10 keV, such as the Au L-edges, remain limited due to weak X-ray absorption cross-sections and correspondingly, low signal levels. Here, multimodal spectro-microscopy has been performed on Au microcrystallites that host both conventional face-centered cubic (fcc) and unconventional non-cubic lattices, particularly important for catalysis. Amplitude and phase contrasts across the $\mathrm{L}_3$ edge are resolved on thin Au crystallites (150 nm to 1.71 $\mu\mathrm{m}$) with 45 nm spatial resolution. The resulting XANES spectra exhibit excellent agreement with standard transmission and fluorescence contrasts, while providing complementary chemical insights with improved spatial resolution. Quantitative retrieval of anomalous phase variations ~12 keV further demonstrates the suitability of this method for high-resolution chemical imaging across a wide range of functional materials.
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All authors Chaitali Sow, Martin Seyrich, Johannes Hagemann, Giridhar U. Kulkarni, Thomas F. Keller, Christian G. Schroer, Andreas Schropp

Materials Science

Collision Cascade Morphology and Defect Nucleation in Pure Tungsten and an Equimolar Refractory W-Ta-Cr-V Alloy

Mark Fedorov, Francisco Javier Dominguez-Gutierrez, Damian Kalita et al. (+1 more)

Collision cascades and defect-formation mechanisms in pure W and the WTaCrV refractory high-entropy alloy were investigated using molecular dynamics simulations over primary knock-on atom (PKA) energies of 0.1-100 keV. Machine-learning tabulated Gaussian approximation potentials (tabGAP), complemented by short-range Ziegler-Biersack-Littmark (ZBL) interactions, were employed. Analysis of the heat spike lifetime, affected region, atomic relocations and surviving defects reveals an importance of subcascade formation and presence of light elements in the alloy. Cascade splitting into subcascades contributes to the distribution of the PKA energy over a larger region in the material, nucleating less dislocations and smaller voids - latter being present only in W and absent in WTaCrV. Since cascade splitting is more frequently observed in WTaCrV, the alloy shows better resistance to primary irradiation damage compared to pure W. Apart from cascade splitting, analysis of interstitials shows that Cr and to lesser extent V, being lighter and smaller elements, accommodate the damage in form of point defects, contributing to the mitigation of formation of the dislocation loops.
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All authors Mark Fedorov, Francisco Javier Dominguez-Gutierrez, Damian Kalita, Łukasz Kurpaska

Materials Science

Can Universal Machine-Learning Interatomic Potentials Capture Small Polarons? A Geometry-Based Benchmark in Transition-Metal Oxides

Vladislav Korostelev, Pjotrs Žguns and Konstantin Klyukin

Small polarons, formed when excess electrons become localized through lattice distortions, play a critical role in charge transport, catalysis, and degradation processes in transition-metal (TM) oxides. Whether universal machine-learning interatomic potentials (uMLIPs), which lack explicit electronic degrees of freedom, can describe them remains unclear. Here, we investigate H-induced electron localization across a range of TM oxides using a geometric descriptor based on TM-O bond-length expansion (Z-score) to identify polaronic distortions. We find that uMLIPs exhibit varying degrees of electron localization, from delocalized states to metastable small polarons, with their behavior strongly dependent on the training data and treatment of electronic correlations. In particular, the absence of Hubbard U corrections for specific TM cations in the training data leads to electron delocalization, in contrast to DFT+U predictions. Although some uMLIPs reproduce polaron hopping barriers during coupled proton-polaron transfer, all of them fail to stabilize spatially separated polaron-donor configurations, instead favoring localization near the H dopant.
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Materials Science · Cross-listed from physics.app-ph

Progress in Cavitated Front Gridline Ag Paste for PERC Solar Cell Architecture

Donald Intal, Sandra Huneycutt, Abasifreke Ebong et al. (+2 more)

