Phonon Interaction Studies - Slim Bi (111) Film Surface Electronic Structure And Electron-phonon Structure

Topological insulating material is a new quantum matetial which have attracted a great deal of attention in recent years. Topological insulators are electronic materials that have a bulk band gap like an ordinary insulator but have protected conducting states on their edge or surface in relationship with the spin-orbit coupling. Ultrathin Bi films have been theoretically predicted to be an elemental two-dimensional topological insulator recently. Investigation of physical properties for ultrathin Bi films especially down to a few-layer regime, is an interesting topic. In this paper, we study the geometry structure, electronic structures, phonon structure and electron-phonon coupling of ultrathin Bi (111) films from first-principles calculations. The main results obtained are as following:(1) The relaxed results of geometry structure show that the long and the short interlayer distances appear alternately, confirming the stable bilayer structure in Bi (111) film. With the increase of the film thickness, the Bi (111) surface states range from semiconducting character to semimetallic and metallic character.(2) By analyzing the zone-center phonons, an anomalous phonon hardening of Eg and A1g two Raman modes in ultrathin Bi films is found and is explained by considering the redistribution of charge density on the surface of Bi film. With increasing the thickness, the frequencies of these two Raman modes decrease gradually toward the values in the bulk.(3) From surface phonon band structures and surface phonon density of states, we find that the softening and hardening of surface phonon modes coexist in ultrathin Bi film. But the softening may be dominated over the hardening.(4) The calculated electron-phonon coupling constant λ in Bi (111) film is much larger than that in the bulk, which might induce surface-localized superconductivity in this two dimensional topological insulator.