INTERPLANE RELAXATION AND THE BILAYER COUPLING IN Y2BA4CU7O15

The origin of the spin gap in the underdoped cuprate superconductors i s still mysterious. Experimental evidence from neutron scattering and NMR experiments indicates that the spin gap might be present only in t he bilayer compounds. A naive calculation for a two plane Heisenberg m odel locates the order-disorder transition only for very large exchang e coupling between the bilayers. We propose a interplane relaxation ex periment which might quantitatively estimate the strength of the bilay er coupling. We make detailed predictions for the size and the tempera ture dependence of the interplane relaxation rate.