5.10: Appendix II- Ideal Bose Gas Condensation
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We begin with the grand canonical Hamiltonian K=H−μN for the ideal Bose gas, K=∑k(ε∗k−μ)b†kb†k−√N∑k(ν∗kb†k+ˉν†kb†k). Here b†k is the creation operator for a boson in a state of wavevector k, hence [b†k,b†k′]=δ∗kk′. The dispersion relation is given by the function ε∗k, which is the energy of a particle with wavevector k. We must have ε∗k−μ≥0 for all k, lest the spectrum of K be unbounded from below. The fields {ν∗k,ˉν∗k} 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/v∗0 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 N∗b.
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(iNlk⋅a∗l)=1 for all l∈{1,…,d}, where a∗l is the lth fundamental direct lattice vector and N∗l is the size of the system in the a∗l direction; d is the dimension of space. The total number of unit cells is N≡∏lN∗l . Thus, quantization entails k=∑l(2πn∗l/N∗l)b∗l , where b∗l is the lth elementary reciprocal lattice vector (a∗l⋅b∗l′=2πδ∗ll′) and n∗l ranges over N∗l 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(ε∗k−μ)β†kβ†k−N∑k|ν†k|2ε∗k−μ, where β†k=b†k−√Nν†kε∗k−μ,β†k=b†k−√Nˉνkε∗k−μ. Note that [β†k,β†k′]=δ†kk′ so the above transformation is canonical. The Landau free energy Ω=−kBTlnΞ , where Ξ=Tre−K/k∗BT, is given by Ω=NkBT∞∫−∞dεg(ε)ln(1−e(μ−ε)/k∗bT)−N∑k|ν†k|2ε†k−μ, where g(ε) is the density of energy states per unit cell, g(ε)=1N∑kδ(ε−ε∗k)to35pt\rightarrowfillN→∞ ∫ˆΩddk(2π)dδ(ε−ε∗k). Note that ψ∗k≡1√N⟨b†k⟩=−1N∂Ω∂ˉν†k=ν†kε∗k−μ. In the condensed phase, ψ∗k 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)+N∑k(ν∗kˉψ∗k+ˉν∗kψ∗k). Note that ∂Y∂ˉν∗k=∂Ω∂ˉν∗k+Nψ∗k=0, by the definition of ψ∗k. Similarly, ∂Y/∂ν∗k=0. We now have Y(T,N,μ,ψ,ˉψ)=NkBT∞∫−∞dεg(ε)ln(1−e(μ−ε)/k∗bT)+N∑k(ε∗k−μ)|ψ†k|2. Therefore, the boson particle number per unit cell is given by the dimensionless density, n=N∗bN=−1N∂Y∂μ=∑k|ψ†k|2+∞∫−∞dεg(ε)e(ε−μ)/k†BT−1, and the condensate amplitude at wavevector k is ν∗k=1N∂Y∂ˉψ∗kAB=(ε∗k−μ)ψ∗k.
Recall that ν∗k acts as an external field. Let the dispersion ε∗k be minimized at k=K . Without loss of generality, we may assume this minimum value is ε∗K=0 . We see that if ν∗k=0 then one of two must be true:
- ψ∗k=0 for all k
- μ=ε∗K , in which case ψ∗K can be nonzero.
Thus, for ν=ˉν=0 and μ>0, we have the usual equation of state, n(T,μ)=∞∫−∞dεg(ε)e(ε−μ)/k†BT−1, which relates the intensive variables n, T, and μ. When μ=0, the equation of state becomes n(T,μ=0)=n∗0⏞∑K|ψ†K|2+n∗>(T)⏞∞∫−∞dεg(ε)eε/k†BT−1, where now the sum is over only those K for which ε∗K=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 n∗0 , where n∗0=∑K|ψ†K|2 is the dimensionless condensate density. If the integral n∗>(T) in Equation [condeqn] is finite, then for n>n∗0(T) we must have n∗0>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 n∗0=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 T∗c for which n=n∗>(T∗c). For T<T∗c , Equation [GDE] has no solution for any μ and we must appeal to Equation [condeqn]. The condensate density, given by n∗0(n,T)=n−n∗>(T) , is then finite for T<T∗c , and vanishes for T≥T∗c .
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.