Antiferromagnetic Ordering in SuperconductingYBa2Cu3O6.5

Commensurate antiferromagnetic ordering has been observed in the superconducting high- $T_c$ cuprate ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.5}$ ( $T_c=55\mathrm{}\mathrm{K}$) by polarized and unpolarized elastic neutron scattering. The magnetic peak intensity exhibits a marked enhancement at $T_c$. Zero-field muon-spin-resonance experiments demonstrate that the staggered magnetization is not truly static but fluctuates on a nanosecond time scale. These results point towards an unusual spin density wave state coexisting with superconductivity.