We have determined the number of transport channels and the value of the transmission coefficients of a single molecular junction by measuring the multiple Andreev reflections (MARs) with a scanning tunneling microscope (STM). By precisely positioning a Nb STM tip to a single ${\mathrm{C}}_{60}$ molecule on Pb(111), a single molecular junction was fabricated in which the ${\mathrm{C}}_{60}$ molecule connects with the two superconducting electrodes. From the subharmonic gap structures arising from MARs in the current-voltage characteristics together with the tunneling spectrum of the ${\mathrm{C}}_{60}$ molecule, we found that unoccupied molecular orbitals of ${\mathrm{C}}_{60}$ extending to the Fermi level provide three electronic transport channels in the molecular junction. We also found that the transmission coefficients depend on the contact geometry of the molecule. These results demonstrate that the combination of the STM imaging with the MARs measurement provides an effective path for investigating the electronic transport properties through a single molecule sandwiched by two superconducting electrodes.

• Received 4 August 2014
• Revised 13 November 2014

DOI:https://doi.org/10.1103/PhysRevB.90.241405