This is a copy of the slides presented at the meeting but not formally written up for the volume.

Recent experiments by Matsushita et al(1) lead to the conclusion that the superconductivity found in semiconducting PbTe doped with Tl ions of a few times 10/20 per cc (the solubility limit) is caused by an excitonic mechanism. They find 1) the superconducting properties are fit by weak-coupled BCS (mean-field) theory 2) the Tcs as a function of doping are roughly two orders of magnitude higher than comparably doped chalcogen semiconductors and an order of magnitude higher than electron-doped SrTiO3 3)No superconductivity is found upon doping in the same concentration range with cations other than Tl. The above facts virtually eliminate the possibility that the superconductivity might be due to local phonon modes to the many valley structure of the valence band. The further findings of Matsushita et al(2) show that the observed low temperature resistance minima are fit by a charge-Kondo mode assuming a near degeneracy of the +1 and +3 Tl ionic states(the +2 state of Tl is at much higher energy as evident from the chemistry of Tl ). The assumption is justified by measurements of the Hall number. The exitonic pairing mechanism is thus simply the exchange of paired electrons between two Tl valence states and the PbTe valence band. Empirical evidence strongly suggests that the TlO charge reservoir layers in Tl high Tc cuprates, and likewise HgO and BiO layers in their respective cuprates are responsible for large enhancements in Tc presumably by exchanging pairs of carriers with the CuO2 layers.1. Y. Matsushita, H. Bluhm, T.H. Geballe, and I.R. Fisher, Phys. Rev. Lett. 94, 157002 (2005).2. Submitted.