Abstract
We present time-of-
ight inelastic neutron scattering measurements at low temperature on powder
samples of the magnetic pyrochlore oxides Tb2Ti2O7 and Tb2Sn2O7. These two materials possess
related, but di erent ground states, with Tb2Sn2O7 displaying \soft" spin ice order below TN
0:87 K, while Tb2Ti2O7 enters a hybrid, glassy-spin ice state below Tg 0:2 K. Our neutron
measurements, performed at T = 1:5 K and 30 K, probe the crystal eld states associated with
the J = 6 states of Tb3+ within the appropriate Fd3m pyrochlore environment. These crystal eld
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 2 increase in the crystal eld bandwidth of the
2J +1 = 13 states in Tb2Ti2O7 compared with Tb2Sn2O7. Our results are consistent with previous
measurements on crystal eld states in Tb2Sn2O7, wherein the ground state doublet corresponds
primarily to mJ = j5i and the rst excited state doublet to mJ = j4i. In contrast, our results on
Tb2Ti2O7 di er markedly from earlier studies, showing that the ground state doublet corresponds
to a signi cant mixture of mJ = j5i, mJ = j4i and mJ = j2i, while the rst excited state
doublet corresponds to a mixture of mJ = j4i, mJ = j5i and mJ = j1i. 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.
