Activated hopping transport in nematic conducting aerogel at low temperatures

TL;DR

This study investigates low-temperature hopping electron transport in nematic aerogels with graphene-coated nanofibers, revealing a transition from variable range hopping to nearest neighbor hopping as the graphene shell thickness decreases.

Contribution

It demonstrates how the dimensionality of hopping transport and energy dependence of localized states change with graphene shell thickness, elucidating the transition from VRH to NNH in these materials.

Findings

Resistivity follows VRH model with variable exponent α

α approaches 1 as graphene shell becomes thinner

Negative magnetoresistance increases with decreasing shell thickness

Abstract

The transport properties of nematic aerogels, which consist of highly oriented Al$_2$O$_3\cdot$SiO$_2$ nanofibers coated with a graphene shell with a large number of defects, are studied. The temperature dependences of the electrical resistivity in the range of 9-40K strictly follow the formula derived to describe the variable range hopping (VRH) conductivity, in which exponent $\alpha$ changes from 0.4 to 0.9 when the number of layers in the graphene shell decreases from 4-6 to 1-2. The dependence of $\alpha$ on the shell thickness can be explained by a simultaneous change in the dimensionality of hopping transport and the character of the energy dependence of the density of localized states near the Fermi level. The fact that $\alpha$ approaches unity at the minimum graphene shell thickness indicates a gradual transition from VRH transport to nearest neighbor hopping (NNH) transport. The magnetoresistance measured at T = 4.2 K is negative, increases significantly with decreasing graphene shell thickness, and is approximated by a formula for the case of weak localization with a good accuracy. The phase coherence lengths are in a reasonable relation with the graphene grain sizes. The conducting aerogels under study complement the well-known set of materials that exhibit hopping electron transport at low temperatures, which is characteristic of media with strong carrier localization, and also a negative magnetoresistance, which usually manifests itself under weak localization conditions.