Year of Publication

2018

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Arts and Sciences

Department

Physics and Astronomy

First Advisor

Dr. Ambrose Seo

Abstract

The layered iridates such as Sr2IrO4 and Sr3Ir2O7, have attracted substantial attention due to their novel electronic states originating from strong spin-orbit coupling and electron-correlation. Recent studies have revealed the possibilities of novel phases such as topological insulators, Weyl semimetals, and even a potential high-Tc superconducting state with a d-wave gap. However, there are still controversial issues regarding the fundamental electronic structure of these systems: the origin of the insulating gap is disputed as arising either from an antiferromagnetic ordering, i.e. Slater scheme or electron-correlation, i.e. Mott scheme. Moreover, it is a formidable task to unveil the physics of layered iridates due to the limited number of available materials for experimental characterizations.

One way to overcome this limit and extend our investigation of the layered iridates is using metastable materials. These materials which are far from their equilibrium state, often have mechanical, electronic, and magnetic properties that different from their thermodynamically stable phases. However, these materials cannot be synthesized using thermodynamic equilibrium processes. One way to synthesize these materials is by using pulsed laser deposition (PLD). PLD is able to generate nonequilibrium material phases through the use of substrate strain and deposition conditions. Using this method, we have synthesized several thermodynamically metastable iridate thin-films and have investigated their electronic and optical properties. Synthesizing and investigating metastable iridates opens a path to expand the tunability further than the ability of the bulk methods.

This thesis consists of four studies on metastable layered iridate thin film systems. In the first study, three-dimensional Mott variable-range hopping transport with decreased characteristic temperatures under lattice strain or isovalent doping has been observed in Sr2IrO4 thin films. Application of lattice strain or isovalent doping exerts metastable chemical pressure in the compounds, which changes both the bandwidth and electronic hopping. The variation of the characteristic temperature under lattice strain or isovalent doping implies that the density of states near the Fermi energy is reconstructed. The increased density of states in the Sr2IrO4 thin films with strain and isovalent doping could facilitate a condition to induce unprecedented electronic properties, opening a way for electronic device applications. In the second study, the effects of tuning the bandwidth via chemical pressure (i.e., Ca and Ba doping) on the optical properties of Sr2IrO4 epitaxial thin films has been investigated. Substitution of Sr by Ca and Ba ions exerts metastable chemical pressure in the system, which changes both the bandwidth and electronic hopping. The optical conductivity results of these thin films suggest that the two-peak-like optical conductivity spectra of the layered iridates originates from the overlap between the optically-forbidden spin-orbit exciton and the inter-site optical transitions within the Jeff = ½ band, which is consistent with the results obtained from a multi-orbital Hubbard model calculation. In the third study, thermodynamically metastable Ca2IrO4 thin- films have been synthesized. Since the perovskite structure of Ca2IrO4 is not thermodynamically stable, its bulk crystals do not exist in nature. We have synthesized the layered perovskite phase Ca2IrO4 thin- films from a polycrystalline hexagonal bulk crystal using an epitaxial stabilization technique. The smaller A-site in this compound compared to Sr2IrO4 and Ba2IrO4, increases the octahedral rotation and tilting, which enhance electron-correlation. The enhanced electron-correlation is consistent with the observation of increased gap energy in this compound. This study suggest that the epitaxial stabilization of metastable-phase thin-films can be used effectively for investigating complex-oxide systems. Finally, structural, transport, and optical properties of tensile strained (Sr1-xLax)3Ir2O7 (x = 0, 0.025, 0.05) thin-films have been investigated. While high-Tc superconductivity is predicted in the system, all of the samples are insulating. The insulating behavior of the La-doped Sr3Ir2O7 thin-films is presumably due to disorder-induced localization and ineffective electron-doping of La, which brings to light the intriguing difference between epitaxial thin films and bulk single crystals of the iridates. These studies thoroughly investigate a wide array of novel electronic and optical phenomena via tuning the relative strengths of electron correlation, electronic bandwidth, and spin-orbit coupling using perturbations such as chemical doping, and the stabilization of metastable phases in the layered iridates.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2018.495

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