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arXiv · 1808.06026

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

Min-Cheol Lee·Choong H. Kim·Inho Kwak·J. Kim·S. Yoon·Byung Cheol Park·Bumjoo Lee·F. Nakamura·C. Sow·Y. Maeno·T. W. Noh·K. W. Kim
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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.

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Min-Cheol Lee, Choong H. Kim, Inho Kwak, J. Kim, S. Yoon, Byung Cheol Park, Bumjoo Lee, F. Nakamura, C. Sow, Y. Maeno, T. W. Noh, K. W. Kim. 2018-08-18. Abnormal $phase$ flip in coherent phonon oscillations of Ca$_2$RuO$_4$. https://doi.org/10.1103/physrevb.98.161115

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