Static magnetic moments revealed by muon spin relaxation and thermodynamic measurements in the quantum spin ice Yb2Ti2O7

We present muon spin relaxation (mu SR) and specific-heat versus temperature C(T) measurements on polycrystalline and single-crystal samples of the pyrochlore magnet Yb2Ti2O7. C(T) exhibits a sharp peak at a T-C of 0.21 and 0.26 K for the single-crystal and polycrystalline samples, respectively. For both samples, the magnetic entropy released between 50 mK and 30 K amounts to R ln 2 per Yb. At temperatures below T-C we observe a steep drop in the asymmetry of the zero-field mu SR time spectra at short time scales, as well as a decoupling of the muon spins from the internal field in longitudinal magnetic fields of <= 0.25 T for both the polycrystalline and single-crystal samples. These muon data are indicative of static magnetic moments. Our results are consistent with the onset of long-range magnetic order in both forms of Yb2Ti2O7.