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) x 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 Bi2Te3. 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.




The experimental work at Universität Würzburg was supported by DFG (through SFB 1170 “ToCoTronics”; project C02). The work at Cologne received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (Grant Agreement No. 741121) and was also supported by DFG (CRC1238 “Control and Dynamics of Quantum Materials,” project A04). Y.S.H. acknowledges support from NSF Grant No. DMR-1255607.

Keywords and Phrases

Superconducting Gap; Superconductivity; Topological Insulators; Topological Phases of Matter; Topological Superconductors

International Standard Serial Number (ISSN)

2469-9950; 2469-9969

Document Type

Article - Journal

Document Version

Final Version

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