Injection of Cooper pairs into quasidiffusive multiwalled carbon nanotubes with weak localization

Abstract

We study the influence of Cooper pair injection into niobium (Nb)/quasidiffusive multiwalled carbon nanotube (MWNT's)/aluminum (Al) junctions possessing weak localization. Evaporation of the Nb electrode on the open top end of MWNT's standing in nanopores of alumina membranes makes end-bonded structures possible, leading to highly transparent Nb/MWNT interfaces. We found a proximity-induced conductance increase (PIC), which was enhanced by the extremely large diffusion constant of the Cooper pair in the MWNT's, with onset temperatures as high as $T=6--9\mathrm{K}.$ In contrast, we clarified that this PIC was very sensitive to the transparency of the MWNT/Al interface at low temperatures. High transparency led to reentrant conductance due to diffusion out of the Cooper pairs, while we successfully found superconductivity at $T=0.6\mathrm{K},$ implying enhanced critical magnetic fields, in some of the low-transparency samples.