acase@tulane.edu

acase@tulane.edu

acase@tulane.edu

Layered ruthenates in the Ruddlesden-Popper series (Sr,Ca) n+1RunO3n+1 exhibit exceptionally rich ground state properties, such as unconventional spin-triplet superconductivity, orbital ordering, and metamagnetic quantum criticality. These unusually rich ground states headline the complex interplay between the charge, spin, lattice and orbital degrees of freedom in ruthenates, and provide fantastic opportunities to study novel quantum phenomena tuned by non-thermal parameters like chemical doping, pressure, and magnetic field We focus on two particular layered ruthenates, each with rich ground state properties. First, Sr4Ru3O10 exhibits an itinerant ferromagnetic ground state, but moderate in-plane magnetic fields induce a metamagnetic transition. The presence of both ferromagnetism and metamagnetism in this material is puzzling and has been the subject of extensive study. The other material studied here, Ca3Ru2O 7 orders antiferromagnetically at 56 K and exhibits a metal-insulator transition at 48 K. With the application of magnetic field this material shows spin-flop transition causing giant magnetoresistance. Neutron scattering studies have revealed a rich magnetic phase diagram with four different magnetic phases In this dissertation we aim to elucidate the orbital dependence of the magnetism in Sr4Ru3O10 and the nature of the complex coupling between spin and charge in Ca3Ru2O 7. In order to achieve these aims we developed a directional angle-resolved magnetotransport technique, as magnetoresistivity anisotropy is a powerful analysis tool. With this technique we revealed the metamagnetism in Sr 4Ru3O10 to be orbital selective, i.e. the metamagnetism and ferromagnetism arise from different orbitals. In Ca3Ru 2O7 we studied the nature of the effects of ferromagnetic and antiferromagnetic coupling on c-axis transport throughout the different magnetic phases in this material by analyzing the magnetoresistivity anisotropy, and have observed a complex interplay between the charge and spin degrees of freedom that were dependent on both temperature and magnetic field By utilizing a relatively simple technique we have probed the nature of the magnetism in two materials. In addition to revealing important physics in these two materials, our results suggest possible avenues of investigation in other ruthenates and strongly correlated systems using the developed technique / acase@tulane.edu

One of the principal aims of theoretical solid state physics is to use an effective one-body Schrodinger equation to predict the energies of various states in atoms, molecules, and solids. The most common procedure involves incorporating the many-body effects of the electrons into a local 'exchange-correlation' potential. Comparison of the resulting one-particle eigenvalues to experimental photoemission data, though, reveals that this local potential happens to place the one-particle energies too high. For insulators and semiconductors, it predicts anomalously small band gaps. And in the case of transition metals, it predicts valence d bands which are too high and too disperse at the end of the transition series One of the problems with the local density approximation is a spurious self-interaction of the electrons which is present due to the approximate nature of its local exchange-correlation potential. This effect can be corrected, resolving the discrepancy for atoms mentioned above. In other words the one-particle eigenvalues agree with experimental removal energies. The self-interaction correction terms, though, are not invariant under unitary transformations of the orbitals. In particular, for a Bloch state in a crystal, they are zero. Previous applications of the self-interaction correction, therefore, usually involved atomic-like schemes or Wannier-orbital methods In this paper, we propose a size-consistent approximation to the self-interaction correction. It reproduces the original SIC results for the energy levels of atoms fairly closely, and also corrects the band gaps in the insulators which were tested, Neon and Sodium Chloride. The method fails in the case of transition metals. Further analysis, though, indicates the necessity to incorporate metallic screening effects into the problem. A crude screened method yields d band positions and dispersions in Copper and Zinc which are a large improvement over their local density counterparts The basic conclusion, then, is that there exists non-zero self-interaction terms in solids in many cases. This implies that in these cases there is a certain degree of localization of the states involved, which is contrary to much of established energy band lore / acase@tulane.edu

The strontium ruthenate Sr3Ru2O 7 is a strongly correlated electron system with intriguing electronic and magnetic properties. Though structurally similar to the widely studied unconventional superconductor Sr2RuO4, its properties are quite different: its ground state appears to be close to a ferromagnetic instability. Moderate applied magnetic fields can induce a metamagnetic transition into a ferromagnetic-like state, defined as a superlinear rise in magnetization at a given value of applied field; there is good evidence that this transition is a zero-temperature quantum phase transition with features distinctly different than ordinary phase transitions. We have taken three avenues towards better understanding the electronic and magnetic properties of Sr3Ru 2O7 and its behavior under magnetic fields: single particle tunneling, doping with nonmagnetic Ti impurities, and critical current measurements of a Sr2RuO4-Sr3Ru2O7 intergrowth Our tunneling measurements reveal an unusual oscillation in tunneling magnetoresistance under applied magnetic fields. This behavior is unexpected within our existing understanding of Sr3Ru2O7, and likely indicates the presence of an unusual surface state on this material. Our studies of Ti doped Sr3Ru2O7 suggest that the ground state of this material is characterized by competing short-range ferromagnetic and antiferromagnetic interactions; nonmagnetic Ti impurities appear to alter the band structure in such a way to reduce the antiferromagnetism, leaving the system in a state dominated by 2D ferromagnetic fluctuations. Finally, our studies of the Sr2RuO4-Sr3Ru 2O7 solid mixture suggest that large regions of Sr3 Ru2O7 may become superconducting when intergrown in this fashion; this finding may also shed light on the pairing mechanism of the unconventional superconductor Sr2RuO4 These three approaches to studying the ground state of the itinerant metamagnet Sr3Ru2O7 reveal the great diversity of electronic and magnetic properties possible in this material. In addition to confirming key issues related to the magnetic ground state, our results suggest several new directions for further study of Sr3Ru 2O7 and related strontium ruthenate materials / acase@tulane.edu

