THE LOCAL-STRUCTURE OF HIGH-TEMPERATURE SUPERCONDUCTORS

We show how XAFS has been successfully used in the determination of the local crystal structure of high-temperature superconductors, with advantages over traditional diffraction techniques. We review the experimental results that yielded the first evidence for an axial-oxygen-centered lattice instability connected with the superconductivity transition. The interpretation of this instability in terms of a dynamical tunneling model suggests the presence of polarons in these materials. XAFS on Tl2Ba2CuO6 and other Tl-based superconductors indicate the presence of local instabilities in the CuO2 planes of these materials, in addition to axial-oxygen instabilities.