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Is phonon a quasiparticle, or is it something different? Samohyl Jan 08:56, 5 Apr 2005 (UTC)
I can't parse this sentence:
Maybe it should read
But since I don't understand the topic :-(, I'm loathe to edit it. Mcswell ( talk) 20:46, 9 June 2008 (UTC)
Phonons are the quanta of classical sound waves and sound waves do not need the notion of atoms. Magnons are the quanta of classical spinwaves, which also do not need elementary spins. Photons inside an isolator are the quanta of classical dressed electromagnetic waves and do not need the notion of electrons for the definition of the refractive index. Plasmons are the quanta of the plasma oscillations and they only need charge density and mass density and no electrons or ions. Polarons are the quanta of the oscillating polarization in a lightly doped semiconductor and also do not need elementary charge or mass.
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)
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)
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)
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)
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 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 am reverting this edit because I think it introduces many factual inaccuracies or misleading implications:
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)
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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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)
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)
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Is phonon a quasiparticle, or is it something different? Samohyl Jan 08:56, 5 Apr 2005 (UTC)
I can't parse this sentence:
Maybe it should read
But since I don't understand the topic :-(, I'm loathe to edit it. Mcswell ( talk) 20:46, 9 June 2008 (UTC)
Phonons are the quanta of classical sound waves and sound waves do not need the notion of atoms. Magnons are the quanta of classical spinwaves, which also do not need elementary spins. Photons inside an isolator are the quanta of classical dressed electromagnetic waves and do not need the notion of electrons for the definition of the refractive index. Plasmons are the quanta of the plasma oscillations and they only need charge density and mass density and no electrons or ions. Polarons are the quanta of the oscillating polarization in a lightly doped semiconductor and also do not need elementary charge or mass.
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)
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)
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)
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)
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 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 am reverting this edit because I think it introduces many factual inaccuracies or misleading implications:
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)
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)
Hello fellow Wikipedians,
I have just modified 3 external links on Quasiparticle. Please take a moment to review my edit. If you have any questions, or need the bot to ignore the links, or the page altogether, please visit this simple FaQ for additional information. I made the following changes:
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Sourcecheck}}
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regular verification using the archive tool instructions below. Editors
have permission to delete these "External links modified" talk page sections if they want to de-clutter talk pages, but see the
RfC before doing mass systematic removals. This message is updated dynamically through the template {{
source check}}
(last update: 5 June 2024).
Cheers.— InternetArchiveBot ( Report bug) 18:15, 20 July 2016 (UTC)
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)
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)