Local moment fluctuations in an optimally-doped high Tc superconductor
  We present results of neutron scattering experiments on YBa2Cu3O6.95 (Tc=93K). Our results indicate that magnetic collective modes due to correlated local moments are present both above and below Tc in optimally doped YBCO. The magnon-like modes are robust and not overdamped by itinerant particle-hole excitations, which may point at a substantial static or slowly fluctuating charge inhomogeneity. We compare the experimental results to predictions of the Fermi liquid (FL) theory in the Random Phase Approximation (RPA). 
D. Reznik, J.-P. Ismer, I. Eremin, L. Pintschovius, M.Arai, Y. Endoh,   T. Masui, S. Tajima