Hidden order in Yb2TI2O7

We report low temperature specific heat and positive muon spin rotation ($\mu$-SR) measurements of both polycrystal and single crystal Yb$_2$Ti$_2$O$_7$. Our zero field (ZF) $\mu$-SR shows little spin relaxation temperature dependence in the polycrystal Yb$_2$Ti$_2$O$_7$, contrast to previously reported results. We observe no collinear ferromagnetic order, rather a hidden order ground state where spin fluctuations remain dynamic down to 16 mK. Single crystal Yb$_2$Ti$_2$O$_7$ zero field $\mu$-SR measurements with the crystallographic $<$111$>$ direction parallel to the initial muon polarization show small but measurable temperature dependence. In addition, our transverse field (TF) $\mu$-SR measurements show the spin susceptibility undergoes a distinct change at temperatures corresponding to the magnetic transition measured in the specific heat.