Spin-resolved STS, ARPES, and XMCD of magnetically doped topological insulator heterostructures

• Applicants: Professor Dr. Philip Hofmann; Dr. Jens Wiebe
• Subject Area: Experimental Condensed Matter Physics
• Term: from 2013 to 2017
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 237526155

• Final Report Year: 2020

Final Report Abstract

Summarizing, during the course of the project, we found fascinating phenomena evoked by the complex competition between spin order and superconducting correlations of Fe-chalcogenide thin films, and their interplay with the spin-related properties of the Bi-based topological insulator substrates they have been grown on. These phenomena have been studied systematically as a function of film thickness and composition of the Fe-chalcogenide and TI materials. • We found s-wave superconductivity in unit cell thin films of FeTe0.5 Se0.5 grown on the TI and, thereby, realized an ideal platform for the future study of topological superconductivity and the associated Majorana bound states. We have found first indications for the latter in the center of magnetic vortices at the surface of the bulk material. • Surprisingly, the thin-film form of the parent compound of the Fe-chalcogenide superconductor family grown on the Bi-based TI reveals a local spatial coexistence of spin order and superconducting correlations. We demonstrated that the spin order has the same structure as in the bulk, but is re-oriented at the surface of the material with respect to the bulk, which probably also has its effect on the formation of superconducting correlations. • For the vertical heterostructures of TIs grown on magnetically bulk-doped TIs we found a Mn-induced gap at the Dirac point of the topological surface state, which is gradually filled for thicker TI layers. • We also realized a new MBE-facility in Hamburg which enables future experiments on MBE grown chalcogenide heterostructures and their in-situ SP-STS investigation without the need of the Se capping-decapping procedure. The results have been published in 13 manuscripts, partly in high-level journals including two in Nature Communications and one in Physical Review Letters.

Publications

• “Nickel: The time-reversal symmetry conserving partner of iron on a chalcogenide topological insulator”, Phys. Rev. B 94, 161114(R) (2016)
  M. Vondráček, L. Cornils, J. Minar, J. Warmuth, M. Michiardi, C. Piamonteze, L. Barreto, J. A. Miwa, M. Bianchi, Ph. Hofmann, L. Zhou, A. Kamlapure, A. A. Khajetoorians, R. Wiesendanger, J.-L. Mi, B.-B. Iversen, S. Mankovsky, St. Borek, H. Ebert, M. Schüler, T. Wehling, J. Wiebe, and J. Honolka
  
• “Topological insulator homojunctions including magnetic layers: The example of n-p type (n-QLs Bi2 Se3 /Mn-Bi2 Se3 ) heterostructures”, Appl. Phys. Lett. 108, 262402 (2016)
  M. Vališka, J. Warmuth, M. Michiardi, M. Vondraček, A. S. Ngankeu, V. Holý , V. Sechovský , G. Springholz, M. Bianchi, J. Wiebe, Ph. Hofmann, and J. Honolka
  
• “Interfacial superconductivity in a bi-collinear antiferromagnetically ordered FeTe monolayer on a topological insulator”, Nature Communications 8, 14074 (2017)
  S. Manna, A. Kamlapure, L. Cornils, T. Hanke, E. M. J. Hedegaard, M. Bremholm, B. B. Iversen, Ph. Hofmann, J. Wiebe, and R. Wiesendanger
  
• “Reorientation of the diagonal double-stripe spin structure at Fe1+y Te bulk and thin-film surfaces”, Nature Communications 8, 13939 (2017)
  T. Hänke, U. R. Singh, L. Cornils, S. Manna, A. Kamlapure, M. Bremholm, E. M. J. Hedegaard, B. B. Iversen, Ph. Hofmann, J. Hu, Z. Mao, J. Wiebe, and R. Wiesendanger
  
• “Spatial variation of the two-fold anisotropic superconducting gap in a monolayer of FeSe0.5 Te0.5 on a topological insulator”, Phys. Rev. B 95, 104509 (2017)
  A. Kamlapure, S. Manna, L. Cornils, T. Hänke, M. Bremholm, Ph. Hofmann, J. Wiebe, and R. Wiesendanger
  
• “Structural and electronic properties of ultrathin FeSe films grown on Bi2 Se3 (0001) studied by STM/STS”, J. Phys.: Condens. Matter 29, 025004 (2017)
  U. R. Singh, J. Warmuth, V. Markmann, J. Wiebe, and R. Wiesendanger
  
• “Domain imaging across the magneto-structural phase transitions in Fe1+y Te”, npj Quantum Materials 3, 21 (2018)
  J. Warmuth, M. Bremholm, Ph. Hofmann, J. Wiebe, and R. Wiesendanger
  
• “Electronic structure of Fe1.08 Te bulk crystals and epitaxial FeTe thin films on Bi2 Te3 ”, J. Phys.: Condens. Matter 95, 065502 (2018)
  F. Arnold, J. Warmuth, M. Michiardi, Jan Fikaček, M. Bianchi, J. Hu, Z. Mao, J. Miwa, U. R. Singh, M. Bremholm, R. Wiesendanger, J. Honolka, T. Wehling, J. Wiebe, and Ph. Hofmann
  
• “Enhanced spin-ordering temperature in ultrathin FeTe films grown on a topological insulator”, Phys. Rev. B 97, 144513 (2018)
  U. R. Singh, J. Warmuth, A. Kamlapure, L. Cornils, M. Bremholm, Ph. Hofmann, J. Wiebe, and R. Wiesendanger
  
• "Influence of an y Anomalous Temperature Dependence of the Phase Coherence Length on the Conductivity of Magnetic Topological Insulators”, Phys. Rev. Lett. 123, 036406 (2019)
  V. Tkáč, K. Výborný, V. Komanický, J. Warmuth, M. Michiardi, A. S. Ngankeu, M. Vondráček, R. Tarasenko, M. Vališka, V. Stetsovych, K. Carva, I. Garate, M. Bianchi, J. Wiebe, V. Holý, Ph. Hofmann, G. Springholz, V. Sechovský, and J. Honolka