Scanning tunneling spectroscopy investigations of superconducting-doped topological insulators
Experimental pitfalls and results
Stefan Wilfert (Julius-Maximilians-Universität Würzburg)
Paolo Sessi (Julius-Maximilians-Universität Würzburg)
Zhiwei Wang (University of Cologne)
Henrik Schmidt (Julius-Maximilians-Universität Würzburg)
M. Carmen Martínez-Velarte (TU Delft - QN/Otte Lab)
Seng Huat Lee (Missouri University of Science and Technology, The Pennsylvania State University)
Yew San Hor (Missouri University of Science and Technology)
Alexander F. Otte (TU Delft - QN/Otte Lab)
Yoichi Ando (University of Cologne)
Weida Wu (Rutgers University, Julius-Maximilians-Universität Würzburg)
Matthias Bode (Julius-Maximilians-Universität Würzburg)
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Abstract
Recently, the doping of topological insulators has attracted significant interest as a potential route towards topological superconductivity. Because many experimental techniques lack sufficient surface sensitivity, however, definite proof of the coexistence of topological surface states and surface superconductivity is still outstanding. Here we report on highly surface sensitive scanning tunneling microscopy and spectroscopy experiments performed on Tl-doped Bi2Te3, a three-dimensional topological insulator which becomes superconducting in the bulk at TC=2.3 K. Landau level spectroscopy as well as quasiparticle interference mapping clearly demonstrated the presence of a topological surface state with a Dirac point energy ED=-(118±1) meV and a Dirac velocity vD=(4.7±0.1)×105 m/s. Tunneling spectra often show a superconducting gap, but temperature- and field-dependent measurements show that both TC and μ0HC strongly deviate from the corresponding bulk values. Furthermore, in spite of a critical field value which clearly points to type-II superconductivity, no Abrikosov lattice could be observed. Experiments performed on normal-metallic Ag(111) prove that the gapped spectrum is caused only by superconducting tips, probably caused by a gentle crash with the sample surface during approach. Nearly identical results were found for the intrinsically n-type compound Nb-doped Bi2Se3. Our results suggest that the superconductivity in superconducting-doped V-VI topological insulators does not extend to the surface where the topological surface state is located.