STRUCTURAL STABILITY OF SR2RUO4

The stability of low-lying phonon modes in the layered perovskite supe rconductor Sr2RuO4 has been investigated by inelastic neutron scatteri ng. The Sigma(3) branch, which ends at the (0.5 0.5 0) zone boundary i n the mode corresponding to the RuO6 octahedron rotation around c, exh ibits 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 anomalou sly broadens with increasing temperature. A flat dispersion of the rot ation branch along c reveals pronounced two-dimensional character; the re is almost no coupling between rotational deformations of neighborin g layers. In contrast, the tilt mode in La2CuO4 that drives the tetrag onal-to-orthorhombic phase transition does not exhibit any anomalous b ehavior in Sr2RuO4.