Talk:Quasiparticle

Phonon
Is phonon a quasiparticle, or is it something different? Samohyl Jan 08:56, 5 Apr 2005 (UTC)


 * It's a "collective mode" excitation, not a quasiparticle. -- CYD


 * It's not as simple as CYD makes it sound. In some cases one can make a clear distinction between quasiparticles and collective modes on the basis of their quantum numbers.  For example, electrons (charge -1, spin 1/2) are clearly different from plasmons (charge 0, spin 0).  An electron is a quasiparticle similar to a free electron, whereas a plasmon is a collective mode involving oscillations of density in an electron gas.  In other cases collective excitations and quasiparticles are not that different and one can, in fact, go continuously from one to the other.  A well-known example is a weakly interacting Bose gas.  Its excitations are phonons at the lowest energies and momenta.  But at higher energies and momenta these excitations are indistinguishable from free particles.  Here quasiparticles and collective modes form a single branch of excitations and can be "deformed" gradually into each other.  -- Oleg Tchernyshyov 02:57, 27 January 2006 (UTC)


 * It can be even worse: sometimes the same object can be viewed as a quasiparticle or as a collective excitation. For instance, a magnon in a ferromagnet can be considered in one of two perfectly equivalent ways: (a) as a mobile defect (a misdirected spin) in a perfect alignment of magnetic moments or (b) as a quantized spin wave that involves the precession of many spins.  In the first case the magnon is treated as a quasiparticle, in the second as a collective excitation.  I stress that both descriptions are equivalent to each other.  -- Oleg Tchernyshyov 03:25, 27 January 2006 (UTC)


 * Added a section about mean-field/plasmas.Goeie 05:11, 16 July 2007 (UTC)

What does this mean?
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)

What does this mean? Part 2
What's with the "do not need"? Is there some deep philosophical point that got lost during the editing process? There are elementary spins, elementary charges, atoms, electrons, etc. Speculating about what aspects of physics would or would not remain the same in the absence of these things (with what to replace them??) seems pointless and is liable to confuse people anyway. I'd like to replace this paragraph with a straightforward description of phonons, plasmons, etc., if no one objects. --Steve (talk) 06:03, 17 July 2008 (UTC)

Go right ahead. That paragraph is just overall weird and confusing. Headbomb {ταλκ – WP Physics: PotW} 06:36, 17 July 2008 (UTC)
 * Seems to me that you could have sound waves through a continuous (that is, not made of atoms) material. But we don't have any of those, so it doesn't seem like it matters much. Gah4 (talk) 23:20, 13 January 2024 (UTC)
 * Seems to me that you could have sound waves through a continuous (that is, not made of atoms) material. But we don't have any of those, so it doesn't seem like it matters much. Gah4 (talk) 23:20, 13 January 2024 (UTC)

Concerning: "What does this mean?"
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)

phonon quasiparticle or collective excitation
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)


 * 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)


 * 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)


 * 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)

This makes no sense. The definition in the article clearly states "In physics, quasiparticle refers to a group of discrete phenomena whose behaviour is characterised as that of a single particle in a system, INCLUDING the effect the particle has on the system. It can be roughly defined as the combination of a particle and its influence on the local environment" This does not describe a phonon which is a quantum of energy in a "normal mode" - in a small crystal this includes ALL the atoms,say, 10^22. Please say what the 'cloud' of other 'entities' is that a particular phonon is interacting with. I am not saying that a phonon can't interact with other things, just, that like a photon it is defined in terms of the normal modes of the WHOLE system,which is what makes it a 'collective excitation'. What makes something a quasiparticle, say an electron in a crystal, is that it interacts with phonons and acquires a 'quasi-momentum' or mass, this does not make the phonons quasiparticles, in my opinion. If many authors are calling a phonon a quasiparticle then it would seem that the term does not have a clear meaning and should be removed, or stated clearly that it is a term of 'vague' usage. RedAcer (talk) 11:53, 7 March 2011 (UTC) — Preceding unsigned comment added by RedAcer (talk • contribs) 11:51, 7 March 2011 (UTC)


 * 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)


 * 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)


 * 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)

This article is screwed up
The second paragraph of the article says "Most many-body systems possess two types of elementary excitations. The first type, the quasiparticles, correspond to single particles whose motions are modified by interactions with the other particles in the system. The second type of excitation corresponds to a collective motion of the system as a whole. These excitations are called collective modes, and they include phenomena such as zero sound, plasmons, and spin waves." THIS CLEARLY STATES THAT THERE ARE TWO TYPES OF EXCITATIONS, the QUASIPARTICLES and the COLLECTIVE EXCITATIONS. By the definition given here a phonon is a collective excitation and NOT a quasiparticle, which is said to be a SINGLE particle modified by its interactions with other particles. RedAcer (talk) 12:02, 7 March 2011 (UTC)


