TB SPIN CORRELATIONS IN PB2SR3TB0.5CA0.5CU3O8

High-resolution neutron spectroscopy and neutron diffraction results o n the magnetic properties of Tb in the high-T-c superconductor Pb2Sr2T b0.5Ca0.5Cu3O8 are presented. The lowest-lying crystalline-electric-fi eld transition, expected at 7 mu eV, is not observed, indicating that the system can still be described in the mean-field approximation at 2 0 K, well above T-N (4 K). The inelastic neutron spectra can be decomp osed into an elastic and a quasi-elastic part, indicating that a singl e Lorentzian does not describe well the magnetic-response function and /or that Tb 2D short-range correlations persist up to temperatures abo ve 100 K.