  Superconductivity is observed in a composite of rhombohedral crystalline
bismuth nanoparticles imbedded in an insulating porous opal host via electrical
transport and AC magnetic susceptibility. The onset of superconductivity in
this system occurs in two steps, with upper critical temperature Tc,U = 4.1 K
and lower transition temperature of Tc,L = 0.7 K, which we attribute to the
granular nature of the composite. The transition at Tc,U is observed to split
into two transitions with the application of a magnetic field, and has upper
critical field extrapolated to T = 0 K of Hc2,1(0) = 0.7 T and Hc2,2(0) = 1.0
T, corresponding to coherence lengths of xi1(0) = 21 nm and xi2(0) = 18 nm,
respectively. We suggest that because of the lack of bulk-like states in the Bi
nanoparticles due to confinement effects, superconductivity originates from
surface states arising from Rashba spin-orbit scattering at the interface. This
prospect suggests that nanostructured Bi may be an interesting system to search
for Majorana fermions.
