Unveiling a \sqrt2\mathit \times\sqrt2\mathit Superstructure in Optimally Doped Bismuth-based Cuprates with Non-contact Atomic Force Microscopy

Non-contact atomic force microscopy, bolstered by strategic functionalization of the tip apex with inorganic molecules such as carbon monoxide (CO), has emerged as a powerful tool for atomic-resolution imaging. Here, we employ this methodology to elucidate the surface topography of the BiO plane in optimally doped Bi₂Sr₂CaCu₂O_8+δ. Single crystal was cleaved and imaged under ultra-high vacuum conditions at 4.5 K. This approach uncovers the precise positioning of Bi atoms, thereby exposing the intrinsic structure of the BiO surface. We present a detailed representation of the BiO lattice using constant-height non-contact atomic force microscopy. With the CO-functionalized tip, a \sqrt2\times \sqrt2 superlattice structure on the BiO plane was observed upon decreasing ∆z in constant height mode, a phenomenon hitherto unreported, while the W metal tip reveals only a 1×1 structure of the BiO surface.