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Superconductivity of Bi Confined in an Opal Host

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.