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978-3-8439-1393-5, Reihe Physik
Charge screening in topological insulators and low-dimensional superconductivity investigated by scanning tunneling microscopy
207 Seiten, Dissertation Universität Hamburg (2013), Softcover, A5
The main topic of this thesis is an investigation of the prototypical topological insulator bismuth selenide by means of scanning tunneling microscopy (STM). A deposition of Rb adatoms leads to a downwards band bending at the surface. By combining results from STM and photoemission spectroscopy, a thermally induced intercalation of Rb atoms into the van der Waals gaps of the substrate is found, which is driven by Coulomb repulsion between the atoms. Intercalated Rb atoms still act as donors, but are protected against environmental influences. Coverage-dependent distributions of positively charged Rb atoms on the surface are further investigated by STM. A statistical analysis yields the strength and distance behavior of the electrostatic potential, which is unexpectedly efficiently screened by free electrons at the surface of the topological insulator. Its knowledge enables a determination of the potential landscape in which the topological surface state electrons reside. Arbitrary potential landscapes can be created by a combination of self-organized growth and atomic manipulation with the STM tip.
The low-dimensional superconductivity of lanthanum films is investigated by scanning tunneling spectroscopy. Fitting of the measured spectra to BCS theory yields the superconducting energy gaps. For the case of thick, bulk-like films, the bulk energy gap and critical temperature of dhcp lanthanum turn out to be considerably higher as compared to previous studies. In thin films the superconductivity is suppressed by the boundary condition for the superconducting wavefunction, leading to a linear decrease of the superconducting transition temperature as a function of the inverse film thickness. Moreover, the experimentally observed, unexpectedly strong scattering rate of tunneling electrons is explained.
All experiments were conducted with a low-temperature STM facility that has been planned and assembled in the course of this PhD work. It enables studies of surfaces, which can be prepared in-situ in the same facility by standard ultra-high vacuum surface science techniques, at a base temperature of 1.2 K and with high spatial and energy resolution.