Electric-Field-Induced in-Plane Anisotropy of Superconducting Transition Temperature for FeSe_0.5Te_0.5 Thin Films

Superconductor/ferroelectric heterostructures are very important for fundamental research and applications since superconductivity can be controlled by electric fields via ferroelectric polarization/piezostrain. Superconductivity is a macroscopic quantum phenomenon and it is commonly believed that the in-plane superconducting transition temperature (T-c) should be isotropic for superconductors with crystal structures of stacking layers (quasi 2D). Here, the observation of piezostrain-induced in-plane anisotropy of T-c in FeSe0.5Te0.5 (FST) thin films grown on ferroelectric single crystals is reported. The as-prepared FST shows a tetragonal phase with equal in-plane lattice parameters and an absence of in-plane anisotropy of T-c. Upon applying electric fields, piezostrain induces a difference in the in-plane lattice parameters (a-b) and in-plane anisotropy of T-c. The in-plane anisotropy of T-c correlates with a-b and becomes more remarkable for larger values of a-b. Some possible extrinsic effects are ruled out by experiments and analysis. A possible mechanism is proposed involving electronic nematicity and inhomogeneous superconductivity in FST to account for this unusual phenomenon. This work is significant for uncovering the exotic nature of unconventional superconductors, as well as applications.