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- ItemDetermination of tip transfer function for quantitative MFM using frequency domain filtering and least squares method(London : Nature Publishing Group, 2019) Nečas, D.; Klapetek, P.; Neu, V.; Havlíček, M.; Puttock, R.; Kazakova, O.; Hu, X.; Zajíčková, L.Magnetic force microscopy has unsurpassed capabilities in analysis of nanoscale and microscale magnetic samples and devices. Similar to other Scanning Probe Microscopy techniques, quantitative analysis remains a challenge. Despite large theoretical and practical progress in this area, present methods are seldom used due to their complexity and lack of systematic understanding of related uncertainties and recommended best practice. Use of the Tip Transfer Function (TTF) is a key concept in making Magnetic Force Microscopy measurements quantitative. We present a numerical study of several aspects of TTF reconstruction using multilayer samples with perpendicular magnetisation. We address the choice of numerical approach, impact of non-periodicity and windowing, suitable conventions for data normalisation and units, criteria for choice of regularisation parameter and experimental effects observed in real measurements. We present a simple regularisation parameter selection method based on TTF width and verify this approach via numerical experiments. Examples of TTF estimation are shown on both 2D and 3D experimental datasets. We give recommendations on best practices for robust TTF estimation, including the choice of windowing function, measurement strategy and dealing with experimental error sources. A method for synthetic MFM data generation, suitable for large scale numerical experiments is also presented.
- ItemAlberta wildfire 2016: Apt contribution from anomalous planetary wave dynamics([London] : Macmillan Publishers Limited, part of Springer Nature, 2018) Petoukhov, Vladimir; Petri, Stefan; Kornhuber, Kai; Thonicke, Kirsten; Coumou, Dim; Schellnhuber, Hans JoachimIn May-June 2016 the Canadian Province of Alberta suffered one of the most devastating wildfires in its history. Here we show that in mid-April to early May 2016 the large-scale circulation in the mid- and high troposphere of the middle and sub-polar latitudes of the northern hemisphere featured a persistent high-amplitude planetary wave structure dominated by the non-dimensional zonal wave number 4. The strongest anticyclonic wing of this structure was located over western Canada. In combination with a very strong El Niño event in winter 2015/2016 this favored highly anomalous, tinder-dry and high-temperature conditions at the surface in that area, entailing an increased fire hazard there. This critically contributed to the ignition of the Alberta Wildfire in May 2016, appearing to be the costliest disaster in Canadian history thus far.
- ItemMesoscale Dzyaloshinskii-Moriya interaction: Geometrical tailoring of the magnetochirality(London : Nature Publishing Group, 2018) Volkov, O.M.; Sheka, D.D.; Gaididei, Y.; Kravchuk, V.P.; Rößler, U.K.; Fassbender, J.; Makarov, D.Crystals with broken inversion symmetry can host fundamentally appealing and technologically relevant periodical or localized chiral magnetic textures. The type of the texture as well as its magnetochiral properties are determined by the intrinsic Dzyaloshinskii-Moriya interaction (DMI), which is a material property and can hardly be changed. Here we put forth a method to create new artificial chiral nanoscale objects with tunable magnetochiral properties from standard magnetic materials by using geometrical manipulations. We introduce a mesoscale Dzyaloshinskii-Moriya interaction that combines the intrinsic spin-orbit and extrinsic curvature-driven DMI terms and depends both on the material and geometrical parameters. The vector of the mesoscale DMI determines magnetochiral properties of any curved magnetic system with broken inversion symmetry. The strength and orientation of this vector can be changed by properly choosing the geometry. For a specific example of nanosized magnetic helix, the same material system with different geometrical parameters can acquire one of three zero-temperature magnetic phases, namely, phase with a quasitangential magnetization state, phase with a periodical state and one intermediate phase with a periodical domain wall state. Our approach paves the way towards the realization of a new class of nanoscale spintronic and spinorbitronic devices with the geometrically tunable magnetochirality.
