How Does the Interface-Induced Superconductivity Emerge in (Bi,Sb)₂Te₃/FeTe Heterostructures?

Recently, the Dirac-fermion-assisted interfacial superconductivity has been discovered in (Bi,Sb)₂Te₃/FeTe heterostructures. In this work, we used molecular beam epitaxy (MBE) to synthesize a series of one quintuple layer (QL) (Bi,Sb)₂Te₃/FeTe heterostructures with different Bi/Sb ratio and performed low-temperature scanning tunneling microscopy and spectroscopy (STM/S) measurements. On the top surface of 1QL (Bi,Sb)₂Te₃, we observed a U-shaped superconducting gap concurrent with the appearance of a periodic modulation in atomic-resolution STM images. This periodic modulation varies in response to changes in the Bi/Sb ratio. We found that an out-of-plane magnetic field can induce a set of in-gap states on 1QL Bi₂Te₃/FeTe, whereas the superconducting gap on 1QL Sb₂Te₃/FeTe remains unchanged under an out-of-plane magnetic field up to 11T. These different behaviors appear to be linked to the influence of crystal and electronic structures as a consequence of altering the Bi/Sb ratio. Our observation strongly suggests that the appearance of the interfacial superconductivity in (Bi,Sb)₂Te₃/FeTe heterostructures is a result of the formation of the periodic modulation across their interface.