Browsing by Author "Schnatmann, Lauritz"
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- 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.
- ItemInterface-Dominated Topological Transport in Nanograined Bulk Bi2 Te3(Weinheim : Wiley-VCH, 2021) Izadi, Sepideh; Han, Jeong Woo; Salloum, Sarah; Wolff, Ulrike; Schnatmann, Lauritz; Asaithambi, Aswin; Matschy, Sebastian; Schlörb, Heike; Reith, Heiko; Perez, Nicolas; Nielsch, Kornelius; Schulz, Stephan; Mittendorff, Martin; Schierning, Gabi3D topological insulators (TI) host surface carriers with extremely high mobility. However, their transport properties are typically dominated by bulk carriers that outnumber the surface carriers by orders of magnitude. A strategy is herein presented to overcome the problem of bulk carrier domination by using 3D TI nanoparticles, which are compacted by hot pressing to macroscopic nanograined bulk samples. Bi2Te3 nanoparticles well known for their excellent thermoelectric and 3D TI properties serve as the model system. As key enabler for this approach, a specific synthesis is applied that creates nanoparticles with a low level of impurities and surface contamination. The compacted nanograined bulk contains a high number of interfaces and grain boundaries. Here it is shown that these samples exhibit metallic-like electrical transport properties and a distinct weak antilocalization. A downward trend in the electrical resistivity at temperatures below 5 K is attributed to an increase in the coherence length by applying the Hikami–Larkin–Nagaoka model. THz time-domain spectroscopy reveals a dominance of the surface transport at low frequencies with a mobility of above 103 cm2 V−1 s−1 even at room temperature. These findings clearly demonstrate that nanograined bulk Bi2Te3 features surface carrier properties that are of importance for technical applications.
- ItemSignatures of a Charge Density Wave Phase and the Chiral Anomaly in the Fermionic Material Cobalt Monosilicide CoSi(Weinheim : Wiley-VCH Verlag GmbH & Co. KG, 2020) Schnatmann, Lauritz; Geishendorf, Kevin; Lammel, Michaela; Damm, Christine; Novikov, Sergey; Thomas, Andy; Burkov, Alexander; Reith, Heiko; Nielsch, Kornelius; Schierning, GabiMaterials with topological electronic states have emerged as one of the most exciting discoveries of condensed quantum matter, hosting quasiparticles with extremely low effective mass and high mobility. Weyl materials contain such topological states in the bulk and additionally have a non-trivial chiral charge. However, despite known quantum effects caused by these chiral states, the interplay between chiral states, and a charge density wave phase, an ordering of the electrons to a correlated phase is not experimentally explored. Indications for the formation of a charge density wave phase in the Weyl material cobalt monosilicide CoSi are observed. Furthermore, the typical signatures of the charge density wave phase together with typical signatures of Weyl fermions in magnetic field dependent electrical transport characterization are investigated. The charge density wave and the chiral contribution to the electrical magneto-transport are separated as well as a suppression of the charge density wave phase is observed in magnetic fields. © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.