# Bose gas

An ideal Bose gas is a quantum-mechanical phase of matter, analogous to a classical ideal gas. It is composed of bosons, which have an integer value of spin, and obey Bose–Einstein statistics. The statistical mechanics of bosons were developed by Satyendra Nath Bose for a photon gas, and extended to massive particles by Albert Einstein who realized that an ideal gas of bosons would form a condensate at a low enough temperature, unlike a classical ideal gas. This condensate is known as a Bose–Einstein condensate.

## Introduction and examples

Bosons are quantum mechanical particles that follow Bose–Einstein statistics, or equivalently, that possess integer spin. These particles can be classified as elementary: these are the Higgs boson, the photon, the gluon, the W/Z and the hypothetical graviton; or composite like the atom of hydrogen, the atom of 16O, the nucleus of deuterium, mesons etc. Additionally, some quasiparticles in more complex systems can also be considered bosons like the plasmons (quanta of charge density waves).

The first model that treated a gas with several bosons, was the photon gas, a gas of photons, developed by Bose. This model lead to a better understanding of Planck's law and the black-body radiation. The photon gas can be easily expanded to any kind of ensemble of massless non-interacting bosons. The phonon gas, also known as Debye model, is an example where the normal modes of vibration of the crystal lattice of a metal, can be treated as effective massless bosons. Peter Debye used the phonon gas model to explain the behaviour of heat capacity of metals at low temperature.

An interesting example of a Bose gas is an ensemble of helium-4 atoms. When a system of 4He atoms is cooled down to temperature near absolute zero, many quantum mechanical effects are present. Below 2.17 kelvins, the ensemble starts to behave as a superfluid, a fluid with almost zero viscosity. The Bose gas is the most simple quantitative model that explains this phase transition. Mainly when a gas of bosons is cooled down, it forms a Bose–Einstein condensate, a state where a large number of bosons occupy the lowest energy, the ground state, and quantum effects are macroscopically visible like wave interference.

The theory of Bose-Einstein condensates and Bose gases can also explain some features of superconductivity where charge carriers couple in pairs (Cooper pairs) and behave like bosons. As a result, superconductors behave like having no electrical resistivity at low temperatures.

The equivalent model for half-integer particles (like electrons or helium-3 atoms), that follow Fermi–Dirac statistics, is called the Fermi gas (an ensemble of non-interacting fermions). At low enough particle number density and temperature, both the Fermi gas and the Bose gas behave like a classical ideal gas.

## Grand canonical ensemble

The thermodynamics of an ideal Bose gas is best calculated using the grand canonical ensemble. The grand partition function for a Bose gas is given by:

${\mathcal {Z}}(z,\beta ,V)=\prod _{i}\left(1-ze^{-\beta \epsilon _{i}}\right)^{-g_{i}}$

where each term in the product corresponds to a particular energy ε; g is the number of states with energy ε; is the absolute activity (or "fugacity"), which may also be expressed in terms of the chemical potential μ by defining:

$z(\beta ,\mu )=e^{\beta \mu }$

and β defined as:

$\beta ={\frac {1}{k_{\rm {B}}T}}$

where kB  is Boltzmann's constant and is the temperature. All thermodynamic quantities may be derived from the grand partition function and we will consider all thermodynamic quantities to be functions of only the three variables , β (or ), and . All partial derivatives are taken with respect to one of these three variables while the other two are held constant. It is more convenient to deal with the dimensionless grand potential defined as:

$\Omega =-\ln({\mathcal {Z}})=\sum _{i}g_{i}\ln \left(1-ze^{-\beta \epsilon _{i}}\right).$

Following the procedure described in the gas in a box article, we can apply the Thomas–Fermi approximation which assumes that the average energy is large compared to the energy difference between levels so that the above sum may be replaced by an integral:

$\Omega \approx \int _{0}^{\infty }\ln \left(1-ze^{-\beta E}\right)\,dg.$

The degeneracy dg  may be expressed for many different situations by the general formula:

$dg={\frac {1}{\Gamma (\alpha )}}\,{\frac {E^{\,\alpha -1}}{E_{\rm {c}}^{\alpha }}}~dE$

where α is a constant, Ec is a critical energy, and Γ is the Gamma function. For example, for a massive Bose gas in a box, α=3/2 and the critical energy is given by:

${\frac {1}{(\beta E_{\rm {c}})^{\alpha }}}={\frac {Vf}{\Lambda ^{3}}}$

where Λ is the thermal wavelength. For a massive Bose gas in a harmonic trap we will have α=3 and the critical energy is given by:

${\frac {1}{(\beta E_{c})^{\alpha }}}={\frac {f}{(\hbar \omega \beta )^{3}}}$

where V(r)=mω2r2/2  is the harmonic potential. It is seen that Ec  is a function of volume only.

