Colloquium: Linda Reichl
Location
Physics : 401
Date & Time
March 30, 2016, 3:30 pm – 4:30 pm
Description
TITLE: Transport Processes and Sound Mode Decay in Dilute Bose-Einstein Condensates
ABSTRACT: At temperatures of order 10-7 Kelvin, a macroscopic number of rubidium atoms in a dilute rubidium gas begin to condense into a single quantum state. Historically, this transition provided the first example of a superfluid state that could be identified as a true Bose-Einstein condensate (BEC). We briefly describe the history surrounding the discovery of superfluidity, and then we discuss the behavior of transport processes in BECs. Monatomic BECs have six hydrodynamic modes, two are damped by viscous effect and the remaining four modes consist of first and second sound. Values for the shear viscosity, the speed of first and second sound, and the lifetimes of the sound modes are obtained using a kinetic equation based on Bogoliubov mean field theory. The two pairs of sound modes decay at very different rates, except in the neighborhood of an avoided crossing of the sound speeds, where the identity of the longest lived modes switches. The speed and lifetime of the longest lived sound mode, predicted by the theory, are consistent with that observed in a recent experiment. This unusual behavior of the sound modes, at very low temperature, may provide may provide a means to determine the temperature of the BEC at extremely low temperatures.
ABSTRACT: At temperatures of order 10-7 Kelvin, a macroscopic number of rubidium atoms in a dilute rubidium gas begin to condense into a single quantum state. Historically, this transition provided the first example of a superfluid state that could be identified as a true Bose-Einstein condensate (BEC). We briefly describe the history surrounding the discovery of superfluidity, and then we discuss the behavior of transport processes in BECs. Monatomic BECs have six hydrodynamic modes, two are damped by viscous effect and the remaining four modes consist of first and second sound. Values for the shear viscosity, the speed of first and second sound, and the lifetimes of the sound modes are obtained using a kinetic equation based on Bogoliubov mean field theory. The two pairs of sound modes decay at very different rates, except in the neighborhood of an avoided crossing of the sound speeds, where the identity of the longest lived modes switches. The speed and lifetime of the longest lived sound mode, predicted by the theory, are consistent with that observed in a recent experiment. This unusual behavior of the sound modes, at very low temperature, may provide may provide a means to determine the temperature of the BEC at extremely low temperatures.