First of all we assume that we are dealing with a weakly quantum gas, i.e. ${\frac {n}{n_{Q}}}<1$. The system is a gran-canonical ensemble. Hence the partition function is:

$Z_{GC}=\sum _{N,E}e^{-\beta (E-N\mu )}g(E,N)=\sum _{n_{1}}\sum _{n_{2}}\dots \left(e^{-\beta \sum _{i=1}^{N}\left(n_{i}\epsilon _{i}-\mu n_{i}\right)}\right)$

Which is equivalent to:

$Z_{GC}=\prod _{i=1}^{N}\left(\sum _{n_{i}}e^{-\beta (\epsilon _{i}-\mu )n_{i}}\right)={\begin{cases}\prod _{i}\left(1+e^{-\beta (\epsilon _{i}-\mu )}\right)\\\prod _{i}\left({\frac {1}{1-e^{-\beta (\epsilon _{i}-\mu )}}}\right)\end{cases}}$

Where the first expression holds for fermions, while the second one for bosons. In a gran canonical ensemble, the thermodynamic potential is given by the *Landau thermodynamic potential*:

$\Omega (V,T,\mu )=F-{\frac {\partial F}{\partial N}}N=F-\mu N=-kT\log Z_{GC}$

Since it can be proved^{[1]} that $G=\mu N$, we also have: $\Omega =-pV$. From where one can derive the equation of state for weakly quantum gases:

$pV=kT\log Z_{GC}=nkT\left(1\pm {\frac {n}{n_{Q}2^{\frac {5}{2}}}}\right)$

Where the plus sign is for fermions, while the minus sign is for bosons.

Electrons in a metal can be idealized as fermions (electrons) in a box. Electrons will occupy energy levels according to the Fermi-Dirac distribution. In particular we have to note that:

$f(\epsilon ){\underset {T\rightarrow 0}{\rightarrow }}\theta (\epsilon -\mu )={\begin{cases}0\;\;\epsilon >\mu \\1\;\;\epsilon <\mu \end{cases}}$

We do define *Fermi energy*, the maximum of the energy of the occupied level: $\epsilon _{F}=\mu (T=0)$. If we define $D(\epsilon )d\epsilon$ as the number of states, per unit volume, with energy in the interval $(\epsilon ;\epsilon +d\epsilon )$ we have^{[2]}:

$\int _{0}^{\epsilon _{F}}D(\epsilon )d\epsilon =n$

The explicit expression of $D(\epsilon )$ can be derived, obtaining:

$D(\epsilon )={\frac {1}{2\pi ^{2}}}\left({\frac {2m}{\hbar ^{2}}}\right)^{\frac {3}{2}}{\sqrt {\epsilon }}$

Meaning that the Fermi energy^{[3]} equals to:

$\epsilon _{F}={\frac {\hbar ^{2}}{2m}}k_{F}^{2}={\frac {\hbar ^{2}}{2m}}\left(3\pi ^{2}n\right)^{\frac {2}{3}}$

Hence we have that $T_{f}={\frac {\epsilon _{f}}{k_{B}}}$. The chemical potential $\mu$ depends on the temperature $T$ with the relation:

$\mu (T)=\epsilon _{F}\left(1-{\frac {\pi ^{2}}{12}}\left({\frac {T}{T_{F}}}\right)^{2}\right)$

The specific heat, at constant volume $V$, is defined as:

$C_{V}={\frac {1}{V}}{\frac {\partial E}{\partial T}}={\frac {\pi ^{2}}{2}}nk{\frac {T}{T_{F}}}$

We now give three important relations that tell us what happens to the system when it happens a change in some quantity that describes it:

- $\Delta E^{2}=kT^{2}C_{V}$ in a canonical ensemble;

- $\Delta V^{2}=-{\frac {\partial V}{\partial p}}kT$ in an isobaric ensemble;

- $\Delta N^{2}={\frac {\partial N}{\partial \mu }}kT$ in a gran-canonical ensemble.

Quantum superfluids can be idealized as bosons in a box. Bosons in a box occupy energy levels according to the Bose-Einstein distribution. Moreover, if we do assume that all the particles occupy the level at $\epsilon =0$, i.e. the lowes energy level, we have:

$0<f_{BE}(\epsilon =0)={\frac {1}{e^{-\beta \mu }-1}}$

Where we do expect that $e^{-\beta \mu }>1$, that is $\mu <0$. Here we are assuming that *a macroscopic number of particles* occupy a certain energy level, more precisely the lowest possible^{[4]}! Actually, this is not a problem since bosons do not have to respect Pauli's exclusion principle. Meaning that the particles density is given by:

$n=n_{0}+\int _{0}^{\infty }{\frac {D(\epsilon )}{e^{\beta (\epsilon -\mu )}-1}}d\epsilon$

Where the contribute of $n_{0}$ is foundamental for temperatures $T<T_{C}$, the Bose-Einstein condensation temperature. We also have that:

$n_{0}(T)=n\left(1-\left({\frac {T}{T_{C}}}\right)^{\frac {3}{2}}\right),\;T<T_{C}$

The condensation temperature can be derived^{[5]} and its value is:

$kT_{C}=n^{\frac {2}{3}}{\frac {\hbar ^{2}}{2m}}4\pi \left({\frac {1}{2,612}}\right)^{\frac {2}{3}}$

And the condition $T<T_{C}$ is equivalent to:

$n>n_{Q}(2,612),\;\;n_{Q}=\left({\frac {mkT}{2\pi \hbar ^{2}}}\right)^{\frac {3}{2}}$

Bose-Einstein condensation is a necessary but not sufficient condition to the superfluidity. For such systems we can define the specific heat, at constant pressure $P$: $C_{P}=T{\frac {\partial S}{\partial T}}$. We have that:

$C_{P}=-T{\frac {\partial ^{2}G}{\partial T^{2}}}$

- ↑ From $G=G(P,T,N)$ and $G=E-TS+pV$, using $dE=TdS-pdV+\mu dN$
- ↑ For a gas, given its density $\rho$, its mass number $A$ (expressed in u.a.m) and its atomic number $Z$ we can write $n={\frac {N}{V}}={\frac {Z\rho }{Am_{p}}}$.
- ↑ We can also note that:
${\frac {E}{V}}={\frac {N}{V}}\left\langle \epsilon \right\rangle =\int _{0}^{\epsilon _{F}}D(\epsilon )\epsilon d\epsilon ={\frac {3}{5}}n\epsilon _{F}$

Meaning that $\left\langle \epsilon \right\rangle ={\frac {3}{5}}\epsilon _{F}$
- ↑ This condition is called
*Bose-Einstein condition.*
- ↑ From
$n=\int _{0}^{\infty }d\epsilon D(\epsilon ){\frac {1}{e^{\frac {\epsilon }{T_{C}k}}-1}}$