Cu nuclear spin-spin coupling in the dimer singlet state in SrCu2(BO3)2
  We report results of nuclear magnetic resonance (NMR) experiments in SrCu2(BO3)2, a quasi two-dimensional spin system with a singlet ground state. When magnetic field is applied along the c-axis, each of the quadrupole split Cu resonance lines splits further into four lines. The spin-echo intensity for some of the split lines oscillates against the separation time between the pai/2 and the pai rf-pulses. These phenomena are due to strong nuclear spin-spin coupling mediated by the electronic spin system, which exists only within a pair of nuclei. Thus the results provides direct evidence for the dimer singlet gorund state in this material. 
K. Kodama, J. Yamazaki, M. Takigawa, H. Kageyama, K. Onizuka and Y.   Ueda