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We present time-of-flight inelastic neutron scattering measurements at lowtemperature on powder samples of the magnetic pyrochlore oxides Tb2Ti2O7 andTb2Sn2O7. These two materials possess related, but different ground states,with Tb2Sn2O7 displaying "soft" spin ice order below Tn~0.87 K, while Tb2Ti2O7enters a hybrid, glassy spin ice state below Tg~0.2 K. Our neutronmeasurements, performed at T=1.5 K and 30 K, probe the crystal field statesassociated with the J=6 states of Tb3+ within the appropriate Fd\bar{3}mpyrochlore environment. These crystal field states determine the size andanisotropy of the Tb3+ magnetic moment in each material's ground state,information that is an essential starting point for any description of thelow-temperature phase behavior and spin dynamics in Tb2Ti2O7 and Tb2Sn2O7.While these two materials have much in common, the cubic stanate lattice isexpanded compared to the cubic titanate lattice. As our measurements show, thistranslates into a factor of ~2 increase in the crystal field bandwidth of the2J+1=13 states in Tb2Ti2O7 compared with Tb2Sn2O7. Our results are consistentwith previous measurements on crystal field states in Tb2Sn2O7, wherein theground state doublet corresponds primarily to m_J=|\pm 5> and the first excitedstate doublet to mJ=|\pm 4>. In contrast, our results on Tb2Ti2O7 differmarkedly from earlier studies, showing that the ground state doubletcorresponds to a significant mixture of mJ=|\pm 5>, |\mp 4>, and |\pm 2>, whilethe first excited state doublet corresponds to a mixture of mJ=|\pm 4>, |\mp5>, and |\pm 1>. We discuss these results in the context of proposed mechanismsfor the failure of Tb2Ti2O7 to develop conventional long-range order down to 50mK.

Neutron spectroscopic study of crystal field excitations in Tb2Ti2O7 and Tb2Sn2O7