2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/16012We present a theory for inelastic neutron scattering (INS) and nuclear magnetic resonance (NMR) experiments in the pseudo-gap regime of the underdoped high-$T_c$ cuprates. We show that superconducting phase fluctuations greatly affect the temperature and frequency dependence of the spin-susceptibility, $χ''$, probed by both experimental techniques. This result explains the appearance of a {\it resonance peak}, observed in INS experiments, below a temperature $T_0 > T_c$. In the same temperature regime, we find that the $^{63}$Cu spin-lattice relaxation rate, $1/T_1$, measured in NMR experiments, is suppressed. Our results are in qualitative agreement with the available experimental data.5 pages, RevTeX, 3 eps figures embedded in the textSuperconductivityStrongly Correlated Electronstext