Cu nuclear spin-spin coupling in the dimer singlet state in SrCu2(BO3)2

We report results of nuclear magnetic resonance experiments in SrCu 2 (BO 3 ) 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 π/2 and π 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 provide direct evidence for the dimer singlet ground state in this material.