Spin dynamics in high-temperature superconducting copper oxides : the contribution of neutron spectroscopy

The technique of inelastic neutron scattering makes it possible to study magnetic correlations in space and time. After a brief review of the technique and the physical quantities to which it gives access, we will present the evolution of the spectrum of spin excitations in high-temperature superconducting copper oxides as a function of the hole doping of the Cu02 planes: we will show how the system gradually moves from localized magnetism to band magnetism. Particular emphasis will be placed on the emergence of a triplet excitation mode in the superconducting state. This mode is also thought to be responsible for numerous anomalies in the charge excitation spectrum, illustrating the presence of strong spin-fermion coupling in these materials. Finally, we will show that the spectrum of magnetic excitations also suggests the presence of "electronic liquid crystal" states in certain materials or portions of the superconducting cuprate phase diagram.