Effect of oxgyen ordering on the spin dynamics of YBa$_2$Cu$_3$O$_{6.5}$*

We use inelastic neutron scattering to study the oxgyen disorder effect in electronic properties of YBCO 6.5. Previous work have shown when copper oxgyen chains in the YBCO 6.5 form ortho-II order, magnetic excitations in YBCO6.5 form a resonance around 33 meV and incommensurate spin flucutations below the resonance. In oder to study oxygen disorder effect on spin excitations of YBCO 6.5, we measured spin dynamics of detwinned Ortho-I YBCO (x=6.5) ($T_c$=48K), whose Cu-O chains are not well ordered. We find that oxygen disorder can have dramatic effect on spin dynamics of YBCO6.5. We discuss similarities and differences in spin dynamics in these two materials and their possible interpretation.