Jeudi 27 Septembre 2012 à 14h
Salle Remy Lemaire, K223
"Quantum order by disorder in XY pyrochlore Er2Ti2O7".
Mike E. Zhitomirsky (SPSMS - INAC CEA Grenoble)
Motivated by recent neutron scattering experiments, we investigate an effective “pseudodipolar” spin-1/2 model for the XY pyrochlore antiferromagnet Er2Ti2O7. While a bond-dependent 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.