Order by Disorder and Energetic Selection of the Ground State in the XY Pyrochlore Antiferromagnet Er2Ti2O7. An Inelastic Neutron Scattering Study

TL;DR

This study investigates the ground state selection mechanism in Er2Ti2O7, showing that anisotropic interactions can explain the observed spin gap, challenging the previous order-by-disorder interpretation.

Contribution

It demonstrates that anisotropic crystal-field interactions can account for the spin gap in Er2Ti2O7, providing an alternative to the order-by-disorder mechanism.

Findings

Observed spin gap of about 15 micro-eV matches predictions from anisotropic interactions.

High-resolution neutron scattering data supports the anisotropic interaction scenario.

Challenges the previous interpretation of the spin gap as evidence of order-by-disorder.

Abstract

Examples of materials where an "order by disorder" mechanism is at play to select a particular ground state are scarce. It has recently been proposed, however, that the antiferromagnetic XY pyrochlore Er2Ti2O7, reveals a most convincing case of this mechanism. Observation of a spin gap at zone centers has recently been interpreted as a corroboration of this physics. In this paper, we argue, however, that the anisotropy generated by the interaction-induced admixing between the crystal-field ground and excited levels provides for an alternative mechanism. It especially predicts the opening of a spin gap of about 15 micro-eV, which is of the same order of magnitude as the one observed experimentally. We report new high resolution inelastic neutron scattering data which can be well understood within this scenario.