- ItemImpact of the precursor chemistry and process conditions on the cell-to-cell variability in 1T-1R based HfO2 RRAM devices(London : Nature Publishing Group, 2018) Grossi, A.; Perez, E.; Zambelli, C.; Olivo, P.; Miranda, E.; Roelofs, R.; Woodruff, J.; Raisanen, P.; Li, W.; Givens, M.; Costina, I.; Schubert, M.A.; Wenger, C.The Resistive RAM (RRAM) technology is currently in a level of maturity that calls for its integration into CMOS compatible memory arrays. This CMOS integration requires a perfect understanding of the cells performance and reliability in relation to the deposition processes used for their manufacturing. In this paper, the impact of the precursor chemistries and process conditions on the performance of HfO2 based memristive cells is studied. An extensive characterization of HfO2 based 1T1R cells, a comparison of the cell-to-cell variability, and reliability study is performed. The cells’ behaviors during forming, set, and reset operations are monitored in order to relate their features to conductive filament properties and process-induced variability of the switching parameters. The modeling of the high resistance state (HRS) is performed by applying the Quantum-Point Contact model to assess the link between the deposition condition and the precursor chemistry with the resulting physical cells characteristics.
- ItemTime-reversal symmetry breaking type-II Weyl state in YbMnBi2(London : Nature Publishing Group, 2019) Borisenko, S.; Evtushinsky, D.; Gibson, Q.; Yaresko, A.; Koepernik, K.; Kim, T.; Ali, M.; van den Brink, J.; Hoesch, M.; Fedorov, A.; Haubold, E.; Kushnirenko, Y.; Soldatov, I.; Schäfer, R.; Cava, R.J.Spectroscopic detection of Dirac and Weyl fermions in real materials is vital for both, promising applications and fundamental bridge between high-energy and condensed-matter physics. While the presence of Dirac and noncentrosymmetric Weyl fermions is well established in many materials, the magnetic Weyl semimetals still escape direct experimental detection. In order to find a time-reversal symmetry breaking Weyl state we design two materials and present here experimental and theoretical evidence of realization of such a state in one of them, YbMnBi2. We model the time-reversal symmetry breaking observed by magnetization and magneto-optical microscopy measurements by canted antiferromagnetism and find a number of Weyl points. Using angle-resolved photoemission, we directly observe two pairs of Weyl points connected by the Fermi arcs. Our results not only provide a fundamental link between the two areas of physics, but also demonstrate the practical way to design novel materials with exotic properties.
- ItemDiscovery of TaFeSb-based half-Heuslers with high thermoelectric performance(London : Nature Publishing Group, 2019) Zhu, H.; Mao, J.; Li, Y.; Sun, J.; Wang, Y.; Zhu, Q.; Li, G.; Song, Q.; Zhou, J.; Fu, Y.; He, R.; Tong, T.; Liu, Z.; Ren, W.; You, L.; Wang, Z.; Luo, J.; Sotnikov, A.; Bao, J.; Nielsch, K.; Chen, G.; Singh, D.J.; Ren, Z.Discovery of thermoelectric materials has long been realized by the Edisonian trial and error approach. However, recent progress in theoretical calculations, including the ability to predict structures of unknown phases along with their thermodynamic stability and functional properties, has enabled the so-called inverse design approach. Compared to the traditional materials discovery, the inverse design approach has the potential to substantially reduce the experimental efforts needed to identify promising compounds with target functionalities. By adopting this approach, here we have discovered several unreported half-Heusler compounds. Among them, the p-type TaFeSb-based half-Heusler demonstrates a record high ZT of ~1.52 at 973 K. Additionally, an ultrahigh average ZT of ~0.93 between 300 and 973 K is achieved. Such an extraordinary thermoelectric performance is further verified by the heat-to-electricity conversion efficiency measurement and a high efficiency of ~11.4% is obtained. Our work demonstrates that the TaFeSb-based half-Heuslers are highly promising for thermoelectric power generation.
- ItemMagnetic origami creates high performance micro devices(London : Nature Publishing Group, 2019) Gabler, F.; Karnaushenko, D.D.; Karnaushenko, D.; Schmidt, O.G.Self-assembly of two-dimensional patterned nanomembranes into three-dimensional micro-architectures has been considered a powerful approach for parallel and scalable manufacturing of the next generation of micro-electronic devices. However, the formation pathway towards the final geometry into which two-dimensional nanomembranes can transform depends on many available degrees of freedom and is plagued by structural inaccuracies. Especially for high-aspect-ratio nanomembranes, the potential energy landscape gives way to a manifold of complex pathways towards misassembly. Therefore, the self-assembly yield and device quality remain low and cannot compete with state-of-the art technologies. Here we present an alternative approach for the assembly of high-aspect-ratio nanomembranes into microelectronic devices with unprecedented control by remotely programming their assembly behavior under the influence of external magnetic fields. This form of magnetic Origami creates micro energy storage devices with excellent performance and high yield unleashing the full potential of magnetic field assisted assembly for on-chip manufacturing processes.
