Orbital susceptibility and magnetic switching in twisted moiré heterostructures
department or school: Science, Faculty of
department or school: Physics and Astronomy, Department of
data provider: DSpace
degree campus: UBCV
creator: Egan, Shannon
issued: 2022-08-30T20:54:05Z
issued: 2022
related degree: Master of Science - MSc
degree grantor: University of British Columbia
description: Twisted moiré materials have opened up a new platform for realizing the quantum anomalous Hall (QAH) effect, with experiments reporting its observation in both twisted graphene and transition metal dichalcogenide heterostructures. A fascinating feature of these moiré QAH devices is the ability to switch the direction of Hall resistivity with very small magnetic fields or currents, making them a candidate for stable and programmable magnetic memory. Describing this phenomenon theoretically requires knowledge of both the orbital magnetization and orbital susceptibility, of which the latter is less well understood. This motivated us to derive a numerical formula for the zero-field orbital susceptibility of a general multi-band moiré continuum model, expressed in terms of quantities which reflect the geometry of the moiré bands. We use this formula to study the conditions for which magnetic switching can occur in moiré bands of twisted homobilayer MoS₂. We predict that the conduction band states of twisted MoS₂ give rise to two pairs of flat Chern bands in each valley: |C| = ±1 bands generated by moiré potential alone; and |C| = ±2 bands generated by a combination of moiré potential and spin-orbit coupling. All of the flat bands carry a large orbital magnetization ~1 Bohr magneton per moiré unit cell. Our Hartree-Fock analysis shows that at 1/2-filling, interactions generically drive the system into a spontaneous time-reversal broken valley-polarized state, yielding an "orbital Chern insulator" with quantized Hall resistivity. By calculating the Fermi sea contribution to orbital susceptibility, we show that the paramagnetism necessary to induce magnetic switching in the orbital Chern insulator state is confined to the gap between the |C| = ±2 Chern bands. The chemical potential and temperature dependence of the orbital susceptibility may help explain outstanding puzzles in the behaviour of moiré quantum Hall devices.
aggregated CHO: https://circle.library.ubc.ca/rest/handle/2429/82610
note: Orbital susceptibility and magneticswitching in twisted moire´heterostructuresbyShannon EganB.Sc., McGill University, 2020A THESIS SUBMITTED IN PARTIAL FULFILLMENT OFTHE REQUIREMENTS FOR THE DEGREE OFMASTER OF SCIENCEinThe Faculty of Graduate and Postdoctoral Studies(Physics)THE UNIVERSITY OF BRITISH COLUMBIA(Vancouver)August 2022© Shannon Egan 2022The following individuals certify that they have read, and recommend to the Faculty ofGraduate and Postdoctoral Studies for acceptance, the thesis entitled:Orbital susceptibility and magnetic switching in twisted moire´ heterostructuressubmitted by Shannon Egan in partial fulfillment of the requirements forthe degree of Master of Sciencein PhysicsExamining Committee:Marcel Franz, Professor, Physics, UBCSupervisorMona Berciu, Professor, Physics, UBCSupervisory Committee MemberiiAbstractTwisted moire´ materials have opened up a new platform for realizing the quantum anoma-lous Hall (QAH) effect, with experiments reporting its observation in both twisted grapheneand transition metal dichalcogenide heterostructures. A fascinating feature of these moire´QAH devices is the ability to switch the direction of Hall resistivity with very small mag-netic fields or currents, making them a candidate for stable and programmable magneticmemory. Describing this phenomenon theoretically requires knowledge of both the orbitalmagnetization and orbital susceptibility, of which the latter is less well understood. Thismotivated us to derive a numerical formula for the zero-field orbital susceptibility of ageneral multi-band moire´ continuum model, expressed in terms of quantities which reflectthe geometry of the moire´ bands. We use this formula to study the conditions for whichmagnetic switching can occur in moire´ bands of twisted homobilayer MoS2. We predictthat the conduction band states of twisted MoS2 give rise to two pairs of flat Chern bandsin each valley: C = ±1 bands generated by moire´ potential alone; and C = ±2 bandsgenerated by a…
has type: Thesis/Dissertation
date issued: 2022-11
is shown at: 10.14288/1.0418440
language: eng
degree discipline: Physics
provider: Vancouver : University of British Columbia Library
publisher: University of British Columbia
rights: Attribution-NonCommercial-NoDerivatives 4.0 International
rights URI: http://creativecommons.org/licenses/by-nc-nd/4.0/
scholar level: Graduate
contributor: Franz, Marcel
type: Text
identifier URI: http://hdl.handle.net/2429/82610