Partially ordered state in stoichiometric Yb2Ti2O7

The nature of the magnetic state below a first order transition at $T_c = 265$ mK in the Quantum Spin Ice Yb$_2$Ti$_2$O$_7$ is hotly debated. It has been proposed as a Quantum Spin Liquid (QSL) ground state, but some studies find evidence for long range ferromagnetic order; results seemingly vary from sample to sample. We will present low temperature neutron measurements on a polycrystalline sample of Yb$_2$Ti$_2$O$_7$ that is known to be stoichiometric. Our measurements reveal 1) there is a change of intensity at nuclear Bragg positions upon warming which does not occur sharply at $T_c$, and which involves an ordered moment size of $\sim$1.1muB (58\% of the saturation moment) and 2) the inelastic excitations below $T_c$ suggest the presence of dispersive modes coexisting with incoherent low energy fluctuations. The data will be compared to Yb$_2$Sn$_2$O$_7$, which shows nearly identical behavior via inelastic neutron scattering. Our results suggest that the ground state in nominally pure Yb$_2$Ti$_2$O$_7$ and Yb$_2$Sn$_2$O$_7$ is not a conventionally ordered ferromagnet, but instead involves only partial polarization of the magnetic moments coexisting with a disordered component, a situation reminiscent of the partially polarized QSL called the Coulomb Ferromagnetic phase.