Quantum order by disorder and accidental soft mode in Er2Ti2O7.

  title={Quantum order by disorder and accidental soft mode in Er2Ti2O7.},
  author={M. E. Zhitomirsky and Mariya V Gvozdikova and Peter C W Holdsworth and Roderich Moessner},
  journal={Physical review letters},
  volume={109 7},
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… 

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