Quantum Behavior of an Atomic Fermi Gas – Brian DeMarco

Date: Thu. May 1st, 2003, 4:15 pm-5:15 pm
Location: Rockefeller 301

The colloquium will cover my graduate work on creating the first Fermi gas of atoms. The magnetic trapping and evaporative cooling techniques used to produce atomic Bose-Einstein condensation were extended to create the first quantum degenerate Fermi gas of atoms. Evaporatively cooling fermionic atoms is hindered by the fundamental difficulty that identical fermionic atoms do not collide at ultra-cold temperatures (less than a few 100 mK). This complication was overcome by magnetically trapping two spin-states of the fermionic atom 40K which undergo the s-wave collisions necessary for rethermalization during cooling.

The unique properties of binary collisions of fermionic atoms were used to make the first measurement of the 40K s-wave triplet scattering length. In particular, by varying the spin mixture of the gas the effect of p-wave and s-wave collisions on rethermalization could be separated at low temperature. The simultaneous measurement of two partial wave collision cross-sections enabled a precise determination of the triplet scattering length, a parameter crucial to cold collision theory. In addition, we made one of the first measurements of the Wigner threshold law for p-wave collisions of neutral scatterers.

A novel two-component cooling scheme was implemented to cool the gas into the quantum degenerate regime where the temperature T is less than the Fermi temperature TF. The effects of quantum degeneracy were first observed in the thermodynamics of a spin-polarized gas. Not only did the gas possess higher energy than expected classically, but the momentum distribution of the gas deviated from the classical expectation. A model was developed of the evaporative cooling process in order understand the limitations of the cooling scheme. Technical improvements to the apparatus motivated by this model have led to progress farther into the degenerate regime (T/TF~0.2) with interacting (spin-mixed) gases. The thermodynamics of interacting gases was studied, and Pauli blocking of collisions was observed through measurements of the dynamics of the spin-mixed gas.

The colloquium will finish with an overview of current efforts to realize a phase transition to a paired, superconducting-like state in an atomic Fermi gas.