Tailoring the spin-orbit interaction at surfaces and interfaces

• Antragsteller: Dr. Christian Reinhard Ast
• Fachliche Zuordnung: Experimentelle Physik der kondensierten Materie
• Förderung: Förderung von 2007 bis 2014
• Projektkennung: Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 43127736

• Erstellungsjahr: 2014

Zusammenfassung der Projektergebnisse

The main research theme of the Emmy-Noether-Group is the study of low dimensional electron systems with a particular focus on the effects of the spin-orbit interaction at surfaces and interfaces. More specifically, we have traced the origin of the giant Rashba-type spin-splitting in the two-dimensional electronic structure of the (Bi,Pb,Sb)/Ag(111) and (Bi,Sb)/Cu(111) surface alloys to a significant contribution from the surface corrugation, i. e. a structural influence. Further, we have shown that the alloy atoms Bi, Pb, Sb can be mixed to arbitrary concentrations to form a mixed surface alloy allowing the spin-splitting as well as the Fermi level to be tuned within a certain range of parameters. Transferring the concept of the giant spin-splitting onto a semiconducting substrate we were able to demonstrate that a substantial Rashba-type spin-splitting can be induced by a monolayer of Bi trimers on a Si(111) substrate. Concerning the recently discovered topological insulators (TIs), we have found that water vapor on the TI Bi2 Se3 induces a reactive chemical doping with the formation of quantum well states with a Rashba-type spin-splitting that coexist with the topologically non-trivial surface state. Exposing the Bi2 Se3 crystal surface to air as well as to residual gas in ultra high vacuum we could show that the water vapor is at least in part responsible for the “aging” effect previously discussed in the literature. Another project has been concerned with the electronic structure of graphene. Here, we were able to induce an atomic p-type doping in the conical band structure of epitaxial graphene on silicon carbide using Sb, Bi, or Au atoms. While for Sb and Bi atoms a continuous doping effect has been observed, only discrete doping levels with ambipolar character (both n-type and p-type) have been observed for Au. In contrast to the Sb and Bi atoms, we believe the Au atoms to intercalate between the graphene layer and the substrate. In addition, a decoupling of the epitaxial graphene layer has been observed in the p-doping phase of the intercalated Au phase. Using variable light polarization we investigated the interference effects of the graphene sublattices in photoemission spectra extending previous models and accessed the initial state wave function coefficients in graphene near the Dirac point. Concerning instrument innovation, we were able to finish a long-term project of constructing a scanning tunneling microscope operating at 15 mK and in high magnetic fields up to 14 T. The applicant (group leader), who is the project leader since 2005, has continued this project during the Emmy-Noether-Group. Using a superconducting Al-tip on a Cu(111) sample we could show from the characteristic superconducting spectrum that the effective spectroscopic resolution is 11.4 µeV (corresponding effective temperature: 37 mK).

Projektbezogene Publikationen (Auswahl)

• Atomic Hole Doping of Graphene, Nano Letters 8, 4603 (2008)
  I. Gierz, C. Riedl, U. Starke, C. R. Ast, and K. Kern
  
• Design criteria for scanning tunneling microscopes to reduce the response to external mechnanical disturbances, Rev. Sci. Instr. 79, 093704 (2008)
  C. R. Ast, M. Assig, A. Ast, K. Kern
  
• Spin-orbit split two-dimensional electron gas with tunable Rashba and Fermi energy, Phys. Rev. B 77, 081407(R) (2008)
  C. R. Ast, D. Pacil´é, L. Moreschini, M. C. Falub, M. Papagno, K. Kern, M. Grioni, J. Henk, A. Ernst, S. Ostanin, and P. Bruno
  
• Tuning of spin-gaps at silicon interfaces, Phys. Rev. Lett. 101, 196805 (2008)
  E. Frantzeskakis, S. Pons, H. Mirhosseini, J. Henk, C. R. Ast, and M. Grioni
  
• Silicon surface with giant spin-splitting, Phys. Rev. Lett. 103, 046803 (2009)
  I. Gierz, T. Suzuki, E. Frantzeskakis, S. Pons, S. Ostanin, A. Ernst, J. Henk, M. Grioni, K. Kern, and C. R. Ast
  
• Electronic decoupling of an epitaxial graphene monolayer by gold intercalation, Phys. Rev. B 81, 235408 (2010)
  I. Gierz, T. Suzuki, R. T. Weitz, D. S. Lee, B. Krauss, C. Riedl, U. Starke, H. Höchst, J. H. Smet, C. R. Ast, and K. Kern
  
• Illuminating the dark corridor in graphene: polarization dependence of angle-resolved photoemission spectroscopy on graphene, Phys. Rev. B 83, 121408(R) (2011)
  I. Gierz, J. Henk, H. Höchst, C. R. Ast, and K. Kern
  
• Reactive chemical doping of the Bi2 Se3 topological insulator, Phys. Rev. Lett. 107, 177602 (2011)
  H. M. Benia, C. Lin, K. Kern, and C. R. Ast
  
• A 10 mK Scanning Tunneling Microscope Operating in Ultra High Vacuum and High Magnetic Fields, Rev. Sci. Instr. 84, 033903 (2013)
  M. Assig, M. Etzkorn, A. Enders, W. Stiepany, C. R. Ast, and K. Kern
  
• A Natural Topological Insulator, Nano Lett. 13, 1179 (2013)
  P. Gehring, H. M. Benia, R. Dinnebier, C. R. Ast, M. Burghard, and K. Kern