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Subject: search.filters.subject.530Subject: search.filters.subject.660Start date: 2025End date: 2025

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    ItemOpen Access
    Ab initio machine-learning unveils strong anharmonicity in non-Arrhenius self-diffusion of tungsten
    (2025) Zhang, Xi; Divinski, Sergiy V.; Grabowski, Blazej
    The knowledge of diffusion mechanisms in materials is crucial for predicting their high-temperature performance and stability, yet accurately capturing the underlying physics like thermal effects remains challenging. In particular, the origin of the experimentally observed non-Arrhenius diffusion behavior has remained elusive, largely due to the lack of effective computational tools. Here we propose an efficient ab initio framework to compute the Gibbs energy of the transition state in vacancy-mediated diffusion including the relevant thermal excitations at the density-functional-theory level. With the aid of a bespoke machine-learning interatomic potential, the temperature-dependent vacancy formation and migration Gibbs energies of the prototype system body-centered cubic (BCC) tungsten are shown to be strongly affected by anharmonicity. This finding explains the physical origin of the experimentally observed non-Arrhenius behavior of tungsten self-diffusion. A remarkable agreement between the calculated and experimental temperature-dependent self-diffusivity and, in particular, its curvature is revealed. The proposed computational framework is robust and broadly applicable, as evidenced by first tests for a hexagonal close-packed (HCP) multicomponent high-entropy alloy. The successful applications underscore the attainability of an accurate ab initio diffusion database.
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    ItemOpen Access
    Femtosecond direct laser writing of conductive and electrically witchable PEDOT:PSS optical nanostructures
    (2025) Ludescher, Dominik; Ruchka, Pavel; Siegle, Leander; Huang, Yanzhe; Flad, Philipp; Ubl, Monika; Ludwigs, Sabine; Hentschel, Mario; Giessen, Harald
    Microscale three‐dimensional (3D)‐printing, with its remarkable precision and ability to create complex structures, has transformed a wide range of applications, from micro‐optics and photonics to endoscopy and quantum technologies. In these fields, miniaturization plays a crucial role in unlocking new capabilities. However, despite these advancements, most 3D‐printed optical structures have remained static, lacking dynamic behavior and tunability. In this study, a novel approach is presented that combines direct laser writing with the electrically switchable optical properties of the conductive polymer poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS). This integration facilitates the creation of dynamic structures directly on 3D‐printed objects, marking a significant step toward adaptive optical devices. The fabrication of electrically tunable structures is demonstrated via direct laser writing using PEDOT:PSS on indium tin oxide (ITO)‐coated glass substrates, as well as beneath and atop static 3D‐printed structures. It is found that electrical conductivity as well as the greyscale behavior of PEDOT:PSS remains intact after direct laser writing. The switching speed, durability, and gradual tunability of the material are explored upon complementary metal‐oxide‐semiconductor (CMOS)‐compatible voltages ranging from -3 to +2 V. In the future, this advancement opens exciting possibilities in adaptive micro‐optics, such as switchable apertures printed directly onto micro‐optical lenses.
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    ItemOpen Access
    Floating zone growth of large tetragonal Ruddlesden-Popper bilayer nickelate YySr3-yNi2-xAlxO7-δ single crystals
    (2025) Yilmaz, Hasan; Sosa-Lizama, Pablo; Knauft, Manuel; Küster, Kathrin; Starke, Ulrich; Isobe, Masahiko; Clemens, Oliver; Aken, Peter A. van ; Suyolcu, Y. E.; Puphal, Pascal
    The discovery of superconductivity under high pressure in Ruddlesden-Popper (RP) type phase bilayer La3Ni22.5+O7 and trilayer La4Ni32.66+O10 has initiated the frontier of nickelate-based superconductors. In this context, RP-type phases within the Sr-Ni-O system offer promising alternatives as they offer unconventional high oxidation states and Sr-T-O comprises the usual RP series. Here, the intrinsic stability of the undoped Sr-Ni-O framework is investigated using density functional theory (DFT). While Sr3Ni2O7 (SNO) is stable synthesis so far requires Al co-substition in Sr3Ni2-xAlxO7-δ (SNAO). Y-doping resulting in YySr3-yNi2-xAlxO7-δ (YSNAO) effectively mitigates the challenge posed by an insulating ground state. This modification yields a substantial reduction in resistivity, with the crystals exhibiting semiconducting behavior. To explore phase formation within the narrow compositional window of the Y-Sr-Ni-Al-O system, single crystals were grown using the optical floating zone (OFZ) technique under an oxygen partial pressure of approximately 10 bar. The optimized growth conditions for YSNAO enabled the production of large (6 × 5 x 3 mm3), high-quality crystals suitable for neutron scattering experiments. In the absence of Al, crystal growth yielded the n = 1 RP phase Sr1.66Y0.33NiO4-δ, for which single crystals were obtained. The structural, chemical, electrical, and magnetic properties of both the as-grown and topochemically reduced YSNAO compounds were comprehensively characterized through diffraction, spectroscopy, transport, and magnetization measurements.