This paper reports on the progress made with the cavitated silver paste on passivated emitter rear contact cells (PERC) to reduce the series resistance. Thus, electrical and microscopy characterizations are used to examine these resistive losses. Fifteen 252 cm2 PERC cells were fabricated for this assessment. Under standard test conditions, these cells achieved a mean conversion efficiency of 21.87% and a maximum of 22.07%. The mean measured fill factor was 79.98%, compared with a pseudo-fill factor of 83.60%, giving a mean deficit of 3.62 percentage points. Fill factor decreased with increasing area-normalized series resistance, which ranged from 0.53 to 0.83 ohm-cm2, while open-circuit voltage and short-circuit current remained unvaried. These electrical trends support resistive current extraction as the principal contributor to fill-factor variation. Electroluminescence imaging revealed localized emission nonuniformity. Cross-sectional scanning transmission electron microscopy showed variation in the glass interfacial layer, while scanning electron microscopy identified micrometer and sub micrometer silver particles. Conductive atomic force microscopy after sequential removal of the bulk metal and glass revealed spatially heterogeneous current response. These observations are consistent with nonuniform electrical access at the contact, where restricted conducting pathways may concentrate on current and increase local voltage drops. The combined characterization identifies interfacial heterogeneity and the connectivity of conducting pathways as priorities for understanding and reducing extraction losses in cavitated silver metallization.
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All authors Donald Intal, Sandra Huneycutt, Abasifreke Ebong, Dana Hankey, Marshall Tibbetts

Materials Science

Evidence of an anisotropic Griffiths phase in a two-dimensional van der Waals magnet

Anupam K. Singh, Dhananjay Nair, Holger Meyerheim et al. (+2 more)

Over the last decade, the emerging field of two-dimensional van der Waals magnetism has focused mainly on long-range-ordered magnetic phases. Disordered magnetic phases, however, remain largely unexplored. Here, we report a highly anisotropic Griffiths phase driven by self-intercalation in the van der Waals magnet Fe5GeTe2. We find evidence of short-range-ordered ferromagnetic clusters embedded in a paramagnetic matrix, which dominate the magnetic response between the Curie temperature and a higher characteristic Griffiths temperature. An anomalous power-law divergence in the magnetic susceptibility, universal Lee Yang scaling, and distinct entropy anomalies provide further evidence for this phase. Strikingly, the non-analytic Griffiths response emerges exclusively under in-plane magnetic fields, whereas out-of-plane fields abruptly alter this behavior. This directional dependence indicates that the local magnetic interactions within nanoscale clusters are closely coupled to the layered lattice geometry. Our findings establish a regime of dimensionally constrained, disorder-driven magnetism and identify self-intercalated van der Waals magnets as a platform for investigating the interplay between magnetic disorder, anisotropy, and reduced dimensionality.
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All authors Anupam K. Singh, Dhananjay Nair, Holger Meyerheim, Stuart S. P. Parkin, Banabir Pal

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

Phase-Coherent Spin and Lattice Modulations in a Y-type Hexaferrite

Oskar Stepančić, Annika Stellhorn, Emma Campillo et al. (+7 more)

Spin-driven multiferroics owe their electric polarization to lattice distortions that follow the magnetic order, yet in incommensurate magnets symmetry normally forbids such a distortion at the fundamental magnetic wavevector, and the distortions themselves are rarely observed. Here we use half-polarized neutron scattering to show that the Y-type hexaferrite Ba$_{0.4}$Sr$_{1.6}$Mg$_2$Fe$_{12}$O$_{22}$ hosts, in its saturated high-field state, a magnetic modulation and a longitudinal lattice modulation at one and the same wavevector, locked in phase. The signature is a large nuclear-magnetic interference asymmetry, $r$ ~ 0.8, of the same sign at the two conjugate satellites and persisting to 275 K. The zone contrast and the field dependence of the satellite indicate an internal, field-induced longitudinal distortion. The modulated state coexists with the transverse-conical magnetoelectric phase and outlives it, revealing a fundamental-wavevector spin-lattice coupling in a magnetic field regime previously regarded as a simple ferrimagnet.
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All authors Oskar Stepančić, Annika Stellhorn, Emma Campillo, Kun Zhai, Niklas Ekström, A. Corani, D. Alba Venero, R. Cubitt, E. Blackburn, L. Shen

