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.