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Tb₂Ti₂O₇ is unique rare-earth pyrochlore in that it does not exhibit theoretically predicted long-range magnetic order down to the lowest measured temperatures below 100 mK. Instead, a fluctuating spin-liquid state with short-range correlations persists, while strong spin-lattice coupling occurs. As a manifestation of this coupling, the two lowest crystal electric field (CEF) level doublets separated by approximately 1.5 meV = 0.42 THz have been shown to couple to a transverse acoustic phonon. Such a vibronic process is accompanied by hybrid magnetoelastic excitations, which have been previously detected by neutron scattering and THz spectroscopy.
We measured absorption of THz radiation in a single crystal at the temperatures 3 K and 60 K combined with magnetic fields up to B = 15 T applied along the local [111] axis. The B dependence of the crystal field excitations gives us the opportunity to accurately describe the CEF spectrum including the vibronic process, and to see how it is transformed by a static magnetic field.