Andreev Spectroscopy of Molecular States in Resonant and Charge Accumulation Regime

Molecular electronics represents the ultimate step of the miniaturization process of the integrated circuits. Including molecules in manmade devices may introduce novel functionalities in nanodevices, such as the possibility to interact with biological environments with tremendous implications in several fields. With these motivations, we present a Bogoliubov-de Gennes description of the transport properties of a Normal-metal/molecule/Superconductor (N/m/S) junction formed by contacting an annular molecule (m) (e.g., a benzene molecule) with a normal (N) and a superconducting electrode (S). Differently from N/m/N junctions, the superconducting correlations in S play a crucial role in determining the transparency of the system. We demonstrate that the zero-bias differential conductance of the device is strongly suppressed in presence of charge accumulation on the molecule, while a resonant transmission with maximum conductance is observed when the charge stored on the molecule is negligible. We explain these findings observing that the space distribution of molecular quantum states is strongly affected by the Andreev mechanism, which represents the dominant scattering process at the m/S interface when the energy of the incident quasi-particle is below the superconducting energy gap. The relevance of these findings is briefly discussed.