Abstract
We grew Sr-doped Bi2Se3 thin films using molecular beam epitaxy, and their high quality was verified using transmission electron microscopy. The thin films exhibited weak antilocalisation behaviours in magneto-resistance measurements, a typical transport signature of topological insulators, but were not superconducting. In addition, the carrier densities of the non-superconducting thin-film samples were similar to those of their superconducting bulk counterparts. Atom-by-atom energy-dispersive X-ray mapping also revealed similar Sr doping structures in the bulk and thin-film samples. Because no qualitative distinction between non-superconducting thin-film and superconducting bulk samples had been found, we turned to a quantitative statistical analysis, which uncovered a key structural difference between the bulk and thin-film samples. The separation between Bi layers in the same quintuple layer was compressed whereas that between the closest Bi layers in two neighbouring quintuple layers was expanded in the thin-film samples compared with the separations in pristine bulk Bi2Se3. In marked contrast, the corresponding changes in the bulk doped samples showed opposite trends. These differences may provide insight into the absence of superconductivity in doped topological insulator thin films.
Highlights
Topological insulators (TIs) and superconductors (TScs) are active research fields in condensed matter physics[1]
The answer to this question is necessary to understand the origin of the superconductivity in doped TIs, but will contribute to the successful growth of superconducting doped TI thin films, which are important for the study of exotic Majorana quasiparticles and the related quantum transport phenomena
It was thought that dopants such as Cu atoms intercalated into van der Waals (vdW) gaps acted as electron donors and that the superconductivity was generated merely through electron doping
Summary
Topological insulators (TIs) and superconductors (TScs) are active research fields in condensed matter physics[1]. Point contact spectroscopy measurements have revealed a zero-bias conductance peak (ZBCP) in Cu-doped Bi2Se3, suggesting a possible mid-gap state that may be related to the Majorana zero mode[10]. To search for smoking-gun type evidence of Majorana zero modes, a number of detection schemas have been proposed, many of which require the preparation of superconducting films from doped TIs18–22. We report our attempt to grow Sr-doped Bi2Se3 thin films using MBE. To explore the differences between the non-superconducting thin films and superconducting bulk samples, we performed atom-by-atom energy-dispersive X-ray spectroscopy (EDX) mapping. The only difference was the opposite trend of expansion/compression of the separation between Bi layers in the Bi2Se3 lattice for the bulk and thin-film doped samples (compared with that in pristine Bi2Se3), which suggests that the emergence of superconductivity in doped Bi2Se3 is possibly related with doping-induced lattice structural change
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