Quantum order by disorder and accidental soft mode in Er2Ti2O7.

Motivated by recent neutron scattering experiments, we derive and study an effective "pseudodipolar" spin-1/2 model for the XY pyrochlore antiferromagnet Er(2)Ti(2)O(7). While a bond-dependent in-plane exchange anisotropy removes any continuous symmetry, it does lead to a one-parameter 'accidental' classical degeneracy. This degeneracy is lifted by quantum fluctuations in favor of the noncoplanar spin structure observed experimentally-a rare experimental instance of quantum order by disorder. A non-Goldstone low-energy mode is present in the excitation spectrum in accordance with inelastic neutron scattering data. Our theory also resolves the puzzle of the experimentally observed continuous ordering transition, absent from previous models.