Scanning Tunnelling Microscopy of Co-impurified Noble Metal Surfaces: Kondo-Effect, Electronic Surface States and Diffusive Atom Transport

Abstract

The scanning tunnelling microscope (STM) was used to investigate the properties of nanoscale structures on (111)-oriented Ag and Cu films. The special focus is on their electronic states and the properties of single transition metal impurities embedded in such films. The (111) noble metal surfaces exhibit electronic surface states that can be described as a two-dimensional non-interacting electron gas located in the topmost crystal layers. Island structures with only monatomic height are capable of confining the surface state electrons, which results in quantization of these states. This quantization is visualized using scanning tunnelling spectroscopy mappings of such islands. While these measurements are possible only at low temperatures, preparatory room temperature studies demonstrate that island structures tend to disintegrate on time scales of several hours. In this context, the dynamics of diffusive mass transport along monatomic step edges has also been investigated in this work. If transition metal impurities such as Co atoms are dissolved in a noble metal film, they form a dilute magnetic alloy. The local moments of the impurity atoms introduce an additional degree of freedom that enables spin-flip scattering of the conduction electrons at the impurity atoms. Towards low temperatures, this spin-flip scattering gives rise to typical anomalies in the fundamental physical properties of the host crystal, known as the Kondo-effect. The tunnelling microscope offers direct access to single surface impurities and allows to locally probe the electronic states in their vicinity. The Kondo-temperature of such a system was determined via tunnelling spectroscopy of single Co impurities in a Cu surface.