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 YBa{sub 2}Cu{sub 3}O{sub 6+{ital x}} and related materials, and use the results to interpret measurements of the optical magnon energy in YBa{sub 2}Cu{sub 3}O{sub 6.2} and of the NMR {open_quote}{open_quote}cross-relaxation{close_quote}{close_quote} rate in Y{sub 2}Ba{sub 4}Cu{sub 7}O{sub 15}. We estimate {ital J}{sub {perpendicular}}{approximately}14 meV in both materials, and {chi}{sub max}/{mu}{sup 2}{sub {ital B}}{approximately}100 states/eV Cu in Y{sub 2}Ba{sub 4}Cu{sub 7}O{sub 15} at {ital 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 YBa{sub 2}Cu{sub 3}O{sub 7} 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 {ital J}{sub {perpendicular}}. {copyright} {ital 1996 The American Physical Society.}