Nonanalytic polarization response of a dipolar glass

The polarization response of the dipolar glass KTa_1-xNbₓO₃ ($x=0.009$) over a range of temperature (1.8 K<T<15 K) near Tg is found to be a nonanalytic function of field, revealing the inadequacy of the usual nonlinear susceptibility description, as well as the incorrectness of even the linear response previously reported in such systems. A model based upon activated switching of two-level systems with a barrier distribution extending to zero energy adequately describes our data. Nonanalytic response may be a general feature of dipolar glasses, as well as other random systems, including magnetic spin glasses.