Structural stability ofSr2RuO4

The stability of low-lying phonon modes in the layered perovskite superconductor ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ has been investigated by inelastic neutron scattering. The ${\Sigma }_3$ branch, which ends at the (0.5 0.5 0) zone boundary in the mode corresponding to the ${\mathrm{RuO}}_6$ octahedron rotation around $c,$ exhibits a sharp drop near the zone boundary. This clearly indicates that ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ is close to a rotational instability. Although the frequency of the rotational mode depends only little on temperature, it anomalously broadens with increasing temperature. A flat dispersion of the rotation branch along $c$ reveals pronounced two-dimensional character; there is almost no coupling between rotational deformations of neighboring layers. In contrast, the tilt mode in ${\mathrm{La}}_2{\mathrm{CuO}}_4$ that drives the tetragonal-to-orthorhombic phase transition does not exhibit any anomalous behavior in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4.$