- ItemDivalent EuRh 2 Si 2 as a reference for the Luttinger theorem and antiferromagnetism in trivalent heavy-fermion YbRh 2 Si 2(London : Nature Publishing Group, 2019) Güttler, M.; Generalov, A.; Fujimori, S.I.; Kummer, K.; Chikina, A.; Seiro, S.; Danzenbächer, S.; Koroteev, Y.M.; Chulkov, E.V.; Radovic, M.; Shi, M.; Plumb, N.C.; Laubschat, C.; Allen, J.W.; Krellner, C.; Geibel, C.; Vyalikh, D.V.Application of the Luttinger theorem to the Kondo lattice YbRh 2 Si 2 suggests that its large 4f-derived Fermi surface (FS) in the paramagnetic (PM) regime should be similar in shape and volume to that of the divalent local-moment antiferromagnet (AFM) EuRh 2 Si 2 in its PM regime. Here we show by angle-resolved photoemission spectroscopy that paramagnetic EuRh 2 Si 2 has a large FS essentially similar to the one seen in YbRh 2 Si 2 down to 1 K. In EuRh 2 Si 2 the onset of AFM order below 24.5 K induces an extensive fragmentation of the FS due to Brillouin zone folding, intersection and resulting hybridization of the Fermi-surface sheets. Our results on EuRh 2 Si 2 indicate that the formation of the AFM state in YbRh 2 Si 2 is very likely also connected with similar changes in the FS, which have to be taken into account in the controversial analysis and discussion of anomalies observed at the quantum critical point in this system.
- ItemEnantiomer-selective magnetization of conglomerates for quantitative chiral separation(Berlin : Springer Nature, 2019) Ye, X.; Cui, J.; Li, B.; Li, N.; Wang, R.; Yan, Z.; Tan, J.; Zhang, J.; Wan, X.Selective crystallization represents one of the most economical and convenient methods to provide large-scale optically pure chiral compounds. Although significant development has been achieved since Pasteur’s separation of sodium ammonium tartrate in 1848, this method is still fundamentally low efficient (low transformation ratio or high labor). Herein, we describe an enantiomer-selective-magnetization strategy for quantitatively separating the crystals of conglomerates by using a kind of magnetic nano-splitters. These nano-splitters would be selectively wrapped into the S-crystals, leading to the formation of the crystals with different physical properties from that of R-crystals. As a result of efficient separation under magnetic field, high purity chiral compounds (99.2 ee% for R-crystals, 95.0 ee% for S-crystals) can be obtained in a simple one-step crystallization process with a high separation yield (95.1%). Moreover, the nano-splitters show expandability and excellent recyclability. We foresee their great potential in developing chiral separation methods used on different scales. © 2019, The Author(s).
- ItemWhen optimization for governing human-environment tipping elements is neither sustainable nor safe(London : Nature Publishing Group, 2018) Barfuss, W.; Donges, J.F.; Lade, S.J.; Kurths, J.Optimizing economic welfare in environmental governance has been criticized for delivering short-term gains at the expense of long-term environmental degradation. Different from economic optimization, the concepts of sustainability and the more recent safe operating space have been used to derive policies in environmental governance. However, a formal comparison between these three policy paradigms is still missing, leaving policy makers uncertain which paradigm to apply. Here, we develop a better understanding of their interrelationships, using a stylized model of human-environment tipping elements. We find that no paradigm guarantees fulfilling requirements imposed by another paradigm and derive simple heuristics for the conditions under which these trade-offs occur. We show that the absence of such a master paradigm is of special relevance for governing real-world tipping systems such as climate, fisheries, and farming, which may reside in a parameter regime where economic optimization is neither sustainable nor safe.