Bose-Einstein condensation occurs at 3 dimensions. However, it is not possible to happen at 1 or 2 dimensions; in fact I am able to prove this myself. What is the explanation for this?
Answer
For simplicity, I will consider non-interacting gases. The idea here is that there is a bound on the density of excited states in 3 and higher dimensions, whereas no such bound exists in 1 and 2 dimensions. This bound enforces a condensation of the gas particles into the ground state for 3 and higher dimensions but not in lower.
The details are a bit cumbersome to write down from scratch, so I will refer to some standard details and sketch out the non-trivial part. Check out eqns (7.31) and (7.32) of Mehran Kardar, Statistical physics of particles, where the average occupation number of a non relativistic gas is derived. Write down the same in d dimensions. If you now convert the integral for the average occupation number into a dimensionless form, you will arrive at the generalization of eqn (7.34) for d− dimensions given by
n=gλdf1d/2(z)
where
f1m(z)=1(m−1)!∫∞0dxxm−1z−1ex−1
This integral is finite for all values of z if d≥3, and hence has a maximum value , whereas the same is not true for lower dimensions. In d≥3, we therefore have a bound on the density of excited states at z=1, given by
nx=gλdf1d/2(z)≤n∗=gλdζd/2
where ζd/2 is the max value of the number density of excited states at z=1.
Since the integral is not finite and therefore no such maximum value exists for d=1,2, hence there is no BEC in lower dimensions for this system.
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