Exploring Phonons in Dielectrics: A Beginner's Guide

[androz]

Hi everybody.
I know a little about phonons in a lattice such as semiconductors, which are energy quanta of the vibrational states of the crystal, but I can't figure out how a phonon can exist in a dielectric or even in a non monocristalline material. Can somenone help me or advise me a book on the subject ?

 

[Tyger]

Imagine a Hydrogen molecule, with just two protons and a pair of electrons. The protons can have vibrational exitations much like two weights coupled by a spring, with the electron pair acting as the spring. Now put many Hydrogen molecules together to form a crystal and you can have local or collective exitations of the protons in the crystal.

 

[androz]

Hi Tyger
Thanks for your answer, but I was wondering of what appens in a dielectric, I mean in a glass (such as silica). I agree that locally it is a crystal so perhaps we can speak of phonons, but locally. Moreover, can a phonon propagate ? Surely in a crystal, but can it propagate between two joined crystals, so in a glass ?

 

[ZapperZ]

Hi Tyger
Thanks for your answer, but I was wondering of what appens in a dielectric, I mean in a glass (such as silica). I agree that locally it is a crystal so perhaps we can speak of phonons, but locally. Moreover, can a phonon propagate ? Surely in a crystal, but can it propagate between two joined crystals, so in a glass ?

Let's go to one extreme where there ISN'T any crystalinity - gas. We already know that sound can propagate in gas simply via the transfer of vibration via collision. Now, go back to a polycrystal. The fact that they are not in a single crystal only imply that the phonon modes are not that well-defined as in single crystals. However, it doesn't mean that there no phonons whatsover. Sound still can propagate through a dielectric (which again is nothing more than a semicoductor but with a large band gap - so if you have no problems with a semiconductor in the first place, you shouldn't have a problem with dielectric in general).

As long as they are some form of bonding or scattering, that should be a sufficient mechanism for such phonons to exist. It is just that such modes are rather very dispersive in a polycrystal.

Zz.

 

[androz]

Thank you Zz
do you mean that in a glass for example (phonon that are not well defined) the band structure of the phonons is extended and that there are more energy band permitted ?

 

[ZapperZ]

Thank you Zz
do you mean that in a glass for example (phonon that are not well defined) the band structure of the phonons is extended and that there are more energy band permitted ?

Glass, or "glassy" state, is one of the most complex structure in condensed matter. So I'm not sure how in-depth you want the explanation to be. The phonon density of state of glass isn't that trivial either. However, take note that you can have a crystalline silicate (or glass). So not all dielectric are polycrystalline.

When you have well-defined phonon modes, then you can have absorption of that frequency of the EM spectra. The fact that glass is often transparent over the visible frequency range means that there is a broad phonon band over such freq. range but no well-defined modes.

Zz.

 

[androz]

Thank you very much Zz. The phonons ever wake up my interest but I didn't ever have a good idea of how it can interact whith EM wave. I think I understant better now. How could I go farther in this area, just for my culture ? Are there articles or books I could read on the subject ?

 

[ZapperZ]

Thank you very much Zz. The phonons ever wake up my interest but I didn't ever have a good idea of how it can interact whith EM wave. I think I understant better now. How could I go farther in this area, just for my culture ? Are there articles or books I could read on the subject ?

Not knowing what your background in physics is, I can't give you specific recommendations. You may, if you haven't done so, pick up an elementary solid state physics text. The fundamentals of phonons and normal modes are usually covered in those texts.

Zz.