Non-Ohmic hopping transport in a-YSi: from isotropic to directed percolation
Résumé
(Received PACS numbers: 05.60+w-64.60.Ak-72.15.Rn-72.20.Ht Electrical transport has been investigated in amorphous Y 0.19 Si 0.81 , from 30 mK to room temperature. Below 2 K, the conductance G exhibits Efros-Shklovskii behavior G ∼ exp[−(T 0 /T) 1/2 ] at zero electric field, where conduction is expected to occur along a very sinuous path (isotropic percolation). The non-linear I-V characteristics are systematically studied up to very high fields, for which the conductance no longer depends on T and for which the current paths are expected to be almost straight (directed percolation). We show that the contributions of electronic and sample heating to those non-linearities are negligible. Then, we show that the conductance dependence as a function of low electric fields (F/T < 5000 V m −1 K −1) is given by G(F, T) = G(0, T) × exp(−eF L/k B T). The order of magnitude (5-10 nm) and the T dependence (∼ T −1/2) of L agrees with theoretical predictions. From the T 0 value and the length characterizing the intermediate field regime, we extract an estimate of the dielectric constant of our system. The very high electric field data do not agree with the prediction I(F) ∼ exp[−(F 0 /F) γ′ ] with γ′ = 1/2 : we find a F dependence of γ′ which could be partly due to tunneling across the mobility edge. In the intermediate electric field domain, we claim that our data evidence both the enhancement of the hopping probability with the field and the influence of the straightening of the paths. The latter effect is due to the gradual transition from isotropic to directed percolation and depends essentially on the statistical properties of the " returns " i.e. of the segments of the paths where the current flows against the electrical force. The critical exponent of this returns contribution which, up to now was unknown both theoretically and experimentally, is found to be β = 1.15 ± 0.10. An estimation of the returns length is also given.
Domaines
Physique [physics]
Origine : Fichiers produits par l'(les) auteur(s)
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