Antiferromagnetic spin fluctuations in superconducting ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6+\mathrm{x}}$ have been studied using inelastic neutron scattering. In a crystal with x=0.5 and $T_c$=50 K, we have observed spin fluctuations at 12 K having a cross section which increases with increasing excitation energy. The data are consistent with highly overdamped spin waves and a very short spin-spin correlation length.

• Received 27 October 1989

DOI:https://doi.org/10.1103/PhysRevLett.64.800