Unconventional magnetic ground state in Yb2Ti2O7

We report low-temperature specific heat and positive muon spin relaxation/rotation (mu SR) measurements on both polycrystalline and single-crystal samples of the pyrochlore magnet Yb2Ti2O7. This material is believed to possess a spin Hamiltonian able to support a quantum spin ice (QSI) ground state. Yb2Ti2O7 displays sample variation in its low-temperature heat capacity and, while our two samples exhibit extremes of this variation, our mu SR measurements indicate a similar disordered low-temperature state down to 16 mK in both. We report little temperature dependence to the muon spin relaxation and no evidence for ferromagnetic order, in contrast to reports by Chang et al. [Nat. Comm. 3, 992 (2012)] and Yasui et al. [J. Phys. Soc. Japan. 72, 11 (2003)]. Transverse field (TF) mu SR measurements show changes in the temperature dependence of the muon Knight shift that coincide with heat capacity anomalies, which, incidentally, prove that the implanted muons are not diffusing in Yb2Ti2O7. From these results, we are led to propose that Yb2Ti2O7 enters an unconventional ground state below T-c similar to 265 mK. As found for all the current leading experimental candidates for a quantum spin liquid state, the precise nature of the state below T-c in Yb2Ti2O7 remains unknown and, at this time, defined by what is not as opposed to what it is: lacking simple periodic long-range order or a frozen spin glass state.