Investigation of Vibron Bound State in Tb₂Ti₂O₇

After the discovery that the rare earth pyrochore Tb₂Ti₂O₇ supports a cooperative paramagnetic phase down to 50 mK, its ground state remains controversial as antiferromagnetic order was expected from pure spin models. An effect that might contribute greatly to the Hamiltonian is the presence of strong magnetoelastic coupling. Various couplings have been identified in the excitation spectrum of Tb₂Ti₂O₇, including a coupling between the first crystal field excitation (CEF1) and acoustic phonons. In some cerium-based intermetallics, coupling between crystal field excitations and optical phonons produces vibrational bound states or vibrons. A similar vibron formed by the coupling of the third crystal field level (CEF3) and an optical phonon was proposed in Tb₂Ti₂O₇ on the basis of inelastic neutron scattering (INS) experiments. Three distinct peaks are visible, the temperature dependence for which distinguishes the CEF3 from two magnetoelastic optic modes (MEOMs) derived from the optical phonon. We present a detailed investigation of vibron modes in a single crystal of Tb₂Ti₂O₇ using polarized and field dependent INS, which clearly demonstrate the presence of the phonon and the tuning of the coupling by modifying the quadrupolar matrix element of the crystal field transition.