Double Vibronic process in the quantum spin ice candidate Tb$_2$Ti$_2$O$_7$ revealed by terahertz spectroscopy

TL;DR

This study uncovers a double vibronic process in Tb$_2$Ti$_2$O$_7$ using terahertz spectroscopy, revealing strong coupling between magnetic and lattice degrees of freedom that may drive quantum fluctuations in the spin liquid state.

Contribution

It demonstrates the existence of a unique double vibronic process in Tb$_2$Ti$_2$O$_7$, linking lattice motion to quantum spin fluctuations through hybrid excitations.

Findings

Hybrid crystal electric field-phonon excitations at 0.67 THz and 0.42 THz.

Strong coupling between magnetic and lattice degrees of freedom.

Lattice motion as a potential driver of quantum spin flips.

Abstract

The origin of quantum fluctuations responsible for the spin liquid state in Tb$_2$Ti$_2$O$_7$ has remained a long standing problem. By synchrotron-based terahertz measurements, we show evidence of strong coupling between the magnetic and lattice degrees of freedom that provides a path to the quantum melting process. As revealed by hybrid crystal electric field-phonon excitations that appear at 0.67 THz below 200 K, and around 0.42 THz below 50 K, the double vibronic process is unique for Tb$^{3+}$ in the titanate family due to adequate energy matching and strong quadrupolar moments. The results suggest that lattice motion can indeed be the driving force behind spin flips within the hybridized ground and first excited states, promoting quantum terms in the effective Hamiltonian that describes Tb$_2$Ti$_2$O$_7$.