#### Abstract

We report results of nuclear magnetic resonance (NMR) 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 provides direct evidence for the dimer singlet gorund state in this material. The discovery of excitation gap and quantaized magnetization plateaus in the quasi-two-dimensional spin system SrCu 2 (BO 3) 2 by Kageyama et al.[1] have since stimulated vast amount of experimental and theoretical work. As shown in figure 1, the Cu 2+ spins (s=1/2) in this compound form a planar network of dimers, which is identical to the Shastry-Sutherland (SS) model[2] when only the nearest neighbor intra-dimer exchange (J) and the second nearest inter-dimer exchange (J ′) interactions are retained. Extensive studies on the SS-model have revealed interesting novel aspects [3-11]. First, the simple direct-product of dimer singlet is an exact eigenstate for any value of J ′ /J and is the ground state when J ′ /J is smaller than a certain critical value [2, 4]. In the opposite limits, J ′ /J ≫ 1, the model reduces to the nearest neighbor Heisenberg model on a square-lattice with the obvious Neel order. It was proposed that the two phases are separated by a first order transition at T = 0 near J ′ /J = 0.7 [4]. For SrCu 2 (BO 3) 2 , the exponential decrease of susceptibility and Cu nuclear spin-lattice relaxation rate at low temperatures[1] indicate that the ground state is singlet with a finite energy gap for excited states. The magnitude of the excitation gap ∆ is determined to be 35K from the inelastic neutron scattering[12] and the electron spin resonance[13] experiments. Secondly, excitations from the dimer singlet ground state in the SS-model have extremely localized character. Using perturbation expansion in J ′ /J, Miyahara et al. found that hopping of an excited triplet from one dimer to another is allowed only from sixth order[4]. A very small dispersion width of 0.2 meV for the magnetic excitations ‡ To whom correspondence should …