 * I agree that the article is "screwed up." I am posting the "in need of an expert" template. Dratman (talk) 17:04, 12 January 2012 (UTC)


 * Can you be more specific? For example, RedAcer is clearly annoyed at something but I don't know what. RedAcer seems to be fully agreeing with what the the article already says. For example, the article already says that the phonon is a collective excitation not a quasiparticle. What's the problem? What is screwed up? --Steve (talk) 21:38, 12 January 2012 (UTC)


 * I am not saying the physics is wrong. I don't know enough to judge that. And probably I should not have seconded the phrase "screwed up." What I am trying to say is that I find the article very confusing and unclear. For example, compare the following two quotations from the article:


 * "In the language of many-body quantum mechanics, a quasiparticle is a type of low-lying excited state of the system... known as an elementary excitation."


 * "In physics, quasiparticles (and related collective excitations)... "


 * Upon initial reading, my impression was that "and related collective excitations" was intended to mean "along with other collective excitations." Now I think it is probably intended to mean something like "and the related phenomena known as collective excitations." If my first interpretation is the intended one, it contradicts "elementary excitations". If my second interpretation is the one intended, it ought to be restated for clarity.


 * The example just given is not in itself a very serious criticism, but it typifies the trouble I have while reading the article. Both quasiparticles and collective excitations seem to be conceptually tricky subjects. As such, they require a logical and well-organized presentation. I do not get that from the present state of the article, to such an extent that I cannot follow its content. At present the article is just not clearly written, in my opinion, and since I am not personally capable of improving it, I am asking that an expert try to do so. Dratman (talk) 23:06, 12 January 2012 (UTC)

I have another thought. Based on a quick perusal of other sources, it appears to me that quasiparticles are probably not bona fide entities of a formal, axiomatized theory. If this indeed turns out to be the case, my request for a really clear explanation is unlikely to be satisfied. In that case, I beg that the summoned expert (for whom I have drawn a geometrically accurate pentagram, burned incense, and so forth) will explain the situation in a calm and gentle way, in order to soothe our anxious brains, if only with mental pabulum. Dratman (talk) 23:36, 12 January 2012 (UTC)


 * I edited the first sentence to (hopefully) be slightly clearer.
 * Quasiparticles are indeed an emergent phenomenon. That doesn't mean they cannot be explained in a clear way! :-) --Steve (talk) 17:59, 13 January 2012 (UTC)


 * Well, I would say emergent phenomena, clearly explained, will of necessity have to be presented in a different style from, say, Euclidean geometry or axiomatic quantum mechanics. But, to be honest, I'm not sure quasiparticles or collective excitations should be called "emergent." In that extended sense, one might say that classical resonant vibration in a column of air is an emergent phenomenon, because the eigenmodes and eigenvalues are nowhere to be found in Newton's laws. Or that the solar system emerges from gravity and condensed matter. I don't think that is exactly the right way to use the concept of emergence.


 * But I am blathering.


 * Bottom line, I am in favor of anything that could make the article hang together as... as an emergent whole. Which I don't think it yet does. Dratman (talk) 00:46, 14 January 2012 (UTC)


 * I agree that phonons are similar to the classical resonant vibrations of a column of air. I would call both "emergent" and you would call neither "emergent". Oh well, it doesn't much matter. I certainly haven't spent much time thinking about it, and it's not addressed by the article except in one noncommittal sentence with references to further reading.


 * I tried to rewrite the article text for clarity, particularly to explain the many-body quantum mechanics jargon. The "history" section is still in poor shape, that's where I dumped the remaining content that I am too lazy and/or ignorant to edit. --Steve (talk) 16:35, 1 February 2012 (UTC)

inaccuracies in last edit
I am reverting this edit because I think it introduces many factual inaccuracies or misleading implications:
 * What quasiparticle has negative mass?
 * It is not easy to correctly interpret the claim that quasiparticles cannot be "directly measured". For example, holes flow through a p-type semiconductor and you "directly measure" the resulting current. The current is quantized and has shot noise statistics characteristic of individual particles. Hall effect, cyclotron resonance, etc. etc. can all be interpreted as relatively direct measurements of the properties of holes. I know what you mean, but it is not described well.
 * The difference between a quasiparticle and collective excitation is certainly not "lack of definition of its position". Neither can have a precisely-defined position, to the same extent and for the same reason.
 * Some things are not explained, like the term "different statistical distribution". Or what a quasiparticle is for that matter...it just says that they can be "defined". (How? Why? By whom?) --Steve (talk) 13:24, 20 October 2012 (UTC)


