Lack of evidence for a singlet crystal-field ground state in the magnetic pyrochlore Tb₂Ti₂O₇

We present high-resolution inelastic neutron scattering data on the candidate spin liquid Tb₂Ti₂O₇. We find that there is no evidence for a zero-field splitting of the ground-state doublet within the 0.2 K resolution of the instrument. This result contrasts with a pair of recent works claiming that the spin-liquid behavior of Tb₂Ti ₂O₇ can be attributed to a 2 K split singlet-singlet single-ion spectrum at low energies. We also reconsider the entropy argument presented by Chapuis as further evidence of a singlet-singlet crystal-field spectrum. We argue that estimates of the low-temperature residual entropy drawn from specific heat measurements are a poor guide to the single-ion spectrum without a proper understanding of the nature of the correlations in the spin-liquid regime.