Adjudicating Conduction Mechanisms in High Performance Carbon Nanotube Fibers

TL;DR

This study investigates the conduction mechanisms in high-performance carbon nanotube fibers, revealing heterogeneous fluctuation-induced tunneling, the effects of doping, and magnetic field responses, ultimately predicting their superior conductivity potential.

Contribution

It provides a comprehensive experimental and theoretical analysis of conduction in CNT fibers, highlighting the role of heterogeneity, doping, and magnetic effects in their transport properties.

Findings

Semi-conducting-like response becomes temperature-independent near absolute zero.

De-doping causes localized hopping conduction contrasting metallic behavior.

Magneto-resistance shows significant anisotropy and Aharonov-Bohm-like effects.

Abstract

The performance of carbon nanotube (CNT) cables, a contender for copper-wire replacement, is tied to its metallic and semi-conducting-like conductivity responses with temperature; the origin of the semi-conducting-like response however is an underappreciated incongruity in literature. With controlled aspect-ratio and doping-degree, over 61 unique cryogenic experiments including anisotropy and Hall measurements, CNT cable performance is explored at extreme temperatures (65 mK) and magnetic fields (60 T). A semi-conducting-like conductivity response with temperature becomes temperature-independent approaching absolute-zero, uniquely demonstrating the necessity of heterogeneous fluctuation induced tunneling; complete de-doping leads to localized hopping, contrasting graphite's pure metallic-like response. High-field magneto-resistance (including +22% longitudinal magneto-resistance near room-temperature) is analyzed with hopping and classical two-band models, both similarly yielding a parameter useful for conductor development. Varying field-orientation angle uncovers two-and four-fold symmetries from Aharonov-Bohm-like corrections to curvature-induced bandgap. Tight-binding calculations using Green's Function formalism model large-scale, coherent transport in commensurate CNT bundles in magnetic field, revealing non-uniform transmission across bundle cross-sections with doping restoring uniformity; independent of doping, transport in bundle-junction-bundle systems are predominantly from CNTs adjacent to the other bundle. The final impact is predicting the ultimate conductivity of heterogeneous CNT cables using temperature and field-dependent transport, surpassing conductivity of traditional metals.