1. Cooper pairs of electrons in superconductors or pairs of 3He atoms in superfluid 3He below 3mK represent a special case of the degenerate Bose regime, which cannot be treated in the framework of the ideal Bose gas model (the average distance between different pairs is small compared to the radius of a pair).
2. Experimentally, it is found that the energy distribution of the He atoms is very different from the ideal Bose distribution of equation (2b). In fact, the uncondensed fraction appears to have an almost classical (Boltzmann) distribution characterized by an effective temperature of about 15°K even when the sample is well below the lambda point at 2.17K. The condensed fraction has been measured recently. It is the order of 10 percent well below Tλ, and it is found to have a temperature variation which is considerably different from that of an ideal Bose-Einstein condensate.
3. A. Einstein, Sitz, pr. Akad. Wiss. Phys. Math. Kl 22., 261 (1924),
4. A. Einstein, Sitz, pr. Akad. Wiss. Phys. Math. Kl 23 3 (1925).
5. F. London, Nature, Lond 141, 643 (1938).