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4.1: Microcanonical Ensemble (μCE)

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The microcanonical distribution function

We have seen how in an ergodic dynamical system, time averages can be replaced by phase space averages:

ergodicityf(φ)t=f(φ)S ,

where

f(φ)t=limT1TT0dtf(φ(t)) .

and

f(φ)S=dμf(φ)δ(EˆH(φ))/dμδ(EˆH(φ)) .

Here ˆH(φ)=ˆH(q,p) is the Hamiltonian, and where δ(x) is the Dirac δ-function1. Thus, averages are taken over a constant energy hypersurface which is a subset of the entire phase space.

We’ve also seen how any phase space distribution ϱ(Λ1,,Λk) which is a function of conserved quantitied Λa(φ) is automatically a stationary (time-independent) solution to Liouville’s equation. Note that the microcanonical distribution,

ϱE(φ)=δ(EˆH(φ))/dμδ(EˆH(φ)) ,

is of this form, since ˆH(φ) is conserved by the dynamics. Linear and angular momentum conservation generally are broken by elastic scattering off the walls of the sample.

So averages in the microcanonical ensemble are computed by evaluating the ratio

A=TrAδ(EˆH)Trδ(EˆH) ,

where Tr means ‘trace’, which entails an integration over all phase space:

TrA(q,p)1N!Ni=1ddpiddqi(2π)dA(q,p) .

Here N is the total number of particles and d is the dimension of physical space in which each particle moves. The factor of 1/N!, which cancels in the ratio between numerator and denominator, is present for indistinguishable particles2. The normalization factor (2π)Nd renders the trace dimensionless. Again, this cancels between numerator and denominator. These factors may then seem arbitrary in the definition of the trace, but we’ll see how they in fact are required from quantum mechanical considerations. So we now adopt the following metric for classical phase space integration:

dμ=1N!Ni=1ddpiddqi(2π)d .

Density of States

The denominator,

D(E)=Trδ(EˆH) ,

is called the density of states. It has dimensions of inverse energy, such that

D(E)ΔE=E+ΔEEdEdμδ(EˆH)=E<ˆH<E+ΔEdμ=\# of states with energies between E and E+ΔE .

Let us now compute D(E) for the nonrelativistic ideal gas. The Hamiltonian is

ˆH(q,p)=Ni=1p2i2m .

We assume that the gas is enclosed in a region of volume V, and we’ll do a purely classical calculation, neglecting discreteness of its quantum spectrum. We must compute

D(E)=1N!Ni=1ddpiddqi(2π)dδ(ENi=1p2i2m) .

We shall calculate D(E) in two ways. The first method utilizes the Laplace transform, Z(β):

Z(β)=L[D(E)]0dEeβED(E)=TreβˆH .

The inverse Laplace transform is then

D(E)=L1[Z(β)]c+icidβ2πieβEZ(β) ,

where c is such that the integration contour is to the right of any singularities of Z(β) in the complex β-plane. We then have

Z(β)=1N!Ni=1ddxiddpi(2π)deβp2i/2m=VNN!(dp2πeβp2/2m)Nd=VNN!(m2π2)Nd/2βNd/2 .

The inverse Laplace transform is then

D(E)=VNN!(m2π2)Nd/2Cdβ2πieβEβNd/2=VNN!(m2π2)Nd/2E12Nd1Γ(Nd/2) ,

exactly as before. The integration contour for the inverse Laplace transform is extended in an infinite semicircle in the left half β-plane. When Nd is even, the function βNd/2 has a simple pole of order Nd/2 at the origin. When Nd is odd, there is a branch cut extending along the negative Reβ axis, and the integration contour must avoid the cut, as shown in Figure 4.1.1. One can check that this results in the same expression above, we may analytically continue from even values of Nd to all positive values of Nd.

For a general system, the Laplace transform, Z(β)=L[D(E)] also is called the partition function. We shall again meet up with Z(β) when we discuss the ordinary canonical ensemble.

clipboard_e314319edc92c19301e61c81a21f9b884.png
Figure 4.1.1: Complex integration contours C for inverse Laplace transform L1[Z(β)]=D(E). When the product dN is odd, there is a branch cut along the negative Reβ axis.

