qdependence of the central peak in the inelastic-neutron-scattering spectrum ofSrTiO3

The recent discovery, by high-resolution x-ray-diffraction measurements, of two length scales in ${\mathrm{SrTiO}}_3$ motivated a reinvestigation by neutron scattering of the soft phonon and associated central peak above the zone-boundary structural-phase-transition temperature (${\mathit{T}}_{\mathit{c}}$=99.5 K). The central peak has an anisotropic q width which is determined by the anisotropy of the slopes of the dispersion curves. The correlation length derived from the energy-integrated intensities of the phonon and the central peak agree well with the short length scale measured in the x-ray studies. The sharp component, corresponding to the long length scale observed in the x-ray experiment just above ${\mathit{T}}_{\mathit{c}}$, is absent in the neutron-scattering measurement. Since the neutron experiment probes the total sample volume, the absence of the narrow component supports the view that it originates in the near-surface region of the crystal.