The ac conductivity σ(ω) of three ionically conducting crystalline materials (${\mathrm{NaClZn}}^{2+}$, ${\mathrm{KTaO}}_3$:${\mathrm{Co}}^{3+}$,${\mathrm{H}}^{+}$, and ${\mathrm{CeO}}_2$:${\mathrm{Gd}}^{3+}$) and two glasses have been studied over an unusually wide temperature range. In all cases, limiting power-law behavior [exponent s=1 in σ(ω)=${\mathrm{\sigma }}_0$+A${\mathrm{\omega }}^{\mathit{s}}$] is observed at relatively low temperatures. It is concluded that this limiting behavior is a universal phenomenon. Available theoretical explanations have been examined and found to be unsatisfactory.

• Received 22 May 1991

DOI:https://doi.org/10.1103/PhysRevLett.67.1559