Professor T. H. Geballe
Dept of Applied Physics
Stanford CA., 94305

"Enhanced Superconductivity- A Survey and the Implications"

The transition temperature Tc of a given superconductor may provide useful clues as to underlying microscopic interactions when its Tc is compared to the magnitude of the Tcs of a comparable group of superconductors. We will discuss systems where Tcs are enhanced, and where chemical and physical properties suggest the presence of local negative U-centers that can serve as pairing centers. The most direct evidence is found in the PbTe doped with Tl where Tc, compared with other known low-density superconducting chalcogenides, is enhanced by two-orders of magnitude. Consideration of Tcs in the High Tc cuprates suggests that factors of two enhancement are due to pairing associated with negative-U centers. Such considerations lead to postulating a novel quasiparticle that is consistent with transport and phase diagram constraints. Finally we will discuss the possibility that a long-sought-after satisfactory explanation for the enhancement of Tc in some amorphous superconductors is associated with negative-U centers.