Interplane coupling in the superconductor Y2Ba4Cu7O15 as revealed by NQR spin-echo double resonance

We report a detailed spin-echo double-resonance study of the planar Cu sites in Y2Ba4Cu7O15 above T-c employing nuclear quadrupole resonance (NQR). The experiments directly reveal the coupling between the inequivalent single planes of a double CuO2 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.