  Superconductivity is a macroscopic quantum phenomenon and it is commonly
believed that the in-plane superconducting transition temperature (Tc) should
be isotropic for superconductors with crystal structures of stacking layers
(quasi two-dimensional). Here, we report the observation of piezostrain-induced
in-plane anisotropy of Tc in FeSe0.5Te0.5 (FST) thin films grown on
ferroelectrics. The as-prepared FST shows tetragonal phase with equal in-plane
lattice parameters and absence of in-plane anisotropy of Tc. Upon applying
electric fields, piezostrain induces difference of the in-plane lattice
parameters (a-b) and in-plane anisotropy of Tc. The in-plane anisotropy of Tc
correlates with a-b and becomes more remarkable for larger values of a-b. Some
possible extrinsic effects were ruled out by experiments and analysis. It is
suggested that electronic nematicity with one direction induced by piezostrain
and inhomogeneous superconductivity in FST are involved in this unusual
phenomenon. This work reveals the exotic behavior of FST and will stimulate
exploration of similar behavior in other unconventional superconductors.
