Surface superconductivity in nanocrystals NbCx encapsulated in the multiwall carbon cages

The critical magnetic field Hc3 for the nanocrystals NbCx encapsulated in the multiwall carbon cages and that Hc2 for the bulk crystals NbCx as a function of t (=T/Tc) are reported. The ratio Hc3/Hc2 increases with increasing t from 1.0+/-0.1 at t=0.67 to 4.1+/-0.5 at t=0.995; the ratio Hc3/Hc2 is insensitive to temperature below t=0.9. The data are discussed in terms of the boundary effects, which correspond to two observed interfaces: a nanocrystal/nanocavity interface and a nanocrystal/carbon-cage interface.