Intramolecular variations of Andreev reflection

Andreev reflection is probed with a scanning tunneling microscope using a gold-coated normal-metal tip and a single nickel-phthalocyanine molecule adsorbed on superconducting Pb(111). Spectroscopy of the differential conductance at tip-molecule separations ranging from tunneling to contact distances unveils the evolution of Andreev reflection at the central Ni ion and its neighboring pyrrole moieties. Both molecular sites behave similarly in the tunneling range, while at contact separations deviations occur. The efficiency of Andreev reflection rises in a less pronounced manner for Ni than for the pyrrole groups. In addition, across Ni the Andreev reflection rate stays well below the maximal achievable value, while across the pyrrole groups it is nearly reached. This difference is associated with the energy shift of the lowest unoccupied molecular orbital, which reflects the progressive hybridization of the molecule with the approaching tip. The orbital energy remains above the Fermi level for Ni and dives below for the pyrrole groups, which is accompanied by the absence (Ni) and presence (pyrrole) of the molecular Kondo effect. A possible interplay of Andreev reflection and Kondo screening is suggested.