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 energ y in YBa2Cu3O6.2, and of the NMR ''cross-relaxation'' rate in Y2Ba4Cu7 O15. We estimate J(perpendicular to) similar to 14 meV in both materia ls, and chi(max)/mu(B)(2) similar to 100 states/eV Cu in Y2Ba4Cu7O15 a t 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 a rgue that the 41 meV excitation observed in superconducting YBa2Cu3O7 is a collective mode pulled down below the superconducting gap by inte ractions, and that the observed antisymmetry under interchange of plan es follows from the non-negligible value of J(perpendicular to).