Interplane relaxation and the bilayer coupling in   Y$_2$Ba$_4$Cu$_7$O$_{15}$
  The origin of the spin gap in the underdoped cuprate superconductors is still mysterious. Experimental evidence from neutron scattering and NMR experiments indicates that the spin gap might be present only in the bilayer compounds. A naive calculation for a two plane Heisenberg model locates the order-disorder transition only for very large exchange coupling between the bilayers. We propose a interplane relaxation experiment which might quantitatively estimate the strength of the bilayer coupling. We make detailed predictions for the size and the temperature dependence of the interplane relaxation rate. 
H. Monien and A. W. Sandvik (ETH Zuerich, Switzerland and UCSB Santa   Barbara, CA)