We studied the atomic and electronic structures of ultrathin Bi(111) films grown on ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ by means of angle-resolved photoemission, first-principles calculations, and low-energy electron diffraction. These Bi films were found to be strained due to the influence of the substrate. Accordingly, the band structure is affected and Bi undergoes a topological phase transition; it is shown that the $Z_2$ topological invariant in three dimensions switches from $+1$ (trivial) to $-1$ (nontrivial or topological). This was clearly confirmed from the change in the surface-state dispersion near the Fermi level. Our discovery offers a method to produce novel topological systems from simple materials.

The structure of multilayer silicene formed on Ag(111) films was studied by low-energy electron diffraction. It turned out that the experimental data cannot be explained by proposed models that only consider buckling of silicon atoms. We have rather found that multilayer silicene on Ag(111) is actually a thin film of bulklike silicon terminated with the Si(111)$\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$-Ag surface. The results are compared to previous works and clearly show the importance to properly understand the structure of the system when discussing its electronic properties.