High-Temperature Superfluidity in an Ultracold Fermi Gas by Martin W. Zwierlein Submitted to the Department of Physics in partial ful¯llment of the requirements for the degree of Doctor of Philosophy at the MASSACHUSETTS INSTITUTE OF TECHNOLOGY October 2006 °c Massachusetts Institute of Technology 2006. All rights reserved. Author.............................................................. Department of Physics October 26, 2006 Certi¯ed by. Wolfgang Ketterle John D. MacAurthur Professor of Physics Thesis Supervisor Accepted by . Thomas J. Greytak Professor of Physics, Associate Department Head for Education High-Temperature Superfluidity in an Ultracold Fermi Gas by Martin W. Zwierlein Submitted to the Department of Physics on October 26, 2006, in partial ful¯llment of the requirements for the degree of Doctor of Philosophy Abstract This thesis presents experiments in which a strongly interacting gas of fermions was brought into the superfluid regime. The strong interactions are induced by a Feshbach scattering resonance that allows to tune the interfermion scattering length via an external magnetic ¯eld. When a Fermi mixture was cooled on the molecular side of such a Feshbach resonance, Bose-Einstein condensation of up to 107 molecules was observed. Subsequently, the crossover region interpolating between such a Bose- Einstein condensate (BEC) of molecules and a Bardeen-Cooper-Schrie®er superfluid of long-range Cooper pairs was studied. Condensates of fermion pairs were detected in a regime where pairing is purely a many-body e®ect, the pairs being stabilized by the presence of the surrounding particles. Superfluidity and phase coherence in these systems was directly demonstrated throughout the crossover via the observation of long-lived, ordered vortex lattices in a rotating Fermi mixture. Finally, superfluidity in imbalanced Fermi mixtures was established, and its Clogston limit was observed for high imbalance. The gas was found to separate into a region of equal densities, surrounded by a shell at unequal densities. Thesis Supervisor: Wolfgang Ketterle Title: John D. MacAurthur Professor of Physics To my family, Erika and Otto, Anne-Julia and Cornel Acknowledgments Over the years at MIT I had the good fortune to learn from and the pleasure to work with an exceptional group of people. These few lines can only be a knowing nod to all of them, while there is no way to fully express my gratitude in writing. First of all, I would like to thank Wolfgang Ketterle for teaching me the ways of a physicist. His sparkling enthusiasm for science is well known to be infectious. The trust he had in every one of us to achieve our goals is what made us believe that we actually might. I treasure every discussion we had, be it about physics or general matters of life. It is my pleasure to thank my fellow graduate students on the lithium project, its ¯rst members and initiators, Zoran Hadzibabic, Claudiu Stan and Subhadeep Gupta, the new crew around Christian Schunck and Andr¶eSchirotzek, and special guest Jamil Abo-Shaeer. None of the work in this thesis would have been possible without them. I consider myself a lucky guy, as all of them have not only been the greatest colleagues, but have also become wonderful friends. When I ¯rst came to MIT as a visiting student from Paris, Zoran took me under his wings. I immediately grew attached to his way of doing science, of ¯guring things out instead of taking them for granted, and having tremendous fun at it. He taught me invaluable lessons about physics and life, and he was a big reason for me to come back for my PhD. We shared a lot of habits, the bizarre schedule, the preference for Miracle burgers with extra chutney, later I copied his addiction to diet coke. Unfortunately I could not copy his wit, but maybe that simply means we need more time to work together. From Claudiu I learned that what has to work well has to look good: His pre- cision and care in building apparatus was the pillar for the reliability and success of our experiments. If you have any question about some unknown, weird-looking and potentially dangerous substance, Claudiu can tell you all about it. I was always amazed by his ability to keep his cool when everything around us seemed to fall apart. The only way you could tell that the problem he just ¯xed could have turned into a major catastrophe was by the number of cigarettes consumed afterwards. When lithium atoms and light were ready for the upgrade of the sodium BEC machine, we teamed up with Deep Gupta, an exceptional scientist and a great teacher. He introduced me to the sodium BEC machine, the dye(t) laser, the subtleties