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978-3-8439-1180-1, Reihe Physik

Georg Wirth Orbital Superfluidity in Excited Bands of an Optical Lattice

342 Seiten, Dissertation Universität Hamburg (2013), Hardcover, B5

Since their first demonstration, optical lattices have raised hopes of emulating the physics of electrons in solids, and with this provide answers to yet unsolved problems like e. g. high-temperature superconductivity. Following R. Feynman's idea of a universal quantum simulator, researchers therefore successively developed additional "tuning knobs" and measurement techniques for their experiments. Up to now however, the orbital degree of freedom remained mostly unexplored.

This thesis presents the first realization of superfluids in excited bands of a bipartite optical lattice. Using a ground-state superfluid it was first shown that the ensemble's many-body wave-function in the experiment is in a good approximation given by single-particle Bloch-functions. By means of a population swapping technique, atoms were transferred into higher bands, in which the particles form a metastable condensate via a collision-aided condensation process. This method was used to prepare superfluids in excited bands with complex-valued order parameters.

To compare the measured data with the single-particle model, a band calculation was applied, which allowed us to identify non-trivial higher orbitals with local angular momentum. In the second band, a complex superposition of two different Bloch states at non-equivalent minima in the Brillouin zone, was discovered. The population of both wave-functions in this state can be adjusted by tuning a small energy difference between the condensation points via an arbitrary distortion of our lattice potential. A mean field analysis shows that the complex superposition is stabilized by the repulsive atomic interaction. In the seventh band, the dynamical growth and subsequent decay of coherence is observed, and decay processes which rethermalize the system are analyzed.