Cross relaxation and the bilayer coupling in YBCO
  There is strong experimental evidence for substantial antiferromagnetic coupling between the CuO$_2$ planes of the bilayers in YBCO compounds. The acoustic mode of the spin excitation spectrum has been measured but the optic mode has not been observed. Theoretical estimates suggest values for the antiferromagnetic interplane coupling as large as 10 meV. It has been proposed that this coupling is responsible for the spin gap in the underdoped YBCO compounds. In the recently synthesized compounds Y$_2$Ba$_4$Cu$_7$O$_{15}$ with alternating single/double chain blocks, it is possible to distinguish between the CuO$_2$ planes in a bilayer. We propose a nuclear magnetic cross relaxation experiment to directly measure the strength of the coupling between the two planes of a bilayer. The temperature dependence and magnitude of this cross relaxation rate is predicted. 
H. Monien and T. M. Rice (Theoretische Physik, ETH Zuerich,   Hoenggerberg, Switzerland)