Abnormal $phase$ flip in coherent phonon oscillations of Ca$_2$RuO$_4$

TL;DR

This study uses optical pump-probe techniques to investigate coherent phonon oscillations in Ca$_2$RuO$_4$, revealing a phase flip and amplitude suppression near a transition temperature linked to orbital ordering.

Contribution

It uncovers a temperature-dependent phase flip in phonon oscillations and links it to lattice distortions and orbital ordering, providing new insights into phase transitions in Ca$_2$RuO$_4$.

Findings

Amplitude suppression of phonon mode at 260 K.

180-degree phase flip of oscillations across the transition.

Large antipolar distortion in low-temperature structure.

Abstract

We employ an optical pump-probe technique to study coherent phonon oscillations in Ca$_2$RuO$_4$. We find that oscillation-amplitude of an $A_g$ symmetric phonon mode is strongly suppressed at 260 K, a putative transition point of orbital ordering. The oscillation also shows a gradual but huge change in its $phase$, such that the oscillation even flips over with a 180$^{\circ}$ change across the temperature. Density functional theory calculations indicate that the $A_g$ phonon has an eigenmode of octahedral distortion with conventional tilting along the $a$-axis and antipolar distortion of apical oxygen. Careful inspection of the lattice captures an unusually large antipolar distortion in low-temperature structures, which may play a crucial role for the phase transition at 260 K.