Superconductivity and vortex structure in Bi2Te3/FeTe0.55Se0.45 heterostructures with varying Bi2Te3 thickness

Using scanning tunneling microscopy, we investigate the superconductivity and vortex properties in topological insulator Bi2Te3 thin films grown on the iron-based superconductor FeTe0.55Se0.45. The proximity-induced superconductivity weakens in the Bi2Te3 film when the thickness of the film increases. Unlike the elongated shape of vortex cores observed in the Bi2Te3 film with 2-quintuple-layer (QL) thickness, the isolated vortex cores exhibit a star shape with six rays in the 1-QL film, and the rays are along the crystalline axes of the film. This is consistent with the sixfold rotational symmetry of the film lattice, and the proximity-induced superconductivity is still topologically trivial in the 1-QL film. At a high magnetic field, when the direction between the two nearest-neighboring vortices deviates from that of any crystalline axes, two cores connect each other by a pair of adjacent rays, forming a new type of electronic structure of vortex cores. On the 3-QL film, the vortex cores elongate along one of the crystalline axes of the Bi2Te3 film, similar to the results obtained on 2-QL films. The elongated vortex cores indicate a twofold symmetry of the superconducting gap induced by topological superconductivity with odd parity. This observation confirms possible topological superconductivity in heterostructures with a thickness of more than 2 QLs. Our results provide rich information for the vortex cores and vortex-bound states on the heterostructures consisting of the topological insulator and the iron-based superconductor.