Anomalous thermal activation of the electron glass dynamics in a-InOx and granular aluminum.

Abstract

In this article, we explore the temperature dependence of the electrical glassy dynamics in insulating amorphous indium oxide (a-InO_x) and granular Al films. We use non-isothermal gate voltage protocols, which can reveal changes in the dynamics induced by the temperature, when logarithmic relaxations devoid of characteristic times are at work. We demonstrate that, contrary to almost 20 years of opposite belief, the dynamics of amorphous indium oxide films in the liquid helium temperature range is thermally activated, i.e. it slows down under cooling and accelerates upon heating. Amorphous indium oxide thus adds to the list of glassy disordered systems in which we already demonstrated thermal activation, which includes granular Al and amorphous Nb_xSi_1-xfilms. Moreover, measurements up to 40 K in a-InO_xand granular Al films reveal the close similarity between the two systems and a very anomalous character of the thermal activation, with an effective activation energy increasing withTasT². We so far have no explanation for it. Its further study and understanding may be important for the physics of electron glasses.