  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.
