Quantum transport in gapped graphene

This thesis focuses on investigating local and nonlocal transport properties in hexagonal boron nitride (hBN)/graphene superlattice Hall bars and graphene Hall bars proximity coupled to the ferrimagnetic insulator yttrium iron garnet (\text{Y}₃\text{Fe}₅\text{O}₁₂ or YIG). The first part describes in detail the pulse laser deposition of atomically flat YIG thin films onto single crystal gadolinium gallium garnet with a magnetization of 144 emu cm^-3. This part also outlines device fabrication procedures including graphene exfoliation and dry transfer, electron beam lithography and metallization of side-contacts, and finally the electrical setup for measuring local and nonlocal transport in graphene. The second part investigates transport properties in hBN/graphene/hBN superlattice Hall bars with a field-effect mobility of up to 220,000 cm² V^-1 s^-1 at 9 K with low charge impurities. By aligning hBN and graphene, a ∼33.7 meV band gap at 9 K is demonstrated at the primary Dirac point in zero magnetic field. Furthermore, the nonlocal resistances approach h/2e², where $h$ is Planck’s constant and e is the electron charge. Nonlocal measurements demonstrate that, below 60 K a spin-degenerate ballistic counter-propagating edge state forms and dominates with a possible secondary contribution from a network of one-dimensional conducting channels with soliton-like domain walls. The spin-degenerate ballistic edge states offer possibilities for electronic applications beyond quantum spin and anomalous Hall effects since a quantized resistance is observed through valley coupling. The third part reports a proximity-induced magnetic exchange field in graphene of the order 60 T by placing graphene on the ferrimagnetic insulator YIG. From electrical transport measurements, the magnetic order and energy gap of the edge modes in graphene are tunable, and a transition between the canted antiferromagnetic and spin-polarized ferromagnetic $ν = 0$ quantum Hall states can be achieved with relatively low magnetic fields ($> 6$ T) at 2.7 K. The fourth part summarizes the key results of the thesis.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2020

Author dc:creator

• Li, Yang

Advisor dc:contributor.advisor

• Robinson, Jason

• Graphene
• Quantum effect
• Edge state
• Nonlocal transport
• Proximity effect

Licence

• https://www.repository.cam.ac.uk/bitstreams/837f6e32-e5bd-4b6b-81fc-011301190621/download
• https://www.rioxx.net/licenses/all-rights-reserved/

Language dc:language

en

DOI dc:identifier.doi

https://doi.org/10.17863/CAM.48578

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/301509