Fluctuations in SrTiO3near the 105-K Phase Transition

Abstract

The temperature dependence of the EPR linewidth of the ${\mathrm{Fe}}^{3+}$ -$V_{\mathrm{O}}$ pair center in SrTi${\mathrm{O}}_3$ has been investigated above and below $T_c=105.6\mathrm{}$ K. It is shown from the angular dependence that the pronounced line broadening near $T_c$ results from rotational fluctuations of the oxygen octahedra. The analysis yields a changeover from a fast-to a slow-fluctuation regime for $T\to T_c^{+}$. The linewidth has a cusp-shaped behavior close to $T_c$ and is finite at $T_c$. From measurements in samples transforming to monodomains below $T_c$ the anisotropy of the fluctuations is investigated. It is found that they are anisotropic even for temperatures within 10 K above $T_c$, where the point symmetry of the crystal is cubic. In this region, the anisotropy is connected with the increasing regions of correlated motions. Well below $T_c$ it is related to the two distinct soft-mode branches. The results are analyzed in terms of the dynamical form factor $S(\overrightarrow{\mathrm{q}}, \omega )$. Within a certain approximation they are found to be consistent, for temperatures above and below $T_c$, with a value of the critical exponent $\nu$ (for the correlation length $\xi$) of $\nu \simeq 0.65$, together with a two-dimensionally anisotropic static-order-parameter susceptibility $\chi (\overrightarrow{\mathrm{q}}, T)$ with an anisotropy parameter $\Delta$ of the order of $\frac{1}{40}$. The changeover from fast- to slow-fluctuation regime yields an estimate of the half-width of the central peak in $S(\overrightarrow{\mathrm{q}}, \omega )$ of 70 MHz at $T=T_c+0.8\mathrm{}\pm 0.3$ K.