Abstract
We report on non equilibrium field effect in insulating amorphous NbSi thin films having different Nb contents and thicknesses. The hallmark of an electron glass, namely the logarithmic growth of a memory dip in conductance versus gate voltage curves, is observed in all the films after a cooling from room temperature to 4.2~K. A very rich phenomenology is demonstrated. While the memory dip width is found to strongly vary with the film parameters, as was also observed in amorphous indium oxide films, screening lengths and temperature dependence of the dynamics are closer to what is observed in granular Al films. Our results demonstrate that the differentiation between continuous and discontinuous systems is not relevant to understand the discrepancies reported between various systems in the electron glass features. We suggest instead that they are not of fundamental nature and stem from differences in the protocols used and in the electrical inhomogeneity length scales within each material.
Highlights
In the past two decades, intriguing out-of-equilibrium phenomena have been reported in the electrical conductance G of several disordered insulating systems [1]
The evolution of the conductance observed at a given Vg can be seen as the sum of two separate contributions: one coming from the part of the film within Lsc from the gate insulator which is influenced by the applied Vg, and another one coming from the rest of the film which is insensitive to Vg changes and which pursues it relaxation induced by the initial cooling
We observed significant background conductance relaxations in 12.5 nm thick films, which reflect a spatial extent of the gate voltage disturbance of only few nanometres
Summary
In the past two decades, intriguing out-of-equilibrium phenomena have been reported in the electrical conductance G of several disordered insulating systems [1]. In granular Al films it was demonstrated that the conductance relaxation triggered by a Vg change to a pristine value is not exactly logarithmic with time and depends on the total time elapsed since the cooling [5] This age dependence of the dynamics or ageing effect underlines the strong similarity between these out-of equilibrium phenomena and what is seen in structural and spin glasses. The large penetration length observed in InOx films was taken as an experimental evidence for electronic avalanches found after a charge injection in numerical simulations of Coulomb glass models [30,31] They seem to contradict the very short screening lengths sometimes invoked in this system [13, 29]. The NbSi results are discussed in light of what has been found in other disordered insulating systems
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