Impurity-induced singlet breaking in SrCu2(BO3)2

We have performed muon spin relaxation (μSR) studies on single crystals of SrCu2(BO3)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 …