Order from structural disorder in $XY$ pyrochlore antiferromagnet $\rm Er_2Ti_2O_7$

TL;DR

This paper investigates how structural disorder, such as vacancies and weak exchange fluctuations, influences magnetic order in the XY pyrochlore antiferromagnet Er2Ti2O7, revealing a mechanism that can stabilize specific magnetic structures.

Contribution

It introduces the concept of 'order by structural disorder' in XY pyrochlore antiferromagnets and predicts impurity-induced stabilization of a particular magnetic state in Er2Ti2O7.

Findings

Structural disorder lifts degeneracy in the ground states.

Nonmagnetic impurities stabilize the coplanar ψ3 state.

Order by structural disorder competes with thermal and quantum fluctuations.

Abstract

Effect of structural disorder is investigated for an $XY$ pyrochlore antiferromagnet with continuous degeneracy of classical ground states. Two types of disorder, vacancies and weakly fluctuating exchange bonds, lift degeneracy selecting the same subset of classical ground states. Analytic and numerical results demonstrate that such an "order by structural disorder" mechanism competes with the effect of thermal and quantum fluctuations. Our theory predicts that a small amount of nonmagnetic impurities in $\rm{Er_2Ti_2O_7}$ will stabilize the coplanar $\psi_3$ ($m_{x^2-y^2}$) magnetic structure as opposed to the $\psi_2$ ($m_{3z^2-r^2}$) state found in pure material.