Lack of evidence for a singlet crystal-field ground state in the magnetic pyrochlore Tb_2Ti_2O_7

摘要

We present high-resolution inelastic neutron scattering data on the candidate spin liquid Tb_2Ti_2O_7. 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 Tb_2Ti_2O_7 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 et al. [Phys. Rev. B 82, 100420 (2010)] 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.