  Although their crystal structures are complex, all high temperature
superconductors contain some crystal planes consisting of only Cu and O atoms
in a square lattice. Superconductivity is believed to originate from strongly
interacting electrons in these CuO2 planes. Substitution of a single impurity
atom at a Cu site creates a simple but powerful perturbation to these
interactions. Detailed knowledge of the effects of such an impurity atom on the
superconducting order parameter and on the quasi-particle local density of
states (LDOS) could allow competing theories of high temperature
superconductivity (HTSC) to be tested at the atomic scale. The fundamental
implications of results from numerous bulk measurements on samples doped with
impurity atoms could also be clarified with such data. Here we describe
scanning tunneling microscopy studies of the effects of individual Zn impurity
atoms located at the Cu site in the high-Tc superconductor Bi2Sr2CaCu2O8+delta.
Tunneling spectroscopy shows intense quasi-particle scattering resonances at
the Zn sites, coincident with strong suppression of superconductivity within
about 1.5 nm. Imaging of the quasi-particle LDOS at these sites reveals the
long sought four-fold symmetric "quasi-particle cloud" aligned with the d-wave
gap nodes. Several unexpected phenomena, which can shed new light on the
atomic-scale response of HTSC to a probe impurity atom, are also observed.
