We employ an optical pump-probe technique to study coherent phonon oscillations in Ca2RuO4. We find that oscillation-amplitude of an Ag 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∘ change across the temperature. Density functional theory calculations indicate that the Ag 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.
@article{arxiv.1808.06026,
title = {Abnormal $phase$ flip in coherent phonon oscillations of Ca$_2$RuO$_4$},
author = {Min-Cheol Lee and Choong H. Kim and Inho Kwak and J. Kim and S. Yoon and Byung Cheol Park and Bumjoo Lee and F. Nakamura and C. Sow and Y. Maeno and T. W. Noh and K. W. Kim},
journal= {arXiv preprint arXiv:1808.06026},
year = {2018}
}