Electronic and topological properties of Bi(110) ultrathin films grown on a Cu(111) substrate

The electronic and topological properties of Bi(110) ultrathin films epitaxially grown on a Cu(111) substrate are investigated using scanning tunneling microscopy/spectroscopy, combined with density functional theory calculations. Bilayer-by-bilayer growth of the ultrathin Bi(110) films is observed, which manifests a structure of black-phosphorus-like Bi bilayers (BLs), with thickness up to 4 BLs. The surface atomic buckling heights in the 1-BL and 2-BL films are clearly identified to depend on the stacking modes with respect to the well-ordered Bi atoms in the adlayer covered on the Cu(111) surface. Our results demonstrate that while the electronic and topological properties of 1-BL films greatly depend on the stacking modes between the Bi(110) bilayer and the adlayer, the 2-BL films show well-decoupled electronic properties from the Cu substrate and nontrivial topologies robust against surface atomic buckling height benefitting from the interbilayer coupling. Our calculations further show that besides the parameter of buckling heights, the topological nontrivial-to-trivial transition can also be induced by changing the interbilayer distance and the vertical intralayer bond length.