In-Gap Spin Excitations and Finite Triplet Lifetimes in the Dilute   Singlet Ground State System SrCu(2-x)Mgx(BO3)2
  High resolution neutron scattering measurements on a single crystal of SrCu(2-x)Mgx(BO3)2 with x=0.05 reveal the presence of new spin excitations within the gap of this quasi-two dimensional, singlet ground state system. Application of a magnetic field induces Zeeman-split states associated with S=1/2 unpaired spins which are antiferromagnetically correlated with the bulk singlet. Substantial broadening of both the one and two-triplet excitations in the doped single crystal is observed, as compared with pure SrCu2(BO3)2. Theoretical calculations using a variational algorithm and a single quenched magnetic vacancy on an infinite lattice are shown to qualitatively account for these effects. 
S. Haravifard, S.R. Dunsiger, S. El Shawish, B.D. Gaulin, H.A.   Dabkowska, M.T.F. Telling and J. Bonca