Structural Stability Of Sr2ruo4

The stability of low-lying phonon modes in the layered perovskite superconductor Sr2RuO4 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 RuO6 octahedron rotation around c, exhibits a sharp drop near the zone boundary. This clearly indicates that Sr2RuO4 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 La2CuO4 that drives the tetragonal-to-orthorhombic phase transition does not exhibit any anomalous behavior in Sr2RuO4.