INVESTIGATIONS OF CROSSED ANDREEV REFLECTION IN HYBRID SUPERCONDUCTOR-FERROMAGNET STRUCTURES BY MADALINA COLCI O'HARA Dipl. de Lie, University of Bucharest, 2000 M.S., University of Illinois at Urbana-Champaign, 2003 DISSERTATION Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Physics in the Graduate College of the University of Illinois at Urbana-Champaign, 2009 Urbana, Illinois Doctoral Committee: Professor James N. Eckstein, Chair Professor Dale J. Van Harlingen, Director of Research Professor Paul G. Kwiat Professor Anthony J. Leggett UMI Number: 3391915 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. UMT Dissertation Publishing UMI 3391915 Copyright 2010 by ProQuest LLC. All rights reserved. This edition of the work is protected against unauthorized copying under Title 17, United States Code. ProQuest LLC 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106-1346 © 2009 Madalina Colci O'Hara Abstract Cooper pair splitting is predicted to occur in hybrid devices where a superconductor is coupled to two ferromagnetic wires placed at a distance less than the superconducting coherence length. In this thesis we search for signatures of this process, called crossed Andreev reflection (CAR), in three device geometries. The first devices studied are lateral spin valves. In these structures, when electrons with energies less than the superconducting energy gap are injected from one ferromagnetic wire into the superconductor, nonlocal transport processes involving the second ferromagnetic wire are predicted to take place. We measure a negative nonlocal voltage in the antiparallel magnetization alignment of the ferromagnetic wires, which is the theoretically predicted signature of CAR. The second type of hybrid devices that we measured consist of two superconducting electrodes connected by two ferromagnetic nanowires placed within a superconducting coherence length of each other, forming an S-FF-S junction. We find that below the critical temperature of the superconductor, the resistance versus temperature curves show re-entrant behavior, with the signal corresponding to antiparallel alignment of the magnetization of ferromagnetic wires distinctly larger than that of the parallel case. We discuss one possible explanation of this result in terms of Cooper pair splitting. We also report the first observation of multiple Andreev reflection peaks in the differential resistance of these devices. The third line of investigation briefly examines superconductor-ferromagnet SQUID- type devices to which we apply an external magnetic field to modulate the phase drop across the junctions. We do not observe coherent effects such as supercurrent or resistance oscillations, but suggest improvements for future research. n Pentru mama, cu dragoste, admirable §i recuno§tinfd 111 Acknowledgments I am indebted to my advisor, Dale Van Harlingen, and thank him for his continuous support and patience during my time as a PhD student. I enjoyed working on exciting research projects and it was a privilege to be exposed to his broad knowledge and impressive physical intuition. Dale's support in times of difficulty was crucial for finishing my work, and I am grateful for it. I also thank my thesis committee, Professors Tony Leggett, Paul Kwiat, and Jim Eckstein, for the time and effort put into my defense exam, for their careful reading of the manuscript, and for offering suggestions and making comments that improved the quality of my dissertation. The successful completion of my PhD work is made possible by the contributions of many people along the way. To start with, Trevis Crane introduced me to the world of fabrication and, despite having this world take over my life, I thank him for his coaching. Lukas Urban trained me in the art of operating dilution fridges. He can always be counted on for solving problems, and together torturing the dilution fridge under the excuse of diagnosing malfunctions was educational and fun; it could have been productive too, were it not for all the chatting about unrelated matters that in the end did not lead to me cooking sauerkraut soup nor to him landing a job. Special thanks go to Tony Banks who has been an amazing resource of fabrication knowledge. A great deal of my gratitude goes to him for keeping the Microfab show going; in particular, for fixing an instrument right away every time I needed it. Our conversations will be very much missed. I am grateful to Martin Stehno for shared knowledge on our research projects, and for critical input on parts of my thesis. His skills in assembling cribs and dressers are very much appreciated as well. However, he will probably be remembered most for