ANGLE-RESOLVED PHOTOEMISSION SPECTROSCOPY OF Sr1−xLaxCuO2 THIN FILMS GROWN BY MOLECULAR-BEAM EPITAXY A Dissertation Presented to the Faculty of the Graduate School of Cornell University in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy by John Wallace Harter May 2013 c 2013 John Wallace Harter ALL RIGHTS RESERVED ANGLE-RESOLVED PHOTOEMISSION SPECTROSCOPY OF Sr1−xLaxCuO2 THIN FILMS GROWN BY MOLECULAR-BEAM EPITAXY John Wallace Harter, Ph.D. Cornell University 2013 Among the multitude of known cuprate material families and associated structures, the archetype is “infinite-layer” ACuO2, where perfectly square and flat CuO2 planes are separated by layers of alkaline earth atoms. The infinite-layer structure is free of mag- netic rare earth ions, oxygen chains, orthorhombic distortions, incommensurate super- structures, ordered vacancies, and other complications that abound among the other ma- terial families. Furthermore, it is the only cuprate that can be made superconducting by both electron and hole doping, making it a potential platform for decoding the complex many-body interactions responsible for high-temperature superconductivity. Research on the infinite-layer compound has been severely hindered by the inability to synthesize bulk single crystals, but recent progress has led to high-quality superconducting thin film samples. Here we report in situ angle-resolved photoemission spectroscopy measurements of epitaxially-stabilized Sr1−xLaxCuO2 thin films grown by molecular-beam epitaxy. At low doping, the material exhibits a dispersive lower Hubbard band typical of other cuprate parent compounds. As carriers are added to the system, a continuous evolution from Mott insulator to superconducting metal is observed as a coherent low-energy band de- velops on top of a concomitant remnant lower Hubbard band, gradually filling in the Mott gap. For x = 0.10, our results reveal a strong coupling between electrons and (π; π) antiferromagnetism, inducing a Fermi surface reconstruction that pushes the nodal states below the Fermi level and realizing nodeless superconductivity. Electron diffraction mea- surements indicate the presence of a surface reconstruction that is consistent with the polar nature of Sr1−xLaxCuO2. Most knowledge about the electron-doped side of the cuprate phase diagram has been deduced by generalizing from a single material family, Re2−xCexCuO4, where robust anti- ferromagnetism has been observed past x ≈ 0.14. In contrast, in all hole-doped cuprates, Neel´ order is rapidly suppressed by x ≈ 0.03, with superconductivity following at higher doping levels. Studies of cuprates, however, often yield material-specific features that are idiosyncratic to particular compounds. By studying a completely different electron- doped cuprate, we can for the first time independently confirm that the cuprate phase dia- gram is fundamentally asymmetric and provide a coherent framework for understanding the generic properties of all electron-doped cuprates. BIOGRAPHICAL SKETCH John Harter was born in Syracuse—just one hour north of Cornell—in the summer of 1984. From a very young age, he demonstrated an unrelenting curiosity about the nat- ural world and dreamed of becoming a scientist. When he was seven years old, John’s parents decided they were fed up with the weather of central New York and moved the family to sunny Florida. There, high school presented John’s first exposure to physics, and he fell in love with its beauty and simplicity. In 2002, he was salutatorian of his high school graduating class and moved to Gainesville to attend the University of Florida. He majored in physics and mathematics after reluctantly dropping a third major—chemistry. During the last year of his undergraduate education, John performed theoretical research on superconductivity under the advisement of Professor Peter Hirschfeld, published his first manuscript, and earned a National Science Foundation Graduate Research Fellow- ship. In 2006, he graduated summa cum laude with Bachelor of Science degrees in physics and mathematics and moved to Ithaca to start graduate school at Cornell University. There he became the first student to join Professor Kyle Shen’s new research group, help- ing to build the laboratory up from scratch. He received a Master of Science degree in physics from Cornell in 2010 while pursuing research on angle-resolved photoemission spectroscopy of thin films grown by molecular-beam epitaxy. The results of this research would eventually form his doctoral dissertation, presented here. iii “The electron is not as simple as it looks.” — Sir William Lawrence Bragg iv ACKNOWLEDGEMENTS Graduate research is rarely a solitary endeavor, especially for an experimentalist. I am blessed to have had many