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===Distinction between quasiparticles and collective excitations=== Usually, an elementary excitation is called a "quasiparticle" if it is a [[fermion]] and a "collective excitation" if it is a [[boson]].<ref name="Kaxiras" /> However, the precise distinction is not universally agreed upon.<ref name="Mattuck" /> There is a difference in the way that quasiparticles and collective excitations are intuitively envisioned.<ref name=Mattuck/> A quasiparticle is usually thought of as being like a [[dressed particle]]: it is built around a real particle at its "core", but the behavior of the particle is affected by the environment. A standard example is the "electron quasiparticle": an electron in a crystal behaves as if it had an [[Effective mass (solid-state physics)|effective mass]] which differs from its real mass. On the other hand, a collective excitation is usually imagined to be a reflection of the aggregate behavior of the system, with no single real particle at its "core". A standard example is the [[phonon]], which characterizes the vibrational motion of every atom in the crystal. However, these two visualizations leave some ambiguity. For example, 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 quantum of a collective [[spin wave]] that involves the precession of many spins. In the first case, the magnon is envisioned as a quasiparticle, in the second case, as a collective excitation. However, both (a) and (b) are equivalent and correct descriptions. As this example shows, the intuitive distinction between a quasiparticle and a collective excitation is not particularly important or fundamental. The problems arising from the collective nature of quasiparticles have also been discussed within the philosophy of science, notably in relation to the identity conditions of quasiparticles and whether they should be considered "real" by the standards of, for example, [[entity realism]].<ref>{{Cite journal |doi = 10.1080/0269859032000169451|title = Manipulative success and the unreal|journal = International Studies in the Philosophy of Science|volume = 17|issue = 3|pages = 245β263|year = 2003|last1 = Gelfert|first1 = Axel|citeseerx = 10.1.1.405.2111|s2cid = 18345614}}</ref><ref>B. Falkenburg, ''Particle Metaphysics'' (The Frontiers Collection), Berlin, Germany: Springer 2007, esp. pp. 243β246.</ref>
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