We 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, $\chi''$, 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.
Categories: cond-mat.supr-con cond-mat.str-el