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5.10: Appendix II- Ideal Bose Gas Condensation

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We begin with the grand canonical Hamiltonian K=HμN for the ideal Bose gas, K=k(εkμ)bkbkNk(νkbk+ˉνkbk). Here bk is the creation operator for a boson in a state of wavevector k, hence [bk,bk]=δkk. The dispersion relation is given by the function εk, which is the energy of a particle with wavevector k. We must have εkμ0 for all k, lest the spectrum of K be unbounded from below. The fields {νk,ˉνk} break a global O(2) symmetry.

Students who have not taken a course in solid state physics can skip the following paragraph, and be aware that N=V/v0 is the total volume of the system in units of a fundamental "unit cell" volume. The thermodynamic limit is then N. Note that N is not the boson particle number, which we’ll call Nb.

Solid state physics boilerplate : We presume a setting in which the real space Hamiltonian is defined by some boson hopping model on a Bravais lattice. The wavevectors k are then restricted to the first Brillouin zone, ˆΩ, and assuming periodic boundary conditions are quantized according to the condition exp(iNlkal)=1 for all l{1,,d}, where al is the lth fundamental direct lattice vector and Nl is the size of the system in the al direction; d is the dimension of space. The total number of unit cells is NlNl . Thus, quantization entails k=l(2πnl/Nl)bl , where bl is the lth elementary reciprocal lattice vector (albl=2πδll) and nl ranges over Nl distinct integers such that the allowed k points form a discrete approximation to ˆΩ .

To solve, we first shift the boson creation and annihilation operators, writing K=k(εkμ)βkβkNk|νk|2εkμ, where βk=bkNνkεkμ,βk=bkNˉνkεkμ. Note that [βk,βk]=δkk so the above transformation is canonical. The Landau free energy Ω=kBTlnΞ , where Ξ=TreK/kBT, is given by Ω=NkBTdεg(ε)ln(1e(με)/kbT)Nk|νk|2εkμ, where g(ε) is the density of energy states per unit cell, g(ε)=1Nkδ(εεk)to35pt\rightarrowfillN ˆΩddk(2π)dδ(εεk). Note that ψk1Nbk=1NΩˉνk=νkεkμ. In the condensed phase, ψk is nonzero.

The Landau free energy (grand potential) is a function Ω(T,N,μ,ν,ˉν). We now make a Legendre transformation, Y(T,N,μ,ψ,ˉψ)=Ω(T,N,μ,ν,ˉν)+Nk(νkˉψk+ˉνkψk). Note that Yˉνk=Ωˉνk+Nψk=0, by the definition of ψk. Similarly, Y/νk=0. We now have Y(T,N,μ,ψ,ˉψ)=NkBTdεg(ε)ln(1e(με)/kbT)+Nk(εkμ)|ψk|2. Therefore, the boson particle number per unit cell is given by the dimensionless density, n=NbN=1NYμ=k|ψk|2+dεg(ε)e(εμ)/kBT1, and the condensate amplitude at wavevector k is νk=1NYˉψkAB=(εkμ)ψk.

Recall that νk acts as an external field. Let the dispersion εk be minimized at k=K . Without loss of generality, we may assume this minimum value is εK=0 . We see that if νk=0 then one of two must be true:

  • ψk=0 for all k
  • μ=εK , in which case ψK can be nonzero.

Thus, for ν=ˉν=0 and μ>0, we have the usual equation of state, n(T,μ)=dεg(ε)e(εμ)/kBT1, which relates the intensive variables n, T, and μ. When μ=0, the equation of state becomes n(T,μ=0)=n0K|ψK|2+n>(T)dεg(ε)eε/kBT1, where now the sum is over only those K for which εK=0 . Typically this set has only one member, K=0, but it is quite possible, due to symmetry reasons, that there are more such K values. This last equation of state is one which relates the intensive variables n, T, and n0 , where n0=K|ψK|2 is the dimensionless condensate density. If the integral n>(T) in Equation [condeqn] is finite, then for n>n0(T) we must have n0>0. Note that, for any T, n>(T) diverges logarithmically whenever g(0) is finite. This means that Equation [GDE] can always be inverted to yield a finite μ(n,T), no matter how large the value of n, in which case there is no condensation and n0=0. If g(ε)εα with α>0, the integral converges and n>(T) is finite and monotonically increasing for all T. Thus, for fixed dimensionless number n, there will be a critical temperature Tc for which n=n>(Tc). For T<Tc , Equation [GDE] has no solution for any μ and we must appeal to Equation [condeqn]. The condensate density, given by n0(n,T)=nn>(T) , is then finite for T<Tc , and vanishes for TTc .

In the condensed phase, the phase of the order parameter ψ inherits its phase from the external field ν, which is taken to zero, in the same way the magnetization in the symmetry-broken phase of an Ising ferromagnet inherits its direction from an applied field h which is taken to zero. The important feature is that in both cases the applied field is taken to zero after the approach to the thermodynamic limit.


This page titled 5.10: Appendix II- Ideal Bose Gas Condensation is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by Daniel Arovas.

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