Self-assembled indium linear chains on the Si(111) surface are found to exhibit instability of the metallic phase and 1D charge density wave (CDW). The room-temperature metallic phase of these chains undergoes a temperature-induced, reversible transition into a semiconducting phase. The 1D CDW along the chains is observed directly in real space by scanning tunneling microscopy at low temperature. The Fermi contours of the metallic phase measured by angle-resolved photoemission exhibit a perfect nesting predicting precisely the CDW periodicity.

Our scanning tunneling microscopy and electron diffraction experiments revealed that a new two-dimensional allotrope of Bi forms on the $\mathrm{S}\mathrm{i}(111)\mathrm{\text{−}}7\times 7$ surface. This pseudocubic $\{012\}$-oriented allotrope is stable up to four atomic layers at room temperature. Above this critical thickness, the entire volume of the film starts to transform into a bulk single-crystal (001) phase, as the bulk contribution in the cohesion becomes dominant. Based on ab initio calculations, we propose that the new allotrope consists of black phosphorus-like puckered layers stabilized by saturating all the $p_z$ dangling bonds in the film.

The electronic structure of Bi(001) ultrathin films (thickness $\ge 7$ bilayers) on $\mathrm{Si}(111)\mathrm{\text{−}}7\times 7$ was studied by angle-resolved photoemission spectroscopy and first-principles calculations. In contrast with the semimetallic nature of bulk Bi, both the experiment and theory demonstrate the metallic character of the films with the Fermi surface formed by spin-orbit-split surface states (SSs) showing little thickness dependence. Below the Fermi level, we clearly detected quantum well states (QWSs) at the $\overline{M}$ point, which were surprisingly found to be non-spin-orbit split; the films are “electronically symmetric” despite the obvious structural nonequivalence of the top and bottom interfaces. We found that the SSs hybridize with the QWSs near $\overline{M}$ and lose their spin-orbit-split character.