Novel Phases in Hetero-Epitaxial and Super-Oxygenated Thin Films of Complex Oxides by Hao Zhang A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy Graduate Department of Physics University of Toronto © Copyright 2018 by Hao Zhang Abstract Novel Phases in Hetero-Epitaxial and Super-Oxygenated Thin Films of Complex Oxides Hao Zhang Doctor of Philosophy Graduate Department of Physics University of Toronto 2018 In this thesis we study structural phase transition and defect structures in complex oxide thin films induced by either heterostructuring or superoxygenation, in an effortto understand their effects on the electronic and superconducting properties. Thin filmswere chosen for the ability to tune the heteroepitaxial strain, as well as their large surface-to- volume ratio. We focus on two families of oxides, including the cuprate superconductors Y-Ba-Cu-O (YBCO), and the Ruddlesden-Popper iridates Srn+1IrnO3n+1. We examine the effect of heteroepitaxial strain on the superconducting critical tem- perature (Tc) of YBa2Cu3O7−δ (YBCO-123) thin film. Strain-induced intergrowth of CuO defect structures is seen in La2=3Ca1=3MnO3 (LCMO)/YBCO-123 bilayer films, and can account for the reduced Tc in such bilayers. Perovskite/YBCO-123/perovskite trilayers with either ferromagnetic LCMO or paramagnetic LaNiO3 (LNO) as clamping layers show similarly strong reduction of the Tc. The Tc reduction is much milder when orthorhombic PrBa2Cu3O7−δ (PBCO) is used as clamping layers instead. These results indicate that heteroepitaxial strain, rather than long-range proximity effect, is responsi- ble for the long length scales of Tc reduction in LCMO/YBCO-123 heterostructures. We carried out superoxygenation experiments on YBCO-123 films in order to search for novel YBCO phases. YBCO-123 films are annealed in high-pressure oxygen, incon- junction with Cu enrichment by solid-state diffusion. The annealed films show clear evidence of phase transformation to Y2Ba4Cu7O15−δ and Y2Ba4Cu8O16, increasing with ii higher degree of Cu enrichment. Regions of exotic phases containing multiple CuO or Y layers are also seen in high-pressure annealed films. Our results demonstrate a novel route of synthesis towards discovering more complex YBCO phases. We also carried out superoxygenation on Sr2IrO4 (SIO) films in an attempt to met- allize the iridates via hole-doping. High-pressure oxygen annealed SIO films show a progressive drop in room-temperature resistivity of up to 3 orders of magnitude, and an evolution towards metallic behavior. The post-annealed films show transformation to SrIr1−xO3, increasing with annealing time, pressure, and reduced film thickness. The evolution towards metallicity is attributed to phase transformation, interstitial oxygens, and Ir vacancies. The Ir vacancies in the most phase-transformed film appears to be structurally-ordered along the c-axis. Our results demonstrate a novel method for hole- doping and phase-transforming the iridates. iii Acknowledgements First and foremost, I would like to express my deepest gratitude and appreciation to my supervisor, Prof. John Wei. Throughout my PhD studies, Prof. Wei has continuously supported, guided, and encouraged me both professionally and personally, for which I am forever grateful. He has been a great mentor and role model to me, and pushed me beyond my limitations over and over again. It has been a pleasure and privilege to be a member of Prof. Wei’s research group. I would like to express my appreciation to current and former members of the Wei group, Igor Fridman, Chris Granstrom, and Charles Zhang for giving me invaluable assistance in the lab as well as constructive scientific insights. This research has benefited from the contributions of many undergraduate students who has worked with our group, including Tianmin Liu, Alexander Su and Ben Xu. I would also like to thank my committee members Prof. Young-June Kim and Prof. Arun Paramekanti, who has contributed time and expertise, and provided valuable guid- ance towards the completion of my thesis. I thank the careful proof-reading and con- structive comments on my thesis by Prof. Hae-Young Kee, Prof. Luyi Yang, and Prof. Alain Pignolet. I would like to acknowledge and thank our collaborators. Prof. Gianluigi Botton and Dr. Nicolas Gauquelin at McMaster University performed microscopy imaging of our thin films; Prof. David Hawthorn and Christopher McMahon at University of Waterloo took x-ray absorption spectroscopy data; Prof. Thomas Gredig and Anh Nguyen at California State University Long Beach performed x-ray reflectivity measurements; Dr. Patrick Clancy took some x-ray diffraction (XRD) data for me and taught me many technical details about XRD; Some of the XRD data were taken by Dr. Abdolkarim Danaei and Dr. Raiden Acosta at Department of Material Science, and Dr. Srebri Petrov at