Evidence of an improper displacive phase transition in Cd$_2$Re$_2$O$_7$ via time-resolved coherent phonon spectroscopy

TL;DR

This study uses ultrafast spectroscopy and theoretical modeling to reveal that the phase transition in Cd$_2$Re$_2$O$_7$ is electronically driven and not due to phonon softening, challenging previous interpretations.

Contribution

It demonstrates that the structural distortion at the transition is secondary and driven electronically, supported by the absence of phonon softening and anomalous phonon lifetime behavior.

Findings

No phonon softening observed at transition

Phonon lifetime decreases linearly to zero near Tc

Supports electronically driven phase transition mechanism

Abstract

We have used a combination of ultrafast coherent phonon spectroscopy, ultrafast thermometry, and time-dependent Landau theory to study the inversion symmetry breaking phase transition at $T_c = 200$ K in the strongly spin-orbit coupled correlated metal Cd$_2$Re$_2$O$_7$. We establish that the structural distortion at $T_c$ is a secondary effect through the absence of any softening of its associated phonon mode, which supports a purely electronically driven mechanism. However, the phonon lifetime exhibits an anomalously strong temperature dependence that decreases linearly to zero near $T_c$. We show that this behavior naturally explains the spurious appearance of phonon softening in previous Raman spectroscopy experiments and should be a prevalent feature of correlated electron systems with linearly coupled order parameters.