Images of an ultracold gas of potassium-40 atoms. Since each atom is composed of an odd number of particles, they are classified as fermions--objects which have values of a quantity known as spin in multiples of 1/2. One of the two types of matter found in nature, fermions have many specific properties. Many of the building blocks of nature, such as electrons, neutrons, and protons, are also fermions.
Ultracold Fermi gases can provide potential insights into the basic properties of such objects as neutron stars, since they are made up of fermions. The gas in the picture is initially trapped in magnetic fields. The images show the gas 15 thousandths of a second (milliseconds) after they are released from the trap. The hotter cloud (left) had 2.5 million potassium-40 atoms at a temperatures of 2.4 microkelvin (millionths of a degree above absolute zero). The colder cloud (right) had approximately 780,000 atoms at 290 nanokelvins (billionths of a degree) above absolute zero--the lowest temperature yet recorded for a gas of fermion atoms.
The images are in false color, with white signifying the highest density regions of atoms and blue/black indicating the lowest density regions. At absolute zero (T) all of the atoms would lie inside the radius marked "Fermi energy" in each cloud.
This illustration contrasts how fermions and bosons occupy levels in an energy well. Bosons all fall into the lowest energy state, forming a Bose Einstein condensate. Fermions, on the other hand, must obey the Pauli exclusion principle, which prohibits two identical particles from occupying the same state.
At a temperature of absolute zero (T=0), fermions stack up in the energy levels, with the energy of the highest-filled state called the Fermi energy (figures courtesy of D.S. Jin, NIST/University of Colorado).
This research is reported by B. DeMarco and D.S. Jin in the 10 September 1999 issue of Science.
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