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

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 ${\Sigma }_4$ octahedral tilt mode softens significantly at the zone boundary of the high temperature tetragonal (HTT) \textit{I4}${}_1$\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 ${\Sigma }_4$ 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}${}_1$\textit{/acd} tetragonal structure of the doped ruthenate.

4 pages, 4 figures

References in corpus (4)

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