Impurity effects in the normal and superconducting state of high T_c superconductors

Julien Bobroff

Laboratoire de Physique des Solides,
Université Paris XI, 91405 Orsay Cedex, France

Magnetic and nonmagnetic impurities offer a unique way to probe magnetic correlations in high Tc cuprates. Above Tc , both types of defects induce a staggered magnetization which reveals the existence of correlations between Cu sites. We will review how local and macroscopic techniques allow to probe such magnetization, and how this can be connected to the magnetic correlations of these systems. In particular, we will show how NMR can be used to get a quantitative determination of these effects both above and below Tc. Above Tc, these measurements allow to estimate the correlation length in these systems, both in the underdoped regime and at optimal doping. Below Tc, this magnetization is seen to persist both for Zn and Ni, downto low temperatures. In addition, below T=30K, new spectral features are observed, due to local density of states near Fermi Level are observed for Zn, not for Ni. These measurements can be directly compared to analogous STM measurements for the first time.