Scanning Tunneling Microscopy and Spectroscopy of Surface States with Different Step Decorations on Re(0001) and Mobility Investigations of Ni on Re(0001) at low Temperatures
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Abstract
In this thesis, topographic and electronic properties of the pristine
Re(0001) surface covered by 0 to 20 atomic layers of Au and Ni are
studied using scanning tunneling microscopy and spectroscopy. Growth,
mobility, surface structures, and surface states of these systems were investigated.
The experimental results represent a step towards a fundamental
understanding of surface properties, which are relevant for a wide range of
applications such as chemical catalysis, quantum information technology,
and spintronics.
In the first part of the thesis, quasiparticle interference patterns formed
by a surface state on Re(0001) were investigated using scanning tunneling
spectroscopy. The Tamm surface state exhibited a Rashba type band splitting,
revealing a strong spin-momentum locking. The energy dispersion
was inferred from Fourier-transformed differential conductivity maps for
occupied and unoccupied states. An analysis of the phase of interference
patterns at step edges revealed a change of sign in the effective energy
barrier for backscattering above and below the Fermi level. The attenuation
of the interference pattern with increasing distance indicated interband
scattering as the significant scattering mechanism. Step decorations by Ni
had a negligible influence on the pattern, excluding spin-flip scattering as a
dominant contribution. The one-dimensional Au/Re line interface, however,
reversed the scattering barrier behavior, indicating a coupling of Au and
Re surface states. Using spin-resolved scanning tunneling spectroscopy, we
investigated the spin-dependent scattering of spin waves.
In the second part of this thesis, a movement of Ni adatoms at 4.6K on a
Re(0001) single crystal was investigated, while the surface was rigid at room
temperature. Measurements at intermediate temperatures revealed increasing
mobility with decreasing temperature. This inverse mobility behavior
can be explained tentatively by a combination of a small force induced
by the tip of a scanning tunneling microscope (STM) and the weakened
bonding of Ni adatoms due to an increase of the mismatch between their
lattice constants at low temperature. To achieve a better understanding of
the physics resulting in this unique behavior, investigations at various temperatures,
coverages, and tunneling parameters were performed. Different
types of movement and decreasing mobility with increasing temperature
and layer thickness were observed. In addition, measurements with a Fe coated
tip on ultrathin Ni films revealed an unexpected spectroscopic contrast.
Close inspection of the spectroscopic and topographic results indicated
that Fe atoms are transferred from the tip to the sample surface during
scanning.