Giant Zero-Bias Conductance and Multiple Andreev Reflections in Carbon Nanotubes with Superconducting Contacts

Superconducting metal leads strongly modify the electronic conductance through carbon nanotubes. Depending on the nanotube transparency, which is controlled by the gate voltage, we observe a strong enhancement of the zero-bias conductance (way above the ballistic limit of 4e/h) or formation of a tunneling gap. Subgap structures corresponding to the Multiple Andreev Reflections are also found. At intermediate transparencies, the zero-bias differential conductance demonstrates unexpected non-monotonic temperature dependence.