First Order Transition in the Spin Dynamics of Geometrically Frustrated   Yb2Ti2O7
  Using neutron diffraction, 170Yb Mossbauer and muSR spectroscopies, we have examined the pyrochlore Yb2Ti2O7 where the Yb3+ S'=1/2 ground state has planar anisotropy. Below ~0.24K, the temperature of the known specific heat lambda transition, there is no long range magnetic order. We show that the transition corresponds to a first order change in the fluctuation rate of the Yb3+ spins. Above the transition temperature, the rate, in the GHz range, follows a thermal excitation law, whereas below, the rate, in the MHz range, is essentially temperature independent indicative of a quantum fluctuation regime. 
J.A.Hodges, P.Bonville, A.Forget, A.Yaouanc, P.Dalmas de Reotier,   G.Andre, M.Rams, K.Krolas, C.Ritter, P.C.M.Gubbens, C.T....