In condensed matter systems electron-electron interactions, negligible in everyday metals, can dramatically alter the electronic behavior of the system. Examples of such altered behavior include... Show moreIn condensed matter systems electron-electron interactions, negligible in everyday metals, can dramatically alter the electronic behavior of the system. Examples of such altered behavior include high-temperature superconductivity and modulation of the electron density. A common feature of this correlation driven behavior is the tendency of the spatial electronic structure to vary on the nanometer scale. In this thesis we explore the nanoscale variation of the electronic structure of various correlated electron systems. We use the wave-like oscillations in the electron density of states to probe fundamental properties of the system providing insights into when various experimental probes disagree with each other. Turning our attention to high-temperature superconductors we find that close to the transition between superconductor and metal a granular superconductor emerges, small nanoscale patches of superconductivity interlaces with a metallic matrix. A careful examination of the wave-like oscillations hints at the presence of spatial ordering of the electrons. Finally we study how the presence of strong interactions can alter the way electrons flow through a material such that concepts usually reserved for everyday fluids become relevant. Show less
In this thesis we use Josephson and noise scanning tunneling microscopy for the study of conventional, unconventional (iron-based) and disordered superconductors. On the one hand, Josephson... Show moreIn this thesis we use Josephson and noise scanning tunneling microscopy for the study of conventional, unconventional (iron-based) and disordered superconductors. On the one hand, Josephson scanning tunneling microscopy allows us to directly visualize the superfluid density with high spatial resolution. On the other hand, noise scanning tunneling microscopy is employed for measuring the shot noise which detects the charge of the carriers forming a superconducting condensate. Show less
Materials with strongly correlated electrons show some of the most mysterious and exotic phases of quantum matter, such as unconventional superconductivity, quantum criticality and strange... Show more Materials with strongly correlated electrons show some of the most mysterious and exotic phases of quantum matter, such as unconventional superconductivity, quantum criticality and strange metal phase. In this thesis, we study strongly-correlated electron materials using spectroscopic-imaging scanning tunneling microscopy. We first describe the design and construction of a novel, ultra-stiff, scanning tunneling microscope that is optimized to have the high signal-to-noise ratio required to study these materials. We then present the discovery of the melting of the Mott insulating phase in the iridate Sr2IrO4 upon electron doping, that results in the formation of puddles of pseudogap and charge order. This is striking similar to the cuprate unconventional superconductors and for the first time we show the universality of these phenomena using scanning tunneling microscopy. We moreover discuss the effect of electric field penetration in a poorly conducting sample, and how this affects STM measurements on lightly doped Mott insulators in general. Finally, we show quasiparticle interference measurements on the correlated metal Sr2RhO4, and we discuss its comparison with photoemission results. Show less
In this thesis, the formation of hexagonal boron nitride (h-BN) __nanomesh__ structures and of graphene on Rhodium (111) is studied experimentally. The structures of h-BN and graphene are extremely... Show moreIn this thesis, the formation of hexagonal boron nitride (h-BN) __nanomesh__ structures and of graphene on Rhodium (111) is studied experimentally. The structures of h-BN and graphene are extremely similar: both of them are single atomic layers with a honeycomb lattice, and the lattice constants are nearly identical. Both materials introduce novel properties and have the potential for a variety of applications. In this thesis, the layers were grown by chemical vapor deposition (CVD) on Rh(111). During growth, the formation processes were tracked by scanning tunneling microscopy (STM). This was performed in situ, namely during deposition at the elevated temperatures, required for the growth. In this way, we have obtained detailed knowledge of the formation mechanisms. In this thesis, basic surface science principles are employed to explain the observed, special growth behavior. Our understanding of the mechanisms at play has enabled us to compose new, improved deposition recipes that result in higher quality nanomesh and graphene layers. This knowledge is not only valuable for these specific systems, but it also deepens our general insights into deposition and growth of atomically thin layers. Show less
This thesis presents scanning tunneling spectroscopy (STS) measurements of the spatial distribution of the density of states (DOS) of materials where electron correlations play a role. In the... Show moreThis thesis presents scanning tunneling spectroscopy (STS) measurements of the spatial distribution of the density of states (DOS) of materials where electron correlations play a role. In the manganite La0.67Ca0.33MnO3, the measurements show that flat films with atomically smooth terraces are electronically homogeneous, while rough films with no apparent terraces are electronically inhomogeneous and respond to applied magnetic fields in a way consistent with the percolation model of colossal magnetoresistance. The flat surfaces appear to have an electronic structure different from the bulk due to the change in symmetry at the surface. In La0.5Sr0.5CoO3 films the thickness drives the electronic homogeneity. A film thinner than a critical thickness t__ was electronically inhomogeneous, while a film thicker than t__ was found to be electronically homogeneous. Both films were rough, indicating that surface morphology plays no role here. STS measurements of the DOS of a ferromagnetic/superconducting bilayer (CuNi alloy/Nb) were performed to probe the phase of the superconducting order parameter induced in the ferromagnet. DOS measurements varied from deeply gapped (zero phase) to flat, with no reproducible signs of inverted spectra (_ phase). The seeming anticorrelation between film morphology and spectral character suggests the presence of Ni-clusters in the CuNi layer. STM/S measurements of the quasiparticle DOS of a ferromagnetic/superconducting bilayer (CuNi alloy/Nb) were performed to probe the zero and pi phases of the order parameter induced in the ferromagnet. DOS measurements on one bilayer varied from deeply gapped (zero phase) to flat, with occasional but irreproducible inverted spectra (pi phase). The seeming anticorrelation between film morphology and spectral character suggests the presence of Ni-clusters in the CuNi layer. Show less
