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

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.