NCAFM2023 Programme Booklet

Wednesday 1500 - 1520

CHARACTERIZING THE ADSORPTION OF SINGLE FE ATOMS ON TWO ARCHETYPICAL TOPOLOGICAL INSULATORS

Adrian Weindl, Christoph Setescak, Emma Grasser, Alexander Liebig, Franz J. Giessibl

1 Faculty of Physics, University of Regensburg, D-93053 Regensburg, Germany Email: adrian.weindl@ur.de

Topological insulators (TIs) are a class of materials whose bulk is insulating, whereas the surface houses topologically-protected metallic states. One of the most ubiquitous approaches to modifying these peculiar surface states is the adsorption of magnetic adatoms. However, most studies employ surface-averaging techniques like ARPES to study their effect. Atomic-scale characterization of magnetic impurities on TI surfaces is therefore quite lacking. Here, we employ combined atomic force microscopy (AFM) and scanning tunneling microscopy (STM) to characterize the adsorption of single Fe atoms on TI surfaces on the picometer scale. Two archetypical TIs, Bi 2 Se 3 and TlBiSe 2 , are studied. After cleaving in vacuum, the surface of TlBiSe 2 is terminated by half a monolayer of thallium atoms sitting on a full selenium layer, while cleaving of Bi 2 Se 3 yields an atomically flat, selenium terminated surface. We observe complex adsorption geometries on both materials, with pronounced vertical relaxation of surface atoms surrounding the Fe adatoms. We simulate AFM data using the probe particle model (PPM) [1] and obtain excellent agreement with the AFM contrast observed in experiment. Using Kelvin probe force spectroscopy (KPFS), we determine the charge state of the Fe adatoms to be positive relative to the surfaces of both TIs. The local density of states (LDOS) near the adatoms is probed using scanning tunneling spectroscopy (STS), revealing distinct signatures as a function of the exact adsorption site.

Fig. Fe on Bi 2 Se 3 . Constant height AFM image (a) of two Fe adatoms adsorbed in hollow sites. The PPM simulated AFM (b) reproduces the experimental AFM data very well. Kelvin parabolas measured on a line across the defects show a drop in the local contact potential difference (c). Differential conductance spectra (d) recorded on the Fe atoms show strong resonances that are unique for the respective adsorption site.

References [1] P. Hapala, G. Kichin, C. Wagner, F. S. Tautz, R. Temirov, P. Jelínek, Phys. Rev. B, 2014, 90 , 085421

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