of the sodium MOT, saddle points (sic) of magnetic trapping and evaporative cooling and to frequent punning. It was wonderful to watch Zoran and Deep bounce ideas o® each other during the optimization of the sympathetic cooling scheme, and see the atom number grow by the minute. We, the youngens, were thrown into cold water when they both graduated a day from each other, but they had left a miraculous heritage, the brightest source of ultracold fermions in the world. Christian ¯rst joined the crew as a diploma student, while I was back in Paris, and he must have experienced the same welcoming warmth I had felt, as he was able to cite the relevant puns even before having watched The Big Lebowski. He now makes the best White Russian in the Northern Hemisphere, as I can certify. He has been a dependable friend, o±ce-mate and lab companion through countless days and nights. Not widely known are his superb musical skills: His harp play is simply outstanding. I wish him and his newly-wed wife Alessandra all the best for the future. In a memorable night in February 2001, I heard a tremendous outcry coming from the BEC II lab. I jumped inside, fearing something terrible had happened, and saw a stunning image on the screen: Jamil Abo-Shaeer and Chandra Raman had just created a vortex lattice in a Bose-Einstein condensate containing about 100 vortices! The image stuck in my mind. Over the next years, Jamil had found in me, his poor o±ce-mate, the ideal subject for his teases. He tricked me, embarrassingly more than once, into believing that I was really heavy (which I am not!), next, that his mouse has a life of its own and that he could solve the Rubik's cube in less than ¯ve minutes. Jamil was our star in soccer. He needed twelve sure chances to score one goal. At least he always scored. My biggest success in grad school was to convince Jamil to defect from BEC II, delay his thesis and "bring the vortices over" to the sodium condensates in BEC I. Which he did. In a day. Within the two months or so he had been a member of our lab I learned experimental skills for a lifetime. I pay this back by dedicating Fig. 6-9 to him. Andr¶e,from the same village in Germany and almost as well-built as me, had been a close friend from day one. His enthusiasm and fearlessness in the lab are by now legendary. When we were almost about to give up on vortices in Fermi gases, but had a beautiful gas cloud on our screen, he asked "should we try it once m...?", I interrupted saying "yes!", and the result was Fig. 6-8a. I will always cherish the times watching Friends in Clinton Street, eating entire cows, "making gym" and playing all sorts of existing or invented sports inside building 26. We were all happy to have Andr¶eon our soccer defense, where he flattened the other team's players with grace. Next time we play squash, I might let him win once or twice. I am grateful for all things I learned from our postdocs Axel GÄorlitz,Kai Dieck- mann and Jamie Kerman, and the newest addition to the lithium lab, Yong-Il Shin. After seminal works on bosons in BEC III, Yong immediately jumped onto the hottest topic in fermion physics and made his lasting mark. His clear way of thinking is ad- mirable, and I pro¯ted enormously from discussions with him. Thanks also go to Sebastian Raupach, philosopher/physicist, who joined us for his diploma thesis in the heyday of fermion pair BEC. He competed with Christian for the title of the nicest guy on earth. Peter Zarth, another diploma student, and new PhD student Ye-yroung Lee impressed me with their enthusiasm and drive to get the ¯rst potassium MOT at MIT, working in the same little room where Zoran, Claudiu and myself produced our ¯rst laser cooled lithium atoms. I owe a lot to the crew of BEC II. The past members Roberto Onofrio, Chandra Raman, Jamil, and Johnny Vogels for developing the robust technique of stirring ultracold atoms, and the new team Kaiwen Xu, Takashi Mukaiyama, Jit Kee Chin, Dan Miller, Yingmei Liu, Christian Sanner and Widagdo Setiawan for letting me steal Jamil for a while. Dan Miller must receive extra thanks for the many parties at his house that helped foster the group spirit, for buying excellent Dunkin' Donuts co®ee, for having me park my belongings at his place and generally for being hilarious. With BEC III we lithium guys are connected in a love-hate relationship, because of the illegal "trade" of optics and other equipment that is constantly going on. They should have put the wall back up a long time ago. Aaron Leanhardt provided the role model for everyone in our group with his work ethics and deep understanding of the most diverse areas of physics.