Pauli blocking in a Fermi gas
(a) The phenomenon of "Pauli blocking" suppresses elastic collisions in a Fermi gas. The high-energy green atom, for example, can only lose energy (green arrow) when it collides with a low-energy blue atom if it there is an unoccupied, lower-energy final state for the green atom to enter. However, all the low-energy states in the Fermi sea of atoms are already occupied, which means that this collision cannot occur.
(b) Pauli blocking observed by studying "collective excitations" in a gas of potassium-40 atoms with two different spins, denoted by the Zeeman quantum numbers mf = 9/2 and 7/2. The excitations are density oscillations, or sound waves, excited by perturbing the trapping potential. The damping time, tau, which is a measure of how fast the collective excitations fade away, falls below the classical value, tauclassical, as the temperature, T, of the gas drops below the Fermi temperature, TF. This indicates that the collisions required to establish the collective nature of the excitation have been suppressed by Pauli blocking.
(c) An absorption image of the two-component Fermi gas after it is released from the magnetic trap and allowed to expand. The relative motion in the horizontal direction of the two different types of atoms was measured by vertically separating them with a magnetic field.