Superconducting proximity effect in single-walled carbon nanotubes between NbN electrodes

We have fabricated network-like single-walled carbon nanotubes with superconducting NbN electrodes. The single-wall carbon nanotubes were synthesized on SiO2 substrates by the thermal chemical vapor deposition technique using Co catalyst. We obtained sufficiently good contact characteristics between the single-walled carbon nanotubes and the NbN electrodes by infrared annealing at heater temperature 700∘C for ∼15 min in vacuum. We observed multiple Andreev reflection by measuring differential resistance as a function of applied voltage below 7 K. This multiple Andreev reflection is due to the proximity effect between NbN electrodes. The superconducting energy gap of NbN, 2Δ, is about 6 meV. The multiple Andreev reflection processes occur around the dips, i.e. ±2Δ/e and ±Δ/e. On the other hand, at above 8 K, the curves of different resistance are changed from dip to peak at zero bias voltage. This behavior is similar to reentrant behavior that has a maximum conductance corresponding to the correlation energy (Thouless energy) below superconducting critical temperature Tc. This reentrant behavior has been studied in normal metal or two-dimensional gas of semiconductor heterostructures coupled to superconductor.