Impurity-induced singlet breaking in SrCu2(BO3)2

We have performed muon spin relaxation $(\ensuremath{\mu}\mathrm{SR})$ studies on single crystals of $\mathrm{Sr}{\mathrm{Cu}}_{2}{(\mathrm{B}{\mathrm{O}}_{3})}_{2}$, a quasi-two-dimensional spin system with a spin-singlet ground state. We find evidence for two different muon sites which we associate with muons located adjacent to the two inequivalent O sites. One site, presumably located in the Cu-O-Cu superexchange path, exhibits a large increase in the frequency shift with decreasing temperature which is unaffected by the onset of the singlet ground state, indicating that the muon has perturbed the Cu-Cu interaction and created two quasifree spins. We have also performed $\ensuremath{\mu}\mathrm{SR}$ on single crystals of $\mathrm{Sr}{\mathrm{Mg}}_{0.05}{\mathrm{Cu}}_{1.95}{(\mathrm{B}{\mathrm{O}}_{3})}_{2}$, ${\mathrm{Sr}}_{0.96}{\mathrm{La}}_{0.04}{\mathrm{Cu}}_{2}{(\mathrm{B}{\mathrm{O}}_{3})}_{2}$, and ${\mathrm{Sr}}_{0.95}{\mathrm{Na}}_{0.05}{\mathrm{Cu}}_{2}{(\mathrm{B}{\mathrm{O}}_{3})}_{2}$. We have found that the frequency shifts of these doped samples are equivalent and contain three branches at low temperatures. Two of these branches map onto the branches observed in the pure sample, and so we attribute the third branch to a dopant effect. Specifically, this third branch represents the case when the muon resides at a site in the superexchange path where a local singlet cannot form due to doping. This leads to the conclusion that doping this system both in and out of the $\mathrm{Cu}\mathrm{B}{\mathrm{O}}_{3}$ planes prevents the formation of some singlets.