THESIS
2022
1 online resource (xiii, 115 pages) : illustrations (some color)
Abstract
Two-dimensional moiré superlattices, as represented by twisted graphene and twisted transition
metal dichalcogenides (TMDCs), provide a new platform to investigate many-body correlation
effects. In this thesis, unconventional electronic transport in moiré superlattices is studied. The
three main topics are: the electrically tunable Berry curvature dipole in twisted graphene, the
unconventional superconductivity in twisted WSe
2 (tWSe
2) and the giant nonlinear Hall effect in
tWSe
2.
For twisted graphene, previous studies mainly focused on the global properties of electronic
wavefunctions. Therefore, our knowledge about the local distribution of the Berry curvature
remains limited. In chapter 3, the Berry curvature distribution in twisted graphene is studied
utilizing the nonlinear Hall eff...[
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Two-dimensional moiré superlattices, as represented by twisted graphene and twisted transition
metal dichalcogenides (TMDCs), provide a new platform to investigate many-body correlation
effects. In this thesis, unconventional electronic transport in moiré superlattices is studied. The
three main topics are: the electrically tunable Berry curvature dipole in twisted graphene, the
unconventional superconductivity in twisted WSe
2 (tWSe
2) and the giant nonlinear Hall effect in
tWSe
2.
For twisted graphene, previous studies mainly focused on the global properties of electronic
wavefunctions. Therefore, our knowledge about the local distribution of the Berry curvature
remains limited. In chapter 3, the Berry curvature distribution in twisted graphene is studied
utilizing the nonlinear Hall effect. Enabled by the sliding of the Berry curvature hotspots, the
application of an out-of-plane displacement field substantially changes the direction and amplitude
of the second harmonic Hall signal. This observation makes twisted systems promising platforms for nonlinear applications.
Compared to graphene, TMDCs have larger Wiger-Seitz radius and relatively stronger electron-electron
interactions. In chapter 4, superconductivity in tWSe
2 is studied. I find that the determined
gap to critical temperature ratio exceeds the weak coupling BCS ratio, and the superconducting
coherent length is comparable to the inter-particle distance. These findings provide strong evidence
for correlation-induced unconventional superconductivity in tWSe
2.
In chapter 5, the effect of electron-electron interactions on the nonlinear Hall effect in tWSe
2 is
studied. I find that the nonlinear Hall signal exhibits a sharp peak near the half-filling with a
generation efficiency at least two orders of magnitude greater than those observed in previous
experiments. Then, I demonstrate that the giant signal can be explained by a mass-diverging type
continuous Mott transition. This work demonstrates not only how interaction effects can couple to
Berry curvature dipoles to produce novel quantum phenomena but also the potential of nonlinear
Hall effect measurements as a new tool for studying quantum criticality.
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