Observation of intra-unit-cell nematic order in epitaxial bilayer FeSe films on SrTiO₃(001)

Epitaxial FeSe thin films provide an ideal platform to probe the interplay of superconductivity and nematicity, due to the absence of long-range magnetic order. Here, we systematically investigate the nematic order in high quality bilayer FeSe/SrTiO₃ films grown by molecular beam epitaxy. Using low temperature scanning tunneling microscopy/spectroscopy, we observe features associated with Se atoms to be elongated along the Fe-Fe lattice direction within a specific energy window, demonstrating symmetry breaking from fourfold to twofold. Detailed analysis of Fourier transformation of the STM images reveals that the intensity of Fe Bragg peak breaks rotational symmetry within each Fe unit cell, indicative of an intra-unit-cell nematicity. Our results provide critical information on nematicity in Fe-based superconductors, an essential element in understanding superconducting transition in these materials.