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.
S. Wilfert and P. Sessi and Z. Wang and H. Schmidt and M. C. Martínez-Velarte and S. H. Lee and Y. S. Hor and A. F. Otte and Y. Ando and W. Wu and M. Bode, "Scanning Tunneling Spectroscopy Investigations of Superconducting-Doped Topological Insulators: Experimental Pitfalls and Results," Physical Review B, vol. 98, no. 8, American Physical Society (APS), Aug 2018.
The definitive version is available at https://doi.org/10.1103/PhysRevB.98.085133
Keywords and Phrases
Superconducting Gap; Superconductivity; Topological Insulators; Topological Phases of Matter; Topological Superconductors
International Standard Serial Number (ISSN)
Article - Journal
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