Type-I superconductivity in carbon-coated Sn nano-spheres

Tin spheres of diameter ∼120 nm and ∼1400 nm coated with sub-nanometer carbon layers were fabricated, using a sonochemical technique. Samples of both spheres reveal a type-I superconducting behavior characterized by super-critical fields and an intermediate state manifested by a gradual increase of the magnetization to zero. However, the small and large tin spheres exhibit a similar critical field, H_c, contrary to the expected increase in H_c in spheres with size smaller than the coherence length (∼230 nm). Analysis of the data shows that a relative high degree of carbon doping in the small tin spheres, eliminates the expected size-effect on H_c. Simulations, based on the time dependent Ginzburg-Landau equations, imply that the intermediate state in both measured samples consists of only one superconducting domain surrounded by a normal domain, whereas a rich multi-domain structure is predicted for larger Sn spheres.