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@INPROCEEDINGS{Karthein:1053065,
      author       = {Karthein, Jan and Buchhorn, Jonas and Toehgiono, Gion and
                      Jalil, Abdur Rehman and Vaßen-Carl, Max and Underwood,
                      Kaycee and Kawano, T. and Otsubo, S. and Ishihara, J. and
                      Kohda, M. and Schüffelgen, Peter and Grützmacher, Detlev
                      and Schäpers, Thomas},
      title        = {{O}ptimization of magnetic topological insulators for
                      superconducting applications},
      reportid     = {FZJ-2026-01405},
      year         = {2025},
      abstract     = {Magnetic topological insulators are predicted to be a
                      promising platform for scalable topological quantum
                      computation [1,2]. In the one-dimensional limit the chiral
                      edge modes on opposite sides of a quantum anomalous Hall
                      system hybridize to form a single helical conducting
                      channel. Proximitizing this channel with an s-wave
                      superconductor leads to the appearance of Majorana zero
                      modes. This work makes an effort to optimize the magnetic
                      topological insulator, composed out of Cr-doped
                      (BixSb1-x)2Te3, for these superconducting applications. One
                      approach is to modify the magnetic properties of the films
                      via controlling the parameters during molecular beam epitaxy
                      growth. Van der Pauw measurements are performed on magnetic
                      topological insulator thin films with different material
                      compositions. This approach allows for a fast optimization
                      of the material and a systematic study of the interplay
                      between growth parameters and magneto-transport signatures.
                      We show how the material can be made charge neutral either
                      by varying the substrate temperature during growth or, more
                      directly, by varying the chromium concentration of the thin
                      films in small amounts to compensate for the residual
                      charges in the underlying (BixSb1-x)2Te3. Furthermore, by
                      lowering the chromium concentration to values much smaller
                      than those typically used for quantum anomalous Hall systems
                      [3] the material can be tuned into a parameter space that,
                      according to theoretical proposals, is beneficial for
                      maximizing the induced superconducting gap [4]. The
                      magnetotransport measurements on these weakly-doped thin
                      films indicate that even at very low chromium concentrations
                      of around $2\%,$ the anomalous Hall resistance can reach up
                      to $60\%$ of the quantized value at a temperature of 1.2 K.
                      This suggests that it may be possible to enter the quantum
                      anomalous Hall regime even with low intrinsic
                      magnetizations. In addition to the material optimization,
                      novel ways to fabricate nanometer-sized magnetic topological
                      insulator/superconductor hybrid devices based on shadow
                      walls and stencil masks are being explored. These techniques
                      avoid lithography and etching steps that are otherwise known
                      to be harmful for the material. Based on this, Hall bars
                      from Cr-doped (BixSb1-x)2Te3 are fabricated in the
                      micrometer and nanometer size ranges and their electric and
                      magnetic properties are investigated. No significant
                      difference was found between the micrometer- and
                      nanometer-sized Hall bars, indicating that the fabrication
                      technique does not change the material composition. This
                      hypothesis is supported by transmission electron microscopy
                      and energy-dispersive X-ray spectroscopy
                      measurements.References[1] C.-Z. Chen, et al., Physical
                      Review B 97, 104504 (2018). [2] X.-L. Qi, et al., Physical
                      Review B 82, 184516 (2010). [3] C.-Z. Chang, et al.,
                      Advanced materials 25, 1065 (2013).[4] D. Burke, et al.,
                      Phys. Rev. B 109, 045138 (2024).},
      month         = {Oct},
      date          = {2025-10-21},
      organization  = {ASPIRE Workshop on Quantum and
                       Topological Materials, Sendai (Japan),
                       21 Oct 2025 - 22 Oct 2025},
      subtyp        = {Invited},
      cin          = {PGI-9},
      cid          = {I:(DE-Juel1)PGI-9-20110106},
      pnm          = {5222 - Exploratory Qubits (POF4-522) / DFG project
                      G:(GEPRIS)491798118 - Magnetische topologische Isolatoren
                      für robuste Majorana Zustände (491798118) / DFG project
                      G:(GEPRIS)390534769 - EXC 2004: Materie und Licht für
                      Quanteninformation (ML4Q) (390534769)},
      pid          = {G:(DE-HGF)POF4-5222 / G:(GEPRIS)491798118 /
                      G:(GEPRIS)390534769},
      typ          = {PUB:(DE-HGF)6},
      url          = {https://juser.fz-juelich.de/record/1053065},
}