Interplane coupling in the superconductor ${\mathrm{Y}}_2$${\mathrm{Ba}}_4$${\mathrm{Cu}}_7$${\mathrm{O}}_{15}$ as revealed by NQR spin-echo double resonance

We report a detailed spin-echo double-resonance study of the planar Cu sites in ${\mathrm{Y}}_2$${\mathrm{Ba}}_4$${\mathrm{Cu}}_7$${\mathrm{O}}_{15}$ above ${\mathit{T}}_{\mathit{c}}$ employing nuclear quadrupole resonance (NQR). The experiments directly reveal the coupling between the inequivalent single planes of a double ${\mathrm{CuO}}_2$ plane. The ratio of particular NQR relaxation rates is a measure for the strength of this interplane coupling with respect to the intraplane coupling. This ratio increases with decreasing temperature. Using recent calculations by Millis and Monien, an interplane coupling constant of about 5 meV is deduced.