Quasiperiodic Pb monolayer on the fivefoldi-Al-Pd-Mnsurface: Structure and electronic properties

Abstract

Lead is one of few elements that adopts a pseudomorphic structure when deposited on quasicrystalline substrates. We present a structural model of quasiperiodic Pb overlayers formed on the fivefold surface of an icosahedral Al-Pd-Mn quasicrystal at two different coverages close to saturation. The skeleton of the Pb monolayer is formed by a network of ``starfish'' (SF) clusters formed at the initial stages of Pb deposition, as studied in detail in our previous work [Ledieu et al., Phys. Rev. B 79, 165430 (2009)]. The atomic structure of the Pb monolayers can be represented as a decorated pentagonal Penrose P1 tiling. The structural models reproduce also the quasiperiodic superstructure of the layers observed in experimental scanning tunneling microscopy (STM) images which is described by the $\ensuremath{\tau}$-scaled $\ensuremath{\tau}\text{P}1$ tiling ($\ensuremath{\tau}$ is the golden mean). The atomic structure underlying the observed $\ensuremath{\tau}$-scaled ``white flower'' motifs and the origin of irregular bright spots in STM images are discussed. The bright spots appear in the centers of the SF clusters centered at substrate sites occupied by Al atoms. The calculated electronic structure shows that the minimum in the density of states (pseudogap) at the Fermi level which is characteristic for the Al-Pd-Mn quasicrystals appears also in the local density of states of the adsorbed Pb monolayer. However, our analysis demonstrates that the formation of this pseudogap is not due to the quasiperiodic arrangement of the Pb atoms but to the strong hybridization of their orbitals with the substrate.