Exploring many-body physics with ultracold atoms Lindsay Leblanc To cite this version: Lindsay Leblanc. Exploring many-body physics with ultracold atoms. Condensed Matter [cond-mat]. University of Toronto, 2010. English. tel-00548894 HAL Id: tel-00548894 https://tel.archives-ouvertes.fr/tel-00548894 Submitted on 20 Dec 2010 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Exploring many-body physics with ultracold atoms by Lindsay Jane LeBlanc A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy Graduate Department of Physics University of Toronto Copyright © 2011 by Lindsay Jane LeBlanc Abstract Exploring many-body physics with ultracold atoms Lindsay Jane LeBlanc, Doctor of Philosophy, 2011 Graduate Department of Physics, University of Toronto The emergence of many-body physical phenomena from the quantum mechanical prop- erties of atoms can be studied using ultracold alkali gases. The ability to manipulate both Bose-Einstein condensates (BECs) and degenerate Fermi gases (DFGs) with designer potential energy landscapes, variable interaction strengths and out-of-equilibrium initial conditions provides the opportunity to investigate collective behaviour under diverse con- ditions. With an appropriately chosen wavelength, optical standing waves provide a lattice po- tential for one target species while ignoring another spectator species. A \tune-in" scheme provides an especially strong potential for the target and works best for Li-Na, Li-K, and K-Na mixtures, while a \tune-out" scheme zeros the potential for the spectator, and is pre- ferred for Li-Cs, K-Rb, Rb-Cs, K-Cs, and 39K-40K mixtures. Species-selective lattices pro- vide unique environments for studying many-body behaviour by allowing for a phonon-like background, providing for effective mass tuning, and presenting opportunities for increasing the phase-space density of one species. Ferromagnetism is manifest in a two-component DFG when the energetically preferred many-body configuration segregates components. Within the local density approximation (LDA), the characteristic energies and the three-body loss rate of the system all give an observable signature of the crossover to this ferromagnetic state in a trapped DFG when interactions are increased beyond kF a(0) = 1:84. Numerical simulations of an extension ii to the LDA that account for magnetization gradients show that a hedgehog spin texture emerges as the lowest energy configuration in the ferromagnetic regime. Explorations of strong interactions in 40K constitute the first steps towards the realization of ferromagnetism in a trapped 40K gas. The many-body dynamics of a 87Rb BEC in a double well potential are driven by spatial phase gradients and depend on the character of the junction. The amplitude and frequency characteristics of the transport across a tunable barrier show a crossover between two paradigms of superfluidity: Josephson plasma oscillations emerge for high barriers, where transport is via tunnelling, while hydrodynamic behaviour dominates for lower bar- riers. The phase dependence of the many-body dynamics is also evident in the observation of macroscopic quantum self trapping. Gross-Pitaevskii calculations facilitate the interpre- tation of system dynamics, but do not describe the observed damping. iii Acknowledgements I was rather delighted the day I found the analogy between the emergence of many-body physics and the working of a community { a general description that says the individual particles of a physical system are better off when they cooperate and work together. While such anthropomorphization may not help solve any physics problems, this analogy provided me a kinship with my atoms, in the knowledge that I, too, benefit from the communities that surround me. In particular, the work presented in this thesis was made possible through the support of my many teachers, colleagues, family and friends. More than anyone else, I owe my thanks to Joseph Thywissen for his support in all of the work that is presented in this thesis. In his role as my thesis supervisor, his mentorship has meant much to me. For the early years and our weekly meetings where I was free to ask my na¨ıve questions, for listening to me and challenging me, for many pieces of good advice, for the spirit of compromise, discussion, and consensus that was always applied to the decisions made in the lab, for the broader picture at the times when I was lost in the details, for an awareness of the impact personality has on the physics, and for confidence in me as I presented work to the wider world that would eventually lead to my moving on from his group, my thanks. Without the labmates who have walked alongside me throughout my PhD, this thesis could not