Bosons and Fermions

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Here, we see the ground states of bosons (left) and fermions (right) in harmonic magnetic traps at 0 Kelvin. On the left, we see, as in Figure 22, that all the bosonic atoms are in the unique lowest quantum energy level of the trap. Note that the spins are all pointing in the same direction as their alignment with the trap's magnetic field is, in fact, what holds them in the trap. Because these atoms are bosons, with integer spin, this does not violate the Pauli exclusion principle. On the right, in strong contrast, each spin-up composite fermionic atom occupies a single energy level. Unlike the pairs of oppositely oriented electrons in Figures 12 and 13, there simply are no spin-down atoms as these are not held in the trap. The situation illustrated on the right is a fully degenerate Fermi gas; if most but not all of the energy levels were occupied, we would have a "partially" or "highly" degenerate gas. The use of the word "degenerate" here simply means that energy levels are occupied from the bottom up. (Unit: 6)