Vogel-Fulcher Scaling of the Susceptibility in a Mixed-Crystal Proton Glass

Low-temperature dispersion of the electric susceptibilities in a mixed crystal of ${\mathrm{Rb}}_{1-x}{\left(\mathrm{N}{\mathrm{H}}_4\right)}_x{\mathrm{H}}_2\mathrm{P}{\mathrm{O}}_4$ ($x=0.35\mathrm{}$) is reported. The frustrated proton ordering of this system relates it to spin-glasses. The measurements reveal a broad distribution of relaxation times. The data scale using a temperature-independent distribution of activation energies linked to the relaxation times by a Vogel-Fulcher law with a freezing temperature of 10 K.