Control of Andreev Reflection via a Single-Molecule Orbital

Charge transport across a single-molecule junction fabricated from a normal-metal tip, a phthalocyanine, and a conventional superconductor in a scanning tunneling microscope is explored as a function of the gradually closed vacuum gap. The phthalocyanine (2H-Pc) molecule and its pyrrolic-hydrogen-abstracted derivative (Pc) exhibit vastly different behavior. Andreev reflection across the 2H-Pc contact exhibits a temporary enhancement that diminishes again with increasing conductance. In contrast, the single-Pc junction lacks Andreev reflection in the same conductance range. Spectroscopies and supporting nonequilibrium Green's function calculations highlight the importance of a molecular orbital close to the Fermi energy for Andreev reflection.