Basic Energy Sciences Distinguished Lecture

There have been thousands of theoretical and experimental investigations of superconductivity in the cuprate family over the past two decades but as yet there is no concensus as to microscopic interactions responsible for the unprecedentedly high Tcs that are found. However it is widely accepted that all the interactions responsible for the superconductivity originate in fully stoichiometric copper oxide layers that have been doped by charge transfer from adjacent layers. I will discuss data reported in the literature for two different families of cuprates that strongly suggest otherwise, namely that optimal doping of the stoichiometric copper oxide layers is insufficient to account for the high Tcs that are observed. The first is the so-called charge-reservoir family that includes layers of negative U-center ions, either Tl or Hg, and is the family in which the highest known Tc’s are found. The second is Sr2CuO(4-v) , a highly overdoped member of the "214" La2CuO4 family originally discovered by Bednorz and Mueller, in which Tc’s are enhanced well beyond what are considered to be optimally doped "214" structures [1]. I will discuss two speculations that might explain the unusual enhancement. Even though lack of single phase material introduces uncertainty at this time, they lead to ideas for enhancing Tc even further.