Our final result, then, is

D(E,V,N)=VNN!(m2π2)Nd/2E12Nd1Γ(Nd/2) .

Here we have emphasized that the density of states is a function of E, V, and N. Using Stirling’s approximation,

lnN!=NlnNN+12lnN+12ln(2π)+O(N1) ,

we may define the statistical entropy,

S(E,V,N)kBlnD(E,V,N)=NkBϕ(EN,VN)+O(lnN) ,

where

ϕ(EN,VN)=d2ln(EN)+ln(VN)+d2ln(mdπ2)+(1+12d) .

Recall kB=1.3806503×1016erg/K is Boltzmann’s constant.

Second method

The second method invokes a mathematical trick. First, let’s rescale pαi2mEuαi. We then have

D(E)=VNN!(2mEh)Nd1EdMuδ(u21+u22++u2M1) .

Here we have written u=(u1,u2,,uM) with M=Nd as a M-dimensional vector. We’ve also used the rule δ(Ex)=E1δ(x) for δ-functions. We can now write

dMu=uM1dudΩM ,

where dΩM is the M-dimensional differential solid angle. We now have our answer:3

D(E)=VNN!(2mh)NdE12Nd112ΩNd .

What remains is for us to compute ΩM, the total solid angle in M dimensions. We do this by a nifty mathematical trick. Consider the integral

IM=dMueu2=ΩM0duuM1eu2=12ΩM0dss12M1es=12ΩMΓ(12M) ,

where s=u2, and where

Γ(z)=0dttz1et

is the Gamma function, which satisfies zΓ(z)=Γ(z+1).4 On the other hand, we can compute IM in Cartesian coordinates, writing

IM=(du1eu21)M=(π)M .

Therefore

ΩM=2πM/2Γ(M/2) .

We thereby obtain Ω2=2π, Ω3=4π, Ω4=2π2, , the first two of which are familiar.

Arbitrariness in the definition of S(E)

Note that D(E) has dimensions of inverse energy, so one might ask how we are to take the logarithm of a dimensionful quantity in Equation ???. We must introduce an energy scale, such as ΔE in Equation 4.1.9, and define ˜D(E;ΔE)=D(E)ΔE and S(E;ΔE)kBln˜D(E;ΔE). The definition of statistical entropy then involves the arbitrary parameter ΔE, however this only affects S(E) in an additive way. That is,

S(E,V,N;ΔE1)=S(E,V,N;ΔE2)+kBln(ΔE1ΔE2) .

Note that the difference between the two definitions of S depends only on the ratio ΔE1/ΔE2, and is independent of E, V, and N.

Ultra-relativistic ideal gas

Consider an ultrarelativistic ideal gas, with single particle dispersion ε(p)=cp. We then have

Z(β)=VNN!ΩNdhNd(0dppd1eβcp)N=VNN!(Γ(d)Ωdcdhdβd)N .

The statistical entropy is S(E,V,N)=kBlnD(E,V,N)=NkBϕ(EN,VN), with

ϕ(EN,VN)=dln(EN)+ln(VN)+ln(ΩdΓ(d)(dhc)d)+(d+1)

Discrete systems

For classical systems where the energy levels are discrete, the states of the system |σ are labeled by a set of discrete quantities {σ1,σ2,}, where each variable σi takes discrete values. The number of ways of configuring the system at fixed energy E is then

Ω(E,N)=σδˆH(σ),E ,

where the sum is over all possible configurations. Here N labels the total number of particles. For example, if we have N spin-12 particles on a lattice which are placed in a magnetic field H, so the individual particle energy is εi=μ0Hσ, where σ=±1, then in a configuration in which N particles have σi=+1 and N=NN particles have σi=1, the energy is E=(NN)μ0H. The number of configurations at fixed energy E is

\ROmega(E,N)={N\choose N\ns_\uar} = {N!\over \big({N\over 2} - {E\over 2\mu\ns_0 H}\big)! \, \big({N\over 2} + {E\over 2\mu\ns_0 H}\big)!}\ ,

since N\ns_{\uar/\dar}={N\over 2}\mp {E\over 2\mu\ns_0 H}. The statistical entropy is S(E,N)=\kB\ln\ROmega(E,N).


This page titled 4.1: Microcanonical Ensemble (μCE) is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by Daniel Arovas.

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