Neutron scattering from a collective spin fluctuation mode in aCuO2bilayer

A resonance in the magnetic scattering of neutrons from superconducting ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ has been observed at a momentum transfer of (π/a,π/a,π/c¯), where a is the in-plane Cu spacing and c¯ is the Cu-Cu bilayer spacing. Here we construct a model of a ${\mathrm{CuO}}_2$ bilayer and show that a collective S=1, antiferromagnetic excitation with q=(π/a,π/a,π/c¯) arises if the gap has ${\mathit{d}}_{{\mathit{x}}^2\mathrm{-}{\mathit{y}}^2}$ symmetry. © 1996 The American Physical Society.