Quantum Institute: Visitor Schedule

The Quantum Lunch is regularly held on Thursdays in the Theoretical Division Conference Room, TA-3, Building 123, Room 121.
For more information, contact Diego Dalvit.
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Thursday, October 20, 2011
12:30 PM - 2:00 PM

Speaker: Chih-Chun Chien (T-4)

Technical Host:

TOPIC: Auxiliary-field-theory approach to ultra-cold atoms and how it solved a 63-year old problem

Abstract
In this talk I will present an auxiliary-field-theory approach developed here in Los Alamos. Already in the leading-order-auxiliary-field (LOAF) theory of repulsive bosons there are several important features. The LOAF theory satisfies three crucial requirements of a consistent theory for bosons: conservativeness, gaplessness, and a second-order Bose-Einstein condensation (BEC) transition. It is the first theory to meet all three requirement since 1947 when Bogoliubov proposed his formulation of BEC for weakly interacting bosons. The LOAF theory succeeds because it treats normal and anomalous densities on equal footing and preserves the symmetry in its approximations. I will compare LOAF results to other popular theories to highlight this point. Another interesting prediction from the LOAF theory is that there is a superfluid phase above the BEC transition temperature and the superfluid density is closely related to the anomalous density, not the BEC condensate density. I will show that the same LOAF framework applies to fermions in BCS-BEC crossover. Surprisingly, Leggett-BCS theory is the only meaningful theory in this framework.

Reference: PRL 105, 240402 (2010); PRA 83, 053622 (2011); PRA 83,053637 (2011); PRA 84, 023603 (2011).