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Quantum effects at molecule metal interfaces

In this talk I will discuss two examples of inherently quantum mechanical phenomena that appear at interfaces between p-conjugated organic molecules and metals. The first is a chemistry-driven magnetic quantum phase transition, the second the novel experimental method of scanning quantum dot microscopy.

(1) When two local moments on a metal surface interact, this is usually discussed in terms of a competition between the Kondo effect and the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction (ref). RKKY is an indirect exchange interaction which is mediated by the conduction electrons. Depending on distance, it favours ferromagnetic or antiferromagnetic alignments of local moments, whereas the Kondo effect tends to quench the moments locally with the help of the conduction electrons. Here we describe a different scenario, in which the competition between the kinetic energy gain due to Kondo scattering and the binding energy gain due to chemical interaction between the moment-carrying orbitals is the driving force of a quantum phase transition in a system of two local moments on a metal surface. Interestingly, in this scenario the Kondo effect favours the alignment while the chemical interaction promotes the local quenching of the moments. We expect this mechanism to be generic and widespread, because it relies only on very general features of chemical interactions and Kondo physics. Moreover, since it is straightforward to engineer the chemistry, the mechanism will allow for an easy tuning of the magnetic interaction between local moments.

(2) A scanning probe technique that enables three-dimensional imaging of local electrostatic potential fields with subnanometer resolution is introduced. Registering single electron charging events of a molecular quantum dot attached to the tip of an atomic force microscope operated at 5 K, equipped with a qPlus tuning fork, we image the quadrupole field of a single molecule. To demonstrate quantitative measurements, we investigate the dipole field of a single metal adatom adsorbed on a metal surface. We show that because of its high sensitivity the technique can probe electrostatic potentials at large distances from their sources, which should allow for the imaging of samples with increased surface roughness.

[1] T. Esat, T. Deilmann, B. Lechtenberg, C. Wagner, P. Krüger, R. Temirov, F. B. Anders, M. Rohlfing, F. S. Tautz, Physical Review B  91, 144415 (2015)

[2] T. Esat, B. Lechtenberg, T. Deilmann, C. Wagner, P. Krüger, R. Temirov, M. Rohlfing, F.B. Anders, F. S. Tautz, submitted

[3] C. Wagner, M.F.B. Green, P. Leinen, T. Deilmann, P. Krüger, M. Rohlfing, R. Temirov, F.S. Tautz, Phys. Rev. Lett. 115, 026101 (2015)