Bilayer coupling in the yttrium-barium family of high-temperature superconductors

We derive the expressions needed to interpret experiments relating to interplane magnetic coupling in YBa2Cu3O6+x and related materials, and use the results to interpret measurements of the optical magnon energy in YBa2Cu3O6.2, and of the NMR ''cross-relaxation'' rate in Y2Ba4Cu7O15. We estimate J(perpendicular to) similar to 14 meV in both materials, and chi(max)/mu(B)(2) similar to 100 states/eV Cu in Y2Ba4Cu7O15 at T = 100 K. We show that there is at present no obvious contradiction between the results of a widely used analysis of NMR experiments and the results of neutron scattering experiments in the Y-Ba system. We argue that the 41 meV excitation observed in superconducting YBa2Cu3O7 is a collective mode pulled down below the superconducting gap by interactions, and that the observed antisymmetry under interchange of planes follows from the non-negligible value of J(perpendicular to).