Development of Model Systems for Studying Negative-Uu Pairing in Superconductors

Thin film deposition of novel materials as model systems. We deposit composite structures where the ability of negative U centers to form or stabilize Cooper pairs can be studied Personnel- T.H. Geballe, Gertjan Koster, Boris Moyzhes, Dave Blank (Twente University, The Netherlands), Arjen Molag(visiting student), Prof. Ian Fisher and student Yanna Matsushita (Materials Science).

It has been noted [3] that the charge reservoir layers of all the highest Tc cuprate superconductors contain Bi, Tl, or Hg and that all of these are well known negative U centers in the solid state. We intend to explore the possibility that the charge reservoir layers are not just passive suppliers of carriers as is commonly believed, but that they actually contain pairing centers which can supplement the pairing on the CuO2 planes.  The initial investigations will be based upon semiconducting PbTe where it has been well established that doping with concentrations of Tl ions (which replace the Pb) of the order of 1% induces superconductivity.  It is expected under some conditions that tunnel junctions containing negative U centers in the barrier will exhibit unusually high IcR products [4].  With the  successful deposition of Hg-cuprates (see above) the role of Hg as a negative-U center will be directly investigated.

The thermodynamic criteria for the existence of negative U centers in oxides generally are being investigated. It seems likely that the same criteria can be used to understand many more properties of oxides such as stability, doping, and vacancy formation [5].

[3] T. H. Geballe and B. Moyzhes, Physica C (2000)
[4] ?, S. Kivelson, T. H. Geballe and B. Moyzhes, submitted (2001)
[5] Gertjan Koster, T. H. Beballe and B. Moyzhes, in preparation (2001)