In many-body theory, the term Green's function (or Green function) is sometimes used interchangeably with correlation function, but refers specifically to correlators of field operators or creation and annihilation operators.
Contents
- Basic definitions
- Two point functions
- Imaginary time ordering and periodicity
- Spectral representation
- Hilbert transform
- Proof of spectral representation
- Non interacting case
- Zero temperature limit
- Noninteracting case
- References
The name comes from the Green's functions used to solve inhomogeneous differential equations, to which they are loosely related. (Specifically, only two-point 'Green's functions' in the case of a non-interacting system are Green's functions in the mathematical sense; the linear operator that they invert is the Hamiltonian operator, which in the non-interacting case is quadratic in the fields.)
Basic definitions
We consider a many-body theory with field operator (annihilation operator written in the position basis)
The Heisenberg operators can be written in terms of Schrödinger operators as
and
Similarly, for the imaginary-time operators,
[Note that the imaginary-time creation operator
In real time, the
where we have used a condensed notation in which
In imaginary time, the corresponding definition is
where
Note regarding signs and normalization used in these definitions: The signs of the Green functions have been chosen so that Fourier transform of the two-point (
and the retarded Green function is
where
is the Matsubara frequency.
Throughout,
(See below for details.)
Two-point functions
The Green function with a single pair of arguments (
where the sum is over the appropriate Matsubara frequencies (and the integral involves an implicit factor of
In real time, we will explicitly indicate the time-ordered function with a superscript T:
The real-time two-point Green function can be written in terms of 'retarded' and 'advanced' Green functions, which will turn out to have simpler analyticity properties. The retarded and advanced Green functions are defined by
and
respectively.
They are related to the time-ordered Green function by
where
is the Bose–Einstein or Fermi–Dirac distribution function.
Imaginary-time ordering and β-periodicity
The thermal Green functions are defined only when both imaginary-time arguments are within the range
Firstly, it depends only on the difference of the imaginary times:
The argument
Secondly,
for
These two properties allow for the Fourier transform representation and its inverse,
Finally, note that
Spectral representation
The propagators in real and imaginary time can both be related to the spectral density (or spectral weight), given by
where |α⟩ refers to a (many-body) eigenstate of the grand-canonical Hamiltonian H − μN, with eigenvalue Eα.
The imaginary-time propagator is then given by
and the retarded propagator by
where the limit as
The advanced propagator is given by the same expression, but with
The time-ordered function can be found in terms of
The thermal propagator
The spectral density can be found very straightforwardly from
where P denotes the Cauchy principal part. This gives
This furthermore implies that
where
The spectral density obeys a sum rule,
which gives
as
Hilbert transform
The similarity of the spectral representations of the imaginary- and real-time Green functions allows us to define the function
which is related to
and
A similar expression obviously holds for
The relation between
Proof of spectral representation
We demonstrate the proof of the spectral representation of the propagator in the case of the thermal Green function, defined as
Due to translational symmetry, it is only necessary to consider
Inserting a complete set of eigenstates gives
Since
Performing the Fourier transform then gives
Momentum conservation allows the final term to be written as (up to possible factors of the volume)
which confirms the expressions for the Green functions in the spectral representation.
The sum rule can be proved by considering the expectation value of the commutator,
and then inserting a complete set of eigenstates into both terms of the commutator:
Swapping the labels in the first term then gives
which is exactly the result of the integration of ρ.
Non-interacting case
In the non-interacting case,
From the commutation relations,
with possible factors of the volume again. The sum, which involves the thermal average of the number operator, then gives simply
The imaginary-time propagator is thus
and the retarded propagator is
Zero-temperature limit
As β→∞, the spectral density becomes
where α = 0 corresponds to the ground state. Note that only the first (second) term contributes when ω is positive (negative).
Basic definitions
We can use 'field operators' as above, or creation and annihilation operators associated with other single-particle states, perhaps eigenstates of the (noninteracting) kinetic energy. We then use
where
with a similar expression for
Two-point functions
These depend only on the difference of their time arguments, so that
and
We can again define retarded and advanced functions in the obvious way; these are related to the time-ordered function in the same way as above.
The same periodicity properties as described in above apply to
and
for
Spectral representation
In this case,
where
The expressions for the Green functions are modified in the obvious ways:
and
Their analyticity properties are identical. The proof follows exactly the same steps, except that the two matrix elements are no longer complex conjugates.
Noninteracting case
If the particular single-particle states that are chosen are `single-particle energy eigenstates', i.e.
then for
so is
and so is
We therefore have
We then rewrite
therefore
use
and the fact that the thermal average of the number operator gives the Bose–Einstein or Fermi–Dirac distribution function.
Finally, the spectral density simplifies to give
so that the thermal Green function is
and the retarded Green function is
Note that the noninteracting Green function is diagonal, but this will not be true in the interacting case.