Spin-polaron excitations in a doped Shastry-Sutherland model

Using variational algorithm on an infinite Shastry-Sutherland (SS) lattice we show that the introduction of a static nonmagnetic impurity into a dimerized ground state leads to a formation of a small, localized spin polaron surrounding the impurity site. Due to a particular symmetry of the SS lattice, the polaron is extremely anisotropic with a short spatial extent. The presence of nonmagnetic impurities leads to a formation of pronounced in-gap peaks in the dynamical spin structure factor, which we attribute to the spin-doublet excitations of a single unpaired spin $S=1/2$ surrounded by triplet fluctuations. Our results are relevant for the description of SrCu$_2$(BO$_3$)$_{2}$ compound when doped with nonmagnetic atoms at Cu sites.