Semiclassical studies of decoherence produced by scattering

The conventional notion of coherent atom-surface scattering originates from the existence of Bragg peaks in elastic scattering. The helium atom acts as a quantum mechanical matter wave that is coherent with itself; the well-defined phase relationship of the particle beam at the different spatial positions at surface impact implies the possibility of different non-specular outgoing beams thanks to the constructive interference of the emitted waves from each surface atom. Moreover, we still observe diffraction peaks when scattering off a lattice at finite temperature, although the peaks are here diminished by the Debye-Waller factor. However, in the case of inelastic scattering, the surface particles are displaced by the scattering atom itself and may then emit or absorb one or more phonons to the scatterer. Acoustic phonons produced by this process are gapless excitations; hence, extremely long-wavelength phonons will contribute vanishingly small shifts in energy and momentum. The difficulty in observing this is exacerbated due to the roughly 1eV resolution of high energy helium scattering experiments. So through phonon excitation the surface has "measured" the particle's presence which acts to destroy quantum coherence, though we still observe diffraction spots which imply coherent scattering. How do we reconcile these disparate viewpoints? We propose a new way of looking at the question of coherence in atom-surface scattering. Instead of considering a single beam of helium particles, we instead use semiclassical techniques to simulate an initially coherent superposition of helium particles with equal probabilities of interacting with the surface or not interacting with the surface. We then evolve the classical mechanical trajectories, and recombine the atoms after scattering to observe the resulting interference pattern. The degree to which phonons are excited in the lattice by the scattering process dictates the fringe contrast of the interference pattern of the resulting beams. We show that for a wide range of conditions, despite the massive change in the momentum perpendicular to the surface, we can still expect to have coherent (in the superposition sense) scattering. Thesis Supervisor: Eric Heller Title: Professor of Physics and Chemistry, Harvard University Thesis Co-Supervisor: Nuh Gedik Title: Professor of Physics Acknowledgements Like most of the students at MIT I have met, I found my experience through my PhD to be a challenging one, often in ways I didn't anticipate, and I would not have made it through the PhD process if it hadn't been for the help and support of many people. I owe them in ways I could never repay. I want to first thank my family for their continual support and encouragement. My father always prioritized the education of his children more than anything else, was always there to lend an ear and continue urging me to fight the good fight and stay the course whenever I was discouraged. My mother has been a kind soul and I was always glad for her encouragement. My little brother has been an emotional bedrock and stalwart chap I could always depend on, and my little sister and my niece have always been a joy to see. I want to thank the core crew that I met in my courses during my first year: Chris, Sasha, Will, and Jeremy. I'll always treasure the memories of all the shenanigans we got up to in Ashdown and around MIT and Boston. While there are many more, some highlights have included discussing our past lives in the IGRE, hoisting the Canadian flag, honoring the Marquis and the one named Groovy, the athena cluster stalking, roadtripping to Royalton and Quebec, trance Eurosexuality, MLP, Doctor Who, the jock++ and no shame lifestyle and the despair. Through thick and thin, whether it was at the "Chestnut Tree Cafe," venting at the gym, obscene XBox games, Ahhnold movie nights, excessively cursed Dominion games, eating vegetable teriyaki at the student center, or late nights experiencing the crash over psets, you have been the best friends I could ask for. I would be negligent if I didn't mention the other people I got to know at MIT: Maksim, Darius, TC, Ben, John, Arturs, Carlos, Lucy, Ariel, Hiro, Colin, and Daniel. There are others I've known whom for one reason or another I've either neglected to remember here or who drifted apart from me, but know that I am grateful for the time that we had together. My experience and friends at athenahealth and my continued friends from UC Davis were also delightful. I would also like to thank the other members of my group, especially Lucas (and Alyssa), Anna, Byron, and Susanne. You guys have become some of my closest friends while in Rick's group and I'll miss talking physics, making dirty jokes and learning from you guys. Even though Lucas will probably roll his eyes at this, you made my last few years much better. I would like to thank my advisors Rick Heller at Harvard and Nuh Gedik at MIT. I have enjoyed working with you, and would like to thank you for the inspiration and support you have given me, as well being patient with me through my research. I would also like to thank the many professors I've TAed for, especially Peter Fisher, Josh Winn, and David Pritchard. It has been a pleasure getting to teach with the best in the world. I finally want to thank Erica for being my favorite adventure buddy over the last few years. Your wit, your smile, your humor and your kindness have made me incredibly happy and I can't imagine where I'd be without you in my life. Your sloth is looking forward to starting the next chapter with you!