Neutron spectroscopic study of crystal field excitations in Tb2Ti2O7 and Tb2Sn2O7

We present time-of-flight inelastic neutron scattering measurements at low temperature on powder samples of the magnetic pyrochlore oxides Tb2Ti2O7 and Tb2Sn2O7. These two materials possess related, but different ground states, with Tb2Sn2O7 displaying soft spin ice order below T-N similar to 0.87 K, while Tb2Ti2O7 enters a hybrid, glassy spin ice state below T-g similar to 0.2 K. Our neutron measurements, performed at T = 1.5 and 30 K, probe the crystal field states associated with the J = 6 states of Tb3+ within the appropriate Fd (3) over barm pyrochlore environment. These crystal field states determine the size and anisotropy of the Tb3+ magnetic moment in each material's ground state, information that is an essential starting point for any description of the low-temperature phase behavior and spin dynamics in Tb2Ti2O7 and Tb2Sn2O7. While these two materials have much in common, the cubic stanate lattice is expanded compared to the cubic titanate lattice. As our measurements show, this translates into a factor of similar to 2 increase in the crystal field bandwidth of the 2J + 1 = 13 states in Tb2Ti2O7 compared with Tb2Sn2O7. Our results are consistent with previous measurements on crystal field states in Tb2Sn2O7, wherein the ground-state doublet corresponds primarily to m(J) = vertical bar +/- 5 > and the first excited state doublet to m(J) = vertical bar +/- 4 >. In contrast, our results on Tb2Ti2O7 differ markedly from earlier studies, showing that the ground-state doublet corresponds to a significant mixture of m(J) = vertical bar +/- 5 >, vertical bar -/+ 4 >, and vertical bar +/- 2 >, while the first excited state doublet corresponds to a mixture of m(J) = vertical bar +/- 4 >, vertical bar -/+ 5 >, and vertical bar +/- 1 >. We discuss these results in the context of proposed mechanisms for the failure of Tb2Ti2O7 to develop conventional long-range order down to 50 mK.