Correlations in moiré heterostructures

Placeholder Show Content


Moiré patterns in van der Waals heterostructures are a readily realizable class of 2D superlattices with electronic properties distinct from those of the parent materials that can be controlled by the composition and relative orientations of layers. The large artificial superlattice generates minibands that disperse on a finer energy scale in a significantly smaller mini Brillouin zone. I begin with transport measurements in highly aligned graphene/hexagonal boron nitride (hBN) heterostructures, a high electron mobility superlattice with miniband edges and Van Hove singularities that are accessible via electrostatic gating. By using recently developed absorptive pinhole collimators, consisting of absorptive sidewalls between a pair of collinear slits, it is possible to populate and detect a narrow window of $k$-states. Low-field transport measurements yield results consistent with ballistic Monte Carlo simulations based on numerical bandstructure calculations when the superlattice has been tuned to exhibit non-circular Fermi surfaces. Subsequent scanning gate measurements spatially image the non-circular Fermi surface, raising the possibility of using absorptive pinhole collimators as geometric valley filters. A number of moiré superlattices have recently emerged that exhibit flat electronic bands, making them ideal for exploring strongly correlated physics. The flat bands in magic-angle twisted bilayer graphene (tBLG) and ABC-trilayer graphene aligned with hBN (ABC-TLG/hBN) have been shown to give rise to correlated insulators [1,2]. An amazing recent discovery is that tBLG becomes superconducting when doped slightly away from two holes per site, reminiscent of high temperature superconductors [3]. Understanding the mechanism of high-transition-temperature (high-$T_c$) superconductivity is a central problem in condensed matter physics. It is highly desirable to study a tunable Hubbard system, in which systematic investigations of the unconventional superconductivity and its evolution with the Hubbard parameters can deepen our understanding of high-$T_c$ superconductivity. I begin with a report of signatures of tunable superconductivity in ABC-TLG/hBN. Unlike in tBLG, theoretical calculations show that under a vertical displacement field, the ABC-TLG/hBN heterostructure features an isolated flat valence miniband associated with a Hubbard model on a triangular superlattice [2,4]. Signatures of superconductivity emerge below 1 kelvin for the electron- and hole-doped sides of the one-quarter-filling correlated state. The electronic behaviour in the ABC-TLG/hBN superlattice is expected to depend sensitively on the interplay between electron--electron interactions and the miniband bandwidth. By varying the vertical displacement field, transitions from the candidate superconductor to correlated insulator and metallic phases are observed. Superconductivity is one of many possible ground states in flat band systems when electron-electron interactions dominate. I present evidence of an incipient Chern insulating state at integer filling of the flat band in both tBLG and ABC-TLG/hBN. Notably, in tBLG, the magnetization of the sample can be reversed by applying a small direct current. Additionally, when tilting a tBLG sample in an external magnetic field, the ferromagnetism is highly anisotropic. Because spin-orbit coupling is negligible in graphene [5], such behavior is unlikely to come from spin, but rather favors theories in which the ferromagnetism is orbital. For an in-plane field larger than $5\ \mathrm{T}$, the out-of-plane magnetization is destroyed, suggesting a transition to a new phase. At integer filling of these strongly correlated moiré systems, it is natural to expect the possibility of spontaneous spin/valley polarization. However, there is the possibility of fractional quantum anomalous Hall insulators at fractional filling if interactions are strong enough. I report evidence of ferromagnetism at fractional filling in both tBLG and ABC-TLG/hBN. These states depend very sensitively on the control parameters in the moiré system, to the point where the results in tBLG were not repeatable between cooldowns of the same device. However, the state in ABC-TLG/hBN is much more robust, exhibiting prominent ferromagnetic hysteresis behavior with large anomalous Hall resistivity in a broad region of density. In ABC-TLG/hBN, not only the magnitude of the anomalous Hall signal, but also the sign of the hysteretic ferromagnetic response can be modulated by tuning the carrier density and displacement field. The plethora of interesting ground states found thus far highlights the exciting opportunities for exploring strongly correlated physics enabled in moiré superlattices.


Type of resource text
Form electronic resource; remote; computer; online resource
Extent 1 online resource.
Place California
Place [Stanford, California]
Publisher [Stanford University]
Copyright date 2020; ©2020
Publication date 2020; 2020
Issuance monographic
Language English


Author Sharpe, Aaron Layne
Degree supervisor Goldhaber-Gordon, David, 1972-
Thesis advisor Goldhaber-Gordon, David, 1972-
Thesis advisor Kastner, Marc A
Thesis advisor Kivelson, Steven
Degree committee member Kastner, Marc A
Degree committee member Kivelson, Steven
Associated with Stanford University, Department of Applied Physics


Genre Theses
Genre Text

Bibliographic information

Statement of responsibility Aaron Sharpe.
Note Submitted to the Department of Applied Physics.
Thesis Thesis Ph.D. Stanford University 2020.
Location electronic resource

Access conditions

© 2020 by Aaron Layne Sharpe
This work is licensed under a Creative Commons Attribution Share Alike 3.0 Unported license (CC BY-SA).

Also listed in

Loading usage metrics...