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S(q, ω) = eT . (6.153)
kT q 2

For the di¬erential cross section of a Boltzmann gas of N non-interacting nuclei
we have according to Eq. (6.153)

d2 σ m2 a2 p 2π
=N n3 S(q, ω)
dˆ d (2π ) p

m2 a2 p 2π q2
dt e’iωt e’ 2M (t kT ’i t)
=N n3 . (6.154)
(2π ) p

Exercise 6.5. Show that the limiting behavior of the total cross section for a Boltz-
mann gas is

d2 σ
σ = dˆ d
dˆ d
4π 0
21 We here follow the conventional notation, although in the standard notation of this chapter
we have S(x, x , ω) = χ> (x, x , ω). According to the ¬‚uctuation“dissipation theorem, the structure
function is related to the density response function according to S(x, x , ω) = 2 n(ω) m χ(x, x , ω).
178 6. Linear response theory

§ Mp 2
mn a 2
⎪ 4πN 2
for 1

⎪ √
2π 2 2m2 kT
⎨ Mp2
mn n
π (1 + 2m 2 k T
= (6.155)

⎪ 4πN
mn a 2 Mp
for 1.
2π 2 2m2 kT
(1 + ) n

The divergent result for low energies is caused by the almost vanishing ¬‚ux of in-
coming neutrons being scattered by the moving nuclei in the gas, and in the opposite
limit we recover the result for scattering o¬ N free and non-interacting nuclei.

For a discussion of the liquid“gas transition, and the phenomenon of critical
opalescence we refer the reader to chapter 7 of reference [1].

6.8 Summary
The non-equilibrium states of a system which allows a description with su¬cient
accuracy by taking into account only the linear response occupies an especially simple
regime. In fact, the non-equilibrium properties of such states could be completely
understood in terms of the ¬‚uctuations characterizing the equilibrium state. Since
the equilibrium state possesses universal properties, so does the dissipative regime of
ever so slight perturbations, a feature with many important practical consequences.
In Chapter 11 we shall return to study the linear response functions, the transport
coe¬cients or conductivities. In particular we shall study the electrical conductivity
of a disordered conductor in the quantum regime and take into account nonlinear
e¬ects in an applied magnetic ¬eld. To discuss such intricacies we shall express
transport coe¬cients in terms of Green™s functions and thereby have the powerful
method of Feynman diagrams at our disposal. The density response function is
for a system of charged particles equivalent to the e¬ective interaction as density
¬‚uctuations are the source of the interaction. The e¬ective interaction in a disordered
conductor is discussed in Chapter 11. In the next two chapters, we shall study general
non-equilibrium states, and universal properties are in general completely lost.

Quantum kinetic equations

In this chapter, the quantum ¬eld theoretic method will be used to derive quantum
kinetic equations. The classical limit can be established, and quantum corrections
can be studied systematically. Of importance is the fact that the treatment allows
us to assess the validity regime of the kinetic equations by diagrammatic estimates.
The quasi-classical Green™s function technique is introduced. It will allow us to go
beyond classical kinetics and, for example, to discuss renormalization e¬ects due to
interactions in a controlled approximation. Thermo-electric e¬ects, being depending
on particle“hole asymmetry, are not tractable in the quasi-classical technique and
are dealt with on a separate basis.1

7.1 Left“right subtracted Dyson equation
In a non-equilibrium situation, the ¬‚uctuation“dissipation relation is no longer valid,
and the kinetic propagator, GK , is no longer speci¬ed by the spectral function and
the quantum statistics of the particles, as for example in Eq. (3.116). To derive
quantum kinetic equations the left and right matrix Dyson equation™s, Eq. (5.66)
and Eq. (5.69), are subtracted giving

[G’1 ’ Σ — G]’ = 0 ,
, (7.1)

the left“right subtracted Dyson equation. The reason behind this trick will soon
become clear. Here we have again used — to signify matrix multiplication in the spa-
tial and time variables, and introduced notation stressing the matrix multiplication
structure in these variables

[A — B]’ = A — B ’ B — A , [A — B]+ = A — B + B — A ,
, , (7.2)

the latter anti-commutation notation to be employed immediately also. The general
quantum kinetic equation is obtained by taking the kinetic or Keldysh component,
1 This chapter, as well and the following chapter, follows the exposition given in references [3]
and [9].

