First-Order Transition in the Spin Dynamics of Geometrically FrustratedYb2Ti2O7

Using neutron diffraction, ${}^{170}\mathrm{Yb}$ Mössbauer and muon spin relaxation spectroscopies, we have examined the pyrochlore ${\mathrm{Yb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$, where the ${\mathrm{Yb}}^{3+}{S}^{\prime }=1/2\mathrm{}$ ground state has planar anisotropy. Below $\sim 0.24\mathrm{K}$, 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 ${\mathrm{Yb}}^{3+}$ 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 temperature independent, indicative of a quantum fluctuation regime.