Superconducting proximity effect and reentrant behaviors in random network carbon nanotubes

We investigated the superconducting proximity effect in a sandwich structure of network-like carbon nanotubes coupled to NbN electrodes. The proximity effect gives rise to multiple Andreev reflection processes and enhanced magnetoconductance fluctuations that are similar to universal conductance fluctuation. Reentrant behavior caused by the proximity effect was observed. The proximity effect correction to the conductance disappears at low temperature and reaches a maximum value at about 8 K, which corresponds to Thouless energy. This reentrant behavior was also observed in the temperature dependance of fluctuation amplitude. These results are compared with theory.