The Spectral Function in Many-Body Physics and its Relation to Quasiparticles recently, I stumbled accross a concept which might be very helpful understanding quasiparticles and effective theories (and might shed light on an the question How to calculate the properties of Photon-Quasiparticles): the spectral function $$A\left(\mathbf{k},\omega \right) \equiv -2\Im G\left(\mathbf{k},\omega \right)$$
as given e.g. in Quasiparticle spectral function in doped graphene (on arXiv).
It is widely used in many-body physics of interacting systems and contains the information equivalent to the Greens function $G$. For free particles, $A$ has a $\delta$-peaked form and gets broader in the case of interactions.
The physical interesting thing is, as I read, quasiparticles of interacting systems can be found if $A$ is also somehow peaked in this case. I don't understand this relationship, hence my question:

What is the relation of the spectral function's peak to the existence of quasiparticles in interacting systems?

Thank you in advance
Sincerely
Robert
 A: Dear Robert, 
the answer to your question is trivial and your statement holds pretty much by definition.
You know, the Green's functions contain terms such as
$$G(\omega) = \frac{K}{\omega-\omega_0+i\epsilon}$$
where $\epsilon$ is an infinitesimal real positive number. The imaginary part of it is
$$-2\Im(G) = 2\pi \delta(\omega-\omega_0)$$
So it's the Dirac delta-function located at the same point $\omega$ which determines the frequency or energy of the particle species. At $\omega_0$, that's where the spectrum is localized in my case. If there are many possible objects, the $G$ and its imaginary part will be sums of many terms.
This delta-function was for a particle of a well-defined mass (or frequency - I omitted the momenta). If the particle is unstable, or otherwise quasi-, the sharp delta-function peak will become a smoother bump, but there's still a bump.
Because you didn't describe what you mean by "peak" more accurately, I can't do it, either. It's a qualitative question and I gave you a qualitative answer.
Cheers
LM
A: Spectral function gives the number of state(or density of state if you divide volume,...etc), The peak means there's a state or there're several degenerate states there. In single particle system, spectral function are only delta function sets at where eigenstates are. Considering the many-body interaction (for ex: electron-electron interaction, electron-phonon interaction...etc in Condensed Matter) into hamiltonian as a perturb term and calculating the approximate solution in some degree, the new eigenstates ket could be called quasiparticle. Sometimes we called this particle as "dressed electron". It's just a approximation which merge those complicated interaction and electron into a "quasiparticle". Thus, the spectral function couldn't be so simple as a set of delta function in single electron system, but relates with the interaction, which add a so-called "self energy" term in spectral function. The real part of self-energy changes the peak position, the imaginary part changes the life time of the state.
you can see the ch1 & ch2 in this book: Green's Functions and Condensed Matter by G.Rickayzen.
Hope this message will help you.
