Toward understanding the second universality—A journey inspired by Arthur Stanley Nowick

Abstract

In 1991, Arthur Stanley Nowick and his co-workers discovered a new universality, now known as nearly constant loss (NCL) or second universality. This mini-review, written in honor of A.S. Nowick, reports on the present authors’ more recent endeavors and advances on their way toward understanding the second-universality phenomenon. In pursuit of that goal, new ideas and new data have led us to new questions and new answers. The essence of the new ideas was to consider time-dependent single-particle potentials, caused by Coulomb interactions and experienced by locally mobile ions. The dynamics of such ions were described in terms of a rate equation, and model conductivity spectra showing the NCL effect could be derived from it, with the help of linear response theory. New data corroborated the predictions made by the model. Also, our experimental data suggested a gradual transition, occurring with decreasing temperature, from a slightly activated NCL-type behavior to the non-activated features of the second universality. Two new questions have emerged. (i) Is it possible to quantify the transition mentioned above? (ii) Is there a crossover of the low-temperature, second-universality ionic conductivity from its characteristic linear frequency dependence to a quadratic one at low frequencies? New answers to our two questions can now be given, both of them in the affirmative. In particular, the crossover in the frequency dependence has been identified as an implication of the localization of the non-activated ionic motion.