Quantum order by disorder and accidental soft mode Er2Ti2O7

Abstract

Motivated by recent neutron scattering experiments, we derive and study an effective "pseudo-dipolar" spin-1/2 model for the XY pyrochlore antiferromagnet Er2Ti2O7. 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 non-coplanar 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.

5 pages, 4 figures, final version

References in corpus (16)

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• Spin Waves and Quantum Criticality in the Frustrated XY Pyrochlore Antiferromagnet Er2Ti2O7
• Energetic selection of ordered states in a model of the Er2Ti2O7 frustrated pyrochlore XY antiferromagnet
• Soft modes in the easy plane pyrochlore antiferromagnet
• Magnetic excitations in the XY-pyrochlore antiferromagnet Er${}_2$Ti${}_2$O${}_7$
• Titanium pyrochlore magnets: how much can be learned from magnetization measurements?
• Field evolution of the magnetic structures in Er${}_2$Ti${}_2$O${}_7$ through the critical point

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• Order by Disorder Spin Wave Gap in the XY Pyrochlore Magnet Er2Ti2O7
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• Nature of finite-temperature transition in anisotropic pyrochlore Er2Ti2O7
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