Unconventional nematicity of the charge order in a monolayer high-Tc iron-based superconductor

Symmetry-breaking electronic phase in unconventional high-temperature (high-T_c) superconductors is a fascinating issue in condensed-matter physics, among which two attractive phases are charge density wave (CDW) phase with four unit-cell periodicity in cuprates and nematic phase breaking the C₄ rotational symmetry in iron-based superconductors (FeSCs). Recently, pair density wave (PDW), an exotic superconducting phase with non-zero momentum Cooper pairs, has been observed in high-T_c cuprates and the monolayer FeSC. However, the interplay between the CDW, PDW and nematic phase remains to be explored. Here, using scanning tunneling microscopy/spectroscopy, we detected commensurate CDW orders with a period of λ = 4a_Fe (a_Fe is the distance between neighboring Fe atoms) in a monolayer high-T_c Fe(Te,Se) film grown on SrTiO₃(001) substrate. Further analyses demonstrate the observed CDW is a smectic order, which breaks both translation and C₄ rotational symmetry. Moreover, the C₄ symmetry breaking of the CDW order is strongest near the superconducting gap, and the CDW order weakens at higher temperatures, in vortex regions under a magnetic field or near defects, indicating the interplay between the smectic CDW and PDW orders. Our works provide a new platform to study the intertwined orders and their interactions in high-T_c superconductors.