Spin pseudo-gap and interplane coupling in Y_2Ba_4Cu_7O_{15}: a ^{63}Cu   nuclear spin-spin relaxation study
  We report measurements of the Gaussian contribution, T_{2G}, to the plane ^{63}Cu nuclear spin--spin relaxation time in the YBa_2Cu_3O_7 and YBa_2Cu_4O_8 blocks of normal and superconducting Y_2Ba_4Cu_7O_{15}. The data confirm our previous results that adjacent CuO_2 planes have different doping levels and that these planes are strongly coupled. -- The static spin susceptibility at the anti-ferromagnetic wave vector exhibits a Curie--Weiss like temperature dependence in the normal state. -- The Y_2Ba_4Cu_7O_{15} data are incompatible with a phase diagram based on a single CuO_2 plane theory and suggest that the appearance of a spin gap implies interplane coupling. Additional data for YBa_2Cu_4O_8 and YBa_2Cu_3O_{6.982} are in accord with the single plane theory. -- The temperature dependence of T_{2G,ind} below T_c excludes isotropic s-wave superconductivity in all three compounds. 
R. Stern, M. Mali, J. Roos and D. Brinkmann (Physik-Institut,   University Zurich, CH-8057 Zurich, Switzerland)