Talk:Quasiparticle

message: I can't parse this sentence:
Both definitions mutually exclude each other as with the former definition collective excitations which are no "real" particles are considered to be quasiparticles.
Maybe it should read
Both definitions mutually exclude each other, since under the former definition collective excitations which are not "real" particles are considered to be quasiparticles.
But since I don't understand the topic :-(, I'm loathe to edit it. Mcswell (talk) 20:46, 9 June 2008 (UTC)
thread_title: What does this mean?

message: I try here to expose what is my understanding in the hope that, with the help of other comments, this will help to clarify the point.
To understand what is a quasi-particle I feel the need to have a definition of what is a particle. For this I take the idea that particles are the elementary particles of the standard model and the ones which can be composed from the of these. All this particles can be though in an ideal situation of single particles propagating in the vacuum. So for each of these I can write a single particle Hamiltonian. Accordingly to this Hamiltonian such particles last forever if we do not consider vacuum fluctuations and processes such as pair productions.
Important point for me: here for vacuum I mean what is vacuum according to our common sense, the vacuum of the universe, none of the fundamental or composite particles is present.
Clearly the notion of quasi-particle appears when we have a many body system with interacting particles, otherwise we could simply speak of particles. Now to describe a quasi-particle I can classify two different "phenomena" different from particles:
A - we start from a different definition of the vacuum and we change basis writing down the hamiltonian (i.e. we choose a basis where the Hamiltonian is diagonal, in the diagonalization we define a new vacuum)
B - we have quasi-particles which does not last forever even without considering the vacuum fluctuations or pair production-like phenomena, but which lasts for a long time.
To possibility "A" belongs all phenomena which are eigenstate of some interacting many-body Hamiltonian such as phonons, magnons, excitons, plasmons, etc..this can be more or less collective phenomena with respect to real particles description, think the electronic excitations in a solid, we can have some eigenstates which are practically indistinguishable from single particles and then move towards collective phenomena (some excitations are 10% collective, some other 30% and so on..)
To possibility "B" belongs instead phenomena which are not eigenstate of the Hamiltonian and so have a specific life-time. I feel that Landau, when introduced the idea of quasi-particle, was thinking to phenomena of this type. For example, always thinking to the excitations of a solid instead of looking at all possible eigenstate of the Hamiltonian we can describe the main peaks of the spectrum as quasi-particles so as energies with a real (the center of the peak) and an imaginary part (the width of the peak). Mathematically this correspond to switch from a 
Lemann rappresentation of the Green's function to the quasi-particle rappresentation through the analitic continuation.
Now with respect to what is written in the article these seem to be the two possibilities considered by the author. Anyway I don't feel that they are mutually exclusive, it's always possible to consider a new vacuum and so a new basis (phonons, magnons, etc..) and then to find phenomena of the kind B.
Dave (talk) 19:04, 2 June 2009 (UTC)
thread_title: Concerning: "What does this mean?"

message: The introduction says that a phonon is well known example of a quasi particle. In the section 'more common examples' a phonon is defined as collective excitation.
In the book 'a guide to Feynman diagrams and the many body problems', by Richard Mattuck, which has seperate chapters on quasi particles and collective excitations a phonon is described as a collective excitation of the whole lattice of atoms. Quote from page 227. "Collective excitations are the quanta associated with collective motions of the system as a whole, such as, phonons which are the quanta of the sound wave. Like quasi particles, collective excitations have particle like properties, BUT, unlike quasi particles, these qualities do not at all resemble those of the original particles of the system."
I will change the introduction to reflect this and remove the contradiction in the article.
RedAcer (talk) 09:46, 6 March 2011 (UTC)
replies: text: That doesn't make sense to me. A hole does "not at all resemble those of the original particles of the system": There is no small positively-charged particle in a crystal. (There are no positrons!) So, is a hole also a collective excitation? --Steve (talk) 20:58, 6 March 2011 (UTC)
replies: text: Not too sure what exactly you're getting at... quasiparticles are collective excitations. And that quote is completely unclear as to what it means. Headbomb {talk / contribs / physics / books} 21:39, 6 March 2011 (UTC)
replies: text: I concur that phonons are often called quasiparticles and this does not contradict in any way the notion of elementary excitations. Quasiparticles resemble real particle in that they have properties analogous to particles, location, momemtum, sometimes even mass. They are collective excitations, because they always effect all surrounding, real or quasi-, particles in the cloud. I have restored the phonon example with some prose modification. Kbrose (talk) 23:36, 6 March 2011 (UTC)

text: I think it's clear that some authors use the term "quasiparticle" generically to refer to everything (phonons, holes, quasi-electrons, etc.), while other authors say that electrons and holes and "quasiparticles" and phonons are "elementary excitations which are closely related to quasiparticles". The distinction between one and the other seems murky though...Mattuck's definition of quasiparticle (quoted above, here is the link) should not include holes, yet he does class holes as quasiparticles. In fact he admits that his definition of quasiparticle isn't quite general: "we include [bogoliubov quasiparticles] here because they are called quasi particles, but actually their structure is quite different...". Here is another source that uses a simpler definition: If it's a fermion, it's a quasiparticle; If it's a boson, it's a collective excitation. It's hard for me to get a handle on what all the definitions are and what is the most mainstream. What are other people's experience? Are there more sources? --Steve (talk) 23:34, 7 March 2011 (UTC)
replies: text:  I've started a thread on sci.physics.research(moderated) asking about current usage of 'quasiparticles'. Research physicist post there so the answers should be reasonable. Looking at a book I used as a grad student doing research on phonons; 'Lattice Vibrations' by Donovan and Angress - In 200 pages quasiparticles are only mentioned on one page where it is clear he paying 'lip-service' by saying 'strictly speaking they should be called  quasiparticles' and then only mentioning them once again, on the same page, describing the limitations. A book I have, containing ~200 research papers on phonon interactions, has only 4 papers where it is mentioned, none of them referring to phonons, but referring to cooper-pairs and other stuff. --RedAcer (talk) 11:22, 11 March 2011 (UTC)
replies: text: I added into the intro the Kaxiras definition, but said it wasn't universal, and gave the Mattuck definition too in a footnote. I agree, it is universally agreed that electrons and holes are quasiparticles not collective excitations and phonons are vice-versa. So I avoided calling phonons quasiparticles. I think other things besides electrons and holes and phonons might be controversial for which category they are. Like, "dressed photons" are quasiparticles under Mattuck's definition but collective excitations under Kaxiras's. --Steve (talk) 19:05, 11 June 2011 (UTC)
thread_title:  phonon quasiparticle or collective excitation 

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thread_title:  External links modified 

message: The article says: This occurs because of the Boltzmann distribution, which implies that very-high-energy thermal fluctuations are unlikely to occur at any given temperature.  Seems to me that should be Fermi-Dirac distribution. Gah4 (talk) 23:29, 13 January 2024 (UTC)
thread_title:  [[Fermi-Dirac distribution]]