Observation of magnetic moments in the superconducting state ofYBa2Cu3O6.6

Neutron scattering measurements for ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.6}$ have identified small magnetic moments that increase in strength as the temperature is reduced below $T^{*}$ and further increase below $T_c.$ An analysis of the data shows the moments are antiferromagnetic between the Cu-O planes with a correlation length of longer than $195\hspace{0.5em}\AA$ in the a-$b$ plane and about $35\hspace{0.5em}\AA$ along the c axis. The origin of the moments is unknown, and their properties are discusssed both in terms of Cu spin magnetism and orbital bond currents.