Kate Ross, Lucile Savary, Leon Balents, Hanna Dabkowska, Yiming Qiu, John Copley and Bruce Gaulin


The nature of the low temperature magnetism in the pyrochlore material, Yb2Ti2O7, has been controversial.  Effective spin-1/2 quantum spins are associated with Yb3+ in Yb2Ti2O7. These are ferromagnetically coupled to each other, and possess XY-like symmetry. This should imply that Yb2Ti2O7 possesses a non-frustrated, conventional ground state, in spite of its underlying lattice having a frustration-inducing tetrahedral motif.  In contrast, many experimental studies have indicated a disordered ground state below a "transition" temperature (Tc~240mK), marked by a specific heat anomaly.  Our neutron scattering results on Yb2Ti2O7, obtained at the NIST Center for Neutron Research, show intriguing short range, low dimensional spin correlations.  Application of a modest magnetic field at T=30mK induces a quantum phase transition to an ordered, polarized state with sharp features appearing in the inelastic neutron scattering. We have successfully described the observed spin wave excitations in the polarized state using a model that relies on a strong anisotropy of nearest neighbor exchange. This model provides an explanation for the geometrically frustrated magnetism in Yb2Ti2O7.  The model also places Yb2Ti2O7 tantalizingly close to two versions of the long sought after Quantum Spin Liquid ground state.