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

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.

10 pages, 6 figures, supplementary information included