Non-ohmic hopping transport in a-YSi: from isotropic to directed percolation

TL;DR

This study investigates variable range hopping in amorphous YSi, revealing how electric fields influence conduction paths from isotropic to directed percolation, with experimental results aligning with some theoretical predictions and highlighting new path segment statistics.

Contribution

It provides the first analysis of path segments where current opposes the electric field, and compares experimental data with theoretical models across different field regimes.

Findings

Conductance follows Efros-Shklovskii law at zero field.

Lattice length scale L matches theoretical predictions at low fields.

High field data deviate from theory, possibly due to tunneling effects.

Abstract

Variable range hopping transport has been investigated in amorphous YSi from 30 mK to 2 K as a function of the temperature T and electric field F. For F=0, where conduction is along sinuous paths (isotropic percolation), the conductance G depends on T according to an Efros-Shklovskii law. The nonlinear I-V's were studied up to very high fields for which G no longer depends on T (directed percolation : current paths along F). We show that heating effects are negligible. Then, we show that for low F values, ln(G(F,T))=ln(G(0,T))-eFL/kT. The order of magnitude (5-10 nm) and the T dependence of L agree with theoretical predictions. From L, we extract the dielectric constant. The very high electric field data do not agree with the theoretical predictions, which could be due to tunneling across the mobility edge. For intermediate electric fields our data evidence the influence of the straightening of the paths (transition from isotropic to directed percolation). The statistical properties of the segments of the paths where the current flows against the electrical force are extracted for the first time.