Neutron scattering from a collective spin fluctuation mode in a CuO2 bilayer

A resonance in the magnetic scattering of neutrons from superconducting YBa2Cu3O7 has been observed at a momentum transfer of (pi/a, pi/a, pi/(c) over bar), where a is the in-plane Cu spacing and (c) over bar is the Cu-Cu bilayer spacing. Here we construct a model of a CuO2 bilayer and show that a collective S=1, antiferromagnetic excitation with q=(pi/a, pi/a, pi/(c) over bar) arises if the gap has d(x2-y2) symmetry.