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.
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