[PDF]Lecture notes - 中国科学院物理研究所
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emuch:
元激发又叫准粒子。对于能量靠近基态的低激发态（这往往是凝聚态物理感兴趣的状态），可以将其看成一些独立的激发单元集合，具有确定的能量和波矢。
元激发的引入使得固体物理中的很多问题可以用统一的观点和方法来描述和处理，是整个固体理论的一个核心内容。
可以这么说，传统的固体理论就是在研究不同性质的元激发。比如集体激发，包括声子，自旋波量子，等离激元等等，和个别激发，比如准电子，准空穴，激子，极化子等等
這是 http://physics.zju.edu.cn/pw/sharely/ppt/%E5%87%9D%E8%81%9A%E6%80%81%E7%89%A9%E7%90%86%E5%9F%BA%E6%9C%AC%E6%A6%82%E5%BF%B52.pdf 的 HTML 檔。
G o o g l e 在網路漫遊時會自動將檔案轉換成 HTML 網頁。
基态通常是充分有序态。激发态显示恢复原本对称显示恢复原本对称各式各样的元激发与拓扑缺陷。
Quasiparticles and collective excitations are a type of low-lying excited state. For example, a crystal at absolute zero is in the ground state, but if one phonon is added to the crystal (in other words, if the crystal is made to vibrate slightly at a particular frequency) then the crystal is now in a low-lying excited state. The single phonon is called an elementary excitation. More generally, low-lying excited states may contain any number of elementary excitations (for example, many phonons, along with other quasiparticles and collective excitations).[3]
When the material is characterized as having "several elementary excitations", this statement presupposes that the different excitations can be combined together. In other words, it presupposes that the excitations can coexist simultaneously and independently. This is never exactly true. For example, a solid with two identical phonons does not have exactly twice the excitation energy of a solid with just one phonon, because the crystal vibration is slightly anharmonic. However, in many materials, the elementary excitations are very close to being independent. Therefore, as a starting point, they are treated as free, independent entities, and then corrections are included via interactions between the elementary excitations, such as "phonon-phonon scattering".
Therefore, using quasiparticles / collective excitations, instead of analyzing 1018 particles, one needs only to deal with only a handful of somewhat-independent elementary excitations. It is therefore a very effective approach to simplify the many-body problem in quantum mechanics. This approach is not useful for all systems however: In strongly correlated materials, the elementary excitations are so far from being independent that it is not even useful as a starting point to treat them as independent.
Distinction between quasiparticles and collective excitations[edit]
Usually, an elementary excitation is called a "quasiparticle" if it is a fermion and a "collective excitation" if it is a boson.[1] However, the precise distinction is not universally agreed.[2]There is a difference in the way that quasiparticles and collective excitations are intuitively envisioned.[2] 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": A real electron particle, in a crystal, behaves as if it had a different 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.[4][5]
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