Critical Dynamics in SrTio3from Paramagnetic Resonance

Paramagnetic resonance line shapes and linewidths of the ${\mathrm{Fe}}^{3+}$-$V_0$ center in SrTi${\mathrm{O}}_3$ above $T_c$ permit the determination of an effective relaxation rate $\Delta {\omega }_l\mathrm{}\left(T\right)\mathrm{}$ for the local rotations of the oxygen octahedra by observation of the crossover from Lorentzian to Gaussian line shapes with different settings of the applied field. We find that $\Delta {\omega }_l$ decreases as $T\to {T_c}^{+}$, clearly indicating critical slowing down. In addition, the relaxation rate ${\Gamma }_c$ for the $R$-point (order-parameter) collective mode is estimated from the line-widths in the region of Lorentzian shapes, indicating a "central peak" of width ${\Gamma }_c\approx 0.6\times {10}^8 invsec\approx 0.1\Delta {\omega }_l$ at $T=T_c+2\mathrm{}$ K.