Invited Paper MI-WeA4
Local Detection and Manipulation of Single Spins and Spin-Orbit Coupling at Surfaces

Wednesday, October 17, 2007, 2:40 pm, Room 619

The spin state of single magnetic atoms and molecules at surfaces is not only of fundamental interest but may play an important role in future atomic-scale technologies. It can be determined via the Kondo resonance by low-temperature scanning tunneling microscopy. The Kondo effect originates from the screening of the spin of a magnetic impurity by the surrounding conduction band electrons and is characterized by a peak in the impurity’s density of states near the Fermi level. As a second impurity is brought into proximity, magnetic interactions between the impurities become important and can modify the Kondo resonance considerably. Here, I demonstrate that it is possible to determine the magnetic interaction between single Co atoms adsorbed on a noble metal surface by measuring the modified Kondo spectrum. The results are compared to theoretical predictions of the magnetic interactions between single atoms. Increasing the interatomic distance of a Cobalt dimer from 2.56 to 8.1 Å we follow the oscillatory transition from ferromagnetic to antiferromagnetic coupling. Adding a third atom to the antiferromagnetically coupled dimer results in the formation of a collective correlated state. I will further demonstrate the ability to tune the coupling of individual cobalt adatoms with their surroundings by controlled attachment of molecular ligands. In the second part of the talk I will show that by scanning tunneling spectroscopy it is possible to extract the strength of the spin-orbit coupling in a two-dimensional energy band from the local density of states. The spin splitting of the surface state induces a singularity in the local density of states which can be detected as a distinct peak in the differential conductance spectrum. From the STS spectrum we can determine the Rashba energy as a measure of the strength of the spin splitting. Its detection and imaging are demonstrated for the surface alloys Bi and Pb on Ag(111), which exhibit particularly large spin-split band structures. The giant spin splitting in these systems opens up interesting perspectives in the field of spintronics.