Dopant structural distortions in high-temperature superconductors: an active or a passive role?

The parent compounds of high-temperature superconductors, such as YBa₂Cu₃O₆ and La₂CuO₄, are strongly interacting electron systems, rendering them insulators with Mott-Hubbard gaps of a few electronvolts. Charge carriers (holes) are introduced by chemical doping, causing an insulator-metal (IM) transition and, at low temperatures, superconductivity. The role of dopants is widely seen as limited to the introduction of holes into the CuO₂ planes (i.e. occupying electronic states derived from Cu and O 2p_x,y atomic orbitals). Most theories of high-T_c superconductivity deal with pairing interactions between these planar holes. Local distortions around dopants are poorly understood, because of the experimental difficulty in obtaining such information, particularly at low doping. This has resulted in the neglect, in most theories, of the effect of such distortions on the chemical and electronic structure of high-T_c superconductors. Angular-resolved X-ray absorption fine structure (XAFS) spectroscopy on oriented samples is an ideal technique to elucidate the dopant distortions. Element specificity, together with a large orientation dependence of the XAFS signal in these layered structures, allows the local structure around dopants to be resolved. Results are presented here on (Sr, Ba) and Ni dopants, which substitute at the La and Cu sites, respectively, of insulating La₂CuO₄. The relevance of the measured local distortions for a complete understanding of the normal and superconducting properties of cuprates is discussed.