have happened. The spirit of cooperation and friendship that permeates our lab not only makes it a pleasant place to work, but contributes to our success. I owe special thanks to Seth Aubin for teaching me much of what I know about running an experiment, and to postdocs Stefan Myrskog, Thorsten Schumm, and Jason McKeever, for their generosity of time and knowledge. To my fellow \Chip Experiment" students, Marcius Extavour and Alma Bardon, my thanks for the time, effort, and insight that have contributed to many of the results in this thesis, to the labmates next door, Dylan Jervis and Dave MacKay, my thanks for thoughtful questions and helpful advice, and to Alan Stummer, my thanks for a certain wizardry with the electronics. Both at the University of Toronto and through interactions with fellow physicists at iv various international conferences and workshops, I have appreciated the sense of collabo- ration and helpfulness that characterizes my physics community. I owe much thanks to Arun Paramekanti for his interest in the work on ferromagnetism, and patiently helping to develop my early calculations into something more serious. Regular interactions with Aephraim Steinberg, Daniel James, and their groups have helped me to broaden my out- look. With Arun and Aephraim, I thank the other members of my examination committee Allan Griffin and Chris Westbrook for their helpful advice on this thesis. Many thanks are due to our collaborators at the University of Trento, Augusto Smerzi and Francesco Piazza, for their simulations of and insight into the double well BEC work. I would also like to thank the Innsbruck Ultracold Atoms group for their hospitality during May 2009. To the many friends who have made the years of graduate school some of the best in my life, my thanks. To Cheryl, for inspiring the move to Toronto and remaining my friend once I got here; to Asya, for giving me a life outside the lab; to Chris, for being a wonderful housemate. To physics-friends, Jean-Michel, Etienne, , Julien, Susanne, Nir, Cristen, Brendan, Lisa, Sarah, and others, for being more social than the stereotypes and bringing both serious conversation and laughter into my life. To Loretta, Celia, Dana, Nikki and the many others from across my life, for friendships that have endured the tests of time and distance. To the Wednesday crew at the Redeemer Lunch Program and the SGS womens' basketball teams through the years, for making Wednesday mornings and Monday nights something to look forward to. Though graduate school took me farther from them, my family's never-ending support has followed me across the country. For always fostering and facilitating my curiosity, I thank my parents Bill and Avril. For becoming some of my closest friends, I thank my sisters Katie and Valerie, and for making our family a bigger and better place through marriage and birth, Brad and Jackson. And though it seems silly to say, I would like to thank the City of Toronto, for being the kind of place where you are free to become the person you are meant to be, and for providing the kind of environment in which the coming together of people from all backgrounds benefits us all. v Preface The work presented in this thesis is the culmination of six years of experimental and theoretical work performed at the Toronto Ultracold Atoms Lab. I have had the opportunity to work with many different people on a variety of projects. The following specifies the specific publications arising from the work discussed in each chapter, and acknowledges the contributions of my collaborators to work presented in this thesis. Chapter 2: A versatile BEC-DFG machine Chapter2 discusses the apparatus used for the experiments in this thesis. Much of the original work building the apparatus was done by Seth Aubin, Stefan Myrskog and Marcius Extavour. Thorsten Schumm and Marcius Extavour were responsible for the construction of the electronics used to produce the radio-frequency dressed potentials. Publications describing this work, as well as the early results from this apparatus, include: S. Aubin et al., Trapping Fermionic 40K and Bosonic 87Rb on a Chip, J. Low Temp. • Phys. 140, 377 (2005). S. Aubin, S. Myrskog, M. Extavour, L. LeBlanc, D. McKay, A. Stummer, and J. H. • Thywissen, Rapid sympathetic cooling to Fermi degeneracy on a chip, Nat. Phys. 2, 384 (2006). M. H. T. Extavour, L. J. LeBlanc, T. Schumm, B. Cieslak, S. Myrskog, A. Stummer, • S. Aubin, and J. H. Thywissen, Dual-species quantum degeneracy of 40K and 87Rb on an atom chip, At. Phys. 20, 241 (2006). Chapter 3: Species-specific optical lattices Chapter3 was work done in collaboration with Joseph Thywissen, who is especially responsible for the discussion of effective mass tuning.