Magnetic Susceptibility and Contactless Resistivity Measurements of Percolating Superconductivity in Tin-Doped Bismuth.

Abstract

Magnetic susceptibility and contactless resistivity measurements were undertaken to determine whether or not the zero-resistance transitions seen previously in tin- and tellurium-doped bismuth were evidence of bulk superconductivity in these systems. An improved technique has been developed which extracts both the magnetic susceptibility and resistivity of a sample from the in-phase and quadrature components of an a.c. mutual inductance measurement. High sensitivity is achieved by a careful design and by using a bridge circuit with a SQUID (superconducting quantum interference device) as a null detector. This allows the use of small a.c. excitation fields (50 (mu)Oe, p-p) for observing weakly coupled superconducting networks which are sensitive to probing fields. The results of this investigation suggest that tin-doped bismuth is not a bulk superconductor but is, instead, an interesting example of an inhomogeneous superconductor comprised of segregated and r and omly distributed tin grains in a semimetal matrix. Superconductivity originating at the tin sites, appears to propagate via proximity effect and "weak-link" coupling, causing broad transitions which extend from about 0.6K down to approximately 12 mK or lower.