Anisotropic conductance oscillations in Pb films on Si(557)

We correlate in this study the growth of Pb films on the stepped Si(557) surface at temperatures of 70 K, studied by low-energy electron diffraction, with the properties of electronic transport, measured by a macroscopic four-point probe technique. Despite a large lattice mismatch, layer-by-layer growth is observed, as most obvious from the characteristic oscillations in conductance with layer periodicity, incipient with the first monolayer both along and across the step direction. These findings demonstrate that lateral misfits (here almost 10%) in heteroepitaxial systems can be effectively compensated by substrate steps and can change the growth mode with respect to flat surfaces. While structurally the layers appear to be isotropic starting already with the third layer, anisotropy is seen in transport up to at least six monolayers with functional dependencies of conductance varying with layer thickness and measurement direction. Maxima of conductance oscillations up to five monolayers do not coincide with completion of individual layers. They are characteristic for the close coupling of structure and quantum effects. © 2010 The American Physical Society.DF

Spin-Bahn-Kopplung in niedrigdimensionalen Strukturen auf Oberflächen : Wachstum und elektronischer Transport

[no abstract

Sensing surface states of Bi films by magnetotransport

Macroscopic magnetotransport measurements at Bi films grown epitaxially on Si(111) substrates have been carried out at 10 K. The magnetoconductance curves reveal two characteristic regimes, which are assigned to magnetotransport by surface and bulk states, respectively. In contrast to bulk, backscattering, i.e., weak localization, is strongly restricted for the charge carriers in the spin-polarized surface bands, and a classical magnetoresistance behavior was found. While the surface-state conductivity was found to be as high as 4×10−4Ω−1/□, the bulk conductivity is extremely low, possibly due to quantum confinement of the bulk band structure. © 2011 American Physical Society.DFGDAA

Diffusing magnetic Tb impurities and magnetotransport in strongly spin-polarized Bi films

As a contribution to electronic transport within strongly spin-polarized surface states and its modification by adsorption of magnetic impurities we studied the adsorption of Tb (atomic magnetic moment 10 μB) on epitaxial Bi(111) films by means of surface sensitive (magneto)conductance and low-energy electron diffraction. Surface diffusion turned out to be non-negligible even at substrate temperatures of 10 K. The Tb adatoms finally nucleate at intrinsic defects of the Bi(111) surface, where the Tb impurities act as dopants but not as scatterers. Nevertheless, time-dependent measurements allowed to determine also single-particle Tb scattering properties, as also supported by simulations of adsorption kinetics and time-dependent conductance. The magnetoconductance properties are characterized by small charge transfer (0.05 e/atom) and strong spin-orbit scattering, which in this case results only in strong reduction of the weak antilocalization effect but not a reversal to weak localization as for Fe and Co [see Lükermann, Sologub, Pfnür, Klein, Horn-von Hoegen, and Tegenkamp, Phys. Rev. B 86, 195432 (2012)]. Although Tb has a magnetic moment, which is by far higher than for adsorbed Fe and Co, it turns out that the f electrons of Tb play essentially no role in scattering of the conduction electrons, yielding an even smaller scattering cross section than that for Fe and Co. The adatom coordination (interstitial or on the surface) may also play an important role. © 2013 American Physical Society.DFGDAA

Magnetotransport in anisotropic Pb films and monolayers

The anisotropy induced by atomic steps of a Si(557) substrate in structure and magnetoconductance of ultrathin Pb films adsorbed on this surface is shown to be effectively shielded as a function of layer thickness, as found out by a combined study of low-energy electron diffraction and macroscopic four-point conductivity measurements as a function of Pb coverage, temperature, and magnetic field. In strong contrast to flat Si(111), substrate steps effectively compensate the lateral misfit (10%), leading to crystalline growth starting from the first monolayer. Multilayers already exceeding four physical monolayers (PML) form isotropic and percolated Pb islands even on this uniaxial surface. This structural anisotropy corresponds well to that found in dc conductivity measurements. As a function of temperature, strong localization effects with clear anisotropy become dominant for coverages below 4 PML. Strong anisotropic magnetotransport was found for Pb-wetting layers close to completion of the physical monolayer caused by an enhanced elastic scattering rate in the direction perpendicular to the step direction. While multilayers are characterized by weak localization, antilocalization is found for all monolayer structures due to strong spin-orbit coupling, which is effectively switched off around 1.3 ML (1 PML) below 78 K, where one-dimensional transport was seen along the step direction. © 2010 The American Physical Society.DF

