When is quasiparticle same as elementary excitation, and when is not?

Question

Can anyone shed light on the comparison between these two concepts?

This post imported from StackExchange Physics at 2014-08-22 05:05 (UCT), posted by SE-user huotuichang

Comments

As it stands this is rather a broad question. The best course would be for you to read around the area a bit and come back to us with any specific questions.

This post imported from StackExchange Physics at 2014-08-22 05:05 (UCT), posted by SE-user John Rennie

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See physics.stackexchange.com/questions/21954/… for an answer.

This post imported from StackExchange Physics at 2014-08-22 05:05 (UCT), posted by SE-user Xiao-Gang Wen

Answer 1

In the context of condensed matter physics:

To make it short, and with the caveat that it is not a universally accepted definition, an elementary excitation may be called a quasiparticle if it is fermionic (e.g. dressed electron), and collective excitation if it is bosonic in nature (e.g. phonon, magnon).

But there is not clear cut and absolute divide between such terms, and you will not get into trouble for using them in a lax manner.

This post imported from StackExchange Physics at 2014-08-22 05:05 (UCT), posted by SE-user Dominique Geffroy

Comments

Hello @IsidoreSeville Then, can some of you provide the definition of "elementary excitation" in condensed matter physics? I only see one sense in which an "elementary excitation" can be elementary. If this answer is correct, then, in my opinion, this condensed matter terminology should be abandoned because it is confusing and useless.

This post imported from StackExchange Physics at 2014-08-22 05:05 (UCT), posted by SE-user drake

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Hi @Drake, I don't know how to rigorously define it. But I would identify elementary excitation with quasi-particle. But there are circumstances in which the elementary excitations are not particle-like but string-like. So I would think the notion of elementary excitation is slightly broader.

This post imported from StackExchange Physics at 2014-08-22 05:05 (UCT), posted by SE-user Isidore Seville

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Yes Isidore, that is my understanding and what I tried to explain (probably clumsily in my answer): in my understanding elementary excitations are a broad term which covers quasi particle and collective excitations. All this is valid in the context of condensed matter physics, which might not be the only valid context, or even the context the question was set in. I have edited my answer to give precise examples and emphasize the condensed matter physics context.

This post imported from StackExchange Physics at 2014-08-22 05:05 (UCT), posted by SE-user Dominique Geffroy

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Thanks for your answer! You clearly place them in categories. But I often see people using them in a rather lax manner, barely distinguishable in terms of Fermion and Boson. Anyway, I think your answer makes sense.

This post imported from StackExchange Physics at 2014-08-22 05:05 (UCT), posted by SE-user huotuichang

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Indeed, I think the terminology is rarely used in a very strict manner.

This post imported from StackExchange Physics at 2014-08-22 05:05 (UCT), posted by SE-user Dominique Geffroy

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Hello, you read correctly, but excitation=particle, while elementary$\neq$collective . Thanks.

This post imported from StackExchange Physics at 2014-08-22 05:05 (UCT), posted by SE-user drake

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I guess we have a different understanding of the question, best would be to have huotuichang's own opinion on the interpretation of his own question, if he still cares about it a few months on. Thanks.

This post imported from StackExchange Physics at 2014-08-22 05:05 (UCT), posted by SE-user Dominique Geffroy

Answer 2

Dictionary for this answer: Excitation = particle; collective excitation = quasi-particle.

Short answer:

Elementary particles are never quasi-particles, by definition of elementary. This does not mean that what now it is thought as an elementary particle could be a quasi-particle of entities to be discovered.

Mathematical answer:

Elementary particles are those that correspond to irreducible representations of the Poincare group.

Physical answer:

Quasi-particles require the existence of an external medium or fields, whereas elementary particles do not. For example, phonons require a solid or a fluid to exist (they are collective modes of the atomic lattice vibration), likewise pions require a quark-antiquark sea. These are not fundamental particles, in the sense that they need the existence of other particles. A closed notion is that of composite particle, for example a molecule is made of atoms which, in turn, are made of a nucleus plus electrons. The difference between quasi and composite particles lies in the fact that quasi-particle are though as collective excitations of many particles (usually of the order on the Avogadro number $\sim 10^{23}$, but there may be far fewer, but not tens), while composite particles are more like building blocks where each constituent may be an elementary particle—such as an electron— or another composite particle—such as an atom— (a molecule is usually made of a few or tens of atoms, an atom usually contains from a few to tens of electrons plus a nucleus). Nevertheless, the difference between both concepts is not sharp; for example, pions are somehow made of quarks and antiquarks, they actually are also collective modes (waves) on the quark-antiquark sea, being quasi-Goldstone bosons of the approximate chiral symmetry.

The difference between elementary and composite particles is tied to the human knowledge at the time. At a certain point, it was though that nuclei were elementary, after that people realized that there were in fact more fundamental constituents (protons and neutrons), and later on quarks and gluons were discovered.

This post imported from StackExchange Physics at 2014-08-22 05:05 (UCT), posted by SE-user drake

Comments

Thank you very much! Could you please explain more about the 'Mathematical answer'? I know a bit of group theory but cannot figure out this link.

This post imported from StackExchange Physics at 2014-08-22 05:05 (UCT), posted by SE-user huotuichang

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@huotuichang The Poincare algebra has two quadratic Casimir invariant: the square of the cuadri-momentum (the particle's mass) and the square of the Pauli- Lubanski (related to the spin or helicity of the particle), which self-commute. Therefore, the eigenvalues of these operators are good label. This classification is originally due to Wigner. Elementary particles are also classified according to the way they transform under the gauge group of the standard model $U_y(1)\times SU_l(2)\times SU_c(3)$

This post imported from StackExchange Physics at 2014-08-22 05:05 (UCT), posted by SE-user drake