Department of Chemistry. Prof. Ambrose Seo at University of Kentucky provided the Sr2IrO4 samples used in the study. I thank Prof. Oscar Bernal, Prof. Guo-Meng Zhao iv and his students Victor Aguilar, Carlos Sanchez, and Bo Truong at California State University Los Angeles for assisting me with Hall effect measurements. Finally I would like to thank Prof. Young-June Kim for kindly letting me use his high-pressure furnace, which is a valuable instrument essential for a significant portion of my research. I am indebted to my family for the emotional support they gave me throughout the years. I would like to thank my parents for their love and support, without which I would never be able to complete this challenging endeavor. I thank my wife Fei for her unconditional love and patience, for helping me staying sane, for giving me strength, and for her understanding and support in every possible ways. Last but not least, I wish to thank my daughter Muyao for giving me motivation to finish my thesis during the last, and the most stressful year of my PhD study. I feel extremely lucky to have them all in my life. v Contents List of Publications vi 1 Introduction 1 1.1 Cuprate superconductors and Y-Ba-Cu-O . 1 1.2 Half-metallic manganites . 7 1.3 Superconducting pairing symmetry and proximity effect . 11 1.4 Ruddlesden-Popper iridates . 15 1.5 Thesis outline . 17 2 Experimental Techniques 20 2.1 Pulsed-laser deposition . 20 2.2 Thickness and roughness measurement by X-ray reflectivity and atomic force microscopy . 25 2.3 High-pressure oxygen annealing . 30 2.4 Electrical transport measurements . 32 3 Attenuation of Superconductivity in Manganite/Cuprate Heterostruc- tures by Epitaxially-Induced CuO Intergrowths 39 3.1 Introduction . 39 3.2 Experiment . 41 3.3 Results . 41 vi 3.4 Discussions . 47 3.5 Conclusion . 48 4 Strain-Induced Tc Reduction in Heteroepitaxial Perovskite/YBa2Cu3O7−δ/ Perovskite Trilayers 50 4.1 Introduction . 50 4.2 Experimental . 52 4.3 Results . 53 4.4 Discussion and outlook . 58 4.5 Conclusion . 60 5 Synthesis of High-Oxidation Y-Ba-Cu-O Phases in Superoxygenated Thin Films 61 5.1 Introduction . 61 5.2 Experimental . 62 5.3 Results and discussions . 64 5.4 Conclusion . 77 6 Phase Transformation and Hole Doping in Superoxygenated Sr2IrO4 Thin Films 78 6.1 Introduction . 78 6.2 Experimental . 80 6.3 Results and discussions . 80 6.4 Conclusion . 94 7 Conclusions and Future Perspectives 95 7.1 Conclusions . 95 7.2 Suggested future experiments . 98 Bibliography 102 vii List of Tables 1.1 Table of some common multi-layer hole-doped cuprates that demonstrate the Tc tends to scale with the lattice complexity . 4 1.2 Superconductor pairing symmetries allowed by Pauli exclusion principle . 12 3.1 Comparison of the ab-plane lattice parameters between YBCO-123, YBCO- 247, LCMO, and LSAT. 48 4.1 Bulk lattice parameters for the oxides used in the trilayer study . 52 5.1 Known and possible phases of the YBCO family of cuprates . 62 viii List of Figures 1.1 Doping phase diagram of the cuprate superconductors . 3 1.2 Lattice structures of YBCO-123, YBCO-247 and YBCO-248 . 5 1.3 Phase diagrams of temperature versus oxygen pressure for bulk YBCO samples . 6 1.4 Density of states close to Fermi energy for a half-metal . 8 1.5 Illustration of the double exchange mechanism in mixed-valence manganites 9 1.6 Lattice structure of the perovskite LCMO . 10 1.7 A typical resistance versus temperature plot of LCMO . 11 1.8 Illustration of long-ranged proximity effect across a superconductor/ferromagnet junction . 13 1.9 Lattice structures of Sr2IrO4 and perovskite SrIrO3 . 15 1.10 The schematic energy diagram of the Ruddlesden-Popper iridates. 16 2.1 The schematic diagram of our PLD setup. 21 2.2 A photograph of the inside of the PLD chamber during film growth with the plume showing. 22 2.3 Comparison of AFM images of unoptimized and optimized YBCO-123 films 24 2.4 The experimental setup of typical XRR measurements . 26 2.5 Comparison between measured XRR data obtained on LNO/YBCO-123/ LNO trilayer films with the simulated XRR spectra . 27 ix 2.6 YBCO-123 film that were etched with HCl solution for thickness measure- ments . 29 2.7 A schematic diagram of the Morris Research HPS-5015 high-pressure furnace 30 2.8 Circuit diagram for four-terminal resistivity measurement using two lock- in amplifiers. 33 2.9 Wiring configurations for van der Pauw measurement used for thin-film resistivity measurements . 34 2.10 The tip of our dipper probe for measuring electrical resistance as a function of temperature. 35 2.11 Wiring diagram for Hall effect measurement with the PPMS . 37 3.1 HAADF-STEM images of a 25 nm/50 nm bilayer LCMO/YBCO-123 film grown on (001)-oriented LSAT substrate . 42 3.2 STEM image of a 25 nm unilayer YBCO-123 films grown on LSAT sub- strate showing negligible amount of double-CuO chains .