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
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
In this thesis I present my research on the physics of some important processes in the production of thin films. I studied physical vapour deposition (PVD) and thin film modification through ion... Show moreIn this thesis I present my research on the physics of some important processes in the production of thin films. I studied physical vapour deposition (PVD) and thin film modification through ion bombardment using a newly developed, high-speed scanning tunneling microscope (STM). The instrument has the special property that it can be tilted and azimuthally rotated to allow atom or ion beams a direct line-of-sight access to the region of the surface that is being imaged by the STM tip. With the microscope I have recorded STM movies (available as supplementary material) that offer a unique insight into the atomic surface processes that occur during thin film growth and ion beam sputtering. The __real-time STM__ was applied to the study of some key steps in the fabrication of Mo-Si multilayer optical coatings. I have investigated the non-idealities of these optics, i.e. the alloying of Mo and Si during deposition and the surface roughness formation of a deposited layer. Furthermore, I successfully used the STM to find a possibility to smooth a rough Mo layer after its deposition by means of ion bombardment. Show less
This thesis describes an STM study of the creation, diffusion and annihilation of missing atoms, so-called surface vacancies, in the Cu(100) surface. Because of the extremely high mobility of... Show moreThis thesis describes an STM study of the creation, diffusion and annihilation of missing atoms, so-called surface vacancies, in the Cu(100) surface. Because of the extremely high mobility of surface vacancies in combination with their extremely low density, we have been forced to use tracer particles, in form of indium atoms incorporated in the topmost copper layer, in order to investigate the behavior of the surface vacancies. In this study we have employed tailor-made geometries in the copper surface, in which indium atoms were surrounded exclusively by upward or by downward terrace ledges. Our STM movies show a striking difference between these two cases, with differences in jump frequencies and average jump lengths of more than one order of magnitude. This allowed us to determine that surface vacancies are primarily created and annihilated at the upper side of terrace ledges, which can be formulated, in analogy with the energetics of ad-atoms, in terms of an Ehrlich-Schwoebel barrier for surface vacancies. Dedicated low-temperature measurements, where surface vacancies have been artificially created, have directly revealed the diffusion characteristics of individual surface vacancies. These measurements allow us to construct the complete energy landscape for the birth, life and death of surface vacancies in Cu(100). Show less
