Order by Disorder and Energetic Selection of the Ground State in the XY   Pyrochlore Antiferromagnet Er2Ti2O7. An Inelastic Neutron Scattering Study
  Examples of materials where an "order by disorder" mechanism is at play to select a particular ground state are scarce. It has recently been proposed, however, that the antiferromagnetic XY pyrochlore Er2Ti2O7, reveals a most convincing case of this mechanism. Observation of a spin gap at zone centers has recently been interpreted as a corroboration of this physics. In this paper, we argue, however, that the anisotropy generated by the interaction-induced admixing between the crystal-field ground and excited levels provides for an alternative mechanism. It especially predicts the opening of a spin gap of about 15 micro-eV, which is of the same order of magnitude as the one observed experimentally. We report new high resolution inelastic neutron scattering data which can be well understood within this scenario. 
S. Petit, J. Robert, S. Guitteny, P. Bonville, C. Decorse, J.   Ollivier, H. Mutka, M.J.P. Gingras, I. Mirebeau
cond-mat.str-el