  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.