We can solve the equation for the grand potential by integrating the Taylor series of the integrand term by term, or by realizing that it is proportional to the Mellin transform of the Li1(z exp(-β E)) where Lis(x) is the polylogarithm function. The solution is:

$\Omega \approx -{\frac {{\textrm {Li}}_{\alpha +1}(z)}{\left(\beta E_{c}\right)^{\alpha }}}.$

The problem with this continuum approximation for a Bose gas is that the ground state has been effectively ignored, giving a degeneracy of zero for zero energy. This inaccuracy becomes serious when dealing with the Bose–Einstein condensate and will be dealt with in the next section.

## Inclusion of the ground state

The total number of particles is found from the grand potential by

$N=-z{\frac {\partial \Omega }{\partial z}}\approx {\frac {{\textrm {Li}}_{\alpha }(z)}{(\beta E_{c})^{\alpha }}}$

The polylogarithm term must remain real and positive, and the maximum value it can possibly have is at z=1 where it is equal to ζ(α) where ζ is the Riemann zeta function. For a fixed , the largest possible value that β can have is a critical value β where

$N={\frac {\zeta (\alpha )}{(\beta _{\rm {c}}E_{\rm {c}})^{\alpha }}}$

This corresponds to a critical temperature Tc=1/kBβc below which the Thomas–Fermi approximation breaks down. The above equation can be solved for the critical temperature:

$T_{\rm {c}}=\left({\frac {N}{\zeta (\alpha )}}\right)^{1/\alpha }{\frac {E_{\rm {c}}}{k_{\rm {B}}}}$

For example, for $\alpha =3/2$  and using the above noted value of ${\textstyle E_{\rm {c}}}$  yields

$T_{\rm {c}}=\left({\frac {N}{Vf\zeta (3/2)}}\right)^{2/3}{\frac {h^{2}}{2\pi mk_{\rm {B}}}}$

Again, we are presently unable to calculate results below the critical temperature, because the particle numbers using the above equation become negative. The problem here is that the Thomas–Fermi approximation has set the degeneracy of the ground state to zero, which is wrong. There is no ground state to accept the condensate and so the equation breaks down. It turns out, however, that the above equation gives a rather accurate estimate of the number of particles in the excited states, and it is not a bad approximation to simply "tack on" a ground state term:

$N=N_{0}+{\frac {{\textrm {Li}}_{\alpha }(z)}{(\beta E_{\rm {c}})^{\alpha }}}$

where N is the number of particles in the ground state condensate:

$N_{0}={\frac {g_{0}\,z}{1-z}}$

Figure 1: Various Bose gas parameters as a function of normalized temperature τ. The value of α is 3/2. Solid lines are for N=10,000, dotted lines are for N=1000. Black lines are the fraction of excited particles, blue are the fraction of condensed particles. The negative of the chemical potential μ is shown in red, and green lines are the values of z. It has been assumed that k =εc=1.

This equation can now be solved down to absolute zero in temperature. Figure 1 shows the results of the solution to this equation for α=3/2, with k=εc=1 which corresponds to a gas of bosons in a box. The solid black line is the fraction of excited states 1-N0/N  for =10,000 and the dotted black line is the solution for =1000. The blue lines are the fraction of condensed particles N0/N  The red lines plot values of the negative of the chemical potential μ and the green lines plot the corresponding values of . The horizontal axis is the normalized temperature τ defined by

$\tau ={\frac {T}{T_{\rm {c}}}}$

It can be seen that each of these parameters become linear in τα in the limit of low temperature and, except for the chemical potential, linear in 1/τα in the limit of high temperature. As the number of particles increases, the condensed and excited fractions tend towards a discontinuity at the critical temperature.

