PhD Oral Exam - Andrew McRae, 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.
Graphene Quantum Strain Transistors And Two-In-One Carbon Nanotube Quantum Transistors
Graphene and carbon nanotubes are ideal for strain engineering in quantum nanoelectromechanical systems due to their long coherence lengths, mechanical strength, and sensitivity to deformations. Mechanical strain induces scalar (δμ_ε) and vector (A) potentials, which directly tune the Hamiltonian, providing precise control of the energy, momentum, and quantum state of electrons in these materials. This strain-tunability could be used to completely suppress ballistic transmission in graphene quantum strain transistors (GQSTs), generate large pseudomagnetic fields (∇×A), or carry quantum information (valleytronics). Thus far, experimental challenges have prevented thorough exploration of quantum transport strain engineering (QTSE). To this end, we have constructed low temperature (T~1K) QTSE instrumentation. Incorporating fabrication methods for ultra-short (~10nm), suspended carbon nanotube and graphene devices, we predict tunable uniaxial strains up to ≈1-10% using this instrumentation.
We first determined the impact of ultra-short channel lengths on transport by measuring unstrained nanotube devices. These formed "two-in-one" quantum transistors with drastically different behaviour for electrons and holes. In a small bandgap nanotube (≈50meV) we observed ballistic transport for electrons, and quantum dot (QD) behaviour for holes, while in larger bandgap nanotubes (≈300meV), we measured asymmetric QD behaviour between electrons and holes. We showed that this transport asymmetry is caused by electron doping in the nanotube contacts, and is greatly enhanced in ultra-short devices. With these contact effects in mind, we developed a realistic applied theoretical model for transport in uniaxially strained ballistic GQSTs. We calculated conductivity for strained ballistic graphene, and found four transport signatures: gate-shifting of the data from the scalar potential, and strong suppression of conductivity, modification of electron-hole conductivity asymmetry, and a rich resonance spectrum from the vector potential. We calculated on/off ratios >〖10〗^4 in realistically achievable GQSTs at sufficient strains. Using our strain instrumentation, we measured transport in strained graphene, observing unambiguously the effects of strain-induced vector and scalar potentials. In graphene QDs, we observed gate-shifting of the charge states with strain, consistent with strong strain-tunable pseudomagnetic fields. In a strained ballistic graphene device, we observed four expected transport signatures discussed above, and using our model, we found extremely good semi-quantitative agreement between theory and experiment.