Creep and relaxation of domain walls under ac electric fields are observed in an ideal model system, periodically poled ${\mathrm{KTiOPO}}_4$, to occur in different regimes, which are separated by dynamic phase transitions at frequencies $f_m(T)=f_{m0}\exp (-\Delta E/k_{B}T)$, with $f_{m0}=3\times {10}^9\mathrm{Hz}$ and $\Delta E=0.6\mathrm{eV}$. Power law dispersion of the creep susceptibility, $\chi \propto 1+(i\omega \tau {)}^{-\beta }$, with $\beta \approx 0.4$, and large nonlinearity encountered at $f<f_m$, is contrasted with Cole-Cole-type relaxational dispersion, $\chi \propto (1+[i\omega \tau {]}^{1-\alpha }{)}^{-1}$, with $\alpha \approx 0.3$, at $f>f_m$.

• Received 26 October 2004

DOI:https://doi.org/10.1103/PhysRevLett.94.117601