kind, intelligent, and considerate people in my life, both professionally and personally. It may be cliche,´ but it is no less true: Without the help and support of others, I could not have gotten this far. I owe a great deal of gratitude to Kyle Shen, who hired me as his first student way back in the spring of 2007. Since then, he has taught me everything I know about ultra- high vacuum, photoelectron spectroscopy, and creating a world-class physics laboratory. Kyle has always been hands-on and down-to-earth, and I am grateful for his enthusiasm, approachability, and general “coolness.” After all, how many professors will buy a flat screen TV, rock-climbing harness, or 3D printer for their lab? I am indebted to all of the current and former members of the Shen group. Eric Monkman, Danny Shai, and Dawei Shen were there from the very beginning, and I still have vivid memories of traveling with them to Penn State. I remember living above Saint’s Cafe during our first summer in State College, before wisely choosing to rent a house in the middle of a corn field but away from the noise of the city. Since those days, Yuefeng Nie, Bulat Burganov, Shouvik Chatterjee, Phil King, Masaki Uchida, Ed Lochocki, and Haofei Wei have joined the group and have made it a pleasure to work in the lab every day (and sometimes nights). The close collaboration that was forged with Darrell Schlom’s research group in the Penn State days has contributed tremendously to my research. I would be remiss if I did not acknowledge Darrell’s guidance and the heroic efforts that Carolina Adamo and Gino Maritato sustained in order to grow thin films for me—Carolina for the SrFeO3/SrTiO3 and ruthenate projects at Penn State, and Gino for the cuprate project that makes up this dissertation. Gino, in particular, has taught me everything I know about growing cuprate films via molecular-beam epitaxy. v Within Clark Hall, I owe thanks to all of the hard-working people that have made research in the physics department possible: Eric Smith, who continues to tolerate our abuse of helium dewars; Leonard Freelove, who has delivered hundreds of packages to our office without misplacing a single one; Stan Carpenter and Rodney Bowman, who have had to put up with crazy construction and machining requests over the years; Linda Hatch, who has had to process more purchase orders than I would like to think about; and Nate Ellis, who introduced me to the joys of machining. I will always remember Nate’s wit, humor, and outstanding teaching skills. The life of a graduate student is sometimes inhumane, especially in a small town like Ithaca. I could not have lasted these seven years without the help (and commiser- ation) of my good friends: Brant, Charlie, Ben, Pablo, Nick, Emily, Antonio, and everyone else I have met while here. The conversations, meals, parties, and heavy drinking that we’ve shared have made all the difference. The Golden Girls theme song says it perfectly: “Thank you for being a friend.” Lastly, and most importantly, I would like to thank my family for all of the support they have given me throughout my life. My parents have always encouraged my love of learning. As a young child, I remember Dad teaching me about evaporation by leaving a shallow dish of salt water out overnight, and I remember Mom ordering hard-to-find educational books over the phone because I had checked them out from the library and wanted to own them. From the moment I was born, my sister has stood up for me. Amanda’s friendship, wisdom, and unwavering support has been invaluable. I could not ask for a better family, and this dissertation is dedicated to them. John W. Harter March 2013 vi TABLE OF CONTENTS Biographical Sketch . iii Quotation . iv Acknowledgements . v Table of Contents . vii List of Tables . x List of Figures . xi 1 Introduction 1 1.1 The many-body problem . 1 1.2 Fermi liquid theory . 2 1.2.1 Quasiparticles . 4 1.3 Outline of the text . 5 2 Angle-Resolved Photoemission Spectroscopy 7 2.1 History . 7 2.1.1 The photoelectric effect . 7 2.1.2 The Einstein equation . 8 2.1.3 History of ARPES . 9 2.2 Theory of photoemission . 10 2.2.1 The three-step model . 11 2.2.2 Fermi’s golden rule and the sudden approximation . 13 2.2.3 The spectral function . 15 2.3 Experimental aspects . 18 2.3.1 Maintaining vacuum . 20 2.3.2 The sample manipulator . 22 2.3.3 The photon source . 25 2.3.4 The electron analyzer . 27 2.4 Analysis of data . 30 2.4.1 Transformation from angle to momentum . 30 2.4.2 Energy referencing . 32 2.4.3 Display of data . 33 3 Molecular-Beam Epitaxy 36 3.1 History . 36 3.1.1 Oxide MBE . 37 3.2 Experimental aspects . 37 3.2.1 Maintaining vacuum . 39 3.2.2 Knudsen effusion cells . 40 3.2.3 Shutters . 43 3.2.4 Ozone generation . 44 3.2.5 Quartz crystal microbalance .