With the use of a high-sensitivity mutual-inductance bridge with a superconducting quantum-interference device detector, ac magnetic susceptibility and contactless resistivity measurements were made down to 12 mK to determine the nature of the zero-resistance transitions seen previously on samples of Sn-doped Bi. The results indicate that this system is not a bulk superconductor but, rather, an interesting example of an inhomogeneous superconductor consisting of segregated tin grains in a semimetallic matrix. Superconductivity originates at the tin sites and propagates via the proximity effect. The emerging superconducting network has a weakly coupled character and can be described reasonably well by a model of randomly distributed expanding superconducting spheres centered on the tin inclusions.

• Received 9 January 1985

DOI:https://doi.org/10.1103/PhysRevB.32.88