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Peter Wahl - Max Planck Institute, Stuttgart

Towards Quantum Criticality in Atomic Point Contacts

Scanning tunneling microscopy and spectroscopy has in the past been used extensively to characterize the single-impurity Kondo physics of single magnetic adatoms on noble metal surface. Attempts to investigate the two-impurity Kondo physics for pairs of atoms at a metal surface have however proven difficult, because the magnetic interaction between neighbouring impurities decays on the order of inter-atomic distances. Here I present an experiment where the two impurities are not sitting on the metal surface but instead one is attached to the tip of the STM and the other sits on the surface. This configuration allows to continuously tune the inter-atomic distance and thereby enables to explore the two-impurity Kondo physics as a function of the coupling. From transport measurements through the two Kondo impurities, we can extract information about the inter-impurity coupling and the ground state of the dimer. While our experiments show a transition from Kondo screening to antiferromagnetic coupling, we do not observe the quantum phase transition which has been predicted to occur in the two impurity Kondo problem. I will discuss possibilities to approach the quantum phase transition experimentally.


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