Pb nanowires on vicinal Si(111) surfaces: Effects of refacetting on transport

The conductance of Pb wires grown by self-assembly on Si(557) has been studied in detail as a function of coverage and of the facet structure. Only for 1.31 ML, corresponding to one physical monolayer on the terraces (steps not covered with Pb), and a perfectly ordered wire array along the [¯1¯12] direction quasi-one-dimensional (1D) transport along the [1¯10] direction is found, corroborating the model of one-dimensional band filling in an adsorbate induced (223) facet structure. The transport results recently shown by Morikawa et al. [Phys. Rev. B 82, 045423 (2010)] can also reproduced by our group. In contrast to what was claimed by them, our results clearly show that either a too small coverage or structural imperfections of the surface are responsible for a metal-insulator transition around 140 K irrespective of the crystallographic direction. The variety of different transport scenarios found is caused by strong adsorbate-induced refacetting into an electronically stabilized (223) orientation, which differs from the macrosocopic orientation of the substrate. The crucial interplay between structure and filling factor explains the extremely small parameter window in which the 1D transport channel can be observed. © 2010 The American Physical Society.DF

Scattering at magnetic and nonmagnetic impurities on surfaces with strong spin-orbit coupling

Adsorption-induced reduction of surface-state conductivity in epitaxial Bi(111) films, a prototype system with large Rashba-induced surface-state splitting, by adsorbed atoms of Bi, Fe, and Co has been investigated by macroscopic surface magnetotransport measurements at a temperature of 10 K. A detailed analysis of magnetotransport, dc transport, and Hall data reveals that the scattering efficiencies for Co and Fe are larger by a factor of 2 than that for Bi. While for the latter charge transfer and change of band filling near the Fermi level are negligible, we find an increase of hole concentration upon Co and Fe adsorption. These atoms act as acceptors and immobilize on average about 0.5 electrons per adsorbed atom. Besides the dominant classical magnetoconductance signal the films show signatures of weak antilocalization, reflecting the strong spin-orbit coupling in Bi(111) surface states. This behavior can be changed to weak localization by the adsorption of high concentrations (0.1 monolayers) of magnetic impurities (Fe,Co), similarly to results found on the topological insulator Bi2Se3. Our results demonstrate that details of chemical bond formation for impurities are crucial for local spin moments and electronic scattering properties. © 2012 American Physical Society.DFGDAA

Unoccupied electronic structure and momentum-dependent scattering dynamics in Pb/Si(557) nanowire arrays

The unoccupied electronic structure of quasi-one-dimensional reconstructions of Pb atoms on a Si(557) surface is investigated by means of femtosecond time- and angle-resolved two-photon photoemission. Two distinct unoccupied electronic states are observed at E-EF=3.55 and 3.30 eV, respectively. Density functional theory calculations reveal that these states are spatially located predominantly on the lead wires and that they are energetically degenerated with an energy window of reduced electronic density of states in Si. We further find momentum-averaged lifetimes of 24 and 35 fs of these two states, respectively. The photoemission yield and the population dynamics depend on the electron momentum component perpendicular to the steps of the Si substrate, and the momentum-dependent dynamics cannot be described by means of rate equations. We conclude that momentum- and direction-dependent dephasing of the electronic excitations, likely caused by elastic scattering at the step edges on the vicinal surface, modifies the excited-state population dynamics in this system. © 2015 American Physical Society.DFG/FOR/170

Barrier-free subsurface incorporation of 3d metal atoms into Bi(111) films

By combining scanning tunneling microscopy with density functional theory it is shown that the Bi(111) surface provides a well-defined incorporation site in the first bilayer that traps highly coordinating atoms such as transition metals (TMs) or noble metals. All deposited atoms assume exactly the same specific sevenfold coordinated subsurface interstitial site while the surface topography remains nearly unchanged. Notably, 3d TMs show a barrier-free incorporation. The observed surface modification by barrier-free subsorption helps to suppress aggregation in clusters. It allows a tuning of the electronic properties not only for the pure Bi(111) surface, but may also be observed for topological insulators formed by substrate-stabilized Bi bilayers. © 2015 American Physical Society.DFG/SFB/616DFG/SPP/1601DFG/Pf238/3