180 7. Quantum kinetic equations

the o¬-diagonal component, of equation Eq. (7.1) giving
i K— i—K
[G’1 ’ eΣ — GK ]’ ’ [ΣK — [Σ , A]+ ’
, , eG]’ = [“ , G ]+ (7.3)
2 2
where we have introduced the spectral weight function

A(1, 1 ) ≡ i(GR (1, 1 ) ’ GA (1, 1 )) (7.4)

eG(1, 1 ) ≡ (G (1, 1 ) + GA (1, 1 )) (7.5)
and similarly for the self-energies

“(1, 1 ) ≡ i(ΣR (1, 1 ) ’ ΣA (1, 1 )) (7.6)

eΣ(1, 1 ) ≡ (Σ (1, 1 ) + ΣA (1, 1 )) . (7.7)
The way we have grouped the self-energy combinations in Eq. (7.3) appears at the
moment rather arbitrary (compare this also with Section 5.7.4). Recall that A and “
can be expressed as A = i(G> ’G< ) and “ = i(Σ> ’Σ< ) and appear on the right side,
whereas eΣ and eG are of a di¬erent nature. We shall later understand the physics
involved in this di¬erence of appearance of the self-energies: those on the left describe
renormalization e¬ects, i.e. e¬ects of virtual processes, whereas those on the right
describe real dissipative collision processes. The presence of the self-energy entails
one having to deal with a complicated set of equations for an in¬nite hierarchy of
the correlation functions, the starting equation being the Dyson equation. Of course,
the general quantum kinetic equation is useless in practice unless an approximate
expression for the self-energy is available.
Notice that in equilibrium, say at temperature T , the exact quantum kinetic
equation is an empty statement since the Green™s functions are related according to
the ¬‚uctuation“dissipation relation, which for the case of fermions reads3
GR (E, p) ’ GA (E, p)
GK (E, p) = tanh (7.8)
and consequently
ΣR (E, p) ’ ΣA (E, p)
ΣK (E, p) = tanh . (7.9)
As a consequence, the two terms on the right in Eq. (7.3) cancel each other and the
terms on the left are trivially zero in an equilibrium state since the convolution — in
this case is commutative.
In a non-equilibrium situation, the ¬‚uctuation“dissipation relation is no longer
valid. Since a Green™s function is a traced quantity, a closed set of equations can
2 The choice of notation re¬‚ects that in the Wigner or mixed coordinates, A and eG will be
purely real functions as shown in Exercise 7.1 on page 182.
3 Displayed for simplicity for the case of a translational invariant state.
7.2. Wigner or mixed coordinates 181

not be obtained, and one gets complicated equations for an in¬nite hierarchy of the
correlation functions. If one, preferably by some controlled approximation, can break
the hierarchy, usually at most at the two-particle correlation level, one obtains quan-
tum kinetic equations, i.e. equations having the form of kinetic equations, but which
contain quantum features which are not included in the classical Boltzmann equation
[22]. One of the earliest applications of the non-equilibrium Green™s function tech-
nique was to derive such kinetic equation [10] [14], though owing to their complicated
structure they leave in general little progress in their solution by analytical means.
However, as we shall see, combined with the diagrammatic estimation technique,
the enterprise has the virtue of giving access to quantitative criteria for the validity
of the so prevalently used Boltzmann equation, and thus not just the unquanti¬ed
statement of lowest-order perturbation theory.
We now embark on the manipulations leading to a form of the quantum kinetic
equation resembling classical kinetic equations. This is done by introducing Wigner

7.2 Wigner or mixed coordinates
To derive quantum kinetic equations resembling the form of classical kinetic equa-
tions, we introduce the mixed or Wigner coordinates
x1 + x1
r = x1 ’ x1
R= , (7.10)
and time variables4
t1 + t1
, t = t 1 ’ t1
T= (7.11)
in order to separate the variables, (r, t), describing the microscopic properties, gov-
erned by the characteristics of the system, from the variables, (R, T ), describing the
macroscopic properties, governed by the non-equilibrium features of the state under
consideration, say as a result of the presence of an applied potential. To implement
this separation of variables, we Fourier transform all functions with respect to the
relative coordinates, say for a Green™s function

G(X, p) ≡ dx e’ipx G(X + x/2, X ’ x/2) (7.12)

where the abbreviated notation has been introduced

X = (T, R) , x = (t, r) (7.13)

xp = ’Et + p · r .
p = (E, p) , (7.14)
We then express the current and density in terms of the mixed variables. The
average charge density, Eq. (3.54), becomes (the factor of two is from the spin of the
4 No danger of confusion with the notation for the temperature should occur.
182 7. Quantum kinetic equations

particles, say electrons)

