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arXiv · 0810.2816

Phonon softening and anomalous mode near the $x_{c}=0.5$ quantum critical point in Ca$_{2-x}$Sr$_{x}$RuO$_4$

R. G. Moore·M. D. Lumsden·M. B. Stone·Jiandi Zhang·Y. Chen·J. W. Lynn·R. Jin·D. Mandrus·E. W. Plummer
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Abstract

Inelastic neutron scattering is used to measure the temperature dependent phonon dispersion in Ca$_{2-x}$Sr$_{x}$RuO$_{4}$ ($x=0.4$, 0.6). The in-plane $Σ_{4}$ octahedral tilt mode softens significantly at the zone boundary of the high temperature tetragonal (HTT) \textit{I4}$_{\mathit{1}}$\textit{/acd} structure as the temperature approaches the transition to a low temperature orthorhombic (LTO) \textit{Pbca} phase. This behavior is similar to that in La$_2$CuO$_4$, but a new inelastic feature that is not found in the cuprate is present. An anomalous phonon mode is observed at energy transfers greater than the $Σ_{4}$ albeit with similar dispersion. This anomalous phonon mode never softens below $\sim 5$ meV, even for temperatures below the HTT-LTO transition. This mode is attributed to the presence of intrinsic structural disorder within the \textit{I4}$_{\mathit{1}}$\textit{/acd} tetragonal structure of the doped ruthenate.

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R. G. Moore, M. D. Lumsden, M. B. Stone, Jiandi Zhang, Y. Chen, J. W. Lynn, R. Jin, D. Mandrus, E. W. Plummer. 2008-10-15. Phonon softening and anomalous mode near the $x_{c}=0.5$ quantum critical point in Ca$_{2-x}$Sr$_{x}$RuO$_4$. https://doi.org/10.1103/physrevb.79.172301

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