Temperature and Magnetic Field Dependence of Spin-Ice Correlations in the Pyrochlore Magnet Tb^2^Ti2O7

1 2 ; 1 2 ; 1 2 scattering develops strongly below 275 mK, and correlates with the opening of a spin gap of 0:06 0:08 meV over most of the Brillouin zone. The concomitant low-lying magnetic spin excitations are weakly dispersive and appear to soften near the 1 ; 1 ; 1 wave vector at 80 mK. The nature of the transition at 275 mK has many characteristics of spin glass behavior, consistent with ac-susceptibility measurements. The application of a magnetic eld of 0.075 T applied along the