Static magnetic moments revealed by muon spin relaxation and thermodynamic measurements in quantum spin ice Yb${}_2$Ti${}_2$O${}_7$

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 Yb${}_2$Ti${}_2$O${}_7$. $C(T)$ exhibits a sharp peak at a $T_{\mathrm{C}}$ 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 $T_{\mathrm{C}}$ 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 $\le 0.25$~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 Yb${}_2$Ti${}_2$O${}_7$.