Coexistence of Superconductivity and Ferromagnetism in Topological Insulator Thin Films*

The emergence of quantum phenomena at the interface of two materials has attracted much attention in the condensed matter field, such as interfacial superconductivity and Fu-Kane topological superconductivity (TSC). In this work, we employed molecular beam epitaxy (MBE) to synthesize a series of Cr-doped (Bi,Sb)₂Te₃/FeTe heterostructures with different thicknesses of the Cr-doped (Bi,Sb)₂Te₃ layer. We first demonstrated the ferromagnetic properties of our samples through electrical transport, magnetic force microscopy (MFM), and reflective magnetic circular dichroism (RMCD) measurements. Next, we performed low-temperature scanning tunneling microscopy and spectroscopy (STM/S) measurements and found that a superconducting gap appears on the top surface of 1 quintuple layer (QL) Cr-doped (Bi,Sb)₂Te₃ but is absent on the FeTe surface. This observation directly confirms the emergence of the interface-induced superconductivity in Cr-doped (Bi,Sb)₂Te₃/FeTe heterostructures. Furthermore, we measured the proximity-induced superconducting gap on the top surface of the ferromagnetic Cr-doped (Bi,Sb)₂Te₃ with different thicknesses. We observed the proximity-induced superconducting gap even on the top surface of 10QL Cr-doped (Bi,Sb)₂Te₃, suggesting a strong proximity effect. The coexistence of ferromagnetism and superconductivity in our Cr-doped (Bi,Sb)₂Te₃/FeTe heterostructures underscores their potential as a promising platform for the exploration of chiral topological superconductivity and chiral Majorana physics.