It is shown that collective glassy behavior occurs not only in strongly substitutionally disordered systems as assumed so far, but also in weakly disordered systems, namely, ${\mathrm{Rb}}_{1-x}{\left({\mathrm{NH}}_4\right)}_x{\mathrm{H}}_2\mathrm{As}{\mathrm{O}}_4$ with $x=0.01\mathrm{}$ and $x=0.02\mathrm{}$. The observed ${}_{}{}^{75}{}_{}{}^{}\mathrm{As}$ NQR and NMR line shapes can only be described quantitatively by the compressible random bond-random field pseudospin Ising model, showing a much greater random bond than random field effect even for $x\to 0$. A discontinuous change in the Edwards-Anderson order parameter has been observed at the transition temperature where ferroelectric long range ordering occurs as predicted by theory.

• Received 5 March 1993

DOI:https://doi.org/10.1103/PhysRevLett.73.276