This thesis presents insights from our study of various correlated electron systems with a scanning tunneling microscope (STM). In ordinary metals, electron-electron interactions exist, but get... Show moreThis thesis presents insights from our study of various correlated electron systems with a scanning tunneling microscope (STM). In ordinary metals, electron-electron interactions exist, but get substantially screened due to the sheer number of electrons. We therefore describe electrons in ordinary metals as a gaseous state of free, or weakly interacting charged particles. This adequately explains their properties, however, this picture does not work for correlated electron materials. The prominent electron-electron interactions present in these materials enable a wide range of exotic electronic phenomena, some of which are presented in this work. In chapter 2, we present STM results measured on an overdoped copper oxide compound and show that the superconducting state that occurs in these materials cannot be described by conventional BCS theory, contrary to what is commonly believed. In chapter 3, we study twisted bilayer graphene devices, and quantify their local twist angle and strain on the nanoscale. In chapter 4, we describe how to build and characterize the hardware needed to do noise spectroscopy measurements in a conventional, low temperature STM setup. Finally, in chapter 5, we present our noise spectroscopy measurements on Sr2IrO4, and explain how random telegraph noise could lead to the observed noise enhancements. Show less
In twisted bilayer graphene (TBG) a moiré pattern forms that introduces a new length scale to the material. At the 'magic' twist angle of 1.1°, this causes a flat band to form, yielding emergent... Show moreIn twisted bilayer graphene (TBG) a moiré pattern forms that introduces a new length scale to the material. At the 'magic' twist angle of 1.1°, this causes a flat band to form, yielding emergent properties such as correlated insulator behavior and superconductivity [1-4]. In general, the moiré structure in TBG varies spatially, influencing the local electronic properties [5-9] and hence the outcome of macroscopic charge transport experiments. In particular, to understand the wide variety observed in the phase diagrams and critical temperatures, a more detailed understanding of the local moiré variation is needed [10]. Here, we study spatial and temporal variations of the moiré pattern in TBG using aberration-corrected Low Energy Electron Microscopy (AC-LEEM) [11,12]. The spatial variation we find is lower than reported previously. At 500°C, we observe thermal fluctuations of the moiré lattice, corresponding to collective atomic displacements of less than 70pm on a time scale of seconds [13], homogenizing the sample. Despite previous concerns, no untwisting of the layers is found, even at temperatures as high as 600°C [14,15]. From these observations, we conclude that thermal annealing can be used to decrease the local disorder in TBG samples. Finally, we report the existence of individual edge dislocations in the atomic and moiré lattice. These topological defects break translation symmetry and are anticipated to exhibit unique local electronic properties. Show less
Transport experiments in twisted bilayer graphene haverevealed multiple superconducting domes separated by cor-related insulating states 1–5 . These properties are generallyassociated with strongly... Show moreTransport experiments in twisted bilayer graphene haverevealed multiple superconducting domes separated by cor-related insulating states 1–5 . These properties are generallyassociated with strongly correlated states in a flat mini-bandof the hexagonal moiré superlattice as was predicted by bandstructure calculations 6–8 . Evidence for the existence of a flatband comes from local tunnelling spectroscopy 9–13 and elec-tronic compressibility measurements 14 , which report two ormore sharp peaks in the density of states that may be asso-ciated with closely spaced Van Hove singularities. However,direct momentum-resolved measurements have proved to bechallenging 15 . Here, we combine different imaging techniquesand angle-resolved photoemission with simultaneous real- andmomentum-space resolution (nano-ARPES) to directly mapthe band dispersion in twisted bilayer graphene devices nearcharge neutrality. Our experiments reveal large areas with ahomogeneous twist angle that support a flat band with a spec-tral weight that is highly localized in momentum space. The flatband is separated from the dispersive Dirac bands, which showmultiple moiré hybridization gaps. These data establish thesalient features of the twisted bilayer graphene band structure. Show less
