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Abnormal phase flip in the coherent phonon oscillations of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>Ca</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi>RuO</mml:mi><mml:mn>4</mml:mn></mml:msub></mml:mrow></mml:math>
We employ an optical pump-probe technique to study coherent phonon oscillations in ${\mathrm{Ca}}_{2}{\mathrm{RuO}}_{4}$. 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 …