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- ItemBerry phase and band structure analysis of the Weyl semimetal NbP(London : Nature Publishing Group, 2016) Sergelius, Philip; Gooth, Johannes; Bäßler, Svenja; Zierold, Robert; Wiegand, Christoph; Niemann, Anna; Reith, Heiko; Shekhar, Chandra; Felser, Claudia; Yan, Binghai; Nielsch, KorneliusWeyl semimetals are often considered the 3D-analogon of graphene or topological insulators. The evaluation of quantum oscillations in these systems remains challenging because there are often multiple conduction bands. We observe de Haas-van Alphen oscillations with several frequencies in a single crystal of the Weyl semimetal niobium phosphide. For each fundamental crystal axis, we can fit the raw data to a superposition of sinusoidal functions, which enables us to calculate the characteristic parameters of all individual bulk conduction bands using Fourier transform with an analysis of the temperature and magnetic field-dependent oscillation amplitude decay. Our experimental results indicate that the band structure consists of Dirac bands with low cyclotron mass, a non-trivial Berry phase and parabolic bands with a higher effective mass and trivial Berry phase.
- ItemThe influence of the in-plane lattice constant on the superconducting transition temperature of FeSe0.7Te0.3 thin films(New York : American Institute of Physics, 2017) Yuan, Feifei; Iida, Kazumasa; Grinenko, Vadim; Chekhonin, Paul; Pukenas, Aurimas; Skrotzki, Werner; Sakoda, Masahito; Naito, Michio; Sala, Alberto; Putti, Marina; Yamashita, Aichi; Takano, Yoshihiko; Shi, Zhixiang; Nielsch, Kornelius; Hühne, RubenEpitaxial Fe(Se,Te) thin films were prepared by pulsed laser deposition on (La0.18Sr0.82)(Al0.59Ta0.41)O3 (LSAT), CaF2-buffered LSAT and bare CaF2 substrates, which exhibit an almost identical in-plane lattice parameter. The composition of all Fe(Se,Te) films were determined to be FeSe0.7Te0.3 by energy dispersive X-ray spectroscopy, irrespective of the substrate. Albeit the lattice parameters of all templates have comparable values, the in-plane lattice parameter of the FeSe0.7Te0.3 films varies significantly. We found that the superconducting transition temperature (Tc) of FeSe0.7Te0.3 thin films is strongly correlated with their a-axis lattice parameter. The highest Tc of over 19 K was observed for the film on bare CaF2 substrate, which is related to unexpectedly large in-plane compressive strain originating mostly from the thermal expansion mismatch between the FeSe0.7Te0.3 film and the substrate.
- ItemRare-earth-free MnAl-C-Ni permanent magnets produced by extrusion of powder milled from bulk(Amsterdam : Elsevier, 2020) Feng, Le; Freudenberger, Jens; Mix, Torsten; Nielsch, Kornelius; Woodcock, Thomas GeorgeRare-earth-free MnAl-C-Ni permanent magnets have been produced for the first time by extruding powders milled from bulk. The resulting materials, fabricated using different conditions, contained a large volume fraction (> 0.92) of the desired τ-phase. In terms of the maximum energy product, the best performance obtained for a whole, transverse section of the extruded material was (BH)max = 46 kJm−3, and was (BH)max = 49 kJm−3 for a sample taken from near the edge of this section. Analysis showed that this material was comparable to the long-established benchmark, comprising MnAl-C-based magnets extruded in industry from bulk or from gas-atomised powder. Such materials are no longer available. The microstructure of the materials produced here consisted of fine, recrystallised grains, which exhibited an <001> fibre texture with intermediate texture quality and of larger, non-recrystallised regions, which contained hierarchical twinning and a high density of defects. The volume fraction and size of the non-recrystallised regions was greatly reduced by decreasing the size of the initial powder particles. This led to a large increase in the squareness factor of the demagnetisation curve and consequently to the high (BH)max values observed.
- ItemOn Curie temperature of B20-MnSi films([London] : Macmillan Publishers Limited, part of Springer Nature, 2022) Li, Zichao; Yuan, Ye; Begeza, Viktor; Rebohle, Lars; Helm, Manfred; Nielsch, Kornelius; Prucnal, Slawomir; Zhou, ShengqiangB20-type MnSi is the prototype magnetic skyrmion material. Thin films of MnSi show a higher Curie temperature than their bulk counterpart. However, it is not yet clear what mechanism leads to the increase of the Curie temperature. In this work, we grow MnSi films on Si(100) and Si(111) substrates with a broad variation in their structures. By controlling the Mn thickness and annealing parameters, the pure MnSi phase of polycrystalline and textured nature as well as the mixed phase of MnSi and MnSi1.7 are obtained. Surprisingly, all these MnSi films show an increased Curie temperature of up to around 43 K. The Curie temperature is likely independent of the structural parameters within our accessibility including the film thickness above a threshold, strain, cell volume and the mixture with MnSi1.7. However, a pronounced phonon softening is observed for all samples, which can tentatively be attributed to slight Mn excess from stoichiometry, leading to the increased Curie temperature.
