PhD Oral Exam - Linxiang Huang, Physics

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

Single-wall carbon nanotubes (SWCNTs) are narrow ribbons of graphene with atomically precise boundary conditions and a single quantum transport channel at realistic charge dopings. Together with their extreme mechanical strength, wide elastic deformation range, and strong electron-mechanical coupling, these properties make them ideal systems to harness quantum transport straintronics (QTS), i.e. using mechanical strain to control quantum transport.

We first adapted an applied theoretical model to study QTS in uniaxially strained quasi-metallic-SWCNT transistors. Mechanical strain adds both scalar ϕε and vector A gauge potentials to the transistor's Hamiltonian. We demonstrate that these potentials tune the charge carriers' propagation angle (the helix angle with respect to the nanotube axis) and create a rich spectrum of quantum interferences in conductance, which can be described as a mechanical Aharonov-Bohm effect. The charge carriers' quantum phase could be controlled by purely mechanical means.

We then fabricated suspended SWCNT transistors with channel lengths of ≈ 30 nm and acquired QTS data over a broad range (≈ 0 to 4%) of in-situ tunable and reversible uniaxial mechanical strain. We present and analyze these detailed charge transport data showing a large mechanical-gating effect of the SWCNT quantum dots (QDs). The precise reversibility of the data, and their agreement with QTS theory, confirm that the nanotubes were strained elastically. We demonstrate that this mechanical control of the QD doping is not due to capacitive-gating effects, but to predictable bandstructure changes described by ϕε and A.

We also present measurements of quantum transport in strained graphene transistors that agreed quantitatively with models based on ϕε and A. Finally, we observed strain-tunable electro-mechanical coupling in both graphene and SWCNT mechanical oscillators.

Our work opens new opportunities to harness quantitative strain effects in quantum transport of low-dimensional materials and could find applications in qubits, oscillators and other quantum devices.