Theses Doctoral

Scanning Tunneling Microscopy of Three Twisted Graphene Heterostructures and the Two-Dimensional Heavy Fermion Material CeSiI

Turkel, Simon Eli

The exploration of physical extremes drives technological innovation. Recent decades have seen a push towards materials engineering at the absolute limit of space with electronic systems that are a single atom thick. When electrons are confined to two-dimensional structures, exotic and often unexpected phenomena emerge due to enhanced interaction effects and crystalline anisotropies. The study of such unconventional phenomena offers the opportunity to extend knowledge of fundamental physics with an eye towards advancing the state of the art in control over quantum matter.

In this thesis we use scanning tunneling microscopy to study the electronic structure of a collection of novel two-dimensional materials: twisted double-bilayer graphene (TDBG), mirror symmetric twisted trilayer graphene (TTG), small angle twisted double trilayer graphene (TDTG), and the van der Waals heavy Fermion material CeSiI. In TDBG, we directly image spontaneous symmetry breaking of the electronic states as a function of carrier density and attribute this to an intrinsic nematic instability of the metallic Fermi liquid.

In TTG, we find evidence for a novel form of lattice relaxation, in which twist angle disorder leads to the formation of moiré lattice defects that can act to lock trilayer devices into a magic angle configuration while strongly modulating the local electronic structure, with implications for the superconducting state. In TDTG, we discover yet another form of lattice relaxation in which a global transformation of the stacking structure creates a net energy reduction, even while the stacking energy density in roughly half of the moiré lattice rises.

Lastly, we show through quasiparticle interference spectroscopy and theoretical modeling that CeSiI hosts a nodal hybridization between itinerant conduction electrons and a lattice of local moments, giving rise to a strong angular dependence of the heavy Fermion mass enhancement in this van der Waals material.

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More About This Work

Academic Units
Physics
Thesis Advisors
Pasupathy, Abhay
Degree
Ph.D., Columbia University
Published Here
October 18, 2023