Research Team Members

John T. Stewart
John P. Gaebler

Research

One of the key questions in ultracold physics is how atoms behave in the crossover regime of the BCS-BEC continuum of quantum mechanical behavior. BCS stands for the Bardeen-Cooper-Schrieffer theory of superconductivity developed in the mid-1950s. BEC stands for Bose-Einstein condensation.

The condensation behavior of Fermi systems evolves smoothly from BEC behavior (where fermions are paired in tightly bound molecules) through the BCS- BEC crossover to BCS behavior, where Cooper pairs of atoms form a superfluid, as shown below. In the crossover region, pairs of fermions interact strongly with each other. Depending on experimental conditions (in particular, changes in the magnetic field), Fermi atom pairs can behave more like molecules or more like Cooper pairs.

Crossover studies use Feshbach resonances and magnetic tuning to investigate atoms in this region. A Feshbach resonance is a special value of a magnetic field around which small changes in field strength have dramatic effects on the scattering length of atoms in an ultracold gas. Recently, our group used Feshbach resonances to look at the momentum distribution of ⁴⁰K atoms throughout the BCS-BEC continuum. As shown below, ⁴⁰K atoms on the BCS side (left side of the image) showed the least variation in velocity. The  colors indicate the density of atoms moving at the same velocity, with white and red corresponding to the highest densities and blue and green to the lowest densities. At the BCS-BEC crossover (third from left), the atomic velocities were more spread out, with lower atom densities at any one velocity, as evidenced by the green and light blue colors. Here the atom pairs were not only getting much smaller, but more pairs were forming. The velocities showed an even broader spread toward the BEC end of the continuum.

Experiments like this one give theorists enough information to evaluate the strengths and weaknesses of different theories explaining crossover physics. For instance, the experimental observations of atomic momentum distribution in the BCS-BEC crossover were quite different from those predicted by superconductivity theory, and  this poses a challenge to theorists to develop a better understanding of crossover physics.

In our most recent experiment, we measured the potential energy of an ultracold trapped gas of ⁴⁰K atoms in the BCS-BEC crossover and investigated the temperature dependence of this energy. We were able to extract a universal many-body parameter β and determine its value. The value was consistent with previous measurements using ⁶Li atoms and with recent crossover theory. Our new work demonstrates the universality of ultracold Fermi gases in the strongly interacting regime.