ρ(R, T ) = ’2ie dE G< (E, p, R, T ) (7.15)
(2π)3 ’∞

and the average electric current density in the presence of a vector potential A,
Eq. (3.57), becomes in terms of the mixed variables

e dp
j(R, T ) = ’ dE (p ’ eA(R, T )) G< (E, p, R, T ) . (7.16)
m ’∞

1K i
G< = G+A (7.17)
2 2
the current and density can also be expressed in terms of the kinetic Green™s function

e dp
j(R, T ) = ’ dE (p ’ eA(R, T )) GK (E, p, R, T ) (7.18)
m ’∞

and for the density (up to a state independent constant)

ρ(R, T ) = ’2ie dE GK (E, p, R, T ) . (7.19)
(2π)3 ’∞

Exercise 7.1. Show that, for an arbitrary non-equilibrium state, retarded and ad-
vanced Green™s functions in the mixed coordinates are related according to

(GR (R, T, p, E))— = GA (R, T, p, E) . (7.20)

As a consequence, the spectral function in the mixed coordinates is a real function,

(ΣR (R, T, p, E))— = ΣA (R, T, p, E) . (7.21)

Note that in the Wigner coordinates, the spectral function is twice the imaginary
part of the advanced Green™s function.
Exercise 7.2. Show for an arbitrary non-equilibrium state the spectral representa-
tion in the mixed coordinates, the Kramers“Kronig relations,

dE GR (X, p ) ’ GA (X, p )
G (X, p) =
’∞’2πi E ’ E (’) i0

dE A(X, p )
p ≡ (p, E ) .
= , (7.22)
’∞ 2π E ’ E (’) i0
7.2. Wigner or mixed coordinates 183

We now show that a convolution C = A — B in the mixed coordinates is given by
’‚p ‚X )
(A — B)(X, p) = e 2 (‚X ‚p A(X, p) B(X, p) , (7.23)

‚X = (’‚T , ∇R ) , ‚p = (’‚E , ∇p )
‚A ‚B ‚A ‚B
≡’ ·
‚X ‚p + (7.25)
‚T ‚E ‚R ‚p
and the upper index refers to the function operated on. Let us here for clarity
distinguish quantities in the mixed coordinates by a tilde
C(X, x) ≡ C(X + x/2, X ’ x/2) = C(x1 , x1 ) . (7.26)

Consider the convolution

C(x1 , x1 ) ≡ dx2 A(x1 , x2 ) B(x2 , x1 ) , (7.27)

which in mixed coordinates becomes

C(X, x) ≡ dx2 A(X + x/2, x2 ) B(x2 , X ’ x/2)

˜ (X + x/2 + x2 ), X + x/2 ’ x2
= dx2 A

˜ (x2 + X ’ x/2), x2 ’ (X ’ x/2)
B . (7.28)
Making the shift of variable

x2 ’ x2 ’ (X ’ x/2) (7.29)

eliminates the X-dependence in the variable at the relative coordinate place, giving

˜ ˜ ˜
dx2 A(X + x2 /2, x ’ x2 ) B(X ’ x/2 + x2 /2, x2 ) .
C(X, x) = (7.30)

In the mixed coordinates we have

dx e’ixp ˜ ˜
dx2 A(X + x2 /2, x ’ x2 ) B(X ’ x/2 + x2 /2, x2 )
C(X, p) =

dp ’ip (x’x2 )
dx e’ixp
= dx2 e A(X + x2 /2, p )

— B(X ’ x/2 + x2 /2, p ) ,
184 7. Quantum kinetic equations

where in the last equality the integrand has been expressed in the mixed coordinates.
Performing a Taylor expansion and partial integrations then leads to Eq. (7.23).
In particular, for the case of interest of slowly varying perturbations, which cor-
responds to the lowest-order Taylor expansion, the convolution becomes
(A — B)(X, p) = A(X, p) B(X, p) + (‚X A(X, p)) ‚p B(X, p)

’ (‚p A(X, p)) ‚X B(X, p) . (7.32)
In the mixed coordinates, the operator part of the inverse Green™s function, G’1
of Eq. (3.68), becomes a simple multiplicative factor

G’1 (E, p, R, T ) = E ’ ξp ’ V (R, T ) , (7.33)

where V (R, T ) is an applied potential, and ξp = p ’ μ is the single-particle energy
measured from the chemical potential, and for quadratic dispersion, such as the case
for the free electron model, p = p2 /2m.

7.3 Gradient approximation
To make progress towards an intelligible and tractable equation, one assumes that the

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