Speaker
Description
The rare-earth pyrochlore Tb₂Ti₂O₇ (TTO) remains one of the most puzzling frustrated magnetic systems. Although its crystal-electric-field (CEF) scheme and exchange topology nominally favour a classical dipolar spin-ice state, neither spin-ice order nor any long-range magnetism emerges down to tens of millikelvin. Increasing evidence points to the decisive role of magneto-elastic interactions, where hybridisation between the non-Kramers Tb³⁺ CEF levels and low-energy phonons generates a vibronic coupling that can melt dipolar order and promote quadrupolar degrees of freedom. This competition has been proposed to stabilise either a quadrupolar-ice-like state or a vibronic quantum spin liquid, sensitively dependent on stoichiometry. Understanding how such spin-lattice entanglement shapes the low-temperature phase remains an outstanding challenge.
Magneto-optical THz spectroscopy has played a central role in developing this picture by resolving the fine structure and symmetry breaking in the first excited CEF manifold. Under applied magnetic field, these levels split in characteristic ways that reveal the underlying anisotropies induced by the lattice. Building on this foundation, we employ polarisation-resolved pump–probe spectroscopy at the ELBE free-electron laser to selectively drive the vibronically coupled Tb³⁺ excitations with intense 0.7 THz pulses, while tracking the induced magnetisation through ultrafast optical Faraday rotation. This approach enables us to follow, in real time, the decay of individually excited Tb ions within the primitive cell and potentially disentangle linear and nonlinear magnetic responses linked to the hybrid CEF–phonon modes.
To interpret the observed relaxation channels, and field-dependent oscillations, we model the Tb³⁺ single-ion dynamics using modern quantum-optical frameworks implemented in QuTiP. Direct comparison between experiment and theory provides new insight into the microscopic vibronic processes that may underpin quadrupolar frustration and the emergence of unconventional spin-liquid physics in Tb₂Ti₂O₇.
| Field of Condensed Matter | Magnetism |
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