- Green's function (many-body theory)
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.
The name comes from the
Green's functionsused to solve inhomogeneous differential equations, to which they are loosely related. (Specifically, only two-point 'Green's functions' are Green's functions in the mathematical sense; the linear operator that they invert is the part of the Hamiltonian operator that is quadratic in the fields.)
patially uniform case
Throughout, is for
bosons and for fermions and denotes either a commutatoror anticommutator as appropriate.
The signs of the Green functions have been chosen so that the thermal Green function for a free particle is:and the retarded Green function is:In this section, functions will have their arguments abbreviated as single numbers: and will be replaced by .
The imaginary-time Heisenberg operators can be written in terms of Schrödinger operators as::where is the grand-canonical Hamiltonian. Similarly, for the real-time operators,:and .
In imaginary time, the general Green function is defined by:where signifies . (The imaginary-time variables are restricted to the range to .)
In real time, the definition is:where signifies .
The Green function with a single pair of arguments () is referred to as the two-point function, or propagator. In the presence of both spatial and temporal translational symmetry, it depends only on the difference of its arguments. Taking the Fourier transform with respect to both space and time gives:where the sum is over the appropriate Matsubara frequencies (and the integral involves an implicit factor of , as usual).
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 -periodicity
The thermal Green functions are defined only when both imaginary-time arguments are within the range to . The two-point Green function has the following properties. (The position or momentum arguments are suppressed in this section.)
Firstly, it depends only on the difference of the imaginary times::The argument is allowed to run from to .
Secondly, is periodic under shifts of . Because of the small domain within which the function is defined, this means just:for . (Note that the function is antiperiodic for fermions.) Time ordering is crucial for this property, which can be proved straightforwardly, using the cyclicity of the trace operation.
These two properties allow for the Fourier transform representation and its inverse,:
Finally, note that has a discontinuity at ; this is consistent with a long-distance behaviour of .
The propagators in real and imaginary time can both be related to the spectral density (or spectral weight), given by:where refers to a (many-body) eigenstate of the grand-canonical Hamiltonian , with eigenvalue .
The imaginary-time propagator is then given by:and the retarded propagator by:where the limit as is implied.
The advanced propagator is given by the same expression, but with in the denominator. The time-ordered function can be found in terms of and . As claimed above, and have simple analyticity properties: the former (latter) has all its poles and discontinuities in the lower (upper) half-plane. The thermal propagator has all its poles and discontinuities on the imaginary axis.
The spectral density can be found very straightforwardly from , using the
Sokhatsky-Weierstrass theorem:where denotes the Cauchy principal part.This gives:
This furthermore implies that obeys the following relationship between its real and imaginary parts::where denotes the principal value of the integral.
The spectral density obeys a sum rule::which gives:as .
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 by:and:A similar expression obviously holds for .
The relation between and is referred to as a Hilbert transform.
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 for , given by:Inserting a complete set of eigenstates gives:
Since and are eigenstates of , the Heisenberg operators can be rewritten in terms of Schr"odinger operators, giving: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 .
In the non-interacting case, is an eigenstate with (grand-canonical) energy , where is the single-particle dispersion relation measured with respect to the chemical potential. The spectral density therefore becomes:
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 , leaving:
The imaginary-time propagator is thus:and the retarded propagator is:
As , the spectral density becomes:where corresponds to the ground state. Note that only the first (second) term contributes when is positive (negative).
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 is the annihilation operator for the single-particle state and is that state's wavefunction in the position basis. This gives:with a similar expression for .
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 . Specifically,:and:for .
In this case,:where and are many-body states.
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.
If the particular single-particle states that are chosen are `single-particle energy eigenstates', ie,:then for an eigenstate::so is ::and so is ::
We therefore have:where:
We then rewrite:and use 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.
* Negele, J.W. and Orland, H.: "Quantum Many-Particle Systems" AddisonWesley (1988).
* Abrikosov, A.A., Gorkov, L.P. and Dzyaloshinski, I.E.: "Methods of Quantum Field Theory in Statistical Physics" Englewood Cliffs: Prentice-Hall (1963).
Linear response function
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