  The significance of spin-lattice coupling in the phase diagram of the quantum
spin-icepyrochlore Tb2Ti2O7 has been a topic of debate for some time. Here, we
focus on the aspect of vibronic coupling, which occurs between the Tb3+
electronic levels and transverse acoustic phonons, by studying their dependence
on a magnetic field applied along the cubic h111i direction. Our experimental
THz spectroscopy measurements, combined with quantitative theoretical quantum
calculations, show that indeed vibronic effects are observed at 3 K. An
analysis incorporating quadrupolar spin-lattice effects in the Hamiltonian is
therefore relevant in this compound, which is no longer optically isotropic but
magnetically birefringent.
