Complex magnetic ordering in TI/FeTe heterostructures: A candidate topological superconductor

The combination of time-reversal symmetry breaking, nontrivial electronic states, and superconductivity represent the key ingredients for a host of exotic physics with relevance for quantum computation. Finding all three properties in a single material system is exceedingly rare, with only a few candidates such as UTe₂. Rather than searching for a single perfect material, an alterative approach is the fabrication of thin film heterostructures which combine aspects of the necessary properties and allow them to interact at the interface. In this context, the emergence of topological insulators such as (Bi,Sb)₂Te₃ with a single topologically protected Dirac cone on the surface offers a unique opportunity. In 2014, Q. L. He et al. discovered that depositing Bi2Te₃ on top of FeTe yielding emergent superconductivity at the interface. An antiferromagnetic analogue of the superconductor FeSe, FeTe films do not exhibit superconductivity when grown individually, and it has been suggested that the long-range magnetic order in FeTe suppresses the superconductivity. This question has remained the subject of considerable controversy ever since.

In 2024, H. Yi et al, demonstrated the coexistence of ferromagnetism and superconductivity in Cr-doped (Bi,Sb)₂Te₃/FeTe heterostructures, realizing the third and final component: time-reversal symmetry breaking. In these heterostructures, we used a combination of neutron scattering and low-energy muon spin relaxation spectroscopy to confirm the existence of long-range magnetic order in both the FeTe and Cr-doped (Bi,Sb)₂Te₃ layers of these films in the superconducting state. By further examining a range of different capping layers with different magnetic order, including Te, MnBi₂Te₄, (Bi,Sb)₂Te₃, we explicitly demonstrate the spatial overlap between the superconducting state and both ferromagnetic and antiferromagnetic order in these systems.We find evidence of direct coupling behaviors between the ferromagnetic order and superconductivity, as well as the emergence of multiple unexpected magnetic states which are not present in either of the constituent systems, including a weak net magnetization within the FeTe layer. We discuss the implications of these findings for the underlying physics, and future directions of the work.