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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 any one 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 occured 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 any one 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 occured 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 [Broken] 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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