Quantum transport in gapped graphene

Change log
Li, Yang 

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 (Y3Fe5O12 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.

Robinson, Jason
Graphene, Quantum effect, Edge state, Nonlocal transport, Proximity effect
Doctor of Philosophy (PhD)
Awarding Institution
University of Cambridge
Cambridge Trust and China Scholarship Committee