the delicious Mozart balls he always brought back from Austria. Gratitude is also due to Dan Bahr for cooling down the dilution fridge for me a few times when I was swamped with other tasks. It is a lot of fun being around him, especially when he takes a deep breath of helium gas or when the dialog-laced storytelling turns on. I extend my appreciation to all DVH group contemporary labmates for providing valuable insights into diverse problems, and for their help and support during the last year of my graduate work. iv Most of my years in Urbana have involved a lot of work, but there has been some life outside the lab, on a few occasions. My first year was particularly fun owing to Micah, Josh, and Tim. I thank them for giving me a nice introduction to American life, and for their patience in repeatedly explaining menu items at restaurants. It was not their fault when in the end, after their frantic coaching, the question "How do you want your eggs?" left me dumbfounded. In addition to culinary help, Micah deserves thanks for his critical reading of a couple of manuscripts I wrote in the first year, after which they were notably improved. Also in the "Urbana" chapter, Francoise is a special mention: she always had an ear to listen to my frustrations and joys, and provided lots of fun conversations; I hope we stay in touch wherever we may be. Some of my strongest friendships were separate from my life in Urbana. My closest friends, Bogdan and Ioana, have offered a great deal of support and understanding during my isolation years in graduate school. I thank them deeply. Many thanks also go out to Joy, my mother-in-law, who has helped me tremendously during the last 100 meters of my graduate school marathon by coming to Urbana to take care of Ethan. I would not have come anywhere close to my achievement if it were not for my parents' sacrifices throughout my life, for which I am eternally grateful. Nu exista cuvinte care sa mult;umeasca indeajuns parinl^ilor mei Ionel si Ioana pentru sacrificiile facute ca sa-mi ofere acces la educa^ia pe care am avut-o. Iubirea, incurajarile §i sfaturile lor au fost de nepre^uit de-a lungul anilor, iar recuno§tint;a mea pentru tot ce au facut este eterna. Tata ar fi fost foarte mandru sa ma vada atat mamica cat §i doctor in fizica, dar din nefericire mama trebuie sa duca mandria pe umerii ei pentru amandoi. In plus fa^a de paring, mul^umesc lui tanti Leana §i nenea Mircea pentru gandurile bune §i pentru felicitarile trimise prin po§ta an de an, cu ocazia fiecarei sarbatori. De asemenea, sunt recunoscatoare lui tanti Ani§oara §i intregii familii Rufa pentru rugaciunile lor §i sus^inerea morala. Finally, thanks to my husband, Tim, "Mr. Amazing, Mr. Incredibly-Superbly-Fantastic-Ness". His support and help for whatever I needed were crucial in completing my graduate work, while his efforts in making sure my eyes stayed focused on the end result have made this dissertation possible. He has touched many parts of this manuscript with his excellent proofreading skills and suggestions for concise writing. Overall, I am grateful to him for his patience and understanding throughout my graduate school journey. I gratefully acknowledge the NSF DMR grant no. 06-05813 and the Department of Physics for various sources of funding over the years that enabled my scientific research to progress. v Table of Contents List of Symbols viii Chapter 1 Introduction 1 Chapter 2 Theoretical Background 4 2.1 Superconductivity Concepts 4 2.2 Ferromagnetism and Spin-dependent Transport 6 2.2.1 Spin Polarized Transport into a Nonmagnetic Metal 7 2.2.2 Magnetoresistance 14 2.2.3 Anisotropic Magnetoresistance 15 Chapter 3 Transport Phenomena in Superconducting Hybrid Structures .... 17 3.1 Superconductor-Normal Metal Heterostructures 17 3.1.1 Non-Equilibrium Superconductivity: Charge Imbalance 17 3.1.2 The Blonder-Tinkham-Klapwijk Model 19 3.1.3 Andreev Reflection 21 3.1.4 The Proximity Effect 23 3.1.5 The Re-entrance Effect 24 3.1.6 The Josephson Effect 25 3.1.7 Multiple Andreev Reflections 26 3.2 Superconductor-Ferromagnet Heterostructures 27 3.2.1 Modified BTK Model for Transport Across an F/S Interface 27 3.2.2 Proximity Effect in Ferromagnets 28 3.2.3 Long-Range Proximity Effect 30 3.2.4 Inverse Proximity Effect 31 3.3 Nonlocal Processes: Crossed Andreev Reflection and Elastic Co-tunneling 32 Chapter 4 Sample Fabrication and Experimental Setup 35 4.1 Sample Design Considerations 35 4.2 Fabrication Techniques: Lithography and Metal Deposition 36 4.3 Measurement Instruments 44 4.3.1 Dilution Refrigerator, Filters and Shielding 44 4.3.2 Electronics: DC and AC Electronics, Magnetic Field Bias, Data Acquisition .