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I am struggling with the concept of phonons and if they should be considered (experimentally) to be treated as collective oscillations of a certain region instead of the whole crystal.

I know the title comes off as a little strange -- since phonons are not localised to anyone atom, but consider an experimental setup where we measure the phonon modes of a crystal, say neutron scattering, brillouin light scattering (BLS), or to lesser precision, raman spectroscopy.

In these techniques, the principle is that a particle, be it photon or neutron, scatters off the crystal, and excites/populates a phonon state.

This is where I start running into problems: as I understand, phonons are characteristic of the whole crystal, so it follows that the scattering event at point A (instantly?) causes the whole crystal to vibrate in such a way that is described by the created phonon... even if the process is not instantaneous, it would suggest that if I had a crystal big enough, the information that a phonon is created could potentially travel faster than the speed of light. Which is kind of problematic.

So I try to get out of this by saying that there is a certain "locality", or loci, around which I need to consider for collective oscillations around where the scattering event occurred to describe the phonon, from an experimental point of view, and take that the infinite lattice/whole crystal is an abstraction to simplify the problem. However, I am not sure (or know where else) to seek clarification on matters like these...

Thank you.

I know the title comes off as a little strange -- since phonons are not localised to anyone atom, but consider an experimental setup where we measure the phonon modes of a crystal, say neutron scattering, brillouin light scattering (BLS), or to lesser precision, raman spectroscopy.

In these techniques, the principle is that a particle, be it photon or neutron, scatters off the crystal, and excites/populates a phonon state.

This is where I start running into problems: as I understand, phonons are characteristic of the whole crystal, so it follows that the scattering event at point A (instantly?) causes the whole crystal to vibrate in such a way that is described by the created phonon... even if the process is not instantaneous, it would suggest that if I had a crystal big enough, the information that a phonon is created could potentially travel faster than the speed of light. Which is kind of problematic.

So I try to get out of this by saying that there is a certain "locality", or loci, around which I need to consider for collective oscillations around where the scattering event occurred to describe the phonon, from an experimental point of view, and take that the infinite lattice/whole crystal is an abstraction to simplify the problem. However, I am not sure (or know where else) to seek clarification on matters like these...

*This question was kind of lodged behind my mind for almost a year whenever I think of MoS2, I think of http://www.researchgate.net/profile/BK_Tay/publication/237068085_From_Bulk_to_Monolayer_MoS2_Evolution_of_Raman_Scattering/links/00b7d52cb4f4fc9049000000.pdf and then this question. In the paper, raman spectroscopy is employed to study the energy dependence of phonon modes on the number of layers, and did so by initially having a multilayered MoS2 crystal, and then scraping off the layers, making something that looks like stairs (figure 1).*

Therefore I reasoned (to myself) that if the phonon mode was indeed describing the whole crystal, then it should not matter where the laser is used, but indeed if there is a certain "locality" that is needed, then the layer dependence can be shown using the steps within the same MoS2 material.

Therefore I reasoned (to myself) that if the phonon mode was indeed describing the whole crystal, then it should not matter where the laser is used, but indeed if there is a certain "locality" that is needed, then the layer dependence can be shown using the steps within the same MoS2 material.

Thank you.

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