Materials Science

Comprehensive Workflow for Uncertainty-Aware Ionic Liquid Solvent Discovery

Alexander Lobo, Urvi Awasthi and Leonid Zhukov

Designing synthesizable materials with targeted properties remains one of the central objectives in materials sciences. Ionic liquids (ILs) are well suited to this goal due to their "tunability", where physical and chemical properties can be precisely customized by modifying their molecular structure, specifically by varying the combination of cations and anions. At the same time, current data-driven property models for ionic liquids often degrade in scarce, noisy, and uneven data regimes, where measurements cover only a small fraction of chemical space and a limited range of thermodynamic conditions. This makes out-of-distribution screening brittle: even when point predictions look plausible, it is difficult to compare candidates consistently, quantify downstream implications, or trust rankings enough to guide molecular discovery. We present a property-prediction and optimal candidate selection workflow that treats data processing, deep learning modeling, and physics-based calculations as a coupled system, linking property prediction with uncertainty estimation and physics-based filtering. By tightening the link between predicted properties and the criteria used for down-selection, the workflow supports reliable out-of-distribution ranking and enables prioritization of previously untested ionic liquid candidates with favorable predicted trade-offs. We demonstrate the application of our workflow by designing novel ionic liquids for CO2 capture from flue gas, predicting their properties and selecting Pareto-optimal candidates based on CO2 solubility, IL viscosity and working capacity.
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Materials Science

Spin-orbit torque effects in all-electrical spin-wave spectroscopy experiments

José Solano Córdova, Jone Mencos, Richard Schlitz et al. (+10 more)

Electrical excitation and detection of propagating spin waves are key elements of magnonic devices. While conventional antennas consisting of non-magnetic metals such as Au or Cu excite spin waves predominantly through radio-frequency (RF) Oersted fields, antennas comprising Pt can additionally generate spin-orbit torques (SOTs) via the spin Hall effect. Recent theoretical work has proposed RF-SOT as an alternative mechanism for coherent spin-wave excitation, predicting distinct characteristics compared with conventional inductive excitation. Here, we investigate Pt antennas for the excitation of coherent spin waves in 105-nm-thick epitaxial $\text{Y}_3\text{Fe}_5\text{O}_{12}$ (YIG) films through two potential mechanisms: an oscillating Oersted field and an oscillating spin current. The raw experimental transmission spectra are strongly affected by fabrication-induced modifications to the YIG film associated with the etching process together with spatially inhomogeneous interface conditions induced by Pt. After removing these contributions, we find the remaining two wave propagation consistent with a combined excitation from Oersted fields and SOTs.
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All authors José Solano Córdova, Jone Mencos, Richard Schlitz, Quentin Rossi, Denise Reustlen, Javed Ali, Sebastian Sailler, Michaela Lammel, Matthieu Bailleul, Daniel Stoeffler, Felix Casanova, Sebastian T. B. Goennenwein, Helena Reichlova

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

Non-volatile Orbital magnetoelectric effect in a two-dimensional electron gas

Yu Chen, Martando Rath, Julien Brehin et al. (+10 more)

Electrical control of orbital magnetism is a central goal of orbitronics. While current-induced orbital polarization has recently been demonstrated in inversion-asymmetric systems, nonvolatile electrical control of an equilibrium orbital moment has remained elusive. Here, using x-ray absorption spectroscopy with in situ electric-field control of the FE polarization, we demonstrate non-volatile switching of the equilibrium orbital magnetization of the oxide q2DEGs formed at the LaAlO$_3$/EuTiO$_3$/Ca-STO (001) interface. X-ray linear dichroism reveals a reversible modulation, driven by the FE-polarization direction, of the Ti-3$d$ crystal-field splitting and orbital occupancy, establishing a ferroelectric-like metallic state. X-ray magnetic circular dichroism shows a polarization-controlled switching of the Eu$^{2+}$ orbital magnetic moment induced by the orbital magnetization of the q2DEG projected onto Eu-5$d$ states and detected at the Eu-$M_{4,5}$ edge through Eu-5$d$/Eu-4$f$ hybridization in the inversion-symmetry-broken interface. These results provide spectroscopic evidence of ferroelectric control of orbital magnetism in an oxide q2DEG.
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All authors Yu Chen, Martando Rath, Julien Brehin, Cinthia Piamonteze, Daniele Preziosi, Payal Wadhwa, Alessio Filippetti, Yuri Fukaya, Mario Cuoco, Filomena Forte, Daniela Stornaiuolo, Manuel Bibes, Marco Salluzzo