Strain superlattice effects on the electronic properties of graphene
Sarkar, Preetha
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https://hdl.handle.net/2142/127377
Description
Title
Strain superlattice effects on the electronic properties of graphene
Author(s)
Sarkar, Preetha
Issue Date
2024-12-02
Director of Research (if dissertation) or Advisor (if thesis)
Mason, Nadya
Doctoral Committee Chair(s)
Wang, Pengjie
Committee Member(s)
van der Zande, Arend
Fradkin, Eduardo
Department of Study
Physics
Discipline
Physics
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Graphene
Strain
Superlattice
Pseudo-magnetic Field
Electron Transport
Van Der Waals Materials
Mesoscopic Physics
Language
eng
Abstract
Strain superlattices, i.e. non-uniform periodic strain profiles, in graphene have been predicted to lead to novel quantum phenomena through the generation of gauge fields called pseudomagnetic fields (PMF). PMF can lift the sublattice symmetry of graphene, and hence break the valley degeneracy of charge carriers. It can create discrete energy levels in the band structure called pseudo-Landau levels and flatten these energy bands enhancing electron-electron interactions. These PMF-induced effects can be manifested in charge transport through zero-field Hall effect, valley Hall effects, topological edge states etc. However, there have been few experimental realizations of custom, reproducible strain superlattice potentials in graphene to study the effects of PMF in transport. In this thesis, we report extensive transport characterization of a graphene strain superlattice system. By stacking graphene on top of a self-assembly of silica nanospheres, a quasi-periodic strain array was generated in graphene. Previous two-terminal transport measurements have already confirmed the existence of a superlattice in this system. Here we present a detailed study of how the pseudomagnetic fields associated with this superlattice modify the transport properties of graphene. In a collaborative scanning tunneling measurement study, we found that PMF generated in this graphene-NS system is not spatially uniform, rather varies periodically, reaching values as high as 55 T on top of the nanospheres. PMF was observed to lead to charge confinement effects in graphene similar to confinement by quantum dots. Formation of pseudo-Landau levels at both fractional and integer values illustrated the enhanced electron-electron correlations in this system. Through two-terminal transport measurements on a strained graphene nanoribbon, we were able to observe the effects of the individual nanospheres more prominently. Charge transport through the nanoribbon was found to be a result of cotunneling processes through an array of nanosphere-defined quantum dots. An unusual valley-split quantum Hall signal due to Landau quantization was observed at moderate external magnetic fields in this strained nanoribbon, clearly illustrating the role of PMF in generating valley polarized edge currents. Further details of the nature of these valley-split counterpropagating edge currents came from four-terminal transport measurements on strained graphene Hall bars, where, apart from the appearance of Landau quantized features at unconventional integer and fractional filling factors, in both longitudinal and Hall resistance, a magnetic field-asymmetric edge current was also observed, indicating sublattice symmetry breaking. We also explored the possibility of incorporating other functional properties of the nanospheres into the strain superlattice by illustrating a transport experiment in which ferromagnetic iron oxide nanosphere self-assemblies, when layered on top of graphene, proximity induced a magnetic superlattice in graphene. Apart from improving our fundamental understanding of the effects of PMF on charge carriers, this research will also have broader implications in the field of graphene valleytronics, which rely on the generation and manipulation of valley polarized currents in graphene.
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