06/08/2026
Doctoral Thesis Defense: Shengjie Yu
Date & Time: June 9, 2026, 1:00 PM
Location: Brockman Hall for Physics BRK 200
Host Department: ECE
Advisor: Prof. Junichiro Kono
Committee: Prof. Geoff Wehmeyer, Prof. Douglas Natelson, Prof. Matteo Pasquali
Title: Electronic Transport in Aligned Carbon Nanotube Assemblies
Abstract:
Carbon nanotubes (C**s) are widely used to study quantum transport in low-dimensional systems because of their one-dimensional electronic structure and long phase-coherence lengths. In practical materials, however, C**s assemble into bundles, fibers, and aligned films, where intertube coupling, disorder, anisotropy, and current-path distribution reshape transport signatures. This dissertation examines how phase-coherent transport appears across this hierarchy in macroscopic CNT fibers, exfoliated CNT bundles, and aligned CNT films, using temperature-dependent transport, magnetotransport, nonlocal measurements, and angular-dependent magnetotransport.
In highly aligned CNT fibers, the direct observations are metallic high-temperature transport and a pronounced positive low-temperature magnetoconductance with field dependence characteristic of weak localization (WL). These measurements show that phase-coherent corrections remain observable in a macroscopic CNT conductor. Fits to the magnetoconductance also show that single-dimensional WL models are insufficient. An effective mixed-dimensional interpretation, combining bundle-scale quasi-one-dimensional coherence with a more extended diffusive contribution, provides a constrained phenomenological description of the fiber response.
Individual CNT bundles exfoliated from the fibers reveal WL-like low-field magnetoconductance, reproducible universal conductance fluctuations (UCF), and nonlocal field-dependent signals over micron-scale separations. Comparing these probes gives the central conceptual result: the WL field scale, UCF amplitude, and nonlocal response define different operational coherence scales. The corresponding interpretation is trajectory dependent, with short, flux-sensitive diffusive loops coexisting with a subset of pathways that remain correlated over longer portions of the bundle.
Aligned CNT films show how current direction, channel length, and field orientation control transport in a planar anisotropic network. Perpendicular transport is hopping-dominated and exhibits a low-temperature dimensional crossover, whereas parallel transport is more conductive and shows low-field magnetotransport compatible with a phenomenological WL-like description. Long-channel devices average over network anisotropy more strongly, while shorter channels make directional magnetotransport more visible; angular-dependent magnetotransport further distinguishes angularly averaged response from anisotropic flux sensitivity.
Overall, the thesis shows that phase-coherent signatures remain experimentally observable in aligned CNT assemblies, but their interpretation depends on the observable and measurement geometry.
Zoom link:
https://riceuniversity.zoom.us/j/92604535244?pwd=vwzT4vkEbkPaMpYsQ6LrBcbXmBawOU.1
Meeting ID: 926 0453 5244
Passcode: 466907
