ON THE LATTICE PROPERTIES AND ELECTRON-LATTICE INTERACTION IN HIGH-TC SUPERCONDUCTORS

The experimental results on the unusual crystalline lattice properties in high temperature superconductors are reviewed. Special attention is paid to their dependence on temperature and doping and their anomalous behaviour associated with the superconducting and metal-insulator transition. The most physically relevant features are: certain atoms fluctuating between different positions in the unit cell, unusually large oscillator strength of certain vibrational modes strongly coupled to a broad spectrum of electronic excitations and the polaronic nature of charge carriers. A synthesis of these experimental results is attempted in view of constructing a coherent picture for the metal-insulator transition and the super-conducting state. We conclude that in the insulating materials the holes are localized on the CuO2 units in the immediate vicinity of the dopant ions. In the metallic materials we expect charge-fluctuations of holes between the units containing the O(4) apex ions (Cu(1)-20(4)-20(1) for YBa2Cu3O7-delta) and the Cu(2)-20(2)-20(3) complex in the CuO2 layers. This charge transfer being linked to large Cu(1)-O(4) bond fluctuations ultimately leads - via a polaronic mechanism - to pairing of holes in the Cu(2)-20(2)-20(3) units.