 * Here is a reference that mentions twice the possibility to define quasiparticles with negative mass: http://www.st-andrews.ac.uk/~jmjk/research/polariton-long.shtml. I'm quite sure about having heard that also in a solid state lecture.
 * I didn't want to say, that you can not measure a flow of quasiparticles, I meant that you can not isolate one of them in the same way you could trap a proton in a penning trap, and that is a key difference between particles and quasiparticles, see this reference: http://www.tf.uni-kiel.de/matwis/amat/semitech_en/kap_9/advanced/t9_1_1.html at the explanation of "Holes". In this page you will also see (and I agree with you), that solids are made of protons, electrons and neutrons, but it is wrong to say that they are elementary particles, as you can see in the page you linked with this term. Finally, and using the same reference again, I find very important to mention that photons are also particles (you can trap exactly one in a resonator and measure its properties, there are moreover recent papers about single-atom single-photon interactions).
 * In a semiconductor you define classically the holes as they would have a position in space (like "positive" electrons). But I agree with you for taking that comment out, it can give the people a wrong idea.
 * With "different statistical distribution" I meant the Fermi- and Bose- statistics (related to quasiparticles and collective excitations), which are already mentioned in the introduction of the article. It is for me redundant to mention them again. I also said that the quasiparticles can be defined by energy, momentum, mass, etc. That depends on the model used.

It is not my intention to be inaccurate, my objective is to explain quasiparticles at the beginning of the article in a way, that a general person (or at least a freshman) could imagine what is going on here and why they are not "normal" particles. Of course we can (and we have to!) formalize things very accurate as the article goes on.

Rolancito (talk) 16:19, 22 October 2012 (UTC)


 * Oops, you're right that proton and neutron are not "elementary". I have edited the wording. :-)
 * The lecture you link to is discussing holes. It is possible to describe these as having negative charge and negative mass, but it is much more common and wiser to describe them as having positive charge and positive mass. I think it is always possible to use a description where all the quasiparticles have positive (or zero) mass, although I could be wrong ... in any case it seems a rather obscure possibility and probably not worth discussing off the bat. I propose to not say anything about whether or not quasiparticles may have negative mass, do you think that's reasonable?
 * I understand that you think it's important to state clearly that a quasiparticle cannot exist floating in space. Do you think the current version is adequate in emphasizing that?
 * Do you think that the distinction between quasiparticles and collective excitations is not sufficiently well explained and accessible in the article right now? If so, do you think it would help to change the order of Sections 1.2 with 1.3?
 * Is there anything else that you find not well explained in the version now? Do you think a college freshman would find the introduction and Section 1.1 confusing? --Steve (talk) 01:20, 23 October 2012 (UTC)

Your argument is convincing, so let's forget about negative masses. I'm happy with the new section, I think we made an improvement here. The order is OK, since section 1.2 motivates both definitions (quasiparticles and collective excitations) needed in the 1.3 one. Thanks for the collaboration :)

Rolancito (talk) 14:11, 23 October 2012 (UTC)

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Solids are made of only three kinds of particles. Suggestion
The General Introduction currently contains the following sentence: "Solids are made of only three kinds of particles: electrons, protons, and neutrons." From the viewpoint of everyday chemistry, that might be close enough. More generally, of course, it is not true. Rather than detour into all the particles of the Standard Model, I suggest something like the following: "Solids are made of only three kinds of (stable, separable) matter particles: electrons, protons, and neutrons." I did not make that change myself as I suspect there is a better way to express the same general idea. Dratman (talk) 15:35, 27 September 2021 (UTC)
 * From a solid state physics (or chemistry) viewpoint, they are electrons and nuclei. That makes two. Unless you have energy to get excited states, that is, collective excitations, of the nucleus. Gah4 (talk) 23:26, 13 January 2024 (UTC)
 * From a solid state physics (or chemistry) viewpoint, they are electrons and nuclei. That makes two. Unless you have energy to get excited states, that is, collective excitations, of the nucleus. Gah4 (talk) 23:26, 13 January 2024 (UTC)

Fermi-Dirac distribution
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)