ON THE LENGTH SCALES OF THE CRITICAL FLUCTUATIONS IN SRTIO3

Using synchrotron radiation we have performed a high wavevector resolution study of the X-ray critical scattering above the cubic-to-tetragonal structural phase transition in SrTiO3. The high incident X-ray flux enabled us to follow the thermal evolution of the critical scattering up to approximately Tc + 10 K, whereas previous high-resolution studies using a rotating anode X-ray source were restricted to working within 2 K of Tc. In contrast to earlier studies, our results show that even when using high resolution there are two length scales in the observed critical scattering: one diffuse component associated with the R-point soft mode; and a second sharper component which has a Lorentzian-squared line shape, isotropic in wavevector, and a width which narrows to the instrumental resolution as T → Tc. These qualitative features of the second component are consistent with the calculated effect of random fields on a weakly first-order transition. © 1990.