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Start date: 2020 × DDC: 530 × Type: article × Publisher: College Park, MD : APS × WGL Institute: IKZ ×

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    Quasi-monocrystalline silicon for low-noise end mirrors in cryogenic gravitational-wave detectors
    (College Park, MD : APS, 2022) Kiessling, Frank M.; Murray, Peter G.; Kinley-Hanlon, Maya; Buchovska, Iryna; Ervik, Torunn K.; Graham, Victoria; Hough, Jim; Johnston, Ross; Pietsch, Mike; Rowan, Sheila; Schnabel, Roman; Tait, Simon C.; Steinlechner, Jessica; Martin, Iain W.
    Mirrors made of silicon have been proposed for use in future cryogenic gravitational-wave detectors, which will be significantly more sensitive than current room-temperature detectors. These mirrors are planned to have diameters of ≈50 cm and a mass of ≈200 kg. While single-crystalline float-zone silicon meets the requirements of low optical absorption and low mechanical loss, the production of this type of material is restricted to sizes much smaller than required. Here we present studies of silicon produced by directional solidification. This material can be grown as quasi-monocrystalline ingots in sizes larger than currently required. We present measurements of a low room-temperature and cryogenic mechanical loss comparable with float-zone silicon. While the optical absorption of our test sample is significantly higher than required, the low mechanical loss motivates research into further absorption reduction in the future. While it is unclear if material pure enough for the transmissive detector input mirrors can be achieved, an absorption level suitable for the highly reflective coated end mirrors seems realistic. Together with the potential to produce samples much larger than ≈50 cm, this material may be of great benefit for realizing silicon-based gravitational-wave detectors.
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    Exploring the intrinsic limit of the charge-carrier-induced increase of the Curie temperature of Lu- and La-doped EuO thin films
    (College Park, MD : APS, 2020) Held, R.; Mairoser, T.; Melville, A.; Mundy, J.A.; Holtz, M.E.; Hodash, D.; Wang, Z.; Heron, J.T.; Dacek, S.T.; Holländer, B.; Muller, D.A.; Schlom, D.G.
    Raising the Curie temperature TC of the highly spin-polarized semiconductor EuO by doping it with rare-earth elements is a strategy to make EuO more technologically relevant to spintronics. The increase of TC with free carrier density n and the surprisingly low dopant activation p, found in Gd-doped EuO thin films [Mairoser et al., Phys. Rev. Lett. 105, 257206 (2010)], raised the important question of whether TC could be considerably enhanced by increasing p. Using a low-temperature growth method for depositing high-quality Lu-doped EuO films we attain high dopant activation (p) values of up to 67%, effectively more than doubling p as compared to adsorption-controlled growth of Lu- and Gd-doped EuO. Relating n, p, and lattice compression of La- and Lu-doped EuO films grown at different temperatures to the TC of these samples allows us to identify several different mechanisms influencing TC and causing an experimental maximum in TC. In addition, scanning transmission electron microscopy in combination with electron energy loss spectroscopy measurements on La-doped EuO indicate that extensive dopant clustering is one, but not the sole reason for dopant deactivation in rare-earth doped EuO films.
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    Inhomogeneous ferromagnetism mimics signatures of the topological Hall effect in SrRuO3 films
    (College Park, MD : APS, 2020) Kim, Gideok; Son, K.; Suyolcu, Y.E.; Miao, L.; Schreiber, N.J.; Nair, H.P.; Putzky, D.; Minola, M.; Christiani, G.; van Aken, P.A.; Shen, K.M.; Schlom, D.G.; Logvenov, G.; Keimer, B.
    Topological transport phenomena in magnetic materials are a major topic of current condensed matter research. One of the most widely studied phenomena is the topological Hall effect (THE), which is generated via spin-orbit interactions between conduction electrons and topological spin textures such as skyrmions. We report a comprehensive set of Hall effect and magnetization measurements on epitaxial films of the prototypical ferromagnetic metal SrRuO3 the magnetic and transport properties of which were systematically modulated by varying the concentration of Ru vacancies. We observe Hall effect anomalies that closely resemble signatures of the THE, but a quantitative analysis demonstrates that they result from inhomogeneities in the ferromagnetic magnetization caused by a nonrandom distribution of Ru vacancies. As such inhomogeneities are difficult to avoid and are rarely characterized independently, our results call into question the identification of topological spin textures in numerous prior transport studies of quantum materials, heterostructures, and devices. Firm conclusions regarding the presence of such textures must meet stringent conditions such as probes that couple directly to the noncollinear magnetization on the atomic scale.