- ItemTransition to the quantum hall regime in InAs nanowire cross-junctions(Bristol : IOP Publ., 2019) Gooth, Johannes; Borg, Mattias; Schmid, Heinz; Bologna, Nicolas; Rossell, Marta D.; Wirths, Stephan; Moselund, Kirsten; Nielsch, Kornelius; Riel, HeikeWe present a low-temperature electrical transport study on four-terminal ballistic InAs nanowire cross-junctions in magnetic fields aligned perpendicular to the cross-plane. Two-terminal longitudinal conductance measurements between opposing contact terminals reveal typical 1D conductance quantization at zero magnetic field. As the magnetic field is applied, the 1D bands evolve into hybrid magneto-electric sub-levels that eventually transform into Landau levels for the widest nanowire devices investigated (width = 100 nm). Hall measurements in a four-terminal configuration on these devices show plateaus in the transverse Hall resistance at high magnetic fields that scale with (ve 2 /h) -1 . e is the elementary charge, h denotes Planck's constant and v is an integer that coincides with the Landau level index determined from the longitudinal conductance measurements. While the 1D conductance quantization in zero magnetic field is fragile against disorder at the NW surface, the plateaus in the Hall resistance at high fields remain robust as expected for a topologically protected Quantum Hall phase. © 2019 IOP Publishing Ltd.
- ItemDensity-Dependence of Surface Transport in Tellurium-Enriched Nanograined Bulk Bi2Te3(Weinheim : Wiley-VCH, 2023) Izadi, Sepideh; Bhattacharya, Ahana; Salloum, Sarah; Han, Jeong Woo; Schnatmann, Lauritz; Wolff, Ulrike; Perez, Nicolas; Bendt, Georg; Ennen, Inga; Hütten, Andreas; Nielsch, Kornelius; Schulz, Stephan; Mittendorff, Martin; Schierning, GabiThree-dimensional topological insulators (3D TI) exhibit conventional parabolic bulk bands and protected Dirac surface states. A thorough investigation of the different transport channels provided by the bulk and surface carriers using macroscopic samples may provide a path toward accessing superior surface transport properties. Bi2Te3 materials make promising 3D TI models; however, due to their complicated defect chemistry, these materials have a high number of charge carriers in the bulk that dominate the transport, even as nanograined structures. To partially control the bulk charge carrier density, herein the synthesis of Te-enriched Bi2Te3 nanoparticles is reported. The resulting nanoparticles are compacted into nanograined pellets of varying porosity to tailor the surface-to-volume ratio, thereby emphasizing the surface transport channels. The nanograined pellets are characterized by a combination of resistivity, Hall- and magneto-conductance measurements together with (THz) time-domain reflectivity measurements. Using the Hikami-Larkin-Nagaoka (HLN) model, a characteristic coherence length of ≈200 nm is reported that is considerably larger than the diameter of the nanograins. The different contributions from the bulk and surface carriers are disentangled by THz spectroscopy, thus emphasizing the dominant role of the surface carriers. The results strongly suggest that the surface transport carriers have overcome the hindrance imposed by nanoparticle boundaries.
- ItemThe Role of Al2O3 ALD Coating on Sn-Based Intermetallic Anodes for Rate Capability and Long-Term Cycling in Lithium-Ion Batteries(Weinheim : Wiley-VCH, 2022) Soltani, Niloofar; Abbas, Syed Muhammad; Hantusch, Martin; Lehmann, Sebastian; Nielsch, Kornelius; Bahrami, Amin; Mikhailova, DariaThe electrochemical performances of CoSn2 and Ni3Sn4 as potential anode materials in lithium-ion batteries (LIBs) are investigated using varying thicknesses of an alumina layer deposited by the atomic layer deposition (ALD) technique. Rate capability results showed that at high current densities, Al2O3-coated CoSn2 and Ni3Sn4 electrodes after 10-ALD cycles outperformed uncoated materials. The charge capacities of coated CoSn2 and Ni3Sn4 electrodes are 571 and 134 mAh g−1, respectively, at a high current density of 5 A g−1, while the capacities of uncoated electrodes are 363 and 11 mAh g−1. When the current density is reduced to 1 A g−1, however, the cycling performances of Al2O3-coated CoSn2 and Ni3Sn4 electrodes fade faster after almost 40 cycles than uncoated electrodes. The explanation is found in the composition of the solid-electrolyte interface (SEI), which strongly depends on the current rate. Thus, X-ray photoelectron spectroscopy analysis of SEI layers on coated samples cycles at a low current density of 0.1 Ag−1, revealed organic carbonates as major products, which probably have a low ionic conductivity. In contrast, the SEI of coated materials cycled at 5 Ag−1 consists mostly of mixed inorganic/organic fluorine-rich Al-F and C-F species facilitating a higher ionic transport, which improves electrochemical performance.
