Magnetoelastic Excitations in the Pyrochlore Spin Liquid Tb₂Ti₂O₇

Tb₂Ti₂O₇ is often referred to as a spin liquid as it does indeed remain in a magnetically disordered phase with spin dynamics down to 0.05 K, but this itself is a surprise since there are strong expectations of magnetic order and/or a structural distortion. However, throughout the spin liquid regime there are also strong signs of magnetoelastic coupling, leading to the suggestion that both spin and structural degrees of freedom are frustrated. Using a combination of time-of-flight spectroscopy at the ILL and triple axis spectroscopy with (TASP+MuPAD at PSI) and without (EIGER at PSI) polarization analysis, we have discovered that 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 Tb₂Ti₂O₇ 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 Tb₂Ti₂O₇ can now be regarded as a Coulomb phase with propagating bosonic spin excitations.