Molecular beam epitaxy of electronic perovskite oxides: BaSnO3, Sr3SnO, and EuxSr1-xSrTiO3
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Molecular beam epitaxy of electronic perovskite oxides: BaSnO3, Sr3SnO, and EuxSr1-xSrTiO3

Abstract

Perovskite oxides are a class of materials known to exhibit a wide range of electronic, magnetic, and other functional properties. In their simplest forms they have the chemical formula ABO3, where the O is oxygen and A and B are cations. Different combinations of the A and B cations lead to a plethora of properties in electronic structure, magnetism, structural transitions, quantum phases, and more. In addition, certain inversions of this relatively simple structure, such as the antiperovskites, open up an entirely new realm of topologically nontrivial materials that offer even more avenues of exploration. Molecular beam epitaxy (MBE) is a thin film growth technique which is known for its ability to produce films of the highest quality (i.e. low defect density) with multiple levels of control, making it an excellent tool for the study of novel perovskite oxides.BaSnO3 is a transparent semiconductor which displays high electron mobilities at room-temperature when single crystals are doped with La. These mobilities are accessed at high carrier densities, opening up possibilities for uses in high power density electronics and transparent conductor applications. Thin films have so far been unable to match the mobilities exhibited by single crystals, however. In this thesis, I will describe studies aimed towards understanding the various mobility-limiting mechanisms at play in thin film BaSnO3. These show that while dislocations may a prominent role in limiting mobility at the moment, ultimately it may be stoichiometry control which sets an upper limit on the mobility that can be achieved in films. Sr3SnO is an antiperovskite oxide which exhibits topologically nontrivial band inversion in its bulk. It has only a protected band crossing at the Fermi level, making it a three-dimensional topological Dirac semimetal (or topological crystalline insulator if a small mass gap is considered). Additionally, superconductivity was discovered in polycrystalline samples, opening up the possibility that Sr3SnO is an intrinsic topological superconductor. In this thesis I will describe my efforts involving getting the initial growth of this material by MBE off the ground. I will also discuss a protective capping scheme developed such that the material would degrade in air once removed from the growth chamber, as it is extremely air sensitive. SrTiO3 is one of the most well-studied perovskite oxides. It is a dilute superconductor, which challenges the traditional electron-phonon pairing mechanisms of BCS theory. It is also an incipient ferroelectric, meaning it is in proximity to a ferroelectric quantum phase transition. In this thesis I will describe experimental work – built on prior progress in the growth of high-quality SrTiO3 thin films – to further examine the coexistence of ferroelectricity and superconductivity in SrTiO3. By growing films alloyed with magnetic Eu (EuxSr1-xTiO3), I will show that superconductivity in SrTiO¬3 is not only remarkably insensitive to magnetism, but may also be more strongly tied to the presence of static polar order than has been previously considered.

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