The equation for the number of particles can be written in terms of the normalized temperature as:

$N={\frac {g_{0}\,z}{1-z}}+N~{\frac {{\textrm {Li}}_{\alpha }(z)}{\zeta (\alpha )}}~\tau ^{\alpha }$

For a given and τ, this equation can be solved for τα and then a series solution for can be found by the method of inversion of series, either in powers of τα or as an asymptotic expansion in inverse powers of τα. From these expansions, we can find the behavior of the gas near T =0 and in the Maxwell–Boltzmann as approaches infinity. In particular, we are interested in the limit as approaches infinity, which can be easily determined from these expansions.

One can also calculate the fluctuations of the occupancy of the states. Below the transition, the fluctuations in the occupancy in the ground state become large. The magnitude however is ensemble-dependent. In the usual grand canonical ensemble treatment of the subject, the fluctuates become of the order of the number of particles. However, in the canonical ensemble, the fluctuations are much smaller.

## Thermodynamics

Adding the ground state to the equation for the particle number corresponds to adding the equivalent ground state term to the grand potential:

$\Omega =g_{0}\ln(1-z)-{\frac {{\textrm {Li}}_{\alpha +1}(z)}{\left(\beta E_{\rm {c}}\right)^{\alpha }}}$

All thermodynamic properties may now be computed from the grand potential. The following table lists various thermodynamic quantities calculated in the limit of low temperature and high temperature, and in the limit of infinite particle number. An equal sign (=) indicates an exact result, while an approximation symbol indicates that only the first few terms of a series in $\tau ^{\alpha }$  is shown.

Quantity General $T\ll T_{c}\,$  $T\gg T_{c}\,$
$z$  $\approx {\frac {\zeta (\alpha )}{\tau ^{\alpha }}}-{\frac {\zeta ^{2}(\alpha )}{2^{\alpha }\tau ^{2\alpha }}}$  $=1\,$
Vapor fraction
$1-{\frac {N_{0}}{N}}\,$
$={\frac {{\textrm {Li}}_{\alpha }(z)}{\zeta (\alpha )}}\,\tau ^{\alpha }$  $=\tau ^{\alpha }\,$  $=1\,$
Equation of state
${\frac {PV\beta }{N}}=-{\frac {\Omega }{N}}\,$
$={\frac {{\textrm {Li}}_{\alpha \!+\!1}(z)}{\zeta (\alpha )}}\,\tau ^{\alpha }$  $={\frac {\zeta (\alpha \!+\!1)}{\zeta (\alpha )}}\,\tau ^{\alpha }$  $\approx 1-{\frac {\zeta (\alpha )}{2^{\alpha \!+\!1}\tau ^{\alpha }}}$
Gibbs Free Energy
$G=\ln(z)\,$
$=\ln(z)\,$  $=0\,$  $\approx \ln \left({\frac {\zeta (\alpha )}{\tau ^{\alpha }}}\right)-{\frac {\zeta (\alpha )}{2^{\alpha }\tau ^{\alpha }}}$

It is seen that all quantities approach the values for a classical ideal gas in the limit of large temperature. The above values can be used to calculate other thermodynamic quantities. For example, the relationship between internal energy and the product of pressure and volume is the same as that for a classical ideal gas over all temperatures:

$U={\frac {\partial \Omega }{\partial \beta }}=\alpha PV$

A similar situation holds for the specific heat at constant volume

$C_{V}={\frac {\partial U}{\partial T}}=k_{\rm {B}}(\alpha +1)\,U\beta$

The entropy is given by:

$TS=U+PV-G\,$

Note that in the limit of high temperature, we have

$TS=(\alpha +1)+\ln \left({\frac {\tau ^{\alpha }}{\zeta (\alpha )}}\right)$

which, for α=3/2 is simply a restatement of the Sackur–Tetrode equation. In one dimension bosons with delta interaction behave as fermions, they obey Pauli exclusion principle. In one dimension Bose gas with delta interaction can be solved exactly by Bethe ansatz. The bulk free energy and thermodynamic potentials were calculated by Chen-Ning Yang. In one dimensional case correlation functions also were evaluated. In one dimension Bose gas is equivalent to quantum non-linear Schrödinger equation.