Theory of high-Tcsuperconductors: Back to small bipolarons

Abstract
The nature of the ground state of carriers in high-Tc metallic oxides is discussed in the light of current theories and new experiments. The latest experiments with isotope substitution, infrared, tunnelling, photoemission, NMR and high-resolution electron-energy-loss spectroscopies favour the bipolaronic nature of high-Tc metallic oxides.