Magnetoelastic excitations in the pyrochlore spin liquid Tb$_2$Ti$_2$O$_7$

TL;DR

This paper reveals that in Tb$_2$Ti$_2$O$_7$, a spin liquid, a hybrid magnetoelastic excitation forms due to coupling between crystal field levels and phonons, suppressing magnetic order and structural distortion.

Contribution

It demonstrates the existence of hybrid magnetoelastic excitations in Tb$_2$Ti$_2$O$_7$, introducing the concept of a magnetoelastic spin liquid with propagating bosonic excitations.

Findings

Discovery of hybrid magnetoelastic excitations in Tb$_2$Ti$_2$O$_7$

Hybridization vanishes in the paramagnetic state

Suppression of magnetic order and structural distortion

Abstract

At low temperatures, Tb$_2$Ti$_2$O$_7$ enters a spin liquid state, despite expectations of magnetic order and/or a structural distortion. Using neutron scattering, we have discovered that in this spin liquid state an excited crystal field level is coupled to a transverse acoustic phonon, forming a hybrid excitation. Magnetic and phononlike branches with identical dispersion relations can be identified, and the hybridization vanishes in the paramagnetic state. We suggest that Tb$_2$Ti$_2$O$_7$ is aptly named a "magnetoelastic spin liquid" and that the hybridization of the excitations suppresses both magnetic ordering and the structural distortion. The spin liquid phase of Tb$_2$Ti$_2$O$_7$ can now be regarded as a Coulomb phase with propagating bosonic spin excitations.