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Degenerate Fermi Gases in a Box Potential

Final Activity Report Summary - FERMIBOX (Degenerate Fermi Gases in a Box Potential)

The confinement and control of ultracold fermionic atoms has marked an important achievement in physics, and the results from these experiments are paramount to the entire realm of physical sciences. By tuning the interaction strength of these atoms, one can access two fundamentally different limits of quantum particles: bosons, where atoms pair into molecules and then condense to form a Bose-Einstein condensate (BEC) and fermions, where atoms can unite in a special many-body state of fermions according to BCS (Bardeen, Copper, Schriefer) theory. The change between these limits is called the BEC-BCS crossover.

We examined the collisional behaviour of a finite-temperature ultracold Fermi gas in the BEC-BCS crossover. At the coldest temperatures, atoms exist in a superfluid state. Here, atoms respond in unison to external stimuli and move without friction; this is analogous to the zero resistance in a superconducting wire. At a certain temperature, superfluidity breaks down and the gas is said to be in the normal state. We have explored the temperature where this transition occurs, previously on the BCS side of the crossover, although current experiments are determining it throughout the crossover.

There is also a surprising wealth of information in the normal state as well. Near the transition temperature, atoms are still strongly influenced by quantum mechanical effects, such as many-body pairing. We explored the fact that although the gas is in the normal state, the time for atoms to collide is so fast, it has nearly the same properties as a superfluid and responds hydrodynamicly to external stimuli. At larger temperatures, the time scale between collisions is long and therefore termed collisionless. By after comparing the results of experiments that probe the collisional behaviour and an experiment that examines the pairing, we conclude that these two quantities may be correlated.

These experiments may have important consequences in developing a theory to solve physical real-world problems such as high-temperature superconductivity.