acase@tulane.edu

acase@tulane.edu

Surface properties of various single-crystalline SnO2 surfaces were studied and the growth of palladium was investigated in the low-coverage regime. Metal - oxide structures play an important role in microelectronics and nanotechnology. They are also widely used in catalysis. Small catalytically-active metal particles on metal oxide substrates are key features in the gas sensing mechanism: they dramatically increase the sensitivity and selectivity of solid-state gas sensors towards target gases. Tin Oxide is widely used in solid-state gas sensors for detection of combustible and toxic gases. Its sensitivity and selectivity strongly depends on catalytic dopants, such as Pd or Pt, on the surface of the material. Thus, the characterization of Pd growth on tin oxide may give new insights into the catalytic and gas sensing mechanisms, and also help to understand fundamental steps that lead to various metal-on-oxide growth modes Upon deposition of Pd onto the reduced (101) surface of a SnO2 single crystal, 1D cluster growth was observed. Starting from very low coverages, one-dimensional Pd clusters grow on the terraces, which indicates that the Pd wets the reduced tin oxide surface. Pd deposition on the oxidized surface results in randomly distributed three-dimensional Pd clusters. The clusters are distributed at step edges and on terraces without any apparent preferential adsorption sites The one-dimensional clusters are imaged in scanning tunneling microscopy (STM) as straight, parallel nanostructures oriented along the [-101] direction, all with the same characteristic width of 0.5 nm and a height of 1 monolayer (ML). X-ray photoelectron spectroscopy (XPS) experiments show no sign of Pd oxidation; i.e. Pd grows as a metal. There is a 0.5 eV shift in the Pd 3d 5/2 core level peak position to lower binding energy that occurs during the initial stages of the growth on the reduced surface. This is an indication of charge transfer from the Pd clusters to the substrate. Coverage-dependent Ultraviolet Photoelectron Spectroscopy (UPS) spectra show that, at submonolayer Pd coverages, a Pd 4d-derived peak appears at the same position (3eV from Fermi edge) in the band gap as the Sn surface state and shifts towards the Fermi edge as coverage increases. Angular resolved photoemission data of the valence band of the clean reduced SnO2 surface and the Pd dosed reduced surface shows a strong correlation between the Sn 5s derived surface state and the Pd 4d state. The position, as well as the shape of Pd 4d peak closely follows the position and the shape of the 5s derived Sn peak in both low-index directions. This is a sign of a strong electronic interaction, hybridization between Pd 4d and Sn 5s derived states Scanning tunneling microscopy experiments on a clean, reduced SnO 2 (100)-(1x1) surface reveal surface defects with zero, one, and two dimensions. Point defects consist of missing SnO/SnO2 units. Line defects are probably crystallographic shear planes that extend to the surface and manifest themselves as rows of atoms, shifted half a unit cell along the [010] direction. Their ends act as preferential nucleation sites for the formation of Pd clusters upon vapor-deposition. Submonolayer coverages of Pd deposited on the reduced surface at room temperature by vapour deposition result in the formation of three-dimensional clusters nucleating on the terraces. Areas of a more reduced surface phase, i.e. elongated 'holes', observed at the surface after annealing to higher temperatures, still with a (1x1) structure and a half-unit-cell deep, form at [001]-oriented step edges Recently, the use of nanobelts and nanoribbons has been suggested as novel materials for gas sensing applications. The large surface-to-volume ratio of the semiconducting metal oxide nanobelts and the congruence of the carrier screening length with their lateral dimensions make them highly sensitive and efficient transducers of surface chemical processes into electrical signal The surface morphology of an individual nanobelts (NB) was studied with STM. Atomically resolved STM images of NBs reveal an 1x1 (101) SnO2 structure on the top surface of the NB. To the best of the author's knowledge, this is the first atomically resolved STM image of SnO2 nanobelts. The thermal stability of the NBs was studied with SEM. The critical temperatures were determined, where structural changes occur in UHV, O2, and air. XPS was used to characterize chemical composition and monitor the cleanness of the NB material. Ca and C contamination was detected on as-grown SnO 2 nanobelts. O plasma, ozone treatment, and annealing in oxygen were used to remove the contaminants / acase@tulane.edu

In this work I have studied a number of technologically important magnetic materials using density functional theory. Two ab-initio methods have been employed to investigate several ferromagnetic materials, the Vienna ab-initio Simulation Package and the layer Korringa-Kohn-Rostoker Method. The systems studied include two series of L10 alloys, and Co3Pt (both form natural superlattices) as well as artificially grown Co1Pt5 superlattices. Experimental difficulties associated with the growth of L10 alloys is discussed, along with theoretical calculations of the magnetic properties as a function of content. Another candidate for perpendicular data storage, the orthorhombic derivative Pmm2 phase of Co 3Pt, has also proven difficult to grow as it appears to be metastable. The possibility of phase stabilization has been explored using first principles methods. In a collaborative effort to improve micromagnetics simulations, the way in which local magnetic properties are affected near grain boundaries of Co1Pt5 supercells has been examined. Results of these first principles calculations have been used as input parameters in micromagnetics simulations. Both the first principles calculations and specifics of the micromagnetic simulations are discussed, in addition to calculations of Cr buffer layers acting to magnetically decouple grains. Conclusions are drawn as to the achievement of the stated research goals / acase@tulane.edu