Atomic scale impurities in metallic nanoscale structures: A combined experimental and theoretical study
Final Report Abstract
In this project, we have investigated single non-magnetic impurities buried in the bulk metal Cu. Single subsurface Ag and Ge atoms induce surface interference patterns, which result from Friedel oscillations around the defect caused by electron scattering and directionally propagating electrons within the host. Experimentally, we have studied these modifications in the local density of states by means of scanning tunneling microscopy and spectroscopy. As a complementary approach, we have carried out theoretical calculations using density functional theory as implemented in the full potential Korringa-Kohn-Rostoker Green function method. We are able to prepare dilute alloys with single impurities of Ag or Ge below the Cu(100) surface, respectively, using a low temperature growth method. Despite weak scattering amplitudes of the nonmagnetic atoms, the surface signatures of impurities buried many layers below the surface can be detected. These interference patterns are defined by the scattering properties of the defect as well as the electronic structure of the host. By comparing both impurity species, our approach allows one to distinguish and characterize both of these components of the sample system. We extract energy-independent effective scattering phase shifts for Ag and Ge impurities in Cu. Both species show very similar values, which is consistent with the fact that the experimental topographic surface signatures strongly resemble each other. This is not expected from the scattering phase shifts and spectra obtained from the ab-initio calculations. Employing an orbital-resolved analysis of the composition of the interference patterns, we can identify incoherent sp scattering processes at the Ge impurity as possible explanation of the experimental data. Spectroscopically, the scattering properties of both investigated species are inconspicuous and the spectral appearance of the surface signatures is mainly given by the band structure of the host. Thus, our approach reveals a real space access to the electronic structure of bulk Cu. Around the Fermi level, we find a kink in the experimental data, which is not found in the simulations and exceeds the wellknown Cu bulk band structure. This additional feature can be described by many-body effects via a Debye self-energy originating from electron-phonon coupling. We find a surprisingly high coupling parameter λ, which is discussed to originate from bulk propagation in the vicinity of the surface. Utilizing impurity scattering, we demonstrate a novel access to probe band structure renormalizations. In summary, the comparison of experimental and calculated data for two fundamentally different nonmagnetic defects in Cu provided, on the one hand, a deeper understanding of the scattering mechanisms of defects in a solid and, on the other hand, a real space access to the host’s electronic structure and renormalizations due to many-body effects.
Publications
- “Absence of a spin-signature from a single Ho adatom as probed by spin-sensitive tunneling”, Nature Communications 7, 10454 (2016)
M. Steinbrecher, A. Sonntag, M. dos Santos Dias, M. Bouhassoune, S. Lounis, J. Wiebe, R. Wiesendanger, A.A. Khajetoorians
(See online at https://doi.org/10.1038/ncomms10454) - “RKKY-like contributions to the magnetic anisotropy energy: 3d adatoms on Pt(111) surface”, Phys. Rev. B 94, 125402 (2016)
M. Bouhassoune, M. dos Santos Dias, B. Zimmermann, P. H. Dederichs, and S. Lounis
(See online at https://doi.org/10.1103/PhysRevB.94.125402) - “Chiral magnetism of magnetic adatoms generated by Rashba electrons”, New J. Phys. 19 023010 (2017)
J. Bouaziz, M. dos Santos Dias, A. Ziane, M. Benakki, S. Blügel and S. Lounis
(See online at https://doi.org/10.1088/1367-2630/aa59e8) - “Scanning Tunneling Spectroscopy of Subsurface Non-Magnetic Impurities in Copper”, Dissertation, Georg-August-Universität Göttingen, 2021
Thomas Kotzott
(See online at https://doi.org/10.53846/goediss-8807)
