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 ins ulators with Mott-Hubbard gaps of a few electronvolts. Charge carriers (hol es) are introduced by chemical doping, causing an insulator-metal (IM) tran sition 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 3d(x2-y2) and O 2p(x,y) ato mic orbitals). Most theories of high-Tc 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 negl ect, in most theories, of the effect of such distortions on the chemical an d electronic structure of high-Tc superconductors. Angular-resolved X-ray a bsorption fine structure (XAFS) spectroscopy on oriented samples is an idea l technique to elucidate the dopant distortions. Element specificity, toget her with a large orientation dependence of the XAFS signal in these layered structures, allows the local structure around dopants to be resolved. Resu lts 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 s uperconducting properties of cuprates is discussed.