The resonant magnetic mode: a common feature of high-T_C superconductors

Inelastic neutron scattering experiments in high-$T_c$ cuprates have evidenced a new magnetic excitation present in the superconducting state. In particular, recent experiments on single layer Tl$_{2}$Ba$_{2}$CuO$_{6+\delta }$, performed near optimum doping ($ T_{c} \sim 90$ K), provide evidence of a sharp magnetic resonant mode below $ T_{c}$, similar to previous reports on the YBCO and BSCCO bilayer systems. This result supports models that ascribe a key role to magnetic excitations in the mechanism of superconductivity.