- ItemSelf‐Patterning of Multifunctional Heusler Membranes by Dewetting(Weinheim : Wiley-VCH, 2021) Lünser, Klara; Diestel, Anett; Nielsch, Kornelius; Fähler, SebastianNi-Mn-based Heusler alloys are an emerging class of materials which enable actuation by (magnetic) shape memory effects, magnetocaloric cooling, and thermomagnetic energy harvesting. Multifunctional materials have a particular advantage for miniaturization since their functionality is already built within the material. However, often complex microtechnological processing is required to bring these materials into shape. Here, self-organized formation of single crystalline membranes having arrays of rectangular holes with high aspect ratio is demonstrated. Dewetting avoids the need for complicated processing and allows to prepare freestanding Ni–Mn–Ga–Co membranes. These membranes are martensitic and magnetic, and their functional properties are not disturbed by self-patterning. Feature sizes of these membranes can be tailored by film thickness and heat treatment, and the tendencies can be explained with dewetting. As an outlook, the advantages of these multifunctional membranes for magnetocaloric and thermomagnetic microsystems are sketched. © 2021 The Authors. Advanced Materials Interfaces published by Wiley-VCH GmbH
- ItemBuilding Hierarchical Martensite(Weinheim : Wiley-VCH, 2020) Schwabe, Stefan; Niemann, Robert; Backen, Anja; Wolf, Daniel; Damm, Christine; Walter, Tina; Seiner, Hanuš; Heczko, Oleg; Nielsch, Kornelius; Fähler, SebastianMartensitic materials show a complex, hierarchical microstructure containing structural domains separated by various types of twin boundaries. Several concepts exist to describe this microstructure on each length scale, however, there is no comprehensive approach bridging the whole range from the nano- up to the macroscopic scale. Here, it is described for a Ni-Mn-based Heusler alloy how this hierarchical microstructure is built from scratch with just one key parameter: the tetragonal distortion of the basic building block at the atomic level. Based on this initial block, five successive levels of nested building blocks are introduced. At each level, a larger building block is formed by twinning the preceding one to minimize the relevant energy contributions locally. This naturally explains the coexistence of different types of twin boundaries. The scale-bridging approach of nested building blocks is compared with experiments in real and reciprocal space. The approach of nested building blocks is versatile as it can be applied to the broad class of functional materials exhibiting diffusionless transformations. © 2020 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH
- ItemThermoelectric Characterization Platform for Electrochemically Deposited Materials(Weinheim : Wiley-VCH Verlag GmbH & Co. KG, 2020) Barati, Vida; Garcia Fernandez, Javier; Geishendorf, Kevin; Schnatmann, Lauritz Ule; Lammel, Michaela; Kunzmann, Alexander; Pérez, Nicolás; Li, Guodong; Schierning, Gabi; Nielsch, Kornelius; Reith, HeikoSuccessful optimization of the thermoelectric (TE) performance of materials, described by the figure of merit zT, is a key enabler for its application in energy harvesting or Peltier cooling devices. While the zT value of bulk materials is accessible by a variety of commercial measurement setups, precise determination of the zT value for thin and thick films remains a great challenge. This is particularly relevant for films synthesized by electrochemical deposition, where the TE material is deposited onto an electrically conductive seed layer causing an in-plane short circuit. Therefore, a platform for full in-plane zT characterization of electrochemically deposited TE materials is developed, eliminating the impact of the electrically conducting seed layer. The characterization is done using a suspended TE material within a transport device which was prepared by photolithography in combination with chemical etching steps. An analytical model to determine the thermal conductivity is developed and the results verified using finite element simulations. Taken together, the full in-plane zT characterization provides an inevitable milestone for material optimization under realistic conditions in TE devices. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim