All-iron-based ferromagnetic/superconductor heterostructures and the origin behind superconducting FeTe

Ferromagnetism (FM) and superconductivity (SC) are the most famous macroscopic quantum phenomena, and researchers have long pursued the quest to couple them in SC/FM heterostructures for novel functionalities. However, placing FM and SC layers nearby leads to the degradation of their original properties due to extrinsic and intrinsic reasons such as interfacial disorder, chemical interdiffusion, and electronic/magnetic proximity effects.

Here, we introduce an SC/FM heterostructure entirely composed of Fe-based chalcogenides: Fe(Te, Se) (FTS) and Fe₃GeTe₂ (FGT). This is the first platform where the same element (Fe) is responsible for both strong FM and high-temperature SC and coexist without degrading their individual properties. Surprisingly, the coexistence of SC and FM was also observed even in non-superconducting FeTe/Fe₃GeTe₂ heterostructures. There is a consensus that the ground state of FeTe is an antiferromagnetic (AFM) Hund metal. However, multiple reports of emerging superconductivity in certain FeTe heterostructures have been reported. Initially, it was proposed that topological surface states may be the critical factor behind the emergence of superconductivity, considering that they were first observed when a topological insulator (TI) was deposited on FeTe. However, with the discovery of similar superconductivity in non-TI-based FeTe heterostructures (such as FeTe/MnTe and CrTe₂/FeTe), it has become evident that topological surface state is not the main factor for the superconductivity. Hence, the origin of superconductivity in FeTe heterostructures remains one of the central mysteries in the Fe chalcogenide system. We will discuss the possible origins behind the emerging superconductivity in FeTe heterostructures.